Patent Grant
8253663
The present invention contains subject matter related to Japanese Patent Application JP 2007-079037 filed in the Japan Patent Office on Mar. 26, 2007, the entire contents of which being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a display apparatus, a method for driving the display apparatus and electronic equipment. More particularly, the present invention relates to a display apparatus of a flat-panel type, in which pixel circuits each including an electro-optical device are laid out to form a matrix, a method for driving the display apparatus and electronic equipment employing the display apparatus.
2. Description of the Related Art
In recent years, in the field of a display apparatus for displaying an image, a display apparatus of a flat-panel type, in which pixels (or pixel circuits) each including a light emitting device are laid out to form a matrix, has been becoming popular very fast. A light emitting device included in each pixel circuit in the display apparatus of a flat-panel type is an electro-optical device of the so-called current-driven type in which the luminance of a light beam emitted by the device changes in accordance with the magnitude of a current flowing through the device. The development of an organic EL (Electro Luminescence) display apparatus employing such electro-optical devices into a commercial product has been making progress. An example of the electro-optical device of the so-called current-driven type is an organic EL device operating on the basis of a phenomenon in which a light beam is generated by the device when an electric field is applied to an organic film.
The organic EL display apparatus has the following characteristics. The organic EL device employed in the EL display apparatus can be driven by an applied voltage not exceeding 10V so that the power consumption of the device is low. In addition, since the organic EL device is a light emitting device, the organic EL display apparatus is capable of displaying an image which is visible in comparison with a liquid crystal display apparatus for displaying an image by controlling the intensity of a light beam generated by a light source known as a backlight in a liquid crystal cell included in every pixel circuit of the liquid crystal display apparatus. On top of that, the organic EL display apparatus can be made light and thin with ease because the organic EL display apparatus does not need illumination members such as the backlight which is necessary for the liquid crystal display apparatus. Furthermore, the organic EL device has an extremely high speed providing a short response time of the order of several microseconds. Thus, a residual image is not generated in an operation to display a moving image.
Much like the liquid crystal display apparatus, a passive matrix method or an active matrix method can be adopted as a method for driving the organic EL display apparatus. However, even though an organic EL display apparatus adopting the passive matrix method has a simple structure, the apparatus raises problems such as difficulties to implement a large display screen having a high resolution. For the reasons described above, an organic EL display apparatus adopting an active matrix method is developed aggressively. In accordance with this active matrix method, an active device is provided in the same pixel circuit as an electro-optical device. The active device is used for controlling a current flowing through the electro-optical device. An example of the active device is an insulated-gate type field effect transistor which is generally a TFT (thin film transistor).
By the way, the I-V characteristic (that is, the current-voltage characteristic) of an organic EL device is known to deteriorate with the lapse of time in the so-called aging process. In a pixel circuit employing an N-channel TFT for controlling a current flowing through the organic EL device, the organic EL device is connected to the source of the transistor which is referred to hereafter as a driving transistor. Thus, when the I-V characteristic of the organic EL device deteriorates, a voltage Vgs appearing between the gate and source of the driving transistor changes. As a result, the intensity of a light beam generated by the organic EL device also changes as well.
To put it more concretely, an electric potential appearing at the source of the driving transistor is determined by the operating points of the driving transistor and the organic EL device. When the I-V characteristic of the organic EL device deteriorates, the operating points of the driving transistor and the organic EL device change. Thus, the electric potential appearing at the source of the driving transistor also changes even if a voltage applied to the gate of the transistor after the operating points of the driving transistor and the organic EL device change is sustained at the same level as that before the operating points of the driving transistor and the organic EL device change. Accordingly, the voltage Vgs appearing between the gate and source of the driving transistor also changes as well, causing a current flowing through the transistor and a current flowing through the organic EL device to vary. As a result, since the current flowing through the organic EL device varies, the intensity of a light beam generated by the organic EL device also changes as well.
In addition, in the case of a pixel circuit employing a poly-silicon TFT, not only does the I-V characteristic of the organic EL device deteriorate with the lapse of time, but the threshold voltage Vth of the driving transistor and the mobility μ of a semiconductor film composing the channel of the transistor also change with the lapse of time. In the following description, the mobility μ of a semiconductor film composing the channel of a driving transistor is referred to as the mobility μ of the driving transistor. On top of that, the threshold voltage Vth and mobility μ of the driving transistor each vary from pixel to pixel due to variations in fabrication process. That is to say, the characteristic of the driving transistor varies from pixel to pixel.
If the threshold voltage Vth and mobility μ of the driving transistor each vary from pixel to pixel, the current flowing through the transistor also varies from pixel-to-pixel. Thus, the luminance of a light beam generated by the organic EL device also varies from pixel to pixel even for the same voltage applied to the gate of each driving transistor. As a result, the screen loses uniformity.
In order to prevent the luminance of a light beam generated by the organic EL device from varying from pixel to pixel even for the same voltage applied to the gate of each driving transistor and, hence, from being affected by deteriorations of the I-V characteristic of the organic EL device and/or changes of the threshold voltage Vth and mobility μ of the driving transistor even if the I-V characteristic deteriorates with the lapse of time and/or the threshold voltage Vth and the mobility μ change with the lapse of time, it is necessary to provide every pixel circuit with a compensation function and a variety of correction functions as is described in documents such as patent reference 1 which is Japanese Patent Laid-open No. 2006-133542. The compensation function is a function to compensate for characteristic variations of the organic EL device. The correction functions include a threshold-voltage correction function and a mobility correction function. The threshold-voltage correction function is a function to make corrections for threshold voltage (Vth) variations of the driving transistor. On the other hand, the mobility correction function is a function to make corrections for mobility (μ) variations of the driving transistor.
As described above, every pixel circuit is provided with the compensation function to compensate for characteristic variations of the organic EL device, the threshold-voltage correction function to make corrections for threshold voltage (Vth) variations of the driving transistor and the mobility correction function to make corrections for mobility (μ) variations of the driving transistor. Thus, it is possible to prevent the luminance of a light beam generated by the organic EL device from varying from pixel to pixel even for the same voltage applied to the gate of each driving transistor and, hence, from being affected by deteriorations of the I-V characteristic of the organic EL device and/or changes of the threshold voltage Vth and mobility μ of the driving transistor even if the I-V characteristic deteriorates with the lapse of time and/or the threshold voltage Vth and the mobility μ change with the lapse of time.
