KR20090048823A - Pixel circuit of organic light emitting display - Google Patents

Abstract

Disclosed is a pixel circuit of an active organic light emitting display device. In a section in which a data signal is applied and the threshold voltage of the driving transistor is stored, the pixel circuit includes two capacitors connected in parallel to the gate terminal of the driving transistor. That is, the storage capacitor and the auxiliary capacitor are provided in the pixel circuit, and the threshold voltage of the driving transistor is stored in the two capacitors. In the light emitting period, the threshold voltage of the driving transistor is compensated, and the influence of the threshold voltage is excluded. In addition, the time constant at the gate terminal is increased by capacitors connected in parallel to the gate terminal of the driving transistor. Therefore, even when a leakage current occurs in the thin film transistor, a constant voltage can be maintained at the gate terminal.

Organic field, OLED, pixel circuit

Description

Pixel Circuit of Organic Light Emitting Display

The present invention relates to an organic light emitting display device, and more particularly, to a pixel circuit of an active organic light emitting display device.

As the recent informatization progresses, the video, information and communication industries have developed dramatically. In particular, in the display field, existing CRTs or liquid crystal displays alone do not satisfy consumers.

The display device proposed as a solution to this problem may be called an organic light emitting device. An organic electroluminescent device is a typical self-luminous device in which electrons and holes are injected from an electrode, and light emission operation occurs when they recombine through an excited state. The organic light emitting display device has various advantages, such as a wide viewing angle, fast response speed, and low power, compared to a conventional liquid crystal display.

In general, organic electroluminescent devices are classified into passive type and active type.

The passive type has a structure in which a light emitting layer is interposed between an anode electrode and a cathode electrode, and does not include an active element such as a thin film transistor.

In addition, the active type has an active element such as a thin film transistor for each pixel, and has an advantage of performing light emission for a predetermined period even without continuously applying a data signal for determining the luminance of the pixel.

In such an active organic light emitting display device, a mechanism for generating a driving current by applying a scan signal and a data signal to each of a plurality of pixels arranged in a matrix form is used. The scan signal is used to select a pixel, and the data signal is used to perform a light emission operation of desired luminance on the selected pixel.

1 is a circuit diagram illustrating a pixel circuit of a conventional organic light emitting display device.

Referring to FIG. 1, a scan signal SEL is applied to a gate terminal of the switching transistor Q1. When the scan signal SEL is activated at a low level, the switching transistor Q1 is turned on and the data signal DATA is applied to the driving transistor Q2 and the storage capacitor Cst.

The data signal DATA is stored through the storage capacitor, and the driving transistor Q2 generates the driving current Idr. By the generated driving current Idr, the organic light emitting diode OLED performs light emitting operation at a predetermined brightness.

The driving current Idr is determined by the following equation.

[Equation 1]

In Equation 1, K is a constant, Vsg is a source-gate voltage of the driving transistor Q2, and | Vth | is an absolute value of the threshold voltage of the driving transistor Q2.

The above-described switching transistors and driving transistors are thin film transistors, and are formed using low temperature poly-silicon (LTPS) technology that forms amorphous silicon on a substrate and then converts the polycrystalline silicon. In the case of forming a transistor on such polycrystalline silicon, the mobility and the threshold voltage of the charge are changed according to the crystal direction of the grain of the channel region. That is, even if the same fabrication process is performed, a problem arises in that the adjacent pixels have different threshold voltages.

When the driving transistors of each pixel have different threshold voltages, a problem occurs in that light emission operations are performed at different luminance levels even when the same data signal is applied. That is, even though the same luminance must be expressed by the same data signal, a problem arises in that different luminance is expressed.

In addition, while the light emitting operation is performed, the scan signal SEL is in an inactive state, and the switching transistor Q1 is turned off. In this case, the driving transistor Q2 forms a driving current according to the voltage stored in the storage capacitor Cst. However, thin film transistors manufactured by the LTPS process have a higher leakage current than transistors manufactured by the conventional semiconductor manufacturing process. Therefore, a phenomenon occurs in which the data signal DATA stored in Cst is changed in accordance with a predetermined time constant. That is, the voltage applied to the gate terminal of the driving transistor Q2 during the light emission operation is changed, and thus the driving current of the driving transistor Q2 is changed with time. As a result, there is a problem that the light emitting characteristics of the organic light emitting diode OLED are changed over time.

An object of the present invention for solving the above problems is to provide a pixel circuit that can eliminate the influence of the threshold voltage of the thin film transistor and minimize the variation of the voltage at the gate terminal of the driving transistor.

