CN101996579A - Pixel driving circuit and method of active organic electroluminescent display - Google Patents

Abstract

The invention discloses pixel driving circuit and method of an active organic electroluminescent display. The pixel driving circuit comprises a driving transistor, four switch transistors, a coupling capacitor, a storage capacitor and a light-emitting diode, wherein a drain electrode of the first transistor is connected with a data wire, a grid electrode of the first transistor is connected with a first scanning control wire, and a source electrode of the first transistor is connected with the C end of the coupling capacitor; a drain electrode of the second transistor is connected with source electrodes of the third transistor and the fourth transistor, a grid electrode of the second transistor is connected with the A ends of the coupling capacitor and the storage capacitor and a drain electrode of the third transistor, and a source electrode of the second transistor is connected with a drain electrode of the fifth transistor and the B end of the storage capacitor and is grounded through the organic light-emitting diode; a grid electrode of the third transistor is connected with a second scanning control wire; a drain electrode of the fourth transistor is connected with a power wire, and a grid electrode of the fourth transistor is connected with a light-emitting control wire; and a grid electrode of the fifth transistor is connected with the first scanning control wire, and a source electrode is grounded. The invention can effectively compensate the unevenness of threshold voltages of the transistors and the degradation of a start voltage of the OLED (Organic Light Emitting Diode), ensures that the brightness of an image displayed by the OLED is even and realizes high contrast.

Description

Pixel driving circuit and driving method of active organic electroluminescence display

technical field

The invention relates to a pixel driving technology of a light emitting diode display, in particular to a pixel driving circuit of an active organic electroluminescent display and a driving method thereof.

Background technique

Organic light-emitting diode (OLED) display has the advantages of small size, self-illumination, large viewing angle, short response time, and low production cost, which has attracted more and more researchers to participate. The passive driving method requires high efficiency and brightness of the OLED light-emitting device. The high-voltage or current pulse driving method makes the working efficiency of the light-emitting device very low and reduces the service life of the OLED, and cannot meet the requirements of high-resolution and large-information display. requirements. For large-screen high-resolution displays, active drive methods are usually used.

At present, the thin film transistors used in active OLEDs mainly include amorphous silicon thin film transistors (a-Si TFT) and polycrystalline silicon thin film transistors (Poly-Si TFT). The carrier mobility of a-Si TFT is low, the size of the device is much larger than that of Poly-Si TFT, and the driving voltage and signal voltage are relatively large. These unfavorable factors will cause the pixel aperture ratio of the display screen to decrease and the life of OLED to be shortened. . Poly-Si TFT has a high carrier mobility. Compared with the amorphous silicon process, its device size can be made smaller, which increases the aperture ratio of the OLED pixel, and can also integrate the peripheral drive circuit of the display on the around the display. The active drive OLED technology based on Poly-Si TFT has become the development direction of OLED display drive in the future.

In actual production, it is difficult to ensure that the threshold voltages of the driving transistor TFTs in each pixel are the same at the current technological level. In the traditional two-transistor unit driving scheme, the non-uniformity of the brightness of the entire display screen will be caused by the non-uniformity of the threshold voltage of each pixel driving transistor. In addition, as the working time increases, the threshold voltage of the driving transistor will also increase accordingly, and the degradation of OLED will also cause the increase of its own turn-on voltage, thereby causing the decrease of the brightness of the display screen. In order to compensate the inhomogeneity of the threshold voltage of TFT driven by each pixel point, many compensation schemes have been proposed. These schemes can be mainly classified into current programming type and voltage programming type. Generally speaking, a current-programmed pixel driving circuit requires a long charging time when displaying low gray scales, which affects its use in large-screen high-resolution displays. In the voltage-programmable pixel drive circuit, a large current will charge the storage capacitor and the equivalent capacitance of the OLED itself during the initialization phase, so the charging time can be greatly reduced. Therefore, the research on voltage-programmable pixel driving has attracted more and more researchers in recent years.

Contents of the invention

The purpose of the present invention is to overcome the shortcoming and deficiency of above-mentioned prior art, provide the pixel driving circuit of active organic electroluminescent display, not only can effectively solve the inhomogeneity of each pixel point driving TFT threshold voltage in the active matrix OLED display, It can also solve the problem of degradation of the OLED turn-on voltage, so that the brightness of the OLED display is uniform.

Another object of the present invention is to provide a driving method for the above-mentioned pixel driving circuit of the active organic electroluminescence display.