As described above, in an organic EL display apparatus with a configuration including pixel circuits each having correction functions such as the threshold-voltage correction function and the mobility correction function, four operations are, carried out periodically on every pixel row. The four operations are: a threshold-voltage correction preparatory operation carried out in order to fix each of the electric potential Vg appearing on the gate of the driving transistor and the electric potential Vs appearing on the source of the driving transistor at a predetermined level; a threshold-voltage correction operation carried out in order to sufficiently raise the electric potential Vs appearing on the source of the driving transistor so as to fix a voltage Vgs appearing between the gate and source of the driving transistor at the threshold voltage Vth of the driving transistor; a signal write operation carried out in order to write an input signal voltage Vsig determined by luminance information as the voltage of a video signal into the pixel circuit; and a mobility correction operation carried out in order to make corrections for the mobility μ of the driving transistor. Details of each of the above operations will be described alter.
If the four operations described above are carried out on each pixel row within a 1H period (where 1H is the length of a horizontal scan period or the length of a horizontal synchronization period), there is a problem of difficulty to allocate sufficient time to each of the threshold-voltage correction operation and the mobility correction operation as a threshold-voltage correction period and a mobility correction period respectively so as to assure that the threshold-voltage correction operation and the mobility correction operation can be carried out with a high degree of reliability. In particular, in an effort to increase the display resolution of the display apparatus, the number of pixel circuits shows a trend of increasing from year to year, inevitably reducing the length of the 1H period. Thus, in the present state of the art, it is difficult to allocate sufficient time to each of the threshold-voltage correction operation and the mobility correction operation as a threshold-voltage correction period and a mobility correction period respectively.
As an example, the above description takes an organic EL display apparatus with a configuration including pixel circuits each having correction functions such as the threshold-voltage correction function and the mobility correction function. It is to be noted, however, that an organic EL display apparatus with a configuration including pixel circuits each having only the threshold-voltage correction function also raises the problem of difficulty to allocate sufficient time to the threshold-voltage correction operation as a threshold-voltage correction period as is the case with the organic EL display apparatus with a configuration including pixel circuits each having both the threshold-voltage correction function and the mobility correction function.
If it is not achieved to allocate sufficient time to each of the threshold-voltage correction operation and the mobility correction operation as a threshold-voltage correction period and a mobility correction period respectively, it is also not achieved to assure that the threshold-voltage correction operation and the mobility correction operation can be carried out with a high degree of reliability. Thus, even if a uniform voltage is applied to the gates of driving transistors, uniformity of the screen is lost due to variations of the luminance of a light beam generated by the organic EL device from pixel to pixel.
In order to solve the problems described above, inventors of the present invention have innovated a display apparatus capable of allocating sufficient time to each of the threshold-voltage correction operation and the mobility correction operation as a threshold-voltage correction period and a mobility correction period respectively so as to assure that the threshold-voltage correction operation and the mobility correction operation can be carried out with a high degree of reliability. In addition, the inventors have also innovated a driving method for the display apparatus and electronic equipment employing the display apparatus.
In accordance with the embodiment of the present invention for solving the problems described above, there is provided a display apparatus employing: a pixel array section including pixel circuits laid out to form a matrix as pixel circuits each having an electro-optical device, a write transistor for carrying out a voltage storing process to sample a video signal and store the sampled video signal into the pixel circuit, a holding capacitor for holding the sampled video signal stored in the pixel circuit by the write transistor, and a driving transistor for driving the electro-optical device on the basis of the video signal held by the voltage holding capacitor. The display apparatus further includes a driving circuit for carrying out a selective scan operation on the pixel circuits in the pixel array section in row units, and a threshold-voltage correction operation to correct variations of the threshold voltage of every driving transistor for each pixel row selected in the selective scan operation. In the display apparatus, before the driving circuit carries out a threshold-voltage correction operation on the pixel row in a horizontal scan period, the driving circuit performs a preparatory operation on the pixel row prior to the horizontal scan period in order to fix each of an electric potential appearing on the gate of the driving transistor and an electric potential appearing on the source of the driving transistor at a predetermined level.
In the display apparatus having the configuration described above as well as electronic equipment employing the display apparatus, as explained above, before a threshold-voltage correction operation is carried out on the pixel row in a horizontal scan period, a preparatory operation is carried out on the pixel row prior to the horizontal scan period in order to fix each of an electric potential appearing on the gate of the driving transistor and an electric potential appearing on the source of the driving transistor at a predetermined level. Thus, it is no longer necessary to allocate sufficient time of the horizontal scan period provided for the pixel row subjected to the threshold-voltage correction operation to the threshold-voltage correction preparatory operation as a threshold-voltage correction preparation period. Therefore, the threshold-voltage correction period set in the horizontal scan period can be prolonged by the threshold-voltage correction preparation period. As a result, it is possible to allocate sufficient time to the threshold-voltage correction operation as a threshold-voltage correction period so as to assure that the threshold-voltage correction operation can be carried out with a high degree of reliability.
In accordance with the embodiment of the present invention, it is possible to allocate sufficient time to the threshold-voltage correction operation as a threshold-voltage correction period so as to assure that the threshold-voltage correction operation in order to suppress variations of the characteristics of the driving transistor and deteriorations of the electro-optical device with the lapse of time can be carried out with a high degree of reliability. Thus, an image having a high quality can be displayed.
Preferred embodiments of the present invention are described in detail by referring to diagrams as follows.
As shown in
The pixel circuits 20 in the pixel array section 30 form a matrix of m rows and n columns. The m rows are connected to m scan lines 31-1 to 31-m respectively as well as m power-supply feed lines 32-1 to 32-m respectively. On the other hand, the n columns are connected to n signal lines 33-1 to 33-n respectively.
