JP5224729B2 - Display device and pixel driving method - Google Patents

Description

  The present invention relates to a display device in which a pixel circuit formed in a portion where a column-shaped signal line, a row-shaped scanning line and a power supply line intersect is arranged in a matrix, and a pixel driving method thereof, for example, light emission The present invention relates to a display device using an organic electroluminescence element (organic EL element) as an 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.

By the way, as pixel circuit configurations using organic EL elements, various configurations have been studied from various viewpoints such as improvement of display quality by eliminating luminance unevenness for each pixel and correspondence to higher frame rates. Yes. For example, various pixel circuit configurations and operations have been proposed in which variations in threshold voltage and mobility of the drive transistor for each pixel are canceled so that luminance unevenness for each pixel can be eliminated.
Here, in the present invention, when a novel circuit configuration and circuit operation developed by the applicant of the present invention is assumed as the pixel circuit of the display device, the threshold voltage cancel operation of the driving transistor is appropriately executed. The purpose is to realize a high-quality display.

The display device of the present invention includes a pixel array in which pixel circuits are arranged in a matrix, signal lines arranged in a column on the pixel array, and scanning lines arranged in a row on the pixel array. When the power supply lines arranged in rows on the pixel array, a main scanner for line sequential scanning the pixel circuits in row units sequentially supplies a scan pulse to disposed the above scan lines in rows, said a first potential and a power supply scanner for supplying a power supply voltage switched by the second potential to disposed the above power supply lines in rows in accordance with the line sequential scanning, the above arranged in rows in accordance with the above line sequential scanning A signal selector for supplying a signal potential and a reference potential to each signal line;
The pixel circuit includes a light emitting element, a sampling transistor, a driving transistor, a storage capacitor, and an auxiliary capacitor. The sampling transistor has a gate connected to the scanning line, one of a source and a drain connected to the signal line, and the other connected to the gate of the driving transistor. The drive transistor has its source connected to the light emitting element, a drain connected to the power supply line. The storage capacitor is connected between the gate and source of the driving transistor. Further, the source of the driving transistor is connected to the power line before x rows (where x ≧ 1) through the auxiliary capacitor .

Further, in the pixel circuit having the above structure, the sampling transistor is turned on by the scan pulse during a period in which the power supply line is set to the second potential and the signal line is set to the reference potential. Preparation for threshold cancellation of the driving transistor is performed in which the gate potential is set to the reference potential and the source potential is set to the second potential. In addition, after the threshold cancellation preparation, the sampling transistor is turned on by the scan pulse in a period in which the signal line is set to the reference potential in a state where the power supply line is set to the first potential. A drive transistor threshold cancellation operation is performed. Further period of the signal potential to the signal lines is applied, by the sampling transistor is turned on by the scanning pulse together with the signal potential is retained in the retention capacitor, the mobility correction operation of the driving transistor is carried out, and thereafter, the by the driving transistor, the current supply from the power supply line in the first potential, supplying a drive current corresponding to the signal potential retained in the retention capacitor to the light emitting element A light emitting operation of the light emitting element is performed.
Here, the timing of the scan pulse is set so that the sampling transistor is non-conductive at the timing when the threshold cancellation preparation is performed and the power line before the x-th row is at the first potential. To be.

  The pixel driving method of the present invention is a driving method for the pixel circuit having the above-described configuration, and the sampling transistor is turned on by the scan pulse during a period in which the power supply line is the second potential and the signal line is the reference potential. The threshold voltage canceling preparation operation of the driving transistor in which the gate potential of the driving transistor is the reference potential and the source potential is the second potential, and the power source line is the first potential, and the signal line is set to the reference potential. Threshold cancellation operation of the driving transistor performed by conducting the sampling transistor by the scanning pulse during the period of the potential, and conduction of the sampling transistor by the scanning pulse during the period of applying the signal potential to the signal line As a result, the signal potential is held in the holding capacitor and the driving is performed. Sampling and mobility correcting operation for performing a mobility correction operation of the transistor, and a driving current corresponding to the signal potential held in the holding capacitor by supplying current from the power supply line at which the driving transistor is at the first potential. Is caused to flow through the light emitting element to perform a light emitting operation for causing the light emitting element to emit light. Then, the timing of the scan pulse is set so that the sampling transistor becomes non-conductive at the timing when the power line before x rows becomes the first potential within the period of the threshold cancel preparation operation.

In the present invention, the pixel circuit samples the signal potential in the holding capacitor by the operation of the sampling transistor and the driving transistor, and performs the light emitting operation by flowing the driving current corresponding to the held signal potential to the light emitting element. Do.
Here, the source of the driving transistor is connected to the power line in the previous x row via the auxiliary capacitor. This is because the source potential is increased by writing the signal potential to the holding capacitor and correcting the mobility. In addition, the capacitance value is supplemented in order to prevent the light emitting element from turning on due to insufficient parasitic capacitance of the light emitting element (for example, organic EL element).
However, when the auxiliary capacitor added to the source of the driving transistor is arranged between the power line before x rows in this way, the threshold cancel operation is started in the pixel circuit before x rows, that is, the power line before x rows is When the first potential is applied, coupling enters the source of the driving transistor. As a result, the source potential rises and a current flows from the source side of the driving transistor to the power supply line. However, since the amount of current depends on the gate-source voltage, the amount of current is relatively small when the gate potential is fixed. Therefore, it takes time until the source potential is pulled back to the second potential. When this is within the threshold cancellation preparation period, the source potential needs to be quickly returned to the second potential (before the threshold cancellation operation is started) in order to properly execute the threshold cancellation operation. Is done.
Therefore, the sampling transistor is made non-conductive at the timing when this coupling enters. That is, the gate of the driving transistor is set in a floating state. Then, as the source potential rises due to coupling, the gate potential also rises. As a result, the gate-source voltage of the driving transistor increases and the current value increases, so that the source voltage can be quickly pulled back to the second potential. It becomes possible.

