KR101073007B1 - Oeld - Google Patents

Abstract

The present invention relates to an organic light emitting display device capable of compensating threshold voltage and mobility deviation and improving aperture ratio.

An organic light emitting display device according to an embodiment of the present invention includes a plurality of pixels arranged in a matrix form, each pixel comprising: a first switching transistor configured to provide a scan signal to a gate and a data signal to a source; A driving transistor having a gate connected to a drain of the first switching transistor and a predetermined first voltage provided to a source; A storage capacitor connected between the gate and the source of the driving transistor; A second switching transistor having a drain and a source connected to the gate and the drain of the driving transistor and a control signal provided to the gate; And an EL element having an anode electrode connected to the drain of the driving transistor and a predetermined second voltage supplied to the cathode electrode.

Active, Matrix, Organic Field, Light Emitting Display

Description

Organic light emitting display device {OELD}             

1 is a structural diagram of a pixel including threshold voltage compensation means in an active matrix organic light emitting display device according to an embodiment of the present invention;

2 is a detailed configuration diagram of a common voltage driver in an organic light emitting display device according to the present invention;

3 is a configuration diagram of an organic light emitting display device of the present invention;

4 is a view showing an operating state of a driving transistor during current sinking in the organic light emitting display device of the present invention;

5 is a configuration diagram of one pixel and a source driver in the organic light emitting display device of the present invention;

6 is an operational waveform diagram of an organic light emitting display device of the present invention;

* Description of the symbols for the main parts of the drawings *

100: pixel portion 200: common voltage driver circuit

300: scan driver circuit 400: source driver circuit

11, 12: switching transistor 13: storage capacitor

14: driving transistor

The present invention relates to an active matrix organic light emitting display device, and more particularly, to an active matrix organic light emitting display device (AMOLED) capable of compensating for variation in threshold voltage and increasing aperture ratio.

In general, in the organic light emitting display device, each pixel is arranged in a matrix and consists of two thin film transistors and one capacitor. In the driving thin film transistor of each pixel, the variation of the threshold voltage Vth occurred due to a process variable. In the related art, a threshold voltage compensation thin film transistor is connected to a driving transistor to compensate for variations in threshold voltages. In order to compensate for the threshold voltage, the aperture ratio is reduced because each pixel requires four or more thin film transistors and one or more capacitors, and a plurality of signal lines for providing a selection signal for switching the thin film transistors. There was this.

A method of driving an organic light emitting display device includes a voltage programming method and a current programming method. In the voltage programming type organic light emitting display device, in order to compensate the threshold voltage of the driving transistor, the threshold voltage compensation thin film transistor is connected to the driving thin film transistor to form the data voltage in the form of Vdata-Vth. Therefore, the current flowing through the EL element is expressed as in Equation (1).

ID = 1/2 * β (VSG-VTH) ² = 1/2 * β (VDD-Vdata + VTH-VTH) ² = 1/2 * β (VDD-Vdata) ²

Therefore, since the current ID flowing through the driving transistor for driving the EL element to the EL element does not relate to the threshold voltage Vth of the driving transistor from Equation 1, the deviation of the threshold voltage due to the process variable is compensated for each pixel. Could. However, in the conventional voltage programming method, the deviation of the threshold voltage of the driving transistor can be compensated for, but there is a problem that the deviation of mobility cannot be compensated.

On the other hand, in the case of the current program method, the drive current is provided from the current source to the drive transistor, and the gate-source voltage (Vgs) of the drive transistor for the current flowing through the drive transistor is provided to the storage capacitor. The deviation was compensated for. However, in the case of the current program method, when a low current is used, it takes a long time to charge the storage capacitor.

As described above, in the organic light emitting display device, a small number of thin film transistors are used to improve the aperture ratio, compensate not only the threshold voltage but also the mobility variation, and the compensation of the deviation of the threshold voltage and mobility during the operation time. Was required.

SUMMARY OF THE INVENTION The present invention has been made to solve the problems of the prior art as described above, and an object thereof is to provide an organic electroluminescent display device that can compensate for variations in threshold voltage and mobility and improve aperture ratio and operation speed.

According to an aspect of the present invention, there is provided an organic light emitting display device in which a plurality of pixels are arranged in a matrix form, each pixel including: a first switching transistor provided with a scan signal at a gate and a data signal at a source; A driving transistor having a gate connected to a drain of the first switching transistor and a predetermined first voltage provided to a source; A storage capacitor connected between the gate and the source of the driving transistor; A second switching transistor having a drain and a source connected to the gate and the drain of the driving transistor and a control signal provided to the gate; An organic light emitting display device comprising an EL element having an anode electrode connected to a drain of the driving transistor and provided with a predetermined second voltage to the cathode electrode, is provided.

In the embodiment of the present invention, the cathode electrode of the EL element becomes a ground level in the scanning section, and a common voltage of the same level as the first voltage is provided in the non-scanning section.