The pixel array section 30 is normally created on a transparent insulation substrate such as a glass substrate and has a panel (flat) structure. Each of the pixel circuits can be created by making use of an amorphous silicon TFT (Thin Film Transistor) or a low-temperature poly-silicon TFT. If a low-temperature poly-silicon TFT is used, the write scan circuit 40, the power-supply feed line scan circuit 50 and the horizontal driving circuit 60 are also created on a display panel (substrate) 70 on which the pixel array section 30 is created.
The write scan circuit 40 typically employs a shift register for shifting (transferring) start pulses sp synchronously with clock pulses ck. In order to carry out an operation to write a video signal into the pixel circuits 20 of the pixel array section 30, the write scan circuit 40 supplies sequential scan signals WS1 to WSm to the scan lines 31-1 to 31-m respectively in order to sequentially scan the pixel circuits 20 in row units in the so-called row sequential scan operation.
The power-supply feed line scan circuit 50 also typically employs, a shift register for shifting (transferring) start pulses sp synchronously with clock pulses ck. The power-supply feed line scan circuit 50 supplies power-supply feed-line electric potentials, DS1 to DSm to the power-supply feed lines 32-1 to 32-m respectively in synchronization with the row sequential scan operation carried out by the write scan circuit 40. The power-supply feed-line electric potentials DS1 to DSm are each switched to a high first electric potential Vccp from a low second electric potential Vini lower than the high first electric potential Vccp.
The horizontal driving circuit 60 properly selects the voltage Vsig representing a video signal or an offset voltage Vofs. The voltage Vsig representing a video signal varies in accordance with luminance information supplied by a signal supplying source (not shown in the figure). The horizontal driving circuit 60 then simultaneously supplies the selected voltage Vsig or Vofs to the pixel circuits 20 of the pixel array section 30 through signal lines 33-1 to 33-n typically in column units. That is to say, the horizontal driving circuit 60 supplies the input signal voltage Vsig (or the offset voltage Vofs) to all pixel circuits on a column simultaneously in the so-called write-line sequential write driving operation.
The offset voltage Vofs is a voltage serving as a reference of the voltage Vsig representing a video signal. Typically, the reference of the voltage Vsig representing a video signal corresponds to the black level of the video signal. In the following description, the voltage Vsig representing a video signal is also referred to as an input signal voltage Vsig or merely a signal voltage Vsig. In addition, the low second electric potential Vini is an electric potential sufficiently lower than the offset voltage Vofs.
(Pixel Circuits)
In the above circuit, the driving transistor 22 and the write transistor 23 are each an N-channel TFT. However, the N-channel conduction type of the driving transistor 22 and the write transistor 23 is no more than a typical one. That is to say, the conduction type of the driving transistor 22 and the write transistor 23 is by no means limited to the N-channel conduction type.
The cathode of the organic EL device 21 is connected to a common power-supply feed line 34 which is connected to all pixel circuits 20. The source of the driving transistor 22 is connected to the anode of the organic EL device 21 and the drain of the driving transistor 22 is connected to a power-supply feed line 32 (or, to be more specific, the corresponding one of the power-supply feed lines 32-1 to 32-m).
The gate of the write transistor 23 is connected to a scan line 31 (or, to be more specific, the corresponding one of the scan lines 31-1 to 31-m). One of the source and drain of the write transistor 23 is connected to a signal line 33 (or, to be more specific, the corresponding one of the signal lines 33-1 to 33-n) whereas the other one of the source and drain of the write transistor 23 is connected to the gate of the driving transistor 22. One terminal of the voltage holding capacitor 24 is also connected to the gate of the driving transistor 22 whereas the other terminal of the voltage holding capacitor 24 is connected to the source of the driving transistor 22 as well as the anode of the organic EL device 21.
In the pixel circuit 20 with a configuration described above, when a scan signal WS generated by the write scan circuit 40 is applied to the gate of the write transistor 23 through a scan line 31, the write transistor 23 enters a conductive state. In this conductive state, the write transistor 23 samples the signal voltage (input signal voltage) Vsig supplied by the horizontal driving circuit 60 through a signal line 33 as a video-signal voltage representing the luminance of a light beam or samples the offset voltage Vofs also supplied by the horizontal driving circuit 60 through the signal line 33 and writes the sampled voltage in the pixel circuit 20. To put it concretely, the write transistor 23 holds the sampled input signal voltage Vsig or the sampled offset voltage Vofs in the voltage holding capacitor 24.
With the electric potential DS of the power-supply feed line 32 (or, to be more specific, the corresponding one of the power-supply feed lines 32-1 to 32-m) set at the high first electric potential Vccp, the driving transistor 22 receives a current from the power-supply feed line 32 and supplies the current to the organic EL device 21 as a driving current for driving the organic EL device 21. The magnitude of the driving current is determined by the input signal voltage Vsig held in the voltage holding capacitor 24.
(Pixel-Circuit Structure)
The organic EL device 21 has an anode electrode 204, an organic layer 205 and a cathode electrode 206. The anode electrode 204 is made of materials including a metal created on the bottom of the dent 203A of the wind insulation film 203. Created on the anode electrode 204, the organic layer 205 includes an electron transport layer 2053, a light emitting layer 2052 and a hole transport layer/hole injection layer 2051. Created on the organic layer 205, the cathode electrode 206 is made of materials including a transparent conductive film common to all pixel circuits 20.
The organic layer 205 of the organic EL device 21 is created by sequentially piling the hole transport layer/hole injection layer 2051, the light emitting layer 2052, the electron transport layer 2053 and an electron injection layer not shown in the figure to form a stacked pile of layers on the anode electrode 204. A current generated by the driving transistor 22 shown in
After an organic EL device 21 is constructed over the glass substrate 201, on which a pixel circuit 20 including a driving transistor 22 and a write transistor 23 have been created, to sandwich the insulation film 202 and the wind insulation film 203 between the organic EL device 21 and the glass-substrate 201 for each pixel circuit 20, a sealing substrate 208 is joined by an adhesive layer 209 to a passivation film 207. In this way, the sealing substrate 208 seals the organic EL device 21 to finally give a display panel 70.
