JP2011191620A - Display device and display driving method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To carry out a grayscale expression in a larger number of levels even without using a signal line driver corresponding to the larger number of grayscale levels. <P>SOLUTION: A signal selector giving a video signal voltage and a reference voltage for threshold correction to a pixel circuit selects, as a reference voltage for threshold correction, one voltage value from a plurality of voltage values in accordance with a grayscale value at which each pixel circuit emits light, and outputs the selected voltage value. The emission luminance of the pixel circuit where the threshold correction is carried out is proportional to the square of a difference between the video signal voltage and the standard voltage for threshold correction. Thereby, light emission in a greater number of grayscale levels is attainable by selecting the standard voltage for threshold correction among the plurality of values without increasing the number of grayscale levels by the video signal voltage. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

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

JP 2003-255856 A JP 2003-271095 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.

Since the organic EL element is a current light emitting element, the color gradation is obtained by controlling the current value flowing through the organic EL element.
For this reason, in the organic EL display, a video signal voltage corresponding to the gradation value of the video data is applied to each pixel circuit, and each pixel circuit supplies a current corresponding to the input video signal voltage to the organic EL element. Shed. Thus, each pixel circuit emits light according to the corresponding signal value.

Currently, there is a demand for higher image quality of video display panels, and there is a strong demand for increasing the number of gray levels. Generally, the greater the number of gradations, the higher the color reproducibility.
However, if the input video data is a multi-gradation signal such as 12 bits or 14 bits, a high-performance driver IC that performs DA conversion is required.
For each pixel circuit, an analog voltage value corresponding to the gradation value of the video data is generated by the driver IC, but a driver IC having an increased number of bits for DA conversion is used to support multiple gradations. This leads to an increase in IC size and a significant cost increase.

  In view of such a point, the present invention performs multi-gradation display without using a high-performance driver IC. That is, it is an object to allow display to be executed by expressing more gradations beyond the number of gradations that the driver IC can output as a video signal voltage value.

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. The pixel circuit has a pixel array arranged in a matrix. Further, a signal selector for supplying a threshold correction reference voltage and a video signal voltage for each pixel circuit as the signal line voltage to each signal line arranged in a row on the pixel array, and on the pixel array A drive control scanner that applies a power pulse to each power control line arranged in a row and applies a drive voltage to the drive transistor of the pixel circuit, and each write arranged in a row on the pixel array A write scanner that applies a scan pulse to the control line to control the sampling transistor of the pixel circuit and to input a threshold correction reference voltage and a video signal voltage to each pixel circuit. The signal selector selects one voltage value from a plurality of voltage values as the threshold correction reference voltage according to the gradation value to be emitted from each pixel circuit, and outputs the selected voltage value to the signal line.
Further, the difference value of each voltage value as the threshold correction reference voltage is smaller than the voltage difference value of the minimum gradation interval as the video signal voltage.
The signal selector selects a voltage value as the video signal voltage according to a value of m bits (n> m) of n-bit gradation values of video data for the pixel circuits, and the n The voltage value as the threshold correction reference voltage is selected according to the (nm) bit value of the bit gradation values.

  The display driving method of the present invention is a display driving method for a display device including a pixel array, a signal selector, a drive control scanner, and a writing scanner. In this display driving method, one voltage value is selected from a plurality of voltage values according to a gradation value to be emitted from the pixel circuit, and is output to the signal line.

In the present invention as described above, by providing a plurality of threshold correction reference voltages without changing the number of gradations based on the video signal voltage, it is possible to perform gradation expression in more stages.
That is, a signal selector that provides a video signal voltage and a threshold correction reference voltage to the pixel circuit selects one voltage value from a plurality of voltage values as a threshold correction reference voltage according to a gradation value to be emitted by each pixel circuit. Output.
The light emission luminance in the pixel circuit where the threshold correction operation is performed is proportional to the square of the difference between the video signal voltage and the threshold correction reference voltage. Accordingly, even if the number of gradations due to the video signal voltage is not increased, a light emission operation with more gradations can be performed by selecting a threshold correction reference voltage from among a plurality of threshold voltages.

