JP2011145479A - Display device, and display driving method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To eliminate a difference in emission luminance due to a difference in gate voltage drop immediately before light emission in pixels in a horizontal direction of a panel, which is caused by blunting of a scanning pulse. <P>SOLUTION: When threshold correction in a plurality of times is carried out in a non-emission period of one light emission cycle of each pixel circuit, in threshold correction in the second time for example, the pulse width of a scanning pulse WS for conducting a sampling transistor is reduced than a pulse width in other threshold correction. This induces difference in a gate-source voltage Vgs of drive transistors in the horizontal direction of the panel when the threshold correction is finished, so as to offset the difference in the gate voltage drop immediately before light emission. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a display device having a pixel array in which pixel circuits are arranged in a matrix, and a display driving method thereof, for example, a display device using an organic electroluminescence element (organic EL element) as a light emitting element.

JP 2007-133282 A JP 2003-255856 A JP 2003-271095 A

For example, as can be seen in Patent Documents 2 and 3, image display apparatuses using organic EL elements as pixels have been developed. Since the organic EL element is a self-luminous element, it has advantages such as higher image visibility than a liquid crystal display, no need for a backlight, and high response speed. Further, the luminance level (gradation) of each light emitting element can be controlled by the value of the current flowing therethrough (so-called current control type).
In the organic EL display, similarly to the liquid crystal display, there are a simple matrix method and an active matrix method as driving methods. Although the former has a simple structure, there is a problem that it is difficult to realize a large-sized and high-definition display. Therefore, the active matrix method is actively developed at present. In this method, a current flowing through a light emitting element in each pixel circuit is controlled by an active element (generally a thin film transistor: TFT) provided in the pixel circuit.

By the way, as a pixel circuit configuration using an organic EL element, improvement in display quality by eliminating luminance unevenness for each pixel, high luminance, high definition, and high frame rate (high frequency) are strongly demanded. Yes. Development of larger panels is also underway.
From these viewpoints, various configurations are being studied. For example, as in Patent Document 1, various pixel circuit configurations and operations have been proposed in which variations in the threshold voltage and mobility of the drive transistor for each pixel are canceled to eliminate luminance unevenness for each pixel.
An object of the present invention is to realize a pixel circuit operation suitable for high frequency and large panel as a display device using an organic EL element.

The display device of the present invention is electrically connected to a light emitting element and a driving transistor that applies a current corresponding to a gate-source voltage to the light emitting element by applying a driving voltage between the drain and the source. A sampling transistor that inputs a signal line voltage to the gate of the drive transistor, and a storage capacitor that is connected between the gate and source of the drive transistor and holds the threshold voltage of the drive transistor and the input video signal voltage. A pixel circuit having a pixel circuit having a matrix circuit and a threshold correction reference voltage and a video signal voltage as the signal line voltage are supplied to a pixel array arranged in a matrix and each signal line arranged in a column on the pixel array. A power supply pulse is applied to the signal selector and each power control line arranged in a row on the pixel array, and the drive transistor of the pixel circuit is supplied. A drive control scanner for applying a drive voltage to the pixel array, and a scanning pulse is applied to each write control line arranged in a row on the pixel array to control the sampling transistor of the pixel circuit and to each pixel circuit The signal line voltage is a write scanner that executes input of the threshold correction reference voltage and video signal voltage of the pixel circuit so that threshold correction is executed a plurality of times during a non-light emission period of one light emission cycle of each pixel circuit. The sampling transistor is turned on by the scan pulse a plurality of times when the threshold correction reference voltage is used, and at least one of the plurality of times of the threshold correction, the scan pulse for turning on the sampling transistor is turned on. And a writing scanner that makes the pulse width shorter than in other threshold corrections.
For example, in the writing scanner, the pulse width of the scanning pulse for turning on the sampling transistor is made shorter in the second threshold correction than in other threshold corrections.
The writing scanner outputs the scan pulse so that the threshold correction is completed a plurality of times immediately before the gate-source voltage of the driving transistor reaches the threshold voltage of the driving transistor.

  As described above, the display drive method of the present invention is a display drive method for a display device including a pixel array, a signal selector, a drive control scanner, and a write scanner. The sampling transistor is turned on by the scan pulse a plurality of times when the signal line voltage is the threshold correction reference voltage so that the threshold correction is performed a plurality of times during the non-light emission period of one light emission cycle. This is a display driving method in which the pulse width of the scanning pulse for turning on the sampling transistor is shorter than that in other threshold corrections at the time of threshold correction at least once.

In the present invention, the threshold correction is performed a plurality of times in each pixel circuit in one light emission cycle so that the threshold correction operation period can be extended even when a high frame rate is achieved. Here, considering the increase in the size of the panel, the waveform of the scan pulse applied becomes duller as the pixel circuit becomes farther from the writing scanner due to the resistance component and the capacitance component of the writing control line.
Due to the difference in the waveform of the scan pulse, the amount of decrease in the gate voltage of the drive transistor caused by the coupling when the scan pulse is turned off immediately after the video signal voltage is written is reduced between the left and right pixel circuits (the pixel circuit close to the write scanner and the far pixel). Circuit). This appears as a gradation in the horizontal direction of the panel.
Therefore, in the present invention, at the time of threshold correction at least once among a plurality of times, the pulse width of the scanning pulse for making the sampling transistor conductive is made shorter than at the time of other threshold corrections. Then, the left and right pixel circuits can have a difference in the gate-source voltage of the driving transistor when a plurality of threshold correction operations are completed. This difference in gate-source voltage cancels the difference in gate voltage drop due to the coupling.

  According to the present invention, it is possible to reduce gradation due to a difference in light emission luminance of each pixel circuit in the left-right direction of the panel (the direction of the write control line) that occurs due to the dullness of the scan pulse waveform on the write control line. Accordingly, there is an effect that it is possible to provide a display device that can prevent display quality from being deteriorated and is adapted to an increase in panel size.

