JP2011145327A - Display device and display driving method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress roughness of a display picture caused by a leakage current in the off period of a sampling transistor. <P>SOLUTION: A first switching element is connected to the gate of a sampling transistor and is turned on/off to control supply of a control signal to turn on/off the sampling transistor. A second switching element is connected between the gate of the sampling transistor and a signal line (source terminal). In a period of keeping the sampling transistor in an off state as a light-emitting period, the second switching element is conducted to make constant the gate-source voltage of the sampling transistor. Thus, an operation point is fixed, thereby preventing influences of the degree of the leakage current on a display picture. <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.

Here, the thin film transistor that performs the switching operation has variations in the amount of leakage current due to the gate-source voltage.
A good transistor has little change in the amount of leakage current even when the gate-source voltage fluctuates. On the other hand, there is a transistor in which the leakage current increases as the gate-source voltage increases.
That is, in practice, the difference in the characteristics of the sampling transistor for each pixel may cause the amount of leakage current of the sampling transistor during light emission to be different for each pixel.
Then, even when all the pixels emit light at the same gradation, the change in the light emission luminance due to the influence of the leakage current varies for each pixel, and the display image becomes a rough image as shown in FIG. End up.

  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.

  In view of the above, an object of the present invention is to suppress variations in leakage current among pixels at the time of light emission of a transistor used for a switching operation in a pixel circuit so that a rough image does not occur.

  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. The sampling transistor for inputting the voltage applied to the signal line to the gate of the driving transistor, and the threshold voltage of the driving transistor connected between the gate and source of the driving transistor and the input video signal voltage are held. A pixel circuit having a storage capacitor, a first switch element that controls supply of a control signal to the gate of the sampling transistor, and a second switch element that connects and disconnects the gate of the sampling transistor and the signal line Have. A signal selector for supplying a video signal voltage and a threshold correction reference voltage as the signal line voltage to each signal line arranged in a column on a pixel array in which the pixel circuits are arranged in a matrix; A drive control scanner that applies power pulses to the power supply control lines arranged in a row on the pixel array and applies a drive voltage to the drive transistors of the pixel circuit, and a row on the pixel array. A scanning pulse is applied to the write control line, the first switch element is turned on, and the second switch element is turned off to make the sampling transistor conductive, and a video signal voltage and a reference voltage to each pixel circuit When the sampling transistor is turned off at least during the light emission period, the first switch is turned off and the second switch element is turned off. The and a write scanner to turn on.

  The display driving method of the present invention is the display driving method of the display device having the pixel circuit configuration, wherein the writing scanner turns on the first switch element and turns off the second switch element. And the video signal voltage and the reference voltage are input to each pixel circuit, and the first switch is turned off and the second switch element is turned off when the sampling transistor is turned off at least in the light emission period. This is a display driving method for turning on.

In the present invention as described above, the first switch element is connected to the gate of the sampling transistor, and the ON / OFF control signal supply of the sampling transistor is controlled by the ON / OFF of the first switch element.
A second switch element is connected between the gate of the sampling transistor and the signal line (source end). In the light emission period, the signal line side of the sampling transistor serves as a source, but the gate and the source of the sampling transistor are short-circuited by turning on the second switch element. That is, by turning on the second switch element during the light emission period, the gate-source voltage of the sampling transistor is fixed, and the operating point does not vary due to variations in transistor characteristics.

According to the present invention, during the light emission period in which the sampling transistor is off, the second switch element is turned on to fix the gate-source voltage of the sampling transistor. As a result, the operating point of the leakage current of the sampling transistor in each pixel circuit is fixed, so that a difference in the amount of leakage current due to variation in the characteristics of the sampling transistor does not occur.
Therefore, the difference in the amount of leakage current of the sampling transistor for each pixel circuit can be suppressed, and the fluctuation in drive current due to the leakage current, that is, the difference in fluctuation in light emission luminance can be suppressed. As a result, it is possible to provide a display device with uniform image quality that is free from deterioration 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 a circuit diagram of a pixel circuit considered in the process leading to the present invention. 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. It is explanatory drawing of operation | movement of the pixel circuit of 1st Embodiment. It is an equivalent circuit diagram of the pixel circuit of the embodiment. It is explanatory drawing of the operating point of the sampling transistor of embodiment. It is explanatory drawing of the pixel circuit of 2nd 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. Pixel Circuit Operation of 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, the first write control lines WSLa1, WSLa2,. , DSL2... DSL (m). These first and second write control lines WSLa and WSSLb and the power supply control line 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.

