KR20080000468A - Pixel circuit of organic light emitting display - Google Patents

Abstract

A pixel circuit of an OLED(ORGANIC LIGHT EMITTING DISPLAY) device is provided to enhance image quality by maintaining constantly the uniformity of brightness of respective pixels. A pixel circuit of an OLED(Organic Light Emitting Display) device includes first, second, third, and fourth transistors(T1,T2,T3,T4), a capacitor(Cgs), and an OLED(OLED). The first transistor transmits data signals from data lines in response to a selection signal from a scan line. The second transistor delivers a first source voltage in response to the selection signal from the scan line. The capacitor stores data signals from the first and second transistors and a second source voltage. The third transistor delivers the second source voltage to the capacitor in response to the control signals from an illuminating control line and compensates for the voltage drop of the first source voltage. The fourth transistor receives voltages from the capacitor and generates driving currents corresponding to the received voltages. The OLED, which is connected between a third source voltage and the fourth transistor, executes illumination operations according to the received driving currents.

Description

Pixel Circuit of Organic Light Emitting Display

1 is a block diagram illustrating a conventional organic light emitting display device.

2 is a circuit diagram illustrating a pixel circuit of a conventional organic light emitting display device.

3 is a circuit diagram illustrating a pixel circuit of an organic light emitting display device according to an exemplary embodiment of the present invention.

4 is a timing diagram illustrating an operation of a pixel circuit of an organic light emitting display device according to an exemplary embodiment of the present invention.

5 is a circuit diagram illustrating a storing step of a pixel circuit of an organic light emitting display device according to an exemplary embodiment of the present invention.

6 is a circuit diagram illustrating a light emitting step of a pixel circuit of an organic light emitting display device according to an exemplary embodiment of the present invention.

The present invention relates to a pixel circuit of an organic light emitting display device.

Recently, the importance of flat panel displays (FPDs) has increased with the development of multimedia. In response, a number of liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), organic light emitting devices (Organic Light Emitting Devices), etc. Branch-type flat panel displays have been put into practical use.

In particular, the organic light emitting display device has a high response time with a response speed of 1 ms or less, low power consumption, and self-emission. In addition, there is no problem in viewing angle, which is advantageous as a moving image display medium regardless of the size of the device. In addition, low-temperature manufacturing is possible, and the manufacturing process is simple based on the existing semiconductor process technology has attracted attention as a next-generation flat panel display device in the future.

In general, an organic light emitting display device is a display device that electrically excites fluorescent organic compounds and emits light, and performs voltage driving or current driving of N × M organic light emitting diodes (OLEDs) arranged in a matrix form. Programming to express an image. The organic light emitting display device may be driven by a passive matrix method and an active matrix method using a thin film transistor. The passive matrix method forms the anode and the cathode to be orthogonal and selects and drives the lines, whereas the active matrix method connects the thin film transistors to each pixel electrode and drives them according to the voltage maintained by the capacitor capacitance connected to the gate electrode of the thin film transistor. That's the way it is.

The organic light emitting display device as described above may be classified into a top light emitting type and a bottom light emitting type according to a direction in which light is emitted from a light emitting diode. An organic light emitting display device may be manufactured.

The inverted top emission type organic light emitting display device includes a light emitting diode in which a cathode, an organic light emitting layer, and an anode are sequentially stacked, and a thin film transistor for driving is connected to the cathode of the organic light emitting diode. Therefore, since the anode which is a transparent electrode is located in the direction from which light is taken out, it has the advantage of high light transmittance and high luminous efficiency.

1 is a block diagram illustrating an organic light emitting display device according to the related art.

Referring to FIG. 1, the organic light emitting display device includes a display panel 110, a scan driver 120, a data driver 130, a controller 140, and a power supply 150.

The display panel 110 includes the data lines D1 -Dm arranged in the first direction and the scan lines S1 -Sn and the data lines (D1 -Dm crossing the first direction and arranged in the second direction). )) And the pixel circuits P11 -Pnm positioned in the pixel area where the scan lines S1 -Sn cross each other.

The control unit 140 outputs control signals to the scan driver 120, the data driver 130, and the power supply unit 150, and the power supply unit 150 includes the scan driver 120, according to the driving control of the controller 140. The voltage required to drive the data driver 130 and the display panel 110 is output.

The scan driver 120 outputs a scan signal to scan lines S1 -Sn connected to the scan driver 120 according to a control signal of the controller 140. As a result, the pixel circuits P11 -Pnm located in the display panel 110 are selected in response to the scan signals S1 -Sn.

The data driver 130 may correspond to the data signals through the data lines D1 -Dm connected to the data driver 130 in synchronization with the scan signal output from the scan driver 120 according to the control signal of the controller 140. Applied to the pixel circuits 110. Accordingly, the display panel 110 displays an image image by emitting light from the pixel circuits P1 -Pnm in response to the data signals.

2 is a circuit diagram illustrating a pixel circuit of an organic light emitting display device according to the related art.