It is explanatory drawing of a structure of the display apparatus of embodiment of this invention. It is a circuit diagram of a pixel circuit of an embodiment. It is explanatory drawing of pixel circuit operation | movement in the case of performing division | segmentation threshold value correction | amendment. It is explanatory drawing of the influence of the waveform blunting of a scanning pulse. It is an equivalent circuit diagram of the process of the light emission operation of one cycle of the pixel circuit. It is an equivalent circuit diagram of the process of the light emission operation of one cycle of the pixel circuit. It is an equivalent circuit diagram of the process of the light emission operation of one cycle of the pixel circuit. FIG. 11 is an explanatory diagram of the operation of the pixel circuit of the embodiment. FIG. 11 is an explanatory diagram of the operation of the pixel circuit of the embodiment.

Hereinafter, embodiments of the present invention will be described in the following order.
[1. Configuration of Display Device and Pixel Circuit]
[2. Pixel circuit operation considered in the process leading to the present invention: division threshold correction]
[3. Pixel Circuit Operation of Embodiment]

[1. Configuration of Display Device and Pixel Circuit]

FIG. 1 shows a configuration of an organic EL display device according to an embodiment.
This organic EL display device includes a pixel circuit 10 that uses an organic EL element as a light emitting element and performs light emission driving by an active matrix method.
As illustrated, the organic EL display device includes a pixel array 20 in which a large number of pixel circuits 10 are arranged in a matrix in the column direction and the row direction (m rows × n columns). Each of the pixel circuits 10 is a light emitting pixel of any one of R (red), G (green), and B (blue), and a color display device is configured by arranging the pixel circuits 10 of each color according to a predetermined rule. .

As a configuration for driving each pixel circuit 10 to emit light, a horizontal selector 11, a drive scanner 12, and a write scanner 13 are provided.
Also, signal lines DTL1, DTL2,... DTL (n), which are selected by the horizontal selector 11 and supply a voltage corresponding to the signal value (gradation value) of the luminance signal as display data to the pixel circuit 10, are on the pixel array. It is arranged in the column direction. The signal lines DTL1, DTL2,... DTL (n) are arranged by the number of columns (n columns) of the pixel circuits 10 arranged in a matrix in the pixel array 20.

  On the pixel array 20, write control lines WSL1, WSL2,... WSL (m) and power supply control lines DSL1, DSL2,. These write control lines WSL and power supply control lines DSL are arranged by the number of rows (m rows) of the pixel circuits 10 arranged in a matrix in the pixel array 20, respectively.

Write control lines WSL (WSL1 to WSL (m)) are driven by the write scanner 13.
The write scanner 13 sequentially supplies scanning pulses WS (WS1, WS2,... WS (m)) to each of the write control lines WSL1 to WSL (m) arranged in rows at a predetermined timing set. Then, the pixel circuit 10 is line-sequentially scanned in units of rows.

The power supply control lines DSL (DSL1 to DSL (m)) are driven by the drive scanner 12. The drive scanner 12 supplies power pulses DS (DS1, DS2,... DS (m)) to the power supply control lines DSL1 to DSL (m) arranged in a row in accordance with the line sequential scanning by the write scanner 13. To do. The power supply pulse DS (DS1, DS2,... DS (m)) is a pulse voltage that switches between two values of the drive voltage Vcc and the initial voltage Vini.
The drive scanner 12 and the write scanner 13 set the timing of the scanning pulse WS and the power supply pulse DS based on the clock ck and the start pulse sp.

The horizontal selector 11 supplies a signal line voltage as an input signal to the pixel circuit 10 to the signal lines DTL1, DTL2,... Arranged in the column direction in accordance with the line sequential scanning by the write scanner 13.
In the present embodiment, the horizontal selector 11 supplies a threshold correction reference voltage Vofs and a video signal voltage Vsig as signal line voltages to each signal line.

  In the display device of this embodiment, an example of a signal selector referred to in the present invention is the horizontal selector 11, an example of a drive control scanner is a drive scanner, and an example of a writing scanner is a write scanner 13. Become.

FIG. 2 shows a configuration example of the pixel circuit 10. The pixel circuits 10 are arranged in a matrix like the pixel circuits 10 in the configuration of FIG.
In FIG. 2, only one pixel circuit 10 arranged at a portion where the signal line DTL intersects with the write control line WSL and the power supply control line DSL is shown for simplification.

  The pixel circuit 10 includes an organic EL element 1 that is a light emitting element, a storage capacitor Cs, a sampling transistor Ts, and an n-channel thin film transistor (TFT) as a driving transistor Td. Note that the capacitance Coled is a parasitic capacitance of the organic EL element 1.

The storage capacitor Cs has one terminal connected to the source (node ND2) of the drive transistor Td and the other terminal connected to the gate (node ND1) of the drive transistor Td.
The light emitting element of the pixel circuit 10 is, for example, the organic EL element 1 having a diode structure, and includes an anode and a cathode. The anode of the organic EL element 1 is connected to the source of the drive transistor Td, and the cathode is connected to a predetermined wiring (cathode potential Vcat).

The sampling transistor Ts has one end of its drain and source connected to the signal line DTL and the other end connected to the gate of the driving transistor Td.
The gate of the sampling transistor Ts is connected to the write control line WSL.
The drain of the drive transistor Td is connected to the power supply control line DSL.

The light emission driving of the organic EL element 1 is basically as follows.
At the timing when the video signal voltage Vsig is applied to the signal line DTL, the sampling transistor Ts is turned on by the scanning pulse WS supplied from the write scanner 13 by the write control line WSL. As a result, the video signal voltage Vsig from the signal line DTL is written to the storage capacitor Cs.

