CN101980330A - Pixel driving circuit of organic light emitting diode - Google Patents

Abstract

A pixel driving circuit of an organic light emitting diode comprises a first switch, a first capacitor, a transistor, a second switch, a second capacitor and an organic light emitting diode. The operation of the pixel driving circuit mainly comprises four stages of resetting, threshold voltage compensation, data writing and light emitting driving. The pixel driving circuit can compensate the threshold voltage of the transistor, so the driving current of the organic light emitting diode is only related to the data voltage and the reference voltage.

Description

Pixel drive circuit for organic light emitting diodes

【Technical field】

The invention relates to a pixel driving circuit of an organic light emitting diode, in particular to a pixel driving circuit of an organic light emitting diode capable of compensating the critical voltage of a transistor.

【Background technique】

Please refer to FIG. 1 . FIG. 1 is a schematic diagram of a display panel of an organic light emitting diode (OLED) in the background art. The display panel 10 includes a data driver 11 , a scan driver 12 and a display array 13 . The data driver 11 controls the data lines DL ₁ to DL _n , and the scan driver 12 controls the scan lines SL ₁ to SL _m . The display array 13 is formed by interlacing data lines DL ₁ to DL _n and scan lines SL ₁ to SL _m , and each interleaved data line and scan line forms a display unit, for example, data line DL ₁ and scan line SL ₁ The display unit 14 is formed. As shown in FIG. 1, the equivalent circuit of the display unit 14 (other display units are also the same) includes a switching transistor T11, a storage capacitor C11, a driving transistor T12, and an organic light emitting diode D11, wherein the switching transistor T11 and the driving transistor T12 are N-type. transistor.

The scan driver 12 sequentially sends scan signals to the scan lines SL ₁ to SL _m , so that only the switching transistors of all display units in a certain column are turned on at the same time, and the switching transistors of all display units in other columns are turned off. The data driver 11 sends corresponding video signals (gray scale values) to a row of display units through the data lines DL ₁ to DL _n according to the image data to be displayed. For example, when the scan driver 12 sends a scan signal to the scan line _SL1 , the switching transistor T11 of the display unit 14 is turned on, and the data driver ₁₁ transmits the corresponding pixel data to the display unit 14 through the data line DL1, and The voltage of the pixel data is stored by the storage capacitor C11. The driving transistor T12 provides a driving current Ids to drive the OLED D11 according to the voltage stored in the storage capacitor C11.

Since the OLED D11 is a current-driven component, the value of the driving current Ids can determine the brightness of the light generated by the OLED D11. The driving current Ids is the current flowing through the driving transistor T12, which can be expressed as formula (1):

$\mathrm{Ids}=\frac{1}{2}k{(\mathrm{Vgs}-\mathrm{Vth})}^2$ Formula 1)

Where k is the conduction parameter of the driving transistor T12, Vgs is the voltage difference between the gate and the source of the driving transistor T12, and Vth is the threshold voltage value of the driving transistor T12.

However, due to the manufacturing process of thin film transistors, in the display array 13 , the driving transistors in each region are electrically different, that is, the threshold voltage values of the driving transistors are different. Therefore, when a plurality of display units in different regions receive pixel data with the same voltage, due to differences in the threshold voltages of the driving transistors, the values of the driving currents provided to the OLEDs in these display units are inconsistent, resulting in organic The brightness generated by the light emitting diodes is different, and the display panel 10 displays uneven images.

【Content of invention】

Therefore, an object of the present invention is to provide a pixel driving circuit of an OLED to solve the above problems.

The invention provides a pixel driving circuit, which includes a first switch, a first capacitor, a transistor, a second switch, a second capacitor and an organic light emitting diode. The first switch has a first end for receiving a data signal, a second end, and a control end for receiving a scan signal. The first capacitor has a first end electrically connected to the second end of the first switch, and a second end. The transistor has a first terminal, a control terminal electrically connected to the first terminal of the first capacitor, and a second terminal electrically connected to the second terminal of the first capacitor. The second switch has a first end electrically connected to a first voltage source, a second end electrically connected to the first end of the transistor, and a control end for receiving a first control signal. The second capacitor has a first end electrically connected to the second end of the transistor, and a second end electrically connected to a second voltage source. The OLED has a first end electrically connected to the second end of the transistor, and a second end electrically connected to the second voltage source.

【Description of drawings】

FIG. 1 is a schematic diagram of an OLED display panel in the prior art.