Referring to FIG. 2, the pixel circuit transmits a data signal from the data line Dm in response to a scan signal from the scan line Sn, and a data signal received through the switching transistor T1. And a driving transistor T2 for generating a driving current according to a data signal stored in the capacitor Cgs, and an organic light emitting diode OLED for performing a light emitting operation according to the driving current. .

Here, the switching transistor T1 and the driving transistor T2 are NMOSs, the source electrode of the driving transistor T2 is connected to the negative power line VSS, and the drain electrode is the cathode of the organic light emitting diode OLED. Is connected to. In addition, the anode of the organic light emitting diode OLED is connected to the positive power line VDD.

In the active matrix type organic light emitting display device including the pixel circuit as described above, the luminance is controlled as the amount of current flowing through the organic light emitting diode OLED, and the amount of current flowing through the organic light emitting diode OLED is expressed by the following equation. Can be.

Therefore, the amount of current flowing through the organic light emitting diode OLED of each pixel circuit is determined by the gate voltage Vg, the threshold voltage Vth, and the negative power supply voltage Vss of the driving transistor T2.

However, since the voltage drop IR Drop occurs in the negative power line VSS supplying power to each pixel circuit, the magnitude of the negative power supply voltage Vss supplied varies according to the position of each pixel circuit. This causes a problem in that the luminance of each pixel is generated by changing the amount of current flowing through the organic light emitting diode OLED of each pixel circuit.

Accordingly, an object of the present invention is to provide a pixel circuit of an organic light emitting display device which can improve the quality of a screen by compensating voltage supplied to each pixel circuit to make luminance uniform.

In order to achieve the above object, the present invention provides a first transistor for transmitting a data signal from a data line in response to a selection signal from a scan line, and a first voltage having a negative voltage in response to a selection signal from a scan line. A second transistor carrying a power supply voltage, a capacitor for receiving and storing a data signal and a first power supply voltage from the first and second transistors, and a second power supply voltage to the capacitor in response to a control signal from a light emitting control line. Thus, a third transistor for compensating the voltage drop of the first power supply voltage, a fourth transistor receiving a voltage from the capacitor and generating a corresponding driving current; And an organic light emitting diode configured to receive a driving current from the fourth transistor to perform a light emitting operation, wherein an anode of the organic light emitting diode is connected to a third power supply voltage which is a positive voltage, and a cathode of the organic light emitting diode is connected to the fourth transistor. A pixel circuit of an organic light emitting display device is provided.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

3 is a circuit diagram illustrating a pixel circuit of an organic light emitting display device according to an exemplary embodiment of the present invention.

Referring to FIG. 3, a pixel circuit according to an exemplary embodiment of the present invention may include first to fourth transistors T1, T2, T3, and T4, a capacitor Cgs, and an organic light emitting diode OLED. ).

The gate electrode of the first transistor T1 is connected to the scan line Sn, and one end thereof is connected to the data line Dm. A capacitor Cgs is connected to the other end of the first transistor T1, and the first transistor T1 transfers a data signal from the data line Dm to one end of the capacitor Cgs.

The gate electrode of the second transistor T2 is connected to the scan line Sn together with the gate electrode of the first transistor T1. One end of the second transistor T2 is connected to the gate electrode of the fourth transistor T4, and the other end thereof is connected to the first power line VSS, which is a negative power line. The second transistor T2 is turned on by the scan signal to transfer the first power supply voltage from the first power supply line VSS to the other end of the capacitor Cgs. Here, the channel width / length W / L of the second transistor T2 may be smaller than the channel width / length W / L of the first and third transistors T1 and T3.

The first transistor T1 and the second transistor T2 are connected to both ends of the capacitor Cgs. Therefore, the capacitor Cgs stores a voltage corresponding to the difference between the data signal applied from the first transistor T1 and the first power voltage applied from the second transistor T2. In addition, the third transistor T3 is connected to the node where the capacitor Cgs and the first transistor T1 are connected.

The gate electrode of the third transistor T3 is connected to the light emission control line Emit. The third transistor T3 transfers the second power supply voltage from the second power supply line Vsus to the capacitor Cgs by a control signal from the light emission control line Emit. Since no current is generated in the second power line Vsus, no voltage drop occurs, and therefore, a constant voltage may be applied to one end of the capacitor Cgs for each pixel.

The gate electrode of the fourth transistor T4 is connected to the other end of the capacitor Cgs so that the fourth transistor T4 is turned on by the voltage stored in the capacitor Cgs. The fourth transistor T4 transfers a driving current corresponding to the difference between the voltage applied from the first power line VSS connected to one end and the voltage applied from the capacitor Cgs to the organic light emitting diode OLED.

The current generated from the fourth transistor T4 is applied to the cathode of the organic light emitting diode OLED, and the third power line VDD, which is a positive power supply voltage, is connected to the anode of the organic light emitting diode OLED. The organic light emitting diode OLED receives a driving current and performs a light emitting operation corresponding thereto.

