JP2011141346A - Display device and display driving method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress variation in a driving current to light-emitting elements, caused by a leakage current when a sampling transistor is turned off. <P>SOLUTION: In a light emission period when the sampling transistor is not energized and the current corresponding to a video signal voltage is applied by the drive transistor to the light-emitting element, a first driving voltage and a second driving voltage lower than the first driving voltage are applied to the drive transistor. The second driving voltage is applied when a signal line has a threshold correction reference voltage. The gate voltage of the drive transistor is temporality reduced by applying the second driving voltage, and the drain-source voltage of the sampling transistor is reduced when the signal line has the threshold correction reference voltage, and accordingly, the leakage current is suppressed. <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 (Electroluminescence) element) as a light emitting element. About.

JP 2003-255856 A JP 2003-271095 A JP 2008-33193 A JP 2008-281671 A JP 2001-222256 A

For example, as can be seen in Patent Documents 1 and 2, image display devices 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.

In the case of the active matrix method, a thin film transistor provided inside a pixel is used as a switching transistor for sampling a video signal (hereinafter referred to as a sampling transistor) in order to apply a video signal voltage as a gradation value to a pixel circuit having an organic EL element. I use it.
When the organic EL element emits light, the video signal voltage supplied to the signal line is taken into a storage capacitor provided at the gate end (control input terminal) of the drive transistor by making the sampling transistor conductive. Then, the drive transistor supplies a drive signal corresponding to the captured video signal voltage to the organic EL element. For example, in an organic EL display device, an input video signal voltage is converted into a current signal by a driving transistor, and the driving current is supplied to an organic EL element.
At this time, in order that the light emission luminance of the organic EL element is unchanged, it is important that the video signal voltage taken in and held in the holding capacitor is constant. For example, in order for the light emission luminance of the organic EL element to be unchanged, it is important that the drive current corresponding to the input video signal voltage is constant.
For this reason, various mechanisms for making the drive current constant have been studied as pixel circuits for organic EL elements (see, for example, Patent Document 3 above).

Here, in a pixel circuit using an organic EL element, after a video signal voltage is written in a storage capacitor, a bootstrap operation is performed to emit light. For this reason, the gate voltage of the drive transistor is higher than the input video signal voltage.
As described above, the video signal voltage is taken into the gate terminal of the driving transistor through the sampling transistor and held in the holding capacitor.
However, when the leakage current of the sampling transistor (and the various transistors performing the switching operation) provided on the gate terminal side of the driving transistor is large, the leakage current flows from the gate terminal of the driving transistor toward the signal line. As a result, the voltage held in the storage capacitor varies depending on the leakage current.
As a result, the drive current for the organic EL element varies, and the light emission luminance cannot be maintained constant.

  Further, as a characteristic of the thin film transistor that performs the switching operation, the higher the drain-source voltage, the larger the leakage current in the off region. For this reason, the leakage current of the sampling transistor easily flows during the period when the voltage of the signal line is low. If the level of this phenomenon is different for each pixel, even if all the pixels emit light at the same gradation, the display image becomes a rough image as shown in FIG.

  This countermeasure is disclosed in the above-mentioned Patent Documents 4 and 5, but the process steps become complicated by changing the LDD (Lightly Doped Drain) structure, and the pixel circuit itself becomes complicated, resulting in cost and yield points. The problem remains that it is disadvantageous.

  Therefore, the present invention suppresses a leakage current at the time of light emission of a transistor used for a switching operation in the pixel circuit, so that the light emission luminance can be kept constant.

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 And a writing scanner for executing input of the threshold correction reference voltage and the video signal voltage. In the drive control scanner, as the drive voltage applied to the drive transistor during a light emission period in which the sampling transistor is non-conductive and the drive transistor applies a current corresponding to a video signal voltage to the light emitting element, A first drive voltage and a second drive voltage lower than the first drive voltage are applied.
In particular, the drive control scanner applies the second drive voltage to the drive transistor when the signal selector supplies the threshold correction reference voltage to the signal line during the light emission period.
In the pixel circuit, an auxiliary capacitor is connected between the drain and gate of the driving transistor.

  In the display driving method of the present invention, the drive control scanner is configured so that the drive transistor is in a light emission period in which the sampling transistor is non-conductive and the drive transistor applies a current corresponding to a video signal voltage to the light emitting element. In the display driving method, the first driving voltage and the second driving voltage lower than the first driving voltage are applied as the driving voltage.