The drive transistor Td causes the current Ids to flow through the organic EL element 1 by supplying current from the power supply control line DSL to which the drive potential Vcc is applied by the drive scanner 12, and causes the organic EL element 1 to emit light.
At this time, the current Ids becomes a value corresponding to the gate-source voltage Vgs of the driving transistor Td (a value corresponding to the voltage held in the holding capacitor Cs), and the organic EL element 1 emits light with luminance corresponding to the current value. To do.
That is, in the case of this pixel circuit 10, by writing the video signal voltage Vsig from the signal line DTL to the storage capacitor Cs, the gate applied voltage of the drive transistor Td is changed, thereby controlling the value of the current flowing through the organic EL element 1. To obtain the gradation of light emission.

Since the drive transistor Td is designed to always operate in the saturation region, the drive transistor Td becomes a constant current source having a value represented by the following expression 1.
Ids = (1/2) · μ · (W / L) · Cox · (Vgs−Vth) ² (Equation 1)
Where Ids is the current flowing between the drain and source of a transistor operating in the saturation region, μ is the mobility, W is the channel width, L is the channel length, Cox is the gate capacitance, and Vth is the threshold voltage of the driving transistor Td. Yes.
As apparent from Equation 1, the drain current Ids is controlled by the gate-source voltage Vgs in the saturation region. Since the gate-source voltage Vgs is kept constant, the drive transistor Td operates as a constant current source, and can emit the organic EL element 1 with constant luminance.

In this way, basically, in each frame period, an operation is performed in which the video signal value (gradation value) Vsig is written in the storage capacitor Cs in the pixel circuit 10, and thereby the driving transistor is selected according to the gradation to be displayed. The gate-source voltage Vgs of Td is determined.
The drive transistor Td functions as a constant current source for the organic EL element 1 by operating in the saturation region, and a current corresponding to the gate-source voltage Vgs is supplied to the organic EL element 1 so that each frame period is The organic EL element 1 emits light with a luminance corresponding to the gradation value of the video signal.

[2. Pixel circuit operation considered in the process leading to the present invention: division threshold correction]

Here, in order to understand the present invention, the pixel circuit operation considered in the process leading to the present invention will be described. This is a circuit operation including a threshold correction operation and a mobility correction operation for compensating for uniformity deterioration due to variations in the threshold and mobility of the driving transistor Td of each pixel circuit 10. In particular, the threshold correction operation is an example in which division threshold correction is performed a plurality of times by dividing within one light emission cycle.

In the pixel circuit operation, the threshold value correction operation and the mobility correction operation itself have been performed conventionally. This necessity will be briefly described.
For example, in a pixel circuit using a polysilicon TFT or the like, the threshold voltage Vth of the drive transistor Td and the mobility μ of the semiconductor thin film constituting the channel of the drive transistor Td may change over time. Further, the transistor characteristics of the threshold voltage Vth and the mobility μ are different for each pixel due to variations in the manufacturing process.
If the threshold voltage and mobility of the drive transistor Td differ from pixel to pixel, the current value flowing through the drive transistor Td varies from pixel to pixel. For this reason, even if the same video signal value (video signal voltage Vsig) is given to all the pixel circuits 10, the light emission luminance of the organic EL element 1 varies from pixel to pixel. As a result, the screen uniformity (uniformity) ) Is damaged.
For this reason, the pixel circuit operation is provided with a correction function for fluctuations in the threshold voltage Vth and the mobility μ.

FIG. 3 shows a timing chart of the operation of the pixel circuit 10 in one light emission cycle (one frame period).
FIG. 3 shows the signal line voltage that the horizontal selector 11 applies to the signal line DTL. In the case of this operation example, the horizontal selector 11 supplies the signal line DTL with the pulse voltage as the threshold correction reference voltage Vofs and the video signal voltage Vsig in one horizontal period (1H) as the signal line voltage.
FIG. 3 shows a power pulse DS supplied from the drive scanner 12 via the power control line DSL. The drive voltage Vcc or the initial voltage Vini is given as the power supply pulse DS.
FIG. 3 shows a scan pulse WS applied to the gate of the sampling transistor Ts by the write scanner 13 via the write control line WSL. The n-channel sampling transistor Ts is turned on when the scanning pulse WS is set to the H level, and is turned off when the scanning pulse WS is set to the L level.
FIG. 3 shows changes in the gate voltage Vg and the source voltage Vs of the drive transistor Td as the voltages of the nodes ND1 and ND2 shown in FIG.

A time point ts in the timing chart of FIG. 3 is a start timing of one cycle in which the organic EL element 1 as a light emitting element is driven to emit light, for example, one frame period of image display.
Before reaching this time point ts (period LT0), light emission of the previous frame is performed. An equivalent circuit of the period LT0 is illustrated in FIG.
That is, the light emission state of the organic EL element 1 is a state where the power supply pulse DS is the drive voltage Vcc and the sampling transistor Ts is turned off. At this time, since the drive transistor Td is set to operate in the saturation region, the current Ids ′ flowing through the organic EL element 1 is expressed by the above-described equation 1 according to the gate-source voltage Vgs of the drive transistor Td. Value.

The operation for light emission of the current frame is started at time ts.
First, the power supply pulse DS is set to the initial potential Vini. FIG. 5B shows an equivalent circuit of the period LT1.
At this time, when the initial potential Vini is smaller than the sum of the threshold voltage Vthel and the cathode voltage Vcat of the organic EL element 1, that is, Vini ≦ Vthel + Vcat, the organic EL element 1 is extinguished and a non-light emitting period is started. At this time, the power supply control line DSL becomes the source of the drive transistor Td. The anode (node ND2) of the organic EL element 1 is charged to the initial potential Vini.

