Patent Grant
8022899
1. Field of the Invention
The present invention relates to EL display apparatus and drive method of EL display apparatus which employs a self-luminous display panel (display apparatus) such as an EL display panel (display apparatus) using organic or inorganic electroluminescent (EL) elements, or the like.
2. Related Art of the Invention
With active-matrix image display apparatus which employ an organic electroluminescent (EL) material or an inorganic EL material as an electrochemical substance, emission brightness changes according to current written into pixels. An EL display panel is of a self-luminous type in which each pixel has a light-emitting element. EL display panels have the advantages of being more viewable than liquid crystal display panels, having high light emission efficiency, requiring no backlighting, having high response speed, etc.
Such an organic EL display panel of an active-matrix type is disclosed in Japanese Patent Laid-Open No. 8-234683.
An equivalent circuit for one pixel of the display panel is shown in
The operation shown in
A driver circuit which drives a pixel configuration in
However, an organic EL display panel uses transistor arrays made of low-temperature or high-temperature polysilicon, and variations in the characteristics of the transistors in the polysilicon transistor arrays of organic EL elements will cause display irregularities.
Current programming mode involves applying a video signal or other current signal (program current) given as a magnitude or intensity of a current to a data signal line, source signal line, or pixel, and applies the resulting current signal using pixel transistors to the EL elements.
Both an act of causing a current to flow into an EL element 15 from a driving transistor 11 and act of causing a current to flow into a driving transistor from an EL element 15 are referred to as applying the current from a driving transistor 11 to an EL element 15. In other words, the current programming mode involves a configuration, circuit configuration, or drive method which applies a current signal (program current) that is approximately proportional to the applied current or obtained by converting the applied current in a predetermined manner, either directly or indirectly to the EL element.
With the pixel configuration illustrated in
The voltage programming mode has a low capability to compensate for variations in transistor characteristics of the pixel 16. Thus, it involves display irregularities resulting from the variations in transistor characteristics. However, the voltage programming mode has a high capability to charge and discharge source signal lines and the like both in low tone and high tone regions. Thus, it can achieve proper image display without causing insufficient writing.
The display irregularities can be reduced by using the configuration of the current programming mode. The current programming mode provides low drive current in low tone regions. Consequently, parasitic capacitance of the source signal line 18 can prevent proper driving.
Incidentally, current programming (mode) is also called current driving and voltage programming (mode) is also called voltage driving.
To solve the conventional problem described above, the present invention has an object to provide an EL display apparatus and a drive method thereof which can eliminate insufficient writing in all tone regions while reducing display irregularities.
To solve the above problem, an EL display apparatus according to the present invention outputs a constant current, for example, from a driving transistor 11a of a pixel and measures a gate terminal potential of the driving transistor 11a via a source signal line 18 while the constant current is being outputted from the driving transistor 11a.
The measured potential is stored in memory after A/D (analog-digital) conversion. Preferably, data on the driving transistors 11a of all pixels are stored in the memory. For display on an EL display panel, voltage data of each pixel are read out of the memory and converted into a reference voltage by D/A (digital-analog) conversion. The reference voltage is applied as a precharge voltage Vp to the source signal line. After that, a program voltage is applied to the source signal line as required. Also, a target tone voltage is obtained by adding or subtracting a tone voltage to/from the reference voltage and applied to the driving transistor 11a of the pixel 16.
According to the present invention, voltage at the gate terminal of the pixel driver transistor is measured with a constant current being applied to the pixel transistor or outputted from the gate terminal of the pixel driver transistor. The gate terminal voltage varies among pixel driver transistors depending on the characteristics of the driver transistors.
Measuring the voltage at the gate terminal of the driver transistor with a constant current being applied to the driver transistor involves measuring the characteristics of the driver transistor. The measured voltage is stored in a memory placed or formed inside or outside a source driver IC (circuit) after A/D conversion. Alternatively, the measured or acquired voltage is sampled and held.
When displaying an image on the EL display apparatus, the voltage data stored in the memory are converted into analog voltage through D/A conversion, a target tone signal is obtained by adding or subtracting a tone voltage using the analog voltage as a reference or origin and is applied to the corresponding pixel. Alternatively, a target tone signal is obtained by adding or subtracting a tone voltage using the sampled and held voltage as a reference or origin and is applied to the corresponding pixel.
Thus, adding a video voltage corresponding to a tone or tone difference to the transistor with the measured voltage as a reference and applying the resulting signal to the transistor means applying a tone signal (voltage signal) acting as a video signal after compensating for the characteristics of the pixel driving transistor.
The gate terminal voltage of the driving transistor to be measured may be added/subtracted to/from the video voltage in real time after measurement but before application of the video voltage to the driving transistor of the pixel. The constant current may be zero (meaning that no current flows). In that case, the corresponding pixel can be selected and the driving transistor of the pixel can be shortened between gate and drain terminals without supplying a constant current Iw to the source signal line 18.
According to the present invention, the constant current Iw is a current set to a predetermined value or controlled to have a predetermined value and does not always need to be constant. That is, it means a current of a predetermined value. A constant current generating circuit may be included in a current tone circuit 154 or a separate constant current generating circuit may be provided. No constant current generating circuit is required for image display when passing the constant current Iw through the source signal line 18, measuring or acquiring the potential of the source signal line 18, and storing the measured or acquired potential as data in a memory or other storage device. That is, the constant current generating circuit is not part of the EL display apparatus.
The current programming mode has the disadvantage that it cannot compensate sufficiently for the characteristics of pixel transistors. However, by using current programming mode in which a constant current is applied to pixel transistors and measuring the gate terminal potential of the transistors, the present invention exercises its ability to compensate for transistor characteristics, which is an advantage of the current programming mode.
According to a first aspect of the present invention, the potential of the source signal line 18 is measured or acquired by selecting a pixel row and applying a constant current not lower than a predetermined level to the source single line 18. The measured potential represents the characteristics of the driving transistors 11 in the selected pixel row. The measured or acquired voltage is applied as a precharge voltage Vp to the source signal line 18 either directly or after an addition/subtraction process, thereby bringing the potential of the source signal line 18 close to a target potential. Then, a program current corresponding to a target video signal is written into the pixel 16.
A tone current for use in programming is determined using, as required, the measured or acquired voltage as a variable value for a function which determines tone for the video signal. The determined tone current is written into the pixel 16, and N-fold driving described with reference to
According to a second aspect of the present invention, the potential of the source signal line 18 is measured by selecting a pixel row and applying a constant current not lower than a predetermined level to the source signal line 18. The measured potential represents the characteristics of the driving transistors 11 in the selected pixel row.
A target tone voltage is determined using the measured voltage as a variable value for a function which determines tone for the video signal. By applying the determined tone voltage to the source signal line 18, the driving transistors in the selected pixel row is programmed so that a target current will flow through the EL element 15. Thus, the signal corresponding to the video signal applied to the pixel 16 is a voltage signal. The use of the voltage signal makes it possible to avoid insufficient writing even in low tone regions.
Thus, by calculating or determining tone voltages through addition or subtraction based on the measured voltages of the source signal lines 18 and applying the tone voltages to pixel transistors, it is possible to demonstrate a feature of voltage driving, i.e., the advantage of avoiding insufficient writing in all tone regions.
Although it is stated herein that the gate terminal voltage of the transistor is measured or held either directly or indirectly by applying a constant current to the transistor, the present invention is not limited to this. Also, in addition to the magnitude of voltage, the amount of changes in the voltage before and after the application of the constant current, speed of voltage changes, difference value of the voltage may also be measured and stored in memory.
The measurement of voltage also includes the act of holding the measured voltage inside or outside a driver circuit after analog-digital conversion (A/D conversion) and configuration therefor as well as the act of holding the voltage as digital data in memory. Also, it includes the act of not only measuring, but also temporarily holding, latching, or storing the voltage in a capacitor or other holding medium and configuration therefor. Besides, the constant current includes a state in which no current (0 A) is applied.
The constant current is not limited to being a fixed value. It may vary in one horizontal scanning period as does a sine waveform. It may have any configuration or value as long as it has a predetermined value when averaged over a certain period.
Some parts of drawings herein are omitted and enlarged and/or reduced herein for ease of understanding and illustration. Besides, the same or similar forms, materials, functions, or operations are denoted by the same reference numbers or characters.
Thin-film transistors are cited herein as driver transistors 11a and switching transistors 11b, and the like, this is not restrictive. Thin-film diodes (TFDs) or ring diodes may be used instead. Also, the present invention is not limited to thin-film elements, and transistors formed on silicon wafers may also be used. Needless to say, transistors may also be FETs, MOS-FETs, MOS transistors, or bipolar transistors. It goes without saying that the present invention may also use diodes, varistors, thyristors, ring diodes, photodiodes, phototransistors, or PLZT elements.
A source driver circuit (IC) 14 is not only a mere driver but may incorporate a power circuit (charge pump circuit, DCDC converter circuit), buffer circuit (including a circuit such as a shift register), level shifting circuit, data conversion circuit, latch circuit, command decoder, address conversion circuit, image memory, etc. A source driver circuit (IC) 14 may be formed on the array board 30 by polysilicon technology.
Although the array board 30 is described as being a glass substrate, it may be made of a silicon wafer. Also, the array board 30 may be made of a metal substrate, silicon or other semiconductor substrate, ceramic substrate, plastic sheet (board) or the like.
Needless to say, the transistors 11, gate driver circuits 12, and source driver circuits (ICs) 14 composing the display panel of the present invention may be formed on a glass substrate or the like and subsequently transferred to another substrate (plastic sheet) by transfer technology.
First, description will be given of configuration and operation of pixels 16 in an EL display apparatus according to the present invention as well as a source driver circuit (IC) 14 and the like.
In a pixel configuration shown in
The gate driver 12 (gate driver circuits 12a and 12b in
With the organic EL pixel configuration illustrated in
Clock signals CLK (CLK1 and CLK2), start signals ST (ST1 and ST2), and the like applied to the gate drivers 12 are applied to the source driver IC (circuit) 14 from the controller circuit 801. The clock signals CLK and start signals ST are applied to the gate driver circuits 12 after having their logic level changed by the source driver IC (circuit) 14. That is, the signals applied to the gate driver circuit 12 are supplied from the source driver IC (circuit) 14.
The gate driver circuit 12a may select not only a single gate signal line 17a, but also a plurality of pixel rows at a time. For example, it may select two gate signal lines 17a at a time. That is, it may select two pixel rows at a time.
In the display area 34, pixels for three primary colors of red (R), green (G), and blue (B) are formed in a matrix. The RGB pixels are formed by color filter deposition. Incidentally, a simple color or cyan, yellow, and magenta may be used instead of R, G, and B. Also, four colors may be used by adding white (R) to R, G, and B. In that case, color filters are used.
The display area 34 may have multiple screens for example, a main screen and sub screen. Separate gate driver circuits are provided for the main screen and sub screen while the source signal lines 18 are shared by the main screen and sub screen. Also, the source driver IC (circuit) 14 is shared by the main screen and sub screen.
In the display area 34, films composing the transistors of the pixel 16 are produced by directing a laser irradiation spot approximately in parallel to the source signal line longitudinally during laser annealing as illustrated in
An on current of transistors is relatively uniform if the transistors are monocrystalline. However, in the case of low-temperature polycrystalline transistors formed by low-temperature polysilicon technology at a temperature not higher than 450 to 550 degrees (centigrade), their threshold varies in a range of ±0.2 V to 0.5 V. The on current flowing through the driving transistors 11a varies accordingly, causing display irregularities. The irregularities are caused not only by variations in the threshold voltage, but also by mobility of the transistors and thickness of gate insulating film. Characteristics also change due to degradation of the transistors 11.
The variations in transistor characteristics are not limited to the transistors formed by low-temperature polysilicon technology, and can occur in transistors formed by high-temperature polysilicon technology at a process temperature of 450 degrees (centigrade) or higher or transistors formed on semiconductor films produced by solid-phase growth method (CGS). Besides, such variations can occur in organic transistors and amorphous silicon transistors.
The present invention is applicable to configurations or drive methods of EL display apparatus or display panels which use transistors and the like formed by any of the above technologies.
The transistors 11 of the pixels 16 in the display panel according to the present invention shown in
In
Incidentally, all the transistors in
However, to produce a panel at low cost, all the transistors 11 of pixels as well as the transistors in the gate driver circuits 12 should be P-channel transistors. By using only P-channel transistors for an array, it is possible to reduce the number of masks to 5, resulting in low costs and high yields.
When the driver transistor 11a and transistors (11b and 11c) of the pixel 16 are P-channel transistors as shown in
This is because the amount of shift in punch-through voltage due to the capacitor 19 and the like is constant and the VGH voltage (off voltage of transistors) and VGL voltage (on voltage of transistors) have fixed values. In current driving mode (current programming mode), the program current for low tone is small, making it difficult to charge and discharge the parasitic capacitance of the source signal lines 18. The generation of the punch-through voltage has the effect of reducing the program current (change the gate voltage potential of transistor 11a in such a way as to prevent current flow). Consequently, a relatively large program current can be applied to the source signal line 18 and a smaller current than the program current can be passed by the driving transistor 11a through the EL element 15. This makes it possible to write a small program current (program current in low tone regions) into the pixel 16.
The punch-through voltage depends on voltage amplitude (Vg=VGH−VGL) of the gate signal line 17a which selects pixels 16. In the current driving mode, it is important that the punch-through voltage work effectively. According to the present invention, magnitude of Vg is 6 V or higher. If Vdd denotes anode voltage and Vss denotes cathode voltage, potential difference Ve (=Vdd−Vss) between the anode voltage and cathode voltage is set equal to or smaller than Vg−0.5 (V).
In the case of a P-channel transistor, VGH is a voltage which turns off (opens) the transistor and VGL is a voltage which turns on (closes) the transistor. In the case of an N-channel transistor, VGL is a voltage which turns off (opens) the transistor and VGH is a voltage which turns on (closes) the transistor.
According to the present invention, the driving transistor 11a, transistor 11b, and the like are not limited to P-channel transistors. However, the present invention is characterized in that the driving transistor 11a (transistor 11b (see
Thus, by using P-channel transistors for both driving transistor 11a and switching transistor 11b of the pixel 16, the present invention provides a unique advantage capability to achieve proper black display (black and low tone regions). Incidentally, when the driving transistor 11a of the pixel 16 is an n-channel transistor, the switching transistor 11b is also an N-channel transistor. That is, it is preferable to use transistors of the same polarity for both driving transistor 11a and switching transistor 11b.
Next, a power supply (voltage) used by the EL display panel according to the present invention will be described with reference to
The gate driver circuit 12a performs on/off control of the gate signal lines 17a. The gate driver circuit 12b performs on/off control of the gate signal lines 17b. For ease of explanation, a pixel configuration in
Each shift register circuit 31 is controlled by a positive-phase and negative-phase clock signals CLKx (CLKxP and CLKxN) and a start pulse (STx), where x is a subscript. Besides, it is preferable to add an enable (ENBL) signal which controls output and non-output from gate signal lines and up/down (UD) signal which turns a shifting direction upside down. Also, it is preferable to install an output terminal to ensure that the start pulse is shifted by the shift register circuit 31 and is outputted.
Incidentally, shift timings of the shift register circuits 31 are controlled by a control signal from a controller circuit (not shown). Also, a level shift circuit 31 which level-shifts external data is incorporated. The clock signals may consist of only positive-phase clock signals. By using only positive-phase clock signals, it is possible to reduce the number of signal lines and thereby reduce bezel width.
Incidentally, shift timings of the shift register circuits 31 are controlled by a control signal from a control IC (not shown). Also, the gate driver circuit 12 incorporates a level shift circuit which level-shifts external data. The clock signals may consist of only positive-phase clock signals. By using only positive-phase clock signals, it is possible to reduce the number of signal lines and thereby reduce bezel width.
Since the shift register circuits 31 have small driving capacity, they cannot drive the gate signal lines 17 directly. Therefore, at least two or more inverter circuits (included in the buffer circuit 32) are formed between output of each shift register circuit 31 and an output gate which drives the gate signal lines 17.
To facilitate understanding, voltage values will be prescribed here. First, the anode voltage Vdd is specified to be 6 V and the cathode voltage Vss is specified to be −9 V (see
It is necessary to decrease VGL1 of the gate driver circuit 12 to make on-resistance of the transistor 11c in
Voltages in various parts of the EL display apparatus according to the present invention will be described with reference to
With the power supply circuit arrangement according to the present invention in
As illustrated in
If the amplitude of the gate signal line 17a which selects the pixel 16 is Vg=VGH−VGL, according to the present invention, Vg is set equal to or larger than 6 V. If the anode voltage is Vdd and the cathode voltage is Vss, the potential difference between the anode voltage and cathode voltage (Ve=Vdd−Vss) is equal to or larger than Vg+2 (V). The VGL voltages may be generated by a charge pump circuit or the like formed on the array board 30 by polysilicon technology. Preferably, an inrush current limiting circuit is installed at the input or output of a DC-DC (direct current-direct current) converter circuit which generates the anode voltage.
Although it has been stated with reference to
When VGL1 is smaller than VGL2, the amplitude of the gate signal line 17a works to increase the punch-through voltage of the gate terminal of the driving transistor 11a, which when combined with the driving method of the present invention, achieves better black display. For example, VGL1=−9 V and VGL2=−3 V.
To increase the magnitude of the program current outputted by the driving transistor 11a, it is necessary to increase the anode voltage Vdd. When the program current is increased, the EL element 15 emits light at high brightness, causing the EL display apparatus to provide a bright display. The bright display is useful when using the EL display apparatus outdoors. However, constant use of the high anode voltage Vdd increases power consumption of the EL display apparatus. Thus, it is desired to minimize a period or state in which the driving transistor 11a outputs a large program current. The present invention increases the anode voltage Vdd when bright display is required. Also, the anode voltage is increased as shown in
It has been stated with reference to
Besides, the anode voltage Vdd and cathode voltage Vss may be varied according to the type or state of displayed images such as moving images or still images. Also, the anode voltage Vdd and cathode voltage Vss may be varied according to external illuminance. In that case, the anode voltage Vdd and the like are increased when the external illuminance is high and the anode voltage Vdd and the like are decreased when the illuminance is low. The illuminance is detected by a PIN photodiode or the like. Incidentally, write condition may change depending on panel temperature when a program voltage or program current is applied. Again, this situation can be dealt with by varying the anode voltage Vdd and the like. The temperature is detected by a thermistor or posistor mounted on an ineffective area (area which does not emit light available for display) at the back of the panel. The present invention employs a method which varies or adjusts the anode voltage Vdd and/or cathode voltage Vss according to display brightness, write condition of a program current, display condition, a lighting ratio, external illuminance, and the like.
When the anode voltage Vdd is varied as described above by generating or controlling the power supply voltage used in the display apparatus, the power supply voltage of the source driver IC (circuit) 14 and Vmin and VGH of the precharge voltage Vp change accordingly. Thus, even if the anode voltage Vdd and the like are varied when a bright display is required, relative values of VGH and precharge voltage Vp change accordingly, making it possible to maintain a proper image display. This method will prove particularly effective when combined with a lighting ratio control method described with reference to
According to the present invention, the anode voltage Vdd and the like shown in
The EL display apparatus and drive method thereof according to the first aspect of the present invention basically consists of two operations: a first operation (write operation) and second operation (light-emitting operation). The first operation is divided into a potential-varying operation which involves forcibly varying the potential of the source signal line 18 (including varying the gate terminal potential of the driving transistor 11a of the pixel 16) by applying the precharge voltage Vp and the like to the source signal line 18 and a current programming operation which involves applying program currents to the driving transistor 11a and the like. Besides, an initial operation of measuring or acquiring the potential of the source signal line 18 by the application of a constant current (which may be 0 A) to the source signal line 18 is performed as required before the first operation.
The second operation involves causing the EL element 15 to emit light by applying a programmed current to the EL element 15 of the pixel 16 or passing a programmed current through the EL element 15 of the pixel 16. In the second operation, the current supplied from the driving transistor 11a to the EL element 15 is passed or blocked by applying an on/off voltage to the gate signal line 17b as required, as described with reference to
The present invention is not limited to application of voltages such as a precharge voltage Vp (or Va or V0). The present invention also includes applying a current (over-current) larger than program current to the source signal line 18 and charging and discharging the source signal line in a short time. An example is described with reference to
In the initial operation, the driving transistor 11a is operated by applying a constant current (predetermined program current) to the pixel driving transistor 11a, and when the operation of the driving transistor 11a enters a steady state, the gate terminal voltage of the driving transistor 11a or voltage of the source signal line 18 are measured. The measured voltage is stored in a memory or the like after A/D conversion. Alternatively, the voltage is held in a sample-and-hold circuit or the like. The acquired voltage is used for the potential-varying operation in the first operation.
Although it has been stated that a constant current is applied in the initial operation, the present invention is not limited to this. Instead of applying a constant current (constant current=0 A), the driving transistor 11a of the selected pixel 16 may be shorted between gate and drain terminals, and then the potential (Va or V0) may be measured or acquired when the driving transistor 11a is offset-cancelled (when the driving transistor 11a is cut off and does not pass a current). Since the source signal line 18 remains electrically connected with the gate terminal of the driving transistor 11a of the pixel 16 when the pixel 16 is selected, this potential can also be acquired by measuring the potential of the source signal line 18.
The EL display apparatus and drive method thereof according to the second aspect of the present invention consists of three operations: an initial operation, first operation (write operation), and second operation (light-emitting operation).
The initial operation is similar to that of the EL display apparatus (panel) and drive method thereof according to the first aspect of the present invention. In the initial operation, the driving transistor 11a is operated by applying a constant current (predetermined program current) to the pixel driving transistor 11a. When the operation of the driving transistor 11a enters a steady state, the gate terminal voltage of the driving transistor 11a or voltage (Va or V0) of the source signal line 18 is measured.
Preferably the constant current is varied according to the tone to be written. The constant current may be 0 A. A constant current of 0 A means that the driving transistor 11a has substantially been offset-cancelled. The measured voltage (Va or V0) is stored in a memory or the like after A/D conversion. Alternatively, the voltage is held in a sample-and-hold circuit or the like. The acquired voltage is used for the potential-varying operation in the first operation.
Preferably, a predetermined voltage is applied to the source signal line 18 before the initial operation to stabilize the potential of the source signal line 18 or set the potential to a predetermined voltage.
In the first operation, the voltage acquired in the initial operation is used as a reference voltage Va (or origin's voltage V0) and a target voltage is determined by adding or subtracting a tone voltage to/from the reference voltage. The determined target voltage is written into the pixel while the pixel is selected.
The second operation involves subjecting a programmed voltage (target voltage) to voltage-current conversion using the driving transistor 11a and applying the resulting current to the EL element 15 of the pixel 16. The target voltage is held in the capacitor 19 of the pixel 16. During the period of the second operation, the current supplied from the driving transistor 11a to the EL element 15 is passed or blocked by applying an on/off voltage to the gate signal line 17b as required. Also, reference current increase/decrease control and duty ratio control (
The present invention is not limited to the use of tone voltage. The present invention also includes applying a current (over-current) to the source signal line 18 and charging and discharging the source signal line in a short time. The potential of the source signal line 18 changes with the application of the current. That is, the application of a current is practically equivalent to the application of a voltage. Any method may be used in the potential-varying operation as long as it varies the potential of the source signal line 18 or the gate terminal potential of the driving transistor 11a.
The above operation translates into a process on the display screen 34 as illustrated in
Basically, it is assumed that the potential of the source signal line 18 is V0 when the constant current is 0 A. The potential of the source signal line 18 at the constant current Ia (Ia is an arbitrary value) is denoted by Va. However, for the sake of convenience or for ease of explanation, V0 may denote the voltage which corresponds to tone 0 in a video signal and Va may denote the voltage which corresponds to tone a in the video signal.
The pixels (pixel row) 61 are not illuminated (non-display pixels (pixel row)) They can be set to a non-illuminated state by opening the transistor 11d of the pixel 16 by controlling the gate driver circuit 12b. To open the transistor 11d, an off voltage is applied to the gate signal line 17b. The position where the gate driver circuit 12 applies an off voltage to the gate signal line 17 is shifted in sync with a horizontal synchronization signal.
The non-illuminated (non-display) state is a state in which no current or only a current lower than a certain level flows through the EL element 15. That is, it means a dimly displayed state. Thus, the non-illuminated pixel row means that the EL elements 15 of the pixel row are passing no current or that they are lit relatively dimly.
That area of the display screen 34 which is displaying nothing (non-illuminated) is referred to as anon-display area 62. That area of the display screen 34 which is displaying something (illuminated) is referred to as a display area 63. The switching transistors 11d of the pixels 16 in the display area 63 are closed and currents are flowing through the EL elements 15. However, in the case of image display in black display, naturally no current flows through the EL elements 15. The area in which the switching transistors 11d are open is a non-display area 62.
In
The present invention is characterized in that screen luminance or brightness is adjusted by varying the ratio between display area 63 and non-display area 62, varying the area ratio of non-display area 62 to the display screen 34, or increasing/decreasing the number of displayed pixels.
The present invention allows the display area 63 on the screen 34 to be divided into multiple parts. Also, it allows the number of pixel divisions of display area 63 or non-display area 62 to be varied between movie display and still-image display. The present invention is characterized in that bands of the non-display area 62 or display area 63 on the screen 34 move from top to bottom or from bottom to top.
Normally, an NTSC frame rate is 60 Hz (60 frames per second, i.e. the time required to refresh the screen is 1/60 second) and a PAL frame rate is 50 Hz (50 frames per second). As shown in
Input signals are accumulated in an image memory before frame rate conversion. Alternatively, input signals of a frame rate between 72 Hz and 150 Hz (both inclusive) are inputted in the display apparatus according to the present invention. The matters concerning the frame rate are also applicable to other examples of the present invention.
In the pixel configuration shown in
During a period when the current flows through the EL element 15, the transistors 11c and 11b turn off and the transistor 11d turns on as shown in
A timing chart is shown in
In an illuminated pixel row in which no on voltage is applied to the gate signal line 17a (non-selected pixel row), an on voltage (VGL) is applied to the gate signal line 17b. A current flows through the EL elements 15 in the pixel row. The EL elements 15 emit light.
In a non-illuminated pixel row in which no on voltage is applied to the gate signal line 17a (non-selected pixel row), an off voltage (VGH) is applied to the gate signal line 17b. No current flows through the EL elements 15 in the pixel row. The EL elements 15 do not emit light.
The above operations are illustrated in
In the pixel configuration shown in
During a next period when the current flows through the EL element 15, the transistors 11c and 11b turn off and the transistor 11d turns on as shown in
When charging and discharging the source signal line 18 rapidly during measurement or acquisition of the Va voltage or when inserting black (inserting a non-display area) in image display to improve viewability of moving pictures, the magnitude of the constant current is increased N times. Consequently, the current flowing through the EL element 15 also increases N times.