In the present invention, the gate potential of the drive transistor is temporarily lowered by applying the second drive voltage to the drive transistor during the light emission period. This lowers the potential on the drain side of the sampling transistor during the light emission period.
Here, in an organic EL display device or the like, a threshold correction reference voltage may be supplied from a signal line to a pixel circuit in order to correct a threshold voltage of a driving transistor before light emission. In this case, for example, the video signal voltage and the threshold correction reference voltage are applied to the signal line during the 1H period.
When the light emitting element emits light in each pixel circuit, the sampling transistor is off. However, when the threshold correction reference voltage, which is a relatively low voltage, is applied to the signal line (that is, the source side of the sampling transistor during the light emission period), the drain-source voltage of the sampling transistor increases. On the other hand, as described above, by temporarily lowering the gate potential of the driving transistor, the drain-source voltage of the sampling transistor can be lowered. This alleviates the operating point of the leakage current of the sampling transistor and can suppress the leakage current. By suppressing the leak current, fluctuations in the drive current due to the leak current are suppressed.

According to the present invention, in the light emission period in which the sampling transistor is off, the period for applying the second drive voltage to the drive transistor is provided, and during that period, the operating point of the leakage current of the sampling transistor is relaxed. In particular, the second drive voltage is applied during a period in which the threshold correction reference voltage is supplied to the signal line.
As a result, the leakage current of the sampling transistor can be suppressed, and fluctuations in the drive current due to the leakage current can be suppressed. As a result, it is possible to provide a display device with uniform image quality that does not deteriorate due to leakage current such as roughness of the display screen.

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 normal pixel circuit operation | movement. It is explanatory drawing of the leakage current in the light emission period. It is explanatory drawing of the influence of the leakage current in the light emission period. FIG. 11 is an explanatory diagram of the operation of the pixel circuit of the embodiment. It is explanatory drawing of the gate voltage fluctuation | variation of embodiment. It is explanatory drawing of the operating point of the sampling transistor of embodiment. It is explanatory drawing of the display state with roughness.

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]
[3. Pixel Circuit Operation of First Embodiment]
[4. Second 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.
In the present embodiment, the power supply pulse DS (DS1, DS2... DS (m)) is a pulse voltage that switches to three values of the first drive voltage Vcc, the second drive voltage Vcc2, 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.
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 voltage 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]

Here, in order to understand the operation of the present embodiment to be described later, the pixel circuit operation considered in the process leading to the present invention will be described first. 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. The threshold correction operation is an example in which divided 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 LT1), light emission of the previous frame is performed.
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 drive 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.
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.

Then, preparation for threshold correction is performed (period LT2).
That is, in the period LT2, 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. As a result, the gate-source voltage Vgs of the drive transistor Td becomes 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 (period LT3). 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. “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.

  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. 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).
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, the drive current Ids according 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.
By the threshold correction operation, 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 driving transistor Td in each pixel circuit 10 and variations in the threshold voltage Vth due to temporal variation. it can. 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 one cycle of pixel circuit operation because of the demand for higher speed (higher frequency) of the display device.
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.

For example, as shown in FIG. 3, one cycle of light emission driving operation is performed, but the following points are problematic.
As described above, as the pixel circuit operation, after the video signal voltage Vsig is written in the storage capacitor Cs, the bootstrap operation is performed to emit light.
Therefore, at the start of the light emission period LT5, the gate voltage of the drive transistor Td becomes a potential equal to or higher than the video signal voltage Vsig, as can be seen from FIG.
Here, in the light emission period LT5, the sampling transistor Ts is turned off. However, as shown in FIG. 4A, there is a leakage current Ileak when the sampling transistor Ts is off.
The leakage current Ileak flows from the gate terminal of the driving transistor Td toward the signal line DTL, so that the voltage held in the holding capacitor Cs varies.
For this reason, the gate-source voltage of the drive transistor Td fluctuates, the drive current Ids fluctuates, and the light emission luminance cannot be kept constant.
Although FIG. 3 shows an ideal state, actually, as shown in FIG. 5, the gate voltage Vg of the drive transistor Td varies due to the influence of the leak current Ileak in the light emission period LT5.

FIG. 4B shows the gate voltage / drain current characteristics of the sampling transistor Ts.
FIG. 4B shows a case where the drain-source voltage (Vds) is different between the straight line and the broken line. However, the higher the drain-source voltage (Vds), the higher the drain current Id (that is, (Leakage current) increases.
As described above, in the light emission period LT5, the gate voltage Vg of the drive transistor Td becomes higher than the video signal voltage Vsig due to bootstrap after writing the video signal voltage Vsig. Therefore, as shown in FIG. 4A, the sampling transistor Ts has a drain (D) on the node ND1 side and a source (S) on the signal line DTL side.
Also in the light emission period LT5, the threshold value correction reference voltage Vofs and the video signal voltage Vsig are supplied to the signal line DTL every 1H period for the pixel circuits of other lines.