After a certain period, preparation for threshold correction is performed (periods LT2a, LT2b). An equivalent circuit is shown in FIG.
That is, in the periods LT2a and LT2b, when the potential of the signal line DTL becomes the threshold correction reference voltage Vofs, the scanning pulse WS is set to the H level, and the sampling transistor Ts is turned on. Therefore, the gate (node ND1) of the drive transistor Td becomes the threshold correction reference voltage Vofs.
The gate-source voltage Vgs of the drive transistor Td is Vgs = Vofs−Vini.
Since the threshold value correction operation cannot be performed unless this Vofs−Vini is larger than the threshold voltage Vth of the drive transistor Td, the initial potential Vini and the reference voltage Vofs are set so that Vofs−Vini> Vth. .
That is, as a preparation for threshold correction, the gate-source voltage of the drive transistor is sufficiently widened than the threshold voltage Vth.

Subsequently, threshold correction (Vth correction) is performed. Here, an example is shown in which threshold correction is performed four times during the periods LT3a to LT3d.
First, during the period LT3a, the first threshold correction (Vth correction) is performed.
In this case, at the timing when the signal line voltage becomes the threshold correction reference voltage Vofs, the write scanner 13 sets the scanning pulse WS to the H level, and the drive scanner 12 sets the power supply pulse DS to the driving voltage Vcc. FIG. 6B shows an equivalent circuit. In this case, the anode (node ND2) of the organic EL element 1 serves as the source of the drive transistor Td, and a current flows. Therefore, the source node rises while the gate (node ND1) of the drive transistor Td is fixed to the threshold correction reference voltage Vofs.
As long as the anode potential of the organic EL element 1 (potential of the node ND2) is equal to or lower than Vcat + Vthel (threshold voltage of the organic EL element 1), the current of the drive transistor Td is used to charge the storage capacitor Cs and the capacitor Coled. “As long as the anode potential of the organic EL element 1 is equal to or lower than Vcat + Vthel” means that the leakage current of the organic EL element 1 is considerably smaller than the current flowing through the drive transistor Td.
For this reason, the potential of the node ND2 (the source potential of the driving transistor Td) increases with time.

This threshold correction is basically an operation of setting the gate-source voltage of the drive transistor Td to the threshold voltage Vth. Therefore, the source potential of the drive transistor Td only needs to be raised until the gate-source voltage of the drive transistor Td reaches the threshold voltage Vth.
However, the gate node can be fixed to the threshold correction reference voltage Vofs only during the period of the signal line voltage = Vofs. Then, depending on the frame rate or the like, sufficient time for the source potential to rise cannot be taken by the threshold correction operation once until the gate-source voltage reaches the threshold voltage Vth. Therefore, the threshold value correction is performed in a plurality of times.

For this reason, the threshold correction as the period LT3a is ended before the signal line voltage = the video signal voltage Vsig. That is, the write scanner 13 once sets the scanning pulse WS to L level and turns off the sampling transistor Ts.
At this time, since both the gate and the source are floating, a current flows between the drain and the source in accordance with the gate-source voltage Vgs and bootstraps. That is, the gate potential and the source potential rise as shown.

Next, in the period LT3b, the second threshold correction is performed. That is, when signal line voltage = threshold correction reference voltage Vofs, the write scanner 13 sets the scanning pulse WS to H level again and turns on the sampling transistor Ts. As a result, the gate voltage of the drive transistor Td = the threshold correction reference voltage Vofs, and the source potential is increased.
Further, the threshold correction operation is paused. Since the gate-source voltage of the drive transistor Td is closer to the threshold voltage Vth in the second threshold correction, the bootstrap amount in the second pause period is smaller than that in the first pause period.
Further, the third threshold correction is performed in the period LT3c, and after a pause, the fourth threshold correction is performed in the period LT3d.
Finally, the gate-source voltage of the drive transistor Td becomes the threshold voltage Vth.
At this time, the source potential (node ND2: anode potential of the organic EL element 1) = Vofs−Vth ≦ Vcat + Vthel. (Vcat is the cathode potential, Vthel is the threshold voltage of the organic EL element 1)
In the case of FIG. 3, after the fourth threshold correction period LT3d, the scanning pulse WS is set to L level, the sampling transistor Ts is turned off, and the threshold correction operation is completed.

  In this example, the threshold correction is performed four times. However, how many times the threshold correction operation is performed can be appropriately determined according to the configuration and operation of the display device. There are also examples of 3 times, 5 times or more.

  Thereafter, during a period LT4 in which the signal line voltage is the video signal voltage Vsig, the write scanner 13 sets the scanning pulse WS to the H level, and writing of the video signal voltage Vsig and mobility correction are performed. That is, the video signal voltage Vsig is input to the gate of the drive transistor Td. An equivalent circuit at this time is shown in FIG.

The gate potential of the drive transistor Td becomes the potential of the video signal voltage Vsig, but current flows because the power supply control line DSL is at the drive voltage Vcc, and the source potential rises with time.
At this time, if the source voltage of the driving transistor Td does not exceed the sum of the threshold voltage Vthel and the cathode voltage Vcat of the organic EL element 1, the current of the driving transistor Td is used to charge the holding capacitor Cs and the capacitor Coled. That is, the condition is that the leakage current of the organic EL element 1 should be much smaller than the current flowing through the drive transistor Td.
At this time, since the threshold value correcting operation of the drive transistor Td is completed, the current flowing through the drive transistor Td reflects the mobility μ.
Specifically, those with high mobility have a large current amount at this time, and the source rises quickly. On the other hand, when the mobility is low, the amount of current is small and the source rises slowly.
As a result, during the period LT4 when the scanning pulse WS is at the H level, the source voltage Vs of the drive transistor Td rises after the sampling transistor Ts is turned on, and when the sampling transistor Ts is turned off, the source voltage Vs becomes the mobility μ The voltage Vs0 reflects the above. The gate-source voltage Vgs of the drive transistor Td is reduced to reflect the mobility (Vgs = Vsig−Vs0), and becomes a voltage that completely corrects the mobility after a predetermined time has elapsed.