If Vx (x is a tone number) is multiplied by 1 as is conventionally done, the source signal line 18 can be charged and discharged rapidly due to the effect of the N-fold constant current. In this case, since the Va voltage used as a reference already provides an N-fold EL current, the Vx voltage used for addition and subtraction should be set taking this point into consideration. The same is true to the target voltage Vc.
For ease of explanation, it is assumed below that the current Iw used to measure the Va voltage is increased N times (the voltage Va used as a reference is also set to make the driving transistor 11a pass an N-fold current) and that Vx added to Va and V0 is also set to make the driving transistor 11a pass an N-fold current through the EL element 15. The brightness of the display screen 34 brought up on the EL display apparatus at a 1-fold current is assumed to be B and the brightness of a light-emitting part at an N-fold current is assumed to be B×N. Incidentally, although it is assumed that N is a number equal to or larger than 1, it goes without saying that the present invention is applicable even when N is smaller than 1.
In
It is assumed that the constant current or program current Iw passed through the EL element 15 is N times the current needed to obtain the average (predetermined) brightness B of the display screen 34. Thus, the EL element 15 illuminates at N times the predetermined brightness (N-B). The illumination period is 1F/N, where 1F is one field (frame). For ease of explanation, it is assumed that there is no blanking period in one field (frame). Practically, however, there are blanking periods, and thus the brightness is not exactly N·B. That is, the EL element 15 emits light at N times the predetermined brightness (N·B) for a period of 1/N 1F. Thus, the display brightness of the display panel averaged over 1F is given by (N·B)×(1/N)=B (i.e., the predetermined brightness).
Incidentally, N may be any number. However, too large a value of N will result in a large instantaneous current flowing through the EL element 15, and thus it is preferable that N is 10 or less. Of course, it goes without saying that N may be 1 (N=1) so that a write pixel row other than the write pixel row 181 will become a display (illuminated) area 63. In that case, the current Iw passed through the EL element 15 should be the current needed to obtain the average (predetermined) brightness B of the display screen 34. Thus, the EL element 15 illuminates (emits light) at the predetermined brightness B.
One reason for passing the constant current or program current Iw which will provide an emission brightness of N*B is to reduce the influence of the parasitic capacitance of the source signal line 18. By passing a large current, it is possible to charge and discharge the parasitic capacitance in a short period of time.
In the above example, an IC consisting mainly of a silicon chip is used for the source driver circuit (IC) 14. However, the present invention is not limited to this and output stage circuits 81 and the like (polysilicon current holding circuits 82) may be formed or constructed directly on the array board 30 using polysilicon technology (CGS technology, low-temperature polysilicon technology, high-temperature polysilicon technology, or the like) as illustrated in
In
As illustrated in
Although it has been stated with reference to
In
As illustrated in
The non-display area 62 is a pixel 16 area of EL elements 15 not illuminated at a given time point. The display area 63 is a pixel 16 area of EL elements 15 illuminated at a given time point. The non-display area 62 and display area 63 are shifted by one pixel row at a time in sync with a horizontal synchronization signal.
The drive method according to the present invention is capable of intermittent display as illustrated in
The drive method according to the present invention controls the current passed through the EL element 15 by simply turning on and off the switching transistor 11d and the like (see
Even to achieve black insertion (intermittent display such as black display), the present invention does not need to speed up the main clock of the circuit. Also, it does not need to elongate a time axis, and thus requires no image memory. Besides, the organic EL element 15 responds quickly, requiring a short time from application of current to light emission. Thus, the present invention is suitable for movie display, and by using intermittent display, it can solve a problem with conventional data-holding display panels (liquid crystal display panels, EL display panels, etc.) in movie display.
Furthermore, in a large display apparatus, if increased wiring length of the source signal line 18 results in increased parasitic capacitance, this can be dealt with by increasing the value of N (N is a value larger than 1). When the value of the program current applied to the source signal line 18 is increased N times, the conduction period of the gate signal line 17b (the transistor 11d) can be set to 1F/N. This makes it possible to apply the present invention to television sets, monitors, and other large display apparatus.
According to one aspect of the present invention, in the EL display panel with a pixel configuration for current driving, the Va voltage or V0 voltage is measured or acquired by passing a constant current through the driving transistor 11a of each pixel or without passing a constant current (Iw=0). The measured/acquired voltage Va or V0 is stored in a memory or the like after A/D conversion. When displaying images, the Va or V0 voltage is read out, subjected to D/A conversion, and applied as the precharge voltage Vp to the source signal line 18. After the application of the precharge voltage Vp, a program current is applied as required.
According to another aspect of the present invention, the Va or V0 voltage is measured by applying a constant current to the driving transistor 11a of each pixel or without passing a current (Iw=0). The measured/acquired voltage is stored in a memory or the like after A/D conversion. When displaying images, the Va or V0 voltage is readout and subjected to D/A conversion, and the target voltage Vc is generated by adding a tone voltage Vx (x is a tone number) to the Va or V0 voltage.
Incidentally, the present invention is not limited to this. For example, a constant current Iw applied when measuring or acquiring the Va voltage may be a current Iwm corresponding to a maximum tone.
When the current Iwm corresponding to the maximum tone is applied to the driving transistor 11a, a voltage Vam is generated at the gate terminal of the driving transistor 11a such that the current for the maximum tone will flow through the driving transistor 11a. The target voltage Vc is generated by subtracting the tone voltage Vx from Vam. The generated voltage Vcm is applied to the gate terminal of the driving transistor 11a.
Thus, an important or unique operation of an important driving method according to the present invention consists of extracting the current flowing through the pixel in current driving mode to the source signal line 18 or measuring the potential of the source signal line 18. This requires a configuration or arrangement in which the driving transistor 11a or the drain terminal or source terminal of the transistor 11b which forms a current mirror with the driving transistor 11a is connected to the source signal line 18 in a DC manner. That is the driving transistors 11 (11a and 11b) must be configured in the above manner. The passage of current through the EL element 15 includes a case in which the current is supplied to the EL element 15 and a case in which the current flows from the EL element 15 into the driving transistors 11.
Thus, in this example, an approximately 1-fold current Ie is passed through the driving transistors 11 based on Va, V0, and Vam. However, the present invention is not limited to this. For example, needless to say, with a driving method which passes current through the EL element 15 only for a period of 1F/N, but does not pass current during the remaining period (1F(N−1)/N), the constant current may be increased N times. That is, the Va voltage which corresponds to the N-fold constant current (reset current) is determined, and the target voltage Vc is generated based on the voltage Va. Incidentally, although the N-fold constant current is cited here, this is not restrictive. N may be any number equal to or larger than 1.
This method is useful especially when the source signal lines 18 have high parasitic capacitance. Also, it is useful for 10-inch or larger EL display apparatus. When the source signal lines 18 have high parasitic capacitance, by increasing the reset current (program current Iw) N times (N should be at least equal to or larger than one), it is possible to correct “insufficient writing” of the constant current Iw.
The drive method according to the present invention allows intermittent display of each of red (R), green (G), and blue (B) as illustrated in
Although the pixel configuration of the present invention in
In the example shown in
A technical idea of the present invention lies in passing the program current, constant current Iw, or the like from the source driver IC (circuit) 14, and thereby compensating for the characteristics of the driving transistor 11b which passes current directly to the driving transistor 11a or indirectly to the EL element 15. This is because the characteristics of the driving transistor 11 is outputted as a gate terminal potential (=the potential of the source signal line 18) by the application of the constant current Iw. Tone current or tone voltage is determined using the outputted voltage as a variable. Thus, the pixel configuration in
With the pixel configuration in
Referring to
The number of driving transistors 11a or 11b is not limited to one and there may be two or more driving transistors 11a or 11b. Examples include a configuration in which five transistors 11a are arranged in parallel or in series. Also, two or more switching transistors 11c, 11d, or the like may be arranged in parallel or in series.
The source driver IC (circuit) 14 as well as a current output circuit for the constant current or program current Iw will be described below.
Each reference current circuit 153 consists of a resister R1 (R1r, R1g, or R1b), operational amplifier 151a, and transistor 167a. The resisters R1 (R1r, R1g, and R1b) are configured such that their values can be set or adjusted separately according to R, G, and B tone currents. The resisters R1 are external resisters installed outside the source driver IC (circuit) 14.
A voltage Vi is applied to a positive terminal c of the operational amplifier by an electronic regulator 152. The voltage Vi is obtained by dividing a stable reference voltage Vs by the resister R, dividing the resulting value by switches S (S1, S2, S3, . . . ), and selecting a generated voltage.
The electronic regulator 152 varies the output voltage Vi by controlling the switches S using an external signal. Thus, it can be regarded as a voltage output circuit which varies output voltages using an external control signal. However, the present invention is not limited to this and the electronic regulator 152 may be an electronic resister which varies internal impedance. Besides, the electronic regulator 152 may vary not only voltage, but also output current. For example, in
The reference current Ic is expressed as (Vs−Vi)/R1. The RGB reference currents Ic (Icr, Icg, and Icb) are adjusted or varied by separate reference current circuits 153. They are varied by respective RGB electronic regulators. Thus, the values of the voltages Vi outputted from the electronic regulators 152 vary with the control signals applied to the electronic regulators 152. The magnitudes of the RGB reference currents vary with the voltages Vi, and the tone currents (program currents) Iw outputted from the terminals 83 vary proportionally.
The generated reference currents Ic (Icr, Icg, and Icb) are applied from the transistors 167a to the transistors 167b. Each transistor 167b forms a current mirror circuit with a transistor group 165c. Incidentally, although the transistor 167b1 is illustrated as a single transistor in
If the number of tones outputted by the source driver IC (circuit) 14 is K and the size of the unit transistor 164 is St (square μm), the unit transistor 164 satisfies the conditions 40≦K/√(St) and St≦300.
The program current Iw is outputted from the transistor group 165c via the output terminal 83. The gate terminal of each unit transistor 164 in the transistor group 165c is connected with the gate terminal of the transistor 167b via gate wiring 163.
As illustrated in
To generate the output current Iw without variations among the terminals 83, it is necessary to operate multiple unit transistors 164. To reduce variations in the output current Iw among terminals 83, the area occupied by the unit transistors 164 which generate the current must be larger than a certain size. Thus, to output the constant current Iw without variations among the terminals 83 (accurately), an output current source should be composed of multiple unit transistors 164 whose total area is larger than a predetermined size. Although the circuit shown in
The unit current is a program current of one unit outputted by the unit transistor 164 according to the magnitude of the reference current Ic. As the reference current Ic changes, the unit current outputted by the unit transistor 164 changes proportionally. This is because the transistor 167b and unit transistor 164 form a current mirror circuit.
The transistor 167b1 in
The unit transistor 164 is a transistor or current source which outputs a program current Iw of one unit or minimum unit. That is, a unit transistor 164 equals a unit current source. Also, a construction or part in which multiple unit transistors 164 gather together to output a program current corresponding to a tone is referred to as a transistor group (current output circuit) 165c.
The magnitude of the unit current can be varied by adjusting the magnitude or intensity of the reference current Ic outputted by the reference current circuit 153. The reference current Ic is adjusted with a built-in electronic regulator 152 or the like of the source driver IC (circuit) 14. The reference current circuit 153 which generates the reference current Ic is provided separately for the R, G, and B circuit.
Each of the R, G, and B transistor groups 165c consists of a set of unit transistors 164. The magnitude of the output current (unit program current) from the unit transistor 164 can be adjusted based on the magnitude of the reference current Ic. By adjusting the magnitude of the reference current Ic, it is possible to separately change or vary the magnitudes of the R, G, and B program currents (constant currents) Iw for each tone. Thus, under ideal conditions in which the R, G, and B unit transistors 164 have the same characteristics, it is possible to achieve a white balance for display images on the EL display apparatus by changing the ratio among the magnitudes of the reference currents Ic in the R, G, and B reference current circuits.
For ease of explanation or for ease of drawing, it is assumed below that the transistor groups 165c in the source driver circuit (IC) 14 have a 6-bit configuration. In
The output current of the unit transistors 164 for each bit is either outputted or not outputted to the output terminal 83 under the on/off control of analog switches 161 (161a to 161f). Each of the analog switches 161a to 161f corresponds to each bit (e.g., one of six bits) in the control signal of the constant current Iw. When the switch 161a which corresponds to the D0 bit is closed, one unit of current is outputted (inputted) from the output terminal 83. The output terminal 83 is connected with a source signal line 18. Similarly, when the switch 161b which corresponds to the D1 bit is closed, two units of current is outputted (inputted) from the output terminal 83.
Similarly, when the switch 161c which corresponds to the D2 bit is closed, four units of current is outputted (inputted) from the output terminal 83. When the switch 161d which corresponds to the D3 bit is closed, eight units of current is outputted (inputted) from the output terminal 83. When the switch 161e which corresponds to the D4 bit is closed, 16 units of current is outputted (inputted) from the output terminal 83. When the switch 161f which corresponds to the D5 bit is closed, 32 units of current is outputted (inputted) from the output terminal 83.
In this way, the switches 161 are opened and closed in a digital fashion according to the bits in the control signal of the constant current and the sum total of unit currents (the program current Iw) is outputted from the output terminal 83.
The program current Iw flows through the internal wiring 162. Potential Vw of the internal wiring 162 becomes the potential of the source signal line 18. The potential of the source signal line 18 causes the constant current Iw to be applied to the source signal line 18. In steady state, it corresponds to the gate terminal voltage of the driving transistor 11a of the pixel 16 (in the pixel configuration in
The unit transistors 164 form a current mirror circuit with the transistor 167b. Incidentally, for ease of understanding, only one transistor 167b is illustrated in
That is, the transistor 167b consists of a group of many unit transistors 164. Needless to say, however, the unit transistors 164 in the transistor group 165c and the unit transistors of the transistor 167b may differ in size and output current characteristics. Needless to say, the transistor 167a may also be constituted of multiple transistors. Incidentally, a constant current output circuit which has unit transistors 164 is referred to as a transistor group 165c.
Thus, by configuring the transistor (167b, 167a, 168a, 168b, 165b, 165c in
The reference current Ic flows through the transistor 167b and a current corresponding to the current mirror ratio of the reference current Ic flows through the unit transistors 164. The 63 unit transistors 164 in
As described with reference to
It has been stated that one or more unit transistors 164 are formed or placed for each bit as illustrated in
However, the present invention is not limited to this. For example, needless to say, a unit transistor 164 which outputs a current corresponding to each bit may be formed or placed for the given bit. Specifically, one transistor which outputs a current twice larger than the 0th bit transistor is formed or placed for the first bit, one transistor which outputs a current four times larger than the 0th bit transistor is formed or placed for the second bit. Alternatively, two transistors each of which output a current twice larger than the first bit transistor may be formed or placed for the second bit.
In the case of 64 tones (6 bits each for R, G, and B), 63 unit transistors 164 are formed as illustrated in
The present invention has the unique advantage that the currents outputted by the transistor group 165c lend themselves to addition. Also, it has the unique property that if the channel width W of the unit transistor 164 is reduced to ½ with the channel length L kept constant, the current passed through the unit transistor 164 is reduced to approximately ½. Similarly, it has the unique property that if the channel width W is reduced to ¼ with the channel length L kept constant, the current passed through the unit transistor 164 is reduced to approximately ¼. Actually, the current is not reduced to exactly 1/n. For ease of explanation, however, it is assumed herein that the channel width W is reduced to 1/n. The spirit of the present invention is to form or place a unit transistor which outputs 1/n of the unit current of a given unit transistor.
In
In the above example, it has been stated that W of the unit transistor 164b is ¼ that of the unit transistor 164. The output current of the unit transistor 164b is ¼ that of the unit transistor 164. If W of the unit transistor 164 is 6 μm, W of the unit transistor 164b should be 1.5 μm, which is ¼ of 6 μm. However, this is the case when the unit transistor 164b has ideal characteristics. Actually, a channel width larger than 1.5 μm, such as 2.0 μm, is used. Generally, with small transistors, their output current and channel width are not proportional to each other. By using a channel width larger than the ideal value of ¼ the channel width of the unit transistor 164, it is possible to make the current of unit transistor 164b, when multiplied by 4, coincide with the current of the unit transistor 164. This will be described in more detail later.
As illustrated in
The low-order two bits consist of transistors (164a and 164b) smaller in size than the higher-order unit transistors 164. Thus, the unit transistors 164a and 164b can output a unit current ½ or ¼ that of the unit transistor 164, respectively. The unit transistors 164a and 164b occupy minimal space. The number of regular unit transistors 164 is 63, which remains unchanged. Thus, even if a 6-bit configuration (64 tones) is changed to an 8-bit configuration (256 tones), there is not much difference in the formation area of the transistor group 165c between
The reasons why there is no increase in the size of the transistor group 165c in the output stage of the source driver IC (circuit) 14 for current programming mode as illustrated in
Also, as exemplified by the unit transistors 164a and 164b illustrated in
Actually, the output current is not reduced to exactly 1/n even if the channel width W is reduced to 1/n. Some correction is necessary. There is not a significant meaning in halving the channel width W, but there is a technical meaning in reducing the output current of the transistor 24a to ½ of the output current of the unit transistor 164. Thus, it is only necessary to reduce the output current to an approximately integral submultiple such as ½ or ¼ by varying the channel length L in addition to the channel width W. Besides, the unit transistors 164, 164a, and 164b illustrated in
When a transistor is operated at the same gate terminal voltage, its output current is reduced to ½ or less if the channel width W is halved. Therefore, according to the present invention, when the sizes of the higher-bit transistors and lower-bit transistors are changed, the transistor sizes are set as follows when they need to be changed.
A small number of sizes such as two sizes are used for the unit transistors 164 of the source driver circuit (IC) 14. The unit transistors 164 are made to have the same channel length L. That is only the channel width W is varied. Alternatively, the unit transistors are formed by varying only one of the channel width W and channel length L. Preferably, less than four sizes or shapes are used for the unit transistors 164 which compose the transistor group 165c. More preferably, less than three sizes or shapes are used.
Transistors are configured such that when a ratio between a first unit output current of a first unit transistor and a second unit output current of a second unit transistor is n (first unit output current:second unit output current=1:n, where n is a value smaller than 1), the following relationship should hold: channel width W1 of the first unit transistor<channel width W2 of the second unit transistor W2×n×a.
When W1×n×a=W2, preferably the following relationship holds: 1.05<a<1.3. A correction coefficient a can be determined easily by forming a test transistor and measuring or assessing it.
According to the present invention, unit transistors 164 smaller than higher-bit unit transistors 164 are formed or placed to create (provide) lower-order bits. The term “smaller” here means that they are smaller in terms of output current than the unit transistors 164 which constitute higher-order bits. Thus, this concept includes smallness of not only channel width W, but also channel length L or other dimensions. This concept also includes the other shape. It has been stated that the output current of the unit transistor 164a is ½ that of the unit transistor 164, but this does not mean that accuracy is required. It is sufficient if the ratio is set within the range of 60% to 140% so that the output current in each bit will not be reversed. That is, the ratio can be roughly ½ or ¼.
It has been shown in
As also illustrated in
The present invention is not limited to the use of common internal wiring 162 for the unit transistors 164 in the transistor group 165. For example, a configuration shown in
The transistor group 165b1 is connected by internal wiring 162a and the transistor group 165b2 is connected by internal wiring 162b. In
In
Although it has been stated with reference to
The minimum output current of the transistor 164 in the source driver circuit (C) 14 is between 0.5 nA and 10 nA (both inclusive). Preferably, the minimum output current of the unit transistor 164 should be between 2 nA and 20 nA (both inclusive) to ensure accuracy of the transistors 164 composing the transistor group 165c in the source driver IC 14.
Also, as illustrated in
Preferably, the transistors 167b or transistor groups 165b are formed or placed on both sides of the transistor group 165c as illustrated in
Although it is stated that the transistor group 165c according to the present invention outputs current, this is not restrictive. For example, the transistor group 165c may output voltage. That is, the transistor group 165c may output voltage in order for the source driver circuit (IC) 14 to perform voltage driving as in the case of a liquid crystal display panel. Besides, the transistor group 165c of an operational amplifier may output voltage. The present invention similarly applies in the case of a voltage-driven EL display panel. Also, although it is stated that selector circuits 222 and 291 are built into a source driver circuit (IC) 14 consisting of a silicon chip, this is not restrictive. For example, the transistor group 165c may be formed directly on a glass array board 30 by polysilicon technology or the like. Alternatively, it may be formed or constructed on a separate chip.
As illustrated in
The precharge voltage Vp is applied to charge and discharge the source signal line 18 or to set the source signal line 18 to a predetermined voltage. The concept of application of the precharge voltage Vp also includes application of the Va or V0 voltage, application of a target tone voltage or program voltage at the beginning of a horizontal scanning period, and application of an overcurrent for the purpose of changing the source signal line potential.
When the counter circuit 212 counts up to the set value, the output period of the precharge voltage Vp is ended. Output from the counter circuit 212 provides input to part a of an AND circuit 213. The precharge voltage Vp can be switched on (applied) and off (not applied). The switching is performed based on a video signal applied to the source signal line 18, the magnitude of a program current or program voltage corresponding to the video signal, a change in the video signal (difference from the video signal applied during the previous horizontal scanning period), the magnitude of a program current or program voltage corresponding to the video signal (change from the program current or program voltage applied during the previous horizontal scanning period).
With the configuration in
The input in part a of the AND circuit 213 is High and the input in terminal b is High, the switch 161a closes, causing the precharge voltage Vp to be applied to the internal wiring 162. When an HI signal is High as well, the switch 161b closes, causing the precharge voltage Vp to be outputted from the output terminal 83.
The selector circuit 222 latches data onto a latch circuit 221 in sequence in sync with a main clock, where the latch circuit 221 corresponds to output stages. The latch circuit 221 consists of two stages: a latch circuit 221a and latch circuit 221b. The latch circuit 221b sends out data to the precharge circuit 214 in sync with a horizontal scanning clock (1H). That is, the selector latches one pixel row of image data and PC data in sequence and stores the data in the latch circuit 221b in sync with the horizontal scanning clock (1H).
Incidentally, in the latch circuit 221 in
When the output of the latch circuit 221b is High, the precharge circuit 214 turns on the switch 161a to output the precharge voltage Vp to the source signal line 18. The transistor group 165c outputs a program current (constant current) to the source signal line 18 according to image data.
Determination as to whether to apply the precharge voltage Vp is made based on the voltage applied to the source signal line 18 (potential held) before the determination. It is made based on the potential difference between the potential applied to the source signal line 18 before the determination and the voltage to be applied next (the potential of the source signal line 18 expected to be generated by the application of a program current potential) or on the amount of change in potential. If there is a small potential difference, for example, if the voltage applied to the pixels in the N-th (N is an integer not smaller than 1, but not larger than the maximum number of pixel rows) pixel row or a change in potential caused by application of a program current is 4.0 V and the voltage to be applied next is 4.1 V, the precharge voltage Vp is applied to the pixels in the (N+1)-th pixel row. Conversely, if there is a potential difference as large as 2.0 V, the precharge voltage Vp is not applied to the pixels in the (N+1)-th pixel row.
According to the present invention, when the driving transistor 11a of the pixel 16 is a P-channel transistor, determination as to whether a precharge voltage Vp is applied is made in the following range. For ease of explanation, let Vdd denote the anode voltage, let Vss denote the cathode voltage, let Vd denote power supply voltage of the source driver IC (circuit) 14, and let GND denote the ground potential of the source driver IC (circuit) 14. Also, let Vn denote the potential (the voltage applied 1H before) held by the source signal line 18 and let Vm denote the voltage (or the target voltage changed by the application of a program current) outputted from the source driver IC (circuit) 14. Incidentally, the anode Vdd, cathode Vss, Vn, and Vm are voltage values with respect to GND. Preferably, the relationship among potentials in
When the driving transistor 11a of the pixel 16 is a P-channel transistor, the precharge voltage Vp is applied to the source signal line 18 or pixel 16 if at least one of the following conditions is satisfied.
0.5≦(Vdd−Vm)/Vdd≦0.9
0.5≦(Vd−Vm)/Vdd≦0.9
0.1≦|(Vn−Vm)|/Vn≦0.3, where 0.5≦(Vd−Vm)/Vdd
When the driving transistor 11a of the pixel 16 is an N-channel transistor, the precharge voltage Vp is applied to the source signal line 18 or pixel 16 if at least one of the following conditions is satisfied. Incidentally, Vn and Vm are Vss-side voltages of negative polarity.
0.5≦|(Vss−Vm)|/Vss≦0.9
0.5≦|(Vss−Vn)|/Vss≦0.9
0.1≦|(Vn−Vm)|/Vn≦0.3, where 0.5≦(Vss−Vm)/Vss
It has been stated in the above example that determination as to whether to apply the precharge voltage Vp is made based on the potential held by the source signal line 18, the voltage to be applied, or the like. Needless to say however, the determination may be made based on the tone of the video signal applied to the pixel 16. According to the present invention, the precharge voltage Vp is applied to each source signal line 18 if at least one of the following conditions is satisfied, where M is the maximum number of tones, N1 is the tone of the video signal applied 1H before, and N2 is the tone of the video signal to be applied next.
1≦N2≦M×0.25
1≦|N2−N1|≦8
The determination about precharging is not limited to precharging on a pixel by pixel basis. For example, determination about precharging may be made in relation to image data of two or more pixel rows. Also, determination may be made taking into consideration image data around the pixel to be precharged (e.g., by means of weighing). Also, determination may be varied between moving images and still images. What is important is that the controller generates a precharge signal based on image data, thereby achieving good versatility.
The determination about precharging is not limited to precharging on a pixel by pixel basis. For example, determination about precharging may be made in relation to image data of two or more pixel rows. Also, determination may be made taking into consideration image data around the pixel to be precharged (e.g., by means of weighing). Also, determination may be varied between moving images and still images. What is important is that the controller generate a precharge signal based on image data, thereby achieving good versatility. The description below will focus on the determination about precharging and precharge mode.
The determination as to whether to carry out precharging may be made based on image data one pixel row before (or the image data applied to the source signal line just before). Suppose, for example, the image data applied to a source signal line 18 changes in the order: white, black, and black. A precharge voltage is applied when the image data changes from white to black. This is because black tone is difficult to write. When writing image data of black twice in succession, no precharge voltage is applied at the time of the second writing of black because the potential of the source signal line 18 remains ready for black display after the first writing of black. The above operation can be accomplished easily by forming (placing) one pixel row of line memory in the controller circuit (IC) 801 (two lines of memory are required because of FIFO).
Although it is stated herein that the precharge voltage Vp (Va or V0) is outputted in the case of precharge driving, this is not restrictive. A current larger than a program current may be written into the source signal line 18 for a period shorter than one horizontal scanning period. That is, a precharge current may be written into the source signal line 18 before writing a program current into the source signal line 18. The precharge current causes voltage changes all the same in a physical sense. Precharging by means of a precharge current is also included within the technical scope of the precharge driving according to the present invention (within the scope of the present invention).
