KR100562663B1 - Pixel circuit of light emitting display device and driving method thereof - Google Patents

Abstract

The present invention relates to a pixel circuit of a light emitting display device and a driving method thereof. In an exemplary embodiment, a pixel circuit of a light emitting display device includes a driving transistor connected between a first power supply voltage and a fourth switching element to supply a driving current to the light emitting device, and a data voltage of a data line to one end of the first storage device. A first switching element, a second switching element for diode-connecting the driving transistor, a third switching element for transmitting a second power supply voltage to a node to which the first switching element and the first storage element are connected, and between the node and the gate of the driving transistor A second storage element that is electrically connected to store the threshold voltage of the driving transistor, and is turned off after the second and third switching elements are turned on and simultaneously with the turn-off of the first switching element or the first switching element is turned off. And a fourth switching element that is turned on later.

Organic EL, pixel circuit, driving, threshold voltage, voltage drop, compensation

Description

A pixel circuit of a light emitting display device and a method of driving the same {Organic electro luminescent pixel circuit and method of driving the same}             

1 is a conceptual diagram of an organic electroluminescent device.

2 is an equivalent circuit diagram of a pixel circuit of a conventional current driving method.

3 is a schematic structural diagram of a light emitting display device according to a first embodiment of the present invention.

4 is an equivalent circuit diagram of a pixel circuit 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 a first driving waveform diagram for driving a pixel circuit according to first and second embodiments of the present invention.

7 is a second driving waveform diagram for driving a pixel circuit according to the first and second embodiments of the present invention.

The present invention relates to a pixel circuit of an active driving type light emitting display device and a driving method thereof.

Recently, various flat panel displays have been developed to reduce weight and volume, which are disadvantages of cathode ray tube displays. Such flat panel displays include, for example, liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), and electroluminescence (EL). And display devices.

Among them, the EL display device is roughly classified into an inorganic EL display device to which an inorganic electroluminescent element is applied and an organic EL display device to which an organic electroluminescent device is applied depending on the material of the light emitting layer. In addition, the EL display device is a self-luminous element that emits light by itself and has advantages such as fast response speed and high luminous efficiency, luminance, and viewing angle. An organic EL display device is a display device that electrically excites fluorescent or phosphorescent organic compounds and emits light. The organic EL display device performs voltage driving or current writing of M * N organic EL devices (Organic EL Device, OELD or Organic Light Emitting Device, OLED). The luminance can be displayed. In the following description, an OLED is formed in one pixel of an electroluminescent element or one pixel of a display panel of an organic EL display device and includes one of a plurality of sub pixel elements.

1 is a conceptual diagram of an organic electroluminescent device (OLED). Referring to FIG. 1, the organic EL element OLED 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 organic EL element is one of current-driven elements. That is, the light emission response of the organic EL device emits light in proportion to the current flowing into the device, but is not necessarily proportional to the voltage induced across the device. Therefore, when designing a pixel or a driving circuit, it is essential to control the current flowing into the device to express the degree of gradation. Therefore, a general active matrix addressing type organic EL display program inserts a constant current source into each pixel circuit so that a constant current can be supplied to the organic EL element for one frame time.

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 VDD to an organic EL element, a source and a gate of the driving transistor ( a storage capacitor Cs connected between the gates, and a switching transistor M2 connected to 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 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 a power supply voltage V _DD and a voltage having both ends of the data voltage of the data signal. Therefore, even when the switching transistor M2 is turned off, the voltage induced in the storage capacitor serves to control the driving current of the driving transistor so that the organic EL element OLED 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.