(Threshold-Voltage Correction Function)
While the horizontal driving circuit 60 is supplying the offset voltage Vofs to each of the signal lines 33 (that is, the signal lines 33-1 to 33-n) after the write transistor 23 has been put in the conductive state, the power-supply feed line scan circuit 50 switches the electric potential DS asserted thereby on the power-supply feed line 32 to the high first electric potential Vccp from the low second electric potential Vini. By switching the electric potential DS appearing on the power-supply feed line 32 to the high first electric potential Vccp from the low second electric potential Vini, a voltage corresponding to the threshold voltage Vth of the driving transistor 22 is held in the voltage holding capacitor 24.
The voltage corresponding to the threshold voltage Vth of the driving transistor 22 needs to be held in the voltage holding capacitor 24 because of a reason described as follows. The characteristics of the driving transistor 22 vary from pixel to pixel due to variations of the process to fabricate the driving transistor 22 and due to characteristic changes with the lapse of time. The characteristics of the driving transistor 22 include the threshold voltage Vth and the mobility μ. The variations in transistor characteristics cause the driving current Ids flowing between the drain and source of the driving transistor 22 to vary from pixel to pixel even if the same electric potential is applied to the gates of the driving transistors 22 of the pixel circuits 20. Thus, the luminance of a light beam generated by the organic EL device 21 also varies from pixel to pixel. In order to cancel (or correct) effects of the variations of the threshold voltage Vth from pixel to pixel, a voltage corresponding to the threshold voltage Vth of the driving transistor 22 needs to be held in the voltage holding capacitor 24 in advance.
The threshold voltage Vth of the driving transistor 22 is corrected as follows. By storing a voltage corresponding to the threshold voltage Vth in the voltage holding capacitor 24 in advance, the threshold voltage Vth of the driving transistor 22 is cancelled by a voltage, which has been held in advance in the voltage holding capacitor 24 as the voltage corresponding to the threshold voltage Vth, in an operation to drive the driving transistor 22 by later applying the input signal voltage Vsig to the gate of the driving transistor 22 through the write transistor 23. In other words, the threshold voltage Vth of the driving transistor 22 is corrected in advance prior to the operation to drive the driving transistor 22 by applying the input signal voltage Vsig to the gate of the driving transistor 22 through the write transistor 23.
The function to hold a voltage corresponding to the threshold voltage Vth of the driving transistor 22 in the voltage holding capacitor 24 in advance is referred to as a threshold-voltage correction function. By carrying out this threshold-voltage correction function, effects of variations in threshold voltage Vth from pixel to pixel can be eliminated in case the threshold voltage Vth of the driving transistor 22 varies from pixel to pixel due to variations of the process to fabricate the driving transistor 22 and due to transistor-characteristic changes with the lapse of time. Thus, the luminance of a light beam generated by the organic EL device 21 can be sustained at a constant value. The principle of the threshold-voltage correction operation will be described later.
(Mobility Correction Function)
The pixel circuit 20 shown in
(Bootstrap Function)
The pixel circuit 20 shown in
That is to say, even if the I-V characteristic of the organic EL device 21 changes with the lapse of time, causing the electric potential Vs appearing on the source of the driving transistor 22 also to vary, the voltage Vgs appearing between the gate and source of the driving transistor 22 is sustained at a constant level by virtue of the operation of the holding capacitor 24. Thus, a driving current flowing through the organic EL device 21 does not change. As a result, the luminance of a light beam generated by the organic EL device 21 can be sustained at a constant value even if the I-V characteristic of the organic EL device 21 changes with the lapse of time. The operation to eliminate fluctuations in luminance is referred to as a bootstrap operation. By virtue of the bootstrap operation, it is possible to display an image with no luminance deteriorations even if the I-V characteristic of the organic EL device 21 changes with the lapse of time.
As is obvious from the above description, the driving circuit is designed into a configuration in which: the write scan circuit 40 and the power-supply feed scan circuit 50 each carry out a selective scan operation on the pixel circuits 20 of the pixel array section 30 in row units; and threshold-voltage correction and mobility correction operations are carried out to correct respectively the threshold voltage Vth and mobility μ of the driving transistor 22 for every selected pixel row in a 1H period.
As described above, the embodiment implementing the organic EL display apparatus 10 provided with correction functions such as the threshold-voltage correction function and the mobility correction function executes a threshold-voltage correction-preparatory operation and a threshold-voltage correction operation for every pixel row selected in a vertical scan operation in a 1H period, before the threshold-voltage correction operation is carried out on the pixel row in the 1H period, the threshold-voltage correction preparatory operation is carried out on the pixel row prior to the 1H period in order to fix each of an electric potential Vg appearing on the gate of the driving transistor 22 and an electric potential Vs appearing on the source of the driving transistor 22 at a predetermined level. In the following description, a pixel row selected in a vertical scan operation is referred to as a correction-subject pixel row, and 1H is the length of a horizontal scan period or the length of a horizontal synchronization period.
(Circuit Operations Carried Out by the Organic EL Display Apparatus)
Operations carried out by the organic EL display apparatus 10 according to the embodiment are explained by referring to timing charts of
In the timing charts shown in
In the timing charts shown in
It is to be noted that the time t5 is a timing with which the electric potential appearing on the signal line 33 of a pixel row immediately preceding the correction-subject pixel row is changed from the input signal voltage Vsig to the offset voltage Vofs. On the other hand, the time t12 is a timing with which the electric potential appearing on the signal line 33 of the correction-subject pixel row is changed from the input signal voltage Vsig to the offset voltage Vofs.
<Light Emitting Period>
In the timing charts shown in
<Threshold-Voltage Correction Preparatory Period>
At the time t1, the pixel circuit 20 enters a new field of a row sequential scan process. At this time, the electric potential DS appearing on the power-supply feed line 32 is switched from the high electric potential Vccp to the low second electric potential Vini sufficiently lower than the offset voltage Vofs appearing on the signal line 33 as shown in
Then, at a time t2, the electric potential WS appearing on the scan line 31 is changed from a low electric potential WS_L to a high electric potential WS_H in order to put the write transistor 23 in a conductive state as shown in
Thus, the voltage Vgs appearing between the gate and source of the driving transistor 22 becomes equal to a difference of (Vofs−Vini). If the difference of (Vofs−Vini) is not greater than the threshold voltage Vth of the driving transistor 22, the threshold-voltage correction operation explained earlier cannot be carried out. It is thus necessary to set an electric-potential relation of (Vofs−Vini)>Vth. The operation initialize the electric potential Vg appearing at the gate of the driving transistor 22 by fixing (or to confirmedly setting) the electric potential Vg at the offset voltage Vofs and the operation to initialize the electric potential Vs appearing at the source of the driving transistor 22 by fixing (or to confirmedly setting) the electric potential Vs at the low second electric potential Vini are referred to as a threshold-voltage correction preparatory operation.
