KR100822205B1 - Pixel circuit and organic light emitting display device comprising the same - Google Patents

Abstract

A pixel circuit and an organic light emitting display device having the same are provided to increase image resolution by minimizing a variation in a driving voltage due to a kick-back phenomenon. A first PMOS(Positive Metal Oxide Semiconductor) transistor(PM1) includes a first electrode to which a source voltage is applied, a second electrode connected to an OLED(Organic Light Emitting Diode), and a gate. A first NMOS(Negative Metal Oxide Semiconductor) transistor(NM1) includes a gate, which is connected between the second electrode and the gate of the first PMOS transistor, such that the first PMOS transistor is connected in a diode shape. A second NMOS transistor(NM2) includes a first electrode to which a data signal is applied, a second electrode connected to the gate electrode of the first PMOS transistor, and a gate to which a scan signal is applied. A capacitor(Cst) includes a first terminal connected to the gate of the first PMOS transistor, and a second terminal connected to the second electrode of the second NMOS transistor. A second PMOS transistor(PM2) includes a first electrode connected to the second electrode of the first PMOS transistor, a second electrode connected to an anode of the OLED, and a gate to which a first light emitting control signal is applied.

Description

Pixel circuit and organic light emitting display device comprising the same

1 is a conceptual diagram of an organic EL device.

2 is a circuit diagram of a pixel circuit employed in a conventional organic light emitting display device.

3 is a circuit diagram of a pixel circuit for threshold voltage compensation according to an embodiment of the present invention.

FIG. 4 is a driving waveform diagram for driving the pixel circuit shown in FIG. 3.

5 is a circuit diagram of a pixel circuit for compensating a kickback phenomenon according to another exemplary embodiment of the present invention.

FIG. 6 is a driving waveform diagram for driving the pixel circuit shown in FIG. 5.

7 is a block diagram illustrating an organic light emitting display device including a pixel circuit according to an exemplary embodiment of the present invention.

The present invention provides a uniform luminance by compensating for variations in the threshold voltage of a driving transistor included in a pixel circuit, and implements a high resolution by minimizing a change in driving voltage caused by a kickback phenomenon of a switching transistor. An organic light emitting display device is included.

In general, an organic light emitting display device is a display device for electrically exciting a fluorescent organic compound to emit light, and is capable of displaying an image by voltage driving or current driving a plurality of organic light emitting cells arranged in a matrix form. These organic light emitting cells have diode characteristics and are called organic light emitting diodes (OLEDs).

1 is a conceptual diagram of an organic light emitting device.

Referring to FIG. 1, the organic light emitting diode has a structure of an anode (ITO), an organic thin film, and a cathode electrode layer (metal). The organic thin film includes an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) in order to balance the electrons and the grains, thereby improving the emission efficiency.

The organic light emitting cell may be driven using a simple matrix method and an active matrix method using a thin film transistor (TFT) or a MOSFET. In the simple matrix method, the anode and the cathode are orthogonal and the line is selected and driven, whereas the active driving method connects a thin film transistor to each indium tin oxide (ITO) pixel electrode and is maintained by a capacitor capacity connected to the gate of the thin film transistor. It is driven according to the voltage. On the other hand, the active driving method is divided into a voltage programming method and a current programming method according to the type of signal applied to write and maintain a voltage in the capacitor.

2 is a circuit diagram of a pixel employed in a conventional organic light emitting display device.

Referring to FIG. 2, a pixel of an organic light emitting diode display includes an organic light emitting diode OLED, two transistors M1 and M2, and one capacitor C _st , and generally the switching transistor M1 and the driving device. As the transistor M2, a thin film transistor TFT is used.

In the pixel circuit of FIG. 2, the first electrode of the switching transistor M1 is connected to the data signal line. At this time, the switching transistor M1 is turned on by the selection signal S [n] applied to the gate electrode, so that the data signal D [m] is applied to the pixel circuit.

On the other hand, the capacitor C _st is connected between the first electrode and the gate electrode of the driving transistor M2 to maintain the data voltage for a predetermined period. In addition, the driving transistor M2 supplies a current corresponding to the voltage applied between both electrodes of the capacitor C _st to the organic light emitting diode OLED.

