KR100562664B1 - Light emitting display device and driving method thereof - Google Patents

Abstract

The present invention relates to a light emitting display device and a driving method thereof. A light emitting display device according to the present invention includes a driving transistor connected between a first power supply voltage and a fifth switching element to supply a current to the light emitting element, a first switching element transferring a data voltage of a data line to one end of the first storage element; A second switching element for applying an initialization voltage to the node A on the output terminal side of the driving transistor, a third switching element for diode-connecting the driving transistor, a first switching element and a first storage element connected before compensating the threshold voltage of the driving transistor A fourth switching element for applying a second power supply voltage to the node C, a second storage element electrically connected between the node C and a gate of the driving transistor, and storing a threshold voltage of the driving transistor, and a third switching element being turned on And a fifth switching element that is turned off before being turned on and turned on after the first switching element is turned off.

Organic EL, display device, pixel circuit, threshold voltage, voltage drop, initialization voltage

Description

Luminescent display and method for driving the same {Organic electro luminescent display and method for driving the same}

1 is a conceptual diagram of an organic electroluminescent device.

2 is a circuit diagram of a pixel circuit of a conventional organic electroluminescent device.

3 is a schematic diagram of an organic electroluminescent display device according to a first embodiment of the present invention.

4 is an equivalent circuit diagram of a pixel circuit of an organic light emitting display device according to a first embodiment of the present invention.

5 is an equivalent circuit diagram of a pixel circuit according to a second exemplary embodiment of the present invention.

6 is an equivalent circuit diagram of a pixel circuit according to a third exemplary embodiment of the present invention.

7 is an equivalent circuit diagram of a pixel circuit according to a fourth exemplary embodiment of the present invention.

8 is a driving waveform diagram for driving a pixel circuit according to the first to fourth embodiments of the present invention.

9 is an equivalent circuit diagram of a pixel circuit according to a fifth exemplary embodiment of the present invention.

10 is an equivalent circuit diagram of a pixel circuit according to a sixth embodiment of the present invention.

11 is a driving waveform diagram for driving a pixel circuit according to the fifth and sixth embodiments of the present invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active light emitting display device and a driving method thereof, and more particularly to an electro luminescent display device (EL).

In general, an organic EL display device is a display device for electrically exciting a fluorescent or phosphorescent organic compound to emit light, and is capable of displaying gray scales by voltage programming or current programming of M * N organic EL elements OLEDs.

1 is a conceptual diagram for an organic EL element OLED. Referring to FIG. 1, the organic EL device basically has a structure of an anode electrode, an organic thin film, and a cathode electrode. The anode electrode and the cathode electrode are formed of a metal such as transparent electrode (ITO) or aluminum (Al) according to a top emission method, a bottom emission method, or a double-sided emission method. The organic thin film has an electron transport layer (ETL), a hole transport layer (HTL), on both sides of the emission layer (EML) to improve the emission efficiency by improving the injection and transport characteristics of electrons and holes. A multilayer structure includes an electron injecting layer (EIL), a hole injecting layer (HIL), a hole blocking layer (HBL) (not shown), and the like. The light emission response characteristic of such an organic EL device emits light in proportion to the current flowing into the device, and is similar to, for example, diode characteristics.

2 is an equivalent circuit diagram of a pixel circuit of a conventional current driving method. Referring to FIG. 2, a conventional pixel circuit includes an organic EL element OLED, a driving transistor M1 for supplying a driving current from a power supply voltage V _DD to an organic EL element, a gate and a source of the driving transistor. a storage capacitor Cs connected between the sources, and a switching transistor M2 connected between the data line Dm and the gate of the driving transistor M1. The gate of the switching transistor M2 is connected to the scan line Sn. The data line Dm and the scan line Sn are respectively connected to a data driver for supplying a data voltage representing an image signal and a scan driver for supplying a selection signal for selecting a driving pixel.