FIG. 2 is a schematic diagram of a first embodiment of a pixel driving circuit of an OLED of the present invention.

FIG. 3 is an operation waveform diagram of the pixel driving circuit in FIG. 2 .

FIG. 4 is a schematic diagram of a second embodiment of the pixel driving circuit of the OLED of the present invention.

FIG. 5 is an operation waveform diagram of the pixel driving circuit shown in FIG. 4 .

[Description of main component symbols]

10 Display Panel

11 Data Driver

12 Scan driver

13 Display array

14 Display unit

DL ₁ ~ DL _n data lines

SL ₁ ～SL _m scanning line

T11 Switching Transistor

T12 Driver Transistor

C11 Storage Capacitor

D11, OD1 Organic Light Emitting Diodes

SW1-SW3 Switches

T1 Transistor

20, 40 Pixel drive circuit

C1, C2 Capacitance

OVDD First Voltage Source

OVSS Second Voltage Source

OVDDH High Voltage

OVDDL low level voltage

G1 Scan signal

P1, S1 Control Signal

Sdata data signal

Vdata Data Voltage

Vref Reference Voltage

Vg The voltage of the control terminal of the transistor

Vs The voltage of the second terminal of the transistor

【Detailed ways】

Please refer to FIG. 2 , which is a schematic diagram of a first embodiment of a pixel driving circuit for an OLED of the present invention. The pixel driving circuit 20 includes a first switch SW1 , a first capacitor C1 , a transistor T1 , a second switch SW2 , a second capacitor C2 and an organic light emitting diode OD1 . The first end of the first switch SW1 receives the data signal Sdata, and the control end of the first switch SW1 receives the scan signal G1. The first end of the first capacitor C1 is electrically connected to the second end of the first switch SW1. The control terminal of the transistor T1 is electrically connected to the first terminal of the first capacitor C1, and the second terminal of the transistor T1 is electrically connected to the second terminal of the first capacitor C1. The first end of the second switch SW2 is electrically connected to the first voltage source OVDD, the second end of the second switch SW2 is electrically connected to the first end of the transistor T1, and the control end of the second switch SW2 receives the first control signal P1 . The first end of the second capacitor C2 is electrically connected to the second end of the transistor T1, and the second end of the second capacitor C2 is electrically connected to the second voltage source OVSS. The first end of the organic light emitting diode OD1 is electrically connected to the second end of the transistor T1, and the second end of the organic light emitting diode OD1 is electrically connected to the second voltage source OVSS. In an embodiment of the present invention, the first switch SW1 , the second switch SW2 and the transistor T1 are N-type transistors. The first voltage source OVDD includes a high level voltage OVDDH and a low level voltage OVDDL. The voltage Vs represents the voltage of the second terminal of the transistor T1, and the voltage Vg represents the voltage of the control terminal of the transistor T1.

Please refer to FIG. 3 , which is an operation waveform diagram of the pixel driving circuit in FIG. 2 . The operation of the pixel driving circuit 20 mainly includes four stages of reset, threshold voltage compensation, data writing and driving to emit light. The first voltage source OVDD provides a low-level voltage OVDDL in the reset phase, and a high-level voltage OVDDH in other phases. The data signal Sdata provides a data voltage Vdata in a data writing phase, and a reference voltage Vref in other phases. The pixel circuit 20 is reset in the period TD1 to set the voltage Vg and the voltage Vs. In the period TD1, the first voltage source OVDD provides the low level voltage OVDDL, the scan signal G1 and the control signal P1 are logic high level, so the first switch SW1 and the second switch SW2 are turned on, and the control terminal of the transistor T1 receives the reference voltage Vref. Since the reference voltage Vref is greater than the low level voltage OVDDL, the transistor T1 is also turned on, and the second terminal of the transistor T1 receives the low level voltage OVDDL. Therefore, the voltage Vg and the voltage Vs of the period TD1 can be expressed as equations (1), (2):

Vg＝Vref Formula (1)

Vs＝OVDDL Formula (2)

The pixel circuit 20 performs threshold voltage compensation in the period TD2. In the period TD2 , the first voltage source OVDD provides the high level voltage OVDDH, and the logic levels of the scan signal G1 and the control signal P1 remain unchanged, so the first switch SW1 and the second switch SW2 remain turned on. Since the first voltage source OVDD is converted from the low-level voltage OVDDL to the high-level voltage OVDDH, when the transistor T1 remains turned on, the voltage difference between the control terminal and the second terminal of the transistor T1 must be greater than the threshold voltage Vth of the transistor T1, So that the voltage Vs will rise to Vref-Vth. Therefore, the voltage Vg and the voltage Vs of the period TD2 can be expressed as equations (3) and (4):