Then, the drain-source voltage (Vds) of the sampling transistor Ts becomes higher when the signal line DTL is at the threshold correction reference voltage Vofs than when the signal line DTL is at the video signal voltage Vsig. Of course, the video signal voltage Vsig cannot be generally described because it varies depending on the gradation value to be displayed. However, as a representative example, the solid line in FIG. It can be considered that signal line DTL = threshold correction reference voltage Vofs.
When the signal line DTL = the threshold correction reference voltage Vofs, the drain-source voltage Vds of the sampling transistor Ts becomes the largest, and at this time, the leakage current Ileak becomes the largest.
In FIG. 5, in the light emission period LT5, the leakage amount is larger when the voltage of the signal line DTL is the video signal voltage Vsig, and the change in the gate voltage of the drive transistor Td is larger than when the threshold correction reference voltage Vofs. ing.

  As described above, the gate current of the driving transistor Td changes due to the leakage current Ileak of the sampling transistor Ts, and the gate-source voltage Vgs of the driving transistor Td varies. Therefore, the drive current Ids to the organic EL element 1 fluctuates, and it becomes difficult to maintain a constant luminance according to the video signal voltage Vsig during the light emission period LT5.

The amount of leakage current depends on the characteristics of each sampling transistor Ts. If the characteristics of the sampling transistor Ts are different for each pixel circuit 10, even if all the pixels emit light by the video signal voltage Vsig of the same gradation, the amount of leak current differs in each pixel circuit 10, The light emission luminance in the pixel circuit 10 does not become constant.
As a result, the display image becomes a rough image as shown in FIG.

[3. Pixel Circuit Operation of First Embodiment]

In the present embodiment, the pixel circuit 10 as shown in FIG. 6 is driven in order to prevent light emission luminance fluctuation due to such a leakage current.
This is because the drive scanner 12 applies the second drive voltage Vcc2 to the power supply control line DSL in addition to the drive voltage Vcc and the initial voltage Vini, and the pixel circuit 10 of each line is subjected to the light emission period LT5. Thus, this is a drive system that applies drive voltages Vcc and Vcc2.

6 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, the scan pulse WS, the power supply pulse DS, the 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 scanning pulse WS by the write scanner 13 are the same as in FIG.
In the case of FIG. 6, the power pulse DS by the drive scanner 12 is different from that in FIG.

The operation of FIG. 6 will be described. However, the periods LT1 to LT4 in FIG. 6 are the same as those in FIG.
After writing the video signal voltage Vsig and correcting the mobility in the period LT4, the scanning pulse WS is set to L level to determine the gate-source voltage Vgs, and the process proceeds to the bootstrap and light emission state (period LT5).
In this case, the drive current Ids according 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)

Here, the light emission period LT5 is continued until the start timing of the pixel circuit operation for the next frame (timing corresponding to the time ts in FIG. 6). At this time, the drive scanner 12 is as shown in the figure. Drive voltages Vcc and Vcc2 are applied to the power supply control line DSL.
That is, the drive scanner 12 basically applies the drive voltage Vcc to the power supply control line DSL and applies it to the drive transistor Td, but the drive voltage Vcc2 is applied within the period when the voltage of the signal line DTL becomes the threshold correction reference voltage Vofs. To give.

During the period in which the second drive voltage Vcc2 is applied to the drive transistor Td, the gate voltage Vg and the source voltage Vs of the drive voltage Vcc temporarily decrease as shown in the figure.
The amount of decrease in the gate voltage Vg at this time is represented by ΔVg.
Further, as shown in FIG. 7A, the gate-drain parasitic capacitance of the driving transistor Td is Cgd, and the gate-source parasitic capacitance is Cgs. Then, the gate voltage drop amount ΔVg is
ΔVg = {Cgd / (C + Cgd)} × (Vcc2−Vcc) (Expression 2)
It becomes.
In Equation 2, “C” is a series capacitance between the node ND1 and the cathode potential Vcat.
C = {(Cs + Cgs) Coled} / (Cs + Cgs + Coled) (Equation 3)
It is.

As described above, the leakage current of the sampling transistor Ts increases when the drain-source voltage is high.
In the light emission period LT5, the drain-source voltage Vds of the sampling transistor Ts becomes high when the threshold correction reference voltage Vofs is applied to the signal line DTL.
In practice, the gate voltage Vg and source voltage Vs of the drive transistor Td as shown in FIG. 5 greatly vary due to leakage when the threshold correction reference voltage Vofs is applied to the signal line DTL, and this makes the emission luminance constant. It is a dominant factor that cannot be maintained.

  Therefore, in the present embodiment, during the period in which the voltage of the signal line DTL is the threshold correction reference voltage Vofs in the light emission period LT5, the voltage value of the power supply pulse DS becomes a voltage lower than the normal drive voltage Vcc. The driving voltage is Vcc2. Thus, using the parasitic capacitance Cgd of the drive transistor Td, the coupling amount (ΔVg) shown in the above equation 2 is input to the gate of the drive transistor Td, and the gate voltage Vg of the drive transistor Td is temporarily lowered.