The first write control line WSLa (WSLa1 to WSLa (m)) is driven by the write scanner 13.
The write scanner 13 sequentially applies the first scanning pulses WSa (WSa1, WSa2... WSa (m) to the write control lines WSLa1 to WSLa (m) arranged in rows at predetermined timings set. ) To scan the pixel circuit 10 line-sequentially in units of rows.
The second write control line WSLb (WSLb1 to WSLb (m)) is also driven by the write scanner 13.
As will be described later, the write scanner 13 applies the second scanning pulse WSb (WSb1, WSb2... WSb (m)) as a logical inversion pulse of the first scanning pulse WSa to each book arranged in rows. Are sequentially supplied to the control lines WSLb1 to WSLb (m).

  The first and second write control lines WSLa, WSSLb are provided in the first embodiment, and only the first write control line WSLa is provided in the second embodiment. Are provided, and the second write control line WSLb becomes unnecessary.

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 pulses WSa and WSb 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 the video signal voltage Vsig and the threshold correction reference voltage Vofs as signal line voltages in one horizontal period (1H period) 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 lines WSLa and WSLB and the power supply control line DSL is shown for the sake of simplicity.

  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. It also has switches Ta and Tb using n-channel TFTs. 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 drain of the drive transistor Td is connected to the power supply control line DSL.
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).

One end of the source and drain of the sampling transistor Ts is connected to the gate of the drive transistor Td, and the other end is connected to the signal line DTL.
The gate of the sampling transistor Ts is connected to the switch Ta. The other end of the switch Ta is connected to a voltage Vws line as a fixed power source. This voltage Vws is a voltage that makes the sampling transistor Ts conductive.

The gate of the switch Ta is connected to the first write control line WSLa, and therefore the switch Ta is ON / OFF controlled by the first scanning pulse WSa.
When the switch Ta is turned on by the first scanning pulse WSa, the voltage Vws is applied to the gate of the sampling transistor Ts, and the sampling transistor Ts becomes conductive.

A switch Tb is connected between the gate of the sampling transistor Ts and the signal line DTL (the source end of the sampling transistor Ts during the light emission period).
The gate of the switch Tb is connected to the second write control line WSLb. Therefore, the switch Tb is ON / OFF controlled by the second scan pulse WSb.
When the switch Tb is turned on by the second scanning pulse WSb, the gate of the sampling transistor Ts and the signal line (source end) are short-circuited.

The light emission driving of the organic EL element 1 is basically as follows.
The sampling transistor Ts is turned on at the timing when the video signal voltage Vsig is applied to the signal line DTL. As a result, the video signal voltage Vsig from the signal line DTL is input to the gate of the drive transistor Td and written to the storage capacitor Cs.

The drive transistor Td causes the drive 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 drive current Ids becomes a value corresponding to the gate-source voltage Vgs of the drive transistor Td (value corresponding to the voltage held in the holding capacitor Cs), and the organic EL element 1 has a luminance corresponding to the current value. Emits light.
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 is clear from Equation 1, the 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.

Here, the operation of the general pixel circuit 10 shown in FIG. 3 will be described.
Compared with the pixel circuit 10 of the present embodiment shown in FIG. 2, the switches Ta and Tb are not provided.
The switch Ta is not provided, the gate of the sampling transistor Ts is connected to the write control line WSL, and the sampling transistor Ts is turned on / off by the scanning pulse WS from the write scanner 13.
Further, since the switch Tb is not provided, the second write control line WSLb for the control is not provided.

The basic light emission operation by applying a current from the drive transistor Td to the organic EL element 1 is the same.
That is, 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 applied 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 functions as a constant current source for the organic EL element 1 by operating in the saturation region, and a current Ids corresponding to the video signal voltage Vsig (gate-source voltage Vgs) written in the storage capacitor Cs. Is passed through the organic EL element 1. As a result, light emission with luminance corresponding to the gradation value of the video signal is performed.

FIG. 4 shows a timing chart of the operation of one light emission cycle (one frame period) of the pixel circuit 10.
FIG. 4 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. 4 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. 4 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. 4 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.

Time ts in the timing chart of FIG. 4 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. 4, 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 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. 5A, 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. 4 shows an ideal state, actually, as shown in FIG. 6, the gate voltage Vg of the driving transistor Td varies due to the influence of the leakage current Ileak in the light emission period LT5.

The leak current increases as the potential difference between the drain end and the source end 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. 5A, the sampling transistor Ts has a drain (D) on the node ND1 side and a source (S) on the signal line DTL side.
Even during light emission in the period LT5, the threshold 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 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.
In FIG. 6, in the light emission period LT5, the amount of leakage is larger when the voltage of the signal line DTL is the threshold value correction reference voltage Vofs than 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. ing.