  According to the present invention, when the pixel circuit configuration in which the source of the driving transistor is connected to the power line in the previous x row via the auxiliary capacitor is used, the power line in the previous x row is driven to the first potential. At the timing when coupling enters the source of the transistor, the source potential can be quickly pulled by making the sampling transistor non-conductive and setting the gate of the driving transistor in a floating state. As a result, even during the threshold cancellation preparation period in which the source potential is to be set to the second potential, the influence of the temporary source potential increase due to the coupling is not carried over until after the threshold cancellation is started. There is an effect that the operation can be appropriately executed, and a display with high quality can be realized in a display device using a pixel circuit having a novel configuration.

Hereinafter, as an embodiment of the display device of the present invention, an example of a display device using organic EL elements will be described in the following order.
[1. Overall Configuration of Display Device of Embodiment]
[2. Pixel circuit and operation in the process leading to the present invention]
[3. Circuit operation before reaching the present invention in the pixel circuit configuration of the embodiment of the present invention]
[4. Pixel Circuit Operation as an Embodiment of the Present Invention]

[1. Overall Configuration of Display Device of Embodiment]

FIG. 1 shows an overall configuration of a display device according to an embodiment. As will be described later, this display device includes a pixel circuit 10 having a compensation function for variation in threshold voltage and mobility of a driving transistor.
As shown in FIG. 1, the display device of this example includes a pixel array unit 20 in which pixel circuits 10 are arranged in a matrix in the column direction and the row direction, a horizontal selector 11, a write scanner 12, and a drive scanner 13. Prepare.
Further, signal lines DTL1, DTL2,..., Which are selected by the horizontal selector 11 and supply video signals corresponding to luminance information as input signals to the pixel circuit 10, are arranged in the column direction with respect to the pixel array unit 20. The signal lines DTL1, DTL2,... Are arranged by the number of columns of the pixel circuits 10 arranged in a matrix in the pixel array unit 20.

Further, scanning lines WSL1, WSL2,... And power supply lines DSL1, DSL2,. These scanning lines WSL and power supply lines DSL are respectively arranged by the number of rows of the pixel circuits 10 arranged in a matrix in the pixel array unit 20.
The scanning lines WSL (WSL1, WSL2,...) Are driven by the write scanner 12. The write scanner 12 sequentially supplies scanning pulses WS (WS1, WS2,...) To the scanning lines WSL1, WSL2,. Are sequentially scanned line by line.
The power supply lines DSL (DSL1, DSL2,...) Are driven by the drive scanner 13. The drive scanner 13 is a power supply voltage that is switched to the first potential (Vcc) and the second potential (Vini) to each of the power supply lines DSL1, DSL2,. Power supply pulses DS (DS1, DS2,...) Are supplied.
The horizontal selector 11 applies a signal potential (Vsig) as an input signal to the pixel circuit 10 and a reference potential for the signal lines DTL1, DTL2,... Arranged in the column direction in accordance with the line sequential scanning by the write scanner 12. (Vofs) is supplied.

[2. Pixel circuit and operation in the process leading to the present invention]

The display device is configured as shown in FIG. 1, but here, the configuration and operation of the pixel circuit 10A considered in the process of reaching the present invention will be described.
FIG. 2 shows the configuration of the pixel circuit 10A. The pixel circuit 10A is arranged in a matrix like the pixel circuit 10 in the configuration of FIG. In FIG. 2, only one pixel circuit 10A arranged at a portion where the signal line DTL, the scanning line WSL, and the power supply line DSL intersect is shown for simplification.

  The pixel circuit 10A includes an organic EL element 1 that is a light emitting element, a single storage capacitor Cs, and two thin film transistors (TFTs) as a sampling transistor Tr1 and a drive transistor Tr2. The sampling transistor Tr1 and the drive transistor Tr2 are n-channel TFTs.

The holding capacitor Cs has one terminal connected to the source of the drive transistor Tr2, and the other terminal connected to the gate of the drive transistor Tr2.
The light emitting element of the pixel circuit 10A 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 s of the drive transistor Tr2, and the cathode is connected to a predetermined ground wiring (cathode potential Vcat h ). Note that the capacitance CEL is a parasitic capacitance of the organic EL element 1.
The sampling transistor Tr1 has one end of its drain and source connected to the signal line DTL, and the other end connected to the gate g of the driving transistor Tr2. The gate of the sampling transistor is connected to the scanning line WSL.
The drain d of the drive transistor Tr2 is connected to the power supply line DSL.