After writing the video signal voltage Vsig and correcting the mobility in this way, the gate-source voltage Vgs is determined, and the process proceeds to the bootstrap and light emission state (period LT5). FIG. 7B shows an equivalent circuit.
That is, the scanning pulse WS is set to L level, the sampling transistor Ts is turned off, writing is completed, and the organic EL element 1 is caused to emit light. In this case, a current Ids corresponding to the gate-source voltage Vgs of the drive transistor Td flows, the potential of the node ND2 rises to the voltage VEL through which the current flows in the organic EL element 1, and the organic EL element 1 emits light. At this time, the sampling transistor Ts is off, and the gate of the drive transistor Td (node ND1) rises at the same time as the potential of the node ND2 rises, so the gate-source voltage Vgs remains constant. (Bootstrap operation)

As described above, the pixel circuit 10 performs the operation for light emission of the organic EL element 1 including the threshold value correction operation and the mobility correction operation as the light emission drive operation of one cycle in one frame period.
A current corresponding to the signal potential Vsig can be supplied to the organic EL element 1 regardless of variations in the threshold voltage Vth of the drive transistor Td in each pixel circuit 10 and fluctuations in the threshold voltage Vth due to temporal fluctuations by the threshold correction operation. . That is, variations in the threshold voltage Vth due to manufacturing or changes over time can be canceled, and high image quality can be maintained without causing uneven brightness on the screen.
In addition, since the drain current varies depending on the mobility of the driving transistor Td, the image quality deteriorates due to variations in the mobility of the driving transistor Td for each pixel circuit 10, but the mobility correction increases or decreases the mobility of the driving transistor Td. In response to this, the source potential Vs is obtained. As a result, the gate-source voltage Vgs is adjusted so as to absorb the variation in mobility of the drive transistor Td of each pixel circuit 10, so that the deterioration in image quality due to the variation in mobility is also eliminated.

Further, the threshold correction operation is divided and performed a plurality of times as a one-cycle pixel circuit operation because of the demand for higher frequency display devices.
As the frame rate is increased, the operation time of the pixel circuit is relatively shortened, so that it is difficult to secure a continuous threshold correction period (signal line voltage = threshold correction reference voltage Vofs period). . Thus, by performing the threshold correction operation in a time-sharing manner as described above, a necessary period is secured as the threshold correction period, and the gate-source voltage of the drive transistor Td is converged to the threshold voltage Vth.

Here, the influence due to the dullness of the waveform of the scanning pulse WS will be described.
As described in FIG. 1, the write control line WSL is arranged in the row direction on the pixel array 20. The influence of the resistance component and the capacitance component existing in the write control line WSL becomes larger as the length of the write control line WSL becomes longer due to, for example, an increase in the panel size. That is, compared with the scanning pulse WS input to the pixel circuit 10 in the column close to the write scanner 13 in FIG. 1, the influence is large on the scanning pulse WS input to the pixel circuit 10 in the column far from the write scanner 13. Thus, the waveform becomes dull.
For example, it is assumed that the scanning pulse WS input to the pixel circuit 10 connected to the signal line DTL1 (the pixel circuit 10 on the write scanner 13 side) has a steep rising and falling waveform as shown in FIG. In this case, the scan pulse WS input to the pixel circuit 10 connected to, for example, the signal line DTL (n) on the open end side of the write control line WSL rises and falls as shown in FIG. It becomes a dull waveform.

Attention is paid to a portion indicated by an arrow CP in FIGS. That is, it is the gate voltage Vg immediately after writing the video signal voltage Vsig and immediately before shifting to the light emitting state.
The writing of the video signal voltage Vsig is completed by setting the scanning pulse WS to the L level and turning off the sampling transistor Ts. When the scanning pulse WS is turned off, a negative coupling is applied to the node ND1. Due to the coupling immediately after the writing, the node ND1 (the gate voltage Vg of the driving transistor Td) temporarily decreases as shown in FIG.

On the other hand, when the scan pulse WS is dull as shown in FIG. 4, the negative coupling is the same, but the voltage drop at the node ND1 becomes moderate as the voltage drop of the scan pulse WS is not steep. As can be seen by comparing FIG. 3 and FIG. 4, the voltage drop amount at the node ND1 at this time varies depending on the bluntness of the waveform of the scan pulse WS.
This means that a difference occurs in the gate-source voltage Vgs of each drive transistor Td in the pixel circuits 10 on the left and right of the panel immediately before light emission.

For example, it is assumed that the entire screen emits light with the same luminance. That is, the video signal voltages Vsig applied to the pixel circuits 10 are all the same value.
If the video signal voltage Vsig is the same in the previous pixel circuit 10, the entire screen should emit light with uniform brightness. However, as described above, there is a difference in the gate voltage fluctuation of the drive transistor Td due to coupling when the scan pulse WS immediately before light emission is off. The gate-source voltage Vgs should be the same between the pixel circuits 10 in the horizontal direction (the direction of the write control line WSL). A phenomenon occurs in which the inter-voltage Vgs increases.
That is, as the display on the screen, the release end side of the write control line WSL becomes brighter, and gradation occurs in the horizontal direction of the screen.

[3. Pixel Circuit Operation of Embodiment]

In the present embodiment, the pixel circuit 10 is operated at the drive timing as shown in FIG. 8 in order to reduce or prevent such deterioration in image quality due to gradation.
This changes the speed of the threshold correction operation according to the dull amount of the scanning pulse WS during the threshold correction operation, and terminates the threshold correction operation in that the threshold cannot be completely corrected. At the end of threshold correction, a difference is made in advance between the gate-source voltage Vgs (= ND1 voltage−ND2 voltage) according to the dull amount of the scanning pulse WS. As a result, the difference in the coupling amount generated after the writing of the next video signal voltage Vsig is canceled and gradation is reduced.

