CN100363967C - pixel driving circuit of active organic light emitting display - Google Patents

Abstract

The invention relates to a pixel driving circuit of an active organic light emitting display, in particular to a current-driven active organic light emitting display which is provided with a plurality of pixel driving circuits with voltage driving design. The pixel driving circuit is connected in parallel to a comparison circuit, and the comparison circuit at least has an organic light emitting diode and a transistor, which are respectively corresponding to the organic light emitting diode and the transistor in the pixel driving circuit. Through the comparison circuit, the drive current value led into the pixel drive circuit and the comparison current value led into the comparison circuit generate a specific proportional relation.

Description

Pixel driving circuit of active organic light emitting display

technical field

The present invention relates to a pixel drive circuit of an Active Matrix Organic Light Emitting Diode (AMOLED), in particular to a current-driven pixel drive circuit of an Active Matrix Organic Light Emitting Diode.

Background technique

With the progress of organic light emitting diode (Organic Light Emitting Diode, OLED) production technology, the production technology of organic light emitting display is also becoming mature. Basically, organic light emitting diodes are arranged in an array in an organic light emitting display, and in order to drive these organic light emitting diodes to generate images, the methods currently used can be mainly divided into simple matrix (Simple Matrix) and active matrix (Active Matrix). Matrix) two kinds. Among them, the active matrix type can better meet the needs of large-size or high-resolution display.

Please refer to FIG. 1 , which is an equivalent circuit diagram of a pixel of a typical voltage-driven active organic light emitting display. The pixel includes an organic light emitting diode OLED, a transistor T1, a transistor T2 and a capacitor C. Wherein, the source of the transistor T1 is connected to a data line (not shown) to pass through the driving voltage signal V _data , and its gate is connected to a scan line (not shown) to pass through the scanning potential Scan. The source of the transistor T2 is connected to the anode of the organic light emitting diode OLED, the drain thereof is connected to a voltage level V _dd , and the gate thereof is connected to the drain of the transistor T1 . The cathode of the organic light emitting diode OLED is connected to another voltage level Vss. Both ends of the capacitor C are respectively connected to the gate of the transistor T2 and the voltage level V _dd .

In order to generate a stable current to maintain a stable luminance of the organic light emitting diode OLED, when the scanning potential Scan from the scanning line is at a high potential, the transistor T1 is turned on, so that the driving voltage signal V _data can be input to the gate of the transistor T2 through the data line Pole and capacitor C. When the scanning potential Scan is at a low potential, the transistor T1 is turned off, and the voltage difference V _cs between the voltage level of the driving voltage signal Vdata and the aforementioned voltage level V _dd is stored in the capacitor C. In this case, the voltage V _gs across the gate and source of the transistor T2 is equivalent to V _dd −V _data . The difference between the cross voltage V _gs and the threshold voltage (Vt) of the transistor T2 determines the magnitude of the current I flowing through the organic light emitting diode OLED.

Please refer to FIG. 2 , which is an equivalent circuit diagram of a pixel of a typical current-driven active OLED display. The pixel includes an organic light emitting diode OLED, a transistor T1, a transistor T2, a transistor T3, a transistor T4, and a capacitor C. Wherein, the source of the transistor T1 is connected to a data line (not shown), so as to pass through a driving current signal I _data , the gate is connected to a scan line (not shown), so as to pass through the scanning potential Scan, and the drain is connected to the source of transistor T2. The gate of the transistor T2 is connected to the gate of the transistor T4, and the drain thereof is connected to the anode of the organic light emitting diode OLED and the source of the transistor T4. The source of the transistor T3 is connected to the data line to feed the driving current signal _Idata , the gate is connected to the scan line to feed the scanning potential Scan, and its drain is connected to the gate of the transistor T2 and the gate of the transistor T4 pole. The source of the transistor T4 is connected to the anode of the organic light emitting diode OLED, and the drain of the transistor T4 is connected to a voltage level Vdd. The cathode of the organic light emitting diode OLED is connected to another voltage level V _ss . Both ends of the capacitor C are respectively connected to the gate of the transistor T4 and the anode of the organic light emitting diode OLED.

