Patent Grant
7616179
1. Field of the Invention
The present invention relates to an organic EL (electroluminescence) display apparatus and a driving method therefor.
2. Description of the Related Art
An organic electroluminescence device utilizing electroluminescence (hereinafter abbreviated as EL) of an organic material has a first electrode, a second electrode, and an organic compound layer sandwiched between the electrodes. The organic compound layer includes a light emission layer and a carrier transport layer which are composed of organic molecules. The organic EL device is driven by a current passing between the electrodes. The luminance of the organic EL device is almost exactly proportional to the current (driving current). An organic EL display apparatus in which organic EL devices are arranged in a matrix form has excellent color reproducibility and excellent responsiveness to an input signal, and is therefore ideal particularly for display of moving color images. Furthermore, the organic EL display apparatus can emit light of high luminance and has a wide viewing angle, and therefore can be used in various environments. As a material used for the organic compound layer, there are low-molecular-weight materials ideal for vacuum deposition, oligomer and polymer materials ideal for spin coating and ink-jet coating. Currently, low-molecular-weight materials are in widespread use. However, oligomer and polymer materials, which are ideal for display on a large screen, will probably be used increasingly in the future.
Examples of a pixel driving method are the passive matrix method and the active matrix method. In the passive matrix method, a current is directly passed between first electrodes formed in a striped pattern and second electrodes formed in a striped pattern, the striped patterns being orthogonal to each other, so as to cause organic EL devices sandwiched between the first and second electrodes to emit light. In the active matrix method, pixel circuits each composed of thin-film transistors (hereinafter abbreviated as TFTs), a capacitor, etc. and each used to drive an organic EL device, are arranged in a matrix form. Image signals are individually transmitted to pixels, and are then maintained in corresponding pixel circuits. Organic EL devices emit light in accordance with the maintained pixel signals, thereby displaying an image. In the case of the active matrix method, image signals to be transmitted to individual pixels are rarely mixed. Accordingly, this method is ideal particularly for a display apparatus with a large screen, high definition, and a large number of pixels.
The active matrix driving method is roughly classified into the voltage programming method and the current programming method. In the voltage programming method, a potential, which serves as an image signal, is directly applied to the gate of a driving TFT and is then maintained. A current passing through the driving TFT is controlled by the potential of the gate thereof. However, the relationship between the current and the potential of the gate varies according to the TFT, and sometimes changes with operating time. Accordingly, in the case of the voltage programming method, luminance is prone to vary from pixel to pixel, and image burn-in is prone to occur. On the other hand, in the case of the current programming method, a current, which serves as an image signal, is passed through a driving TFT included in each pixel just before an image is displayed, and the gate potential of the driving TFT at that time is maintained. Accordingly, variation of driving TFT characteristics and a change in driving TFT characteristics with time have little effect on display of an image compared with the voltage programming method.
Each of the pixel circuits 100 includes an organic EL device 106 one of whose electrodes is connected to one of the first constant voltage sources 101, a driving TFT 107 whose drain is connected to the other electrode of the organic EL device 106, a voltage maintaining unit 108 for maintaining a gate-to-source voltage of the driving TFT 107, a first switch 109 disposed between the gate and drain of the driving TFT 107, a second switch 110 disposed between the source of the driving TFT 107 and one of the signal lines 103, and a third switch 111 disposed between the source of the driving TFT 107 and one of the second constant voltage sources 102.
In a programming period, that is, a signal write period, the first switch 109 and the second switch 110 are closed and the third switch 111 is opened so as to provide a signal current for the source of the driving TFT 107 in accordance with an image signal transmitted from one of the signal current sources 104. The source-to-gate voltage at that time is maintained in the voltage (capacitance in
In an image display period, the first switch 109 and the second switch 110 are opened, and the third switch 111 is closed. Consequently, a current passes through the driving TFT 107 in accordance with the source-to-gate voltage determined and maintained in the signal write period, whereby the organic EL device 106 emits light.
The driving TFT 107 shown in
The pixel circuits 100 are formed on a glass substrate using amorphous silicon or polysilicon. However, a metal oxide such as InGaZnO disclosed in WO 05/088726 may be used.