Vg＝Vref Formula (3)

Vs＝Vref-Vth Formula (4)

The pixel circuit 20 performs data writing in the period TD3. In the period TD3, the logic level of the scanning signal G1 remains unchanged, and the control signal P1 is converted from a logic high level to a logic low level, so the first switch SW1 remains on, and the second switch SW2 is turned off. At this time, the data signal Sdata provides The data voltage Vdata is transmitted to the control terminal of the transistor T1 through the first switch SW1. When the control terminal of the transistor T1 is converted from the reference voltage Vref to the data voltage Vdata, due to the coupling effect of the capacitor C1, the second terminal of the transistor T1 will generate a voltage difference ΔV, as shown in formula (5). Therefore, the voltage Vg and the voltage Vs of the period TD3 can be expressed as equations (6), (7):

$\mathrm{\&Delta;V}=\frac{C1}{\mathrm{<figure-callout\; id="1"\; label="C"\; filenames="HSA00000346238300021.png,HSA00000346238300051.png"\; state="\{\{state\}\}">C</figure-callout>}1+C2}(\mathrm{Vdata}-\mathrm{Vref})$ Formula (5)

Vg＝Vdata Formula (6)

Vs＝Vref-Vth+ΔV Formula (7)

The pixel circuit 20 is driven to emit light during the period TD4. In the period TD4, the scan signal G1 is switched from a logic high level to a logic low level, and the control signal P1 is switched from a logic low level to a logic high level, so the first switch SW1 is turned off, the second switch SW2 is turned on, and the voltage Vg and voltage Vs can be expressed as equations (8), (9):

Vg＝Vdata+OVSS+VOLED-Vref+Vth-ΔV Formula (8)

Vs＝OVSS+VLED Formula (9)

The voltage VOLED is the voltage difference between the first terminal and the second terminal of the organic light emitting diode OD1, and the current I _OLED driving the organic light emitting diode OD1 is determined by the transistor T1, as shown in formula (10):

I _OLED ＝k(Vgs-Vth) ² formula (10)

The voltage Vgs is the voltage difference between the control terminal and the second terminal of the transistor T1. According to formulas (8) and (9), the voltage Vgs can be expressed as formula (11):

Vgs＝Vdata-Vref+Vth-ΔV Formula (11)

Therefore, according to equations (5), (10), and (11), the current I _OLED can be rewritten as equation (12):

$I_{\mathrm{OLED}}=k{\left[\frac{C2}{\mathrm{<figure-callout\; id="1"\; label="C"\; filenames="HSA00000346238300021.png,HSA00000346238300051.png"\; state="\{\{state\}\}">C</figure-callout>}1+C2}(\mathrm{Vdata}-\mathrm{Vref})\right]}^2$ Formula (12)

From formula (12), it can be seen that the driving current I _OLED of the organic light emitting diode OD1 is only related to the data voltage Vdata and the reference voltage Vref, mainly because the pixel driving circuit 20 compensates the threshold voltage Vth of the transistor T1 .

Please refer to FIG. 4 . FIG. 4 is a schematic diagram of a second embodiment of the pixel driving circuit of the organic light emitting diode of the present invention. In the first embodiment, the first voltage source OVDD of the pixel driving circuit 20 can provide the low level voltage OVDDL or the high level voltage OVDDH, that is, the first voltage source OVDD is an AC voltage source. In the second embodiment, the pixel driving circuit 40 replaces the first voltage source OVDD with two DC voltage sources, and the two DC voltage sources are respectively used to provide the low level voltage OVDDL and the high level voltage OVDDH. 40 further includes a third switch SW3 , the third switch SW3 is controlled by the control signal S1 , and the pixel driving circuit 40 can switch the low level voltage OVDDL and the high level voltage OVDDH through the third switch SW3 and the first switch SW1 .