The driving operation of the conventional pixel circuit will be described below. First, when the enable signal is input to the n-th scan line Sn of the display panel, the switching transistor M2 is turned on. The data signal of the data line Dm is applied to the gate of the driving transistor M1, and at the same time, the storage capacitor is induced with the power supply voltage V _DD and the voltage having the data voltage of the data signal as both ends of the voltage. Therefore, even when the switching transistor M2 is turned off, the voltage induced in the storage capacitor acts to control the driving current of the driving transistor so that the organic EL element displays a specific luminance corresponding to the driving current. At this time, the current flowing through the organic EL device is as shown in Equation 1 below.

Here, I _OLED is a current flowing through the organic EL device, V _GS is a voltage between the gate and the source of the driving transistor M1, V _TH is a threshold voltage of the driving transistor M1, and beta (β) is a constant value. .

As shown in Equation 1, according to the pixel circuit shown in Fig. 2, the driving current is supplied to the organic EL element corresponding to the voltage difference between the power supply voltage and the data voltage, and the organic EL element corresponding to the supplied current substantially. Will emit light. At this time, the applied data voltage has a multi-level value in a predetermined range in order to express the gray scale.

However, in the conventional current driving type pixel circuit, there is a problem in that it is difficult to obtain a high gradation due to variation in threshold voltage and electron mobility of the thin film transistor caused by nonuniformity of the manufacturing process. For example, when driving a thin film transistor of a pixel at 3V, a voltage must be applied to the control electrode of the thin film transistor at intervals of about 12 ms in order to express a gray level of 256 levels, that is, an 8-bit gray level. If the threshold voltage variation of the thin film transistor is 100 kV due to the nonuniformity of the providing process, it is difficult to express the high gradation of the organic EL element because the voltage interval of 12 kV cannot be controlled. In addition, when the beta value in Equation 1 is changed due to the variation in electron mobility of the thin film transistor generated in the manufacturing process, it is difficult to control the thin film transistor and thus it is difficult to express higher gradation.

On the other hand, in the pixel circuit of the current programming method, which is a kind of the current driving method, the uniformity of the pixel is improved by arranging a current source for supplying a uniform current to the pixel circuit through the entire data line. Therefore, if the current programming method is used, even if the driving transistors in each pixel have non-uniform voltage-current characteristics, relatively uniform display characteristics can be obtained.

However, in general, the current flowing through the organic EL element is a fine current. Therefore, in the conventional pixel circuit of the current write method, it is necessary to control the pixel circuit as a fine data current, so that it takes a long time to charge the data line. For example, assuming that the data line load capacitance is 30 mA, a few milliseconds of time are required to charge the load of the data line with a data current of tens of thousands to hundreds of mA. This is a problem that the charging time is not enough considering the line selection time of several tens of kilowatts.

On the other hand, in the case where a polycrystalline silicon thin film transistor (hereinafter referred to as 'Poly-Si TFT') is formed in a pixel circuit of an organic EL display device in the current thin film transistor manufacturing process, the driving circuit of the organic EL display device is a display panel. It is possible to embed in. However, in such a conventional manufacturing method, for example, software having a limited processing window of a laser energy density for obtaining coarse grains and a hardware aspect due to, for example, uneven dose of the laser beam itself. The two factors of the conventional aspect have a process problem that causes the non-uniformity of grain size of the poly-Si thin film constituting the active layer. This non-uniformity of grain size results in a shift in the threshold voltage of the driving transistor. As described above, since it is very difficult to obtain uniform voltage-current characteristics due to the characteristics of the poly-Si TFT manufacturing process, the organic EL display device is characterized by non-uniformity of electrical characteristics of the Poly-Si TFT which is frequently used in display panels and driving circuits. Due to this, there is a problem that it is difficult to express uniform luminance. Therefore, in order to obtain high quality display characteristics, it is very important to develop driving circuits considering characteristics of thin film transistors.

As described above, since the threshold voltages of the driving transistors supplying current to the organic EL elements in the organic EL display apparatus are slightly different for each position on the process and the wafer, it is necessary to compensate for the nonuniformity of the threshold voltages for each pixel in order to uniformly induce current. You must design a circuit that can Otherwise, a problem arises in that the luminance of the organic EL element is different for each pixel even with a slight change in the threshold voltage.

Moreover, since the organic EL element is a current driving type element, the larger the screen size of the display device, the more difficult and complicated the design of the driving circuit becomes. In addition, the current write method has many advantages over the current driving method of FIG. 2 as described above. However, as the screen size of the display device increases, the length of the data line becomes longer and the resistive and capacitive loads increase, so driving of a low gray level may not be possible using only the current write method.