It said switching transistor (M1) is the ground is turned on, the data voltage applied through the data signal line is stored in the capacitor (C _st), since stored in the switching transistor and the capacitor (C _st) in the case that (M1) is turned off data A current corresponding to the voltage is applied to the organic light emitting diode OLED through the driving transistor M2. Accordingly, the organic light emitting diode OLED maintains light emission for a predetermined period even when the switching transistor M1 is turned off.

At this time, the current flowing through the OLED is represented by Equation 1 below.

[Equation 1]

In Equation 1, I _OLED is a current flowing through the OLED, V _gs is a voltage between the gate electrode and the first electrode of the driving transistor M2, V _th is a threshold voltage of the driving transistor M2, V _DD represents a power supply voltage, V _data represents a data voltage, and β represents a gain factor.

However, in the pixel circuit of the voltage write method as described above, a deviation of the threshold voltage V _th occurs in the manufacturing process of the driving transistor such as the driving transistor, which causes a problem that it is difficult to obtain a screen of uniform brightness. That is, when a predetermined driving transistor has an absolute value of a high threshold voltage, as mentioned in Equation 1, the I _OLED value is lowered, and the organic light emitting diode OLED emits dark light by the low I _OLED . do. In addition, as the switching transistor M1 is turned off, a kick-back phenomenon occurs in which charges used in channel formation are leaked to the capacitor, thereby preventing accurate display of the screen.

An object of the present invention is to compensate for the variation in the threshold voltage of the driving transistor included in the pixel circuit to express a uniform brightness, and to minimize the change of the driving voltage caused by the kick-back phenomenon to achieve high resolution. A pixel circuit and an organic light emitting display device including the same are provided.

A first PMOS transistor includes a first electrode to which a power supply voltage is applied, a second electrode electrically connected to an organic light emitting device, and a gate; A first NMOS transistor connected between the second electrode and the gate of the first PMOS transistor to diode-connect the first transistor; A second NMOS transistor having a first electrode to which a data signal is applied, a second electrode electrically connected to the gate electrode of the first PMOS transistor, and a gate to which a scan signal is applied; And a capacitor having a first terminal connected to a gate terminal of the first PMOS transistor, and a second terminal connected to a second electrode of the second NMOS transistor.

The pixel circuit of the organic light emitting diode display includes a first electrode connected to a second electrode of the first PMOS transistor, a second electrode connected to an anode of the organic light emitting diode, and a gate to which a first emission control signal is applied. It may further include a second PMOS transistor.

The pixel circuit of the organic light emitting diode display may include a third PMOS transistor including a first electrode to which a sustain voltage is applied, a second electrode to be connected to the second terminal of the capacitor, and a gate to which a second emission control signal is applied. It may further include.

The second and third PMOS transistors may be turned off while the first and second NMOS transistors are turned on.

In addition, the present invention provides a display device comprising: a plurality of pixels each having the pixel circuit of claim 1 and an organic light emitting element controlled by the pixel circuit; A plurality of data lines transferring the data signals to the plurality of pixels; A plurality of scan lines configured to transfer the scan signals to the plurality of pixels; And a plurality of emission scan lines which transmit the emission control signals to the plurality of pixels.

The organic light emitting diode display may further include a data driver connected to the plurality of data lines to supply the data signal.

The organic light emitting diode display may further include a scan driver connected to the plurality of scan lines to supply the scan signal.

The organic light emitting diode display may further include an emission driver connected to the plurality of emission scan lines to supply the emission control signal.

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

3 is a circuit diagram of a pixel circuit for threshold voltage compensation according to an embodiment of the present invention.

Referring to FIG. 3, the pixel circuit controls the organic light emitting diode OLED by supplying a current to the organic light emitting diode OLED displaying colors of red, green, blue, and white. In other words, the pixel circuit is a data signal, a scan signal, and a light emission control signal applied to the data line D [m], the scan line S [n], and the emission scan lines E1 [n], E2 [n], respectively. By controlling the organic light emitting device (OLED) connected between the first power supply voltage (V _DD ) and the second power supply voltage (V _SS ).

The pixel circuit includes a first transistor PM1 that is a driving transistor, second to fifth transistors PM2 to PM5 that are switching transistors, and a capacitor Cst.