  Thereby, the voltage difference applied between the drain and source of the sampling transistor Ts can be reduced, and the operating point where the leakage current flows can be relaxed as shown in FIG. That is, in FIG. 8, the voltage VWSL is assumed to be the L level potential of the scanning pulse WS, and the operating point of the sampling transistor Ts when off is moved from P1 to P2.

For this reason, the leakage current of the sampling transistor Ts is suppressed, and the fluctuation of the drive current Ids with respect to the organic EL element 1 due to the leakage current can be suppressed.
As a result, it is possible to provide an organic EL display device with uniform image quality that is free from deterioration due to leakage current such as roughness on the screen display.

As shown in FIG. 6, after applying the second drive voltage Vcc2, the drive scanner 12 returns to the first drive voltage Vcc during the period when the signal line DTL becomes the video signal voltage Vsig. .
When the second drive voltage Vcc2 is applied, the gate voltage Vg of the drive transistor Td is lowered by a coupling amount according to the parasitic capacitance Cgd as described above, and the source voltage Vs of the drive transistor Td is also a capacitance. A reduction in proportion occurs.
However, when returning to the first drive voltage Vcc, the original gate voltage Vg and source voltage Vs are restored, and the gate-source voltage Vgs is maintained.

That is, the operation of the present embodiment is driven by applying the second drive voltage Vcc2 when the signal line DTL = threshold correction reference voltage Vofs, which is assumed to cause a level of leakage that is problematic for maintaining the light emission luminance. The gate voltage Vg of the transistor Td is lowered. This suppresses the leakage of the sampling transistor Ts and maintains the gate-source voltage Vgs of the driving transistor Td.
Also, from this, the second drive voltage Vcc2 is an operation in the off region of the sampling transistor Ts in consideration of the gate voltage drop amount ΔVg of the drive transistor Td derived by the above equation 2 and the potential of the threshold correction reference voltage Vofs. The point should be set so that the leakage current becomes an operating point at which there is no problem.

[4. Second Embodiment]

FIG. 7B shows a pixel circuit 10 as a second embodiment.
This is obtained by adding an auxiliary capacitance Ca between the gate and drain of the drive transistor Td to the pixel circuit configuration of the first embodiment shown in FIGS.
The light emission driving operation for one cycle for the pixel circuit is the same as in FIG.

In this case, the gate voltage drop amount ΔVg of the drive transistor Td by applying the second drive voltage Vcc2 is as follows.
ΔVg = {(Cgd + Ca) / (C + Cgd + Ca)} × (Vcc2−Vcc)
... (Formula 4)

That is, the auxiliary capacitor Ca is added as one of the parameters that determine the gate voltage drop amount ΔVg.
This means that the coupling amount entering the gate of the driving transistor can be adjusted by the auxiliary capacitor Ca. That is, by selecting the capacitance value of the auxiliary capacitor Ca, it is possible to increase the ease and flexibility of setting each power supply voltage (Vofs, Vcc2, etc.), which is preferable in design.

Although the embodiments have been described above, the present invention is not limited to the above examples.
In the light emission driving example of FIG. 6, the threshold value correction is performed four times within one light emission cycle. However, how many times the threshold value correction operation is performed can be appropriately determined according to the configuration and operation of the display device. For example, there are examples of 2 times, 3 times, 5 times or more.
As described above, the voltage value of the second drive voltage Vcc2 may be appropriately determined from the viewpoint of suppressing the leakage current in order to make the light emission luminance constant.
It is also conceivable that the timing at which the second drive voltage Vcc2 is applied completely coincides with the period of the threshold correction reference voltage Vofs in the signal line DTL.

  DESCRIPTION OF SYMBOLS 1 Organic EL element, 10 pixel circuit, 11 horizontal selector, 12 drive scanner, 13 light scanner, 20 pixel array part, Cs holding capacity, Ts sampling transistor, Td drive transistor, parasitic capacity of Cgd, Cgs drive transistor, Ca auxiliary capacity

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. A writing scanner to
With
In the drive control scanner, as the drive voltage to be applied to the drive transistor during a light emission period in which the sampling transistor is non-conductive and the drive transistor applies a current according to a video signal voltage to the light emitting element, A display device that provides a driving voltage of 1 and a second driving voltage lower than the first driving voltage.   2. The drive control scanner applies the second drive voltage to the drive transistor when the signal selector supplies the threshold correction reference voltage to the signal line during the light emission period. Display device.   The display device according to claim 2, wherein in the pixel circuit, an auxiliary capacitor is connected between a drain and a gate of the driving transistor. 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 a light emission period in which the sampling transistor is turned off and the drive transistor applies a current corresponding to a video signal voltage to the light emitting element, the drive control scanner uses the drive transistor as the drive voltage as a first voltage. And a display driving method for providing a second driving voltage lower than the first driving voltage.

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