Therefore, there is a need for a suitable driving method and driving circuit along with improving the characteristics of the driving transistor element.

The technical task of the present invention is to more reliably compensate for variations in threshold voltages of the driving transistors to express high gray levels.

In addition, an object of the present invention is to improve the luminance uniformity of a display screen by eliminating an image defect caused by operating characteristics of a driving transistor of a pixel circuit.

According to an aspect of the present invention, there is provided a pixel circuit formed in a plurality of data lines for transmitting a data voltage representing an image signal, a plurality of scan lines for transmitting a selection signal, and a plurality of pixels defined by the data lines and the scan lines, respectively. A light emitting display device includes: a driving transistor for supplying a driving current for emitting light emitting elements by a pixel circuit thereof, a first storage element storing a first voltage corresponding to a voltage difference between the first power supply voltage and a data voltage; A first switching element transferring a data voltage to one end of the first storage element in response to the first selection signal, a second switching element transferring an initialization voltage to an output terminal of the driving transistor in response to the second selection signal, and a third A third switching element for diode-connecting the driving transistor in response to the selection signal and a first switching in response to the third selection signal A fourth switching element transferring a second power supply voltage to a node to which the device and the first storage device are connected; and a second voltage electrically connected between the node and a gate of the driving transistor and corresponding to a threshold voltage of the driving transistor. A second storage element for storing, and a fifth switching element electrically connected between the driving transistor and the light emitting element.

One end of the second switching element is preferably connected to the third scan line or the reference voltage line.

Preferably, the second switching element is a PMOS transistor or an NMOS transistor.

The fifth switching element is turned off by the fourth selection signal of the disable level of the fourth scanning line before the turn-on of the third and fourth switching elements and the first of the enable level of the fourth scanning line after the first switching element is turned off. It is preferable to turn on by the 4 select signal.

According to another aspect of the present invention, a method of driving a light emitting display device includes: a plurality of data lines for transmitting a data voltage, a plurality of scan lines for transmitting a selection signal, and a driving transistor and a scan line for supplying current to the light emitting device; Driving a light emitting display device including a plurality of pixel circuits having a series circuit of a first to a fifth switching element that is controlled to be turned off and the gate-source voltage of the driving transistor for a predetermined period and a second storage element; A method comprising: applying an enable level select signal to a second scan line to initialize an output terminal of the drive transistor when an enable level select signal is applied to the third scan line to compensate for a threshold voltage of the drive transistor; Between the first switching element and the first storage element while an enable level selection signal is applied to the Compensating the threshold voltage of the driving transistor by charging a voltage corresponding to the threshold voltage of the driving transistor to a second storage element connected between a node of the gate and the gate of the driving transistor; and a selection signal having an enable level is applied to the first scan line. Charging a first voltage corresponding to the data voltage to a first storage element connected to the data line while being applied, and a series circuit of the first and second storage elements while the selection signal of the enable level of the fourth scan line is applied Controlling the current supplied to the light emitting device by the voltage charged in the.

(Example)

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily practice the present invention. However, the present invention may be embodied in various forms and should not be construed as limited to the embodiments set forth herein.

In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention. Like parts are designated by like reference numerals throughout the specification. When a part is connected to another part, this includes not only a directly connected part but also a case where another part is connected in between.

First, an organic EL display device according to a first embodiment of the present invention will be described with reference to FIG. 3. 3 is a schematic configuration diagram of an organic EL display device according to a first embodiment of the present invention. In FIG. 3, for convenience of description, the first and second power supply voltage lines supplying the first and second power supply voltages V _DD and V _SUS connected to the pixel circuit are omitted (see FIG. 4).

Referring to FIG. 3, an organic EL display device according to an exemplary embodiment of the present invention includes an organic EL display panel 200, a scan driver 300, and a data driver 400. The organic EL display panel 200 includes a plurality of scan lines S1-Sn, P1-Pn, C1-Cn, and E1-En extending from the scan driver 300 in the horizontal direction, and the data driver 400. For example, a plurality of data lines D1 -Dm and a plurality of pixel circuits 100 extending in the vertical direction are included. Each pixel circuit 100 is formed in a pixel region defined by a plurality of scan lines and a plurality of data lines.