)는 상수 값을 나타낸다.Where I _OLED is the current flowing through the organic EL element, V _GS is the voltage between the gate and the source of transistor M1, V _TH is the threshold voltage of transistor M1, and beta (

) Represents 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 applied data voltage, and the organic EL element substantially corresponds to the supplied current. The EL element emits light. At this time, the applied data voltage has a multi-level value in a predetermined range in order to express 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 12 mV in order to express 256 levels of gray levels, that is, 8-bit gray levels. If the variation of the threshold voltage of the thin film transistor is 100 mV due to the nonuniformity of, it is difficult to express the high gradation of the organic EL element because the interval of 12 mV cannot be controlled. In addition, when the beta value in Equation 1 is changed due to the deviation of the electron mobility, it is difficult to control the thin film transistor, which makes it difficult to express higher gray levels.

On the other hand, in the current programming pixel circuit, which is a type of current driving method, a current source for supplying a uniform current to the pixel circuit through the entire data line is arranged to improve the uniformity of the pixel. Therefore, if the current programming method is used, even if the driving transistors in each pixel have non-uniform voltage-current characteristics, 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 30pF, several milliseconds of time are required to charge the data line load with data currents of several tens of nA to several hundred nA. This is a problem that the charging time is not enough considering the line selection time of several tens of us.

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 in which the processing window of the laser energy density to obtain coarse grains and hardware aspects due to, for example, uneven dose of the laser beam itself is extremely limited. Two factors of the general 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 of 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, the nonuniformity of the threshold voltages for each pixel is used to uniformly induce current. You must design a circuit that can compensate. 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, driving of low gray levels may not be possible using only the current write method in a situation in which the length of the data line becomes longer and the resistive and capacitive loads increase. .

Therefore, there is a need for a suitable driving scheme and driving circuit while improving the characteristics of the driving transistor element.

The technical problem of the present invention is to express a high gradation by reliably compensating a deviation of a threshold voltage of a driving transistor.

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 exemplary embodiment of the present invention, a pixel circuit of a light emitting display device includes a driving transistor for supplying a driving current to emit light of a light emitting element, and a first voltage for storing a first voltage corresponding to a voltage difference between a first power supply voltage and a data voltage. A storage element, a first switching element for transmitting a data voltage to one end of the first storage element in response to the first selection signal, a second switching element for diode-connecting the driving transistor in response to the second selection signal, and a second A third switching element applying 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 selection signal, and a second selection signal electrically connected between the node and the gate of the driving transistor; A second storage element for storing a second voltage corresponding to the threshold voltage of the driving transistor while being applied, and a driving transistor upon application of the second selection signal; And wherein the response to a third selection signal after the selection signal 2 is applied so that the anode voltage of the light emitting element to the drain of the register and a fourth switching element is turned off.

According to another aspect of the present invention, a method of driving a light emitting display device includes a plurality of data lines transferring a data voltage, a plurality of scan lines transferring a selection signal, and a driving transistor and a scan line supplying current to the light emitting device. A method of driving a light emitting display device including a plurality of pixel circuits having a series circuit of a first capacitor and a second capacitor for maintaining a gate-source voltage of a driving transistor and a first to fourth switching element controlled off. After the selection signal is applied to the second scan line to compensate for the threshold voltage of the driving transistor, the fourth switching element connected between the driving transistor and the light emitting element is turned off with the selection signal of the disable level of the third scan line. Reliably diode-connecting the transistor and enabling the second scan line Compensating the threshold voltage of the driving transistor by charging a second voltage corresponding to the threshold voltage of the driving transistor to a second capacitor in the pixel circuit connected to the first scan line and the data line while the selection signal of the bell is applied; Charging the first capacitor with a first voltage corresponding to the data voltage of the data line while the selection signal is applied to the scan line, and a series of first and second capacitors while the selection signal of the enable level of the third scan line is applied Controlling the current supplied to the light emitting element with the voltage charged in the circuit.

In the step of controlling the current supplied to the light emitting element, application of the selection signal of the enable level of the nth light emitting line is preferably performed simultaneously with the selection signal of the nth scanning line being blocked.

In the step of controlling the current supplied to the light emitting element, application of the selection signal of the enable level of the nth light emitting line is preferably performed after the selection signal of the nth scanning line is turned off.