Then, at a time t3, the scan signal WS appearing on the scan line 31 is changed from a high electric potential WS_H to a low electric potential WS_L in order to end the threshold-voltage correction preparatory operation. In this way, the threshold-voltage correction preparatory operation is carried out on the correction-subject pixel row prior to a time t4 earlier than the start of the 1H period for the correction-subject pixel row.
Later on, at a time t4, in order to carry out each of an operation to write the input signal voltage Vsig and a mobility correction operation on a pixel row immediately preceding the correction-subject pixel row, the electric potential appearing on the signal line 33 provided for the immediately preceding pixel row is changed from the offset voltage Vofs to the input signal voltage Vsig. The operation to switch the electric potential appearing on the signal line 33 provided for the immediately preceding pixel row from the offset voltage Vofs to the input signal voltage Vsig is an operation carried out on the pixel row immediately preceding the correction-subject pixel row. Thus, after the time t4, each write transistor 23 on the correction-subject pixel row is sustained in a non-conductive state as shown in
Then, at a time t5, the electric potential appearing on the signal line 33 provided for the pixel row immediately preceding the correction-subject pixel row is changed back from the input signal voltage Vsig to the offset voltage Vofs in order to start the 1H period for the correction-subject pixel row.
Subsequently, at a time t6, the scan signal WS appearing on the scan line 31 is changed back to a high electric potential WS_H from a low electric potential WS_L in order to put the write transistor 23 in a conductive state as shown in
<Threshold-Voltage Correction Period>
Then, at the time t7, the electric potential DS appearing on the power-supply feed line 32 is changed from the low second electric potential Vini to the high first electric potential Vccp. Since the write transistor 23 is in a conductive state at that time, the electric potential Vs appearing on the source of the driving transistor 22 starts to rise. In due course of time, the electric potential Vs appearing on the source of the driving transistor 22 rises to an electric potential of (Vofs−Vth) as shown in
Here, for the sake of convenience, a period during which a voltage corresponding to the threshold voltage Vth of the driving transistor 22 is held in the voltage holding capacitor 24 is referred to as a threshold-voltage correction period. It is to be noted that, in order to flow a current exclusively to the voltage holding capacitor 24 and no current to the organic EL device 21 during the threshold-voltage correction period, the organic EL device 21 needs to be put in a cutoff state by setting the common power-supply feed line 34 at the electric potential Vcath.
Then, at a time t8, the electric potential WS appearing on the scan line 31 is changed from the high electric potential WS_H to the low electric potential WS_L in order to put the write transistor 23 in a non-conductive state as shown in
<Write Period and Mobility Correction Period>
Then, at a time t9, the electric potential appearing on the signal line 33 is changed from the offset voltage Vofs to the input signal voltage Vsig. Subsequently, at a time t10, the scan signal WS appearing on the scan line 31 is changed from the low electric potential WS_L to the high electric potential WS_H in order to put the write transistor 23 in a conductive state as shown in
In actuality, the write transistor 23 stores the input signal voltage Vsig in the holding capacitor 24 employed in the pixel circuit 20, setting the electric potential Vg appearing on the gate of the driving transistor 22 at the input signal voltage Vsig. Then, in an operation carried out to drive the driving transistor 22 by making use of the input signal voltage Vsig set on the gate of the driving transistor 22, the threshold voltage Vth of the driving transistor 22 is cancelled by the voltage held in advance in the holding capacitor 24 as a voltage corresponding to the threshold voltage Vth of the driving transistor 22, performing a threshold-voltage correction process.
At that time, since the organic EL device 21 is initially in a cutoff (high-impedance) state, a drain-source current Ids flowing from the power supply to the driving transistor 22 in accordance with an input signal voltage Vsig proceeds to the parasite capacitor Cel of the organic EL device 21. That is to say, a process to electrically charge the parasite capacitor Cel is started.
The process to electrically charge the parasite capacitor Cel causes the electric potential Vs appearing on the source of the driving transistor 22 to rise with the lapse of time. At that time, variations of the threshold voltage Vth of the driving transistor 22 have already been corrected. However, the drain-source current Ids flowing through the driving transistor 22 is dependent on the mobility μ of the driving transistor 22.
In due course of time, the electric potential Vs appearing on the source of the driving transistor 22 rises to a level of (Vofs−Vth+ΔV), making the voltage Vgs appearing between the gate and source of the driving transistor 22 equal to (Vsig−Vofs+Vth−ΔV). That is to say, the level of (Vsig−Vofs+Vth−ΔV) at which the voltage Vgs appearing between the gate and source of the driving transistor 22 is set is a result of a negative feedback to subtract the increase ΔV of the electric potential Vs appearing on the source of the driving transistor 22 from a voltage (Vsig−Vofs+Vth) held by the voltage holding capacitor 24. In other words, the negative feedback works to electrically discharge the voltage holding capacitor 24. Thus, the increase ΔV in electric potential Vs is the feedback quantity of the negative feedback.
By feeding back a negative feedback quantity ΔV proportional to the drain-source current Ids flowing through the driving transistor 22 to the gate of the driving transistor 22 as described above, that is, by applying the negative feedback quantity ΔV to the voltage Vgs appearing between the gate and source of the driving transistor 22, the dependence of the drain-source current Ids flowing through the driving transistor 22 on the mobility μ is eliminated. That is to say, a mobility correction operation is carried out to correct the variations of the mobility μ.