In the precharge driving according to the present invention, a predetermined voltage is applied to the source signal line 18. It has been stated that the source driver IC outputs a program current. However, in the precharge driving according to the present invention, output voltage may be varied according to the tone. In that case, the precharge voltage outputted to the source signal line 18 is a program voltage. A circuit configuration in which a voltage tone circuit 231 for precharge voltage is incorporated in the source driver IC is shown in
Although the voltage tone circuit 231 is described as being an arrangement on an operation which outputs tone voltage such as program voltage, the present invention is not limited to this. It is also used in referring to a circuit which outputs a predetermined constant voltage or program voltage. Besides, it is used in referring to a sample-and-hold circuit. In short, it is a circuit which can output voltage values at multiple stages. However, if the precharge voltage Vp is a fixed value, the voltage tone circuit 231 may be configured to output one type of voltage. This configuration is also included in the concept of the voltage tone circuit 231. Besides, the electronic regulator 152, which can vary or adjust output voltage using external input data, is also a voltage tone circuit. Furthermore, a D/A (digital-analog conversion) circuit 391 is also a voltage tone circuit.
Incidentally, the voltage tone circuit 231 includes not only circuits which output an analog voltage in response to input of a digital signal, but also circuits which output an analog voltage after impedance conversion, amplification, or reduction. In a wide sense, the voltage tone circuit 231 also includes circuits which select and output one predetermined voltage or multiple voltages. In short, the voltage tone circuit 231 can be understood as a constant voltage source.
Although it is stated that basically the current tone circuit 154 outputs tone current such as program current, the present invention is not limited to this. The term “current tone circuit” is also used in referring to a circuit (constant current output circuit) which outputs a predetermined constant current. Also, it is used in referring to a constant current source because any circuit configuration which can output tone current can output a constant current of a predetermined value such as 1 μA or 0.5 μA.
Naturally, it goes without saying that the current tone circuit 154 may be simplified and configured as a constant current circuit which outputs a constant current Iw. In order to measure Va or V0, it is sufficient to apply the constant current Iw. Needless to say, this function can be achieved by either the tone current circuit 154 or a simplified constant current circuit. Incidentally, in terms of the tone current, the program current Iw can be regarded as a constant current.
The voltage tone circuit 231 is constituted, for example, of a sample-and-hold circuit. Also, it is constituted of a D/A conversion circuit or the like as required. The tone voltage is converted into a precharge voltage by the D/A conversion circuit based on digital video data. The resulting precharge voltage is sampled and held by a sample-and-hold circuit 241 and applied to a terminal of the switch 161a via an operational amplifier.
There is no need to construct or form a D/A conversion circuit for each voltage tone circuit 231, and output from a D/A conversion circuit provided outside the source driver circuit (IC) 14 may be sampled and held in each voltage tone circuit 231. Also, the D/A conversion circuit may be formed by polysilicon technology.
As illustrated in
The program voltage is temporarily held in Cc capacitance and outputted from a buffer amplifier 151a. The outputted voltage is allocated to the output terminals 83 (output terminals 83a, 83b, 83c, 83d. . . , 83n, 83a, 83b, 83c, . . . 83n. . . ) in sequence by the sample-and-hold circuit (illustrated as a switching circuit, in this example) 241. The allocation is performed in sync with the clock CLK. Incidentally, according to the present invention, the program voltage can be allocated to a desired terminal by an 8-bit address signal PADRS. The configuration in which the program voltage can be allocated to a desired output terminal 83 (the use of eight bits allows the program voltage to be allocated to any of 256 terminals) according to the address signal PADRS makes it possible to apply a new program voltage only to a terminal which needs rewriting of program voltage. The allocation of program voltage can be randomized. The program voltage is held (sampled) in capacitance C, the output from the buffer circuit 151b is applied to the output terminal 83 or blocked under the control of a switch Sp. The switch Sp corresponds to the switch 161a in
Specifically, the current tone circuit 154 corresponds to the circuit configuration in
The precharge voltage Vp outputted from the voltage tone circuit 231 is applied at the beginning of a horizontal scanning period (1H) (indicated by symbol A) as illustrated in
Preferably, period A during which a precharge voltage signal is applied is between 1/100 and ½ (both inclusive) of one horizontal scanning period (1H) or between 0.2 μsec and 40 μsec (both inclusive). More preferably, it is between 1/100 and ⅕ (both inclusive) of one horizontal scanning period (1H) or between 0.2 μsec and 10 μsec (both inclusive). The remaining part of the horizontal scanning period (1H) is period B which is an application period of the program current. If period A is too short, the source signal line 18 is not charged and discharged sufficiently, resulting in insufficient writing. On the other hand, if period A is too long, the current application period (B) becomes too short to apply the program current sufficiently. Consequently, the current in the driving transistor 11a cannot be corrected sufficiently.
It is preferable that the voltage application period (period A) is started from the beginning of 1H, but this is not restrictive. For example, it may be started from the blanking period at the end of 1H. Also, period A may be provided in the middle of 1H (horizontal scanning period). That is, the voltage application period may be provided in any part of 1H. Preferably, however, the voltage application period is provided within 1/(4H) (=0.25H) from the beginning of 1H.
Although in the example in
As can be seen from
Referring to
When the potential of the source signal line 18 is close to Vdd (in high tone regions), current programming may be performed for the entire period of 1H.
According to the present invention, the driving transistor 11a is described as being a P-channel transistor, but this is not restrictive and it goes without saying that the driving transistor 11a may be an N-channel transistor. The driving transistor 11a is assumed to be a P-channel transistor only for ease of explanation.
In the example of the present invention, voltage programming is mainly used for writing of pixels in low tone regions and current programming is mainly used for writing in high and halftone regions. That is, this example has the best of current driving and voltage driving. Voltage is used for display in a predetermined tone in low tone regions because in current driving, the voltage applied (by voltage driving or precharge driving) at the beginning of 1H becomes predominant due to weakness of write current (the precharge driving and voltage driving are conceptually identical, but if they have to be distinguished, it can be said that relatively few types of voltage are applied in precharge driving while many types of voltage are applied in voltage driving).
In halftone regions, after a voltage is written, deviations in the voltage are compensated for by program current. That is, the program current becomes predominant (current driving is predominant). In high tone regions, a program current is written. There is no need to apply a program voltage. Any applied voltage would be rewritten by a program current anyway. That is, the current driving is overwhelmingly predominant. Needless to say, a voltage may be applied, of course.
The output from the voltage tone circuit and output from the current tone circuit (including the precharge circuit) can be short-circuited at the output terminal 83 because the current tone circuit has a high impedance. That is, since the current tone circuit has a high impedance, even if a voltage is applied to the current tone circuit from the voltage tone circuit, there will be no circuit problem (such as a short circuit resulting in an overcurrent).
Although it has been stated that the present invention switches between voltage output and current output modes, this is not restrictive. Needless to say, a voltage may be applied to the output terminal 83 from the voltage tone circuit 231 by turning on the switch 161 (see
A program current may be outputted from the current tone circuit 154 by closing the switch 161 with a voltage applied to the output terminal 83. There will be no circuit problem because the current tone circuit 154 has a high impedance. This is also included in the act of switching between voltage driving mode and current driving mode according to the present invention. The present invention takes advantage of properties of current circuits and voltage circuits. This is a unique feature not encountered in other driver circuits.
As illustrated in
In the example of the present invention in
For example, if video signal data consist of D(2), D(3), and D(8), since the Pcntl signal is High, a voltage is outputted to the source signal line 18 from the voltage tone circuit 231 (period A). When Pcntl is Low, first a voltage and then a program current are outputted to the source signal line 18. The period during which a voltage is outputted is indicated by A and the period during which a current is outputted is indicated by B. The period A during which a voltage is outputted is controlled by the Ptc signal. The Ptc signal controls on/off operation of the switch 161 in
It has been stated that only voltage driving mode is used when the Pcntl signal is High and that both voltage driving and current driving modes are used when the Pcntl signal is Low. Preferably, the period during which a voltage is applied is varied with the lighting ratio or tone. In the case of low tones, it is not possible to write program current into pixels completely by current driving alone. Thus, it is preferable to use voltage driving. If the period during which a voltage is applied is extended, the voltage driving mode becomes predominant even in a combined voltage/current driving mode, making it possible to write a low tone into pixels properly. At a low lighting ratio, many pixels are in low-tone mode. Thus, even in a low-tone mode (at a low lighting ratio), if the period during which a voltage is applied is extended, the voltage driving mode becomes predominant even in a combined voltage/current driving mode, making it possible to write a low tone into pixels properly.
In this way, even in a combined voltage/current driving mode, it is preferable to vary the period of voltage driving according to the lighting ratio or the tone data (video data) written into the pixels. That is, it is preferable to ensure, through control, adjustment, or equipment configuration, that the period of voltage driving mode will be increased when the current passed through the EL element 15 is decreased (in a low lighting ratio range, according to the present invention) and that the period of voltage driving mode will be decreased or reduced to zero when the current passed through the EL element 15 is increased (in a high lighting ratio range, according to the present invention).
Although it has been stated with reference to
DATA held by the first latch circuit is loaded onto a second latch circuit 221b at a second stage in response to a load signal (LD). DATA held by the latch circuit 221b becomes input for the voltage tone circuit 231 and current tone circuit 154. One bit in the precharge signal is a signal for switching between the program voltage from the voltage tone circuit 231 and program current from the current tone circuit 154. The precharge signal controls a switching circuit (such as the switch 161 in
Incidentally, since the sample-and-hold circuit in the voltage tone circuit operates relatively slowly, needless to say, the voltage tone circuit may be composed of three stages of latch circuits by adding one stage of latch circuits for sampling-and-holding. Also, the switching circuit 291 may be formed on the array board 30 by polysilicon technology.
In the above example, the precharge voltages Vp (Va and V0) are generated in the source driver IC (circuit) 14 and applied therefrom to the source signal line 18, but the present invention is not limited to this. For example, needless to say, transistor elements for precharge voltages may be formed on the array board 30 and the precharge voltages Vp applied to the precharge voltage lines may be applied to the source signal lines 18 through on/off control of the transistor elements.
In
In the above example, the precharge voltages Vp (Va and V0) have been described as being close to the anode voltage Vdd (from Vdd to Vdd−3 V), but the precharge voltages Vp may be close to the cathode voltage (from Vss to Vss+3 V) depending on the pixel configuration. For example, in case that the driving transistor 11a is an N-channel transistor, the driving transistor 11a is a P-channel transistor and current programming is performed by a discharge current (or sink current in the case of the pixel configuration in
In current driving, insufficient writing is mainly caused by parasitic capacitance Cs of the source signal lines 18 as illustrated in
For ease of explanation, it is assumed below that the driving transistor 11a of the pixel 16 is a P-channel transistor and that current programming is performed by a sink current (current absorbed into the source driver circuit (IC) 14).
Incidentally, the relationship is reversed when the driving transistor 11a is an N-channel transistor or the driving transistor 11a is current-programmed by a discharge (source) current (current discharged from the source driver IC (circuit) 14). In that case, the unit transistors 164 formed in the source driver IC (circuit) 14 are P-channel transistors. That is, although the use of sink current is cited herein, when using discharge (source) current, the pixel configuration or operation as well as the configuration or operation of the source driver IC (circuit) 14 are reversed. This will be readily understood by those skilled in the art, and thus description whereof will be omitted.
As illustrated in
A change from white display (high tone display) to black display (low tone display) mainly involves operation of the driving transistor 11a of the pixel 16. Although the source driver circuit (IC) 14 outputs a black display current, it is too weak to work effectively. The driving transistor 11a operates and charges the parasitic capacitance Cs, bringing it to the same potential as the program current Id2 (Iw). As the parasitic capacitance Cs is charged, the potential of the source signal line 18 rises. Consequently, the gate terminal potential of the driving transistor 11a of the pixel 16 rises, causing current programming to be performed in such a way as to pass the program current Iw.
However, with the driving in
Precharge driving is used to correct insufficient writing of program current. However, with this method alone, it may be difficult to change from white display to black display in
To deal with this situation, the present invention increases the program current outputted from the source driver circuit (IC) 14 in the first half of 1H. In the second half, the regular program current Iw is outputted. However, the regular program current is increased N times in the case of
The drive method (driver or driving method) described below is referred to as overcurrent driving. Needless to say, the overcurrent driving may be combined with another driver or driving method according to the present invention. For example, it is conceivable to apply the precharge voltage Vp, perform overcurrent driving, and then apply a program current (program current driving). Alternatively, it is conceivable to perform overcurrent driving without applying the precharge voltage Vp and then perform program current driving.
Incidentally, the overcurrent driving is a method which involves charging and discharging the source signal line 18, and thus its technical idea is included in the concept of precharge voltage driving.
Incidentally, the overcurrent may be either a discharge current or sink current. The overcurrent driving is performed according to the polarity of the driving transistor 11a of the pixel 16. If the driving transistor 11a of the pixel 16 is a P-channel transistor, the overcurrent flows into the source driver IC (circuit) 14 (sink current) and if the driving transistor 11a of the pixel 16 is an N-channel transistor, the overcurrent is discharged from the source driver IC (circuit) 14 (source current). The overcurrent driving is not performed with respect to all the pixels 16, but performed depending on a tone value applied to the given pixel 16, the potential of the source signal line 18, or a change in potential expected to be caused by the tone applied next. Besides, the magnitude and application period of the overcurrent are varied.
Similarly, a current circuit consisting of 64 unit transistors 164 is designated as a unit transistor group 321g and indicated by ‘64’ and a current circuit consisting of 128 unit transistors 164 is designated as a unit transistor group 321h and indicated by ‘128.’ However, as described with reference to
One set of the unit transistor group 321 (321a to 321h) is the transistor group 165c. For ease of drawing and explanation, it is assumed that each of the unit transistor groups 321 is 8-bit. Needless to say, the transistor groups 321 may be 6-bit or 10-bit.
The unit transistor groups 321 are formed separately for R, G, and B. The number of bits to be formed may be varied among R, G, and B. For example, a 6-bit configuration may be used for R and B while using an 8-bit configuration for G which needs a large number of tones. It is also preferable to vary the magnitude of overcurrent among R, G, and B. For example, there can be a configuration or method in which the magnitude of overcurrent is increased for R and B while decreasing the magnitude of overcurrent for G. The above items also apply to other examples of the present invention. They also apply to the transistor group 165c and transistor group 165b.
In the configuration in
The most significant bit is used to control overcurrent (generate overcurrent) for the following reasons. First, for ease of explanation, it is assumed that a change from the 1st tone to the 4th tone is made. Also, it is assumed that the number of tones is 256 (8 bits each for R, G, and B).
Even when making a change from the 1st tone to a white tone, insufficient writing of program current does not occur if the change is from the first tone to a tone not lower than the middle tone (e.g., the 128th or higher tone). This is because the program current is relatively large and the parasitic capacitance Cs is charged and discharged relatively quickly.
However, when making a change from the 1st tone to a tone lower than the middle tone (e.g., the 127th or lower tone), the program current is too small to charge and discharge the parasitic capacitance Cs in a 1H period. Thus, it is necessary to improve the method for making a change from the 1st tone to a tone lower than the middle tone such as from the 1st tone to the 4th tone. The overcurrent driving according to the present invention is performed for that purpose.
Since the target tone is lower than the middle tone as described above, the most significant bit is not used to specify the program current. That is, when making a change from the 1st tone, the target tone is lower than ‘01111111’ (The switch D7 for the most significant bit is always off). The present invention performs overcurrent driving by keeping the most significant bit off.
If the original tone (tone before the change) is tone 1, one unit transistor 164 operates and the switch D0 turns on. If the target tone is tone 4, the switch D2 operates, bringing four unit transistors 164 into operation. However, the four unit transistors 164 are not enough to discharge the parasitic capacitance Cs to a target level. Thus, the switch D7 is closed to bring the unit transistor group 321h into operation.
Incidentally, the D7 switch may be operated together with the D2 switch (i.e., the D7 and D2 switches may be turned on in the first half or at the beginning of 1H while turning on only the D2 switch in the second half) Alternatively, only the switch D7 may be turned on in the first half or at the beginning of 1H while turning on only the switch D2 in the second half.
When the D7 switch turns on, 128 unit transistors 164 operate (or unit currents corresponding to 128 unit transistors are outputted). Thus, the parasitic capacitance Cs is discharged 32 (=128/4) times faster than when only the D2 switch is used. This makes it possible to improve the writing of program current.
Determination as to whether to turn on the switch D7 is made separately for R, G, and B video data by a controller circuit (IC) (not shown). A decision bit KDATA is applied to the source driver circuit (IC) 14 by the controller circuit (IC). KDATA is, for example, 5-bit. KDATA consists of MSB (1 bit) and low-order 4 bits. When the MSB of KDATA is 0 (Low), overcurrent driving is not performed. When the MSB of KDATA is 1 (High), overcurrent driving is performed. That is, overcurrent driving is performed, and then a program current corresponding to the target tone is applied.
A precharge bit is used to specify whether to apply the precharge voltage Vp. When the precharge bit is 0 (Low), the precharge voltage Vp is not applied. When the precharge bit is 1 (High), the precharge voltage Vp is applied. Also, overcurrent driving is performed according to the setting of KDATA, and then a program current corresponding to the target tone is applied.
The low-order 4 bits of KDATA specifies the application duration of the overcurrent in 15 steps. Overcurrent driving using 16 durations is performed based on this value. Thus, the value of the low-order 4 bits of KDATA specifies the time period during which the D5 bit is turned on.
KDATA is held in the latch circuit 221 for a period of 1H. The counter circuit 212 is reset in response to HD (horizontal synchronization signal) and counts in sync with a clock CLK. Data are compared between the counter circuit 212 and latch circuit 221. When the count value of the counter circuit 212 is smaller than the data value (the low-order 4 bits of KDATA) of the latch circuit 221, the AND circuit 213 continues to output an on voltage to the internal wiring 162b, maintaining the on state of the switch D5. Thus, current flows from the unit transistors 164 in the unit transistor group 321h to the internal wiring 162a and source signal line 18. Incidentally, the switch 161b is closed during current programming while the switch 161a is closed and the switch 161b is open during precharge driving.
A comparison circuit 331 compares the video data 1H before and the current video data, and thereby derives values of KDATA. The values derived are the value of the MSB (1 bit) which specifies whether to perform overcurrent driving and the value of the low-order 4 bits which specifies the application duration of the overcurrent. Also, the precharge bit is available to specify, as required, whether to apply the precharge voltage Vp. Besides, settings may be made, as required, to specify which of the switches D0 to D7 to turn on (to close) in overcurrent driving or to specify the magnitude of the precharge voltage Vp.
The video data DATA is transferred to the source driver circuit (IC) 14. The controller IC (circuit) transfers an upper count limit CNT of the counter circuit 212 to the source driver circuit (IC) 14.
KDATA is determined by the comparison circuit 331 based on the video data before the change (data 1H before) and video data after the change (current data). The data 1H before is the current potential of the source signal line 18. The current data is the target potential of the source signal line 18 to change to. Since the potential of the source signal line 18 corresponds to the tone of the video data, it may be determined based on the video data.
As illustrated in
According to the present invention, I is increased during overcurrent driving. In any case, however, increased I may cause the target potential of the source signal line 18 to be exceeded. Thus, when performing overcurrent driving, it is necessary to take the potential difference V into consideration. KDATA is determined based on the current potential of the source signal line 18 and the target potential of the source signal line 18 found from the next video data (current video data (video data to be applied next=(after the change vertical direction in FIG. 34))).
KDATA may specify the duration for which the D7 switch is kept on or the magnitude of the current used in overcurrent driving. Alternatively, it may specify a combination of the ON time of the D7 switch (the longer the ON time, the longer the duration of application of the overcurrent to the source signal line 18, increasing an effective value of the overcurrent) and the magnitude of the overcurrent (the larger the overcurrent, the larger the effective value of the overcurrent applied to the source signal line 18). For ease of explanation, it is assumed that KDATA specifies the ON time of the D7 switch first.
The comparison circuit 331 compares the video data 1H before and the video data after the change (see
KDATA is specified when the video data 1H before is in a low tone region (preferably the region containing the lower ⅛ of all tones starting from the 0th tone—for example, if there are 256 tones, this is between the 0th tone and 32nd tone) and the video data after the change is not higher than the halftone region (preferably the region containing the lower ½ of all the tones starting from the 1st tone—for example, if there are 256 tones, this is between the 1st tone and 128th tone). The value to be specified is determined by taking in to consideration a VI characteristic curve of the driving transistor 11a. There is a large potential difference between the Vdd voltage of the source signal line 18 and V0 voltage for the 0th tone (complete black display). Also, there is a large potential difference between the V0 voltage and V1 voltage for the 1st tone. There is a considerably smaller potential difference between the V2 voltage for the 2nd tone and the V1 voltage than the potential difference between the V0 voltage and V1 voltage. In this way, the potential differences between V3 and V2, between V4 and V3, and so on decrease gradually with increases in tone due to nothing but the non-linear VI characteristics of the driving transistor 11a.
The potential difference between tones is proportional to the amount of discharge of the parasitic capacitance Cs. Thus, it changes with the application duration of the program current, i.e., the application duration and magnitude of the overcurrent Id in the case of overcurrent driving. For example, even though the tone difference between V0 (tone 0) 1H before and V1 (tone 1) after the change is small, the application duration of the overcurrent Id cannot be decreased because of the large potential difference.
Conversely, even if atone difference is large, there are cases in which it is not necessary to increase the overcurrent. For example, the potential difference between the potential V10 of tone 10 and potential V32 of tone 32 is small and the program current Iw of tone 32 is large, and thus the parasitic capacitance Cs can be charged and discharged in a short time.
In
When changing from the 0th tone (1H before) to the 0th tone (after the change), KDATA is 0 because there is no change in potential. When changing from the 0th tone (1H before) to the 1st tone (after the change), it is necessary to change from the V0 potential to the V1 potential. Since the V1 voltage minus V0 voltage is large, the MSB of KDATA is set to 1 and the low-order 4 bits are set to 15—the highest value (as an example). This is because the change in the potential of the source signal line 18 is large. A change from the 1st tone (1H before) to the 2nd tone (after the change) involves a change from the V1 potential to the V2 potential. Since the V2 voltage minus V1 voltage is relatively large, the low-order 4 bits of KDATA are set to 12—a value close to the highest value (as an example). This is because the change in the potential of the source signal line 18 is large. A change from the 3rd tone (1H before) to the 4th tone (after the change) involves a change from the V3 potential to the V4 potential. However, since the V4 voltage minus V3 voltage is relatively small, the low-order 4 bits of KDATA are set to a small value of 2. This is because the change in the potential of the source signal line 18 can be small and the parasitic capacitance Cs can be charged and discharged relatively quickly, making it possible to write the target program current into the pixel 16.
When changing from a low tone region to a region not lower than the halftone, both MSB and low-order 4 bits of KDATA can be set to 0. This is because the program current corresponding to the tone after the change is large and the potential of the source signal line 18 can be brought to or close to the target potential within a period of 1H. For example, when changing from the 2nd tone to the 38th tone, KDATA=0.
When changing to a lower tone, overcurrent driving is not performed. When changing from the 38th tone to the 2nd tone, the MSB of KDATA is set to 0 and the low-order 4 bits are set to 0. This is illustrated in
It is useful to combine the overcurrent driving method according to the present invention with the N-fold driving method, duty ratio control driving method, or the like described with reference to
As described above, KDATA is determined by the control IC (circuit) and it is transmitted to the source driver circuit (IC) 14 in response to a differential signal. The transmitted KDATA is held in the latch circuit 221 shown in
To determine the relationship in the table in
According to the present invention, the magnitude of the program current Iw varies in proportion to the magnitude of reference current. Thus, the magnitude of the overcurrent in overcurrent driving in
The technical idea of the overcurrent driving method according to the present invention consists in setting the magnitude of overcurrent, application duration (application period), and effective value of overcurrent according to the magnitude of program current, output current from the driving transistor 11a, etc. and combining overcurrent driving and precharge driving.
The comparison circuit 331 or comparison instrument makes comparisons separately for R, G, and B video data. Needless to say, however, KDATA may be calculated by determining the brightness (Y value) from RGB data. That is, instead of simply making comparisons for R, G, and B, KDATA may be calculated, determined, or computed by taking into consideration continuity, periodicity, and variation ratio of tone data as well as chromaticity changes and brightness changes. Also, it goes without saying that KDATA may be derived by taking video data or similar data of surrounding pixels into consideration rather than on a pixel-by-pixel basis. For example, possible methods include one that divides the display screen 34 into multiple blocks and determines KDATA by taking into consideration the video data or the like in each block.
In
If the application of overcurrent is short, the target potential of the source signal line 18 cannot be reached. In overcurrent driving, needless to say, it is preferable that the target potential of the source signal line 18 be reached. However, it is not necessary to reach the target potential of the source signal line by overcurrent driving alone. Regular current driving may be used after performing overcurrent driving in the first half of 1H. Then, any error caused by the overcurrent driving can be corrected by the program current used in the regular current driving. Thus, it is preferable to set the target potential for overcurrent driving a little smaller than the target potential of the source signal line 18. It is a feature of the present invention that any deviation caused by overcurrent driving can be corrected by program current for the video signal.
The source driver circuit (IC) 14 operates at a constant current. Thus, a constant program current Iw flows for the period from t2 to t3. As the parasitic capacitance Cs is charged and discharged by the program current Iw until the target potential is reached, a current I flows through the driving transistor 11a of the pixel 16, maintaining the potential of the source signal line 18 so that the target program current Iw will flow. Thus, the predetermined program current Iw is kept flowing through the driving transistor 11a. In this way, accuracy is not required of overcurrent in overcurrent driving. The lack of accuracy is compensated for by the driving transistor 11a of the pixel 16.
The source driver circuit (IC) 14 operates at a constant current. Thus, a constant program current Iw flows for the period from t4 to t3. As the parasitic capacitance Cs is charged and discharged by the program current Iw until the target potential is reached, a current I flows through the driving transistor 11a of the pixel 16, maintaining the potential of the source signal line 18 so that the target program current Iw will flow. Thus, the predetermined program current Iw is kept flowing through driving transistor 11a. In this way, accuracy is not required of overcurrent in overcurrent driving. The lack of accuracy is compensated for by the driving transistor 11a of the pixel 16.
In this way, the number of operating unit transistors 164 and the magnitude of unit current per unit transistor 164 are fixed. Thus, the charging and discharging time of the parasitic capacitance Cs and the potential of the source signal line 18 can be controlled in proportion to the ON period of the D7 switch. Incidentally, although it is assumed for ease of explanation that the parasitic capacitance Cs is charged and discharged by the overcurrent, this is not restrictive considering leakage of the switch transistor of the pixel 16.
Thus, it is a unique feature of the present invention that the magnitude of overcurrent can be known from the number of operating unit transistors 164. Since the write time t can be expressed as T=ACV/I (where A is a proportionality constant, C is the parasitic capacitance, V is the potential difference, and I is the program current), the value of KDATA can be determined logically from the parasitic capacitance (known at the time of array design), VI characteristics of the driving transistor 11a (known at the time of array design), and the like.