An organic EL display apparatus is often used in mobile apparatuses such as mobile telephones or digital cameras. Accordingly, power consumption is required to be reduced. In order to reduce power consumption during the image display period, it is advantageous that a power supply voltage (a voltage between the first and second constant voltage sources in
In the case of a known pixel circuit compliant with the current programming method, a driving TFT is diode-connected at the time of current programming. Subsequently, a signal current is externally provided for the driving TFT, whereby a gate-to-source voltage of the driving TFT is determined and maintained. In the image display period, a current that is the same as the signal current is passed through an organic EL device in accordance with the maintained gate-to-source voltage.
When the organic EL device emits light at the maximum luminance, the gate-to-source voltage of the driving TFT and the current passing through the organic EL device also become maxima. At least the sum of the voltage across the organic EL device when it emits light at the maximum luminance and the gate-to-source voltage of the driving TFT at that time is required as a power supply voltage. A voltage lower than the sum cannot be used.
In order to further reduce power consumption of the organic EL display apparatus, a new driving circuit is required instead of the known driving circuit compliant with the current programming method.
Like the above-described power supply voltage, a voltage of an output terminal of a signal current source also becomes the maximum value when the organic EL device emits light at the maximum luminance. Accordingly, at least the sum of the voltage across the organic EL device when it emits light at the maximum luminance and the gate-to-source voltage of the driving TFT at that time is required as a power supply voltage used to drive the signal current source. From the viewpoint of power saving, the power supply voltage for the signal current source is preferably reduced.
The present invention provides a driving method for an organic EL display apparatus capable of accurately performing image display with less power and a driving circuit suitable for performing the driving method.
According to an aspect of the present invention, there is provided a driving method of an organic EL device in which, when the organic EL device emits light, a source and a drain of a driving transistor and an anode and a cathode of the organic EL device are connected in series between first and second constant voltage sources and a current flows between the anode and the cathode of the organic EL device in accordance with a gate-to-source voltage of the driving transistor. The gate-to-source voltage of the driving transistor being set in the following steps: (1) disconnecting the series connection of the driving transistor and the organic EL device at the source of the driving transistor; (2) connecting a third constant voltage source maintaining a potential different from a potential of each of the first and second constant voltage sources to a gate of the driving transistor; (3) connecting the source of the driving transistor to a signal current source and passing a signal current between the source and the drain of the driving transistor to generate a voltage between the gate and the source of the driving transistor in a capacitor disposed between the gate and the source of the driving transistor; (4) disconnecting the gate of the driving transistor from the third constant voltage source; (5) disconnecting the source of the driving transistor from the signal current source; and (6) reconnecting the source of the driving transistor to recover the series connection of the driving transistor and the organic EL device.
According to another aspect of the present invention, there is provided an organic EL display apparatus including: an organic EL device having two terminals, an anode and a cathode; a driving transistor having three terminals, a gate, a source, and a drain; a capacitor disposed between the gate and the source of the driving transistor; first, second, and third constant voltage sources each maintaining a constant voltage; a signal current source providing a signal current; a first switch disposed between the gate of the driving transistor and the third constant voltage source; a second switch disposed between the source of the driving transistor and the signal current source; a third switch disposed between the source of the driving transistor and the second constant voltage source; and an opening and closing control portion for controlling opening and closing of the first to third switches.
In a driving circuit according to an embodiment of the present invention, when an image signal is written into a driving TFT that controls a current to be sent to an organic EL device, a potential determined in advance is externally applied to the gate of the driving TFT. Consequently, even if a signal current source and a power supply voltage at the time of image display are lowered, the driving TFT can operate in a saturation region. Even if a TFT having incomplete saturation characteristics is used, a current passing through the organic EL device in an image display period can be more accurately written. Thus, image display can be accurately achieved with less power. In addition, the driving circuit has a simple configuration and is adaptable to various TFTs. Accordingly, it can be easily produced and can be used to achieve high-definition organic EL display apparatuses with large screens.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Operation of Circuit Programming Circuit
For comparison between a driving circuit according to an embodiment of the present invention and a known driving circuit, first, a known current programming circuit shown in
The pixel circuits 100 are connected to the corresponding first constant voltage sources 101 and the corresponding second constant voltage sources 102. The pixel circuits 100 in the same column are connected to one of the signal lines 103. The pixel circuits 100 in the same row are connected to one of scanning lines 114. The opening and closing of the first switch 109, the second switch 110, and the third switch 111 are controlled in accordance with a potential applied to one of the scanning lines 114.