The light emission driving of the organic EL element 1 is basically as follows.
At the timing when the signal potential Vsig is applied to the signal line DTL, the sampling transistor Tr1 is turned on by the scanning pulse WS supplied from the write scanner 12 by the scanning line WSL, so that the input signal Vsig from the signal line DTL is stored in the storage capacitor Cs. Is written to. The drive transistor Tr2 causes a current corresponding to the signal potential held in the holding capacitor Cs to flow through the organic EL element 1 by supplying current from the power supply line DSL to which the first potential Vcc is applied by the drive scanner 13, and the organic EL element 1 The element 1 is caused to emit light.

  Further, in the pixel circuit 10A, an operation for canceling the influence of the variation in the threshold voltage Vth of the drive transistor Tr2 (hereinafter referred to as a Vth cancel operation) is performed prior to the current driving of the organic EL element 1. Further, as described above, the input signal Vsig from the signal line DTL is written to the storage capacitor Cs, and at the same time, the mobility correction operation for canceling the influence of the mobility variation of the drive transistor Tr2 is also performed.

The operation of the pixel circuit 10A will be described with reference to FIG.
FIG. 3 shows potentials (signal potential Vsig and reference potential Vofs) applied to the signal line DTL by the horizontal selector 11 as DTL input signals.
Further, a pulse applied to the scanning line WSL by the write scanner 12 is shown as the scanning pulse WS. By this scan pulse WS, the sampling transistor Tr1 is controlled to be conductive / non-conductive.
In addition, a power supply voltage applied to the power supply line DSL by the drive scanner 13 is shown as the power supply pulse DS. As the power supply voltage, the drive scanner 13 supplies the first potential Vcc and the second potential Vini so as to be switched at a predetermined timing.
Also shown are fluctuations in the gate potential Vg and source potential Vs of the driving transistor.

A time point t0 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.
First, preparation for the Vth cancel operation is performed from time t0. Therefore, the drive scanner 13 sets the power supply pulse DS to the second potential Vini at time t0. Further, the scanning pulse WS is set to the H level by the write scanner 12 during the period when the signal line DTL is set to the reference potential Vofs.
By setting the power supply pulse DS of the power supply line DSL to the second potential Vini, the source potential Vs of the drive transistor Tr2 is lowered to the second potential Vini and fixed.
Further, when the signal line DTL is at the reference potential Vofs, the scanning pulse WS is set to the H level and the sampling transistor Tr1 is turned on, so that the gate potential Vg of the drive transistor Tr2 is set to a fixed potential of the voltage Vofs. .
In this way, the gate-source voltage Vgs of the driving transistor Tr2 is opened to be equal to or higher than the threshold voltage Vth to prepare for Vth cancellation.
At time t1 and time t2, the scanning pulse WS is set to H level and the sampling transistor Tr1 is turned on during the period in which the signal line DTL is set to the reference potential Vofs. This is because the gate potential Vg of the driving transistor Tr2 is set. This is performed in order to more reliably fix the reference potential Vofs.

Next, the Vth cancel operation starts from time t3. At this time, the power supply pulse DS is set to the first potential Vcc by the drive scanner 13 while the gate potential Vg of the drive transistor Tr2 is fixed to the reference potential Vofs, thereby increasing the source potential Vs.
However, at this time, since the source potential Vs does not exceed the threshold value of the organic EL element 1 and the sampling transistor Tr1 is made non-conductive during the period when the DTL input signal is the signal potential Vsig, the write scanner 12 The scanning pulse WS is intermittently turned on during the period when the line DTL is at the reference potential Vofs. As a result, as shown as periods tb, tc, td, and te, the Vth cancel operation is performed divided in periods.
This Vth cancel operation is completed when the gate-source voltage Vgs of the drive transistor Tr2 = the threshold voltage Vth.

Thereafter, at the timing (period tf) when the signal line DTL becomes the signal potential Vsig, the scanning pulse WS is turned on, so that the signal potential Vsig is written into the storage capacitor Cs. This period tf also serves as a mobility correction period for the drive transistor Tr2.
In this period tf, the source potential Vs rises according to the mobility of the drive transistor Tr2. That is, if the mobility of the drive transistor Tr2 is large, the increase amount of the source potential Vs is large, and if the mobility is small, the increase amount of the source potential Vs is small. This results in an operation of adjusting the gate-source voltage Vgs of the drive transistor Tr2 in the light emission period according to the mobility.

となる。但し、Ｉｄｓは飽和領域で動作するトランジスタのドレイン・ソース間に流れる電流、μは移動度、Ｗはチャネル幅、Ｌはチャネル長、Ｃｏｘはゲート容量、Ｖｔｈは駆動トランジスタＴｒ２の閾値電圧、Ｖｇｓは駆動トランジスタＴｒ２のゲート−ソース間電圧を表わしている。
この（数１）からわかるように、電流Ｉｄｓは駆動トランジスタＴｒ２のゲート−ソース間電圧Ｖｇｓの２乗値に依存するため、電流Ｉｄｓとゲート−ソース間電圧Ｖｇｓの関係は図４のようになる。 Thereafter, when the source potential Vs becomes a potential exceeding the threshold value of the organic EL element 1, the organic EL element 1 emits light.
That is, the driving transistor Tr2 causes a driving current to flow according to the potential held in the holding capacitor Cs, and causes the organic EL element 1 to emit light. At this time, the source potential Vs of the driving transistor Tr2 is held at a predetermined operating point.
Since the first potential Vcc is applied from the power supply line DSL to the drain of the drive transistor Tr2, and is set to always operate in the saturation region, the drive transistor Tr2 functions as a constant current source, and the organic EL element 1 The current Ids flowing through the transistor depends on the gate-source voltage Vgs of the drive transistor Tr2.