The example in
FIG. 36(A1) shows operation of the D7 switch. FIG. 36(A2) shows operation of the D6 switch. FIG. 36(A3) shows changes in the potential of the source signal line 18. In
Similarly, FIG. 36(B1) shows operation of the D7 switch, FIG. 36(B2) shows operation of the D6 switch, and FIG. 36(B3) shows changes in the potential of the source signal line 18. In
Incidentally, the above example concerns sink current. When the driving transistors 11a are N-channel transistors, P-channel transistors are used as the unit transistors 164 in the source driver IC (circuit) 14. Thus, output current (overcurrent) from the unit transistors 164 are discharged to the source signal line 18.
Although a case in which the source driver IC (circuit) 14 handles sink current has been cited in the above example, the present invention is not limited to this. The use of source current (discharge current), which is made possible by simple modification of the wording in the above description, is also included in the scope of the present invention.
Similarly, FIG. 36(C1) shows operation of the D7 switch, FIG. 36(C2) shows operation of the D6 switch, and FIG. 36(C3) shows changes in the potential of the source signal line 18. In
Thus, by operating multiple switches including their ON periods based on KDATA, thereby changing or adjusting the number of operating unit transistors 164 or magnitudes of unit currents, it is possible to set or change the source signal line to an appropriate potential.
Although it has been stated with reference to
The switches to be operated are not limited to D7 and D6. It goes without saying that other switches such as D7 may be selected simultaneously for operation or control. In the case of period a, the D7 switch is kept on for a period of 1/(2H) to apply the overcurrent composed of 128 unit currents to the source signal line 18 for overcurrent driving.
In the case of period b, the D7 and D6 switches are kept on for a period of 1/(2H) to apply the overcurrent composed of 128+64 unit currents to the source signal line 18 for overcurrent driving.
In the case of period c, the D7, D6, and D5 switches are kept on for a period of 1/(2H) to apply the overcurrent composed of 128+64+32 unit currents to the source signal line 18 for overcurrent driving.
In the case of period c, the D7, D6, and D5 switches and another video data switch (D2 switch in the case of 4th-tone video data) are kept on for a period of 1/(2H) to apply the overcurrent composed of 128+64+32+α unit currents to the source signal line 18 for overcurrent driving.
The above example is a method which involves generating overcurrents in predetermined periods by controlling the switch D7 and the like as described with reference to
According to the present invention, the source driver circuit (IC) 14 incorporates the transistor group 165c which can output unit currents (program currents) corresponding to tones as the switches D are turned on and off. Thus, by outputting a program current corresponding to a predetermined tone from the transistor group 165c and thereby operating the driving transistor 11a of the pixel 16, it is possible to make settings or adjustments such that the driving transistor 11a of the pixel 16 can pass the program current.
During this operation, with the pixel configuration illustrated in
The precharge voltage Vp is applied to the source signal line 18 from the source driver IC (circuit) 14 and an appropriate pixel row is selected by applying an on voltage to the gate signal line 17a of the pixel row. The precharge voltage Vp is applied to the gate terminal of the driving transistor 11a of the pixel 16, thereby programming (setting) the driving transistor 11a to pass program current Iw. Thus, if the precharge voltage Vp is applied according to the characteristics of the driving transistor 11a of the pixel 16, the driving transistor 11a is programmed with the program current Iw accurately. Since the precharge voltage Vp is a voltage, even if the source signal line 18 has parasitic capacitance, the potential of the source signal line 18 can be charged and discharged in sequence. This makes it possible to take advantage of precharge driving.
According to the present invention, both the program current corresponding to a video tone signal and constant current are denoted by Iw. This is because the constant current Iw is generated by the source driver IC (circuit) 14 using an element of the same structure as the one used to generate the program current and the constant current is generated when the program current corresponding to a tone is set to a predetermined level.
As described above, the potential of the source signal line 18 measured by applying a constant current (predetermined current) Iw to the source signal line 18 has been referred to as a precharge voltage Vp. Also, the voltage applied in period A in
Thus, if the potential at which the driving transistor 11a of each pixel 16 passes a program current Iw can be measured or determined and can be used as the precharge voltage Vp during programming (writing of a tone), it is possible to write the tone into the pixel 16 quickly without being affected by the parasitic capacitance of the source signal line 18. Of course, if a program current Iw is applied after the application of the precharge voltage Vp, the pixel can be programmed with high accuracy.
That is, according to the present invention, the constant current Iw (including Iw=0 A) is applied to the driving transistor 11a, and the gate terminal potential of the driving transistor 11a at that time is measured or acquired via the source signal line 18. The measured/acquired potential is applied as a precharge voltage Vp to the source signal line 18—either directly or after computation or other predetermined processing—to perform tone writing (voltage programming or current programming) by reflecting the characteristics of the driving transistor 11a of the pixel 16.
According to the present invention, a constant current Iw is applied to pixel transistors or outputted from the pixels driver transistor 11a, and the gate terminal voltage of the pixels driver transistor 11a is measured with the constant current Iw being applied or outputted. The gate terminal voltage of the pixels driver transistor 11a varies with the characteristics of the driver transistor 11a. Thus, measuring the gate terminal voltage of the driver transistor 11a with a constant current being applied to the driver transistor 11a is equivalent to measuring the characteristics of the driver transistor 11a.
The measured voltage is stored after A/D conversion in a memory placed or formed inside or outside the source driver IC (circuit) 14. When displaying an image on the EL display apparatus, the voltage data stored in the memory are converted into analog voltage through D/A conversion, and then either the analog voltage is applied directly as a precharge voltage Vp to the corresponding pixel or a target tone signal (precharge voltage Vp) obtained by adding or subtracting a tone voltage to/from the analog voltage (used as a base or origin) is applied to the corresponding pixel.
Thus, adding a video voltage corresponding to a tone or tone difference to the measured voltage used as a base and applying the resulting signal to the transistor 11a means applying a tone signal (voltage signal) acting as a video signal after compensating for the characteristics of the pixel driving transistor 11a.
The measured or acquired gate terminal voltage of the driving transistor 11a may be applied to the driving transistor of the pixel after the measurement either directly or after addition or subtraction of the video voltage in real time. The constant current Iw may be 0 A (meaning that no constant current flows). In that case, the appropriate pixel can be selected and the driving transistor 11a of the pixel can be shorted between gate and drain terminals.
The current programming mode has the disadvantage that it cannot compensate sufficiently for the characteristics of pixel transistors 11a. However, by using current programming mode in which a constant current is applied to pixel transistors 11a and measuring the gate terminal potential of the transistors, the present invention exercises its ability to compensate for transistor characteristics, which is an advantage of the current programming mode.
The use of a constant current Iw not lower than a predetermined level makes it possible to eliminate the problem of insufficient writing in low tone regions (low current regions), which is a weak point of the current programming mode. The video signal applied to the pixels during video display is a voltage signal, which makes it possible to avoid insufficient writing even in low tone regions. That is, by calculating or determining tone voltages through addition or subtraction of a voltage based on the measured voltages and applying the tone voltages to pixel transistors 11a, it is possible to demonstrate a feature of voltage driving, i.e., the advantage of avoiding insufficient writing in all tone regions.
Although it is stated herein that the gate terminal voltage of the transistor 11a is measured or held either directly or indirectly by applying a constant current to the transistor 11a, the present invention is not limited to this. Also, in addition to the magnitude of voltage, the amount of changes in the voltage before and after the application of the constant current, speed of voltage changes, and difference value of the voltage may also be measured and stored in memory. In short, any data may be used as long as the data are related to generation of the precharge voltage Vp.
The measurement of voltage also includes the act of holding the measured voltage inside or outside a driver circuit after analog-digital conversion (A/D conversion) and configuration therefor as well as the act of holding the voltage as digital data in memory. Also, it includes the act of not only measuring, but also temporarily holding, latching, or storing the voltage in a capacitor or other holding medium and configuration therefor. Incidentally, the constant current Iw includes 0 A.
The pixel 16 must be configured such that the output current of the driving transistor 11a can be inputted and outputted to/from the source signal line 18 as shown in
The above operation or configuration makes it possible to pass a program current corresponding to the precharge voltage Vp through the driving transistor 11a of the pixel 16. By measuring the potential of source signal line 18 at this time, it is possible to acquire the precharge voltage Vp corresponding to the predetermined tone.
Although the above example involves applying a constant current (including 0) to the driving transistor 11a of each pixel 16 and measuring the precharge voltage Vp at which the driving transistor 11a of the pixel 16 passes the constant current Iw, the presentinvention is not limited to this. Although the characteristics of the driving transistors 11a formed in a matrix on an array 30 vary from lot to lot, the characteristics do not vary much among arrays within each lot. The gate terminal potential Vp of a particular transistor in an array 30 may be measured by passing a constant current (including 0) through the transistor and then the Vp may be applied to the driving transistors 11a of the other pixels. Although the applied Vp deviates more or less from the voltage at which the driving transistor 11a of the pixel passes the constant current Iw, there will be no problem because a program current is applied subsequently.
According to the present invention, the gate terminal voltage of the driving transistor 11a is varied such that a program current (constant current) corresponding to the tone needed to set the precharge voltage Vp is outputted from the source driver circuit (IC) 14 and passed through the driving transistor 11a. Then, the gate terminal voltage of the driving transistor 11a is measured and fed back as a precharge voltage Vp. This operation or setup makes it possible to set an accurate precharge voltage Vp by feeding back the characteristics of the source driver circuit (IC) 14 and characteristics of the array.
A method for acquiring a precharge voltage Vp accurately will be described below with reference to drawings. Incidentally, it is assumed that the precharge voltage Vp is a program voltage and the gate terminal voltage of the driving transistor 11a. A target current is supplied to the EL element 15 by the application of the program voltage.
As an example of measuring or acquiring the precharge voltage Vp, a method for applying a constant current to measurement pixels 16s formed or placed on an array 30 will be described first. Measurement pixels 16s are formed on the periphery (an area which does not contribute to image display) of the display screen 34 and the like. Of course the pixels 16 used for image display may be used as the measurement pixels 16s.
In the measurement pixel 16s, when an on voltage is applied to the gate signal line 17a and a program current is applied to the source signal line 18, the driving transistor 11a comes into operation and its gate terminal voltage changes. At this time, the precharge voltage Vp can be acquired by reading the potential of the source signal line 18.
For example, to acquire the precharge voltage V1 for tone 1, the program current (normally, the output current from a single unit transistor) corresponding to tone 1 is applied to the source signal line 18, thereby bringing the driving transistor 11a of the measurement pixel 16s into operation. The precharge voltage V1 can be acquired by measuring the potential of the source signal line 18 at the completion of the operation.
Although it has been stated in the example of the present invention that the potential of the source signal line 18 is measured, the present invention is not limited to this. For example, measurements may be taken by pressing a probe stylus against the gate terminal of the driving transistor 11a of the pixel 16. Also, the gate terminal potentials of the driving transistors 11a of multiple pixels 16 may be measured/determined simultaneously or on average, for example, by selecting two or more pixel rows at a time rather than measuring the gate terminal potential of a single driving transistor 11a. The Vp voltage is measured by scanning the positions of the selected gate signal line 17a in sequence by controlling the gate driver circuit 12.
Although it has been stated in the example of the present invention that voltages are measured, the concept of measurement includes the acts of holding, obtaining, or determining voltages. That is, any configuration, form, or method may be used as long as it can use acquired potential of the source signal line 18 as the precharge voltage Vp. For example, possible configurations include one that uses the potential of the source signal line 18s by sampling and holding it, one that performs analog-digital conversion (A/D conversion) of the analog potential of the source signal line 18s and uses the resulting digital data directly as precharge voltage V0 to V5, one that uses results of analog conversion as V0 to V5, and one that uses the potential of source signal line 18s as V0 to V5 by feeding it back directly.
Needless to say, in the method according to the present invention, the acquired/measured potential, voltage, or potential change of the source signal line 18s may be padded, multiplied by a fixed ratio, weighted, or level-shifted, or may undergo predetermined processing or have another voltage value added or subtracted. Also, it goes without saying that a desired value may be obtained by averaging multiple measurements. Besides, a target voltage may be predicted or inferred from changes in the potential of the source signal line 18s. For ease of explanation, it is assumed herein that the concept of measurement includes the above-mentioned concepts, methods, or configurations.
The precharge voltage V0 to V5 can be used not only as the precharge voltage Vp, but also for voltage driving or gamma curve generation. Thus, the technical idea of the present invention is applicable not only to current programming mode (driving), but also to voltage programming mode (driving).
In
Suppose, for example, when a pixel 16s is selected by the application of an on voltage (VGL) to the gate signal line 17a and a constant current Iw is passed through the driving transistor 11a, the gate terminal potential of the driving transistor 11a is 3.8 V. Next, an off voltage (VGH) is applied to the gate signal line 17a to complete selection of the pixel 16. Consequently, the potential of the gate signal line 17a changes from VGL to VGH. As a result, the gate terminal potential of the driving transistor 11a shifts toward the anode potential Vdd as the potential penetrates into the capacitors 19a and 19b. For example, if the change in the potential caused by the penetration is 0.5 V, the gate terminal potential of the driving transistor 11a becomes 4.3 V (=3.8V+0.5V), holding the driving transistor 11a in a state in which it passes a current smaller than Iw.
The above example means that the pixel 16 can be configured such that a current smaller than the constant current Iw will flow. Current driving is not good at writing a small program current into the pixel 16. However, the above configuration or operation allows programming with a small current. This is a big advantage.
By forming the voltage measuring circuit 381 in the source driver IC (circuit) 14, it is possible to acquire the precharge voltage Vp from an output terminal 83s connected to the source signal line 18s. Thus, it is not necessary to form a new output terminal 83 to measure the precharge voltage Vp. Also, if the source driver IC (circuit) 14 is formed or constructed from a semiconductor chip, a sample-and-hold circuit, operational amplifier, analog switch, and the like used to measure the precharge voltage Vp can be built, formed, or constructed accurately with a small area.
A program current generating circuit which outputs the precharge voltage Vp for measurement has a configuration similar to that of the current tone circuit 154 which outputs the program current. The current tone circuit has been described with reference to
The gate driver circuit 12a controls the gate signal line 17a1 which selects measurement pixels 16s and the gate signal line 17a2 (which corresponds to the gate signal line 17a in
The current tone circuit 154 outputs a program current corresponding to tone 0. The program current Iw corresponding to tone 0 is 0. Thus, the switch 161b (see
The voltage measuring circuit 381 measures the voltage of the source signal line 18s and holds it in the voltage tone circuit 231. Alternatively, it stores the measured or acquired value in memory. The held precharge voltage V0 is used as the V0 voltage in
Similarly, the current tone circuit 154 outputs a program current Iw corresponding to tone 1. The program current corresponding to tone 1 is an output current (one unit current) of one unit transistor 164. One unit of program current is supplied to the source signal line 18s to select the gate signal line 17a1. However, when measuring the precharge voltages V0 to V5 successively, the gate signal line 17a1 may remain selected. The driving transistor 11a of the measurement pixel 16s operates such that one unit of program current will flow through the source signal line 18s in steady state. The steady-state flow of the unit current causes the potential of the source signal line 18s to change in such a way as to maintain the steady-state unit current. The driving transistor 11a charges or discharges the source signal line 18s until one unit current flows steadily.
When the potential of the source signal line 18s settles to a fixed value, the potential V1 of the source signal line 18s is measured by operating the voltage measuring circuit 381. Of course, the measured voltage V1 may be used as the precharge voltage Vp after the potential of the source signal line 18s stabilizes, by keeping the voltage measuring circuit 381 in operation.
Although it is stated that the gate signal line 17a1 remains deselected while the voltage measuring circuit 381 is measuring the voltage V1, it goes without saying that the gate signal line 17a1 may remain selected. The voltage measuring circuit 381 measures the voltage V1 of the source signal line 18s and holds it in the voltage tone circuit 231 or stores it in memory. The measured V1 voltage is used as the V1 voltage in
The same applies to the precharge voltage V2. The current tone circuit 154 outputs a program current corresponding to tone 8 (see
Eight units of program current are supplied to the source signal line 18s to select the gate signal line 17a1. The driving transistor 11a of the measurement pixel 16s operates such that eight units of program current will flow through the source signal line 18s in steady state. The steady-state flow of the unit current causes the potential of the source signal line 18s to change in such a way as to maintain the steady-state unit current.
When the potential of the source signal line 18s settles to a fixed value or after the time when it is estimated to settle to a fixed value, the potential of the source signal line 18s is measured by operating the voltage measuring circuit 381. Of course, by keeping the voltage measuring circuit 381 in operation, the measurement may be taken when the potential of the source signal line 18s stabilizes or after the time when it is estimated to stabilize. Measurements may be taken even when the source signal line 18 is changing if the steady-state potential of the source signal line 18 can be estimated. The measured voltage is used as the precharge voltage Vp (=V2). The voltage measuring circuit 381 measures the voltage (precharge voltage V2) of the source signal line 18s and holds it in the voltage tone circuit 231.
Similar operation is performed for V3 which is the precharge voltage Vp for tone 32, V4 which is the precharge voltage Vp for tone 128, and V5 which is the precharge voltage Vp for tone 255.
Although it has been stated in the above example that the precharge voltages Vp are measured in order from V0 to V5, this order is not restrictive. The precharge voltages may be measured in order form V5 to V0. Alternatively, they may be measured randomly. Besides, it is not strictly necessary to measure all V0 to V5. For example, only V0, V3, and V5 may be measured and the potentials of V1, V2, and V4 may be calculated from the voltage values of V0, V3, and V5. Alternatively, unit currents corresponding to the precharge voltages Vp may be applied to the source signal line 18s after setting the potential of the source signal line 18s to a predetermined level by the application of a fixed voltage (black voltage or reset voltage) to the source signal line 18s. Also, the precharge voltages V0 to V5 may be measured by taking and averaging multiple measurements. Besides, only the precharge voltage Vp=V0 may be measured. V0 is the precharge voltage Vp when the constant current Iw is 0 A and corresponds to tone 0. Thus, it is the origin of the gamma curve. Once the origin can be measured or determined, other tones (tones 1 to 255 in the case of 8 bits) can be generated easily.
Measuring time may be varied among precharge voltages Vp such as measuring the precharge voltage V0 for a longer time and measuring the precharge voltage V5 for a shorter time. This is because a small current (constant current Iw) flows into the source signal line 18s and the potential of the source signal line 18s changes slowly in the case of the precharge voltage V1 and the like.
On the other hand, in the case of the precharge voltage V5 and the like, a large current (constant current Iw) flows into the source signal line 18s and the potential of the source signal line 18s changes quickly. Incidentally, it is not strictly necessary to generate the constant current Iw in the source driver IC (circuit) 14. A constant current Iw outputted by a constant current generating circuit formed or placed outside the source driver IC (circuit) 14 may be supplied to the pixel 16 either directly or via the source driver IC (circuit) 14.
In the present invention shown in
The program current Iw from the source driver circuit (IC) 14 is supplied to the driving transistor 11a in the measurement pixel 16s. The resulting precharge voltage Vp is then measured. Consequently, the precharge voltages V0 to V5 reflect the characteristics of the driving transistor 11a in the pixel 16 in the array board 30. Further, with respect to the dependence on temperature, the driving transistor 11a reflects the temperature at which the display panel in accordance with the present invention is driven.
As described above, in the present invention, the source driver IC (circuit) 14 generates an accurate program current. The program current corresponds to a tone used to actually display an image on the display apparatus. Accordingly, the size and costs of the entire source driver circuit (IC) 14 can be reduced. Further, the measurement pixel 16s is produced or formed in the array board 30, in which the pixels 16 are formed. The measurement pixels 16s are formed simultaneously with the pixels 16, which display images (during the same process or step). Further, when the same program current is applied to the pixel 16 and to the measurement pixel 16s, the source signal line 18 has substantially the same potential as the source signal line 18s.
The driving transistor 11a in the pixel 16 and the driving transistor 11a in the measurement pixel 16s are configured or formed so as to have the same characteristics. In order that the driving transistor 11a in the pixel 16 and the driving transistor 11a in the measurement pixel 16s may have the same characteristics, the pixels 16 and 16s may basically have the same configuration or layout. It is easiest and preferable to configure the driving transistors 11a so that they have a channel width W and a channel length L. In the present invention, the driving transistor 11a in the pixel 16 and the driving transistor 11a in the measurement pixel 16s have the same size and shape.
In the above embodiment, the program current is a sink current. However, the present invention is not limited to this. If the driving transistor 11a in the pixel 16 is an N-channel transistor, the program current is a source current. In this case, the unit transistors 164 constituting the transistor group 165c are composed of P-channel transistors.
The driving transistor 11a in the measurement pixel 16s is operated by the program current to vary the potential of the source signal line 18s. The potential of the source signal line 18 corresponding to the program current is defined as Vp. The Vp voltage is measured by a voltage measuring circuit 381. The voltage is converted into digital data by the A/D conversion circuit 391. The digital data is then accumulated or held in the memory or holding circuit (latch circuit or the like). The held data is applied to the voltage tone circuit 231. The voltage tone circuit 231 executes a digital-analog (DA) conversion to obtain a precharge voltage Vp, which is then applied to the source signal line 18.
In the above description, the precharge voltage Vp is applied to the source signal line 18. However, the present invention is not limited to this. For example, a probe needle may be brought into pressure contact with the gate terminal of the driving transistor 11a in the pixel 16 or with the pixel electrode of the EL element 15. The precharge voltage Vp may then be applied to the probe needle.
The measuring precharge voltage VP output to the source signal line 18 may be converted into digital data directly by the A/D conversion circuit 391 not through the voltage measuring circuit 381. That is, in the description of the present invention, the voltage measuring circuit 381 is formed, located, and used or operated. However, any configuration or instrument may be used provided that it can acquire a voltage for the source signal line 18s or source signal line 18 using a certain configuration, instrument, or method. For example, a sample hold circuit may be used to sample and hold the precharge voltage Vp for a given period.
The transistor group 165s, the voltage measuring circuit 381, and the like may be separated from the source driver circuit (IC) and formed on another chip (IC); the transistor group 165s, the voltage measuring circuit 381, and the like conduct the program current through the source signal line 18s. This chip (IC) is mounted on the array board 30 by the COG technique. Alternatively, the TAB technique may be used for mounting.
In the embodiment shown in
Each measurement pixel 16s is used to measure precharge voltages V0 to V5. By averaging each of the precharge voltages V0 to V5 measured via the plurality of measurement pixels 16s, it is possible to determine accurate precharge voltage values V0 to V5.
Different precharge voltages Vp may be assigned to the respective measurement pixels 16s; the measurement pixel 16s1 is used to measure the precharge voltage V0, the measurement pixel 16s2 is used to measure the precharge voltage V1, the measurement pixel 16s3 is used to measure the precharge voltage V2, . . . the measurement pixel 16s6 is used to measure the precharge voltage V5.
The precharge voltage Vp to which each measurement pixel 16s is assigned may change every given period of time. For example, during the first period, the measurement pixel 16s1 is used to measure the precharge voltage V0, the measurement pixel 16s2 is used to measure the precharge voltage V1, the measurement pixel 16s3 is used to measure the precharge voltage V2, . . . the measurement pixel 16s6 is used to measure the precharge voltage V5.
During the second period, the measurement pixel 16s1 is used to measure the precharge voltage V5, the measurement pixel 16s2 is used to measure the precharge voltage V4, the measurement pixel 16s3 is used to measure the precharge voltage V3, . . . the measurement pixel 16s6 is used to measure the precharge voltage V0.
The period may be one or more frames or a fraction of one frame. Alternatively, the gate signal lines 17a may be sequentially selected in synchronism with the scanning of the gate signal lines 17b. That is, the period in which one gate signal line 17a is selected is one horizontal scan period (1H).
As shown in
In the embodiment in
An output current from the transistor group 165s in
A variation in the magnitude of the program current varies the potential of the source signal line 18s. The source signal line 18s has a parasitic capacitance and thus requires a certain period before its potential reaches a target value. In
In
In the above description, the precharge voltage Vp is measured or acquired in association with the predetermined tone. However, the present invention is not limited to this. The precharge voltage Vp may be measured (acquired) for all tones (for example, for 256 tones, the 0th to 255th tones). Appropriate voltage driving can be achieved by using the precharge voltage Vp as a tone signal.
In the above embodiment, at least three precharge voltages Vp are measured. However, the maximum tone, the tone 255 (for 256 tones), and the minimum tone, the tone 0, may be measured so as to generate a precharge voltage Vp midway between the voltages corresponding to the maximum and minimum tones.
The driving scheme shown in
An output from the averaging circuit 443 is input to the operational amplifier 151 to reduce impedance. The output is then input to the electronic regulator 152. The electronic regulator 152 uses the resistor R to divide the input precharge voltage Vp=V0, V55 to generate a precharge voltage (V0 to V255) corresponding to the tone.
As shown in
As described in
As shown in
As shown in
The numbers in the transistor groups 165c and 165s in
In the configuration shown in
The electronic regulator 152 in
In the embodiment shown in
In
The potential of the source signal line 18 may of course be detected by specifying a particular pixel row or column such as the first pixel row or column as shown in
For the precharge voltage V1, the transistor group 165c connected to each output terminal 83 outputs a program current I1. The potential of the source signal line 18 varies to the value corresponding to the program current I1. Under these conditions, the switches S0 to Sn (n is the maximum number value of the output terminal 83) are sequentially closed. The potential of the source signal line 18 is applied to the source signal line potential detection line 501. The measured voltage is defined as Vsd1 and is transmitted to a controller circuit (IC) 801. The controller circuit (IC) 801 causes this voltage data to be stored in the memory (SRAM or EEPROM) 502 as the potential Vst1 of the source signal line 18 for the program current I1. The Vst1 corresponds to the precharge voltage Vp=V1.
For the precharge voltage Vp=V2, the transistor group 165c connected to each output terminal 83 outputs the program current Iw=12. Under these conditions, the switches S0 to Sn (n is the maximum number value of the output terminal 83) are sequentially closed. The potential of the source signal line 18 is applied to the source signal line potential detection line 501. The measured voltage is defined as Vsd2 and is transmitted to a controller circuit (IC) 801. The subsequent steps are as described above.
The thus measured precharge voltages V0 to V5 are transmitted to the source driver circuit (IC) 14 as the set values Vst of the precharge voltage Vp and as required. The precharge voltages V0 to V5 are then used as set values for the electronic regulator 152.
The above configuration allows the variation of the transistor group 165c to vary the program current Iw, used to measure the precharge voltage Vp. This enables the precharge voltage Vp to be measured more flexibly and appropriately.
As shown in
If a plurality of source driver circuits (IC) 14 are used, the voltage measuring circuit 381 is constructed or formed in each source driver circuit (IC) 14. Then, the voltage measuring circuit 381 in one of the plurality of source driver circuits (IC) 14 is operated. The precharge voltage Vp from the voltage measuring circuit 381 is supplied or applied to the other source driver circuit (IC) 14.