According to the present invention, a voltage selected according to a gradation value for which a plurality of voltage values are to be displayed is supplied to the pixel circuit as a threshold correction reference voltage.
As a result, gradation expression exceeding the number of gradations by the video signal voltage output from the signal line driver in the signal selector can be realized, so that high color reproducibility can be realized at low cost.

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. FIG. 10 is an explanatory diagram of the pixel circuit operation of the embodiment. It is explanatory drawing of multi-gradation of embodiment. It is explanatory drawing of selection of the reference voltage for threshold value correction | amendment of embodiment. It is explanatory drawing of a structure of the horizontal selector of embodiment.

Hereinafter, embodiments of the present invention will be described in the following order.
[1. Configuration of Display Device and Pixel Circuit]
[2. Basic pixel circuit operation]
[3. Pixel Circuit Operation of Embodiment]

[1. Configuration of Display Device and Pixel Circuit]

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

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

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

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

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

The horizontal selector 11 supplies a signal line voltage as an input signal to the pixel circuit 10 to the signal lines DTL1, DTL2,... Arranged in the column direction in accordance with the line sequential scanning by the write scanner 13.
In the present embodiment, the horizontal selector 11 supplies a threshold correction reference voltage Vofs and a video signal voltage Vsig as signal line voltages to each signal line.
The video signal voltage Vsig is a voltage value based on the gradation value as the value of the video data input to the horizontal selector 11.
The threshold correction reference voltage Vofs has two types of voltage values in the present embodiment, and the horizontal selector 11 uses the threshold correction reference voltage Vofs1 as the first voltage value and the second voltage value. Is selectively output as a threshold value correction reference voltage Vofs2. The threshold correction reference voltages Vofs1, Vofs2 are selected according to the gradation value as the value of the video data.

  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 which is a light emitting element, a storage capacitor Cs, a sampling transistor Ts, an n-channel thin film transistor (TFT) as a drive transistor Td, and an auxiliary capacitor Csub. 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).
Further, an auxiliary capacitor Csub is connected in parallel with the organic EL element 1.

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

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

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

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

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

[2. Basic pixel circuit operation]

Here, in order to understand the present invention, the basic operation of the pixel circuit 10 will be described. This is a circuit operation including a threshold correction operation and a mobility correction operation for compensating for uniformity deterioration due to variations in the threshold and mobility of the driving transistor Td of each pixel circuit 10.

In 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 light emission cycle (each frame period) of the pixel circuit 10.
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 uses the threshold correction reference voltage Vofs as a single predetermined voltage value and the pulse voltage as the video signal voltage Vsig as a signal line voltage in one horizontal period (1H) as a signal line. Give to DTL.
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 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 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, 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 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.
In the period LT1, preparation for extinction and threshold correction is performed.
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.
Further, the gate potential (node ND1) of the drive transistor Td decreases to a certain potential Vg ′ in accordance with the decrease in the source potential.

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

Subsequently, threshold correction (Vth correction) is performed in the period LT2.
That is, while the signal line voltage is the threshold correction reference voltage Vofs, the write scanner 13 maintains the H level of the scanning pulse WS. The drive scanner 12 sets the power supply pulse DS to the drive 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 (node ND2) 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 charges the holding capacitor Cs, the parasitic capacitor Coled, and the auxiliary capacitor Csub. Used for. “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 an operation in which the gate-source voltage of the drive transistor Td is set to the threshold voltage Vth. Therefore, the source potential of the drive transistor Td is raised until the gate-source voltage of the drive transistor Td reaches the threshold voltage Vth.
After a certain period of time, the gate-source voltage of the drive transistor Td becomes the threshold voltage Vth.
In this example, the threshold correction operation is performed once. However, the threshold correction operation is divided and performed a plurality of times in order to secure time for the gate-source voltage of the drive transistor Td to be the threshold voltage Vth. There is also.