In the exemplary embodiment of the present invention, a common voltage having the same level as the first voltage is applied to the cathode electrode of the EL element of the pixel connected to the same scan line among the plurality of pixels, and the voltage of the ground level during the non-scanning period. The same current is applied to all pixels connected to the same scan line.

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

1 is a block diagram of an active matrix organic light emitting display device according to an exemplary embodiment of the present invention, and illustrates one pixel.

Referring to FIG. 1, in an active matrix organic light emitting display device according to an exemplary embodiment of the present invention, each pixel 10 includes two switching transistors 11, 12, one driving transistor 14, and one pixel. A capacitor 13 and one EL element 15 are provided. In the first switching transistor 11, a scan signal SCAN is applied to a gate, a data voltage Vdata is provided to a source, and a drain is provided to a gate of the driving transistor 14. The second switching transistor 12 is provided with a control signal A1 at a gate thereof, and a drain and a source thereof are connected to the gate and the drain of the driving transistor 14.

The storage capacitor 13 is connected between the gate and the source of the driving transistor 14. The driving transistor 14 is provided with a voltage VOUT at the source, a data signal Vdata is provided at the gate through the first switching transistor 11, and an anode of the EL element 15 is connected to the drain. The common voltage Vcom is provided to the cathode of the EL element 15.

FIG. 2 is a detailed view of a common voltage driver for providing a common voltage Vcom to each pixel of FIG. 1.

Referring to FIG. 2, the common voltage driver 20 includes an operational amplifier 21 for providing a voltage VOUT to a non-inverting terminal and an output terminal connected to an inverting terminal to provide a voltage VOUT as an output signal. The NMOS transistor 22 for switching the output VOUT of the operational amplifier 21 to the output terminal Vcom according to the provided scan signal SCAN, and the output terminal Vcom to ground level in accordance with the scan signal SCAN. And a switch 23 for making it.

The operation of the pixel 10 having the above-described configuration will be described with reference to FIG. 1 or FIG. 2.

When the scan signal SCAN is at a high level and is not a scanning section, the voltage VOUT provided through the operational amplifier 21 of the common voltage driver 20 is turned on and the switch 23 is turned on. ) Is provided to the output terminal Vcom through the NMOS transistor 22. Therefore, the common voltage driver 20 provides the common voltage Vcom to the pixel 10.

At this time, the first switching transistor 11 of the pixel 10 is turned off and the second switching transistor 12 is also turned off by the control signal A1, and the common voltage, that is, the VOUT voltage, is used as the cathode of the EL element 15. Since this is provided, no current flows through the EL element 15.

On the other hand, in the scanning section in which the scan signal SCAN becomes low level, since the NMOS transistor 22 is turned off and the switch 23 is turned on, the common voltage driver 20 brings the output terminal Vcom to the ground level. Thus, the cathode electrode of the EL element 15 is at ground level.

At this time, when the first switching transistor 11 is turned on by the scan signal SCAN and the control signal A1 is turned low during the scanning period, the second switching transistor 12 is turned on, and thus the driving transistor 14 is turned on. A current corresponding to the data signal Vdata flows to the EL element 15. Therefore, as illustrated in FIG. 4, all of the pixels P1-P4 connected to one common line flow the same current.

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

Referring to FIG. 3, the organic light emitting display device of the present invention includes a pixel unit 100, a common voltage driver circuit 200 for providing a common voltage Vcom to the pixel unit 100, and the pixel unit 100. Scan driver circuit 300 for providing scan signals S1, S2, S3, ..., and a source driver circuit 400 for providing a data signal Vdata to the pixel unit 100. do.

In the pixel unit 100, a plurality of pixels P1, P2, P3,... Are arranged in a matrix form, and each pixel P1, P2, P3,... Is configured as shown in FIG. 1. . Each pixel P1, P2, P3,... Is provided with scan signals S1, S2, S3, ... from the scan driver circuit 300, and the common voltage Vcom from the common voltage driver circuit 200. Is provided, and the data signal Vdata is provided from the source driver circuit 400. In the common voltage driver circuit 200, the common voltage driver 20 as shown in FIG. 2 is arranged on the common voltage line to provide a common voltage Vcom to each common voltage line. Therefore, the same current flows in the plurality of pixels connected to one common line during the scanning period.

5 illustrates a pixel and a source driver circuit for providing a data signal to the pixel in the organic light emitting display device according to the exemplary embodiment of the present invention.

Referring to FIG. 5, the source driver circuit includes NMOS transistors NM1 and NM2 connected between a source of the first switching transistor 11 and a ground, and a PMOS transistor connected in series between a power supply voltage VEE and ground. P1M1-PM4, the transfer gate TR1 for transferring the potential of the node Vx2 to the non-inverting input of the output buffer OB, the second transfer gate TR2 for transferring the data signal Vdata, and A capacitor C1 for storing the data signal Vdata transmitted through the second transfer gate TR2, an NMOS transistor NM3 connected between the capacitor C1 and ground, and the output buffer OB. And a third transfer gate TR3 for delivering the output of the node to the node Vx1.