FIG. 8 shows a timing chart of the operation of one light emission cycle (one frame period) of the pixel circuit 10 as in FIG. As in FIG. 3, the signal line voltage, power supply pulse, scanning pulse WS, node ND1 (gate voltage Vg of the drive transistor Td), and ND2 (source voltage Vs of the drive transistor Td) are shown.
The driving of the signal line DTL (signal line voltage) by the horizontal selector 11 and the power supply pulse DS by the drive scanner 12 are the same as in FIG.
In the case of FIG. 8, the pulse width of a part of the scanning pulse WS by the write scanner 13 is different from that in FIG. That is, the pulse width of the second threshold correction (period LT3b) is shorter than the pulse width of the other threshold correction periods (periods LT3a, LT3c, LT3d).

The operation of FIG. 8 will be described. FIG. 8 assumes the operation of the pixel circuit 10 close to the write scanner 13 when the pulse waveform of the scanning pulse WS is not dull.
A time point ts in the timing chart of FIG. 8 is a start timing of one cycle in which the organic EL element 1 as a light emitting element is driven to emit light, for example, one frame period of image display.
Before reaching this time point ts (period LT0), light emission of the previous frame is performed (similar to the case of FIG. 3).

The operation for light emission of the current frame is started at time ts.
First, the power supply pulse DS is set to the initial potential Vini. At this time, when the initial potential Vini is smaller than the sum of the threshold voltage Vthel and the cathode voltage Vcat of the organic EL element 1, that is, Vini ≦ Vthel + Vcat, the organic EL element 1 is extinguished and a non-light emitting period is started. At this time, the power supply control line DSL becomes the source of the drive transistor Td. The anode (node ND2) of the organic EL element 1 is charged to the initial potential Vini.

After a certain period, preparation for threshold correction is performed (periods LT2a, LT2b).
That is, in the periods LT2a and LT2b, when the potential of the signal line DTL becomes the threshold correction reference voltage Vofs, the scanning pulse WS is set to the H level, and the sampling transistor Ts is turned on. Therefore, the gate (node ND1) of the drive transistor Td becomes the threshold correction reference voltage Vofs. Therefore, the gate-source voltage Vgs of the driving transistor Td is Vofs−Vini.
Since the threshold value correction operation cannot be performed unless this Vofs−Vini is larger than the threshold voltage Vth of the drive transistor Td, the initial potential Vini and the reference voltage Vofs are set so that Vofs−Vini> Vth. .
That is, as a preparation for threshold correction, the gate-source voltage of the drive transistor is sufficiently widened than the threshold voltage Vth.

Subsequently, threshold correction (Vth correction) is performed. Here, an example is shown in which threshold correction is performed four times during the periods LT3a to LT3d.
First, during the period LT3a, the first threshold correction (Vth correction) is performed.
In this case, at the timing when the signal line voltage becomes the threshold correction reference voltage Vofs, the write scanner 13 sets the scanning pulse WS to the H level, and the drive scanner 12 sets the power supply pulse DS to the driving voltage Vcc. In this case, the anode (node ND2) of the organic EL element 1 serves as the source of the drive transistor Td, and a current flows. Therefore, the source node rises while the gate (node ND1) of the drive transistor Td is fixed to the threshold correction reference voltage Vofs.
As long as the anode potential of the organic EL element 1 (potential of the node ND2) is equal to or lower than Vcat + Vthel (threshold voltage of the organic EL element 1), the current of the drive transistor Td is used to charge the storage capacitor Cs and the capacitor Coled. For this reason, the potential of the node ND2 (the source potential of the driving transistor Td) increases with time.

The threshold correction as the period LT3a is ended when the write scanner 13 once sets the scanning pulse WS to the L level and the sampling transistor Ts is turned off before the signal line voltage = the video signal voltage Vsig.
At this time, since both the gate and the source are floating, a current flows between the drain and the source in accordance with the gate-source voltage Vgs and bootstraps. That is, the gate potential and the source potential rise as shown.

Next, in the period LT3b, the second threshold correction is performed. That is, when signal line voltage = threshold correction reference voltage Vofs, the write scanner 13 sets the scanning pulse WS to H level again and turns on the sampling transistor Ts.
However, in the case of the second threshold correction, the write scanner 13 has the same meaning as the correction of the scanning pulse WS. The write scanner 13 sets the pulse width of the scanning pulse WS to the first (and third, fourth) threshold correction. It is made shorter than the pulse width.
As described above, FIG. 8 assumes a case in which the pulse waveform of the scanning pulse WS is not dull. In this case, the sampling transistor Ts is rapidly turned on even if the pulse width of the scanning pulse WS is short. Done. As a result, the gate voltage of the drive transistor Td = the threshold correction reference voltage Vofs, and the source potential is increased.

Further, the threshold correction operation is paused. Since the gate-source voltage of the drive transistor Td is closer to the threshold voltage Vth in the second threshold correction, the bootstrap amount in the second pause period is smaller than that in the first pause period.
Further, the third threshold correction is performed in the period LT3c, and after a pause, the fourth threshold correction is performed in the period LT3d.

After the fourth threshold correction period LT3d, the scanning pulse WS is set to L level, the sampling transistor Ts is turned off, and the threshold correction operation is completed.
In this example, the gate-source voltage of the drive transistor Td is finally set to the threshold voltage Vth + ΔV1 by performing the threshold correction four times. That is, four total threshold correction times are set so that the threshold correction is terminated immediately before the gate-source voltage Vgs reaches the threshold voltage Vth.