The first electrode of the first transistor PM1 is connected to a power supply voltage, the second electrode is electrically connected to the organic light emitting element, and the gate is connected to the first node N1. The first transistor PM1 is a driving transistor and supplies a current corresponding to the voltage applied between the first terminal and the second terminal of the capacitor Cst to the organic light emitting diode OLED during the period in which the second transistor PM2 is on. Perform the function of feeding.

The first electrode of the second transistor PM2 is connected to the second electrode of the first transistor PM1, the second electrode is connected to the anode of the organic light emitting element OLED, and the gate thereof is the first emission scan line E1. [n]). The second transistor PM2 supplies a function of supplying a current flowing to the first transistor PM1 to the organic light emitting diode OLED in response to the first emission signal applied to the first emission scan line E1 [n]. Perform.

The first electrode of the third transistor PM3 is connected to the sustain voltage Vsus, the second electrode is connected to the second node N2, and the gate is connected to the second emission scan line E2 [n]. . The third transistor PM3 performs a function of applying the sustain voltage Vsus to the second node N2 in response to the second emission signal applied to the second emission scan line E2 [n].

The gate of the fourth transistor PM4 is connected to the scan line S [n], the first electrode is connected to the third node N3, and the second electrode is connected to the first node N1. The fourth transistor PM4 functions to diode-connect the first transistor PM1 by connecting a gate and a drain of the first transistor PM1 in response to a scan signal applied to the scan line S [n].

The gate of the fifth transistor PM5 is connected to the second emission scan line E2 [n], the first electrode is connected to the data line D [m], and the second electrode is connected to the second node N2. do. The fifth transistor PM5 performs a function of applying a data voltage applied to the data line D [m] to the second node N2 in response to a scan signal applied to the scan line S [n].

The first terminal of the capacitor Cst is connected to the first node N1 and the second terminal is connected to the second node N2. The capacitor Cst charges the amount of charge corresponding to the threshold voltage of the first transistor PM1 while the fourth and fifth transistors PM4 and PM5 are on, and the fourth and fifth transistors PM4 and PM5 are turned off. The threshold voltage is maintained for a period of time.

The anode of the OLED is connected to the second electrode of the second transistor PM2, and the cathode of the OLED is connected to the reference voltage V _SS so that the second transistor PM2 is turned on. On to emit light corresponding to the amount of current applied from the first transistor PM1. In this case, the reference voltage V _SS is a voltage having a lower level than the power supply voltage V _DD , and a ground voltage and a negative voltage may be used.

FIG. 4 is a driving waveform diagram for driving the pixel circuit shown in FIG. 3.

In the first period T1, the first emission signal E1 [n] is low, and the scan signal S [n] and the second emission signal E2 [n] are high. As a result, the second transistor PM2 is turned on, and the third to fifth transistors PM3, PM4, and PM5 are turned off. During this period, the charge stored in the capacitor C _st is cleared.

In the second period T2, the scan signal S [n] is low, and the first emission signal E1 [n] and the second emission signal E2 [n] are high. As a result, the fourth and fifth transistors PM4 and PM5 are turned on, and the second and third transistors PM2 and PM3 are turned off. In this period, the current flowing through the first transistor PM1 becomes OA, so the voltage V _GS between the gate and the source of the driving transistor MD becomes a threshold voltage, that is,-| V _TH |, and the first terminal of the capacitor Cst. The voltage of becomes V _DATA- | V _TH |

In the third period T3, the second emission signal E2 [n] is low, and the scan signal S [n] and the first emission signal E1 [n] are high. As a result, the third transistor PM3 is turned on, and the second, fourth, and fifth transistors PM2, PM4, and PM5 are turned off. The voltage at the first terminal in the floating state of the capacitor Cst becomes V _DATA- | V _TH |

In the fourth period T4, the first emission signal E1 [n] and the second emission signal E2 [n] are low, and the scan signal S [n] is high. As a result, the second and third transistors PM2 and PM3 are turned on, and the fourth and fifth switching transistors PM4 and PM5 are turned off. In this period, since the voltage between the gate and the source of the first transistor PM1 is maintained as in Equation 2, the current I _OLED flowing through the sustain light emitting element _OLED is as in Equation 3.

[Equation 2]

[Equation 3]

As a result, uniform luminance can be expressed by compensating for variation in threshold voltage.