In
The master and slave modes are thus set for the source driver circuit (IC) 14 because the source signal line 18s or measurement pixel 16s used to measure the precharge voltage Vp is formed at a position outside the display screen 34. The measurement pixel 16s is thus constructed at an end of the display screen 34. Accordingly, the source driver circuit (IC) 14 used to measure the precharge voltage Vp is selected to lie at an end of the display screen 34 (in
If the source signal line 18s and the measurement pixel 16s can be formed at the opposite ends of the display screen 34, as shown in
To set the source driver circuit (IC) 14a to the master mode, the switch Sa is closed, the source driver circuit (IC) 14d is set to the slave mode, and the switch Sb is opened. The other source driver circuits (IC) 14 (14b and 14c) operate in the slave mode. To set the source driver circuit (IC) 14d to the master mode, the switch Sb is closed, the source driver circuit (IC) 14a is set to the slave mode, and the switch Sa is opened. The other source driver circuits (IC) 14 (14b and 14c) always operate in the slave mode.
It is also possible to always fix the source driver circuit (IC) 14a or 14d to the master mode. However, alternatively operating the source driver circuits (IC) 14a and 14d in the master mode averages the precharge voltage Vp and thus produces good results.
The switching is carried out periodically, for example, for each field or frame. Of course, the switching may be based on periods; it may be carried out during each horizontal scan period. Further, two or more source driver circuits (IC) 14 may operate in the master mode. For example, if four source driver circuits (IC) 14 operate in the master mode, they may control one switch S so as to apply the precharge voltage Vp to the other source driver circuits (IC) 14.
For example, for the first frame, the source driver circuit (IC) 14a is set to the master mode, the switch Sa is closed, the source driver circuit (IC) 14d is set to the slave mode, and the switch Sb is opened. The other source driver circuits (IC) 14 (14b and 14c) operate in the slave mode. For the second frame succeeding the first frame, the source driver circuit (IC) 14d is set to the master mode, the switch Sb is closed, the source driver circuit (IC) 14a is set to the slave mode, and the switch Sa is opened. Similarly, for the third frame succeeding the second frame, the source driver circuit (IC) 14a is set to the master mode, the switch Sa is closed, the source driver circuit (IC) 14d is set to the slave mode, and the switch Sb is opened. The other source driver circuits (IC) 14 (14b and 14c) operate in the slave mode.
In another embodiment shown in
The transistor group 165s may be constructed or placed inside a voltage measuring circuit (IC) 521 in
The scheme described in the above embodiments applies a program current from one transistor group 165s to one measurement pixel 16s to acquire a plurality of precharge voltages Vp. The present invention is not limited to this. As shown in
In the configuration in
Likewise, ‘32’ in the transistor group 165s means 32 unit transistors generating a precharge voltage V3 (unit transistor group 32). ‘128’ in the transistor group 165s means 128 unit transistors generating a precharge voltage V4 (unit transistor group 128). ‘255’ in the transistor group 165s means 255 unit transistors generating a precharge voltage V5 (unit transistor group 255).
A transistor group 165s1 outputs the program current I1. A transistor group 165s8 outputs a program current I8. Similarly, a transistor group 165s32 outputs a program current I32. A transistor group 165s128 outputs a program current I128. A transistor group 165s255 outputs a program current I255.
A unit transistor group 165s0 is exceptional and has no unit transistors. That is, the current Iw=0. A voltage measuring circuit 381a measuring the precharge voltage V0 is connected to a source signal line 18s0. A measurement pixel 16s0 is also connected to the source signal line 18s0. The measurement pixel 16s0 sets the voltage corresponding to the precharge voltage V0 for the source signal line 18s0. The voltage measuring circuit 381a measures and outputs the precharge voltage V0.
One unit transistor is formed or placed in the unit transistor group 165s1. Alternatively, the unit transistor group 165s1 can output a program current corresponding to the tone 1. The unit transistor group 165s1 has a voltage measuring circuit 381b connected to a source signal line 18s1 to measure the precharge voltage V1. The measurement pixel 16s1 is also connected to the unit transistor group 165s1. When the program current Iw corresponding to the tone 1 is applied to the measurement pixel 16s1, the measurement pixel 16s1 sets or adjusts the voltage corresponding to the precharge voltage V1 for the source signal line 18s1, or is operated. The voltage measuring circuit 381b measures and outputs the precharge voltage V1.
Eight unit transistors are formed or placed in the unit transistor group 165s8. Alternatively, the unit transistor group 165s8 can output the program current Iw corresponding to the tone 8. For example, one transistor is formed which has a channel width eight times as large as that of the unit transistor. However, the transistor group 165s is advantageously composed of a set of identical unit transistors 164 like the transistor group 165c. This prevents the output constant current Iw from varying.
The unit transistor group 165s8 has a voltage measuring circuit 381c connected to a source signal line 18s8 to measure the precharge voltage V2. The measurement pixel 16s2 is also connected to the unit transistor group 165s8. When the program current Iw=I8 corresponding to the tone 8 is applied to the measurement pixel 16s2, the measurement pixel 16s2 sets or adjusts the voltage corresponding to the precharge voltage V2 for a source signal line 18s2, or is operated. The voltage measuring circuit 381c measures and outputs the precharge voltage V2.
Likewise, the unit transistor group 165s32 has a voltage measuring circuit 381d connected to a source signal line 18s3 to measure the precharge voltage V3. The measurement pixel 16s3 is also connected to the unit transistor group 165s32. When the program current Iw=I32 corresponding to the tone 32 is applied to the measurement pixel 16s3, the measurement pixel 16s3 sets or adjusts the voltage corresponding to the precharge voltage V3 for a source signal line 18s3, or is operated. The voltage measuring circuit 381d measures and outputs the precharge voltage V3.
The unit transistor group 165s128 has a voltage measuring circuit 381e connected to a source signal line 18s4 to measure the precharge voltage V4. The measurement pixel 16s4 is also connected to the unit transistor group 165s128. When the program current Iw=I128 corresponding to a tone 128 is applied to the measurement pixel 16s4, the measurement pixel 16s4 sets or adjusts the voltage corresponding to the precharge voltage V4 for a source signal line 18s4, or is operated. The voltage measuring circuit 381e measures and outputs the precharge voltage V4.
Likewise, the unit transistor group 165s255 has a voltage measuring circuit 381f connected to a source signal line 18s5 to measure the precharge voltage V5. The measurement pixel 16s5 is also connected to the unit transistor group 165s255. When the program current Iw=I255 corresponding to the tone 255 is applied to the measurement pixel 16s5, the measurement pixel 16s5 sets or adjusts the voltage corresponding to the precharge voltage V5 for a source signal line 18s5, or is operated. The voltage measuring circuit 381f measures and outputs the precharge voltage V5.
First pixels in accordance with the present invention are formed in a matrix to display an image. Second pixels in accordance with the present invention correspond to, for example, the pixels 16s0, 16s1, 16s8, 16s32, 16s128, and 16s255 in
The first and second pixels in accordance with the present invention correspond to examples of the pixels in accordance with the present invention.
In the above embodiments, the source signal line 18s and the measurement pixel 16s are formed, and the program current Iw is applied to the source signal line 18s. The voltage measuring circuit 381 then measures the potential of the source signal line 18s. However, the present invention is not limited to this. For example, the program current Iw may be applied to the source signal line 18 and pixel 16 formed in the display screen 34. The potential of the source signal line 18 may then be measured to acquire the precharge voltage Vp.
In addition to these arrangements,
Of course, if the program current Iw allowing the precharge voltage Vp to be measured is applied to all or a plurality of the source signal lines 18, the switches S connected to the corresponding source signal lines are selected or sequentially selected to apply the precharge voltage Vp to the voltage measuring circuit 381.
The circuit for generating a program current Iw may be constructed or located outside the source driver IC (circuit) 14. A constant current output by the circuit generating a program current Iw is applied to the source signal line 18. Further, the constant current is not limited to a fixed value. Of course, the constant current may be varied every given period of time. The constant current may be varied like a pulse. The above mentioned are applicable to the other embodiments of the present invention.
In
The driving transistor 11a in each pixel 16 outputs the program current corresponding to the tone 1, to each source signal line 18. At this time, the switch is closed which connects to the source signal line 18 subjected to the program current corresponding to the tone 1. Then, each source signal line is short-circuited by voltage wiring 611. Consequently, the potentials of the source signal lines 18 have the same value. This voltage V1 is obtained by averaging the precharge voltages Vp from the source signal lines 18, which correspond to the tone 1. Therefore, a good precharge voltage V1 can be acquired by allowing the voltage measuring circuit 3811 to measure the precharge voltage V1 across the voltage wiring 611. This applies to the measurement of the precharge voltages Vp corresponding to the other tones.
In the above embodiments, the precharge voltage Vp is acquired by applying the program current Iw (including Iw=0 (A)) corresponding to the tone to all source signal lines 18 and closing all switches S. However, the present invention is not limited to this. Of course, the precharge voltage Vp may be acquired by applying the program current corresponding to the tone to an arbitrary number of source signal lines 18 and closing the arbitrary selected switches S. For example, the following process may be executed. The switches for the even-numbered source signal lines 18 are closed, and the voltage Vp is measured. In the next timing, the switches for the odd-numbered source signal lines 18 are closed, and the voltage Vp is measured. It is also possible to sequentially select two or four switches to allow the precharge voltage Vp to be sequentially measured.
The program current corresponding to the same tone need not be applied to all source signal lines 18. For example, the following process may be executed. The program current corresponding to the tone 1 is applied to the odd-numbered transistor groups 165. The program current corresponding to the tone 32 is applied to the even-numbered transistor groups 165. The switches connected to the odd-numbered source signal lines 18 are then closed. The precharge voltage V1 corresponding to the tone 1 is then measured. The switches connected to the even-numbered source signal lines 18 are then closed. The precharge voltage V3 corresponding to the tone 32 is then measured.
The number of source signal lines 18 selected need not be equal to that of switches selected. If the program current is applied to 32 source signal lines 18, only the switches connected to 16 source signal lines 18 may be selected and closed. Further, the time required to measure the precharge voltage Vp is effectively reduced by applying the constant current Iw to the source signal line 18 before the corresponding switch S is closed.
Of course, the program current applied to each source signal line 18 and corresponding to the tone may be sequentially varied so as to allow the sequential measurement of the precharge voltage Vp. Further, the apparatus is preferably configured or operated so as to periodically change the source signal line 18 to measure each precharge voltage Vp rather than using the fixed source signal line 18 to measure a precharge voltage Vp in a particular tone.
A measurement period and a wait period (period of time before measurement is carried out) for the precharge voltage Vp to be measured preferably vary with the tone. The apparatus is provided with a program function for varying the wait period in accordance with an instruction from the controller circuit (IC) 801. The wait time is varied because, for example, the V1 voltage involves a smaller program current and thus requires a long time for completion of a change in the potential of the source signal line 18. Furthermore, the V5 voltage corresponding to the tone 255 involves a larger program current and thus requires only a short time for completion of a change in the potential of the source signal line 18. This leads to the need of little wait time. Further, the formation of a plurality of voltage measuring circuits 381 enables a plurality of precharge voltages Vp to be simultaneously measured. This makes it possible to reduce the time (period) required for measurement of the precharge voltage Vp.
In the embodiment shown in
Basically, the precharge voltage Vp is acquired during a blanking period corresponding to one field or frame or one horizontal scan period, as shown in
As shown in
Moreover, a measured precharge voltage Vp or a precharge voltage Vp measured during the preceding operation may be digitalized and stored in the memory of the display apparatus. During the next operation, a precharge voltage Vp may be generated using the stored digital data as an initial voltage (start voltage). Alternatively, on the basis of the digital data corresponding to the precharge voltage Vp, the corresponding constant current Iw is calculated or determined and then applied to the source signal line 18.
In the embodiment shown in
In the embodiments of the present invention, the voltage measuring circuit 381 measures the voltage across the source signal line 18. However, the present invention is not limited to this. The voltage measuring circuit 381 may measure the voltage across any line as long as the potential can be falsely varied as with the source signal line 18. For example, the voltage measuring circuit 381 may measure the voltage across separately formed wiring. Alternatively, the voltage measuring circuit 381 may be connected directly to the gate terminal of the driving transistor 11a in the measurement pixel 16s. Alternatively, a probe needle may be brought into pressure contact with the gate terminal of the driving transistor 11a in the pixel 16 to measure the potential (voltage).
The function of the voltage measuring circuit 381 is not limited to the measurement of the potential (voltage) across the source signal line 18. The voltage measuring circuit 381 may determine the precharge voltage Vp from charges in or electric fields from the source signal line 18. Alternatively, the voltage measuring circuit 381 may determine the precharge voltage Vp from the speed of a variation in the magnitude of charges or electric fields. For example, a pickup coil may be placed on the pixel 16 so that the precharge voltage Vp can be indirectly acquired on the basis of the magnitude of lines of electric force emitted by the pixel 16. It is also possible to irradiate the pixel 16 with electron beams to measure the magnitude of electrons.
In the above embodiments, the program current is applied to one measurement pixel 16s so that the voltage measuring circuit 381 can measure the potential across the source signal line 18. The present invention is not limited to this. For example, as shown in
In
The transistor group 165cb applies, to the pixel 16b, the program current corresponding to a predetermined precharge voltage Vp to be measured. The driving transistor 11a in the pixel 16b conducts the program current to charge or discharge the source signal line 18b so that its voltage reaches a value corresponding to the program current. Similarly, the transistor group 165 cc applies, to the pixel 16c, the program current corresponding to a predetermined precharge voltage Vp to be measured. The driving transistor 11a in the pixel 16c conducts the program current to charge or discharge the source signal line 18c so that its voltage reaches a value corresponding to the program current.
The voltage measuring circuit 381 closes the switch Sa to measure the precharge voltage Vp held in the source signal line 18a. The voltage measuring circuit 381 also closes the switch Sb to measure the precharge voltage Vp held in the source signal line 18b. Likewise, the voltage measuring circuit 381 closes the switch Sc to measure the precharge voltage Vp held in the source signal line 18c.
Further, the voltage measuring circuit 381 simultaneously selects any of the plurality of switches S (Sa to Sn). Selection of a plurality of switches S averages the precharge voltages Vp held in the selected plurality of source signal lines 18. This enables the acquisition of a precharge voltage Vp reflecting the characteristics of the driving transistor 11a in the display area.
As described above, the present invention allows a plurality of pixels 16 to be selected so as to enable the measurement of the precharge voltage Vp held in each of the corresponding source signal lines 18. Alternatively, a plurality of source signal lines 18 may be selected so as to enable their precharge voltages Vp to be measured. Alternatively, an n-times program current may be applied to one or more pixels 16 to operate the driving transistors 11a in the pixels 16 to charge or discharge the corresponding source signal lines 18 so that their potentials can be measured. The measured potentials across the source signal lines 18 are calculated to acquire precharge voltages Vp.
The internal wiring 162 in the source driver IC (circuit) 14 is connected to the source signal line 18 via an output terminal 83. The present invention acquires the precharge voltage Vp by measuring the potential of the source signal line 18 or the potential across the internal wiring 162 in the source driver IC (circuit) 14. However, the precharge voltage Vp measured (acquired) by the voltage measuring circuit 381 may not be used as it is. For example, the precharge voltage Vp corresponding to the 0 or 1 tone realizes a perfect black display. This precharge voltage Vp thus needs to be biased toward the anode (shifted toward the anode voltage) compared to that acquired by using the transistor group 165 to apply the program current corresponding to the 0 or 1 tone. This applies to the case where the driving transistor 11a is of the P channel type and where the source terminal of the transistor is connected to the anode terminal.
A calculation circuit 651 adds the HDATA and MDATA together to obtain target VDATA. The digital data VDATA is converted into analog data, which is then output as a precharge voltage Vp or input to the electronic regulator 152. In this operation, the HDATA and MDATA are added together. However, the VDATA may be determined by a subtraction. The VDATA may of course be determined by weighting the HDATA or MDATA at a given rate. This of course applies to the other embodiments of the present invention.
Description has been given of the method of processing measured data or the like by converting it into a digital signal. However, the present invention is not limited to this. As shown in
The HDATA and VDATA shown in
The precharge voltages V0 to V5 are acquired by applying the corresponding program current Iw to the pixel 16. In
The transistor group 165cb outputs the program current corresponding to the voltage V1. The voltage measuring circuit 381 measures and outputs the voltage V1. Similarly, the transistor group 165cb outputs the program current corresponding to the voltage V2. The voltage measuring circuit 381 measures and outputs the voltage V2. This operation is repeated until the V5 is processed. Once the V5 is processed, the same process is executed again starting with the operation of measuring (acquiring) the V0.
In
In
In the above embodiments, the voltage measuring circuit 381 measures the potential across the source signal line 18. The concept or operation of the voltage measuring circuit 381 or the memory storing operation involve a sample hold circuit as shown in
As shown in
The switch S2 is closed to apply the precharge voltage Vp to the capacitor C. Even if the switch S2 is subsequently closed, the precharge voltage Vp is held. The precharge voltage Vp has its impedance reduced by the operational amplifier 151 and is then output. The switch S1 is closed to hold the precharge voltage Vp in Cn. The held precharge voltage Vp is applied to the electronic regulator 152 and others. The above configuration or scheme also involves the voltage measuring circuit 381. The configuration in
In the above configuration, the transistor group 165s and others are formed into a semiconductor chip. However, as shown in
As shown in
In the display panel, independent transistor groups 165c are formed for R, G, and B. The precharge voltage Vp=V0 corresponding to the tone 0 can be shared by the R, G, and B. The V1 to Vn are set for other precharge voltage Vp. The R, G, and B offer different light emission efficiencies for the program current Iw. Of course, if the same or substantially the same program current is used for the R, G, and B, then for each tone, the same precharge voltage Vp may be used for the R, G, and B.
If the precharge voltage Vp varies among the R, G, and B, the apparatus is configured as shown in
The program current from the transistor group 165cR is applied to the measurement pixel 16S by closing the switch SaR. Closing the switch SaR closes the SbR to apply the potential across the source signal line 18 to an R voltage measuring circuit 381R, which measures or acquires a precharge voltage V0R to VmR (m is the maximum number value of the precharge voltage Vp).
The program current from the transistor group 165cG is applied to the measurement pixel 16S by closing the switch SaG. Closing the switch SaG closes the SbG to apply the potential across the source signal line 18 to a G voltage measuring circuit 381G, which measures or acquires a precharge voltage V0G to VmG.
The program current from the transistor group 165cB is applied to the measurement pixel 16S by closing the switch SaB. Closing the switch SaB closes the SbB to apply the potential across the source signal line 18 to a B voltage measuring circuit 381B, which measures or acquires a precharge voltage V0B to VmB.
One voltage measuring circuit 381 may be used as each of the voltage measuring circuits 381R, 381G, and 381B. The internal wiring 162 and the measurement pixel 16S may each be separated into three pieces for the R, G, and B. Further, as shown in
The program current I output by the transistor group 165s or 165c may be multiplied by n before output. The multiplication by n is illustrated in
As shown in
In
In
The present invention uses the measurement pixel 16s or the pixel 16 to acquire a precharge voltage Vp. However, means is required for achieving this scheme if the pixel 16 or the like from which the precharge voltage Vp is to be acquired is defective. The defective pixel does not output a normal precharge voltage Vp or provides no precharge voltage Vp. Means is also required for achieving the scheme if the driving transistor 11a from which the precharge voltage Vp is to be acquired exhibits abnormal characteristics.
The present invention achieves this object by forming a plurality of pixels 16s from which the precharge voltage Vp is to be acquired and selecting a normal one of the plurality of pixels 16s. This is illustrated in
The measurement pixel 16s to be selected is selected or set in advance by measuring the characteristics of the plurality of pixels 16s. The selected or set information is held in the nonvolatile memory as close information for the switch S (S1 to S4). A switch S (S1, S2, S3, or S4) to be selected by default is predetermined.
Of course, n switches S may be closed, with an n-times program current applied, as shown in
The measurement pixels 16s may be formed in a matrix as shown in
The pixels 16s formed in a matrix are configured in the same manner as that for the display screen 34. The gate driver circuit 12s is connected to or formed adjacent to the measurement pixels 16s in the pixel row direction. The transistor group 165s in the source driver circuit (IC) 14 is connected to or formed adjacent to the measurement pixels 16s in the pixel column direction. The measurement pixel 16s to be selected is determined under the control of the selected source signal line 18 and gate driver 12s. The source signal line 18 the precharge voltage Vp of which is to be measured is determined under the control of the voltage measuring circuit 381.
Selection of the measurement pixel row by the gate driver circuit 12s is controlled by the ST3 and CLK3 as in the case of the ST1 and CLK1 (also see
The gate driver circuit 12s selects a prespecified (predetermined) gate signal line 17s (having functions similar to those of the gate signal line 17a) to operate the driving transistors 11a in the selected pixel row. The measurement pixel row to be selected and the measurement pixel to be selected are preselected by measuring the characteristics of a plurality of pixels 16s. The selected information is held in the nonvolatile memory. A default measurement pixel row or pixel 16s is also predetermined. The program current Iw is applied to the measurement pixel row under the control of the source driver circuit (IC) 14.
As in the case of
In
In
In
When the program current Iw corresponding to the precharge voltage V1 is applied to the source signal line 18s, the measurement pixel 16s1 is selected. The voltage measuring circuit 381b then measures and applies the voltage V1 to the electronic regulator 152 and the like. Similarly, when the program current corresponding to the precharge voltage V2 is applied to the source signal line 18s, the measurement pixel 16s2 is selected. The voltage measuring circuit 381c then measures the precharge voltage V2. When the program current corresponding to the precharge voltage V3 is applied to the source signal line 18s, the measurement pixel 16s3 is selected. The voltage measuring circuit 381d then measures the precharge voltage V3. When the program current corresponding to the precharge voltage V4 is applied to the source signal line 18s, the measurement pixel 16s4 is selected. The voltage measuring circuit 381e then measures the precharge voltage V4. When the program current corresponding to the precharge voltage V5 is applied to the source signal line 18s, the measurement pixel 16s5 is selected. The voltage measuring circuit 381f then measures and applies the precharge voltage V5 to the electronic regulator 152 and the like.
The present invention is not limited to the configuration shown in
The above embodiment acquires the precharge voltage Vp by operating the measurement pixel 16s or the pixel 16. However, the precharge voltage Vp may be generated outside the panel before application. For example, as shown in
The following are controlled out by the controller circuit (IC) 801 as shown in
In
As described above, the present invention measures the precharge voltage Vp by applying the constant current Iw (including 0 (A) corresponding to the tone 0) to the source signal line 18 and operating the driving transistor 11a. The precharge voltage Vp is measured before or during the display of an image; the measurement before the display includes inspections or adjustments carried out immediately after the manufacture of the panel and the period during the display includes a blanking period, the beginning of one horizontal scan period and the like.
The measured or acquired precharge voltage Vp is applied during the A period, described with reference to
For example, it is assumed that the constant current Iw 16 corresponding to the tone 16 is applied to the driving transistor 11a in the pixel 16, so that the precharge voltage Vp 16 corresponding to the constant current Iw=16 is measured. Then, to apply the tone 32 to the pixel 16, the potential difference Vsd between the tones 32 and 16 is added to the precharge voltage Vp16 to determine a precharge voltage Vp32, which is then applied to the source signal line 18 during the A period. The potential difference Vsd between tones such as the tones 32 and 16 to be added to the precharge voltage Vp 16 is predetermined by measuring the characteristics of a standard driving transistor 11a.
Further, the above embodiment of the present invention effectively determines the precharge voltage Vp=V0 corresponding to the tone 0. This is because the same precharge voltage Vp=V0 is used for the R, G, and B pixels if the driving transistors 11a in the R, G, and B pixels exhibit the same characteristics. That is, the precharge voltage Vp=V0 can be used as an origin voltage.
The V0 voltage is obtained by selecting the pixel 16 and applying the on voltage to the gate signal line 17a to short circuit the gate and drain terminals of the target driving transistor 11a. Since the applied program current Iw=0 (A), each source signal line 18 is electrically disconnected from the source driver IC (circuit) 14 (floating state). The driving transistor 11a changes the potential across the source signal line 18 so as to avoid passing a current through itself. The potential at which no current flows through the driving transistor 11a (cutoff state) is the V0 voltage.
The measured or acquired V0 voltage may contain the effect of a punch-through voltage or the like. Accordingly, the target precharge voltage Vp=V0 is obtained by addition or subtraction of a given voltage or multiplication of a given ratio.
The precharge voltage Vp=V0 is applied during the A period as shown in
In
Below the low tone areas, a small program current is used for the tone. Consequently, these tone areas are significantly affected by the parasitic capacitance of the source signal line 18, resulting in frequent insufficient writing. Therefore, a high program accuracy cannot be achieved. Application of the precharge voltage Vp=V0 sets the potential across the source signal line 18 at the value for the tone 0. Even if a program current for a low tone area is programmed for the driving transistor 11a, the potential across the source signal line 18 varies from the value for the tone 0. This reduces the amount of charges involved in the charging or discharging of the source signal line 18. The potential can therefore be changed to the value for the target low tone area.
In
If the V0 voltage reflects the characteristics of the driving transistor 11a, the potential varied by the overcurrent driving reflects the characteristics of the driving transistor 11a. This is because the potential varies linearly under the overcurrent driving. Consequently, even with a variation in characteristics among the driving transistors 11a in each of the pixels 16 formed in a matrix, an even image can be displayed by application of the precharge voltage Vp=V0 corresponding to the tone 0 for each driving transistor 11a.
During the B period, the program current is applied which corresponds to the tone to be displayed on the pixel 16. If the target potential has been optimally reached by application of the precharge voltage Vp=V0 and overcurrent, the potential does not change during the period B. Even if the target potential has not been reached, it can be accurately reached by applying the program current during the B period (compensation). This enables the application of a current accurately programmed in the EL element 15 in the pixel 16.
In
In
As described above, the following are determined on the basis of the tone written to the pixel 16 or a change in potential: whether or not to apply the precharge voltage Vp at the beginning of a horizontal scan period and whether or not to carry out overcurrent driving.
The optimum V0 voltage varies with the temperature of the panel. The precharge voltage Vp=V1, V2, V3, . . . also varies with the temperature. Thus, preferably, the temperature of the panel is monitored (using a temperature sensor such as a thermistor) and multiplied by a correction factor for temperature to determine a V0 voltage, which is then applied during the A period.
Further, the precharge voltage Vp applied during the A period is preferably changed or adjusted on the basis of the written tone or potential or a change from the source signal line potential during the preceding horizontal scan period or from the tone written to the pixel during the preceding horizontal scan period. Further, the V0 voltage need not necessarily be applied and the applied voltage may be changed in association with the written tone.