Thereafter, during a period LT4 in which the signal line voltage is the video signal voltage Vsig, the write scanner 13 sets the scanning pulse WS to the H level, and writing of the video signal voltage Vsig and mobility correction are performed. That is, the video signal voltage Vsig is input to the gate of the drive transistor Td.
The gate potential of the drive transistor Td becomes the potential of the video signal voltage Vsig, but current flows because the power supply control line DSL is at the drive voltage Vcc, and the source potential rises with time.
At this time, if the source voltage of the driving transistor Td does not exceed the sum of the threshold voltage Vthel and the cathode voltage Vcat of the organic EL element 1, the current of the driving transistor Td is used to charge the holding capacitor Cs and the capacitor Coled. The current flowing through the driving transistor Td reflects the mobility μ.
That is, when the mobility is high, the amount of current at this time is large and the source rises quickly. On the other hand, when the mobility is low, the amount of current is small and the source rises slowly. As a result, during the period LT4 when the scanning pulse WS is at the H level, the source voltage Vs of the drive transistor Td rises after the sampling transistor Ts is turned on, and when the sampling transistor Ts is turned off, the source voltage Vs becomes the mobility μ The voltage Vs0 reflects the above. The gate-source voltage Vgs of the driving transistor Td is reduced to reflect the mobility μ (Vgs = Vsig−Vs0), and becomes a voltage that completely corrects the mobility μ after a predetermined time has elapsed.

After writing the video signal voltage Vsig and correcting the mobility in this way, the gate-source voltage Vgs is determined, and the process proceeds to the bootstrap and light emission state (period LT5).
That is, the scanning pulse WS is set to L level, the sampling transistor Ts is turned off, writing is completed, and the organic EL element 1 is caused to emit light. In this case, a current Ids corresponding to the gate-source voltage Vgs of the drive transistor Td flows, the potential of the node ND2 rises to the voltage VEL through which the current flows in the organic EL element 1, and the organic EL element 1 emits light. At this time, the sampling transistor Ts is off, and the gate of the drive transistor Td (node ND1) rises at the same time as the potential of the node ND2 rises, so the gate-source voltage Vgs remains constant. (Bootstrap operation)

As described above, the pixel circuit 10 performs the operation for light emission of the organic EL element 1 including the threshold value correction operation and the mobility correction operation as the light emission drive operation of one cycle in one frame period.
Here, in the case of this example, as described above, the pulse width of the scanning pulse WS for executing the second threshold correction is made narrower than in the case of other threshold corrections.
Thereby, the gradation generated on the screen in the direction of the write control line WSL is reduced or eliminated. This point will be described.

  FIG. 9 shows operation waveforms as seen in the pixel circuit 10 on the open end side of the write control line WSL, as in FIG. In this case, the pulse waveform of the scanning pulse WS is dull as shown in the figure. Further, in the period LT3b in which the second threshold correction is performed, the pulse width of the scanning pulse WS is shortened as shown in FIG. 8, but this is due to the waveform dullness on the open end side of the write control line WSL as shown in FIG. turn into.

First, attention is paid to the part indicated by the arrow CP in FIGS. Similar to the case described above with reference to FIGS. 3 and 4, the negative pulse is applied to the node ND1 when the scanning pulse WS becomes L level immediately after the video signal voltage Vsig is written and the sampling transistor Ts is turned off. .
In the case of FIG. 8, the fall of the scan pulse WS is steep, so that the voltage drop of the node ND1 due to coupling is rapid, whereas in the case of FIG. 9, the fall of the scan pulse WS is gradual and the node ND1 due to coupling is slow. The voltage drop becomes moderate.
As a result, the voltage drop amount of the node ND1 at this time becomes smaller toward the release end side of the write control line WSL.

Here, the second threshold correction operation in the period LT3b in the case of FIG.
When the scanning pulse WS becomes H level, the threshold correction reference voltage Vofs is written to the node ND1, and the width of the scanning pulse WS at this time is short as described above. As a result, the scanning pulse WS does not rise sufficiently as shown in FIG. 9 on the release end side of the write control line WSL.
This is due to the difference in the write amount of the threshold correction reference voltage Vofs to the node ND1 between the side near the write scanner 13 (hereinafter referred to as the scanner end side) and the release end side (hereinafter referred to as the release end side) of the write control line WSL. Will be attached.
Specifically, the amount of writing is high because the amount of rounding is small on the scanner end side, and the amount of writing is small because the amount of rounding is large on the open end side. The small writing amount means that the node ND1 does not fall down to the threshold correction reference voltage Vofs.
As a result, during the second threshold correction, the node ND1 approaches the threshold correction reference voltage Vofs on the scanner end side, and has a higher potential on the release end side than on the scanner end side.

Then, when the scanning pulse WS becomes L level and the sampling transistor Ts is turned off and the rest period after the second threshold correction is entered, bootstrap is performed according to the gate-source voltage Vgs. At this time, as described above, since the node ND1 has a higher potential on the open end side than the scanner end side, the open end side boots up more greatly, and the nodes ND1 and ND2 rise.
Therefore, the scanner end side operates so that the gate-source voltage Vgs is larger, and the open end side operates so that the gate-source voltage Vgs is smaller.

  Thereafter, threshold correction is performed for each of the periods LT3c and LT3d, but the threshold correction is set to such an extent that the threshold correction is not sufficient, so that the difference applied in the second threshold correction is maintained to some extent.

Then, when the threshold correction is completed, there is a difference in the gate-source voltage Vgs between the scanner end side and the release end side.
In FIG. 8, it is described that the gate-source voltage Vgs at the end of threshold correction is Vgs = Vth + ΔV1 as the operation on the scanner end side, but the gate-source voltage Vgs on the open end side in FIG. ing. ΔV1> ΔV2. That is, the gate-source voltage Vgs is larger on the scanner end side than on the open end side. The reason why the gate-source voltage Vgs on the open end side is small is that the source voltage Vg at the boot end after the second threshold correction is larger on the open end side as described above.