However, the kick-back in which the charges used to form the channel of the fourth and fifth transistors PM4 and PM5 are leaked to the capacitor C _st while the fourth and fifth transistors PM4 and PM5 are turned off in the period T3. (Kick_back) phenomenon occurs. The leakage current affects the voltage across the capacitor (C _st), is the voltage across the capacitor (C _st) increases. Subsequently, a current corresponding to the voltage stored in the capacitor C _st flows to the organic light emitting diode OLED in the period T4 to emit light. As described above, the voltage across the capacitor C _st is changed to eventually cause the organic light emitting diode. The current I _OLED flowing in the _OLED also changes. In addition, as the voltage across the capacitor C _st increases, the fourth and fifth transistors PM4 and PM5 may not be turned off, but remain turned on, thereby causing data to be mixed.

If the change in the voltage across the capacitor (C _st ) due to the kick-back phenomenon, there is a problem in the high resolution implementation. In order to implement a high resolution to implement the size of the capacitor (C _st) to a minimum, so when the kick according to reduce the size of the capacitor (C _st) - the greater the voltage variation due to the back phenomenon. Therefore, the only way to reduce the voltage change caused by the kick-back phenomenon is to increase the size of the capacitor (C _st ), which is also unsuitable for high resolution.

5 is a circuit diagram of a pixel circuit for compensating a kickback phenomenon according to another exemplary embodiment of the present invention.

Referring to FIG. 5, the fourth and fifth transistors PM4 and PM5 of FIG. 3 are replaced with the first and second NMOS transistors NM1 and NM2. That is, the pixel circuit includes the first PMOS transistor PM1 serving as the driving transistor, the second and third PMOS transistors PM2 and PM3 serving as the switching transistor, the first and second NMOS transistors NM1 and NM2 and the capacitor Cst. Include.

3, the first electrode of the first PMOS transistor PM1 is connected to a power supply voltage, the second electrode is electrically connected to the organic light emitting device, and the gate is connected to the first node N1. In addition, the first electrode of the second PMOS transistor PM2 is connected to the second electrode of the first PMOS transistor PM1, the second electrode is connected to the anode of the organic light emitting diode OLED, and the gate is of the first emission. It is connected to the Sean scan line E1 [n]. In addition, a first electrode of the third PMOS transistor PM3 is connected to the sustain voltage Vsus, a second electrode is connected to the second node N2, and a gate thereof is the second emission scan line E2 [n]. Is connected to.

The gate of the first NMOS transistor NM1 is connected to the scan line S [n], the first electrode is connected to the third node N3, and the second electrode is connected to the first node N1. The first NMOS transistor NM1 diode-connects the first PMOS transistor PM1 by connecting a gate and a drain of the first transistor PM1 in response to a scan signal applied to the scan line S [n]. do.

The gate of the second NMOS transistor NM2 is connected to the second emission scan line E2 [n], the first electrode is connected to the data line D [m], and the second electrode is connected to the second node N2. Connected. The second NMOS transistor NM2 performs a function of applying a data voltage applied to the data line D [m] to the second node N2 in response to a scan signal applied to the scan line S [n]. .

FIG. 6 is a driving waveform diagram for driving the pixel circuit shown in FIG. 5.

Referring to FIG. 6, it can be seen that the driving waveform of FIG. 4 is the same except that the scan signal S [n] in the T2 section is high instead of low.

The pixel circuit of FIG. 5 having the above configuration can solve the kickback problem of the conventional pixel by dropping the voltage across the capacitor Cst, which can rise by the kickback. No matter how much the voltage across the capacitor Cst drops, the first and second NMOS transistors NM1 and NM2 do not fall below the turn-off voltage (-6V) of the first and second NMOS transistors NM1 and NM2. ) Can't turn off and stays on.

7 is a block diagram illustrating an organic light emitting display device including a pixel circuit according to an exemplary embodiment of the present invention.

Referring to FIG. 7, the OLED display includes an image display unit 71, a scan driver 72, an emission driver 73, and a data driver 74.

The image display portion 71 includes N ㅧ M pixels 75, N scan lines S [1]… S [N] formed in the row direction, and N first emission scan lines E1 [1]… E1 [N ]) And N second emission scanning lines E2 [1]… E2 [N], M data lines D [1]… D [M] formed in the column direction, and M power lines V [1]. ]… V [M]).