Please refer to FIG. 5 , which is an operation waveform diagram of the pixel driving circuit in FIG. 4 . The operation principle of the pixel driving circuit 40 is the same as that of the first embodiment, mainly including four stages of reset, threshold voltage compensation, data writing and driving to emit light. In the first embodiment, the first voltage source OVDD provides the low-level voltage OVDDL during the reset phase, and provides the high-level voltage OVDDH during the remaining phases. Therefore, in the second embodiment, when the pixel driving circuit 40 is reset in the period TD1, the control signal P1 is at a logic low level, and the control signal S1 is at a logic high level, so the second switch SW2 is turned off, and the third switch SW2 is turned off. The switch SW3 is turned on, and the low level voltage OVDDL is transmitted to the transistor T1 through the third switch SW3. On the other hand, when the pixel driving circuit 40 performs critical voltage compensation in the period TD2 and drives light in TD4, the control signal P1 is at a logic high level, and the control signal S1 is at a logic low level, so the second switch SW2 is turned on, The third switch SW3 is turned off, and at this time the high level voltage OVDDH is transmitted to the transistor T1 through the second switch SW2. In addition, when the pixel driving circuit 40 performs data writing in the time period TD32, the control signal P1 and the control signal S1 are logic low, so the second switch SW2 and the third switch SW3 are turned off. Therefore, the voltage Vg and the voltage Vs of the pixel driving circuit 40 in the four stages of reset, threshold voltage compensation, data writing and driving light emission are completely the same as those of the first embodiment.

To sum up, the pixel driving circuit of the organic light emitting diode of the present invention includes a first switch, a first capacitor, a transistor, a second switch, a second capacitor and an organic light emitting diode. The operation of the pixel driving circuit mainly includes four stages of reset, threshold voltage compensation, data writing and driving to emit light. The pixel driving circuit can compensate the threshold voltage of the transistor, so the driving current of the OLED is only related to the data voltage and the reference voltage. Therefore, the pixel driving circuit of the organic light emitting diode of the present invention compensates the inconsistency of the driving current caused by the difference in the threshold voltage of the transistor, can improve the difference in brightness produced by the organic light emitting diode, and avoid the unevenness of the display panel of the organic light emitting diode. picture.

The above descriptions are only preferred embodiments of the present invention, and all equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope of the present invention.

Claims (9)

1. pixel-driving circuit comprises:

One first switch has one first end and is used for receiving a data-signal, one second end, and a control end is used for receiving the one scan signal;

One first electric capacity has second end that one first end is electrically connected at this first switch, and one second end;

One transistor has one first end, and a control end is electrically connected at first end of this first electric capacity, and one second end is electrically connected at second end of this first electric capacity;

One second switch has one first end and is electrically connected at one first voltage source, and one second end is electrically connected at this transistorized first end, and a control end is used for receiving one first control signal;

One second electric capacity has one first end and be electrically connected at this transistorized second end, and one second end is electrically connected at one second voltage source; And

One Organic Light Emitting Diode has one first end and be electrically connected at this transistorized second end, and one second end is electrically connected at this second voltage source.

2. pixel-driving circuit according to claim 1 is characterized in that, this first switch, this second switch and this transistor are the N transistor npn npn.

3. pixel-driving circuit according to claim 1 is characterized in that other comprises:

One the 3rd switch has first end and is electrically connected at a tertiary voltage source, and one second end is electrically connected at this transistorized first end, and a control end is used for receiving one second control signal.

4. pixel-driving circuit according to claim 3 is characterized in that, this first voltage source is used to provide one first an accurate voltage, and this tertiary voltage source is used to provide one second an accurate voltage.

5. pixel-driving circuit according to claim 1 is characterized in that, this first voltage source is used to provide one first an accurate voltage and one second an accurate voltage.

6. pixel-driving circuit according to claim 5, it is characterized in that, when this first switch and this second switch were unlocked, this data-signal transferred to this transistorized control end via this first switch with a reference voltage, and this transistorized second termination is received this second accurate voltage.

7. pixel-driving circuit according to claim 6, it is characterized in that, when this first voltage source switched to this first accurate voltage by this second accurate voltage, the voltage of this transistorized second end produced according to the critical voltage of this reference voltage and this crystal.

8. pixel-driving circuit according to claim 7 is characterized in that, when this first switch is unlocked and this second switch when being closed, this data-signal transfers to this transistorized control end via this first switch with a data voltage.

9. pixel-driving circuit according to claim 8 is characterized in that, when this first switch is closed and this second switch when being unlocked, this Organic Light Emitting Diode is by according to current drives that this data voltage and this reference voltage produced and luminous.

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