 (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 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 extends from the scan driver 300 in a vertical direction, for example, from a plurality of scan lines S1 -Sn, C1-Cn, and E1-En extending in the horizontal direction and from the data driver 400. A plurality of data lines D1 -Dm and a plurality of pixel circuits 100 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 C1 -Cn, and the third scan line E1 -En. To feed. The selection signal transmitted to each switching element through the first to third scan lines is represented as a first selection signal, a second selection signal, and a third selection signal, respectively, so that each switching element is turned on or off.

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. Between the data driver 400 and the organic EL display panel 200, a plurality of output terminals of the data driver 400 and a plurality of data lines may be effectively connected, and a demultiplexer may be used to prevent a voltage drop caused by a difference in length of the data lines. May be installed.

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 100 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. In FIG. 4, for convenience of description, a pixel circuit connected to an n th first scan line Sn, an n th third scan line En, and an m th data line Dm is illustrated. In FIG. 4, the second scan line Cn is denoted as Sn-1 in that the second scan line Cn is used by commonly connecting the n−1 th first scan line Sn−1.

Referring to FIG. 4, the pixel circuit 100 includes an organic EL element OLED, a driving transistor M1, four switching elements S1, S2, S3, and S4, and two capacitors C1 and C2. . 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 Sn-1 transfers a second selection signal to the second switching element S2 for diode-connecting the driving transistor M1 which transfers a driving current to the organic EL element OLED. In addition, the second scan line Sn-1 transfers a second selection signal to the third switching element S3 connected between the node between the first switching element S1 and the first capacitor C1 and the second power supply voltage. . The third scan line En transfers a third selection signal to the fourth switching element S4 connected between the driving transistor M1 and the organic EL element OLED.

In the driving transistor M1, a source is connected to the first power supply voltage V _DD , and a drain is connected to one end of the second switching element S2 and the fourth switching element S4.

The first capacitor C1 is connected between the source of the driving transistor M1 and the node C and receives a first voltage corresponding to the voltage difference between the first power supply voltage and the data voltage applied to the node C from the data line Dm. Save it. 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 to compensate for the threshold voltage of the driving transistor M1. 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.

Accordingly, 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 and the gate of the driving transistor M1 and diode-connects the driving transistor M1 in response to a second selection signal from the second scan line Sn-1. The third switching device S3 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 second selection signal, which is a selection signal input to the second switching device. do. 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 third switching element S3.

The fourth switching element S4 is connected between the drain of the driving transistor M1 and the anode of the organic EL element OLED. The fourth switching element S4 transfers a current flowing in the driving transistor M1 to the organic EL element OLED in response to the third selection signal of the third scanning line En. The organic EL element OLED is connected between the fourth switching element S4 and the reference voltage V _SS and emits light substantially corresponding to the amount of current flowing through the driving transistor M1. The reference voltage V _SS supplied from the reference voltage line refers to a voltage induced on the cathode side of the organic EL element OLED.

In the present embodiment, the first to fourth switching elements S1, S2, S3, and S4 are simply described as switching elements. However, the first to fourth switching elements are preferably formed of transistors. Hereinafter, an embodiment in which the first to fourth switching elements S1, S2, S3, and S4 are implemented as PMOS transistors will be described in detail 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.

Referring to FIG. 5, the pixel circuit according to the second embodiment of the present invention may use the second to fifth transistors M2 instead of the first to fourth switching elements S1, S2, S3, and S4 in the pixel circuit of FIG. 4. , M3, M4, and M5) are formed in the same structure as in the first embodiment. The second to fifth transistors M2, M3, M4, and M5 are formed of PMOS transistors.