FIG. 5B shows the characteristics of the gate-source voltage (Vgs) and the drain current Id of the sampling transistor Ts.
In FIG. 5B, a normal transistor with a straight line and a transistor with a large leakage current are shown with a broken line. Here, “normal” means a transistor in which the amount of leakage current hardly changes even when the gate-source voltage varies in the off region.
Depending on the individual transistors, as indicated by the broken line, the drain current (that is, the leakage current) in the off region varies greatly depending on the gate-source voltage.
Since the characteristics of the sampling transistor Ts vary for each pixel circuit 10, the amount of leakage current of the sampling transistor Ts during the light emission period differs as described above.
In the pixel circuit in which the leakage current of the sampling transistor Ts is large, the gate voltage drop of the drive transistor Td is large, and the change in the gate-source voltage Vgs of the drive transistor Td is large. For this reason, the change in light emission luminance becomes large. On the other hand, in the pixel circuit where the leakage current of the sampling transistor Ts is small, the change in the light emission luminance is small or almost none.
Then, even when all the pixels emit light at the same gradation, the change in the light emission luminance due to the influence of the leak current varies for each pixel, and the display image becomes a rough image as shown in FIG. End up.

[3. Pixel Circuit Operation of First Embodiment]

In the present embodiment, in order to prevent the deterioration of the uniformity of the screen display due to such a variation in leak current for each pixel, the configuration described in FIGS. 1 and 2 is adopted, and the pixel circuit 10 as shown in FIG. Drive.
This is because the write scanner 13 turns on the switch Ta and turns off the switch Tb to turn on the sampling transistor Ts, and the video signal voltage Vsig and the threshold correction reference voltage Vofs are input to the pixel circuit 10. When the sampling transistor Ts is turned off during the light emission period, the switch Ta is turned off and the switch Tb is turned on.

FIG. 7 shows a timing chart of the operation of one light emission cycle (one frame period) of the pixel circuit 10 as in FIG.
The signal line voltage applied to the signal line DTL by the horizontal selector 11 becomes a pulse voltage as the threshold correction reference voltage Vofs and the video signal voltage Vsig in one horizontal period (1H) as in the example of FIG.

FIG. 7 shows a scanning pulse WSa applied to the gate of the switch Ta by the write scanner 13 via the first write control line WSLa. The switch Ta of the n-channel TFT is turned on when the scanning pulse WSa is set to H level, and is turned off when the scanning pulse WSa is set to L level.
When the switch Ta is turned on as described above, the voltage Vws is applied to the gate of the sampling transistor Ts, and the sampling transistor Ts is turned on.

A scan pulse WSb applied to the gate of the switch Tb by the write scanner 13 via the second write control line WSLb is shown. The switch Tb by the n-channel TFT is turned on when the scanning pulse WSb is set to the H level, and is turned off when the scanning pulse WSb is set to the L level.
When the switch Tb is turned on, the gate of the sampling transistor Ts and the signal line (source end during the light emission period) are short-circuited.

As can be seen from FIG. 7, the scan pulse WSb is a logic inversion pulse of the scan pulse WSa.
When the scanning pulse WSa is at the H level and the scanning pulse WSb is at the L level, the sampling transistor Ts is turned on and its gate is disconnected from the signal line DTL as shown in the equivalent circuit of FIG. Yes.
When the scanning pulse WSa is at the L level and the scanning pulse WSb is at the H level, the sampling transistor Ts is turned off as shown in FIG. 8B. At this time, the gate is short-circuited to the signal line DTL. The

FIG. 7 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. 7 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.

  The operation of FIG. 7 will be described. However, the periods LT1 to LT4 in FIG. 7 are the same as the basic operations in the periods LT1 to LT4 in FIG. For this reason, a detailed description is avoided, but the differences are described.

First, in the present embodiment, the switch Ta is connected to the gate of the sampling transistor Ts, and the switch Tb is connected between the gate of the sampling transistor Ts and the signal line DTL. Therefore, when the sampling transistor Ts is brought into a conducting state, the switch Ta needs to be turned on while the switch Tb needs to be turned off.
Therefore, when the threshold correction reference voltage Vofs is input from the signal line DTL to the node ND1 in the period LT2 and the periods LT3a to LT3d in FIG. 7, the scanning pulse WSa is set to the H level and the scanning pulse WSb is set to the L level.
In the period LT4, the video signal voltage Vsig is input to the node ND1 from the signal line DTL. Therefore, also in this period, the scanning pulse WSa is at the H level and the scanning pulse WSb is at the L level.