The object of the present invention is achieved through the following solutions:

A pixel drive circuit for an active organic electroluminescence display, the drive circuit comprising:

The first transistor: its drain is connected to the data line, its gate is connected to the first scanning control line, and its source is connected to the C terminal of the coupling capacitor. The first transistor controls the coupling capacitor to write the grayscale data voltage to the gate of the second transistor. ;

The second transistor: its drain is connected to the sources of the third and fourth transistors, the gate is connected to the A terminal of the coupling capacitor and the storage capacitor and the drain of the third transistor, and the source is connected to the drain of the fifth transistor and the terminal of the storage capacitor The B terminal is connected to the ground line through an organic light emitting diode, and the second transistor drives the OLED to emit light;

The third transistor: its gate is connected to the second scanning control line, the third transistor provides a charging path through the fourth transistor, and provides a discharging path through the second transistor;

The fourth transistor: its drain is connected to the power line, and the gate is connected to the light-emitting control line. The fourth transistor controls the charging path through the third transistor, and controls the OLED to emit light through the second transistor;

Fifth transistor: its gate is connected to the first scanning control line, and its source is grounded. The fifth transistor provides a discharge path to prevent the OLED from emitting light during the threshold voltage storage stage.

The above-mentioned first transistor, second transistor, third transistor, fourth transistor and fifth transistor are any one of polysilicon thin film transistors, amorphous silicon thin film transistors, zinc oxide-based thin film transistors or organic thin film transistors.

When the pixel driving circuit works, the first transistor, the third transistor, the fourth transistor and the fifth transistor all work in the linear region, and the second transistor which plays a driving role works in the saturation region. The driving method of the pixel driving circuit of the above-mentioned active organic electroluminescent display comprises the following steps:

Reset stage: the first scan control line, the second scan control line and the light emission control line are at high level, and the current recharges point A through the third transistor and the fourth transistor;

Threshold voltage storage stage: the first scanning control line and the second scanning control line still maintain the original high level, the light-emitting control line jumps to low level, and the potential of point A is discharged to the threshold voltage of the second transistor;

Grayscale data voltage writing stage: the first scanning control line is at high level, the second scanning control line and light emission control line are at low level, and the data voltage is written into the gate A point of the second transistor through the coupling capacitor;

OLED light-emitting stage: the first scanning control line and the second scanning control line are at low level, the light-emitting control line is at high level, the second transistor drives the OLED to emit light, and the voltage difference between the two ends of the storage capacitor remains unchanged, that is, point A and point B The voltage difference at the point remains constant;

In the threshold voltage storage stage, the storage capacitor stores the threshold voltage of the second transistor; in the data voltage writing stage, the storage capacitor stores the grayscale data voltage;

In the above-mentioned OLED light-emitting stage, the storage capacitor keeps the storage voltage at its two ends unchanged, so that the flowing current is constant without being affected by the turn-on voltage of the OLED.

Compared with the prior art, the present invention has the following advantages and effects: (1) The circuit of the present invention is simple and reliable, not only capable of compensating the difference in threshold voltage of transistors between pixels and the non-uniformity of display brightness caused by OLED degradation, but also due to the The adopted design structure enables the pixels to be completely black during the non-working period, thereby greatly improving the contrast of the display; (2) the present invention adopts a storage capacitor and a coupling capacitor, so that the threshold voltage storage and the grayscale data writing The input is performed separately, which can improve the accuracy of pixel threshold voltage compensation, and is suitable for large-size and high-resolution display devices.

Description of drawings

Fig. 1 is the schematic diagram of the pixel driving circuit of the active organic electroluminescence display of the present invention; Among the figure: the first transistor T1, the second transistor T2, the third transistor T3, the fourth transistor T4, the fifth transistor T5, the coupling capacitance C1, storage capacitor C2, first scan control line Vscan1, second scan control line Vscan2, light emission control line Vems, power line Vdd, ground line Vss, data line Vdata, organic light emitting diode OLED.

FIG. 2 is a signal timing diagram of FIG. 1 .

Detailed ways

The present invention will be described in further detail below in conjunction with the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

Example

As shown in FIG. 1, the pixel driving circuit of the active organic electroluminescent display of the present invention includes a first transistor T1, a second transistor T2, a third transistor T3, a fourth transistor T4, a fifth transistor T5, a coupling capacitor C1, The storage capacitor C2, the first scan control line Vscan1, the second scan control line Vscan2, the light emission control line Vems, the power line Vdd, the ground line Vss, the data line Vdata, and the organic light emitting diode OLED.