The second to fifth transistors M2, M3, M4, and M5 will be described below. The first scan line Sn is connected to a gate of the second transistor M2, and the data line Dm is connected to a source thereof. A second scan line Sn-1 is connected to a gate of the third transistor M3, a drain of the driving transistor M1 is connected to a source thereof, and a gate of the driving transistor M1 is connected to a drain thereof. A second scan line Sn-1 is connected to a gate of the fourth transistor M4, and a second power supply voltage line for supplying a second power supply voltage V _SUS is connected to a source thereof. The drains of the second and fourth transistors M2 and M4 and one end of the first and second capacitors C1 and C2 are commonly connected at the node C. A drain of the driving transistor M1 is connected to the source of the fifth transistor M5, and an anode of the organic EL element OLED is connected to the drain thereof.

Next, the operation of the pixel circuit according to the exemplary embodiment of the present invention will be described in more detail with reference to FIGS. 6 and 7. 6 and 7 are driving waveform diagrams for driving the pixel circuit according to the first and second embodiments of the present invention.

5 to 7, the third and fourth transistors M3 and M4 are turned on by the second selection signal of the low level voltage applied from the second scan line Sn-1 at a time t1. At this time, the fifth transistor M5 is turned on by the third select signal of the low level voltage already applied to the third scan line En.

When the third transistor M3 is turned on in the turned-on state of the fifth transistor M5, the organic EL element OLED is connected to a node A, which is a connection point between the drain of the first transistor M1 and the source of the third transistor M3. The anode side voltage is induced. The anode side voltage of the organic EL element OLED is usually a voltage lower than the first power supply voltage. For example, the voltage of the node A is at least a voltage capable of diode-operating the driving transistor M1, that is, 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 voltage V _DD . do. In this configuration, the driving transistor is diode-connected together with the turn-on of the third transistor M3.

When the third transistor M3 is turned on, the drain and the gate of the driving transistor M1 are commonly connected, and the driving transistor M1 is diode connected. Therefore, the gate voltage (voltage of node B) of the driving transistor M1 becomes substantially equal to the voltage of node A by diode connection.

Here, after the third and fourth transistors M3 and M4 are turned on, the fifth transistor M5 is turned off in response to a high level third select signal from the third scan line En at time t2. This prevents the current flowing to the node A through the driving transistor M1 from flowing to the organic EL element OLED by the fifth transistor M5, thereby preventing the current of the driving transistor M1 including the second capacitor C2. This is for smoothly forming the threshold voltage compensation circuit. If the fifth transistor M5 is not turned on when the third transistor M3 is turned on, the driving transistor M1 may not be diode-connected according to the voltage of the node A. Therefore, in the present invention, when the threshold voltage compensation circuit of the driving transistor M1 is formed, the driving transistor is always diode-connected together with the turn-on of the third transistor M3.

By the above configuration, the gate (node B) of the driving transistor M1 is induced with the voltage obtained by subtracting the threshold voltage of the driving transistor M1 from the first power supply voltage V _DD . In other words, the voltage of the 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 fourth transistor M4 is turned on at the time t1, 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 line length 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 a different first power supply voltage occurs in each pixel circuit of the organic EL display device, different voltages are induced in a capacitor that induces a constant voltage between the source and gate of the driving transistor M1, and thus each pixel. There is a problem that the luminance nonuniformity of the display is generated by supplying different driving currents to the organic EL element in the circuit. Therefore, in the pixel circuit according to the present invention, the second power supply voltage is applied to the node C to minimize the problem of the voltage drop of the first power supply voltage.

In addition, 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. 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 third and fourth transistors M3 and M4 are turned off in response to the high level second selection signal from the second scan line Sn-1 at time t3, the node B or the driving transistor M1 is turned off. The gate 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 t4, 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. On the other hand, when the second transistor M2 is turned on, the data voltage is applied to the node C from the data line Dm. Therefore, the voltage of the node B facing the node C with the second capacitor C2 interposed therebetween is boosted by the amount of voltage change 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.

Voltage (V _B) of the boosted node B as illustrated in Equation (3) is smaller swing (swing) the width of the driver transistor (M1). Thus, power consumption by the driving transistor M1 is reduced.