As a result, for the periods LT2 to LT4, the on / off control of the sampling transistor Ts is exactly the same as the operation described in FIG.
Then, as described with reference to FIG. 4, threshold correction preparation is performed in the period LT2, and the threshold correction operation is performed, for example, divided into four periods LT3 (LT3a to LT3d). Thereafter, in the period LT4, the writing of the video signal voltage Vsig and the mobility correction are performed.

In this embodiment, after performing the video signal voltage Vsig writing and mobility correction in the period LT4 as described above, the scanning pulse WSa is set to the L level, and the sampling transistor Ts is turned off. As a result, the gate-source voltage Vgs of the drive transistor Td is determined, and the bootstrap and light emission states (period LT5) are entered.
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, regarding the sampling transistor Ts, when the writing of the video signal voltage Vsig and the mobility correction in the period LT4 are performed, the on state shown in FIG.
Thereafter, in the period LT5, the scanning pulse WSa is at the L level and the scanning pulse WSb is at the H level, so that the state shown in FIG.
That is, the sampling transistor Ts is off, but the connection end of the gate and the signal line DTL is short-circuited.
In the state of FIG. 8B, the light emission operation in the period LT5 is continued.

As described above, the sampling transistor Ts varies in characteristics between the gate-source voltage and the drain current for each individual.
In the period LT5, the potential on the node ND1 (gate of the driving transistor Td) side is higher than the video signal voltage Vsig due to bootstrap after the video signal voltage Vsig is written. In this case, with respect to the sampling transistor Ts, the node ND1 side becomes the drain and the signal line DTL side becomes the source, and a leak current flows from the node ND1 to the signal line DTL side. This amount of leakage current varies depending on the characteristics of the gate-source voltage and the drain current in the off region of the sampling transistor Ts.
Here, in the present embodiment, when the sampling transistor Ts is off, the switch Tb is on. That is, during the light emission period LT5, the gate and source of the sampling transistor Ts are connected, and the gate-source voltage Vgs of the sampling transistor Ts is fixed. As a result, the operating point of the leakage current is fixed.

This will be described with reference to FIG.
FIG. 9 shows the gate-source voltage versus drain current characteristics of a normal transistor and a transistor with a large leakage current, as in FIG. 5B.
If the switch Tb does not exist, the gate voltage of the sampling transistor Ts when turned off is the L level potential of the scanning pulse WSa. On the other hand, the source (signal line DTL side) potential is the potential applied to the signal line DTL, that is, the video signal voltage Vsig or the threshold correction reference voltage Vofs.
If the switch Tb does not exist as described above (for example, in the case of the circuit of FIG. 3), when the voltage of the signal line DTL = the video signal voltage Vsig, the operating point is P2 for each individual sampling transistor Ts. , P3. Further, when the voltage of the signal line DTL = the threshold correction reference voltage Vofs, the operating points are different as P4 and P5 for each individual sampling transistor Ts.
That is, the amount of leakage current differs for each individual sampling transistor Ts. This means that the amount of change in emission luminance differs for each pixel circuit 10.

In contrast, in the present embodiment, when the sampling transistor Ts is turned off, the gate and the source are connected by the switch Tb. As a result, the gate-source voltage Vgs of the sampling transistor Ts is fixed.
If it says in FIG. 9, it will be in the state of the operating point P1. That is, the gate-source voltage Vgs of the sampling transistor Ts does not fluctuate regardless of the voltage of the signal line DTL, so that the amount of leakage current at the time of off does not vary from individual to individual depending on the characteristics.
In other words, since the operating point does not change depending on the voltage of the signal line DTL, it is less likely to be affected by the magnitude of the leakage current due to variations in the characteristics of the gate-source voltage versus the drain current of the sampling transistor Ts.

Then, in the period LT5, it is reduced or eliminated that the amount of leakage current of the sampling transistor Ts is different for each pixel and the amount of change in emission luminance is different.
As described above, since the variation in the light emission luminance for each pixel circuit 10 does not vary, there is no such phenomenon that the screen as shown in FIG. 11 becomes rough when all the pixels emit light with the same gradation value. become.
Therefore, in this embodiment, it is possible to provide an organic EL display device having a uniform image quality with no roughness on the screen display.

[4. Pixel Circuit Operation of Second Embodiment]

FIG. 10 shows a configuration of the pixel circuit 10 as the second embodiment.
In the case of the second embodiment, the switch Ta and the switch Tb are formed of transistors having conduction logics opposite to each other. That is, the switch Ta is an n-channel TFT and the switch Tb is a p-channel TFT.
The gates of the switches Ta and Tb are both connected to a common write control line WSLa.