4 is a timing diagram illustrating an operation of a pixel circuit of an organic light emitting display device according to an embodiment of the present invention shown in FIG. 3, and FIGS. 5 and 6 are organic light emitting display devices shown in FIG. 3. Circuit diagrams for explaining a storage section I and a light emission section II of the pixel circuit in FIG.

3 to 5, in the storage period I, when the selection signal of the “high level” is applied from the scan line Sn, the first transistor T1 and the second transistor T2 are turned on. . As a result, the data signal Vdata from the data line Dm is transmitted to one end of the capacitor Cgs through the first transistor T1. At this time, a "low level" control signal is applied to the emission control line Emit.

When the second transistor T2 is turned on, the first power voltage Vss is applied to the other end of the capacitor Cgs from the first power line VSS connected to one end of the second transistor T2. Therefore, the voltage Cc corresponding to the difference between the first power voltage Vss and the data signal Vdata is stored in the capacitor Cgs. At this time, the voltage applied to the node "a" is Vss, and the voltage Vc stored in the capacitor Cgs may be expressed as follows.

Meanwhile, when the second transistor T2 is turned on, the first power supply voltage Vss is applied to the gate and the source electrode of the fourth transistor T4. Accordingly, the fourth transistor T4 is turned off and the gate-source voltage of the fourth transistor T4 becomes "0". As a result, charges trapped in the fourth transistor T4 by the previous data signal are removed, thereby reducing the afterimage effect caused by the previous data signal.

In addition, since the channel width / length W / L of the second transistor T2 is formed to be smaller than the channel width / length W / L of the first or third transistors T and T3, the second transistor It is possible to reduce the leakage current generated by external light at the turn-on of T2. Therefore, the voltage value stored in the capacitor Cgs does not change when the second transistor T2 is turned on.

3, 4, and 6, when the control signal of "high level" is applied from the emission control line Emit to the gate electrode of the third transistor T3 in the emission period II, the third transistor is applied. The second power supply voltage is supplied to one end of the capacitor Cgs from the second power supply line Vsus connected to one end of the T3.

In this case, the voltage Va applied to the other end of the capacitor Cgs connected to the gate electrode of the fourth transistor T4, that is, the node “a” is as follows.

The voltage is transferred to the gate electrode of the fourth transistor T4, and the fourth transistor T4 generates the following amount of driving current.

In Equation 4, since the first power supply voltage Vss is canceled, the amount of current supplied to the organic light emitting diode OLED is determined by the data signal and the second power supply line. As mentioned above. The second power line can supply a predetermined voltage to each pixel. Therefore, the problem of uneven supply voltage due to the voltage drop of the first power line VSS can be solved, and the problem of uneven brightness of each pixel can be solved.

As described above, the present invention can solve the problem of voltage drop occurring in the first power line VSS by compensating the first power voltage Vss for each pixel. In addition, since the gate voltage of the driving transistor T4 is initialized in the step of compensating the first power voltage Vss, the present invention can prevent the afterimage effect caused by the previous data signal. In addition, according to the present invention, the channel width / length (W / L) of the second transistor T2 is made small to prevent the occurrence of leakage current.

While the invention has been shown and described with reference to specific embodiments thereof, the invention is not so limited, and the invention may be varied without departing from the spirit or scope of the invention as set forth in the claims below. It will be readily apparent to those skilled in the art that the present invention may be modified and changed.

As described above, the present invention can provide a front-emitting organic light emitting display device having an invert structure in which the luminance uniformity of each pixel is secured and the screen quality is improved.

Claims (7)

A first transistor for transferring a data signal from the data line in response to the select signal from the scan line; A second transistor delivering a first power supply voltage that is a negative voltage in response to a selection signal from the scan line; A capacitor for receiving and storing a data signal and a first power voltage from the first and second transistors; A third transistor configured to transfer a second power supply voltage to the capacitor in response to a control signal from an emission control line to compensate for a voltage drop of the first power supply voltage; A fourth transistor configured to receive a voltage stored in the capacitor and generate a corresponding driving current; And An organic light emitting diode configured to perform a light emitting operation by receiving a driving current from the fourth transistor, And an anode of the organic light emitting diode is connected to a third power supply voltage which is a positive voltage, and a cathode of the organic light emitting diode is connected to the fourth transistor. The method of claim 1, And the first and second transistors are commonly connected to the scan line. The method of claim 1, And the control signal is at a low level while the selection signal is at a high level. The method of claim 1, And a channel width / length (W / L) of the second transistor is smaller than a channel width / length (W / L) of the first or third transistor. The method of claim 1, And a high level signal is applied from the scan line, the data signal and a first power supply voltage are applied to both ends of the capacitor, and the fourth transistor is turned off. The method of claim 5, When a low level signal is applied from the scan line and a high level control signal is applied from the light emission control line, a second power supply voltage is applied to one end of the capacitor to change the voltage at the other end of the capacitor. The transistor is applied to the changed voltage to generate a driving current of the pixel circuit of the organic light emitting display device. The method of claim 1, The pixel circuit of an organic light emitting display device, wherein the first to fourth transistors are NMOS.

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