The V0 voltage corresponding to the 0 tone is determined by the driving transistor 11a in the pixel 16. The driving transistor 11a normally has the same size for the R, G, and B. Accordingly, the V0 voltage is the same for the R, G, and B. The charging or discharging of the parasitic capacitance Cs is often based on the V0 voltage. Therefore, the V0 voltage corresponds to the origin (tone 0) in the current or voltage driving scheme.
The above embodiment acquires the precharge voltage Vp from the potential across the source signal line 18 or the like. However, the precharge voltage Vp can be acquired from potential other than that across the source signal line 18. For simplification, description will be given of a scheme for determining the precharge voltage Vp=V0.
The V0 voltage can be measured, acquired, or determined using the configuration shown in
In
The V0′ voltage applied to the source signal line 18 is written to the pixel 16. The V0′ voltage is adjusted so that the set maximum current value Im becomes equal to (or smaller than) a target value. The maximum current Im is the current value I0 corresponding to the tone 0, ideally I0=0 (A). However, setting the current value I0 exactly at 0 (A) is difficult. Further, when the current value at the tone 0 is too close to zero, the potential at the tone 0 is too close to the anode voltage Vdd. This makes it difficult to change to another tone during the next horizontal scan period. Therefore, the Im, the maximum value of the I0, is set.
The V0 voltage is defined as the V0 voltage applied to the source signal line 18 when the Im reaches the target value. In the pixel configuration shown in
The I0 current providing the proper V0 voltage is preferably such that when the diagonal length of the display area of the display panel is defined as d (inches), the unit of I0 is mA, and K=10/d, K=at least 0.2 and at most 2. More preferably, K is at least 0.3 and at most 1.0. The I0 current is set as the Im. These settings enable a good black display to be achieved. Furthermore, even if precharge driving (overcurrent driving) is carried out so as to change the 0 tone to another tone, the tone change can be properly effected.
As described above, the V0′ voltage is changed and the I0 current is measured in association with the change. When the value of the I0 current meets the range of the K (the I0 is equal to or smaller than the Im), the V0′ voltage being applied to the source signal line 18 is set to the precharge voltage V0.
The precharge voltage V0 is also preferably acquired as shown in
In
The voltage measuring circuit 381 is connected to the wiring 845 to measure the potential across the wiring 845. The voltage measuring circuit 381 thus measures the potential across the source signal line 18 via a probe 843. Since the constant current source 844 outputs the zero current, no current is applied to the source signal line 18. That is, the source signal line 18 provides the precharge voltage V0 (tone 0).
In
The data held in the ROM 502 is also composed of 8 bits. The ROM data and the data from the voltage measuring circuit 381 are added together by the addition (or subtraction) circuit 651. In general, the addition data shifts the data toward the anode voltage side.
After the addition, the data is composed of 9 bits. The data is converted into analog data by the D/A (digital-analog conversion) circuit 391. The analog data is then subjected to temperature compensation by a temperature compensation circuit 851 which detects a panel temperature. The compensated data is then applied to the source driver circuit (IC) 14. The temperature compensation circuit 851 is required because the precharge voltage Vp is used for voltage driving and thus depends on temperature. This is in turn because the current flowing through the driving transistor 11a varies with temperature in spite of the fixed value of the gate terminal potential. In
Another driving scheme in accordance with the present invention will be described with reference to
The current tone circuit 154 need not output program currents corresponding to all tones. The current tone circuit 154 has only to be able to output currents for particular tones such as the 128th, 64th, 0th, 1st, and 255th tones. Of course, the voltage tone circuit 231 is desirably configured to be able to output all tone voltages. Further, the voltage tone circuit 231 can desirably output program voltages for lower tones (127th or lower tones).
For easy description, the current tone circuit 154 is formed or constructed inside the source driver circuit (IC) 14. However, the present invention is not limited to this. For example, a circuit generating a constant current Iw=I1 may be provided outside the source driver circuit (IC) 14 to supply the constant current I1 to the source signal line 18 via the switch circuit so as to allow the measurement of the gate terminal voltage (source signal line 18) V1 of the driving transistor 11a in the pixel 16. Further, the measured voltage may be written to an EEPROM placed outside the source driver circuit (IC) 14 so that a V-I curve of the driving transistor 11a in the pixel 16 can be generated from the written data. Of course, the above measurements may be carried out during a panel adjusting process before panel shipment.
First, description will be given of a measuring step which measures or generates driving voltage data. The measuring step is executed while image display is not being carried out, for example, immediately after power-on. The measuring step is also carried out so as not to affect the image display.
As previously described, with the driving scheme in accordance with the present invention, the pixel configuration needs to be of the current driving type as shown in
In the voltage driving pixel (for example, the pixel configuration shown in
The present invention uses the current driving pixel configuration to apply a program voltage to the pixel to carry out voltage driving (application of the program voltage). The present invention measures the voltage for a characteristic curve of the driving transistor 11a in at least one pixel 16 and generates a characteristic curve corresponding to voltage driving on the basis of the measured voltage, to drive the pixel. The voltage scheme in accordance with the preset invention is the same as or similar to the voltage offset cancel type in that the voltage V0 for the tone 0 is measured or generated and in that voltage program data is generated on the basis of the voltage V0 for the tone 0 to drive the pixel.
Of course, the present invention is not limited to the voltage corresponding to the tone 0. However, voltage offsetting can be precisely carried out by accurately measuring the voltage value for the tone 0. If the tone 0 is not used, a characteristic curve is preferably obtained using a voltage value measured or determined in connection with an intermediate tone (at least one-eighth and at most half of the maximum tone). This is because the characteristics of the driving transistor vary significantly within this range.
A voltage-current (V-I) characteristic curve of the driving transistor 11a (a transistor that supplies a current to the EL element 15 or a transistor that defines a current flowing through the above described transistor) can be generated by calculating a polynominal or referencing a matrix table or a lookup table 931. The above described process may be sequentially executed for each video signal data or may be pre-executed. The voltage-current (V-I) characteristic curve need not be obtained for all video signal data but may be determined intermittently or at intervals. This is because the video signal data and the characteristics of the driving transistors or the like in the array 30 are approximate among pixels located close to one another.
The above configuration enables the EL display apparatus in accordance with the present invention to carry out both voltage driving and current driving. The EL display apparatus can thus carry out voltage+current driving (see
The configuration shown in
In the above description, the potential across the source signal line 18 is measured. The present invention is not limited to this. For example, the potential across the source signal line 18 may be approximately measured or estimated by detecting the migration of charges or measuring the intensity of electric fields. Further, the present invention is not limited to the potential across the source signal line 18. Any configuration may be used provided that it can directly or indirectly measure the gate terminal voltage of the driving transistor 11 in the pixel 16.
The present invention is also characterized by controlling the gate driver circuit 12a to sequentially select the gate signal line 17a and sequentially measuring the gate terminal voltages of the driving transistors 11a in the selected pixel row. That is, the present invention selects a pixel row, applies a specified constant current to the source signal line 18, and then measures the gate terminal voltages of the driving transistors in the selected pixel row. A sufficient time is used for the measurement. The V-I characteristics of each of the driving transistors are estimated on the basis of the corresponding measured gate terminal voltage. The video signal is converted into a program voltage on the basis of the estimated V-I curve. The program voltage is applied to the source signal line during image display.
The switch Sx (x=1 to n) is formed in each source signal line 18. The switch Sx is normally formed of an analog switch. The switch Sx only detects voltages and rarely conducts current. The switch Sx may thus be small-sized and has only to offer a high impedance.
The switch Sx may be configured to be able to input or output a potential from an A terminal to each source signal line 18 as shown in
In the above description, the switch Sx is formed in each source signal line 18. However, the present invention is not limited to this. For example, the switch Sx may be formed only in the odd number-th source signal lines 18. Alternatively, the switch Sx may be formed in the multiple-of-four numbered source signal lines 18. Alternatively, depending on the configuration of the display panel, a switch or the like may be formed on or connected to the gate signal line 17.
As described with reference to
In the diagram in
As shown in
The block diagram in
The A/D conversion circuit 391 digitalizes and holds the voltage applied to the source signal line 18 (applied to the output terminal 83) in the memory 502 in the source driver circuit (IC) 14. Each memory contains 8 bits, and the number of memories 502 produced or formed is equal to that of the pixels.
In the above description, the A/D conversion circuit 391 digitalizes the voltage applied to the output terminal 83 (potential across the source signal line 18=voltage at the gate terminal of the driving transistor 11a). However, the present invention is not limited to this. If an analog signal can be sampled and held so that voltage tone data can be generated from the analog signal, the A/D conversion circuit 391 is not required. In
The EL display panel (display apparatus) in accordance with the present invention uses the source driver circuit (IC) 14 in accordance with the present invention. In
This enables the measurement of the program voltage V1 that allows the flow of the constant current I1. The program voltage V1 is a point on a characteristic curve (gate voltage-output current (V−1) curve) of the driving transistor 11a. The characteristic curve can be estimated on the basis of the V1. The program voltage V1 may be an arbitrary point on the characteristic curve. The voltage V0, corresponding to the tone 0, may also be used. However, the constant current corresponding to the tone 0 is zero. The V0 is the voltage at the gate terminal of the driving transistor 11a obtained at the current 0.
The pixels 16 on the display screen 34 have varying characteristics owing to a laser anneal characteristic variation or the like. However, it is possible to pass the constant current I1 through the driving transistor 11a and measure the V1 voltage so that the characteristics of each pixel can be determined on the basis of the magnitude of the V1 voltage. Consequently, a characteristic curve of each pixel 16 can be determined on the basis of the magnitude of the V1 voltage. The characteristic curve is determined in real time by converting V1 data with reference to the matrix table or lookup table 931. Alternatively, the characteristic curve may be calculated using a monomial or polynominal.
Video tone program data is thus obtained. That is, the video tone data is converted into voltage tone program data in accordance with the estimated or measured V-I curve. The conversion is carried out for each pixel 16. To obtain more accurate voltage tone data, the current tone circuit 154 may generate a plurality of constant currents each of which is passed through the corresponding pixel 16 on the display screen 34 so as to allow the measurement of the potential across the corresponding source signal line 18.
To measure the voltage V1, the output terminals 83a to 83n provide the constant current I1 to select the gate driver circuit 12a so as to supply the I1 current to the driving transistors 11a in the selected pixel 16 row. Under the above described condition, the selector circuit 222 sequentially selects the switches S1 to Sn. The A/D conversion circuit 391 measures the potential across the source signal line 18. 8-bit voltage data resulting from a digital conversion by the A/D conversion circuit 391 is stored in a SRAM in a matrix as shown in
In
The voltage data need not be stored for all pixels 16. For example, the voltage data may be stored for some selected pixels as shown in
The above embodiment uses the source driver circuit (IC) 14 to supply a constant current I1 of 1 or 0.5 μA to the source signal line 18 or driving transistor 11a so as to allow the measurement of the potential V1 across the source signal line 18. Alternatively, the potential may be estimated. Alternatively, the voltage at the gate terminal of the driving transistor 11a of the appropriate pixel 16 is measured. Further, the potential V0 across the source signal line 18 is measured, which corresponds to the passage of no constant current (see
The V0 data on each pixel 16 or the V0 and V1 data on each pixel 16 is stored in the source driver circuit (IC) 14. Voltage values for the other tones are generated from the stored V0 data or V0 and V1 data in association with the video signal data as required. The program voltage generated is applied to the source signal line 18. The applied program voltage is applied to the gate terminal of the driving transistor 11a in each pixel 16 in synchronism with the gate driver circuit 12a. The program voltage is then held for a period corresponding to one field (frame).
Alternatively, only the V0 may be measured so that a characteristic curve can be estimated on the basis of the V0 to determine a voltage tone. As shown in
If a characteristic curve is determined on the basis of the V0 voltage or the like, the inclination of the characteristic curve (V-I curve) to the V0 voltage may be fixed.
In
As shown by a solid and dot lines in
The above embodiment measures I=1 μA to estimate a V-I curve so as to allow each tone current to be calculated. If the I can be measured at a plurality of points such as 0 μA (corresponding to the tone 0), 2 μA, and 0.5 μA and the voltage at the gate terminal of the driving transistor 11a which corresponds to each of the current values can be measured, better V-I curves can be determined to realize good image display with even characteristics.
With the driving method, the display panel, and the flat display apparatus using the display apparatus and the display panel in accordance with the present invention, the V0 or V1 voltage or the I1 current is measured or the corresponding data is determined so as to allow a V-I curve of driving transistor 11a or the like to be estimated or generated on the basis of the measured or determined data. Of course, it is also possible to determine or estimate a V-I curve on the basis of appropriate data and then accumulate the program current or voltage corresponding to each tone in the memory or the like so as to allow data corresponding to the program voltage or current to be read from the memory (storage instrument) and applied to the pixel 16.
The display panel in accordance with the present invention, during other than display period, applies a predetermined constant current to each pixel 16 via the current tone circuit 154 or the like to acquire the gate voltage V of a transistor which corresponds to the constant current, the transistor supplying a current to the EL element 15 such as the driving transistor 11a or performing a similar operation. The voltage V acquired is one or more voltage data. The voltage data is used to determine tone voltage data corresponding to a video signal generated by the voltage tone circuit 231. Alternatively, the voltage V acquired may be used. Of course, the predetermined constant current may be generated outside the source driver circuit (IC) 14 and then supplied to each source signal line 18.
The tone voltage data is applied during the A period shown in
This also applies to the case where the V0 or V1 voltage or a larger voltage value is measured. In the above description, a characteristic curve is generated from the measured V0 or V1 voltage. However, the voltage data measured from the source signal line 18 is not used as it is. For example, the pixel configuration shown in
The following are carried out at the time of power on: the measurement of the voltage across the source signal line 18 and determination of a tone voltage from the measured potential. That is, these operations are performed before image display.
The following may be carried out during a vertical or horizontal blanking period: the measurement of the voltage across the source signal line 18 and determination of a tone voltage from the measured potential.
The V0 voltage corresponding to the 0th tone may be measured at the time of power on as shown in
The V0 voltage or the like which corresponds to a lower tone portion is measured by applying a very small constant current (program current) to the source signal line 18. These voltages are thus affected by the parasitic capacitance of the source signal line 18 and have large time constants. Accordingly, the voltage corresponding to the lower tone portion is measured over a sufficient time by delaying a clock for the gate driver circuit 12a. Therefore, the voltage corresponding to the lower tone portion is preferably measured at the time of power on or the like.
In the embodiment of the present invention, the source driver circuit (IC) 14 outputs the constant current Iw (including the Iw=0 (A)) (either a discharge or sink current) to select the pixel 16. The constant current Iw or the constant current Iw in a substantially steady state flows through the driving transistor 11a in the pixel 16. Under this condition, the potential across the source signal line 18 or the potential at the gate or drain terminal of the driving transistor 11a is measured or acquired. Measurement or acquisition of the potential need not necessarily be carried out in a steady potential state. If the steady state is estimated or predicted, the measurement or acquisition may be carried out in a varying state to determine the potential in the steady state.
The above embodiment applies the constant current Iw and measures the potential across the source signal line 18 to determine the characteristics of the driving transistor 11a in the pixel 16. However, the reverse operation may be performed in order to determine the characteristics of the driving transistor 11a. That is, a predetermined constant voltage Va is applied to the source signal line 18 or to the driving transistor 11a in the pixel 16. A current Ia is then measured which flows through the driving transistor 11a when the constant voltage Va is applied. The current Ia depends on the characteristics of the selected driving transistor 11a. Accordingly, the current Ia enables the determination of characteristics of the driving transistor 11a. The measured or acquired current Ia is subjected to a current-voltage conversion and then to an A/D conversion, with the resulting data held in the storage instrument such as the memory. Of course, the above description is applicable to the other embodiments of the present invention.
The above embodiment passes the constant current through all pixels on the display screen 34 and then measures the potential across the source signal line 18 in each pixel (voltage at the gate terminal of the driving transistor 11a in each pixel 16). However, the present invention is not limited to this. Measurement need not be carried on all pixels because the characteristics of an arbitrary pixel are similar to those of pixels around this pixel.
For example, in
Not all pixels 16 need be subjected to the operation of applying the constant current to the pixel 16 to change the potential across the source signal line 18. Further, the measurement is not limited to the adjacent pixel 16. For example, the characteristics of every two pixels 16 may be measured. It is also possible to select even-numbered pixel columns and to measure the characteristics of the driving transistors 11a in the even-numbered pixel columns so as to allow the characteristics of the driving transistors 11a in odd-numbered pixel columns to be determined on the basis of the results of the measurement. It is also possible to select even-numbered pixel rows and to measure the characteristics of the driving transistors 11a in the even-numbered pixel rows so as to allow the characteristics of the driving transistors 11a in odd-numbered pixel rows to be determined on the basis of the results of the measurement. The above process may be executed for every plural pixel rows or every plural pixel columns.
It is not necessary to select one pixel row at a time. Further, the measurement of the potential across the source signal line need not be carried out on one pixel at a time. For example, as shown in
The source driver circuit (IC) 14 supplies the constant current Iw=2×11 to pixels 16(2) and 16(3). A current output by the pixel 16(2) and a current output by the pixel 16(3) are added together to obtain the current 2×11. However, the current output by the pixel 16(2) may be different from the current output by the pixel 16(3). The potential across the source signal line 18 corresponds to the balance between the potentials of the gate terminals of driving transistors 11a in the pixels 16(2) and 16(3). The potential is often the average of the potentials of the gate terminals of driving transistors 11a in the pixels 16(2) and 16(3). However, since adjacent pixels have approximate characteristics, voltage tone data obtained from potentials measured by the A/D conversion circuit 391 pose no problems in a practical sense.
If a plurality of pixel rows are selected, they need not be adjacent to each other as shown in
The above embodiment passes a current through the driving transistor 11a and measures the voltage at the gate terminal of the driving transistor 11a through which the current is flowing. However, the present invention is not limited to this. For example, a current meter (not shown) is connected to a Vss terminal (cathode terminal) connected to or formed at each pixel column. Then, the V0 voltage corresponding to the 0th tone is applied and adjusted so that the current flowing through the current meter during application of the V0 voltage is zeroed or has a very small value. This enables the program voltage V0 corresponding to the tone 0 to be accurately determined.
Alternatively, by adjusting the voltage applied to the driving transistor 11a so that a current of 1 μA is measured by the current meter, it is possible to measure the voltage causing the conductance of 1 μA. A more accurate V-I curve can be estimated or determined by measuring the relationship between the voltage and current at a plurality of points.
The above embodiment simultaneously selects a plurality of pixel rows. Of course, a plurality of pixel columns may be simultaneously selected instead.
The above embodiment simultaneously selects a plurality of pixel rows and applies the constant current Iw to these pixel rows to allow the measurement or acquisition of a potential characteristic corresponding to the average of the potentials at the gate terminals of the driving transistors 11a in the plurality of pixel rows. That is, the embodiment measures the average of the potentials at the gate terminals of the driving transistors 11a in the plurality of pixel rows.
The above embodiment selects a plurality of pixel rows or columns, applies the constant current Iw to these pixel rows or columns, and measures the potentials across the corresponding source signal lines 18 to determine the characteristics of the driving transistors 11a in the pixels 16. However, the reverse operation may be performed in order to determine the characteristics of the driving transistor 11a. That is, the predetermined constant voltage Va is applied to the source signal line 18 or to the driving transistor 11a in the pixel 16. The current Ia is then measured which flows through a selected plurality of the driving transistors 11a when the voltage Va is applied. The current Ia depends on the characteristics of the selected driving transistors 1a. Accordingly, the current Ia enables the determination of characteristics of the driving transistor 11a. The measured or acquired current Ia is subjected to a current-voltage conversion and then to an A/D conversion, with the resulting data held in the storage instrument such as the memory. Of course, the description in this paragraph is applicable to the other embodiments of the present invention.
The present invention can be implemented with a pixel configuration based on the voltage driving scheme such as the one shown in
With the voltage driving, the predetermined voltage V1 is applied to the gate terminal of the driving transistor 11a. The current I resulting from the voltage V1 is measured at a cathode Vss terminal. For example, the current meter is connected to the Vss terminal (cathode terminal) connected to or formed at each pixel row. Alternatively, as shown in
The position at which the pickup resistor R is inserted into the path is not limited to the cathode terminal but may be the anode terminal. Alternatively, the current may be measured at both the cathode and anode terminals. Further, the present invention is not limited to the direct measurement of the current I1. A pickup coil or the like may be used for the measurement. Alternatively, measurement may be carried out on electric lines of force. When a high accuracy is not particularly required, it is possible to short circuit a plurality or all of the cathode or anode terminals and to connect current meters to the short-circuited positions. That is, any method which measures the current I1 may be used provided that it enables the direct or indirect measurement or determination of the current I1.
As described above, the voltage tone circuit 231 applies the known voltage V1 to the source signal lines 18 for the driving transistor 11a. The output current I1 corresponding to this voltage is then measured. Of course, it is possible to select one or more source signal lines 18 and to apply a known voltage to these source signal lines 18. Alternatively, a plurality of pixel rows may be simultaneously selected or scanned and selected. Consequently, the resulting relationship is reverse to that shown in
The embodiment shown in
Selection of each switch S allows the cathode current I1 (or anode current) to flow through the selected pixel 16 into the resistor R. A plurality of switches S may be simultaneously selected. Voltages generated at the opposite ends of the resistor R by the cathode current or the like are digitalized by the A/D conversion circuit 391 and stored in the memory 502. The tone voltage corresponding to the program voltage is calculated or determined on the basis of the stored data. Of course, the current meter may be used to measure the cathode current I1 or the like. The tone 0 of course involves zero voltages generated at the opposite ends of the resistor R. The cathode current may flow in a discharge direction. The present invention is applicable in any case.
The voltage driving requires the voltage V1 to be applied to the driving transistor 11a. Further, the current I1 resulting from the voltage V1 is measured at the Vdd terminal. For example, as shown in
As described above, the voltage tone circuit 231 applies the known voltage V1 to the source signal line 18 for the driving transistor 11a. The output (input) current I1 corresponding to this voltage is then measured. Of course, it is possible to select one or more source signal lines 18 and to apply a known voltage to these source signal lines 18. Consequently, the resulting relationship is reverse to that shown in
With the V0 voltage, the program voltage V0, corresponding to the tone 0, can be accurately determined by adjusting the applied V0 so that the current flowing through the current meter during application of the V0 voltage is zeroed or has a very small value. In this case, the output voltage from the voltage tone circuit 231 is varied and adjusted to zero. Further, the voltage Vx, applied to the driving transistor 11a, is adjusted so that, for example, 1 μA flows through the driving transistor 11a. A more accurate V-I curve can be estimated or determined by measuring the relationship between the voltage V and current at a plurality of points.
As in the case of
Selection of each switch S allows the anode current to flow through the selected pixel 16. The anode current generates voltages at the opposite ends of the resistor R. The resulting voltages are digitalized by the A/D conversion circuit 391 and stored in the memory 502. The tone voltage corresponding to the program voltage is calculated or determined on the basis of the stored data. Of course, the current meter may be used to measure the cathode current I1 or the like. The tone 0 of course involves zero voltages generated at the opposite ends of the resistor R.
In
Application of the voltage Vx to the source signal line 18 causes the cathode current I1 from the driving transistor 11a to flow through the source signal line 18. The cathode current I1 is converted into a voltage by the pickup resistor R, with the voltage then measured. The voltage Vx, applied to the source signal line 18, is adjusted so as to allow the measurement of the voltage V=I1×R.
As in the case of
Selection of each switch S allows the anode current to flow through the selected pixel 16. The anode current generates voltages at the opposite ends of the resistor R. The voltages applied to the source signal line 18 are digitalized by the A/D conversion circuit 391 and stored in the memory 502. The tone voltage corresponding to the program voltage is calculated or determined on the basis of the stored data. The other arrangements are similar to those in
In the embodiment shown in
Further, the present invention stores the measured voltage or current in the flash memory or the like. On the basis of the stored data, the program voltage or current corresponding to the video signal is determined and applied to the pixel 16. Therefore, the embodiment of the present invention is applicable to both pixel configurations, that is, the current program shown in
The measured or acquired voltage data V is stored in the flash memory or the like and then transferred to the memory in the controller circuit (IC) 801. A program voltage or current corresponding to the video data is then generated. However, data cannot be read fast from the flash memory. The present invention mounts a plurality of flash memories 1051 in the display apparatus as shown in
Of course, the technical concept of the present invention described above can be combined with the other embodiments of the present invention. Further, of course, the technical concept of the present invention can be used to construct a semiconductor such as the source driver circuit (IC) 14, a display panel, or a display apparatus. Furthermore, the switch S, resistor R, A/D conversion circuit 391, voltage tone circuit 231, and the like may be formed directly on the array board 30 using a polysilicon technique.
For easy description, the above embodiment accumulates the measured voltage or current data in the memory. However, any memory may be used for the present invention provided that it can temporarily hold data in a digital or analog form. For example, the memory may be a sample hold circuit that samples analog data. Of course, the concept of the memory includes a semiconductor such as a flash memory, a SRAM, or a DRAM. The memory may be constructed inside or outside the source driver IC (circuit) 14. The above description is applicable to the other embodiments of the present invention.
As described above, the present invention applies or supplies a voltage or current to the driving transistor 11a and measures a current or voltage output by the driving transistor or the like (in the current mirror pixel configuration shown in
The present invention applies a known voltage or current to each source signal line 18 and measures an output current or voltage. Alternatively, the present invention adjusts the voltage or current applied to the source signal line 18 so that the output current or voltage has a predetermined value. The present invention thus determines or estimates a V-I curve of the driving transistor 11 supplying a current to the EL element 15. The present invention therefore determines the program voltage or current corresponding to each tone.
The above embodiment enables a V-I curve of each driving transistor 11a to be accurately determined. The voltage determined is a program voltage or current. Each program current or voltage corresponds to the video signal.
As shown in
A predetermined voltage is added to or subtracted from the measured voltage or the measured voltage is corrected, so as to correct the punch-through voltage. The measured voltage is further processed so as to be compatible with the gamma curve or EL characteristic of the video data. The measured voltage thus becomes a precharge voltage Vp serving as tone data on the video signal. Since the precharge voltage Vp corresponds to multi-bit video data, it will be called VDATA in the description below. Further, since the VDATA is a voltage programmed (written) into the pixel 16, it may be called a program voltage VDATA.
As shown in
The program voltage VDATA (precharge voltage Vp) is obtained by applying the constant current Iw (including Iw=0 (A)) to the source signal line 18 to pass the constant current Iw through the driving transistor 11a and then measuring the potential across the source signal line 18. The program voltage VDATA is corrected on the basis of the characteristics (V-I curve) of the driving transistor 11a. The applied program voltage VDATA reflects a variation in characteristics among the driving transistors 11a in the respective pixels 16.