When the gate-source voltage of the drive transistor Td becomes the threshold voltage Vth at the end of the period LT2, 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)
At this time, the write scanner 13 sets the scanning pulse WS to L level, the sampling transistor Ts is turned off, and the threshold correction operation is completed.

  Thereafter, during a period LT3 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 drive 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 drive transistor Td charges the holding capacitor Cs, the parasitic capacitor Coled, and the auxiliary capacitor Csub. Used for. 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 reflects. The gate-source voltage Vgs of the driving transistor Td is reduced to reflect the mobility, 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 bootstrap and light emission states are entered.
That is, the scanning pulse WS is set to L level, the sampling transistor Ts is turned off, writing is completed, and the organic EL element 1 is caused to emit light.
In this case, a current Ids corresponding to the gate-source voltage Vgs of the drive transistor Td flows, the potential of the node ND2 rises to a voltage at which the current flows in the organic EL element 1, and the organic EL element 1 emits light. At this time, the sampling transistor Ts is off, and the gate of the drive transistor Td (node ND1) rises at the same time as the potential of the node ND2 rises, so the gate-source voltage Vgs remains constant. (Bootstrap operation)

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

[3. Pixel Circuit Operation of Embodiment]

The basic pixel circuit operation is as described above. In this case, the light emission gradation in each pixel circuit 10 is determined by the gate-source voltage Vgs of the drive transistor Td. The gate-source voltage Vgs is determined by the video signal voltage Vsig input from the signal line DTL in the period LT3. That is, the light emission gradation is controlled by the value of the video signal voltage Vsig. In other words, the gradation of the video signal voltage Vsig has to be multi-staged in order to realize light emission with more gradations. However, as described above, the cost of the signal line driver of the horizontal selector 11 is increased in that case.
Therefore, in the present embodiment, the horizontal selector 11 keeps the video signal voltage Vsig itself as it is, and selectively outputs, for example, two voltage values Vofs1 and Vofs2 as the threshold correction reference voltage Vofs. Thus, light emission with a higher gradation is realized.

FIG. 4 shows the pixel circuit operation of the embodiment. 4 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.
Note that threshold correction reference voltages Vofs1, Vofs2 are output as the driving of the signal line DTL (signal line voltage) by the horizontal selector 11.
The scanning pulse WS and the power supply pulse DS are the same as those in FIG.

  In one light-emission cycle, the operations of the periods LT1, LT2, and LT3 are basically the same as those in FIG. 3, and thus redundant description is avoided. In FIG. 4, as the signal line voltage output by the horizontal selector 11, As for the threshold correction reference voltage, there are two types of voltage values, Vofs1 and Vofs2. In the figure, as the threshold correction reference voltage Vofs, the solid line indicates the voltage value Vofs1, and the alternate long and short dash line indicates the voltage value Vofs2.

From when the threshold correction operation is prepared near the end of the period LT1, during the period LT2, the sampling transistor Ts is turned on by the scanning pulse WS, and the threshold correction reference voltage Vofs is input to the gate of the drive transistor Td. .
In this period, when the horizontal selector 11 applies the threshold correction reference voltage Vofs1 (solid line) to the signal line DTL, the gate voltage Vg and the source voltage Vs change as indicated by the solid line in the figure. That is, the threshold value correction is performed in the period LT2 as described with reference to FIG. In this case, the gate voltage Vg at the time of threshold correction in the period LT2 is Vg = Vofs1, and when the threshold correction is completed, the source voltage Vs = Vofs1-Vth. Thereafter, writing of the video signal voltage Vsig and mobility correction are performed in a period LT3, and then bootstrap and light emission are performed.
On the other hand, when the horizontal selector 11 applies the threshold correction reference voltage Vofs2 (one-dot chain line) to the signal line DTL, the gate voltage Vg and the source voltage Vs change as indicated by the one-dot chain line in the figure. Similarly, in this case, threshold correction is performed in the period LT2, but the gate voltage Vg = Vofs2 at the time of threshold correction in the period LT2 is obtained, and at the completion of the threshold correction, the source voltage Vs = Vofs2-Vth. Thereafter, writing of the video signal voltage Vsig and mobility correction are performed in a period LT3, and then bootstrap and light emission are performed.

