KR101194860B1 - Pixel Circuit of Organic Light Emitting Display - Google Patents

Abstract

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, a second transistor for delivering a first power supply voltage in response to a selection signal from a scan line, and a first transistor and a first transistor. A capacitor for receiving and storing a data signal and a first power supply voltage from a second transistor, and a third transistor for delivering a second power supply voltage to one end of the capacitor in response to a control signal from a light emission control line, and receiving and driving a voltage from the capacitor A pixel circuit of an organic light emitting display device including a fourth transistor for generating a current and an organic light emitting diode for emitting light according to a driving current applied from the fourth transistor is provided.

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.

7 is a graph illustrating a result of simulating 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.

2. Description of the Related Art In recent years, the importance of flat panel displays (FPDs) has been increasing 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 so as to be orthogonal and selects and drives the line, whereas the active matrix method connects the thin film transistor to each indium tin oxide (ITO) pixel electrode and is connected by a capacitor capacitance connected to the gate electrode of the thin film transistor. It is driven by the maintained voltage.

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 MS. A capacitor (Cgs) for storing a; a driving transistor (MD) 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 source electrode of the driving transistor MD is connected to the first power line VDD supplying positive power, and the second electrode of the organic light emitting diode OLED is connected to the second power line supplying negative power. VSS).

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.

I _OLED = 1 / 2β (Vgs-Vth) ² = 1 / 2β (Vg-Vdd-Vth) ²

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 first power supply voltage Vdd of the driving transistor MD.

However, since the voltage drop IR Drop occurs in the first power line VDD supplying power to each pixel circuit, the magnitude of the first power supply voltage Vdd supplied to each pixel may vary depending on the position of each pixel circuit. Change. This changes the amount of current flowing through the organic light emitting diode (OED) of each pixel circuit, which causes a problem of uneven brightness.

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 that transfers a data signal from a data line in response to a selection signal from a scan line, and a first power supply voltage in response to a selection signal from a scan line. A second transistor, a capacitor for receiving and storing a data signal and a first power supply voltage from the first and second transistors, and a third power supply for supplying a second power supply voltage to one end of the capacitor in response to a control signal from a light emission control line. A pixel circuit of an organic light emitting display device including a transistor, a fourth transistor for generating a driving current by applying a voltage from a capacitor, and an organic light emitting diode for emitting light according to the driving current applied from the fourth transistor are 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 includes first to fourth transistors T1, T2, T3, and T4, a capacitor C1, 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 C1 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 C1.

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 VDD. The second transistor T2 is turned on by the scan signal to transfer the first power voltage from the first power line VDD to the other end of the capacitor C1. 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 C1. Therefore, the capacitor C1 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, a third transistor T3 is connected to a node connected to the capacitor C1 and the first transistor T1.

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 C1 by a control signal from the light emission control line Emit.

The gate electrode of the fourth transistor T4 is connected to the other end of the capacitor C1, and the fourth transistor T4 is turned on by the voltage stored in the capacitor C1. The fourth transistor T4 transfers a driving current corresponding to the difference between the first power supply voltage connected to one end and the voltage applied from the capacitor C1 to the organic light emitting diode OLED. The organic light emitting diode OLED, which receives the driving current, performs a corresponding light emitting operation.

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 and a light emitting section of a pixel circuit of FIG.

3 to 5, in the storage period I, when a low level select signal is applied from the scan line Sn, the first transistor T1 and the second transistor T2 are turned on. Thus, the data signal Vdata from the data line Dm is transmitted to one end of the capacitor C1 through the first transistor T1. At this time, a high level control signal is applied to the emission control line Emit.

When the second transistor T2 is turned on, the first power voltage Vdd is applied to the other end of the capacitor C1 from the first power line VDD connected to one end of the second transistor T2. Therefore, the voltage Vc corresponding to the difference between the first power supply voltage Vdd and the data signal Vdata is stored in the capacitor C1.

Vc = Vdd-Vdata

On the other hand, when the second transistor T2 is turned on, the first power supply voltage Vdd is applied to the gate and the source electrode of the fourth transistor T4. Therefore, since the fourth transistor T4 is turned off and the gate-source voltage of the fourth transistor T4 becomes 0, the charges trapped in the fourth transistor T4 are removed, thereby remaining after the previous data signal. The effect can be reduced.

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, since the voltage value stored in the capacitor C1 does not change when the second transistor T2 is turned on, it is possible to prevent blackening of the organic light emitting diode OLED.

3, 4, and 6, when the low level control signal 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 T3. The second power supply voltage is supplied to one end of the capacitor (C1) from the second power line (Vsus) connected to one end of.

At this time, the voltage Va applied to the other end of the capacitor C1 connected to the gate electrode of the fourth transistor T4 is as follows.

Va = Vdd-Vdata + Vsus

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.

I _OLED = 1 / 2β (Va-Vdd-Vth) ² = 1 / 2β (Vdd-Vdata + Vsus-Vdd-Vth) ²

= 1 / 2β (-Vdata + Vsus-Vth) ²

In Equation 4, since the first power supply voltage Vdd 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. Therefore, it is possible to solve the problem of non-uniformity of the supply voltage due to the voltage drop of the first power line VDD, thereby solving the non-uniformity of luminance of each pixel.

7 is a graph illustrating simulation results of a pixel circuit of an organic light emitting display device according to an exemplary embodiment of the present invention.

In the simulation of the pixel circuit of the organic light emitting display device according to the exemplary embodiment of the present invention, when the amount of current flowing through the organic light emitting diode OLED is changed to 9 to 10V, The change in the 140 kHz area was measured.

Referring to FIG. 7, when the pixel circuit of the organic light emitting display device according to the exemplary embodiment of the present invention is applied, the amount of current flowing through the organic light emitting diode is 2% or less.

As described above, the present invention can solve the problem of voltage drop occurring in the first power line VDD by compensating the first power voltage Vdd for each pixel. In addition, since the gate voltage of the driving transistor T4 is initialized in the compensating the first power supply voltage Vdd, the afterimage effect of the previous data signal can be prevented. In addition, according to the present invention, the channel width / length (W / L) of the second transistor T2 is made small to prevent blackening of the organic light emitting diode OLED due to leakage current.

As described above, the present invention can improve the quality of the screen of the organic light emitting display device by securing the luminance uniformity of each pixel of the organic light emitting display device and preventing the afterimage effect.

Claims (6)

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 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 delivering a second power supply voltage to one end of the capacitor in response to a control signal from an emission control line; A fourth transistor configured to receive a voltage from the capacitor to generate a driving current; And An organic light emitting diode which emits light according to a driving current applied from the fourth transistor, A drain electrode of the first transistor and a drain electrode of the third transistor are respectively connected to one end of the capacitor, and a drain electrode of the second transistor and a gate electrode of the fourth transistor are respectively connected to the other end of the capacitor. 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 the first and second transistors are commonly connected to the scan line. The method of claim 1, And the control signal is at a second level while the selection signal is at a first level. delete The method of claim 1, And a data signal and a first power supply voltage are applied to both ends of the capacitor when the selection signal is applied from the scan line, and the fourth transistor is turned off. 6. The method of claim 5, When a 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, and the fourth transistor is configured to generate a driving current by receiving the changed voltage. Pixel Circuit of Electroluminescent Display.

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