In the period LT5, the video signal voltage Vsig is written in this state, and mobility correction is performed. Thereafter, the sampling transistor Ts is turned off, and at this time, coupling is applied to the node ND1. As described above, this negative coupling has a large scanner end and a small panel end. This difference in coupling amount is canceled out by the difference in gate-source voltage Vgs at the end of threshold correction.
After the video signal voltage Vsig is written, the bootstrap is performed, and a current flows through the organic EL element 1 in accordance with the gate-source voltage Vgs. However, due to the above cancellation, the gate-source voltage Vgs is canceled by the coupling fluctuation. Thus, the emission luminance difference is reduced or eliminated. For this reason, gradation in the write control line direction is reduced or eliminated.

As described above, in the present embodiment, the write scanner 13 performs at least one threshold correction (second time in this example) among the plurality of threshold corrections performed during the non-light emission period of one light emission cycle of each pixel circuit. In this case, the pulse width of the scanning pulse WS is made shorter than in other threshold corrections.
The write scanner 13 outputs the scan pulse WS so that the threshold correction is completed a plurality of times immediately before the gate-source voltage Vgs of the drive transistor Td reaches the threshold voltage Vth of the drive transistor Td.
By such an operation, gradation that occurs in the direction of the writing control line on the screen due to the influence of the blunting of the pulse waveform of the scanning pulse WS can be reduced or eliminated, and deterioration in image quality can be prevented.
In addition, the gradation can be reduced simply by changing the pulse width of the scanning pulse WS at the time of the second threshold correction, and this is a very simple and useful technique.

Although the embodiment has been described above, the present invention is not limited to the above example.
In the above example, the threshold correction is performed four times within one light emission cycle. However, how many times the threshold correction operation is performed can be appropriately determined according to the configuration and operation of the display device. For example, there are 2 times, 3 times, 5 times or more.
The shortening of the pulse width of the scanning pulse WS is not limited to the second threshold correction.
In the present invention, a scan pulse with a short pulse width is input during threshold correction, and as a result, a difference is generated in the gate-source voltage Vgs, and the difference in gate voltage fluctuation due to coupling is canceled by the difference. In that sense, the position and the number of times to shorten the pulse width of the scanning pulse WS are not particularly limited. For example, at the time of the third threshold correction, the pulse width may be shortened at the first threshold correction. Also, the pulse width may be shortened a plurality of times, such as during the second and third threshold corrections.
However, in practice, it is most preferable to shorten the pulse width of the scan pulse WS during the second threshold correction in the sense that it is easy to make a difference in the amount of increase in the source voltage at the bootstrap after the threshold correction. .

  DESCRIPTION OF SYMBOLS 1 Organic EL element, 10 pixel circuit, 11 horizontal selector, 12 drive scanner, 13 write scanner, 20 pixel array part, Cs holding capacity, Ts sampling transistor, Td drive transistor

Claims (4)

The driving voltage is applied between the light-emitting element and the drain-source to apply a current corresponding to the gate-source voltage to the light-emitting element, and the signal line voltage is driven to be electrically connected to the light-emitting element. A pixel circuit having a sampling transistor that is input to the gate of a transistor and a storage capacitor that is connected between the gate and source of the drive transistor and that holds the threshold voltage of the drive transistor and the input video signal voltage A pixel array arranged in
A signal selector for supplying a threshold correction reference voltage and a video signal voltage as the signal line voltage to each signal line arranged in a row on the pixel array;
A drive control scanner that applies a power pulse to each power control line arranged in a row on the pixel array and applies a drive voltage to the drive transistor of the pixel circuit;
A scanning pulse is applied to each write control line arranged in a row on the pixel array to control the sampling transistor of the pixel circuit, and a threshold correction reference voltage and a video signal voltage are input to each pixel circuit. And a plurality of scans when the signal line voltage is the threshold correction reference voltage so that the threshold correction is performed a plurality of times during a non-emission period of one light emission cycle of each pixel circuit. The sampling transistor is turned on by a pulse, and at the time of threshold correction at least once among a plurality of times, the pulse width of the scanning pulse for turning on the sampling transistor is made shorter than at the time of other threshold corrections. A writing scanner;
A display device comprising:   The display device according to claim 1, wherein the writing scanner makes a pulse width of a scanning pulse for conducting the sampling transistor shorter in the second threshold correction than in another threshold correction.   The write scanner outputs the scan pulse so that the threshold correction is completed a plurality of times immediately before the gate-source voltage of the driving transistor reaches the threshold voltage of the driving transistor. The display device described. The driving voltage is applied between the light-emitting element and the drain-source to apply a current corresponding to the gate-source voltage to the light-emitting element, and the signal line voltage is driven to be electrically connected to the light-emitting element. A pixel circuit having a sampling transistor that is input to the gate of a transistor and a storage capacitor that is connected between the gate and source of the drive transistor and that holds the threshold voltage of the drive transistor and the input video signal voltage A pixel array arranged in
A signal selector for supplying a threshold correction reference voltage and a video signal voltage as the signal line voltage to each signal line arranged in a row on the pixel array;
A drive control scanner that applies a power pulse to each power control line arranged in a row on the pixel array and applies a drive voltage to the drive transistor of the pixel circuit;
A scanning pulse is applied to each write control line arranged in a row on the pixel array to control the sampling transistor of the pixel circuit, and a threshold correction reference voltage and a video signal voltage are input to each pixel circuit. As a display driving method of a display device provided with a writing scanner to be
In order to cause the writing scanner to perform threshold correction a plurality of times during a non-light emission period of one light emission cycle of each pixel circuit, when the signal line voltage is the threshold correction reference voltage, the write scanner performs the scan pulse a plurality of times. Display drive for turning on the sampling transistor and reducing the pulse width of the scanning pulse for turning on the sampling transistor to be shorter than that for other threshold corrections at the time of at least one threshold correction among a plurality of times. Method.

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