  The reason why the number of gradations can be increased by selectively providing the pixel circuit 10 with two kinds of voltage values (Vofs1, Vofs2) as the threshold correction reference voltage Vofs is as follows.

The current characteristic of the drive transistor Td is proportional to the square of (Vgs−Vth), as can be seen from the above-described equation 1.
Here, since threshold correction is performed in the driving method of FIG. 4, the current of the driving transistor Td is proportional to the square of (Vsig−Vofs).
That is, the output current characteristic is a quadratic curve with Vofs as the origin, as shown in FIG.
This means that the gradation of the output current (luminance) has the same number of gradations as the gradation of the video signal voltage Vsig.

Here, in the present embodiment, for example, the threshold correction reference voltage Vofs has two values (Vofs1, Vofs2). That is, depending on the value of the threshold correction reference voltage Vofs, two characteristic curves as shown in FIG. 5B are obtained in which the current characteristics are shifted in parallel by the difference of Vofs.
Each curve has the same number of output gradations as the conventional one, but the difference between Vofs1 and Vofs2 is set to a potential smaller than the voltage difference corresponding to the minimum gradation interval of the video signal voltage Vsig, that is, the minimum bit of the video signal voltage Vsig. Then, the total number of output gradations can be doubled.
That is, as the gradation value, the gradation value indicated by the broken line and the gradation value indicated by the alternate long and short dash line in FIG. 5B can be expressed for one Vsig value.
Thus, even with the conventional number of signal line driver gradations, the output gradation can be made more multi-bit.
The voltage difference between Vofs1 and Vofs2 is preferably an intermediate voltage between the voltage differences of the minimum bits of the video signal voltage Vsig.

FIG. 6 shows the increase in the number of bits. In this embodiment, the number of bits of input video data = (number of bits of Vofs) + (number of bits of Vsig) can be set.
For example, when the video signal voltage Vsig is 9 bits and the threshold correction reference voltage Vofs is two types of Vofs1 and Vofs2, if the bit is 1 bit, 10-bit gradation can be expressed. In other words, even if the driver for driving the signal line DTL is compatible with 9 bits, gradation expression with 10 bits is possible.

  FIG. 7 shows a configuration example of the horizontal selector 11. In the horizontal selector 11, for example, the video signal input unit 80, the output value setting units 81-1 to 81-n, and the signal line drivers 82-1 to 82- corresponding to the signal lines DTL1 to DTL (n). n.

Video data is supplied to the video signal input unit 80 from a video signal processing system (not shown). For example, video data is 10-bit data.
The video signal input unit 80 functions as a line buffer, and transfers video data to be given to each pixel circuit 10 to the output value setting units 81-1 to 81-n for each horizontal line.
In the output value setting units 81-1 to 81-n, the values of the video signal voltage Vsig and the threshold correction reference voltage Vofs are set as shown in FIG. 6 according to the input video data value.
For example, if the input video data (gradation value) is “0”, the video signal voltage Vsig = 0 and the threshold correction reference voltage = Vofs1. If the input video data (gradation value) is “1”, the video signal voltage Vsig = 0 and the threshold correction reference voltage = Vofs2. If the input video data (gradation value) is “1023”, the video signal voltage Vsig = 511 and the threshold correction reference voltage = Vofs2.
As described above, the output value setting units 81-1 to 81-n set the voltage value of the video signal voltage Vsig with the upper 9 bits for the 10-bit video data, and correct the threshold according to the lower 1 bit. Vofs1 or Vofs2 is selected as the voltage value of the reference voltage Vofs for use.