The scan driver 300 controls the selection signal for driving each pixel circuit, and controls the selected selection signal to the first scan line S1 -Sn, the second scan line P1 -Pn, and the third scan line C1 -Cn. Or it supplies to 4th scanning line E1-En. The selection signal transmitted to each switching element through the first to fourth scan lines is represented by a first selection signal, a second selection signal, a third selection signal and a fourth selection signal, respectively, and functions to turn each switching element on or off. do.

The data driver 400 controls the data voltage representing the image signal and supplies the controlled data voltage to each of the data lines D1 -Dm.

The scan driver 300 and the data driver 400 are preferably formed on the same substrate as the organic EL display panel 200.

Next, the pixel circuit 100A will be described in more detail with reference to FIG. 4. 4 is an equivalent circuit diagram of a pixel circuit according to a first embodiment of the present invention. 4 illustrates a pixel circuit connected to an n th scan line Sn, an n th scan line Pn, an n th scan line En, and an m th data line Dm for convenience of description. It was. In addition, the third scan line Cn is denoted as Sn-1 in that the first scan line Sn-1 selected in advance is used in common connection.

Referring to FIG. 4, the pixel circuit 100A includes an organic EL element OLED, a driving transistor M1, five switching elements S1, S2, S3, S4 and S5, and two capacitors C1 and C2. Include. The driving transistor M1 is formed of a PMOS transistor.

The first scan line Sn transfers a first selection signal to the first switching element S1 connected between the data line Dm and the first capacitor C1. The second scan line Pn transfers a second selection signal to the second switching element S2 connected between the drain of the driving transistor M1 and the reference voltage line supplying the reference voltage V _SS . The third scan line Sn-1 transfers a third selection signal to the third switching element S3 for diode-connecting the driving transistor M1 supplying the driving current to the organic EL element OLED. In addition, the third scan line Sn-1 is connected between the node C to which the first switching element S1 and the first capacitor C1 are connected, and the second power voltage line to supply the second power voltage V _SUS . The third selection signal is transferred to the fourth switching device S4. The fourth scan line En transfers a fourth selection signal to the fifth switching element S5 connected between the driving transistor M1 and the organic EL element OLED.

The gate of the driving transistor M1 is connected to the node B, the source is connected to the first power supply voltage V _DD , and the drain thereof is the second switching device S2 and the third switching device. It is connected to one end of S3 and the fifth switching element S5.

The first capacitor C1 is connected between the source of the driving transistor M1 and the node C and stores a first voltage corresponding to the voltage difference between the first power supply voltage and the data voltage. The second capacitor C2 is connected between the node C and the gate of the driving transistor M1 and stores a second voltage corresponding to the threshold voltage of the driving transistor M1 to compensate for the threshold voltage of the driving transistor M1. do. The second voltage becomes substantially the same voltage as the threshold voltage of the driving transistor M1. The series circuit of the first and second capacitors C1 and C2 is connected between the source and the gate of the driving transistor M1. The series circuit of the first and second capacitors C1 and C2 maintains the source-gate voltage of the driving transistor M1 at a specific voltage level so that the driving transistor M1 supplies a constant current regardless of the threshold voltage. Act as a function.

The first switching element S1 is connected between the data line Dm and the node C and transfers the data voltage of the data line Dm to the node C in response to the first selection signal from the first scan line Sn. The second switching element S2 is connected between the drain of the driving transistor M1 and the reference voltage line and induces the reference voltage V _SS to the node A in response to the second selection signal from the second scan line Pn.

The third switching element S3 is connected between the drain and the gate of the driving transistor M1 and diode-connects the driving transistor M1 in response to a third selection signal from the third scan line Sn-1. The fourth switching device S4 is connected between the second power supply voltage line and the node C and applies the second power supply voltage V _SUS to the node C in response to the third selection signal transmitted to the third switching device S3. . The node C becomes a common connection point of one end of the first capacitor C1, one end of the second capacitor C2, one end of the first switching element S1, and one end of the fourth switching element S3.