In order to generate a stable current so that the organic light-emitting diode OLED maintains a stable luminance, the scanning potential Scan first turns on the transistors T1 and T3 through the scanning line, so that the driving current signal I _data is provided to the transistor T2 and the capacitor C, and in the capacitor C A voltage level V _cs corresponding to the driving current signal I _data is formed. And when the scan potential from the scan line S1 is low, the transistors T1 and T3 are turned off. Meanwhile, since the transistor T2 and the transistor T4 are located on the corresponding mirror circuit (relative to the capacitor C and the organic light emitting diode OLED). Therefore, under the condition that the transistors T2 and T4 have the same electrical characteristics, the voltage level V _cs stored in the capacitor C will cause a current I corresponding to the aforementioned driving current signal I _data , which is provided to the OLED through the transistor T4 OLED, so that it emits light.

As mentioned above, and please also refer to FIG. 1 , in a voltage-driven pixel, the magnitude of the current I flowing through the OLED is affected by the threshold voltage Vt. Moreover, during the operation of the transistor T2, the threshold voltage Vt tends to increase due to the accumulation of internal charges, thereby causing the current I flowing through the organic light emitting diode OLED to decrease, resulting in a decrease in luminous brightness.

On the contrary, in the current-driven pixel shown in FIG. 2, the magnitude of the current flowing through the organic light-emitting diode OLED is determined by the driving current signal I, and will not be affected by the variation of the threshold voltage values of the transistors T2 and T4. . However, in order to provide current-driven characteristics, four transistors T1, T2, T3, and T4 must be designed in the pixel, resulting in increased manufacturing costs and poor pixel aperture ratio.

How to avoid the impact of the variation of the threshold voltage of the transistor on the luminance of the OLED, and reduce the number of electronic components in the pixel to improve the aperture ratio will have a very important impact on the development of the OLED.

Contents of the invention

The main purpose of the present invention is to overcome the disadvantage of poor aperture ratio of the pixels of the existing current-driven organic light-emitting display, and to propose a solution.

Another object of the present invention is to overcome the disadvantages of traditional voltage-driven organic light-emitting display pixels that the luminous brightness is difficult to maintain stable, and to propose a solution.

The present invention provides an active organic light-emitting display, which has a plurality of pixel driving circuits designed for voltage driving, but uses a driving current to drive these pixel driving circuits. The characteristic of this active organic light emitting display is that the above-mentioned pixel driving circuit is connected in parallel to a comparison circuit, and the comparison circuit has at least one organic light emitting diode and a transistor, respectively corresponding to the organic light emitting diodes in the pixel driving circuit with transistors. Through the comparison circuit, a specific proportional relationship can be generated between the driving current value passed into the pixel driving circuit and the comparison current value passed into the comparison circuit.

The present invention also provides a pixel driving circuit for an active organic light emitting display, the driving circuit includes: at least one pixel driving circuit, each pixel driving circuit includes: a first transistor, the source of which is connected to A data line, the gate of which is connected to a scan line; a second transistor, whose gate is connected to the drain of the first transistor, and the drain of the second transistor is connected to a first voltage level; a capacitor, a first end of which is connected to the drain of the second transistor, and a second end of which is connected to the source of the first transistor and the gate of the second transistor; an organic light emitting diode, whose anode is Connected to the source of the second transistor, the cathode of the organic light-emitting diode is connected to a second voltage level; at least one comparison circuit is electrically connected to the pixel driving circuit, and each comparison circuit includes: a A comparison transistor, corresponding to the second transistor, whose gate and drain are connected to the data line; a comparison organic light emitting diode, corresponding to the organic light emitting diode, whose anode is connected to the source of the comparison transistor , the cathode of which is supplied with the second voltage level.

The second voltage level is ground.

Description of drawings

FIG. 1 is an equivalent circuit diagram of a pixel of a typical voltage-driven active organic light-emitting display;

2 is an equivalent circuit diagram of a pixel of a typical current-driven active organic light-emitting display;

3 is a schematic block diagram of a preferred embodiment of the driving circuit of the active organic light emitting display of the present invention;

FIG. 4 is an equivalent circuit diagram of a single pixel driving circuit in FIG. 3 and a comparison circuit connected in parallel to the pixel driving circuit through a data line.

Symbol Description:

Drive circuit ~ 100

Data Drive Unit ~ 120

Scan drive unit ~ 140

Power supply unit ~ 150

Pixel drive circuit ~ 160

Data line ~ 122

scan line 142

Comparison circuit ~ 180

Power cord ~ 152

Detailed ways

The advantages and spirit of the present invention can be further understood through the following detailed description of preferred embodiments of the invention and the accompanying drawings.