The drain of the first transistor T1 is connected to the data line Vdata, the gate is connected to the first scanning control line Vscan1, and the source is connected to the C terminal of the coupling capacitor C1, and the first transistor controls the coupling capacitor to write to the gate of the second transistor. Grayscale data voltage;

The second transistor T2, its drain is connected to the sources of the third and fourth transistors T3 and T4, the gate is connected to the A terminal of the coupling capacitor C1 and the storage capacitor C2 and the drain of the third transistor T3, and the source is connected to the fifth transistor The drain of T5 and the B terminal of the storage capacitor C2 are connected to the ground line Vss through an organic light emitting diode, and the second transistor drives the OLED to emit light;

The gate of the third transistor T3 is connected to the second scanning control line Vscan2, the third transistor provides a charging path through the fourth transistor, and provides a discharging path through the second transistor;

The drain of the fourth transistor T4 is connected to the power line Vdd, and the gate is connected to the light emission control line Vems. The fourth transistor controls the charging path through the third transistor, and controls the OLED to emit light through the second transistor;

The gate of the fifth transistor is connected to the first scan control line Vscan1 and the source is grounded to the Vss. This fifth transistor provides a discharge path to prevent the OLED from emitting light during the threshold voltage storage stage.

The first transistor T1, the second transistor T2, the third transistor T3, the fourth transistor T4 and the fifth transistor T5 are any one of polysilicon thin film transistors, amorphous silicon thin film transistors, zinc oxide-based thin film transistors or organic thin film transistors kind of transistor.

When the pixel driving circuit is working, the first transistor T1 , the third transistor T3 , the fourth transistor T4 and the fifth transistor T5 all work in the linear region, and the second transistor T2 which plays a driving role works in the saturation region. The input of each signal line is shown in Figure 2. The pixel driving circuit is driven in the following steps in each frame:

Reset stage: the first scan control line Vscan1 , the second scan control line Vscan2 and the light emission control line Vems are at high level, and the grayscale data voltage Vdata is zero. All transistors are turned on, and the current recharges point A through the third transistor T3 and the fourth transistor T4. When the charging reaches a certain fixed value, the fifth transistor T5 is turned on at the same time to make the OLED anode voltage zero, which prevents the OLED phase glow;

Threshold voltage storage stage: the first scanning control line Vscan1 and the second scanning control line Vscan2 still maintain the original high level, the light emission control line Vems jumps to low level, and the grayscale data voltage Vdata is zero. The potential at point A is discharged to the threshold voltage of the second transistor T2 through the third transistor T3, the second transistor T2, and the fifth transistor T5, and at the same time, the fifth transistor T5 is turned on so that the OLED anode voltage is still zero, which prevents the OLED from emitting light at this stage ;

Grayscale data voltage writing stage: the first scan control line Vscan1 is at high level, the second scan control line Vscan2 and the light emission control line Vems are at low level, the grayscale data voltage Vdata jumps from zero to a certain positive value, The grayscale data voltage is written into the gate A point of the second transistor through the coupling capacitor C1, and at the same time, the fifth transistor T5 is turned on to make the OLED anode voltage continue to be zero, which prevents the OLED from emitting light at this stage;

OLED light-emitting stage: the first scanning control line Vscan1 and the second scanning control line Vscan2 are at low level, the light-emitting control line Vems is at high level, the second transistor drives the OLED to emit light, and the voltage difference between the two ends of the storage capacitor remains unchanged, that is, The voltage difference between point A and point B in Figure 1 remains unchanged. Specifically, at this stage: the first scanning control line Vscan1 and the second scanning control line Vscan2 are at low level, the light emission control line Vems is at high level, the first transistor T1, the third transistor T3 and the fifth transistor T5 are all turned off, The second transistor T2 drives the OLED to emit light, and the voltage difference between the two terminals of the storage capacitor C2 remains constant during the light emitting stage, thereby maintaining the brightness of the OLED unchanged within one frame time.

In the aforementioned threshold voltage storage stage, the storage capacitor C2 in the pixel driving circuit stores the threshold voltage of the second transistor T2;

In the above grayscale data voltage writing stage, the storage capacitor C2 in the pixel driving circuit stores the written grayscale data voltage;

In the above-mentioned OLED light emitting stage, the storage capacitor C2 in the pixel driving circuit keeps the stored voltage at both ends of it unchanged, so that the current flowing through T2 is constant without being affected by the turn-on voltage of the OLED.

(Vdata＞0)，并且保持到下一帧。对于不同像素点，阈值电压的差异性会导致存储电容C2两端的电压差不同，但是由下面的公式可以推导出流过OLED的电流确是相同的：In the pixel driving circuit of the present invention, the non-uniformity of the threshold voltage Vth of each pixel point and the degradation of the OLED will not affect the brightness difference of the light emitting device OLED. The brightness of the light-emitting device OLED is proportional to the magnitude of the current flowing through it. In the data voltage writing phase, for each pixel point, the voltage difference across the storage capacitor C2 (that is, the gate-source voltage of the second transistor T2) has been fixed at

(Vdata>0), and keep it until the next frame. For different pixels, the difference in threshold voltage will cause the voltage difference across the storage capacitor C2 to be different, but the current flowing through the OLED can be deduced from the following formula to be the same:

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; OLED</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; \&beta;</span>}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; gs</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; the\; th</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}$