The fifth switching element S5 is connected between the drain of the driving transistor M1 and the anode of the organic EL element OLED. The fifth switching element S5 transfers the current from the driving transistor M1 to the organic EL element OLED in response to the fourth selection signal of the fourth scanning line En. The organic EL element OLED is connected between the fifth switching element S5 and the reference voltage line and emits light substantially corresponding to the amount of current flowing through the driving transistor M1. Usually, the reference voltage V _SS supplied from the reference voltage line refers to the voltage induced on the cathode side of the organic EL element OLED.

According to the pixel circuit of the present invention, even when the threshold voltage of the driving transistor M1 is very large, it is not necessary to lower the reference voltage V _SS of the pixel, thereby reducing power consumption.

Specifically, in the conventional pixel circuit, when the threshold voltage of the TFT driving transistor is very large, the threshold voltage of the driving transistor may be compensated by lowering the cathode voltage, that is, the reference voltage, of the organic EL element connected to the output terminal of the driving transistor. However, the present invention is made to initialize the output terminal of the driving transistor, so that even when the threshold voltage of the TFT driving transistor is very large, it is not necessary to lower the reference voltage of the organic EL element. Therefore, the present invention does not need to increase the voltage for driving the pixel, thereby reducing the power consumption.

Next, the pixel circuit 100B will be described with reference to FIG. 5. 5 is an equivalent circuit diagram of a pixel circuit according to a second exemplary embodiment of the present invention. In the pixel circuit according to the second exemplary embodiment of the present invention, one end of the second switching element S2 is connected to the third scan line Sn-1, and the other end of the pixel circuit of FIG. 4 is drained from the driving transistor M1. Except for being connected to, it has the same structure as the pixel circuit of the first embodiment.

Referring to FIG. 5, the second switching element S2 is connected between the third scan line Sn-1 and the drain of the driving transistor M1. The second switching element S2 drains the voltage of the third scan line Sn- 1 as the initialization voltage of the node A in response to the second selection signal of the enable level of the second scan line Pn. To pass on. In this case, the voltage of the third scan line Sn-1 may be a third select signal having a low level.

As described above, in the present invention, an initialization voltage for the threshold voltage compensation circuit is applied to the node A before the threshold voltage of the driving transistor M1 in the pixel circuits 100A and 100B is compensated for. Therefore, the driving transistor M1 is more stably diode-connected when the third switching device S3 is turned on by the third selection signal. In other words, in the pixel circuits 100A and 100B according to the first and second embodiments of the present invention, by forcibly applying an initialization voltage to the output terminal of the driving transistor M1, that is, the node A, the organic EL element OLED is applied. The driving transistor M1 can be diode-connected with high stability even when the abnormal voltage generated at the anode side is induced at the node A or when a voltage having a magnitude higher or similar to the first power supply voltage is induced at the node A. Therefore, the threshold voltage of the driving transistor in the pixel circuit is reliably compensated. Here, the initialization voltage refers to a voltage lower than a voltage obtained by subtracting the absolute value of the threshold voltage of the driving transistor M1 from the first power supply voltage V _DD . As shown in FIGS. 4 and 5, the organic EL element OLED The reference voltage V _SS induced on the cathode side of or the low level voltage of the third scan line Sn-1 may be used.

In the pixel circuits of the first and second embodiments described above, the first to fifth switching elements S1, S2, S3, S4, and S5 have been described as general switching elements. However, the first to fifth switching elements are preferably formed of transistors. Hereinafter, third and fourth embodiments in which the first to fifth switching elements S1, S2, S3, S4, and S5 are implemented as PMOS transistors will be described in detail with reference to FIGS. 6 to 8.

6 and 7 are equivalent circuit diagrams of a pixel circuit according to third and fourth embodiments of the present invention. In the pixel circuit according to the fourth embodiment of the present invention, except that one end of the third switching element is connected to the third scan line instead of one end of the third switching element as in the third embodiment. It has the same structure as the pixel circuit of the third embodiment. 8 is a driving waveform diagram for driving a pixel circuit according to the first to fourth embodiments of the present invention. 6 and 7 illustrate pixel circuits connected to an n th scan line Sn and an m th data line Dm for convenience of description.