Each pixel 75 includes an organic light emitting element and a pixel circuit. Scan lines S [1]… S [N], first emission scan lines E1 [1]… E1 [N], and second emission scan lines E2 [1]… E2 [N] are pixels 75 ), A selection signal, a first emission signal, and a second emission signal are respectively transmitted. In addition, the data lines D [1]… D [M] and the power supply lines V [1]… V [M] transfer data signals and power supply voltages to the pixels 75, respectively.

The data driver 74 applies a data signal to the data lines D [1]… D [M]. The data signal may be output from a voltage source or a current source in the data driver.

The scan driver 72 applies a selection signal to the scan lines S [1]… S [N]. The selection signal is sequentially applied to the scan lines, and the data signal is applied to the pixel circuit in accordance with the selection signals.

The emission driver 73 applies emission signals to the first emission scan lines E1 [1] ... E1 [N] and the second emission scan lines E2 [1] ... E2 [N]. The driving current is applied to the organic light emitting element according to the voltage stored in the reservoir (capacitor) in the pixel circuit by the emission signals, and the organic light emitting element emits light.

The scan driver 72, the emission driver 73, and / or the data driver 74 described above may be electrically connected to an image display 71 such as a display panel through wire bonding or the like, and may also be connected to the image display 71. The tape may be mounted in the form of a chip such as a tape carrier package (TCP) that is attached to and electrically connected to the tape carrier package. In addition, the scan driver 72, the emission driver 73, and / or the data driver 74 may be bonded to the image display unit 71 and electrically connected to a flexible printed circuit board (FPC) or film. It can be mounted in the form, such a structure is commonly referred to as a chip on film (COF) method. In addition, the scan driver 72, the emission driver 73, and / or the data driver 74 may be directly mounted on the glass substrate of the image display unit 71, and the same layers as the scan line, data line, and TFT may be disposed on the glass substrate. It may be mounted in the driving circuit formed by.

As described above, the pixel circuit and the organic light emitting display device including the same according to the present invention can compensate for the variation in the threshold voltage of the driving transistor included in the pixel circuit to express uniform luminance as well as kick-back. It is possible to realize high resolution by minimizing the change in driving voltage caused by the back phenomenon.

Although the present invention has been described with reference to the embodiments shown in the drawings, this is merely exemplary, and it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

Claims (9)

A first PMOS transistor having a first electrode to which a power supply voltage is applied, a second electrode electrically connected to the organic light emitting element, and a gate; A first NMOS transistor having a gate connected between the second electrode and the gate of the first PMOS transistor to diode-connect the first PMOS transistor; A second NMOS transistor having a first electrode to which a data signal is applied, a second electrode electrically connected to the gate electrode of the first PMOS transistor, and a gate to which a scan signal is applied; A capacitor having a first terminal connected to the gate terminal of the first PMOS transistor and a second terminal connected to the second electrode of the second NMOS transistor; And Organic comprising a second PMOS transistor having a first electrode connected to a second electrode of the first PMOS transistor, a second electrode connected to an anode of the organic light emitting element, and a gate to which a first emission control signal is applied. Pixel circuit of light emitting display device. delete The method of claim 1, And a third PMOS transistor having a first electrode to which a sustain voltage is applied, a second electrode connected to the second terminal of the capacitor, and a gate to which a second light emission control signal is applied. Pixel circuit of display device. The method of claim 3, wherein And the second and third PMOS transistors are turned off while the first and second NMOS transistors are turned on. A plurality of pixels each having a pixel circuit of claim 1 and an organic light emitting element controlled by the pixel circuit; A plurality of data lines transferring the data signals to the plurality of pixels; A plurality of scan lines configured to transfer the scan signals to the plurality of pixels; And And a plurality of emission scan lines to transmit the first emission control signal to the plurality of pixels. The method of claim 5, And a data driver connected to the plurality of data lines and configured to supply the data signals. The method of claim 5, And a scan driver connected to the plurality of scan lines and configured to supply the scan signals. The method of claim 5, And an emission driver connected to the plurality of emission scan lines and configured to supply the emission control signal. The method of claim 1, And a scan signal is applied to the gate of the first NMOS transistor and the gate of the second NMOS transistor in common.

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