The VDATA has an error 0 (no errors; for example, application of the Iw uniquely determines the Va on the basis of the V-I curve) at a characteristic position (for example, Va) on the V-I curve. The error 0 means that the error is cancelled at a particular position (for example, Va). At a position preceding or following the particular one (for example, Va), the VDATA deviates from the ideal value, resulting in an error with respect to the ideal characteristics. However, at the particular position, the VDATA operates at the ideal value. This scheme is called voltage offset canceling for the following reason: the measured voltage of the source signal line 18 (voltage being processed so as to be compatible with the video tone) is applied to cancel the error, and an error with respect to the ideal value occurs with respect to the voltage position (for example, Va).
The program voltage VDATA has a voltage offset canceled value. The program voltage VDATA during the A period causes the source signal line 18 to be charged or discharged so as to pass the target current through the EL element 15. The highest accuracy is achieved by an offset voltage (for example, Va) and its neighborhoods. The error with respect to the target current increases consistently with the distance from the offset voltage.
After applying the program voltage VDATA during the A period, the present invention applies a program current IDATA during a B period (during which an overcurrent is applied as required as shown in
The present invention applies the program current IDATA during the B period. By applying the VDATA at a position different from the neighborhood of the offset voltage, it is possible to perform an ideal, error-free (accurate) write operation through the program current IDATA applied during the B period, in spite of an increase in the error with respect to the target current (target value written into the pixel 16).
The IDATA is converted into a program current by the current tone circuit 154, and the resulting current is supplied to the source signal line 18. The supply occurs during the B period shown in
In
The above embodiment carries out the voltage+current program driving so that voltage offset canceling seems to be executed in the low tone areas, whereas current program driving is executed in the high tone areas. Consequently, the effects of the voltage driving and current driving are produced in a supplementary manner.
The present invention performs processes or operations using, as an origin, the voltage Vbb, which turns the driving transistor off (avoids the passage of a current through the driving transistor). That is, the VDATA sets the Vbb voltage at zero and increments by 9 bits (512 pieces). On the other hand, the IDATA, the program current, is zero when no current flows through the EL element. The IDATA thus has its origin at zero and increments by 8 bits (256 pieces).
The configuration shown in
As shown in
The pixels are produced so that a combination of an R, G, and B pixels constitute a square. Each of the R, G, and B pixels is oblong. Accordingly, annealing with an oblong laser irradiation spot 1092 makes it possible to prevent a variation in characteristics among the transistors 11 from occurring within one pixel. The characteristics (mobility, Vt, S value, and the like) of the transistors 11 connected to one source signal line 18 can also be made uniform (that is, the transistors 11 connected to the adjacent source signal lines 18 may have different characteristics but the characteristics of the transistors 11 connected to one source signal line can be made almost uniform).
In general, the laser irradiation spot 1092 has a fixed length of, for example, 10 inches. Since this laser irradiation spot 1092 is moved, the panel needs to be placed within the movable range of one laser irradiation spot 1092 (that is, laser irradiation spots 1092 are prevented from overlapping in the center of the display screen 34 of the panel).
The configuration shown in
The laser anneal method described with reference to
The matching characteristics of the driving transistors 11a connected to one source signal line 18 are advantageous for the current driving in the respects described below. For example, with white raster display, almost the same current is passed through the transistors 11a in the adjacent pixels. This reduces a variation in the amplitude of a current output by the source driver IC (circuit) 14. If the transistors 11a in
If the transistors 11a connected to one source signal line 18 have almost the same characteristics, the potential across the corresponding source signal line 18 varies insignificantly. This means that the V0 or Vbb voltages of the pixels located along the source signal line 18 may have substantially the same value. Further, substantially the same V-I characteristic allows the Va voltages or the like of the pixels to have the same value. That is, the pixels located along the source signal line 18 may be considered to have a substantially equal V-I characteristic.
That is, if laser beams are applied parallel to the gate signal line 7, while the laser irradiation range 1092 moves perpendicularly to the gate signal line 17, then the V0 voltages of the pixels located along the gate signal line 17 may have substantially the same value. Further, since the V-I characteristics of the pixels are substantially equal, their V1 voltages and the like may also be the same. That is, of course, the embodiments described below are applied on the assumption that the pixels located along the gate signal line 17 have a substantially equal V-I characteristic.
In the description below, the V0 voltage means the voltage for the tone 0. In a broad sense, the V0 voltage also means the Vt voltage, the Vbb voltage, and the like. The V0 voltage is for the tone 0 and thus corresponds to a perfect black display. Thus, the relationship between the V0 voltage and the video signal is easy to understand. The description below is thus based on the V0 voltage. Actually, a current starts to flow through the driving transistor 11a at the Vt voltage. The Vbb voltage enables the ideal black display.
Producing an array as shown in
The V0 voltage varies among the driving transistors 11a. In an embodiment described below with reference to
However, the use of a common VDATA for two pixels may result in a reduced resolution. To solve this problem, an even- and odd-numbered pixel rows (shown by dotted lines) have the same VDATA in the second F (Field (Frame)), following the first F (Field (Frame)) as shown in
The averaging method involves applying the constant current (including the 0 current) to the source signal line 18 for each pixel column, sequentially selecting the first to last pixel rows, and measuring the V0 or V1 voltage across the source signal line 18. After the measurement, the V1 or V0 voltage obtained is averaged to determine the program voltages V0 or V1.
Naturally, a common V0 voltage and the like may be arranged in a matrix (block) form as shown in
In the embodiment shown in
The above embodiment applies the constant current Iw to the source signal line 18 and measures the voltage (Va, V0 voltage, or the like) corresponding to the constant current Iw. The measured or acquired voltage value is used as a reference or processing is executed, to determine a V-I curve indicative of the characteristics of each driving transistor 11a or all driving transistors 11a within the display area or of the average of the characteristics of the driving transistors 11a.
The embodiment of the present invention measures the V0 voltage of each pixel. However, the present invention is not limited to this. For example, if the array 30 is formed as shown in
The above embodiment uses a scheme which measures the V0, V1, Va, or the like, determines or calculates a V-I curve, and carries out the voltage+current driving or the like. However, the present invention is not limited to this. For example, an embodiment shown in
In
To measure the V0 or V1, described with reference to
For example, the switch SI2 is closed, and the constant current Ix is applied to the source signal line 18. The switch S2 is closed, and the potential Vx across the source signal line 18 is measured. The measured Vx is subjected to an A/D conversion by the A/D conversion circuit 391 and held in the memory 502.
The above operation is performed on all or selected required source signal lines 18. The potentials across the source signal lines 18 is measured and held in the memory 502. A V-I curve or a rising voltage is generated on the basis of the held data. The voltage tone circuit 231, the current tone circuit, or the like is then used to carry out voltage driving, voltage+current driving, or overcurrent+tone current driving (see
Each current tone circuit 154 has a fixed output constant current value. For example, the current tone circuit 154a outputs a constant current I1, the current tone circuit 154b outputs a constant current I2, and the current tone circuit 154c outputs a constant current I3. The switch SW1 is used to select one of the current tone circuits 154. The relative magnitude of the constant current output by the current tone circuit 154 can be varied by a resistor installed outside the source driver circuit 14.
As described in
In
Further, no current is passed through the source signal line 18. That is, the source driver circuit 14 opens a switch 161b shown in
Of course, as shown in
Further, as shown in
As described above, the averaged voltage V0 or V1 or the like can be measured by simultaneously selecting a plurality of pixel rows and applying the constant current to them. This eliminates the need for a post-averaging process or the like.
The present invention is not limited to the case where the measured V0 or V1 voltages or the like are subjected to an A/D conversion and then stored in the memory 502 or the like and are read from the memory and then subjected to a D/A conversion. The V0s or V1s obtained by measurement or the like are processed so as to be compatible with the display state (for example, black display of the 0th tone). For example, a given value is added to or subtracted from the V0s or V1s obtained by measurement or the like. Alternatively, the V0s or V1s obtained by measurement or the like may be divided or multiplied by a given value. Alternatively, they may be corrected on the basis of the panel temperature or the like.
For example, for V0=4.1 V obtained by measurement on the source signal line S1 and V0=3.9 V obtained by measurement on the source signal line S2, a given value of 0.2 V is added to the V0s. Thus, 4.3 V is applied to the source signal line S1, and 4.1 V is applied to the source signal line S2 as a voltage for the 0th tone. After application of the voltage for the 0th tone, current precharging with the voltage Vp is carried out, followed by application of a tone current.
Of course, as shown in
In the present invention, as described with reference to
As shown in
The voltage applied (output) to each source signal line 18 is connected to or arranged so as to reach all switches S1 to S160 in the source driver IC (circuit) 14a and all switches S161 to S320 in the source driver IC (circuit) 14b. The potentials across the 320 source signal lines 18 are output to the single A/D conversion circuit 391. Voltage wiring 1222 in the switch S is extended in a traverse direction in each source driver IC (circuit) 14. The source driver ICs (circuits) 14a and 14b are connected together via an a and b terminals of the source driver IC (circuit) 14.
The current tone circuit 154a in the source driver IC (circuit) 14a constitutes a current mirror circuit together with a transistor 168a. A current flowing to the transistor 168a is adjusted by the external resistor R1 (see
The cascade circuit 1221a generates two identical constant currents and supplies one of them to the transistor 168a, while supplying the other to a cascade circuit 1221b in the source driver IC (circuit) 14b via terminals c and d. This configuration supplies the same current to both transistors 168a and 168b. Accordingly, an output current from the current tone circuit 154a in the source driver IC (circuit) 14a is adjusted or varied by the resistor R1. The same current is also applied to the current tone circuit 154b in the source driver IC (circuit) 14b. This allows the same constant current to be supplied to the 320 source signal lines 18.
The measured V0 or V1 voltage or the like is output from the terminal c and subjected to an analog-digital conversion by the A/D conversion circuit 391. The resulting data is stored in the memory 502 such as the EEPROM. The following data is stored in the memory 502: V0 data indicative of one absolute value and Vs data indicative of the difference between the V0 data and the voltage across the source signal line. Specifically, when V0=1.5 V, if the source signal line S1 has a voltage value of 1.6 V, the difference Vs1=0.1 V is stored, if the source signal line S2 has a voltage value of 1.7 V, the difference Vs2=0.2 V is stored, . . . , if the source signal line Sn has a voltage value of 1.4 V, the difference Vs1=−0.1 V is stored in the EEPROM 502. The difference data or the like may of course be subjected to a JPEG compression or the like. The EEPROM 502 also stores panel characteristic data (gamma curves and the like) and control DATA (timing signals for the gate signal lines and the like).
Data V0x in the EEPROM 502 is transferred to a memory area in the controller circuit (IC) 801 through a three-line serial bus in accordance with a control signal from the controller circuit (IC) 801. The stored data is transferred to the sample hold circuit 241 in accordance with a clock SCLK that is slower than the CLK of the digital video signal DATA; the clock SCLK operates at a half rate equal to or lower than that of a normal clock. The digital data V0x is converted into analog voltage data V0x by a D/A conversion circuit 1241.
On the other hand, the digital video signal DATA is applied to the controller circuit (IC) 801 in synchronism with the CLK. The controller circuit (IC) 801 processes the digital video signal DATA so that it is compatible with an input format for the source driver IC (circuit) 14. The digital video signal DATA is then applied to the source driver IC (circuit) 14 in synchronism with a clock MCLK.
The above embodiment supplies the constant current to each pixel 16 involved in display and measures or calculates or acquires the potential across the corresponding source signal line 18. However, the present invention is not limited to this. For example, as shown in
As shown in
The voltage tone circuit 231 may be composed of a sample hold circuit as shown in
The data stored in the EEPROM 502 is periodically read by the controller circuit 801 and converted into analog data by the D/A conversion circuit 1241. On this occasion, the value is corrected so as to be compatible with precharging. The sample hold circuit 241 samples and holds the data. The sample hold circuit is used because of its small scale, contributing to reducing the chip size of the source driver IC (circuit) 14.
The sampled and held voltage is applied to each source signal line 18 in synchronism with a synchronous signal during the 1H. However, the sample hold voltage is not applied to source signal lines 18 to which the voltage need not be output. After the required application, a precharge process is executed on areas involving a change in tone and requiring current or voltage precharging. After the precharge process or the output of the sample hold voltage, the current tone circuit 154 outputs a driving current corresponding to the video signal, to the source signal line 18.
As described above, after applying the sample hold voltage to the source signal line 18 as required, the present invention executes current or voltage precharging as required. This driving scheme subsequently applies the tone current to the source signal line 18. In the above description, the signals are applied to the source signal line 18. However, of course, this means that the signals are applied or supplied to the pixel 16 or the driving transistor 11a in the pixel.
In
The above embodiment applies the program current after the precharge voltage Vp (program voltage VDATA). The present invention is not limited to this. For example, the apparatus may be completely voltage driven as shown in
In
An operation of the driving transistor 11a in the selected pixel 16 sets the potential across the source signal line 18 at the Vx. The potential is at the V0 if the constant current is not supplied. Further, the potential is at the Vx if the constant current Ix is supplied. x corresponds to the tone and ranges from 1 to 255 (for 8-bit display).
The potential Vx (including the V0) across the source signal line 18 is subjected to an A/D conversion by the A/D conversion circuit 391 and held in the EEPROM 502. An output from the EEPROM 502 is subjected to a gamma process or the like corresponding to the video data, on the basis of the Vx voltage. The output is then applied to each source signal line 18.
The embodiment shown in
Another embodiment of the present invention will be described below. A driver circuit and an EL display apparatus using the driver circuit in accordance with the present invention are configurations or methods which comprise a constant current generating circuit that outputs a constant current applied to a transistor driving an EL element, a voltage holding circuit that measures or holds, for a predetermined period, the voltage at a gate terminal of the transistor to which the constant current is being applied, and a voltage applying circuit which adds or subtracts a predetermined voltage signal to or from the voltage that is, for example, held in the voltage holding circuit or which executes a predetermined process on the voltage, the voltage applying circuit then applying the voltage to the gate terminal of the transistor.
The switches SW1, SW2, SW3, SW4, and SW5 are formed for or arranged at the respective outputs. Further, a capacitor 1341 and the buffer 151 are formed or arranged. The capacitor 1341 may have any configuration provided that it has a function which cuts a DC component. Alternatively, the capacitor 1341 may have any configuration provided that it can shift the potential. The buffer 151 may have any configuration provided that its input section a offers a high impedance, while its output section b offers a low impedance. The buffer 151 may be, for example, a buffer amplifier or an operational amplifier. In addition, an emitter follower circuit may be constructed using transistor elements.
As in the above embodiments, the pixels 16 on the EL display panel (EL display apparatus) in accordance with the present invention are each structured so that one pixel 16 is formed of four transistors 11 and the EL element 15 as shown in
The present invention is characterized by passing the program current (constant current Iw) through the driving transistor 11a in the pixel 16 and measuring or holding, for a given period, the potential at the gate terminal of the driving transistor 11a through which the program current is flowing. The present invention is also characterized by adding or subtracting the tone voltage to or from the gate terminal potential and writing the resulting voltage to the gate terminal of the driving transistor 11a in the pixel.
A first operation stores the value of a current flowing through the EL element 15. First, current tone circuit 154 in the source driver IC (circuit) 14 applies the predetermined constant current to the source signal line 18.
The current tone circuit 154 is composed of, by way of example, the operational amplifier 151, transistor 167, and resistor R. The electronic regulator 152 is connected to a plus side terminal of the operational amplifier 151. The electronic regulator operates as a D/A conversion circuit that converts digital data DATA into analog data V. The output voltage V of the electronic regulator 152 is varied by the setting data (digital data) DATA. The current Iw flowing through the source signal line 18 has a value equal to the output voltage V of the electronic regulator 152 divided by the resistor R.
In the present invention, the electronic regulator 152 may be removed and the constant current may be applied to the source signal line 18 by using a resistor partial pressure circuit or the like to generate a voltage V at the +terminal and applying the voltage V to the operational amplifier 151. Further, the constant current need not necessarily be generated by the current tone circuit 154. Any circuit can be used provided that it can generate a predetermined or given range of constant currents. For example, an emitter follower circuit can also generate a constant current.
The constant current Iw includes the state of the current 0 (passage of no current). In the pixel configuration shown in
While the source driver IC (circuit) 14 is applying the program current Iw to the source signal line 18, the transistors 11b and 11c are on (closed) as shown in
As shown
The subsequent voltage read operation closes the switch SW1 to apply the program current (constant current) Iw to the source signal line 18. At this time, the switches SW3, SW4, and SW5 are opened, while the switch SW2 is closed (see
The program current Iw flows through the driving transistor 11a in the pixel 16 shown in
The magnitude of the program current (constant current) Iw may be zero but is preferably set at least one eighth of and at most two thirds of the maximum tone current. To reduce a write time, the magnitude of the program current may be set at least equal to and at most ten times as large as the maximum tone current. The maximum tone current is the magnitude of the current flowing through the EL element 15 for the maximum tone or the magnitude of the program current programmed in the pixel 16. For example, for 256 tones, the maximum tone current is programmed in the EL element 15 for the 255th tone (tone number starts from the 0 tone).
A small program current (constant current) requires a long time to charge or discharge the parasitic capacity of the source signal line 18. A variation in the gate potential of the driving transistor 11a does not converge in an initial short time during one horizontal scan period (1H period). A large program current (constant current) hinders characteristic compensations in low tone areas in which the effect of a variation in characteristics among the driving transistors 11a is likely to appear in an image display. In the embodiment described above, a constant current that is at least one eighth of and at most two thirds of the maximum tone current is applied to the pixel 16. However, this range may be expressed by tone numbers. The read operation has been described.
The above operation reads and loads the gate terminal potential of the driving transistor 11a into the a section of the capacitor 1341. Alternatively, the gate terminal potential is held in the a section of the capacitor 1341. The embodiment shown in
The next operation applies the tone voltage using the read voltage as a reference (center or origin) (see
When the operational amplifier 151 offers a gain of 1, the voltage in the a section is applied to the source signal line 18 via the switch SW5. Since the transistors 11b and 11c in the pixel 16 are closed during one selected horizontal scan period (1H period), the read gate terminal voltage of the driving transistor 11a is, under this condition, applied to the gate terminal of the driving transistor 11a in the pixel 16 again. Accordingly, the driving transistor 11a conducts the current corresponding to the constant current to the EL element 15. In this state, a variation in characteristics among the driving transistor 11a is compensated for, and the constant current (programmed current) is precisely flowing through the EL element 15. Of course, the Va voltage varies among the pixels depending on the characteristics of the driving transistors 11a. However, the program current (constant current) is precisely applied to and passed through the EL element 15.
The voltage tone circuit 231 outputs the tone voltage Vx corresponding to each tone. The tone voltage Vx corresponds to the tone number of the video signal. The tone voltage Vx may be considered to be the video signal. The image can be displayed by applying, as a program voltage, the tone voltage Vx to the driving transistor 11a as it is or after executing a given process (proportional process, shift process, addition or subtraction process, or the like) on the tone voltage Vx.
The tone voltage Vx is applied to the c section of the capacitor 1341 via the switch SW4. The potential Va in the a section of the capacitor 1341 is shifted by the tone voltage Vx output by the voltage tone circuit 231. Accordingly, the potential in the a section is ideally Va+Vx.
The Va+Vx voltage is output after having its impedance reduced by the operational amplifier 151, offering a gain of 1. The Va+Vx voltage is applied to the source signal line 18 via the switch SW5 and output terminal 83 and then to the gate terminal of the driving transistor 11a in the pixel 16. Therefore, the driving transistor 11a applies the current corresponding to the Va+Vx to the EL element 15.
In
In
In
The constant current (tone current) output by the constant current source 1312 need not necessarily have a constant value. The constant current enables the output of a plurality of tones such as 64 or 256 tones or a plurality of current magnitudes. Further, the apparatus may be configured so that the constant current has a value changing at intervals of one horizontal scan period (1H). Furthermore, the apparatus may be configured so that the constant current has a value varying with the pixel in synchronism with a dot clock. The constant current source 1312 may be replaced with the current tone circuit 154.
The tone voltage Va may be replaced with the tone number. For example, the Va voltage corresponds to the 128th of 256 tones, and the Vx=Vc−Va corresponds to a voltage for 64 tones. The voltage tone circuit 231 outputs the Vx to allow the Vc to cover 128+64=192 tones. Provided that the Vx acts in one direction and that the Va−Vx corresponds to a voltage for 64 tones, the voltage tone circuit 231 outputs the Vx to allow the Vb to cover 128-64=64 tones. In
The Va voltage may zero the output current from the driving transistor 11a. In this case, the current tone circuit 154 outputs the 0 current (current tone circuit 154 is unwanted). The on voltage is applied to the gate signal line 17a connected to the corresponding selected pixel 16. Application of the on voltage to the gate signal line 17a causes the driving transistor 11a to vary the gate terminal potential so as to zero the current passed through the EL element 15. The potential V0 is held in the a section of the operational amplifier 151; the potential V0 zeroes the current passed through the EL element 15. The voltage tone circuit 231 outputs a plus side voltage, which is then added to the voltage held in the a section. The resulting voltage is then output to the b section of the operational amplifier 151 (see
As shown in
The second operation shown in
The above operations are performed by the gate driver circuit 12 by sequentially selecting the pixel rows. That is, one of the pixels rows is selected during each horizontal scan period. First, at the beginning of a horizontal scan period, the constant current is applied to the selected pixel row. With the constant current applied, the Va required to pass the constant current through the driving transistor 11a is read or held in the a section. Then, the tone voltage is added to or subtracted from the Va voltage, and the resulting voltage is applied to the gate terminal of the driving transistor 11a. This completes one horizontal scan period. The selected pixel row applies a current to the EL element 15 during a predetermined period after the next horizontal scan period. The EL element 15 thus emits light.
Then, during the next horizontal scan period, the next adjacent pixel row is selected. The pixel row is selected during this horizontal scan period, and the constant current is applied to the pixel row selected at the beginning of the horizontal scan period. The Va required to pass the constant current through the driving transistor 11a is read. Then, the tone voltage is added to or subtracted from the Va voltage, and the resulting voltage is applied to the gate terminal of the driving transistor 11a. This completes one horizontal scan period.
The magnitude of the constant current Iw applied to each pixel 16 may be varied or changed or adjusted depending on the magnitude of the current Ie passed through the EL element 15 in each pixel 16, the difference between the present current and the rewritten current, a lighting period, or the like. The magnitude of the constant current Iw may be varied or changed or adjusted depending on the ratio (lighting ratio) of the current used for each image display to the maximum current used for the entire display screen 34. In particular, the maximum value is defined as 100%, and the constant current Iw is preferably increased when the current value becomes 25% or less. That is, the magnitude of the constant current Iw is varied (controlled) depending on the lighting ratio.
The amplification scale of the operational amplifier 151 may be varied depending on the magnitude of the current passed through the EL element 15 in each pixel 16, the difference between the present current and the rewritten current, a lighting period, or the like. Alternatively, the period during which the constant current is applied may be varied. The amplification factor of the tone voltage Vx output by the voltage tone circuit 231 may be varied depending on the magnitude of the current passed through the EL element 15 in each pixel 16, the difference between the present current and the rewritten current, a lighting period, or the like. Further, Va and V0 voltages may be corrected by a given amount so that the corrected Va and V0 can be used as reference voltages. Further, the switch SW2 and the like may be omitted.
The apparatus shown in
The configuration shown in
Further, in
In
In
Further, in the above description, the A/D conversion circuit 391 applies the measured or held voltage to the addition circuit 651 as digital data. However, the present invention is not limited to this. For example, the digital data from the A/D conversion circuit 391 may be held in a memory circuit (not shown) constructed or formed outside or inside the source driver IC (circuit) 14. The digital data is read and applied or output to the addition circuit 651 as required.
The potential across the source signal line 18 is varied by the voltage or current output by the source driver IC (circuit) 14. The potential across the source signal line 18 is basically rewritten at intervals of one horizontal scan period. The present invention applies the constant current to the driving transistor 11a at the beginning of a horizontal scan period (1H) to operate the driving transistor 11a and measures, acquires, or holds the gate potential of the driving transistor 11a having completed the operation and brought into a steady state. A variation in characteristics among the driving transistors 11ais compensated for by applying the tone voltage to the driving transistor 11a on the basis of the voltage obtained by measurement or the like.
The constant current Iw need not necessarily steadily have a predetermined constant value during one horizontal scan period (1H period). For example, the constant current Iw may have a large value at the beginning of application, and after a given period, may be set to a predetermined value. This operation enables the parasitic capacitance of the source signal line 18 or the like to be charged or discharged in a short time. That is, the constant current Iw may be varied among multiple levels during one H period. Further, the multiple levels of the constant current may be changed or altered depending on the potential across the source signal line 18.
In order to vary the potential at the gate terminal of the driving transistor 11a to compensate for a variation of characteristics among the driving transistors 11a, it is first necessary to use the constant current Iw (and of course the operation of the driving transistor 11a) to charge or discharge the parasitic capacitance of the source signal line 18. A charging or discharging time depends on the potential across the source signal line 18 during the preceding horizontal scan period. Thus, charging or discharging may not be completed within a predetermined time depending on the potential across the source signal line 18.
To solve this problem, the present invention applies the precharge voltage Vp to the source signal line 18 at the beginning of one horizontal scan period (1H). As described later, the apparatus is configured to form a precharge voltage Vp in the source driver IC (circuit) 14 to allow a predetermined voltage to be applied to the source signal line 18.
The present invention is not limited to application of the precharge voltage Vp during the A period. If the potential across the source signal line 18 is within a predetermined range before application of the precharge voltage Vp, the precharge voltage Vp need not be applied. As described above, whether or not to apply the precharge voltage Vp is determined or adjusted depending on the potential across the source signal line 18, the magnitude of the applied precharge voltage Vp, the difference between the applied precharge voltage Vp and the potential across the source signal line 18, or the applied tone value.
The precharge voltage Vp is set at a value close to the anode voltage Vdd that is closer to the Va or V0 voltage. The precharge voltage Vp may have a predetermined fixed value or the apparatus may be configured to be able to vary or adjust the precharge voltage Vp depending on the Va or V0 voltage.
Each of the first to third Hs (first to third horizontal scan periods) is one horizontal scan period. Further, the first to third Hs (first to third horizontal scan periods) corresponds to the order in which the pixel rows are selected. With n pixel rows, one field (frame) period is composed of n horizontal scan periods (pixel rows) and a blanking period. The precharge voltage Vp is applied during the first A period of each horizontal scan period. This allows the precharge voltage Vp to be instantaneously set regardless of the potential across the source signal line 18 during the preceding H.