Referring to FIG. 6, a pixel circuit according to a third exemplary embodiment of the present invention may include an organic EL element OLED, a first transistor M1, second to sixth transistors M2, M3, M4, M5, and M6. And first and second storage elements C1 and C2. The first transistor M1 is a driving transistor for supplying a driving current corresponding to the source-gate voltage of the first transistor M1 to the organic EL element. The second to sixth transistors M2 to M6 serve as switching elements for electrically connecting or disconnecting both ends thereof. The first to sixth transistors M1 to M6 are formed of PMOS transistors.

The gate of the first transistor M1 is connected to the node B, its source is connected to the first power supply voltage line supplying the first power supply voltage V _DD , and its drain is connected to the node A. The gate of the second transistor M2 is connected to the first scan line Sn and its source is connected to the data line Dm. A gate of the third transistor M3 is connected to the second scan line Pn, a source thereof is connected to a reference voltage line, and a drain thereof is connected to the drain of the first transistor M1. The gate of the fourth transistor M4 is connected to the third scan line Sn-1, the source of which is connected to the drain of the driving transistor M1, and the drain of which is connected to the gate of the driving transistor M1. A gate of the fifth transistor M5 is connected to the third scan line Sn-1, and a source thereof is connected to a second power supply voltage line supplying a second power supply voltage V _SUS . The gate of the sixth transistor M6 is connected to the fourth scan line En, the source thereof is connected to the drain of the first transistor M1, and the drain thereof is connected to the anode of the organic EL element OLED. The drains of the second and fifth transistors M2 and M5 and one end of the first and second storage elements are commonly connected at the node C.

Meanwhile, as shown in FIG. 7, in the third switching device M3, the gate of the device is connected to the second scan line Pn, the source thereof is connected to the third scan line Sn-1, and the drain thereof is the first. It may be connected to the drain side (node A) of the transistor M1.

Next, an operation of the pixel circuit according to the first to fourth embodiments of the present invention will be described in detail with reference to FIG. 8. First, the sixth transistor M6 is turned off at the time t1 by the fourth select signal of the high level from the fourth scan line En. In operation t2, the third transistor M3 is turned on by the low level second select signal from the second scan line Pn. In addition, the fourth and fifth transistors M4 and M5 are turned on by the low level third select signal applied from the third scan line Sn-1 at time t2. When the fourth transistor M4 is turned on, the first transistor M1 is diode connected. When the fifth transistor M5 is turned on, the second power supply voltage is applied to the node C.

The node A on the drain side of the driving transistor M1 is induced with a voltage lower than the voltage obtained by subtracting the absolute value of the threshold voltage of the driving transistor from the first power supply voltage. With this arrangement, in the present invention, even when the voltage of the node A is changed by other voltages or abnormal voltages introduced into the pixel circuit, the node A on the output terminal side of the driving transistor M1 is compensated before compensating the threshold voltage of the driving transistor. By forcibly initializing, the driving transistor can be diode-coupled with high stability to compensate for the threshold voltage.

Next, at a time t3, the third transistor M3 is turned off in response to the second selection signal of the high level of the second scan line Pn. The gate (node B) of the driving transistor M1 is induced with a voltage obtained by subtracting the absolute value of the threshold voltage of the driving transistor M1 from the first power supply voltage V _DD . The voltage at node B is expressed by Equation 2.

Here, V _B represents the voltage of the node B, V _DD represents the first power supply voltage, and | V _TH | represents the absolute value of the threshold voltage of the driving transistor.

On the other hand, when the fifth transistor M5 is turned on as described above, the second power supply voltage V _SUS is applied to the node C. This configuration compensates for the voltage drop of the first power supply voltage V _DD . In detail, a voltage drop is generated in the first power supply voltage line providing the first power supply voltage V _DD to each pixel according to the length of the conductive line from the voltage source to each pixel. In this case, each pixel circuit displays different luminance according to the deviation of the voltage drop. In fact, when a voltage drop of the first power supply voltage occurs in each pixel circuit of the organic EL display device, different voltages are stored in a capacitor that induces a constant voltage between the source and gate of the driving transistor M1, and thus each pixel circuit By supplying different driving currents to the organic EL element, there is a problem that luminance unevenness of the display occurs. Therefore, in the pixel circuit according to the present invention, the second power supply voltage is applied to the node C to reduce the influence of the voltage drop of the first power supply voltage.