An image signal 112, which has been transmitted as a time series signal, is input into the signal current sources 104 at the same time. However, at a certain point, the image signal 112 is input into only one of the signal current sources 104 in a specific column that has been selected on the basis of a signal transmitted from a horizontal shift register 113. The horizontal shift register 113 sequentially selects the signal current sources 104 so as to input an image signal into the signal current sources 104 in all columns.
A unique signal current is output from each of the signal current sources 104 to one of the signal lines 103. The pixel circuits 100 in the same column are connected to one of the signal lines 103. However, at a certain point, the signal current on one of the signal lines 103 is input into only one of the pixel circuits 100 in a specific row that has been selected on the basis of a signal transmitted from a vertical shift register 115 to one of the scanning lines 114. At that time, the pixel circuits 100 in the same column and in rows other than the selected row are electrically separated from the one of the signal lines 103. The vertical shift register 115 sequentially selects the pixel circuits 100 in a vertical direction so as to input the signal current into the pixel circuits 100 in all rows.
Operation sequences 504, 505, and 506 denote operation sequences of the first, second, and third switches, respectively. The level shown in each of the operation sequences 504 to 506 does not represent the actual level of a gate voltage, but rather simply a high level represents the switch being closed (ON) and a low level represents the switch being opened (OFF).
The change in a gate-to-source voltage 507 of a driving TFT and the change in a driving current 508 of the driving TFT are shown. Here, in the gate-to-source voltage 507, a dotted line represents a source potential, and a solid line represents a gate potential. The channel conductivity type of the driving TFT 107 is p channel. Accordingly, if the gate potential is lower than the source potential by a threshold voltage, the driving current 508 passes from a source to a drain.
A case shown in
A curve 602 represents the drain current of the driving TFT 107 when the gate-to-source voltage is maintained constant. As the drain potential increases in the negative direction, that is, as the source-to-drain voltage increases, the drain current increases. However, when the source-to-drain voltage is equal to or larger than a predetermined voltage 613 (hereinafter referred to as a saturation drain voltage), the drain current is maintained substantially constant. In
A dotted line 603 represents a relationship between the source-to-drain voltage and the drain current in a state in which the drain and source of the driving TFT 107 are short-circuited (diode-connected).
In a saturation region in which the drain current is maintained constant, the drain potential is lower than a channel potential at the drain terminal of a channel. That is, the pinch-off state in which the p-n junction is reverse-biased occurs. The saturation drain voltage 613 is a pinch-off start voltage, that is, a drain voltage when the channel potential is equal to the drain potential at the drain terminal. The gate potential is lower than the channel potential by a threshold voltage. Accordingly, the source-to-drain voltage providing the drain current characteristics 602 is larger than the saturation drain voltage 613 by the threshold voltage. When the TFT is diode-connected, the drain voltage is therefore higher than the saturation drain voltage 613 by the threshold voltage. The above corresponds to a first operation point 605.
A curve 611 shown in
In the signal write period 502, the first switch 109 and the second switch 110 are closed and the third switch 111 shown in
The gate potential of the driving TFT 107 is determined by the source-to-gate voltage of the drain current characteristics 602 depicted by a curve passing through the first operation point 605. This source-to-gate voltage is used to determine an electric charge of the capacitor 108 disposed between the source and the gate.
A voltage drop from the output terminal of one of the signal current sources 104 to the first constant voltage source in the signal write period 502 is the sum of a source-to-drain voltage 606 of the driving TFT and the voltage across the organic EL device 608. Accordingly, the voltage drop corresponds to a magnitude represented by a double sided arrow 609 shown in
In the image display period 503, the first switch 109 and the second switch 110 are opened and the third switch 111 is closed. The source potential of the driving TFT 107 is equal to the potential of one of the second constant voltage sources 102. Since the first switch is opened, the capacitor 108 maintains a charge determined in the signal write period and the source-to-gate voltage of the driving TFT 107.