To put it more concretely, the higher the input signal voltage Vsig representing the video signal is, the larger the drain-source current Ids flowing through the driving transistor 22 becomes and, hence, the larger the absolute value of the feedback quantity ΔV of the negative feedback becomes. In the following description, the feedback quantity ΔV of the negative feedback is also referred to as a correction quantity ΔV. Thus, the mobility correction operation is carried out in accordance with the level of the luminance of a light beam generated by the organic EL device 21. In addition, with the input signal voltage Vsig of the video signal kept at a constant value, the larger the mobility μ of the driving transistor 22 is, the larger the absolute value of the feedback quantity ΔV of the negative feedback becomes. Thus, variations of the mobility μ from pixel to pixel can be eliminated.
<Light Emitting Period>
Then, at a time t11, the electric potential WS appearing on the scan line 31 is changed from a high electric potential WS_H to a low electric potential WS_L in order to put the write transistor 23 in a non-conductive state as shown in
The increase of the electric potential appearing on the anode of the organic EL device 21 is no other than an increase of the electric potential Vs appearing on the source of the driving transistor 22. As the electric potential Vs appearing on the source of the driving transistor 22 rises, the electric potential Vg appearing on the gate of the driving transistor 22 also rises as well in an interlocked manner due to a bootstrap operation of the voltage holding capacitor 24. At that time, the increase of the electric potential Vg appearing on the gate of the driving transistor 22 is equal to the increase of the electric potential Vs appearing on the source of the driving transistor 22. Therefore, in a light emitting period, the voltage Vgs appearing between the gate and source of the driving transistor 22 is sustained at the level of (Vsig−Vofs+Vth−ΔV). Then, at a time t12, the electric potential appearing on the signal line 33 changes from the input signal voltage Vsig representing the video signal to the offset voltage Vofs.
(Principle of the Threshold-Voltage Correction)
The principle of an operation to correct the threshold voltage Vth of the driving transistor 22 is explained as follows. Designed to operate in a saturated region, the driving transistor 22 functions as a constant current source. Thus, the driving transistor 22 supplies a driving current Ids to the organic EL device 21. Also referred to hereafter as a drain-source current Ids, the driving current Ids has a fixed magnitude expressed by following Eq. (1).
Ids=(½)*μ(W/L)Cox(Vgs−Vth)2 (1)
Notation W denotes the channel width of the driving transistor 22, notation L denotes the channel length of the driving transistor 22 and notation Cox denotes a gate capacity per unit area of the driving transistor 22.
In the case of the pixel (or the pixel circuit) 20 having the configuration described above, on the other hand, the gate-source voltage Vgs appearing between the gate and source of the driving transistor 22 is (Vsig−Vofs+Vth−ΔV) as described above. Inserting (Vsig−Vofs+Vth−ΔV) into Eq. (1) as a substitute for the gate-source voltage Vgs yields the following expression of the drain-source current Ids:
Ids=(½)*μ(W/L)Cox(Vsig−Vofs−ΔV)2 (2)
That is to say, the term of the threshold voltage Vth of the driving transistor 22 is eliminated from Eq. (1) in a process referred to as the threshold-voltage correction operation to result in a drain-source current Ids expressed by Eq. (2). In other words, by virtue of the threshold-voltage correction operation, the drain-source current Ids supplied by the driving transistor 22 to the organic EL device 21 no longer depends on the threshold voltage Vth of the driving transistor 22. Thus, for a given gate-source voltage Vgs appearing between the gate and source, the drain-source current Ids does not change even if the threshold voltage Vth of the driving transistor 22 varies from pixel to pixel due to variations of the process to fabricate the driving transistor 22 and/or due to changes with the lapse of time. As a result, for a given gate-source voltage Vgs appearing between the gate and source, the organic EL device 21 generates a light beam with a luminance that does not vary from pixel to pixel and does not vary with the lapse of time.
(Principle of the Mobility Correction)
Next, the principle of an operation to correct the mobility of the driving transistor 22 is explained as follows.
If there is a difference in mobility μ of the driving transistor 22 between pixel circuits A and B, unless a process to correct the mobility μ in one way or another is carried out, the drain-source current Ids1′ flowing through the driving transistor 22 in pixel circuit A having a relatively large mobility μ of the driving transistor 22 is much greater than the drain-source current Ids2′ flowing through the driving transistor 22 in pixel circuit B having a relatively small mobility μ of the driving transistor 22 even if input signal voltages Vsig of the same level are applied to pixel circuits A and B. If the drain-source current Ids flowing in a pixel circuit is much different from the drain-source current Ids flowing in another pixel circuit due to mobility (μ) variations from pixel to pixel as described above, pixel-circuit uniformity is lost.
As is obvious from the transistor characteristic equation expressed by Eq. (1) given before, the larger the mobility μ is, the larger the drain-source current Ids becomes. Thus, the larger the mobility μ is, the larger the feedback quantity ΔV of the negative feedback becomes. As shown in
To put it concretely, if the mobility correction process making use of the feedback quantity ΔV1 is carried out on pixel circuit A having a driving transistor 22 with a relatively large mobility μ, the drain-source current Ids flowing through the driving transistor 22 is much reduced from the drain-source current Ids1′ to a drain-source current Ids1. If the mobility correction process making use of the feedback quantity ΔV2 is carried out on pixel circuit B having a driving transistor 22 with a relatively small mobility μ, on the other hand, the drain-source current Ids flowing through the driving transistor 22 is reduced from the drain-source current Ids2′ to a drain-source current Ids2 but the reduction of the drain-source current Ids is not so large as pixel circuit A. This is because the feedback quantity ΔV2 applied to pixel circuit B is smaller than the feedback quantity ΔV1 applied to pixel circuit A. As a result, the drain-source current Ids1 flowing through the driving transistor 22 of pixel circuit A becomes approximately equal to the drain-source current Ids2 flowing through the driving transistor 22 of pixel circuit B by virtue of the mobility correction process carried out on the mobility μ.