Further, according to the above configuration, the second capacitor C2 connected between the node B and the node C stores the second voltage by the voltage V _B of the node B and the voltage of the node C induced at both ends thereof. do. For example, when the second power supply voltage V _SUS is commonly connected to the first power supply voltage V _DD , the second capacitor C2 has substantially the same size as the threshold voltage of the driving transistor M1. The voltage is stored.

Next, when the fourth and fifth transistors M4 and M5 are turned off in response to the high level third select signal from the third scan line Sn-1 at time t4, the gate (node) of the driving transistor M1 is turned off. B) is in a floating state.

Next, when the second transistor M2 is turned on by the low level first selection signal of the first scan line Sn at time t5, the data voltage of the data line Dm is applied to the node C. The first capacitor C1 stores a first voltage corresponding to a voltage difference between the first power voltage V _DD and the data voltage. When the data voltage is applied to the node C, the voltage of the node B facing the node C with the second capacitor C2 interposed therebetween is boosted by the voltage change amount of the node C. The voltage of the node C is represented by the amount of voltage change applied to the node C, that is, the voltage difference between the data voltage and the second power supply voltage V _SUS . Therefore, a new voltage (V _B) of the node B are set as shown in equation (3).

Where V _B is the voltage of the node B, V _DD is the first power supply voltage, | V _TH | is the absolute value of the threshold voltage of the driving transistor M1, V _SUS is the second power supply voltage, and V _DATA is the data voltage. Indicates.

Next, at a time t6, the second transistor M2 is turned off in response to the high first select signal of the first scan line Sn. Thereafter, the sixth transistor M6 is turned on in response to the fourth selection signal of the low level of the fourth scan line En. Meanwhile, the sixth transistor M6 may be turned on at the same time as the second transistor M2 within a range that does not affect precharge and programming of the pixel circuit.

This configuration enables the driving transistor M1 to supply the driving current set by the series circuit of the first and second capacitors to the organic EL element OLED more independently of its threshold voltage. The driving current of the driving transistor M1 is expressed by equations (4) and (5).

Where I _OLED is the drive current flowing through the drive transistor M1, V _DD is the first power supply voltage, | V _TH | is the absolute value of the threshold voltage of the drive transistor, V _SUS is the second power supply voltage, and V _DATA is The data voltage of the data line Dm is shown.

As in Equation 5, the driving current I _OLED is controlled only by the second power supply voltage V _SUS and the data voltage V _DATA . Therefore, the luminance of the organic EL element can be controlled to reduce the nonuniformity of the display device.

As described above, in the present invention, the diode connection of the driving transistor can be stably set at all times by initializing the output terminal of the driving transistor while the path to the organic EL element is blocked before compensating the threshold voltage of the driving transistor. With this arrangement, the present invention forcibly forms node A, which is the output terminal side of the driving transistor, as an initialization voltage before compensation of the threshold voltage even when an abnormal voltage generated inside or outside the pixel circuit changes the potential of the node A. As a result, the threshold voltage of the driving transistor can be uniformly and stably compensated.

As an example, in the present invention, when the output end side node of the driving transistor of the pixel circuit is initialized, a large current flowing through the organic EL element can be prevented to reduce the phenomenon that the black level is not displayed properly. If the output terminal of the driving transistor is initialized while the current flows from the node of the output terminal side of the driving transistor to the organic EL element, the current flowing through the organic EL element for a short time during precharging is very large and the black level ) Is not properly represented. Therefore, in the above-described case, there is a problem in that the contrast ratio of the display device is reduced. However, the present invention can reduce the phenomenon that the black level of the display screen is not displayed properly during precharging by initializing the output terminal node of the driving transistor while the driving transistor and the EL element are electrically separated. Therefore, the present invention can increase the contrast ratio of the light emitting display device.