It becomes. 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, Vth is the threshold voltage of the driving transistor Tr2, and Vgs is It represents the gate-source voltage of the drive transistor Tr2.
As can be seen from this (Equation 1), the current Ids depends on the square value of the gate-source voltage Vgs of the drive transistor Tr2, and therefore the relationship between the current Ids and the gate-source voltage Vgs is as shown in FIG. .

In the saturation region, the drain current Ids of the drive transistor Tr2 is controlled by the gate-source voltage Vgs, but the gate-source voltage Vgs (= Vsig + Vth) of the drive transistor Tr2 is constant due to the action of the storage capacitor Cs. The transistor Tr2 operates as a constant current source that allows a constant current to flow through the organic EL element 1.
As a result, the anode potential (source potential Vs) of the organic EL element 1 rises to a voltage at which a current flows through the organic EL element 1, and the organic EL element 1 emits light. That is, light emission at a luminance corresponding to the signal voltage Vsig in the current frame is started.

As described above, the pixel circuit 10A performs an operation for light emission of the organic EL element 1 including the Vth cancel operation and the mobility correction in one frame period.
A current corresponding to the signal potential Vsig can be applied to the organic EL element 1 regardless of variations in the threshold voltage Vth of the drive transistor Tr2 in each pixel circuit 10A and fluctuations in the threshold voltage Vth due to temporal fluctuations by the Vth cancellation 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 Tr2, the image quality deteriorates due to variations in the mobility of the driving transistor Tr2 for each pixel circuit 10A. However, the mobility correction increases or decreases the mobility of the driving transistor Tr2. Accordingly, the source potential Vs is obtained, and as a result, the gate-source voltage Vgs is absorbed so as to absorb the variation in mobility of the driving transistor Tr2 of each pixel circuit 10A. Therefore, the image quality is also deteriorated due to the variation in mobility. It will be resolved.

[3. Circuit operation before reaching the present invention in the pixel circuit configuration of the embodiment of the present invention]

However, the pixel circuit 10A shown in FIG. 2 has the following disadvantages, and therefore the configuration of the pixel circuit 10 corresponding to the present embodiment has been studied.

In the pixel circuit 10A, as described with reference to FIG. 3, the variation in the threshold voltage Vth of the driving transistor Tr2 is canceled by being divided by the scanning pulse WS in a period, and the signal potential Vsig is written to the storage capacitor Cs. At the same time, mobility correction was performed.
Here, the source potential Vs of the drive transistor Tr2 rises during writing to the holding capacitor Cs and the mobility correction shown as the period tf in FIG. 3, but the capacity of the organic EL element 1 (parasitic capacity CEL) is insufficient. Then, the source potential Vs exceeds the threshold value of the organic EL element 1, and the organic EL element 1 is turned on. This affects the image quality as luminance unevenness.

Therefore, as shown in FIG. 5, a pixel circuit 10 having a configuration corresponding to the present embodiment has been considered. This is obtained by adding an auxiliary capacitor Csub to the configuration of the pixel circuit 10A described above.
In FIG. 5, four pixel circuits 10 (m, n−1), 10 as the pixel circuits 10 in the m-th column, the m + 1-th column, the (n−1) -th row, and the n-th row in the pixel array unit 20 of FIG. (M + 1, n-1), 10 (m, n), 10 (m + 1, n) are shown. That is, in the pixel circuit 10 at a portion where the signal lines DTL (m) and DTL (m + 1) intersect with the scanning lines WSL (n−1) and WSL (n) and the power supply lines DSL (n−1) and DSL (n). is there.

In each pixel circuit 10, the source s of the drive transistor Tr <b> 2 is connected to the power line DSL one row before through the auxiliary capacitor Csub.
For example, looking at the pixel circuit 10 (m, n) that is the intersection of the power supply line DSL (n) and the signal line DTL (m), the source s of the drive transistor Tr2 is the power supply of the previous row via the auxiliary capacitor Csub. It is connected to the line DSL (n−1). Similarly, in each of the other pixel circuits 10, the auxiliary capacitor Csub is connected to the power line DSL in the previous row.
By providing the auxiliary capacitor Csub in this manner, the shortage of the capacity of the organic EL element 1 is compensated, and the source potential Vs is set to the organic value when writing the signal potential Vsig to the holding capacitor Cs and correcting the mobility as described above. It can be avoided that the EL element 1 is turned on beyond the threshold value.

ここで、ΔＶｃｃは、前行の電源線ＤＳＬ(n-1)の変動分であり、即ち第２電位Ｖｉｎｉから第１電位Ｖｃｃに切り替わる際の電圧変動分である（ΔＶｃｃ＝Ｖｃｃ−Ｖｉｎｉ）。
このカップリングの影響によって、適正にＶｔｈキャンセル動作の準備ができなくなり、Ｖｔｈキャンセル動作が十分に発揮されないことが生ずる。 However, in the configuration of the pixel circuit 10 in FIG. 5, the same operation as in FIG. 3 is performed, but in that case, the following inconvenience occurs.
Since the auxiliary capacitor Csub is formed between the organic EL element 1 and the power supply line DSL in the previous stage in order to supplement the capacity of the organic EL element 1, coupling occurs in the Vth cancel operation in the pixel circuit 10 in the previous stage.
The pixel circuit 10 (m, n) in FIG. 5 will be described. When the power supply line DSL (n-1) in the previous row is set to the first potential Vcc, coupling occurs in the source of the driving transistor Tr2, and the source The potential Vs increases. The increase ΔVs of the source potential Vs due to the coupling is as follows.