Accordingly, the drain current characteristics in the signal write period 502 and the drain current characteristics in the image display period 503 are the same, and are represented by the same curve 602 in
Since the diode-connection of the driving TFT 107 is disconnected, a drain voltage cannot be determined by the characteristics at the time of diode-connection 603. Alternatively, since the source potential is fixed to the potential of the second constant voltage source, a current zero position of the organic EL device current and voltage characteristics 611 is determined by a voltage between the first and second constant voltage sources (the magnitude represented by an double sided arrow 610 in
When the driving TFT 107 has complete saturation characteristics, the voltage between the first and second constant voltage sources (hereinafter referred to as a power supply voltage) should be set so that it can be equal to or larger than the signal source voltage 609. Consequently, even if the operation point 605 is changed to the operation point 612, a driving current is not changed.
The case in which the signal current 604 is the maximum value has been described with reference to
In
In the above description, it was assumed that the driving TFT had complete saturation characteristics as shown in
It can be considered that as the screens of display apparatuses increase in size, semiconductors such as metal oxides (InGaZnO disclosed in WO 05/088726, amorphous silicon, and ZnO) and organic semiconductors (polythiophene and pentacene), with which display apparatuses having large screens can be easily produced, will be in widespread use. TFTs composed of these semiconductors sometimes have incomplete saturation characteristics. Polysilicon TFTs that have been widely used are also prone to have incomplete saturation characteristics if the channel length is shortened so as to achieve high definition.
A case in which the driving TFT 107 has incomplete saturation characteristics will be described with reference to
In order to pass a predetermined drain current (Ids) through a TFT, a gate-to-source voltage (Vgs) is generally required to be equal to or larger than a threshold voltage (Vth) of a driving TFT. In the case of a low-temperature polysilicon TFT, Vth is usually approximately 1 to 3V. A curve 802 shown in
In a non-saturation region 815 (Vds≦Vgs−Vth), it is generally known that Ids increases with Vds in accordance with the following equation 1.
Ids=k{2(Vgs−Vth)−Vds}·Vds Equation 1
Here, k denotes a constant determined by the configuration of a TFT or the characteristics of a semiconductor used. Equation 1 corresponds to a quadratic curve in which the maximum point is a saturation drain voltage 813 (=Vgs−Vth). The maximum value (saturation drain current) of Ids is obtained using the following equation 2.
Ids=k(Vgs−Vth)2 Equation 2
A locus 817 represents the locus of the saturation drain voltage 813 when Vgs is changed. The saturation drain voltage is approximately 5 to 10V when high-luminance display is performed. In a saturation region 816 (Vds>Vgs−Vth), Ids is a constant value obtained by Equation 2, and depends on Vgs, but does not depend on Vds.
Accordingly, in order to make Ids conform to a predetermined signal current 804 in the image display period, an operation point is required to be in the saturation region 816. The minimum Vds required for that purpose may be lower than Vgs by Vth from the viewpoint of the definition of the saturation drain voltage 813. However, since the first switch 109 is closed and Vds is equal to Vgs when diode-connection is performed, a first operation point 805 larger than the saturation drain voltage 813 by Vth has to be added. Accordingly, the voltage and current characteristics of the driving TFT at the time of diode-connection correspond to a curve 803 obtained by shifting the locus 817 of the saturation drain voltage by Vth in a high-voltage direction. Conversely, if Vds is lowered to the saturation drain voltage 813 during the diode-connection, Vgs is also lowered and Ids becomes lower than the signal current 804. As a result, a signal cannot be accurately written.
If Vgs and Vds can be separately set, Vds can be lowered to the saturation drain voltage 813 with Vgs maintained. In order to feed the signal current 604 from one of the signal current sources 104 in
A circuit according to a first embodiment of the present invention which is based on the above-described concepts will be described. The same reference numerals are used for components having the same functions as those of
An organic EL device and a driving circuit therefor in
The drain of the driving transistor 107 is connected to the anode of the organic EL device, and the cathode of the organic EL device is connected to the first constant voltage source.
The differences between the circuits shown in
The driving TFT 107 shown in
In
In
The circuit shown in
A period 500 or 501 for displaying a single image includes the signal write period 502 and the image display period 503.
In the image display period 503, the first switch 109 and the second switch 110 are in an off state, that is, are open, and the third switch 111 is in an on state, that is, is connected. At that time, the source and the drain of the driving TFT 107 and the anode and the cathode of the organic EL device 106 are connected in series between one of the first constant voltage sources 101 and one of the second constant voltage sources 102, whereby a current path including these terminals is created. At that time, the gate-to-source current of the driving TFT 107 passes between the anode and the cathode of the organic EL device 106, thereby causing the organic EL device 106 to emit light.