Please refer to FIG. 3 , which is a schematic block diagram of a preferred embodiment of the driving circuit 100 of the active organic light emitting display of the present invention. The driving circuit 100 includes a data driving unit 120 , a scanning driving unit 140 , a power supply unit 150 , a plurality of pixel driving circuits 160 and a plurality of comparison circuits 180 . Wherein, each pixel driving circuit 160 is arranged in a row. The data driving unit 120 is connected to the pixel driving circuit 160 and the comparison circuit 180 of each row respectively through a plurality of data lines 122 . The scanning driving unit 140 is respectively connected to the pixel driving circuits 160 of each column through a plurality of scanning lines 142 . The power supply unit 150 provides the power required by each pixel driving unit 160 to drive its internal organic light emitting diodes to emit light. The bottom end of the pixel driving circuit 160 in each row is coupled to a comparison circuit 180 through the data line 122 . The comparison circuit 180 is used to produce the effect of the mirror circuit in the conventional current-driven pixel as shown in FIG. 2 .

Please refer to FIG. 4 , which is an equivalent circuit diagram of a single pixel driving circuit 160 in FIG. 3 and a comparison circuit 180 connected in parallel to the pixel driving circuit 160 through the data line 122 . As shown in the figure, the pixel driving circuit 160 includes a first transistor T1, a second transistor T2, a capacitor C and an organic light emitting diode OLED. Wherein, the source of the first transistor T1 is connected to the data line 122 , and the gate is connected to the scan line 142 . The gate of the second transistor T2 is connected to the drain of the first transistor T1 , and the drain of the second transistor T2 is connected to the power supply unit 150 through the power line 152 for a first voltage level Vdd. One end of the capacitor C is connected to the drain of the second transistor T2, and the other end is connected to the source of the first transistor T1 and the gate of the second transistor T2. An anode of the organic light emitting diode OLED is connected to the source of the second transistor T2, and a cathode thereof is connected to a second voltage level Vss, and the second voltage level is grounded.

The comparison circuit 180 includes a comparison transistor Tm and a comparison organic light emitting diode OLEDm. Wherein, the comparison transistor Tm corresponds to the second transistor T2 in the pixel driving circuit 160 , and the gate of the comparison transistor Tm is connected to the data line 122 , and its drain is also connected to the data line 122 . The comparison organic light emitting diode OLEDm is corresponding to the organic light emitting diode OLED in the pixel driving circuit 160, and its anode is connected to the source of the above comparison transistor Tm, and its cathode is connected to the above second voltage level Vss .

When a given drive current signal I is input into the comparison circuit 180, the difference between the cross voltage V _gs' between the gate and the source of the comparison transistor Tm and the threshold voltage V _t' of the comparison transistor Tm (V _gs' -V _t' ) is also determined accordingly. In this case, the voltage level of the data line 122 is V _data =V _gs' +V _oled' +V _ss . Wherein, V _oled' is the cross voltage between the anode and the cathode of the comparative organic light emitting diode OLEDm.

At the same time, a scanning potential is input into the pixel driving circuit 160 through the scanning line 142 to turn on the first transistor T1, so as to store the first voltage level Vdd and the voltage level V _data of the data line 122 in the capacitor C. pressure difference. In this case, the voltage V _gs across the gate and source of the second transistor T2 is equivalent to V _data -V _oled -V _ss , where V _oled is the voltage across the anode and cathode of the organic light emitting diode OLED.

Since the voltage level of the data line 142 is V _data =V _gs' +V _oled' +V _ss , the voltage across the gate and source of the second transistor T2 is V _gs =V _data -V _oled -V _ss . Therefore, it can be estimated that the voltage Vgs across the gate and source of the second transistor T2 is equivalent to V _gs' -(V _oled -V _oled' ). The difference between the cross voltage V _gs and the threshold voltage Vt of the second transistor T2 determines the magnitude of the current I′ flowing through the organic light emitting diode OLED.