To sum up, if pixel circuits A and B with different mobilities μ exist, the feedback quantity ΔV1 applied to pixel circuit A having a driving transistor 22 with a relatively large mobility μ is greater than the feedback quantity ΔV2 applied to pixel circuit B having a driving transistor 22 with a relatively small mobility μ. That is to say, the larger the mobility μ of a pixel circuit is, the larger the feedback quantity ΔV applied to the pixel circuit becomes and the larger the decrease in drain-source current Ids becomes. Thus, by negatively feeding the drain-source current Ids of the driving transistor 22 back to the side of the input signal voltage Vsig, the magnitudes of the drain-source currents Ids flowing through driving transistors 22 included in pixel circuits as transistors having different mobilities μ can be made uniform. As a result, variations in mobility μ can be eliminated in the mobility correction process.
To be more specific,
For the case in which the threshold-voltage correction operation is carried out but the mobility correction operation is not, on the other hand, for the same input signal voltage Vsig, the difference in drain-source current Ids between pixel circuits A and B is reduced to a certain degree even though the difference still exists as shown in
For the case in which both the threshold-voltage correction operation and the mobility correction operation are carried out, for the same input signal voltage Vsig, the difference in drain-source current Ids between pixel circuits A and B is all but zero as shown in
As described above, in the embodiment implementing the organic EL display apparatus 10 provided with correction functions such as the threshold-voltage correction function and the mobility correction function, instead of executing a threshold-voltage correction preparatory operation and a threshold-voltage correction operation for every correction-subject pixel row in the same 1H period, before the threshold-voltage correction operation is carried out on the correction-subject pixel row in the 1H period, the threshold-voltage correction preparatory operation is performed on the correction-subject pixel row prior to the 1H period in order to fix each of an electric potential Vg appearing on the gate of the driving transistor 22 and an electric potential Vs appearing on the source of the driving transistor 22 at a predetermined level. In the threshold-voltage correction preparatory operation, the electric potential Vg appearing on the gate of the driving transistor 22 and the electric potential Vs appearing on the source of the driving transistor 22 are fixed at typically the offset voltage Vofs and the low second electric potential Vini respectively. Thus, it is no longer necessary to allocate sufficient time of the 1H period provided for the correction-subject pixel row to the threshold-voltage correction preparatory operation as a threshold-voltage correction preparation period. As a result, the threshold-voltage correction period and the mobility correction period can be prolonged totally by the threshold-voltage correction preparation period of the threshold-voltage correction preparatory operation.
Therefore, since it is possible to allocate sufficient time to each of the threshold-voltage correction operation and the mobility correction operation as a threshold-voltage correction period and a mobility correction period respectively, it is also possible to assure that the threshold-voltage correction operation and the mobility correction operation can each be carried out with a high degree of reliability. Accordingly, since variations of the characteristics of the driving transistor 22 from pixel to pixel and deteriorations of the organic EL device 21 with the lapse of time can be suppressed effectively, a high-quality uniform image with neither unevenness nor shading can be displayed. As described earlier, the characteristics of the driving transistor 22 include the threshold voltage Vth and mobility μ of the driving transistor 22. Also as explained before, the variations in transistor characteristics are attributed to variations of the process to fabricate the driving transistor 22 and characteristic changes with the lapse of time.
In particular, a threshold-voltage correction preparatory operation performed on a correction-subject pixel row prior to an 1H period in which a threshold-voltage correction operation is carried out on the correction-subject pixel row is an operation that is optimal for an operation to drive a display apparatus described as follows.
There is a rising demand for a high-resolution display apparatus employed in mobile electronic equipment such as a hand phone for displaying for example a fine map and characters. In addition, as the resolution of the display apparatus is raised, the horizontal scan period having a length of 1H is shortened. Thus, it becomes difficult to allocate sufficient time of the horizontal scan period having a length of 1H to each of the threshold-voltage correction operation and the mobility correction operation as a threshold-voltage correction period and a mobility correction period respectively.
As described above, the higher resolution of an organic EL display apparatus entails a larger number of pixel circuits and a short horizontal scan period having a length of 1H in comparison with the horizontal scan period for the resolution in the related art in the related art. Nevertheless, if a driving method is applied to such an organic EL display apparatus as a method whereby a threshold-voltage correction preparatory operation is performed on a correction-subject pixel row prior to an 1H period allocated to a threshold-voltage correction operation carried out on the correction-subject pixel row, it is possible to allocate sufficient time to each of the threshold-voltage correction operation and the mobility correction operation as a threshold-voltage correction period and a mobility correction period respectively. It is thus possible to assure that the threshold-voltage correction operation and the mobility correction operation can each be carried out with a high degree of reliability. Accordingly, since variations of the characteristics of the driving transistor 22 from pixel to pixel and deteriorations of the organic EL device 21 with the lapse of time can be suppressed effectively, an image having a high quality can be displayed.
In addition, in order to reduce the cost, even an organic EL display apparatus employing pixel circuits 20 each including a driving transistor having a small mobility μ also adopts a method whereby a threshold-voltage correction preparatory operation is performed on a correction-subject pixel row prior to an 1H period allocated to a threshold-voltage correction operation carried out on the correction-subject pixel row so that it is possible to allocate sufficient time to each of the threshold-voltage correction operation and the mobility correction operation as a threshold-voltage correction period and a mobility correction period respectively. It is thus possible to assure that the threshold-voltage correction operation and the mobility correction operation can each be carried out with a high degree of reliability. Accordingly, since variations of the characteristics of the driving transistor 22 from pixel to pixel and deteriorations of the organic EL device 21 with the lapse of time can be suppressed effectively, an image having a high quality can be displayed. An example of the transistor having a small mobility μ is a transistor made of a-Si (amorphous silicon).
<Organic EL Display Apparatus Adopting a Selector Driving Method>
The organic EL display apparatus 10 according to the embodiment described above as an example has a configuration in which the horizontal driving circuit 60 is implemented on the display panel 70. However, it is also possible to design an organic EL display apparatus 10 into a configuration in which the horizontal driving circuit 60 is provided externally to the display panel 70 and supplies video signals to the signal lines 33 (that is, signal lines 33-1 to 33-n) on the display panel 70 through external wires.
In the case of the configuration in which a video signal is received from a source external to the display panel 70, the external wires and the signal lines 33 are wired separately for each of the R (red), G (green) and B (blue) colors to result in the so-called Full HD (High Definition) having a resolution of 1,920×1,080. With such a high resolution, however, 5,760 (=1,920×3) external wires are required. That is to say, the number of wires each used as an external wire is large.