The present invention for achieving the above object, the first switching transistor for transmitting a data signal to the first node according to the selection signal; A driving transistor connected between the first node and a second node and configured to generate a driving current according to the data signal; A second switching transistor connected between the second node and a third node which is a gate terminal of the driving transistor and diode-connecting the driving transistor according to the selection signal; A storage capacitor connected between the third node and a positive power supply voltage and configured to compensate for a threshold voltage of the driving transistor; An auxiliary capacitor connected to the third node and connected in parallel with the storage capacitor to increase a time constant at the third node; An initialization transistor connected to the third node and configured to perform an initialization operation on the storage capacitor and the auxiliary capacitor according to an initialization control signal; A first emission control transistor coupled between the second node and a positive power supply voltage and configured to perform an on / off operation according to an emission control signal; A second light emission control transistor connected between the first node and a fourth node and transferring the driving current generated by the driving transistor; And an organic light emitting diode connected between the fourth node and a negative power supply voltage and configured to perform a light emitting operation corresponding to the old coin.

According to the present invention, the influence of the threshold voltage in the driving transistor of the pixel circuit is excluded. An auxiliary capacitor is also connected in parallel to the storage capacitor to increase the capacitance seen by the gate terminal of the driving transistor. This increases the time constant at the gate terminal of the driving transistor. Therefore, even if a leakage current is generated in the thin film transistor due to the LTPS process, the variation of the voltage of the gate terminal affecting the driving current of the driving transistor is minimized. That is, the driving current corresponding to the data signal to be applied can be formed.

As the inventive concept allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to the specific disclosed form, it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention. In describing the drawings, similar reference numerals are used for similar elements.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art. Terms such as those defined in the commonly used dictionaries should be construed as having meanings consistent with the meanings in the context of the related art and shall not be construed in ideal or excessively formal meanings unless expressly defined in this application. Do not.

Hereinafter, with reference to the accompanying drawings, it will be described in detail a preferred embodiment of the present invention.

2 is a circuit diagram illustrating a pixel circuit according to a preferred embodiment of the present invention.

Referring to FIG. 2, the pixel circuit according to the present exemplary embodiment includes a first switching transistor T1, a second switching transistor T2, an initialization transistor T3, a driving transistor T4, a first emission control transistor T5, a second emission control transistor T6, and a storage capacitor. Cst, auxiliary capacitor Ccom and light emitting diode OLED.

The first switching transistor T1 is connected between the line to which the data signal DATA is applied and the first node N1. In addition, the on / off operation is performed by the scan signal SEL. That is, when the first switching transistor T1 is turned on by the scan signal SEL, the data signal DATA is transferred to the first node N1 through the first switching transistor.

The second switching transistor T2 is connected between the second node N2 and the third node N3. In addition, the gate terminal of the second switching transistor T2 is connected to the scan signal SEL to perform an on / off operation according to the scan signal SEL. Therefore, the gate terminals of the first switching transistor T1 and the second switching transistor T2 are commonly connected to the scan signal SEL.

The initialization transistor T3 is connected between the initialization signal VI and the third node N3. In addition, the on / off operation is performed according to the initialization control signal INT.

The driving transistor T4 is connected between the first node N1 and the second node N2. In addition, the gate terminal of the driving transistor T4 generates the driving current Idr in accordance with the voltage of the third node N3. In addition, the driving transistor T4 is diode-connected according to the turn-on operation of the second switching transistor T2.

The first light emission control transistor T5 is connected between the positive power supply voltage ELVDD and the second node N2 and performs an on / off operation by the light emission control signal EM.

The second light emission control transistor T6 is connected between the first node N1 and the fourth node N4. In addition, the on / off operation is performed by the emission control signal EM in the same manner as the first emission control transistor T5.

The storage capacitor Cst is connected between the third node N3 and the positive power supply voltage ELVDD. In addition, the auxiliary capacitor Ccom is connected between the third node N3 and the initialization control signal INT.

The organic light emitting diode OLED is connected between the fourth node N4 and the negative power supply voltage ELVSS. The organic light emitting diode OLED emits light with a predetermined brightness by the driving current Idr generated in the driving transistor T4.

FIG. 3 is a timing diagram for describing an operation of the pixel circuit shown in FIG. 2.

Referring to FIG. 3, the pixel circuit is driven by dividing into three operation sections.

First, in the initialization period P1, the initialization signal VI is transferred to the storage capacitor Cst and the auxiliary capacitor Ccom through the initialization transistor T3. Therefore, in the initialization period P1, the voltage of the third node N3 is initialized to VI.