Here, ΔVcc is a fluctuation amount of the power supply line DSL (n−1) in the previous row, that is, a voltage fluctuation amount when the second potential Vini is switched to the first potential Vcc (ΔVcc = Vcc−Vini).
Due to the influence of this coupling, properly will not be ready for Vth cancel operation, it occurs Vth cancel operation is not such be sufficiently exhibited.

Hereinafter, with reference to the pixel circuit 10 (m, n), the operation when coupling is performed will be described with reference to FIGS.
Time t0, t1, t2, t3 of Fig. 6 is similar to the time t0, t1, t2, t3 as shown in FIG. That is, at time t0, the power pulse DS (n) in the power line DSL (n) is set to the second potential Vini, and preparation for Vth cancellation is started. That is, the source potential Vs of the driving transistor Tr2 is lowered to the second potential Vini. Further, the scanning pulse WS (n) of the scanning line WSL (n) is set to the H level, the sampling transistor Tr1 is turned on, and the gate potential Vg of the driving transistor Tr2 is set to the reference potential Vofs.
Thereafter, even at time points t1 and t2, the scanning pulse WS (n) of the scanning line WSL (n) is set to the H level when the signal line DTL is set to the reference potential Vofs. so that Keru suppress Yue the gate potential Vg of Tr2 to the reference potential Vofs.
Thereafter, the power supply pulse DS (n) of the power supply line DSL (n) is set to the first potential Vcc at time t3, and the Vth cancel operation in the periods tb, tc, td, and te is executed as described in FIG. .

Here, the power pulse DS (n−1) shown in FIG. 6 is the power pulse of the power line DSL (n−1) of the (n−1) th row, and the power pulse DS (n−1) is The second potential Vini rises to the first potential Vcc at the timing when the Vth cancel operation is started in the pixel circuit 10 (m, n-1) in the (n-1) th row. For example, normally, the power pulse DS (n−1) rises to the first potential Vcc at a timing 1H (one horizontal period) before the timing when the power pulse DS (n) becomes the first potential Vcc. The timing at which the power supply pulse DS (n−1) for the pixel circuit 10 in the previous row becomes the first potential Vcc is shown as a time point t21. At time t21 when the power supply pulse DS (n-1) becomes the first potential Vcc, the scanning pulse WS (n) is at the H level, that is, the period in which the sampling transistor Tr1 is conductive.
At time t21, the pixel circuit 10 (m, n) is in the period of preparation for Vth cancellation and is the timing at which the above-described coupling is entered.

A waveform obtained by enlarging the time axis around the time t21 is shown in FIG. 8, and equivalent circuits before and after coupling are shown in FIGS. 7 (a) and 7 (b).
The scanning pulse WS (n) is set to the H level at the time point t2 shown in FIG. 8, as described in FIGS. 3 and 6, while the signal line DTL is set to the reference potential Vofs, the sampling transistor Tr1 is set. This is to make it conductive so that the gate potential Vg of the drive transistor Tr2 is properly maintained at the reference potential Vofs. FIG. 7A shows an equivalent circuit when the scanning pulse WS (n) is set to the H level at the time point t2.
Since the gate potential Vg of the drive transistor Tr2 is kept at the reference potential Vofs and the power supply pulse DS (n) = the second potential Vini is continued, the source potential Vs is fixed at the second potential Vini.

Here, at time t21, when the power pulse DS (n−1) of the (n−1) th row, which is the previous stage, rises to the first potential Vcc, the above-described coupling is entered, and the source potential Vs rises to Vini + ΔVs. The variation ΔVs of the source potential Vs is as described above (Equation 2).
At this time, the power supply line DSL (n) = the second potential Vini.
Therefore, as shown as current Ids in FIG. 7B, a current flows from the source side to the power supply line DSL (n).
At this time, since the sampling transistor Tr1 is conductive, the gate potential Vg is fixed at the reference potential Vofs. That is, the variation ΔVg = 0 of the gate potential Vg at the time when the coupling enters.

By the way, the time t21 at which this coupling enters is in the period of Vth cancellation preparation. In the Vth cancellation preparation operation, the gate potential Vg is set to the reference potential Vofs and the source potential Vs is set to the second potential Vini. This is an operation aiming to open the gate-source voltage Vgs above the threshold voltage Vth by fixing to V.
Here, when coupling occurs at time t21, the source potential Vs instantaneously increases to (Vini + ΔVs), but as the current Ids flows as shown in FIG. 7B, the source potential Vs decreases. .
The problem is whether or not the source potential Vs has returned to the second potential Vini at the time t3 when the Vth cancel operation is started.

Looking at the source potential Vs in FIG. 8, the source potential Vs rises to (Vini + ΔVs) by coupling at time t21, and then the current Ids described in FIG. It is falling.
However, the decrease in the source potential Vs is relatively slow, and the source potential Vs is not returned to the second potential Vini at the time t3 when the Vth cancel operation is started.
Then, in preparation for Vth cancellation, the gate-source voltage Vgs of the drive transistor Tr2 is not sufficiently opened.
In the following, the term “source potential pulling” is used to return the source potential Vs to the second potential Vini, and the expression “source potential pulling is slow / fast” is used.