Next, at a time t5, the second transistor M2 is turned off in response to the first selection signal of the high level of the first scan line Sn. The fifth transistor M5 is turned on in response to the third select signal of the low level of the third scan line En.

At this time, the fifth transistor M5 is turned on at time t6 after the second transistor M2 is turned off at time t5 as shown in FIG. 6. On the other hand, the fifth transistor M5 is turned on at a time t5 when the second transistor M2 is turned off, as shown in FIG. 7. In this case, the first storage device C1 may limit the driving current of the driving transistor M1 during the low-level selection period of the first scan line Sn when the fifth transistor M5 is turned off. In this case, a time for storing the first voltage corresponding to the source-gate voltage of N) may be obtained.

By the above process, the gate of the driving transistor M1 is in a floating state, and at the same time, the source-gate voltage of the driving transistor M1 is reduced. Therefore, the driving transistor M1 can supply the driving current set by the series circuit of the first and second capacitors to the organic EL element OLED regardless 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. The data voltage of the 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 third selection signal of the third scanning line Em is controlled in consideration of the driving timings of the first and second selection signals, whereby a plurality of pixel circuits in the display panel generate threshold voltages of the driving transistor M1. It is possible to effectively control the compensation circuit of the deviation of or the voltage drop of the first power supply voltage (V _DD ). Thus, the nonuniformity of the organic EL display device can be further reduced.

Meanwhile, in the pixel circuit of the light emitting display device according to the second embodiment of the present invention, the fifth transistor M5 may be formed as an NMOS transistor. In this case, the fifth transistor M5 is turned on in response to the third select signal of the high level of the third scan line En and is turned off in response to the third select signal of the low level. Such a configuration will be apparent to those skilled in the art, so a 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.

According to the present invention, it is possible to reliably compensate for the threshold voltage of the driving transistor which supplies the driving current to the organic EL element by controlling the scanning signal of the pixel circuit. Therefore, a high gradation and high uniformity light emitting display device can be provided.

According to the present invention, the threshold voltage compensation of the driving transistor can be more stably controlled by controlling the scanning signal of the pixel circuit, thereby eliminating an image defect according to the operating characteristics of the driving transistor of the pixel circuit, thereby improving the luminance uniformity of the display screen. have.

Claims (4)

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 diode-connecting the driving transistor in response to a second selection signal; A third switching element applying 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 second 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 while the second selection signal is applied; And A light emitting display including a fourth switching element which is turned off in response to a third selection signal after the second selection signal is applied such that an anode voltage of the light emitting element is applied to a drain of a driving transistor when the second selection signal is applied; Pixel circuit of the device. 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 fourth switching elements controlled on and off by the scan lines; A method of driving a light emitting display device including a plurality of pixel circuits having a series circuit of a first capacitor and a second capacitor for maintaining a gate-source voltage of a driving transistor for a period of time. After the selection signal is applied to the second scan line to compensate for the threshold voltage of the driving transistor, the fourth switching device connected between the driving transistor and the light emitting device is turned off with the selection signal of the disable level of the third scan line. Thereby reliably diode-connecting the driving transistor; While the enable level select signal is applied to the second scan line, the second capacitor in the pixel circuit connected to the first scan line and the data line is charged with a second voltage corresponding to the threshold voltage of the driving transistor to drive the driving transistor. Compensating the threshold voltage of the signal; Charging the first capacitor with a first voltage corresponding to the data voltage of the data line while a selection signal is applied to the first scan line; And Controlling a current supplied to the light emitting element with a voltage charged in a series circuit of the first and second capacitors while the selection signal of the enable level of the third scan line is applied. . The method of claim 2, And controlling the current supplied to the light emitting device to apply the selection signal of the enable level of the nth light emitting line simultaneously with the selection signal of the nth scanning line being blocked. The method of claim 2, And controlling the current supplied to the light emitting device, wherein the selection signal of the enable level of the nth light emitting line is performed after the selection signal of the nth scanning line is turned off.

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