The value of this current is determined by a gate-to-source voltage of the driving TFT 107. The gate-to-source voltage of the driving TFT 107 is set in the signal write period 502 antecedent to the image display period 503.
In the signal write period 502, first, the third switch 111 is turned off, so that the series connection between the driving TFT 107 and the organic EL device 106 is separated at the source of the driving TFT 107. At the same time, the first switch 109 and the second switch 110 are turned on, so that the gate of the driving TFT 107 is connected to one of the third constant voltage sources (V3) 105 and the source thereof is connected to one of the signal current sources 104. At that time, a signal current passes through between the source and the drain of the driving TFT 107, so that a gate-to-source voltage of the driving TFT 107 occurs in accordance with the signal current. This voltage is maintained in the capacitor 108 disposed between the gate and the source of the driving TFT 107.
Subsequently, the first switch 109 is opened so as to disconnect the gate of the driving TFT 107 from one of the third constant voltage sources (V3) 105. At the same time, or after some delay, the second switch 110 is turned off and the third switch 111 is turned on. Consequently, the source of the driving TFT 107 is disconnected from one of the signal current sources 104, and the driving TFT 107 and the organic EL device 106 are reconnected in series.
A voltage drop 908 occurs between electrodes of the organic EL device 106 in accordance with the signal current 904. If display is performed at the maximum luminance, the voltage drop 908 is typically approximately 3 to 5V. Accordingly, Vds of the driving TFT 107 is obtained by subtracting the voltage drop 908 of the organic EL device from an output voltage 909 of one of the signal current sources 104. However, in order to ensure an operation in the saturation region, Vds is required to be larger than a saturation drain voltage 913.
As the signal current 904 increases, a corresponding saturation drain voltage increases. An assumed maximum saturation drain voltage becomes therefore a value corresponding to an assumed maximum signal current. If a line 904 shown in
For example, in the known circuit, if the saturation drain voltage of the driving TFT is 6V, the threshold voltage is 2V, and the voltage drop of the organic EL device is 4V, the signal current source had to output a voltage of 12V as the maximum voltage. However, the maximum voltage at an output terminal of the signal current source required in the typical example of the present invention is only 10V. The first operation point may be another point between the first operation point 905 shown in
If a power supply voltage 910 and the output voltage 909 of the signal current source are matched when image display is performed, power consumption at the time of image display can be reduced. In addition, in the case of the display at the maximum luminance shown in
Thus, a gate-to-source voltage at the time of programming is maintained by setting a gate voltage at the time of the programming as a fixed potential irrespective of a signal current and then disconnecting a gate from a constant voltage source. When image display is performed, a source terminal is disconnected from a signal current source and is then connected to a constant power supply voltage of a second feeder. Consequently, a current flows from the source terminal to a drain terminal in accordance with the gate-to-source voltage, whereby an organic EL device can be driven. According to this method, the fixed potential of the second feeder can be set to a voltage that is the sum of a saturation drain voltage of a TFT and a voltage across the organic EL device, both of which are obtained when the maximum signal current flows within the range of a change in the signal current. This power supply voltage is lower than a power supply voltage obtained using the known driving method in which a TFT is diode-connected at the time of current programming by a threshold voltage. Consequently, power consumption can be reduced.
A circuit according to another embodiment of the present invention will be described with reference to
In
The circuit shown in
The operation of the circuit shown in
First, in the case of high-luminance display shown in
Vds=(The potential of the second constant voltage source)−(The potential of the third constant voltage source)+Vgs Equation 3
This voltage Vds is defined as a first drain-to-source voltage. This voltage Vds is required to be in the saturation region of the driving TFT 107. Accordingly, the potential of the third constant voltage source can be determined so that Vds obtained when the maximum signal current flows is matched with the first operation point 905 shown in
In the image display period, the first switch 109 is opened. After that, the potential written in the voltage maintaining unit 108 is not changed even if the potentials of the first to third constant voltage sources are changed and a potential drop occurs near the pixel circuit due to a wiring resistance in the display apparatus. Since the second switch is opened and the third switch is closed, the source of the driving TFT 107 is disconnected from one of the signal lines 103. However, since Vgs is maintained, the shape of a curve is not changed. The organic EL device 106 and the driving TFT 107 are connected in series between one of the first constant voltage sources 101 and one of the second constant voltage sources 102. Since the signal current 904 also passes through the organic EL device, the voltage drop 908 occurs between electrodes thereof in accordance with the characteristics 907. Accordingly, Vds of the driving TFT 107 is obtained using the following equation 4.