를 커패시터들에 저장한다.In the compensation period P2, the data signal DATA is transferred to the storage capacitor Cst and the auxiliary capacitor Ccom. That is, the absolute value of the threshold voltage of the data signal DATA and the driving transistor T4.

Is stored in the capacitors.

가 반영된 상태이다. 따라서, 구동 전류 Idr에서는 구동 트랜지스터 T4의 문턱전압의 영향이 배제된다.Subsequently, in the emission period P3, the emission control signal EM is activated to turn on the emission control transistors to generate the driving current Idr. The organic light emitting diode OLED emits light at a predetermined luminance by the generated driving current Idr. The absolute value of the threshold voltage is applied to the voltage of the third node N3 applied to the driving transistor T4 to generate the driving current Idr.

Is reflected. Therefore, the influence of the threshold voltage of the drive transistor T4 is excluded from the drive current Idr.

4 is a circuit diagram illustrating an operation in an initialization period of the pixel circuit shown in FIG. 2 according to an exemplary embodiment of the present invention.

Referring to FIG. 4, a solid line indicates a portion where signals are applied, and a dotted line indicates a portion where application of a signal is blocked by turning off the transistor.

First, in the initialization section P1 in FIG. 4, only the initialization control signal INT is activated. Therefore, only the initialization transistor T3 is turned on by the initialization control signal INT having the high level. The initialization signal VI is applied to the third node N3 through the turned-on initialization transistor T3. Therefore, the third node N3 having the storage capacitor Cst and the auxiliary capacitor Ccom connected to each other is initialized to VI level.

FIG. 5 is a circuit diagram illustrating an operation of a data signal input section of a pixel circuit shown in FIG. 2 according to a preferred embodiment of the present invention.

Referring to FIG. 5, a solid line indicates a portion where signals are applied, and a dotted line indicates a portion where signal application is blocked by the transistor off.

In the compensation period P2 of FIG. 3, the initialization signal INT is deactivated, the scan signal SEL is activated, and the data signal DATA is input. The initialization transistor T3 is turned off by the disabled initialization signal INT.

로 세팅된다. 이러한 제3 노드 N3의 전압은 스토리지 커패시터 Cst 및 보조 커패시터 Ccom의 일측단에 저장되는 전압이 된다. 따라서, 스토리지 커패시터 Cst에는 ELVDD-(DATA-

)의 전압차가 저장되며, 보조 커패시터 Ccom에는 INT-(DATA-

)의 전압차가 저장된다.In addition, the first switching transistor T1 and the second switching transistor T2 are turned on by the activated scan signal SEL. The data signal DATA is applied to the first node N1 through the turned-on first switching transistor T1. In addition, the driving transistor T4 is diode-connected by the turned-on second switching transistor T2. When the driving transistor T4 is diode-connected, the voltage of the third node N3 which is the gate terminal is DATA-.

Is set to. The voltage of the third node N3 becomes a voltage stored at one end of the storage capacitor Cst and the auxiliary capacitor Ccom. Therefore, the storage capacitor Cst includes ELVDD- (DATA-

) And the auxiliary capacitor Ccom has INT- (DATA-

Voltage difference is stored.

FIG. 6 is a circuit diagram illustrating an operation in a light emission period of a pixel circuit of FIG. 2 according to a preferred embodiment of the present invention.

Referring to FIG. 6, a solid line indicates a portion where signals are applied, and a dotted line indicates a portion where application of a signal is blocked by turning off the transistor.

In FIG. 3, the scan signal SEL is deactivated and the emission control signal EM is activated in the emission section P3. The first emission control transistor T5 and the second emission control transistor T6 are turned on by the activated emission control signal EM. Thus, a current path is formed between the positive power supply voltage ELVDD and the negative power supply voltage ELVSS.

In addition, the first select transistor T1 and the second select transistor T2 are turned off by the deactivated scan signal SEL. Inflow of the data signal DATA is blocked by the turned-off first select transistor T1.

에 의해 구동 트랜지스터 T4는 구동 전류 Idr을 발생시킨다. 예컨대, 상기 구동 트랜지스터 T4가 활성영역에서 동작하는 경우, 구동 전류 Idr은 하기의 수학식 2로 표현된다.Voltage DATA- of previously stored third node N3

By this, the drive transistor T4 generates the drive current Idr. For example, when the driving transistor T4 operates in the active region, the driving current Idr is represented by the following equation (2).