If the pulling of the source potential is delayed as in the case of FIG. 8, preparation for Vth cancellation is not performed properly, and as a result, the Vth cancellation operation does not function properly. Therefore, it is required to quickly pull the source potential immediately after the source potential Vs rises due to coupling.

[4. Pixel Circuit Operation as an Embodiment of the Present Invention]

The display device of the embodiment has the configuration shown in FIG. 5 as the pixel circuit 10 in the configuration of FIG. Basically, the operation described in FIG. 3 is performed.
As described above, when the circuit configuration of FIG. 5 is adopted, it may occur that the pulling of the source after coupling is delayed and preparation for a sufficient Vth cancel operation cannot be performed. In the configuration of the pixel circuit 10 of FIG. 5, immediately after the source potential Vs rises due to the coupling during the Vth cancel preparation period, the source potential is quickly pulled so that the Vth cancel preparation operation is appropriately performed. To do.

The operation as the embodiment will be described with reference to FIG. 9, FIG. 10, and FIG.
FIG. 9 shows the input signal by the signal line DTL, the scanning pulse WS (n), the power pulse DS (n), and the power pulse DS (n−1) one row before, as in FIG. Yes.
In the present embodiment, the timing at which the write scanner 12 sets the scanning pulse WS (n) to the H level is shifted, and the scanning pulse WS (n) is set to the L level at the time t21 when coupling is started.
That is, in FIG. 9, the scan pulse WS (n) is set to the H level at time t0 and time t1, as in FIGS. 3 and 6, but the power pulse DS (n−1) one row before is In the vicinity of the time point t21 at which the first potential Vcc is reached, the timing of the scan pulse WS is shifted so that the scan pulse WS (n) is at the H level during the time point t2 ′ to t22 ′.
That is, when coupling is entered at time t21, the sampling transistor Tr1 is turned off and the gate of the drive transistor Tr2 is brought into a floating state.

FIG. 10A is an equivalent circuit during the period from time t2 ′ to t22 ′. At this time, as in FIG. 7A, the sampling transistor Tr1 is turned on, and the gate potential Vg = Vofs and the source potential Vs = Vini.
FIG. 10B shows a state after coupling is entered at time t21. The sampling transistor Tr1 is non-conductive. At this time, the source potential Vs rises due to the coupling, whereby a current Ids flows from the source side toward the power supply line DSL (n) as shown in the figure.
Here, the amount of current Ids is considered.

のようになる。
また、駆動トランジスタＴｒ２のゲート電位Ｖｇは、この場合フローティング状態であるためソース電位Ｖｓの上昇と共に上昇する。図１０（ｃ）のように容量成分Ｃｄ、Ｃｇｄ、Ｃｇｓを考えると、ゲート電位Ｖｇの変動分ΔＶｇは、

のようになる。
このようにソース電位Ｖｓの上昇とともにゲート電位Ｖｇが上昇することは、ゲート−ソース間電圧Ｖｇｓが開くことを意味する。
但しこの図１０（ｂ）の電流Ｉｄｓが流れる場合は、ソースｓがドレイン、ドレインｄがソースとして働くため、ここでいうゲート−ソース間電圧Ｖｇｓとは、ゲートｇ−電源線ＤＳＬ（ｎ）の間の電圧である。 As shown in FIG. 10B, at the point of coupling, the sampling transistor Tr1 is non-conductive and the gate g of the drive transistor Tr2 is in a floating state.
In this state, the variation ΔVs of the source potential Vs due to coupling is

become that way.
In addition, since the gate potential Vg of the drive transistor Tr2 is in a floating state in this case, the gate potential Vg rises with an increase in the source potential Vs. Considering the capacitance components Cd, Cgd, and Cgs as shown in FIG. 10C, the variation ΔVg of the gate potential Vg is

become that way.
Thus, the rise of the gate potential Vg with the rise of the source potential Vs means that the gate-source voltage Vgs is opened.
However, when the current Ids of FIG. 10B flows, the source s serves as the drain and the drain d serves as the source. Therefore, the gate-source voltage Vgs here is the gate g-power supply line DSL (n). Is the voltage between.

Here, it compares with FIG.7 (b) mentioned above. In the case of FIG. 7B, the gate potential Vg is fixed to the reference potential Vofs. Therefore, the gate-source voltage Vgs that affects the current Ids is a potential difference between the reference potential Vofs and the second potential Vini of the power supply line DSL (n).
On the other hand, in the state of FIG. 10B, the gate-source voltage Vgs that influences the current Ids when coupling occurs is the gate potential Vg = reference potential Vofs + ΔVg and the second of the power supply line DSL (n). This is the potential difference of the potential Vini.
That is, in the case of FIG. 10B, the gate-source voltage Vgs is sufficiently opened as compared with FIG. 7B.
As can be seen from the above (Formula 1) and FIG. 4, the gate-source voltage Vgs affects the current value as the current Ids by its square value. Therefore, in the case of FIG. 10B, the current Ids becomes sufficiently large, and as a result, the pulling of the source can be accelerated.