Vds=(The power supply voltage 910)−(The voltage drop 908 of the organic EL device) Equation 4
This voltage Vds is defined as a second drain-to-source voltage.
When the first and second drain-to-source voltages are matched, even if a transistor has incomplete saturation characteristics, a current value at the time of programming and a current value at the time of light emission can be matched.
The potential of the third constant voltage source is obtained using equation 5 based on equations 3 and 4 on the basis of GND as follows.
(The gate-to-source voltage Vgs when a predetermined current flows)+(The voltage drop 908 in the organic EL device at that time) Equation 5
This equation can be also represented as (the saturation drain voltage 913+the threshold voltage+the voltage drop 907 in the organic EL device) on the basis of conditions required for a saturation operation of the driving transistor. The saturation drain voltage 913 and the voltage drop 907 in the organic EL device depend on the level of the signal current 904. After the signal current 904 is determined in accordance with a luminance level used for display, the value of the saturation drain voltage 913 and the value of the voltage drop 907 in the organic EL device are determined as represented by the double sided arrows 906 and 908 in
The maximum potential of the third constant voltage source is calculated by obtaining a current with the maximum luminance level to be used for display as the signal current 904 and using equation 5. If the power supply voltage is 15V, Vgs at the time of the maximum luminance is 7V, and the voltage drop of the organic EL device is 6V, the third feeding potential becomes 13V using equation 5. An EL current at that time is in exact agreement with the programming current.
When the third feeding potential is determined and fixed in such a manner, if the luminance is lowered, the first drain-to-source voltage obtained by equation 3 decreases and the second drain-to-source voltage obtained by equation 4 increases. They become different from each other. That is, the operation point at the time of programming and the operation point at the time of light emission become different from each other. If the transistor has incomplete saturation characteristics, the programming current is not matched with the EL current.
In the case of the known circuit shown in
If the signal current decreases, Vgs becomes 5V, and the voltage drop of the organic EL device becomes 4V while the potential of the third constant voltage source is 13V, the first drain-to-source voltage becomes 7V and the second drain-to-source voltage becomes 11V. That is, Vds increases by +4V. If a transistor has incomplete saturation characteristics, and if Ids increases at the ratio of 3%/V in accordance with Vds in a saturation region, a luminance error of 12% occurs. This value is smaller than a luminance error of 18% in the circuit shown in
Conversely, If the potential of the third constant voltage source is optimized for the signal current 904′ in
Here, the signal current is raised to the level of the signal current 904 with the above-described settings maintained. Since Vgs is 7V and the voltage drop of the organic EL device is 6V, the first drain-to-source voltage becomes 13V using equation 5 and the second drain-to-source voltage becomes 9V using equation 4. That is, Vds decreases by 4V. If Ids of a TFT increases at the ratio of 3%/V in accordance with Vds, a luminance error of −12% occurs.
In the case of a general image, it is desirable that optimization be performed for the luminance of a geometric average of the maximum value and the minimum value. A case in which optimization is performed for the luminance level that is approximately one-third of the maximum luminance level will be shown in
As described previously, the luminance error due to incomplete saturation characteristics of a driving TFT cannot be completely prevented. However, for example, in the case of a high-luminance document image having a white background, settings described in the case of high-luminance display can be performed. In the case of a low-luminance document image having a gray background, settings described in the case of low-luminance display can be performed. Consequently, luminance nonuniformity is suppressed. In the case of an image with an average luminance level, settings described with reference to
In the circuits shown in
However, as described previously, at the time of programming, that is, when generating a voltage between the gate and the source of the driving transistor in accordance with a signal, a method different from that used by the circuit shown in
In the first circuit shown in
If the polarity of the driving transistor is reversed, that is, the driving transistor is an n-channel transistor, the direction of a current is reversed. Accordingly, the anode and the cathode of the organic EL device in
In the second circuit shown in
If the polarity of the driving transistor is reversed, that is, the driving transistor is a p-channel transistor, the direction of a current is reversed. Accordingly, the anode and the cathode of the organic EL device in
According to this method, the fixed potential of the second constant voltage source can be set to a voltage that is the sum of a saturation drain voltage of a TFT and a voltage across the organic EL device, both of which are obtained when the maximum signal current within the range of a change in the signal current flows. This power supply voltage is lower than a power supply voltage obtained using the known driving method in which a TFT is diode-connected at the time of current programming by a threshold voltage. Consequently, power consumption can be reduced.