Meanwhile, in the pixel circuit of the light emitting display device according to the third and fourth embodiments of the present invention, the third transistor M3 may be formed as an NMOS transistor instead of the PMOS transistor as shown in FIGS. 9 and 10. A driving waveform diagram for the pixel circuit of this light emitting display device is shown in FIG. In the present exemplary embodiment, the third transistor M3 is turned on in response to the high level second selection signal of the second scan line Pn and is turned off in response to the low level second selection signal. Other configurations are the same as those of the pixel circuits of the third and fourth embodiments, and detailed description thereof will be omitted.

The driving transistor and the switching element according to the embodiment of the present invention described above are preferably formed of a thin film transistor (TFT). In addition, the switching transistor according to the present invention may use a dual gate thin film transistor having excellent characteristics as a switching element such as to reduce leakage current when the transistor is turned off. Such a configuration will be apparent to those skilled in the art, so a detailed description thereof will be omitted.

Although a preferred embodiment of the present invention has been described in detail, the present invention is not limited to the above embodiment, various modifications are possible by those skilled in the art within the scope of the technical idea of the present invention. Do.

As described above, according to the present invention, there is an advantage that the threshold voltage of the driving transistor for supplying the driving current to the EL element can be more reliably compensated by connecting the switching element for transmitting the initialization voltage to the compensation circuit of the driving transistor.

According to the present invention, even when the threshold voltage of the driving transistor is very large, it is not necessary to lower the voltage for driving the pixel, thereby reducing the power consumption.

According to the present invention, by applying an initialization voltage to the compensation circuit of the driving transistor, even when an abnormal voltage is generated from inside and outside the pixel circuit to raise the potential of the output terminal of the driving transistor, the deviation of the threshold voltage of the driving transistor is reliably compensated for. There is an advantage that the nonuniformity of the pixel can be reduced.

According to the present invention, since the large current flowing to the EL element is blocked at the initialization of the node of the output terminal of the driving transistor, the phenomenon that the black level is not displayed properly on the display screen can be reduced.

Claims (5)

A driving transistor for supplying a driving current for emitting the light emitting element; A first storage element for storing a first voltage corresponding to a voltage difference between the first power supply voltage and the data voltage; A first switching element transferring the data voltage to one end of the first storage element in response to a first selection signal; A second switching element transferring an initialization voltage to an output terminal of the driving transistor in response to a second selection signal; A third switching element diode-connecting the driving transistor in response to a third selection signal; A fourth switching element transferring a second power supply voltage to a node to which the first switching element and the first storage element are connected in response to the third selection signal; A second storage element electrically connected between the node and a gate of the driving transistor and storing a second voltage corresponding to a threshold voltage of the driving transistor; And And a fifth switching element electrically connected between the driving transistor and the light emitting element. The method of claim 1, One end of the second switching element is connected to the third scan line or the reference voltage line. The method of claim 1, The second switching element is a PMOS transistor or an NMOS transistor. The method of claim 1, The fifth switching device is turned off by the fourth selection signal of the disable level of the fourth scanning line before turning on the third and fourth switching devices and after the first switching device is turned off. A light emitting display device that is turned on by a fourth selection signal having an enable level. A plurality of data lines for transmitting a data voltage, a plurality of scan lines for transmitting a selection signal, a driving transistor for supplying current to the light emitting device, and first to fifth switching elements that are turned on and off by the scanning lines and the driving A method of driving a light emitting display device including a plurality of pixel circuits having a series circuit of a first storage element and a second storage element for maintaining a gate-source voltage of a transistor for a period of time. Initializing an output terminal of the driving transistor by applying an enable level selection signal to a second scan line when a selection level signal is applied to a third scan line to compensate for a threshold voltage of the driving transistor; Threshold of the driving transistor to the second storage element connected between the node between the first switching element and the first storage element and the gate of the driving transistor while an enable level selection signal is applied to the third scan line. Compensating the threshold voltage of the driving transistor by charging a voltage corresponding to the voltage; Charging a first voltage corresponding to the data voltage to the first storage device connected to the data line while a select signal having an enable level is applied to the first scan line; And Controlling a current supplied to the light emitting element by a voltage charged in a series circuit of the first and second storage elements while the selection signal of the enable level of the fourth scan line is applied. Driving method.

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