As the pixel circuit operation, it can be considered that the second scanning pulse WSb does not exist in FIG.
If the first scanning pulse WSa is at H level, the switch Ta is turned on and the switch Tb is turned off. If the first scanning pulse WSa is at L level, the switch Ta is turned off and the switch Tb is turned on.
Based on the control of the sampling transistor Ts by the scanning pulse WSa, one cycle of light emission driving is performed as in the case of FIG.

If the configuration of the second embodiment is adopted, the screen roughness can be reduced or eliminated as in the first embodiment, and the display device configuration is simplified because the write control line WSLb is unnecessary. Can be achieved.
As a modification of FIG. 10, the switch Ta may be constituted by a p-channel TFT and the switch Tb may be constituted by an n-channel TFT. In that case, the switches Ta and Tb may be controlled by the logic of the scanning pulse WSb in FIG.

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. 7, 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.
Further, for example, in the operation example of FIG. 7, the scan pulse WSb is a logic inversion pulse of the scan pulse WSa, but the scan pulse WSb is a pulse that is continuously at the L level during the period LT2 to LT4, for example. Also good. That is, any control is possible as long as the scanning pulse WSb is set to the H level at least during the period LT5 and the gate and source of the sampling transistor Ts are connected.

  DESCRIPTION OF SYMBOLS 1 Organic EL element, 10 pixel circuit, 11 horizontal selector, 12 drive scanner, 13 light scanner, 20 pixel array part, Cs retention capacity, Ts sampling transistor, Td drive transistor, Ta, Tb switch

Claims (4)

When a drive voltage is applied between the light-emitting element and the drain-source, a drive transistor that applies current to the light-emitting element in accordance with the gate-source voltage is connected to the signal line by being conducted. A sampling transistor that inputs a voltage to the gate of the driving transistor, a storage capacitor that is connected between the gate and source of the driving transistor and holds the threshold voltage of the driving transistor and the input video signal voltage, and the sampling transistor A pixel circuit having a first switch element that controls supply of a control signal to the gate of the first switching element, and a second switch element that connects and disconnects between the gate of the sampling transistor and the signal line;
A signal selector for supplying a video signal voltage and a threshold correction reference voltage as the signal line voltage to each signal line arranged in a column on a pixel array in which the pixel circuits are arranged in a matrix;
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 write control lines arranged in rows on the pixel array, the first switch element is turned on, and the second switch element is turned off to make the sampling transistor conductive, and each pixel Writing in which the video signal voltage and the reference voltage are input to the circuit and the first switch is turned off and the second switch element is turned on when the sampling transistor is turned off at least during the light emission period. A scanner,
A display device comprising: As the write control line, a first write control line corresponding to the first switch element and a second write control line corresponding to the second switch element are provided,
The write scanner applies a first scan pulse to the first write control line to perform on / off control of the first switch element, and performs a second scan on the second write control line. The display device according to claim 1, wherein a pulse is applied to perform on / off control of the second switch element. The first switch element and the second switch element are formed by transistors having opposite conduction logics, and the gate of the first switch element and the gate of the second switch element are shared writing. Connected to the control line,
The display device according to claim 1, wherein the writing scanner performs on / off control of the first and second switch elements by applying a scanning pulse to the writing control line. When a drive voltage is applied between the light-emitting element and the drain-source, a drive transistor that applies current to the light-emitting element in accordance with the gate-source voltage is connected to the signal line by being conducted. A sampling transistor that inputs a voltage to the gate of the driving transistor, a storage capacitor that is connected between the gate and source of the driving transistor and holds the threshold voltage of the driving transistor and the input video signal voltage, and the sampling transistor A pixel circuit having a first switch element that controls supply of a control signal to the gate of the first switching element, and a second switch element that connects and disconnects between the gate of the sampling transistor and the signal line;
A signal selector for supplying a video signal voltage and a threshold correction reference voltage as the signal line voltage to each signal line arranged in a column on a pixel array in which the pixel circuits are arranged in a matrix;
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 writing scanner that applies a scanning pulse to writing control lines arranged in rows on the pixel array to control on / off of the first and second switch elements;
As a display driving method for a display device comprising:
The writing scanner turns on the first switch element and turns off the second switch element to turn on the sampling transistor and execute input of a video signal voltage and a reference voltage to each pixel circuit. A display driving method in which the first switch is turned off and the second switch element is turned on when the sampling transistor is turned off at least in the light emission period.

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