[Equation 2]

의 영향은 배제된다.As shown in Equation 2, the threshold voltage of the driving transistor T4 at the driving current Idr.

The influence of is excluded.

In addition to the storage capacitor Cst, the auxiliary capacitor Ccom is connected in parallel with the third node N3. Therefore, the capacitance seen from the third node N3 becomes Cst + Ccom, and maintains the voltage of the third node N3 applied to the gate terminal of the driving transistor T4 for a predetermined period in the light emitting period for generating the driving current.

That is, the time constant at the third node N3 is increased by the capacitors connected in parallel, and the gate voltage of the driving transistor T4 can be kept constant by increasing the time constant.

Typically, transistors in an active organic light emitting device are formed using a Low Temperature Poly-Silicon (LTPS) process. That is, the thin film transistors are formed of polysilicon by forming a silicon thin film in an amorphous state and then irradiating a laser. When the transistor is formed using the LTPS process, a high leakage current is generated as compared with the transistor formed on the semiconductor substrate. That is, even when the transistor is turned off through the control signal, a leakage current is generated in the transistor in the off state.

In the present invention, capacitors are disposed in parallel to the third node N3, which is the gate terminal of the driving transistor T4, and the time constant is increased by using the increased capacitance. Therefore, despite the occurrence of the leakage current by the increased time constant, it is possible to maintain a constant gate voltage and to exclude the influence of the threshold voltage of the driving transistor T4.

In addition, in FIGS. 2-6, transistors T1, T2, and T3 are depicted as NMOS. However, it may be composed of PMOS transistors instead of NMOS transistors. If the transistors T1, T2, and T3 are PMOS, the initialization control signal INT and the scan signal SEL should be applied in an inverted state in the timing diagram of FIG.

1 is a circuit diagram illustrating a pixel circuit of a conventional organic light emitting display device.

2 is a circuit diagram illustrating a pixel circuit according to a preferred embodiment of the present invention.

FIG. 3 is a timing diagram for describing an operation of the pixel circuit shown in FIG. 2.

4 is a circuit diagram illustrating an operation in an initialization period of the pixel circuit shown in FIG. 2 according to an exemplary embodiment of the present invention.

FIG. 5 is a circuit diagram illustrating an operation of a data signal input section of a pixel circuit shown in FIG. 2 according to a preferred embodiment of the present invention.

FIG. 6 is a circuit diagram illustrating an operation in a light emission period of a pixel circuit of FIG. 2 according to a preferred embodiment of the present invention.

Claims (5)

A first switching transistor for transmitting a data signal to the first node according to the selection signal; A driving transistor connected between the first node and a second node and configured to generate a driving current according to the data signal; A second switching transistor connected between the second node and a third node which is a gate terminal of the driving transistor and diode-connecting the driving transistor according to the selection signal; A storage capacitor connected between the third node and a positive power supply voltage and configured to compensate for a threshold voltage of the driving transistor; An auxiliary capacitor connected to the third node and connected in parallel with the storage capacitor to increase a time constant at the third node; An initialization transistor connected to the third node and configured to perform an initialization operation on the storage capacitor and the auxiliary capacitor according to an initialization control signal; A first emission control transistor coupled between the second node and a positive power supply voltage and configured to perform an on / off operation according to an emission control signal; A second light emission control transistor connected between the first node and a fourth node and transferring the driving current generated by the driving transistor; And And an organic light emitting diode connected between the fourth node and a negative power supply voltage and configured to perform a light emitting operation corresponding to the driving current. The method of claim 1, wherein the pixel circuit, An initialization period in which the initialization transistor is turned on to initialize the storage capacitor and the auxiliary capacitor; A compensation period for turning on the first switching transistor and the second switching transistor to store threshold voltages of the data signal and the driving transistor in the third node; And turning on the first light emitting control transistor and the second light emitting control transistor to divide the driving current into light emitting sections for supplying the driving light to the organic light emitting diodes. The method of claim 2, wherein the auxiliary capacitor stores a difference between the data signal and the threshold voltage of the driving transistor through the third node in the compensation period, and compensates the threshold voltage of the driving transistor in the emission period. A pixel circuit of an organic light emitting display device. 3. The organic light emitting device of claim 2, wherein the auxiliary capacitor is connected in parallel to the talker capacitor with respect to the third node in the light emitting period, thereby increasing a time constant at the third node. Pixel circuit. The pixel circuit of claim 1, wherein the first switching transistor, the second switching transistor, and the initialization transistor are NMOS.

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