FIG. 11 shows an enlarged view around time t21 in this example.
The scanning pulse WS (n) is set to the H level during the period from the time point t2 ′ to t22 ′, which is shifted from the time point t21.
At the time t21, the power pulse DS (n-1) one stage before becomes the first potential Vcc and coupling occurs. At this time, the source potential Vs rises by ΔVs (the above formula 3) as shown in the figure, Along with this, the gate potential Vg rises by ΔVg (the above formula 4).
In this case, the gate potential Vg rises to sufficiently open the gate-source voltage Vgs, and the current Ids increases. As a result, the source potential Vs is quickly returned to the second potential Vini as shown in the figure. The gate potential Vg is also the reference potential Vofs.

  When the source potential is pulled earlier, for example, when the Vth cancel operation is started at time t3, which is 1H after time t21, the source potential Vs is returned to the second potential Vini, and thus the appropriate Vth is obtained. The cancel operation is executed.

That is, according to the present embodiment, when adopting a pixel circuit configuration in which the source s of the drive transistor Tr2 is connected to the power line DS (n−1) one row before through the auxiliary capacitor Csub, one row is used. At the time t21 when the previous power supply line DS (n-1) becomes the first potential Vcc and the source s is coupled, the sampling transistor Tr1 is made non-conductive and the gate g of the drive transistor Tr2 is brought into a floating state. The source potential Vs can be pulled quickly. As a result, even if the source potential Vs is a threshold cancellation preparation period in which the second potential Vini should be set, the influence of the temporary source potential increase due to coupling is not carried over until the threshold cancellation is started. There is an effect that the threshold canceling operation can be properly executed, and a display with high quality can be realized as a display device.
In addition, being able to pull the source potential quickly is preferable in view of dealing with a higher frame rate for image display. That is, even when the 1H period is shortened, the Vth cancel operation can be appropriately executed.

Note that it is also preferable to increase the rate of increase of the gate potential Vg in order to pull the source potential faster.
The variation amount ΔVg of the gate potential Vg is as shown in the above (Equation 4). From this (Equation 4), it is understood that ΔVg increases as the storage capacitor Cs is increased.
Therefore, the rate of increase of the gate potential Vg when coupling is performed can be increased by setting the capacitance value of the storage capacitor Cs. As the fluctuation amount ΔVg of the gate potential Vg increases, the gate-source voltage Vgs (the voltage between the gate and the power supply line DSL (n) in this case) increases accordingly, and the current Ids can be increased. Because it can, you can pull the source faster.
In particular, when the 1H period is shortened due to the high frame rate, it is appropriate to increase the rate of increase of the gate potential in this way.

In the circuit configuration shown in FIG. 5, the source s of the drive transistor Tr2 is connected to the power line DSL one row before via the auxiliary capacitor Csub. For example, the source s is connected to the power line DSL two rows before. You may do it.
The configuration in this case is shown in FIG. In FIG. 12, six pixel circuits 10 (m, m) as the pixel circuits 10 in the m-th column, the (m + 1) -th column, the (n-2) -th row, the (n-1) -th row, and the n-th row in the pixel array unit 20 of FIG. n-2), 10 (m + 1, n-2), 10 (m, n-1), 10 (m + 1, n-1), 10 (m, n), 10 (m + 1, n) ). That is, the signal lines DTL (m), DTL (m + 1), the scanning lines WSL (n-2), WSL (n-1), WSL (n) and the power supply lines DSL (n-2), DSL (n-1). , DSL (n).

In each pixel circuit 10, the source s of the drive transistor Tr2 is connected to the power line DSL two rows before through the auxiliary capacitor Csub.
For example, looking at the pixel circuit 10 (m, n), the source s of the drive transistor Tr2 is connected to the power line DSL (n-2) two rows before through the auxiliary capacitor Csub. Similarly, in each of the other pixel circuits 10, the auxiliary capacitor Csub is connected to the power line DSL two rows before.

FIG. 13 shows the operation timing in this case as in FIG. That is, from the viewpoint of the pixel circuit 10 (m, n), the power line DSL (n−2) in the previous two rows is set to the first potential Vcc at time t23, which is 2H before the start of the Vth cancel operation at time t3. Standing up, coupling will enter at this time.
Here, as shown in the figure, at time t21, the scanning pulse WS (n) is set to be at the L level, so that when the coupling is entered, the gate of the driving transistor Tr2 is in a floating state. As in the case of the above-described embodiment, the source can be pulled quickly.

Further, with this configuration, a time margin of 2H period is created from the time when coupling is started until the time when the Vth cancel operation is started. In other words, the time margin for completing the pulling of the source increases. Thus, by increasing the margin of the time for pulling the source, it is possible to more reliably prepare for Vth cancellation and to make the Vth cancellation operation more reliable.
In particular, when the high frame rate is advanced and the time as the 1H period is shortened, it is preferable to set the source drawing time to the 2H period in this way.
Of course, it is also conceivable to connect the auxiliary capacitor Csub to the power supply line DSL three or more rows before and further widen the margin of the period for completing the pulling of the source.