From the foregoing, it can be seen that the drive circuit of the present invention is characterized in that:

Since the organic light-emitting diode OLEDm and the organic light-emitting diode OLED have similar operation times, the voltage rises between the anode and the cathode are also similar, that is to say, V _oled is approximately equal to V _oled′ . As mentioned above, the voltage across the gate and source of the second transistor T2 is V _gs =V _gs' -(V _oled -V _oled' ), it can be deduced that the voltage across the gate and source of the second transistor T2 V _gs is equivalent to the cross-voltage V _gs′ between the gate and the source of the comparison transistor Tm. Also, the cross voltage V _gs' between the gate and source of the comparison transistor Tm is determined by the driving current signal I, and the magnitude of the cross voltage V _gs between the gate and source of the second transistor determines the flow through the organic The magnitude of the current I' of the light-emitting diode OLED. Therefore, the present invention can directly determine the magnitude of the current I' flowing through the organic light emitting diode according to the driving current signal I, and avoid the influence of the voltage change across the organic light emitting diode OLED on the magnitude of the current I'.

Since the comparison transistor Tm and the second transistor T2 have similar operation times, the threshold voltage V _t′ and the rising range of V _t of the two are also similar. As mentioned above, the voltage V _gs across the gate and source of the second transistor T2 is equivalent to the voltage Vg _s′ across the gate and source of the comparative transistor Tm. Therefore, comparing the transistor Tm and the second transistor T2, the difference between the gate-source cross voltage and the threshold voltage (V _gs −V _t ) is equivalent to (V _gs′ −V _t′ ). It can be seen that, under the condition that the channel width-to-length ratio (W/L) of the comparison transistor T2 and the second transistor Tm are the same, the magnitude of the current I' flowing through the organic light emitting diode OLED will not be affected by the critical voltage V _t With the effect of V _t' rising, it will be the same as the magnitude of the driving current signal I.

As mentioned above, in the second transistor T2 and the comparison transistor Tm, the difference (V _gs -V _t ) between the gate and the source and the threshold voltage is equivalent to (V _gs' -V _t' ) . Comparing the channel width-to-length ratios of the transistor Tm and the second transistor T2 will affect the proportional relationship between the driving current signal I and the current I′. For example, if the channel width-to-length ratio of the comparison transistor Tm is twice the channel width-to-length ratio of the second transistor T2, under the same cross voltage (that is, V _gs is approximately equal to V _gs' ), The driving current signal I flowing through the comparison transistor Tm is approximately twice the current I' flowing through the second transistor T2. And using this concept, even in the case of low luminance (that is, the current I' flowing through the second transistor T2 in the pixel driving circuit 160 is weak), the data line 122 can still maintain a relatively large The current signal I is driven to provide a fast enough charging speed for the capacitor C of the pixel driving circuit 160 to ensure that the capacitor C is charged to a preset potential value.

As shown in the third and fourth figures, although the drive circuit 100 of the present invention is driven by current, each pixel drive circuit 160 only needs two transistors T1 and T2, rather than the conventional current-driven pixel of FIG. , four transistors must be used. Therefore, the design of the driving circuit 100 of the present invention can increase the aperture ratio of each pixel.

Claims (6)

1. A pixel drive circuit for an active organic light emitting display, characterized in that the drive circuit comprises: At least one pixel driving circuit, each pixel driving circuit includes: A first transistor, its source is connected to a data line, its gate is connected to a scan line; a second transistor, the gate of which is connected to the drain of the first transistor, and the drain of the second transistor is connected to a first voltage level; a capacitor, a first terminal of which is connected to the drain of the second transistor, and a second terminal of which is connected to the source of the first transistor and the gate of the second transistor; An organic light emitting diode, the anode of which is connected to the source of the second transistor, and the cathode of the organic light emitting diode is connected to a second voltage level; At least one comparing circuit is electrically connected to the pixel driving circuit, and each comparing circuit includes: a comparison transistor, corresponding to the second transistor, whose gate and drain are connected to the data line; A comparison organic light emitting diode, corresponding to the organic light emitting diode, its anode is connected to the source of the comparison transistor, and its cathode is connected to the second voltage level. 2 . The pixel driving circuit of an active organic light emitting display according to claim 1 , wherein the channel width-to-length ratios of the comparison transistor and the second transistor have a specific proportional relationship. 3 . The pixel driving circuit of an active organic light emitting display according to claim 1 , wherein the channel width-to-length ratios of the comparison transistor and the second transistor are the same. 4. The pixel driving circuit of an active organic light emitting display according to claim 1, wherein each comparison circuit is corresponding to the data line. 5. The pixel driving circuit of an active organic light emitting display according to claim 1, wherein the first voltage level is provided to the second transistor through a power line. 6. The pixel driving circuit of an active organic light emitting display according to claim 1, wherein the second voltage level is grounded.

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