The precharge voltage Vp is preferably set at the V0, corresponding to the tone 0. This enables a good black display to be realized. Of course, the Vp voltage varies depending on a variation in characteristics among the driving transistors 11a in the respective pixels 16. Panel characteristics may be evaluated or measured to determine the voltage V0 corresponding to the constant current Ia=0 (A) so that the voltage V0 can be used as the precharge voltage Vp. As described above, the present invention pre-measures the V0 voltage or the like to determine the precharge voltage Vp in order to set a constant convergence time. The V0 voltage can be measured or acquired by the embodiment described in
During a B period of the horizontal scan period, which follows the A period, the current tone circuit 154 outputs the constant current Iw. The constant current Iw may also be applied during the A period. The constant current Iw may also be 0 (A). The constant current Iw=0(A) corresponds to the V0 in
Of course, the Va voltage varies depending on a variation in characteristics among the driving transistors 11a in the respective pixels 16. The voltage can be precisely written by determining and using the voltage Va corresponding to the constant current Ia which is determined by evaluating or measuring the panel characteristics. The Va voltage can be measured or acquired by the embodiment described in
During a C period, which follows the B period, a target voltage Vc as a video signal is applied. Accordingly, the target tone Vc=Va+Vx, based on the Va, is applied to the source signal line 18. In the example shown in
The above embodiment applies the precharge voltage Vp at the beginning of the horizontal scan period and then the target voltage V. The present invention is not limited to this. It is also possible to apply the precharge voltage Vp at the beginning of the horizontal scan period and then the target program current.
In the embodiment shown in
The precharge voltage Vp applied during the A period is a program voltage corresponding to a written video signal or an approximate voltage. The precharge voltage Vp allows the driving transistor 11a in the pixel 16 to be programmed so as to provide a current equal or close to the target tone current (program current) Iw.
During the first H (first horizontal scan period), the precharge voltage Vp=Vp1, which is applied to the source signal line 18. Application of the precharge voltage Vp1 sets the potential across the source signal line 18 at a target or similar voltage in a short time. Alternatively, application of the precharge voltage Vp1 sets the potential at the gate terminal of the driving transistor 11a in each pixel in the selected pixel row.
During the second H (second scan period), the precharge voltage Vp=Vp2. During the third H, the precharge voltage Vp=Vp3. The applied precharge voltage Vp has a value corresponding to the video signal written to the pixel 16. The precharge voltage Vp applied during the A period often deviates from the target value. The causes of the deviation include the dependence of the driving transistor 11a on temperature, the degradation of the driving transistor 11a, and the like. However, the present invention applies the program current Iw during the B period, following the A period. Application of the program current makes it possible to compensate for the dependence on temperature.
Accordingly, voltage driving during the A period enables the source signal line 18 to be charged or discharged in a short time to write an accurate program current to the pixel 16 during the B period. Further, since the potential is already set equal or close to the target value during the A period, the program current Iw insignificantly changes the potential. In spite of small program currents Iw in low tone areas, an insufficient writing (failure to reach the target value) is avoided, thus enabling accurate tone current settings. The program current Iw is output by the current tone circuit 154.
A program current Iw1 for the first H which is applied during the B period sets the potential across the source signal line 18 at the V1. The potential V1 is applied to and held in the gate terminal of the driving transistor 11a in the pixel 16.
The driving transistor 1a is programmed to pass the program current Iw1 through itself. During the second 2H (next pixel row), a program current Iw2 sets the potential across the source signal line 18 at the V2. The potential V2 is applied to and held in the gate terminal of the driving transistor 11a in the pixel 16. The driving transistor 11a is programmed to pass the program current Iw2 through itself.
Similarly, during the third H for the third row, a program current Iw3 sets the potential across the source signal line 18 at the V3. The potential V3 is applied to and held in the gate terminal of the driving transistor 11a in the pixel 16. The driving transistor 11a is programmed to pass the program current Iw3 through itself.
The precharge voltage Vp may correspond to the V0 voltage (tone 0). Even in this case, the V0 of the precharge voltage Vp applies the voltage corresponding to the characteristics of the driving transistor 11a in each pixel 16 (this has already described with reference to
The length of the A period may be varied depending on the difference between the applied program voltage or the potential across the source signal line 18 and the potential of the written program voltage. For example, if the difference between the potential across the source signal line 18 and the applied precharge voltage Vp is at least 1.5 V, for example, the former is 2.5 V, whereas the latter is 4.1 V, the A period is 10 μsec. If the difference between the potential across the source signal line 18 and the applied precharge voltage Vp is at least 1.0 V and at most 1.5 V, for example, the former is 3.0 V, whereas the latter is 4.1 V, the A period is 6 μsec. Further, if the tone difference is at least 15, for example, the video signal applied to the pixels in the preceding pixel row corresponds to the tone 5, whereas the video signal applied to the pixels in the succeeding pixel row corresponds to the tone 21, then the A period is 10 μsec. If the tone difference is at least 10 and at most 15, for example, the video signal applied to the pixels in the preceding pixel row corresponds to the tone 10, whereas the video signal applied to the pixels in the succeeding pixel row corresponds to the tone 21, then the A period is 6 μsec.
The precharge voltage Vp and the program current Iw are not limited to a DC voltage and a DC, respectively, but may be a rectangular wave, a triangular wave, an AC, or a sine wave. The signal applied during the B period may be the program voltage output by the voltage tone circuit 231. The present embodiment uses the voltage driving both for the precharge voltage Vp and for the program voltage during the B period. Further, the voltage driving may of course be combined with duty ratio driving shown in
The above description is of course applied to the other embodiments of the present invention.
The circuit configuration shown in
Then, the switches SW2 and SW3 are opened, while the switches SW4 and SW5 are closed, and the voltage tone circuit 231 is operated to apply the precharge voltage Vp. During the A period, the precharge voltage Vp is applied to the source signal line 18 via the buffer circuit 151. The precharge voltage Vp is applied to the gate terminal of the driving transistor 11a. The gain of the buffer circuit 151 is set in association with the duty ratio shown in
During the B period, the switches SW2 and SW5 are opened, and the current tone circuit 154 is operated to apply the program current Iw to the source signal line 18.
After application of the program current Iw, the voltage tone circuit 231 may be operated to apply the tone voltage Vx. The above description is of course applied to the other embodiments of the present invention.
The other embodiments will be described below with reference to the drawings. In
The precharge voltage Vp is set close to the voltage corresponding to the maximum tone. The precharge voltage Vp may have a predetermined fixed value or the apparatus may be configured to be able to vary or adjust the precharge voltage Vp depending on the Va or V0 voltage.
Each of the first to third Hs (first to third horizontal scan periods) corresponds to one horizontal scan period as is the case of
The constant current Iw may also be applied during the A period. The constant current Iw flows from the driving transistor 11a in the pixel 16 into the current tone circuit 154 via the source signal line 18. The constant current Iw sets the voltage at the gate terminal of the driving transistor 11a in the pixel 16 at the Va.
Of course, the Va voltage varies depending on a variation in characteristics among the driving transistors 11a in the respective pixels 16. However, the potential difference between the Va voltage and the Vp voltage is almost constant. Consequently, application of the precharge voltage Vp changes the Vp to the Va during application of the constant current regardless of the potential across the source signal line 18 during the preceding H. Therefore, the convergence time is substantially constant.
During the C period, which follows the B period, the target voltage Vc as a video signal is applied. Accordingly, the target tone Vc=Va+Vx, based on the Va, is applied to the source signal line 18. In the example shown in
Current and voltage data needs to be transferred to the source driver IC (circuit) 14 in order to vary the constant current Iw depending on the tone or among multiple levels or to vary the tone voltage Vx with the pixel.
In the embodiment shown in
The embodiment shown in
In
The embodiment shown in
It is also possible to sequentially select a plurality of pixel rows, apply the constant current Iw to each of the pixel rows, and measure the voltage Va or V0 or the like, during one H period. An example of a driving method selects the first pixel row and applies the constant current Iw to the pixel row during the former half of one H and then selects the second pixel row during the latter half of that H.
In the above description, the Va (see
If the V0 voltage is set so that no current flows through the driving transistor 11a (offset voltage) and the tone voltage Vx is applied on the basis of this V0 as shown in
In
The ground potential is applied to the c section of the capacitor 1341 to reset the capacitor 1341. Then, as shown in
The a section of the capacitor 1341 holds the voltage (=gate terminal voltage of the driving transistor 11a) that prevents current from flowing from the driving transistor 11a to the EL element 15. Also during this period, the appropriate pixel row is selected. The gate terminal potential of the driving transistor 11a in each of the pixels 16 in the appropriate pixel row is kept offset (even closure of the transistor 11d does not allow current to flow through the EL element 15).
Then, as shown in
The voltage Vx output to the voltage tone circuit 231 shifts the potential in the a section of the capacitor 1341. The voltage shift in the a section adds to the V0 and Vx voltages. This completes one horizontal scan period. In the selected pixel row, a current is applied to the EL element 15, which thus emits light, during the next horizontal scan period.
The driving scheme in accordance with the present invention described with reference to
The description of the above embodiment of the present invention focuses on the measurement of the Va or V0 and the addition or subtraction of the Vx voltage to or from the Va or V0 voltage, with the resulting voltage applied to the driving transistor 11a in the pixel 16. The description below focuses on image display provided by the EL display apparatus in accordance with the present invention.
The present invention measures (acquires) the potential (f in
In
The second operation closes the transistors 11b and 11c, while opening the transistor 11d.
The embodiment shown in
When the constant current Iw corresponding to the maximum tone Iwm is applied to the driving transistor 11a, a voltage Vam is generated at the gate terminal of the driving transistor 11a so that the maximum tone current can flow through the driving transistor 11a. The tone voltage Vx is subtracted from the Vam to generate a target voltage Vc. The voltage Vcm generated is applied to the gate terminal of the driving transistor 11a.
As described above, the present invention relates to the EL display panel in which the current driving pixels (configured or arranged so that the drain or source terminal of the driving transistor 11a or the transistor 11b current-mirror-coupled to the driving transistor 1a is connected to the source signal line 18 so as to allow a DC to flow from the driving transistor 11a to the source signal line 18, or configured so that the current flowing through the driving transistor 11 (11a or 11b) can be passed to the source signal line 18, from which the current can be extracted, or the current can be input to the driving transistor 11a through the source signal line 18) are arranged in a matrix. The constant current is applied to the driving transistor 11 (or flows through the driving transistor 11). Then, in a substantially steady state, the potential at the gate terminal of the driving transistor 11 is measured (acquired).
The voltage corresponding to the tone voltage is added to or subtracted from the measured (acquired) potential, serving as a reference (origin or relative position) to generate a target voltage Vc. The target voltage generated is applied to the gate terminal of the driving transistor 11 or the like so that the driving transistor 11 can pass the current corresponding to the target voltage to the Element 15. Passing the current to the EL element 15 includes both the supply of the current to the EL element 15 and the flow of the current from the EL element 15 into the driving transistor 11.
In the above embodiment, a substantially unchanged constant current Ie is passed to the driving transistor 11 on the basis of the Va, V0, and Vam. However, the present invention is not limited to this. For example, of course, the constant current may be multiplied by N on the basis of the driving scheme that passes a current to the EL element 15 only during the 1F/N period, while passing no current during the other periods (1F(N−1)/N), as described with reference to
This method is particularly effective if the source signal line 18 has a large parasitic capacitance. It is also effective if the EL display apparatus is large and at least 10 inches in size. If the source signal line 18 has a large parasitic capacitance, an “insufficient writing” with the constant current Iw can be improved by multiplying the reset current (program current Iw) by N (at least avoiding reducing the reset current).
As described above, the display apparatus in accordance with the present invention is based on the scheme which uses the pixel configuration based on the current program to carry out the voltage driving.
The above embodiment applies the constant current Iw to the pixel 16, measures the potential across the source signal line 18, and uses the measured or acquired voltage to program the EL display apparatus. The constant current Iw can be adjusted using the reference current. Further, in
The specification of the present invention calls the ratio of the display area 63 to the entire display screen 34 a duty ratio. That is, the duty ratio is the area of the display area 63/the area of the entire display screen 34, or the number of gate signal lines 17b to which the on voltage is applied/the total number of gate signal lines 17b, or the number of selected pixel rows connected to the gate signal lines 17b to which the on voltage is applied/the total number of pixel rows on the display screen 34.
In the description of the present specification, it is assumed that the duty ratio control or the like is varied depending on the lighting ratio. However, the lighting ratio does not have a fixed meaning. For example, a low lighting ratio means not only that a small current flows through the display screen 34 but also that an image is composed of a large number of pixels displaying low tones. That is, a video constituting the display screen 34 is mostly composed of dark pixels (low tone pixels).
Accordingly, a low lighting ratio corresponds to a large number of video data with low tones resulting from a histogram process on video data constituting the screen. A high lighting ratio means a large current flowing through the display screen 34 but also means a large number of pixels with high tones which constitute the image. That is, the video constituting the display screen 34 has a large number of bright pixels (pixels with high tones). A high lighting ratio corresponds to a large number of video data with high tones resulting from a histogram process on the video data constituting the screen. That is, control based on the lighting ratio is the same as or similar to control based on the distribution of the tones of the pixels or a histogram distribution.
Thus, in some cases, the control based on the lighting ratio corresponds to control based on the distribution of tones in the image (low lighting ratio=a large number of low-tone pixels, high lighting ratio=a large number of high-tone pixels). For example, increasing a reference current ratio with decreasing lighting ratio while reducing the duty ratio with increasing lighting ratio corresponds to increasing the reference current ratio with increasing number of pixels with low tones while reducing the duty ration with increasing pixels with high tones. Accordingly, increasing the reference current ratio with decreasing lighting ratio while reducing the duty ration with increasing lighting ratio has a meaning that is the same as or similar to that of increasing the reference current ratio with increasing number of pixels with low tones while reducing the duty ratio with increasing pixels with high tones, or the former is an operation or control that is the same as or similar to that of the latter.
Further, for example, multiplying the reference current ratio by N at a predetermined low lighting ratio or lower and setting the number of selection signal lines at N has a meaning that is the same as or similar to that of multiplying the reference current ratio by N and setting the number of selection signal lines at N when the number of pixels with low tones is above a constant, or the former is an operation or control that is the same as or similar to that of the latter.
Furthermore, for example, carrying out driving in a duty ratio of 1:1 and reducing the duty ratio step by step or smoothly at a predetermined high lighting ratio or higher normally has a meaning that is the same as or similar to that of carrying out driving in a duty ratio of 1:1 while the number of pixels with low tones or high tones is within a given range and reducing the duty ratio step by step or smoothly when the number of pixels with high tones has at least a given value, or the former is an operation or control that is the same as or similar to that of the latter.
As shown in
In low lighting ratios, an insufficient writing with the program current is marked in a low tone area. However, increasing the reference current in the low lighting ratio area enables an increase in program current in proportion to the reference current as shown in
In
The relationships with the reference current ratio, the duty ratio and the lighting ratio are preferably fixed as described below. This is because degradation may be accelerated by an increase in the amount of flickers generated or the self-heating of the panel. In an area with a lighting ratio of at most 30%, the duty ratio×reference current ratio (A) is preferably at least 0.7 and at most 1.4, more preferably at least 0.8 and at most 1.2. Preferable control or setting for an area with a lighting ratio of at most 80% is such that the duty ratio×reference current ratio (A) is at least 0.1 and at most 0.8, more preferably at least 0.2 and at most 0.6.
If the duty ratio×reference current ratio is defined as A when the lighting ratio is 50%, preferable control or setting for an area with a lighting ratio of at most 30% is such that the duty ratio×reference current ratio×A is at least 0.7 and at most 1.4, more preferably at least 0.8 and at most 1.2. Further, preferable control or setting for an area with a lighting ratio of at most 80% is such that the duty ratio×reference current ratio×A is at least 0.1 and at most 0.8, more preferably at least 0.2 and at most 0.6.
However, a variation in reference current is required for the overcurrent driving, described with reference to
To solve this problem, as shown in
In
Even if the precharge current ratio is varied step by step, the reference current ratio is varied depending on the precharge current ratio. Further, the speed at which the reference or precharge current ratio or the like varies preferably has a low pass filter characteristic (the ratio does not follow a fast variation in lighting ratio). Preferably, the speed also has a hysteresis characteristic (once the ratio changes, it no longer changes even though the lighting ratio returns to its original value).
The above description (step-by-step variation and the provision of the hysteresis characteristic) is also applicable to the duty ratio.
As described above, the duty ratio, reference current ratio, and precharge current ratio are controlled so as to have correlations. The following correlations are established. The duty ratio×reference current ratio results in a constant. The reference current ratio×precharge current ratio also results in a constant. Consequently, the duty ratio×(1/precharge current ratio) also results in a constant or a substantial constant.
In the embodiment shown in
The embodiment shown in
The precharge voltage V0, the offset voltage corresponding to the 0th tone, is output from the output terminal 83 by closing the switch 161a. The precharge current Id and program current Iw are output from the output terminal 83 by closing the switch 161b. The switches 161a and 161b are exclusively controlled by an inverter 1484 so as not to be simultaneously closed.
Logic data is applied to the inverter 1481 by a precharge-period determining section 1483. That is, the precharge-period determining section 1483 uses a set value for current precharge pulse length to control the inverter 1483.
The embodiment changes the reference current ratio from 1 to 2. The magnitude (ratio) of the precharge current is changed from 1 to ½. For example, if settings are such that, in a reference current ratio of 1, a precharge current control circuit 1482 closes the switch S7, when the reference current ratio changes to 2, the precharge current control circuit 1482 performs control such that the switch S6 is closed. This is because the magnitude of the precharge current obtained while the switch S6 is closed is double that obtained while the switch S7 is closed. The charge in precharge current during the change in reference current ratio from 1 to 2 can be linearly adjusted by controlling the switches S0 to S7.
The above operation makes it possible to set or control such that the precharge current ratio×reference current ratio results in the constant (c). That is, C=the precharge current ratio×referent current ratio. Further, the magnitude of the precharge current can be adjusted by combining adjustment of the precharge current period and selection from the switches S.
Even when the reference current is varied depending on the lighting ratio such as the range of low lighting ratios, the precharge current can be properly realized by varying the relative value of magnitude of the precharge current depending on the lighting ratio, as shown in
An increase in the magnitude of the reference current increases the magnitude of a current flowing through the EL element 15 as well as the voltage between the channels (S-D) of the driving transistor 11a. Consequently, an increase in reference current ratio requires an increase in the absolute value between the anode voltage (Vdd) and the cathode voltage (Vss).
An increase in the absolute value between the anode voltage (Vdd) and the cathode voltage (Vss) increases the power consumption of the EL display apparatus. An increase in the power consumption results in heat generation and degradation of the EL display apparatus. The present invention increases the reference current depending on the lighting ratio, particularly within the range of low lighting ratios, in order to prevent insufficient writings. An increase in the magnitude of the reference current in the low lighting ratio area requires an increase in the absolute value between the anode voltage (Vdd) and the cathode voltage (Vss). However, conventional voltage generating circuits keep the anode voltage (Vdd) ands cathode voltage (Vss) values fixed regardless of the lighting ratio. This increases the amount of current consumed, particularly in the high lighting ratio area, thus disadvantageously causing the EL display apparatus to generate heat.
To solve this problem, as shown in
In
As described above, the present invention is characterized in that the power supply voltage of the EL display apparatus is varied depending on the lighting ratio. In particular, the power supply voltage is varied depending on the reference current. Further, this driving scheme varies the power supply voltage (at least one of the anode voltage Vdd and the cathode voltage Vss) depending on the lighting ratio. Furthermore, the power supply voltage is varied depending on the magnitude of the precharge current. Alternatively, the absolute values of the anode voltage Vdd and cathode voltage Vss are increased. In particular, the absolute value of the power supply voltage (anode voltage Vdd and cathode voltage Vss) is increased in the low lighting ratio area.
A method which increases the absolute value of the power supply voltage is easy. The power supply IC is normally subjected to pulse control. An increase in the frequency of an applied pulse (pulse generated inside the power supply IC) raises the voltage. A decrease in the frequency of an applied pulse (pulse generated or oscillated inside the power supply IC) lowers the voltage. Accordingly, the magnitude of the voltage output by the power supply IC can be easily controlled by performing pulse control on the power supply IC.
On the contrary, when an area with a large reference current is assumed to be a reference, the present invention corresponds to a driving scheme that lowers the power supply voltage (at least one of the anode voltage Vdd and cathode voltage Vss) depending on the lighting ratio. That is, the power supply voltage is lowered in the high lighting ratio area. Further, the power supply voltage is lowered depending on the magnitude of the precharge current. Alternatively, the absolute values of the anode voltage Vdd and cathode voltage Vss are increased. That is, the power supply voltage is reduced consistently with the magnitude of the precharge current. In particular, this scheme reduces the absolute value of the power supply voltage (anode voltage Vdd and cathode voltage Vss) in the high lighting ratio area.
With a single power supply shown in
In
Further, the cathode and anode voltages may be simultaneously varied. Furthermore, control may of course be performed such that the absolute values of the cathode and anode voltages are varied.
The variation in cathode voltage is adjusted on the basis of the voltage division ratio of external resistors for the power supply IC. Accordingly, the resistor value can be altered or varied step by step by using the switch circuit to switch among and select from the plurality of resistors. Further, the cathode voltage can be varied almost linearly with the lighting ratio by using an electronic regulator having other steps or the like.
Further, the speed at which the voltage such as the cathode or anode voltage value varies preferably has a low pass filter characteristic (the voltage does not follow a fast variation in lighting ratio). Preferably, the speed also has a hysteresis characteristic (once the cathode or anode voltage value changes, it no longer changes even though the lighting ratio returns to its original value).
The embodiments of the present invention pass the constant current through the source signal line 18 or the like or hold the source signal line 18 in a high impedance state and measure the V1 or V0 voltage or the like. The measured voltage is held in the EEPROM, ROM, or the like as voltage data (current data) or in the source driver IC (circuit) 14 or the like. However, in order to hold all voltage data or the like, it is necessary to hold an enormous amount of data. Thus, a compression technique may be used to hold the data in the ROM 502 or the like.
For example, a still image compressing technique or format such as JPEG may be used. In particular, the distribution of characteristics of the transistor 11a is not random but approximates that of characteristics of a peripheral part. Thus, the technique for compressing image data may be used to carry out proper compression. Further, a motion picture compressing technique such as MPEG may of course be used. The description in this paragraph is of course applicable to the other embodiments of the present invention.
Description will be given of apparatuses or the like which use the EL display panel, the EL display apparatus, or the method for driving the EL display panel or apparatus in accordance with the present invention. Apparatuses described below implement the previously described apparatuses or methods in accordance with the present invention.
The EL display panel in accordance with the present invention is also used as a display monitor. The angle of the display section 184 can be freely adjusted via a supporting point 1531. While out of use, the display section 184 is housed in a storage section 1533.
The EL display apparatus in accordance with the present embodiment is applicable not only to the video camera but also to an electronic camera or a still camera such as the one shown in
It is possible to combine the pixel configurations, the display panels (display apparatuses), the circuits constituting the display panels (display apparatuses), the methods for controlling the display panels (display apparatuses), or the technical concepts for the display panels (display apparatuses) in accordance with the present invention which are described with reference to
It is possible to mutually combine the power supply circuit configurations, the methods for controlling the power supply circuit, or the technical concepts for the power supply circuit described with reference to
It is possible to combine the source driver ICs (circuits), the circuits constituting the source driver ICs (circuits), the methods for controlling the source driver ICs (circuits), or the technical concepts for the source driver ICs (circuits) in accordance with the present invention which are described with reference to
It is possible to combine the driving and control methods or the technical concepts for the methods in accordance with the present invention which are described with reference to
The above described invention is applicable to the display apparatuses described in
Moreover, it is possible to partly or entirely combine the pixel configurations or display panels (display apparatuses), the methods for controlling the pixel configurations, or the technical concepts for the pixel configurations, the driving and control methods for the display panels (display apparatuses), or the technical concepts for the display panels (display apparatuses), the driver circuits such as the source driver IC (circuit) and gate driver IC (circuit) or the controller ICs (circuits) or their control circuits (including the gate driver circuit or the like), the methods for adjusting or controlling these circuits, or the technical concepts for these circuits, the technical concepts for the inspection (evaluation) apparatuses and methods, or the like, which are described above. Further, of course, they are applicable to one another or may be mutually configured or formed. Furthermore, the technical concepts for the adjustment methods in accordance with the present invention and the like are of course applicable to the display panels or apparatuses or the like in accordance with the present invention. Additionally, these technical concepts and others may be partly or entirely combined together.
The technical concepts for the display apparatuses or the driving or control methods or schemes described in the embodiments of the present invention are also applicable to a video camera, a projector, a three-dimensional (3D) television, a projection television, a field emission display (FED), an SED (display developed by Canon and Toshiba), a PDP (Plasma Display Panel), or the like. The technical concepts are also applicable to a viewfinder, a main monitor and a submonitor or a clock display section of a cellular phone, a PHS, a portable information terminal and its monitor, a digital camera, a satellite television, or a satellite mobile television and its monitor. The technical concepts are also applicable to an electrophotographic system, a head mount display, a direct-vision monitor display, a notebook personal computer, a video camera, a digital still camera, or an electronic still camera. The technical concepts are also applicable to a monitor of a cash dispenser, a public telephone, a video telephone, a personal computer, a watch and its display apparatus, or the like. The technical concepts are also applicable to an instrument that generates information such as bar codes. The technical concepts and others may be partly or entirely combined together.
Of course, the present invention is also applicable or expandable to a display monitor of an electric appliance such as a rice cooker, a display section of a car audio system, a speed meter for a car, a display section of a shaver, a pocket game instrument and its monitor, the number of a telephone receiver, a display monitor such as an indicator of a measuring instrument in a factory, a destination display monitor for a train, a substitution of a neon display apparatus, a backlight for display panel or a lighting device for domestic or business use, a ceiling light, window glass, a lighting device such as a headlight of a car, or the like. The lighting device is preferably configured to vary color temperature. The color temperature can be varied by forming R, G, and B pixels that are striped or arranged in a dot matrix and adjusting currents passed through the pixels. The present invention is also applicable to a display apparatus for advertisement or posters, an RGB signal, an alarm display light, or the like. These technical concepts or the like may be partly or entirely combined together.
The self light emitting element, display apparatus, or organic EL display apparatus in accordance with the present invention is effectively used as a light source for a scanner. An RGB dot matrix is used as a light source to illuminate an object to read an image from the object. Of course, monochromatic tones may be used instead of the RGB. Further, of course, the display apparatus in accordance with the present invention can be configured to output light of a single or narrow-band wavelength. A laser display apparatus is thus obtained or may be used for a certain application. Band narrowing can be achieved by using an interference effect or an optical filter.
The present invention is not limited to the above embodiments. In implementation, many variations or changes may be made to the embodiments without departing from the spirit of the present invention. Further, the embodiments may be properly combined together if possible. In this case, the combinations produce characteristic effects.
The EL display apparatus and the method for driving the EL display apparatus in accordance with the present invention are effective in avoiding insufficient writings all over the tone area while reducing display unevenness. The present invention is thus useful for a self-emission display panel (display apparatus) such as an EL display panel (display apparatus) using an organic or inorganic electroluminescence (EL) element or the like, a driving method and apparatus for these display panels, and a display apparatus using these display panels.
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