$<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; \&beta;</span>}{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; the\; th</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; C</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="C"\; filenames="HSA00000321824300012.png"\; state="\{\{state\}\}">C</figure-callout></span>}\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; C</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; data</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; the\; th</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}$

$<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; \&beta;</span>}{<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; C</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="C"\; filenames="HSA00000321824300012.png"\; state="\{\{state\}\}">C</figure-callout></span>}\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; C</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; data</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}$

in I _OLED is the current flowing through the second transistor T2 during the light-emitting phase of the OLED. When other parameters remain unchanged, its magnitude is only related to Vdata, and has nothing to do with Vth and the conduction voltage of the OLED. In the formula, μ _n is the electron mobility C _OX is the insulation layer capacitance per unit area; L and W are the channel length and width of the second transistor T2 respectively; Vth is the threshold voltage of the second transistor T2; Vgs is the gate-source voltage of the second transistor T2. In addition, the voltage across the OLED is zero in the non-working period of the pixel circuit, so the pixel circuit can be completely black in the non-working period, thereby greatly improving the contrast of the display.

Of course, those skilled in the art can also make appropriate changes to the pixel drive circuit structure and drive method proposed above in the present invention, for example, appropriately change the type (P-type or N-type) of each switch transistor in the pixel circuit, and connect the source and drain of each transistor to The electrical connection relationship of the poles is exchanged, etc.

The present invention can be preferably carried out as described above.

The above-mentioned embodiment is only a preferred embodiment of the present invention, but the embodiment of the present invention is not limited by the above-mentioned embodiment, and any other changes, modifications, substitutions, combinations, Simplifications should be equivalent replacement methods, and all are included in the protection scope of the present invention.

Claims (5)

1. A pixel drive circuit for an active organic electroluminescence display, characterized in that the drive circuit comprises: The first transistor: its drain is connected to the data line, its gate is connected to the first scanning control line, and its source is connected to the C terminal of the coupling capacitor. The first transistor controls the coupling capacitor to write the grayscale data voltage to the gate of the second transistor. ; The second transistor: its drain is connected to the sources of the third and fourth transistors, the gate is connected to the A terminal of the coupling capacitor and the storage capacitor and the drain of the third transistor, and the source is connected to the drain of the fifth transistor and the terminal of the storage capacitor The B terminal is connected to the ground line through an organic light emitting diode, and the second transistor drives the OLED to emit light; The third transistor: its gate is connected to the second scanning control line, the third transistor provides a charging path through the fourth transistor, and provides a discharging path through the second transistor; The fourth transistor: its drain is connected to the power line, and the gate is connected to the light-emitting control line. The fourth transistor controls the charging path through the third transistor, and controls the OLED to emit light through the second transistor; Fifth transistor: its gate is connected to the first scanning control line, and its source is grounded. The fifth transistor provides a discharge path to prevent the OLED from emitting light during the threshold voltage storage stage. 2. The pixel drive circuit of an active organic electroluminescent display according to claim 1, wherein the first transistor, the second transistor, the third transistor, the fourth transistor and the fifth transistor are polysilicon thin films Transistors, amorphous silicon thin film transistors, zinc oxide-based thin film transistors, or organic thin film transistors. 3. The driving method of the pixel driving circuit of the active organic electroluminescent display according to claim 1 or 2, characterized in that, comprising the following steps: Reset stage: the first scan control line, the second scan control line and the light emission control line are at high level, and the current recharges point A through the third transistor and the fourth transistor; Threshold voltage storage stage: the first scanning control line and the second scanning control line still maintain the original high level, the light-emitting control line jumps to low level, and the potential of point A is discharged to the threshold voltage of the second transistor; Grayscale data voltage writing phase: the first scanning control line is at high level, the second scanning control line and light emission control line are at low level, and the grayscale data voltage is written into the gate A point of the second transistor through the coupling capacitor ; OLED light-emitting stage: the first scanning control line and the second scanning control line are at low level, the light-emitting control line is at high level, the second transistor drives the OLED to emit light, and the voltage difference between the two ends of the storage capacitor remains unchanged, that is, point A and point B The voltage difference at the point remains constant; 4. The driving method according to claim 3, wherein the second transistor works in a saturation region, and the first transistor, the third transistor, the fourth transistor, and the fifth transistor work in a linear region. 5. The driving method according to claim 3, characterized in that, In the above threshold voltage storage stage, the storage capacitor stores the threshold voltage of the second transistor; In the phase of writing the grayscale data voltage, the storage capacitor stores and writes the grayscale data voltage; In the above-mentioned OLED light-emitting stage, the storage capacitor keeps the storage voltage at its two ends unchanged, so that the flowing current is constant without being affected by the turn-on voltage of the OLED.

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