It is explanatory drawing of a structure of the display apparatus of embodiment of this invention. It is explanatory drawing of the pixel circuit structure before reaching embodiment. It is explanatory drawing of the basic operation | movement of the pixel circuit before reaching embodiment, and the pixel circuit of embodiment. It is explanatory drawing of the Ids-Vgs characteristic of a drive transistor. It is explanatory drawing of the pixel circuit of embodiment. It is explanatory drawing of the circuit operation | movement before reaching embodiment. It is an equalization circuit diagram in circuit operation before reaching an embodiment. It is a wave form chart in circuit operation before reaching an embodiment. It is explanatory drawing of the circuit operation | movement of embodiment. It is an equalization circuit diagram in circuit operation of an embodiment. It is a wave form chart in circuit operation of an embodiment. It is explanatory drawing of the pixel circuit of other embodiment. It is explanatory drawing of the circuit operation | movement of other embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Organic EL element, 10 pixel circuit, 11 horizontal selector, 12 light scanner, 13 drive scanner, 20 pixel array part, Cs holding capacity, Tr1 sampling transistor, Tr2 drive transistor, Csub auxiliary capacity

Claims (2)

A pixel array in which pixel circuits each having a light emitting element, a sampling transistor, a driving transistor, a storage capacitor, and an auxiliary capacitor are arranged in a matrix;
On the pixel array, signal lines arranged in a row,
Scan lines arranged in rows on the pixel array;
On the pixel array, power supply lines arranged in rows,
Sequentially supplying a scan pulse to disposed the above scan lines in rows, a main scanner for line sequential scanning the pixel circuits on a row basis,
A power supply scanner for supplying a power supply voltage is switched between a first potential and a second potential to said power supply lines arranged in rows in accordance with the above line-sequential scanning,
And a signal selector for supplying a signal potential and a reference potential disposed the above-mentioned signal lines in rows in accordance with the above line-sequential scanning,
In the above pixel circuit,
The sampling transistor has a gate connected to the scanning line, one of a source and a drain connected to the signal line, the other connected to the gate of the driving transistor,
The drive transistor has its source connected to the light emitting element, a drain connected to the power supply line,
The storage capacitor is connected between the gate and source of the driving transistor,
The source of the driving transistor is connected to the power line before x rows (where x ≧ 1) through the auxiliary capacitor,
The sampling transistor is turned on by the scan pulse during a period in which the power supply line is set to the second potential and the signal line is set to the reference potential, so that the gate potential of the driving transistor is set to the reference potential, Preparation for threshold cancellation of the drive transistor, where the source potential is the second potential,
After the threshold cancellation preparation, the sampling transistor is turned on by the scan pulse in a period in which the signal line is set to the reference potential in a state where the power supply line is set to the first potential. The threshold cancellation operation of the transistor is performed,
While the signal potential is applied to the signal line, the sampling transistor is turned on by the scan pulse, whereby the signal potential is held in the holding capacitor and the mobility correction operation of the driving transistor is performed. Done,
The driving transistor causes the light emitting operation of the light emitting element by causing the driving current corresponding to the signal potential held in the storage capacitor to flow through the light emitting element by supplying current from the power supply line at the first potential. To be done,
The timing of the scan pulse is set so that the sampling transistor is non-conductive at the timing when the threshold cancellation preparation is performed and at the timing when the power line before the x row becomes the first potential. Viewing apparatus that.   A pixel array in which pixel circuits each having a light emitting element, a sampling transistor, a driving transistor, a storage capacitor, and an auxiliary capacitor are arranged in a matrix;
  On the pixel array, signal lines arranged in a row,
  Scan lines arranged in rows on the pixel array;
  On the pixel array, power supply lines arranged in rows,
  A main scanner that sequentially supplies a scanning pulse to each of the scanning lines arranged in a row to scan the pixel circuit line by line;
  A power supply scanner for supplying a power supply voltage that is switched between a first potential and a second potential to each of the power supply lines arranged in a row in accordance with the line sequential scanning;
  A signal selector for supplying a signal potential and a reference potential to each of the signal lines arranged in a row in accordance with the line sequential scanning;
  In the above pixel circuit,
  The sampling transistor has a gate connected to the scanning line, one of a source and a drain connected to the signal line, the other connected to the gate of the driving transistor,
  The drive transistor has a source connected to the light emitting element, a drain connected to the power supply line,
  The storage capacitor is connected between the gate and source of the driving transistor,
  As a pixel driving method of a display device in which the source of the driving transistor is connected to the power line before x rows (where x ≧ 1) through the auxiliary capacitor,
  The sampling transistor is turned on by the scan pulse during a period in which the power supply line is the second potential and the signal line is the reference potential, and the gate potential of the driving transistor is the reference potential and the source potential is the second potential. A threshold cancel preparation operation of the drive transistor, which is a potential, and
  A threshold canceling operation of the driving transistor performed by conducting the sampling transistor by the scan pulse during a period in which the signal line is set to the reference potential in a state where the power supply line is set to the first potential;
  The sampling transistor is turned on by the scanning pulse during a period in which the signal potential is applied to the signal line, thereby holding the signal potential in the storage capacitor and performing a mobility correction operation of the driving transistor. Sampling and mobility correction operations;
  Light emission that causes the light-emitting element to emit light by causing the drive transistor to flow a drive current corresponding to the signal potential held in the storage capacitor to the light-emitting element by supplying current from the power supply line at the first potential. Operation and
  And run
  A pixel driving method in which the timing of the scan pulse is set so that the sampling transistor becomes non-conductive at the timing when the power line before the x-th row becomes the first potential within the period of the threshold cancel preparation operation.

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