In the following, description will be made by providing specific circuits as examples. First to fourth examples are specific examples of the circuit shown in
A p-channel TFT is used for each of the driving TFT, the first switch 109, and the second switch 110, and an n-channel TFT is used for the third switch 111. The gate of each TFT is connected to a corresponding one of the scanning lines 114. When a low-level signal is applied from the vertical shift register 115 to one of the scanning lines 114, the first switch 109 and the second switch 110 are closed and the third switch 111 is opened. When a high-level signal is applied to one of the scanning lines 114, operations of all switches are reversed. Accordingly, the sequences shown in
An n-channel TFT is used for each of the driving TFT, the first switch 109, and the second switch 110, and a p-channel TFT is used for the third switch 111. The gate of each TFT is connected to a corresponding one of the scanning lines 114. When a high-level signal is applied from the vertical shift register 115 to one of the scanning lines 114, the first switch 109 and the second switch 110 are closed and the third switch 111 is opened. When a low-level signal is applied to one of the scanning lines 114, operations of all switches are reversed. Accordingly, the sequences shown in
The circuits of the first and second examples individually have an additional component, the third constant voltage power source 105, as compared with a known current programming circuit disclosed in U.S. Pat. No. 6,229,506. However, the number of circuit elements such as TFTs does not increase. Accordingly, the circuits can be easily produced and can be said to be practical circuits.
A capacitor is widely used as the voltage maintaining unit 108 in the circuit shown in
In the image display period 503, the written potential is not usually changed, because the first switch 109 is opened. However, if the third switch 111 is closed before the first switch 109 is opened and the source of the driving TFT 107 is connected to one of the second constant voltage power sources 102, a current may flow into the voltage maintaining unit 108 and the appropriately written potential may be changed. As shown in
Like the circuits shown in
Furthermore, as shown in
An n-channel TFT is used for each of the driving TFT and the third switch 111, and a p-channel TFT is used for each of the first switch 109 and the second switch 110. The gate of each TFT is connected to a corresponding one of the scanning lines 114. When a high-level signal is applied from the vertical shift register 115 to one of the scanning lines 114, the first switch 109 and the second switch 110 are closed and the third switch 111 is opened. When a low-level signal is applied to one of the scanning lines 114, operations of all switches are reversed. Accordingly, the sequences shown in
A p-channel TFT is used for each of the driving TFT and the third switch 111, and an n-channel TFT is used for each of the first switch 109 and the second switch 110. The gate of each TFT is connected to a corresponding one of the scanning lines 114. When a low-level signal is applied from the vertical shift register 115 to one of the scanning lines 114, the first switch 109 and the second switch 110 are closed and the third switch 111 is opened. When a high-level signal is applied to one of the scanning lines 114, operations of all switches are reversed. Accordingly, the sequences shown in
A capacitor is widely used as the voltage maintaining unit 108 in the circuit shown in
In the image display period 503, the written potential is not usually changed, because the first switch 109 is opened. However, if the third switch 111 is closed before the first switch 109 is opened and the source of the driving TFT 107 is connected to the organic EL device 106, a current may flow into the voltage maintaining unit 108 and the appropriately written potential may be changed. As shown in
Like the circuits shown in
Like the case described in the fourth example with reference to
In a circuit shown in
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions.
This application claims the benefit of Japanese Applications No. 2006-098009 filed Mar. 31, 2006 and No. 2006-098010 filed Mar. 31, 2006, which are hereby incorporated by reference herein in their entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-098009 | Mar 2006 | JP | national |
| 2006-098010 | Mar 2006 | JP | national |
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