The operation of the organic light emitting display device having the above configuration will be described with reference to the operation waveform diagram of FIG. 6.

When the second switching transistor 12 for threshold voltage compensation is turned on by the control signal A1 during the scanning period and the cathode electrode of the EL element 15 is brought to ground level by the common voltage driver 20, FIG. 4. As shown in Fig. 1, the driving current flows, and the current is determined by the bias signal bias provided to the gate of the NMOS transistor NM2, and is made equal to the maximum current Imax.

The current flowing through the driving transistor 14 determines the voltage of the node Vx1, which has information about the mobility and threshold voltage of the driving transistor. The voltage Vx1 is provided to the gate of the PMOS transistor PM1 and charged to the capacitor C1 in the form of a source follow. At this time, the PMOS transistors PM2 and PM3 and the NMOS transistor NM3 are turned on by the control signals Ch2 and Ch2b.

When the charging operation of the capacitor C1 is completed, the PMOS transistors PM2 and PM3 and the NMOS transistor NM3 are turned off, and the data signal Vdata is transmitted through the transfer gate TR2 by the control signals ch4 and ch4b. ) Is passed. Thereafter, the NMOS transistor NM1 and the first switching transistor 11 are turned off, and the output signal of the output buffer OB is transferred to the pixel P1 through the transfer gate TR5 by the control signals ch5 and ch5b. Is provided.

At this time, the operation for the data compensation is expressed as a formula.

The maximum current Imax is as shown in Equation 2 below.

The detected maximum voltage Vmax is shown in Equation 3 below.

When the detected maximum voltage and the data signal Vdata are input, the current Ipixel of each pixel is expressed by Equation 4 below.

At this time, Vdata is a data voltage to be made, and the gray level is changed according to n.

In an embodiment of the present invention, the output buffer OB is offset-corrected, and Vdata is a beta value of the average value of the panel. As described above, the voltage driving method can improve the speed and accuracy of data charging. As the number of switching lines is reduced, parasitic parameters can be reduced.

The following table shows current deviations with and without mobility deviation when data signal Vdata is applied as in the present invention. It can be seen from the table below that even when the mobility changes, the variation in current is small. Therefore, it is possible to apply the driving current to the degree of power rather than several times.

(Table 1)

n Current deviation adjustment (8 bits) Deviation Mobility deviation
If present (β + 0) 197.9894 197.9848 * 10nA / 256
= 7.7339nA 0% Mobility deviation
Without (β + Δβ) 197.40398 197.40398 * 10nA / 256
= 7.7111nA 100- (7.7111nA / 7.7338nA) * 100 = 293.52E-3%

In the above table, n is obtained by arranging a sequence of data signals. The standard for applying a data signal is an 8-bit gray reference, with a maximum current of 2 mA, and a current deviation per gray of 10 mA.

According to the organic light emitting display device of the present invention as described above, according to the use of a small number of thin film transistors, the aperture ratio can be increased, the mobility and the deviation of the threshold voltage can be corrected, and the charging time can be reduced according to the voltage driving method. Can be.

Although described above with reference to a preferred embodiment of the present invention, those skilled in the art will be variously modified and changed within the scope of the invention without departing from the spirit and scope of the invention described in the claims below I can understand that you can.

Claims (3)

In an organic light emitting display device in which a plurality of pixels are arranged in a matrix form, Each pixel includes: a first switching transistor configured to provide a scan signal at a gate thereof and a data signal at a source thereof; A driving transistor having a gate connected to a drain of the first switching transistor and a predetermined first voltage provided to a source; A storage capacitor connected between the gate and the source of the driving transistor; A second switching transistor having a drain and a source connected to the gate and the drain of the driving transistor and a control signal provided to the gate; And An EL device connected to the drain of the driving transistor and provided with a predetermined second voltage to the cathode; The voltage at the same level as the first voltage is applied to the cathode electrode of the EL element of the pixel connected to the same scan line among the plurality of pixels while the scanning signal is at a high level, and the scanning signal is at a low level. level), a voltage of ground level is applied to all pixels connected to the same scan line so that the same current flows. The method of claim 1, And the cathode electrode of the EL element has a ground level in the scanning section and a common voltage at the same level as the first voltage in the non-scanning section. The method of claim 1, The voltage of the same level as the first voltage is applied to the cathode electrode of the EL element of the pixel connected to the same scan line among the plurality of pixels, and the voltage of the ground level is applied to the same scan line during the scanning period. An organic light emitting display device, characterized in that the pixels all flow the same current.

Images