In order to reduce the number of wires each used as an external wire, the configuration adopts a selector driving method whereby a plurality of signal lines 33 provided on the display panel 70 as a group or a set are assigned to one output of a driver IC serving as the horizontal driving circuit 60 external to the display panel 70. Then, the signal lines 33 assigned to the output are selected sequentially one line after another on a time-division basis and video signals generated at different levels along the time axis as the output of the driver IC are allocated and supplied to the set of signal lines 33 in an operation to drive the signal lines 33. This selector driving method is also referred to as a time-division driving method.
To put it concretely, in accordance with the selector driving method, each output of the IC driver serving as the horizontal driving circuit 60 is assigned to x signal lines 33 provided on the display panel 70 where x is integer equal to or greater than 2. Then, the x signal lines 33 assigned to an output of the IC driver are selected sequentially one line after another during x time divisions allocated to the signal lines 33 respectively. By adopting the selector driving method, the number of outputs of the IC driver and the number of wires each serving as an external wire can each be reduced to 1/x times the total number of signal lines 33.
A typical configuration adopting the selector driving method is shown in
In the case of an organic EL display apparatus adopting the selector driving method (or the time-division driving method), however, it is necessary to provide a signal-line electric-potential write period for asserting different input signal voltages Vsig representing the R, G and B video signals on every three signal lines 33 respectively of the signal lines 33-1 to 33-n through the selector switches SEL_R, SEL_G and SEL_B as shown in timing charts of
As described above, in the case of an organic EL display apparatus 10′ adopting the selector driving method whereby typically R, G and B video signals are written into three pixel circuits (i.e., R, G and B pixel circuits) respectively during a 1H period, it is necessary to provide a signal-line electric-potential write period for asserting the input signal voltages Vsig representing the R, G and B video signals on every three signal lines 33 respectively of the signal lines 33-1 to 33-n. Nevertheless, if the organic EL display apparatus 10′ adopts the method whereby a threshold-voltage correction preparatory operation is performed on a correction-subject pixel row prior to an 1H period allocated to a threshold-voltage correction operation carried out on the correction-subject pixel row, it is possible to allocate sufficient time to the threshold-voltage correction operation and the mobility correction operation as a threshold-voltage correction period and a mobility correction period respectively. Accordingly, since variations of the characteristics of the driving transistor 22 from pixel to pixel and deteriorations of the organic EL device 21 with the lapse of time can be suppressed effectively, an image having a high quality can be displayed.
The embodiment described above implements a typical organic EL display apparatus 10 provided with both a threshold-voltage correction function and a mobility correction function. However, the organic EL display apparatus can be provided with only the threshold-voltage correction function without the mobility correction function. Even in the case of such an organic EL display apparatus, the threshold-voltage correction preparatory operation can be performed on a correction-subject pixel row prior to an 1H period allocated to a threshold-voltage correction operation carried out on the correction-subject pixel row so that it is possible to allocate long time of the 1H period to the threshold-voltage correction operation as a threshold-voltage correction period in comparison with the case in which a threshold-voltage correction preparatory operation is performed on a correction-subject pixel row in the same 1H period as a threshold-voltage correction preparatory operation carried out on the same correction-subject pixel row. Thus, the threshold-voltage correction operation can be carried out with a high degree of reliability.
In addition, the embodiment described above implements a typical organic EL display apparatus 10 having a configuration in which each pixel circuit 20 employs two transistors, i. e. the write transistor 23 and the driving transistor 22, whereas a mobility correction operation is carried out in the same period as an operation to write the input signal voltage Vsig into the pixel circuit 20. It is to be noted, however, that the scope of the present invention is by no means limited to this embodiment. For example, as disclosed in patent reference 1, the present invention can also be applied to an organic EL display apparatus designed into a configuration in which each pixel circuit 20 is further provided with a switching transistor connected in series to the driving transistor 22 as a transistor for controlling the light emitting period/the no-light emitting period of the organic EL device 21 and for carrying out a mobility correction operation prior to the operation to write the input signal voltage Vsig into the pixel circuit 20.
If the mobility correction operation is carried out in the same period as the operation to write the input signal voltage Vsig into the pixel circuit 20 as is the case with the configuration of the organic EL display apparatus 10 according to the embodiment, however, it is not necessary to allocate time to the signal write operation as a signal write period separated from the mobility correction period. Thus, the embodiment described above offers a merit that it is possible to allocate sufficiently long time to each of the threshold-voltage correction operation and the mobility correction operation as a threshold-voltage correction period and a mobility correction period respectively.
In addition, the embodiment described above implements a typical organic EL display apparatus 10 employing organic EL devices each serving as the electro-optical device of a pixel circuit 20. It is to be noted, however, that the scope of the present invention is by no means limited to this embodiment. That is to say, the present invention can also be applied to a general display apparatus employing any current-driven electro-optical devices (or any current-driven light emitting devices) as long as the electro-optical devices each generates a light beam with the luminance thereof determined by a driving current flowing through the device.
[Typical Applications]
The display apparatus according to the embodiments described above are typically applied to various kinds of electronic equipment shown in
It is to be noted that the display apparatuses according to the present invention include a display apparatus having a sealed module configuration. In a typical sealed module configuration, a display module pasted to an opposed member such as a piece of transparent glass corresponds to the pixel array section 30. On the opposed transparent member, it is also possible to provide a color filter, a protection film, a light shielding film described earlier and another component. It is also worth noting that, on the display module, it is possible to provide a circuit or an FPC (Flexible Printed Circuit). The circuit is used for inputting a signal from an external source and supplying the signal to the pixel array section 30 and used for outputting a signal received from the pixel array section 30 to an external target.
In addition, it should be understood by those skilled in the art that a variety of modifications, combinations, sub-combinations and alterations may occur, depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-079037 | Mar 2007 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 20070115225 | Uchino et al. | May 2007 | A1 |
| Number | Date | Country |
|---|---|---|
| 2002-215096 | Jul 2002 | JP |
| 2006-133542 | May 2006 | JP |
| 2008-122632 | May 2008 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20080238901 A1 | Oct 2008 | US |