The output value setting units 81-1 to 81-n output the set threshold correction reference voltage Vofs and the video signal voltage Vsig to the signal line drivers 82-1 to 82-n at predetermined timings, respectively. To do.
That is, if the gradation value corresponding to a certain pixel circuit 10 is “1023”, the output value setting unit 81- (x) corrects the threshold at the timing corresponding to the period LT2 in FIG. The reference voltage Vofs2 is applied to the signal line driver 82- (x), and the voltage value Vofs2 is applied to the signal line DTL. Further, at the timing corresponding to the period LT3 in the pixel circuit 10, the video signal voltage Vsig = 511 is applied to the signal line driver 82- (x), and the video signal voltage Vsig corresponding to “511” is applied to the signal line DTL.

As described above, each pixel circuit 10 emits light at a gradation specified by the threshold correction reference voltage Vofs and the video signal voltage Vsig.
In the example of FIG. 6, although each of the signal line drivers 82-1 to 82-n is 9-bit compatible, each pixel circuit 10 can express 10-bit gradation. In other words, a 9-bit signal line driver can be used even when a 10-bit gradation is performed, and the cost can be reduced.

The same can be said for a smaller number of gradations, and the cost can be reduced. For example, when the input video data is 8 bits, the signal line driver IC corresponding to 8 bits has been conventionally used. However, by using the method of the present embodiment, the signal line driver IC is compatible with 6 bits or 4 bits. A compatible IC can be used.
In general, the cost of a signal line driver is proportional to the number of gradations. Therefore, by applying this embodiment, a low bit driver IC can be used, and the cost can be reduced.

The number of types of voltage values as the threshold correction reference voltage Vofs is two types of Vofs1 and Vofs2 in the above example, but of course, other types may be used. The number of types of voltage values as the threshold correction reference voltage Vofs may be determined according to the number of video data gradations (number of bits) and the number of bits corresponding to the signal line driver.
For example, when a 6-bit compatible signal line driver is used when the video data has an 8-bit gradation, the remaining 2 bits may be expressed by the threshold correction reference voltage Vofs. In this case, four types of voltage values Vofs1 to Vofs4 are used as the threshold correction reference voltage Vofs.

  Although the embodiment has been described above, the present invention is not limited to the above example. For example, the configuration of the pixel circuit 10 is not limited to that shown in FIG. 2, and various examples are conceivable. The present invention can be applied to any pixel circuit configuration in which a threshold correction operation using the threshold correction reference voltage Vofs is performed.

  DESCRIPTION OF SYMBOLS 1 Organic EL element, 10 pixel circuit, 11 horizontal selector, 12 drive scanner, 13 light scanner, 20 pixel array part, 81-1 to 81- (n) output value setting part, 82-1 to 82- (n) signal Line driver, Cs holding capacitor, Ts sampling transistor, Td drive transistor

Claims (4)

The driving voltage is applied between the light-emitting element and the drain-source to apply a current corresponding to the gate-source voltage to the light-emitting element, and the signal line voltage is driven to be electrically connected to the light-emitting element. A pixel circuit having a sampling transistor that is input to the gate of a transistor and a storage capacitor that is connected between the gate and source of the drive transistor and that holds the threshold voltage of the drive transistor and the input video signal voltage A pixel array arranged in
A signal selector for supplying a threshold correction reference voltage and a video signal voltage for each pixel circuit 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
The display device, wherein the signal selector selects one voltage value from a plurality of voltage values as the threshold correction reference voltage according to a gradation value to be emitted from each pixel circuit, and outputs the selected voltage value to the signal line.   The display device according to claim 1, wherein a difference value of each voltage value as the threshold correction reference voltage is smaller than a voltage difference value of a minimum gradation interval as the video signal voltage.   The signal selector selects a voltage value as the video signal voltage according to a value of m bits (n> m) of n-bit gradation values of video data for the pixel circuits, and the n bits The display device according to claim 2, wherein a voltage value as the threshold correction reference voltage is selected in accordance with a value of (nm) bits among the gradation values. 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 for each pixel circuit 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
As a display driving method for a display device comprising:
A display driving method in which the signal selector selects one voltage value from a plurality of voltage values as the threshold correction reference voltage according to a gradation value to be emitted from each pixel circuit and outputs the selected voltage value to the signal line.

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