KR100741979B1 - Pixel Circuit of Organic Electroluminescence Display Device - Google Patents

Abstract

Disclosed is a pixel circuit of an organic electroluminescent display device in which an emission control line is removed to improve an aperture ratio. The pixel circuit of the present invention diode-connects the light emission control transistor, removes the light emission control line, and changes the level state of the reference voltage connected to the cathode electrode of the organic EL element. That is, by changing the reference voltage to the high level while the low level previous scan signal and the current scan signal are supplied, the diode is turned off while the threshold voltage compensation and the data voltage are applied to block the driving current. Therefore, the pixel circuit of the present invention can improve the aperture ratio by eliminating the threshold voltage as well as eliminating the light emission control line.

Description

Pixel Circuit of Organic Electroluminescence Display Device

1 is a circuit diagram illustrating a pixel of a conventional organic electroluminescent display.

FIG. 2 is a timing diagram for driving the pixel illustrated in FIG. 1.

3 is a circuit diagram illustrating a pixel circuit of an organic light emitting display device according to a first embodiment of the present invention.

4 is a circuit diagram illustrating a pixel circuit of an organic light emitting display device according to a second embodiment of the present invention.

5 is a timing diagram illustrating an operation of a pixel according to the first and second embodiments of the present invention illustrated in FIGS. 3 and 4.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electroluminescent display, and more particularly, to a pixel circuit of an organic electroluminescent display, in which an emission control line is removed to improve an aperture ratio.

Recently, as interest in flat panel displays (FPDs) increases, development of various types of flat panel displays has been actively conducted. Typical flat panel display devices include liquid crystal displays (LCDs), plasma display panels (PDPs), organic electroluminescence displays (Organic Electroluminescence Display Devices), and the like.

Among the flat panel display devices, the organic light emitting display device is a self-luminous device, which has no backlight such as an LCD, is thinner, has a response speed of several tens of [ns], has a wide viewing angle, and has a high contrast ratio.

The above-described method of driving the organic light emitting display device includes a passive matrix driving method and an active matrix driving method. The passive matrix driving scheme is, for example, a driving scheme in which a pixel connected to a scan line of a specific row receives a current only for a selected time and has a corresponding luminance. The active matrix driving scheme refers to a driving scheme for storing a voltage for displaying a predetermined gray scale in a capacitor and applying the stored voltage to the pixel for the entire frame time, for example. The active matrix driving method is divided into a voltage programming method and a current programming method according to the type of a signal applied to store a voltage in a capacitor.

In addition, an organic electroluminescent display device uses an organic light emitting diode (OLED), which is a current driving element, unlike an LCD using a voltage driven liquid crystal. The luminance is adjusted according to the amount of light emitted. Therefore, the organic electroluminescent display needs a pixel circuit for generating a driving current.

1 is a circuit diagram illustrating a pixel of a conventional organic electroluminescent display.

Referring to FIG. 1, a conventional pixel includes a pixel driver 10, a light emission control transistor M5, and an organic EL, in which four transistors M1, M2, M3, and M4 and two capacitors Cst and Cvth are electrically connected. It has a device OLED.

The pixel driver 10 is connected to the data line DATA [m], the previous scan line SCAN [n-1], the current scan line SCAN [n], and the power supply voltage line ELVDD, and generates a driving current to be supplied to the organic EL device OLED. do. The pixel driver 10 will be described later.

The organic EL device OLED is connected between the pixel driver 10 and the reference voltage line ELVSS, and receives the driving current supplied from the pixel driver 10 to emit light with a corresponding luminance.

A light emission control transistor M5 is connected between the pixel driver 10 and the organic EL element OLED, and is turned on / off in accordance with a light emission control signal from a light emission control line EMI [n] connected to a gate terminal. The driving current output from (10) is supplied or cut off to the organic EL element OLED.

Although the pixel driver 10 is basically composed of two transistors and one capacitor, the pixel driver 10 forms a compensation circuit to solve the variation in the threshold voltage (Vth) of the driving transistor and removes the variation in the threshold voltage. Shall be.

That is, the pixel driver 10 illustrated in FIG. 1 has four transistors M1, M2, M3, and M4 and two capacitors Cst and Cvth as described above.

The driving transistor M1 is connected between the power supply voltage line ELVDD and the light emission control transistor M5 and outputs a driving current corresponding to the voltage difference between the voltage applied to the gate terminal connected to the node N2 and the power supply voltage ELVDD. .

The threshold voltage compensation transistor M2 is connected between the gate and the drain of the driving transistor M1 and diode-connects the driving transistor M1 in response to a scanning signal from the previous scan line SCAN [n-1]. . Therefore, the threshold voltage of the driving transistor M1 is stored in the second capacitor Cvth.

The switching transistor M3 is connected between the data line DATA [m] and the node N1 and transfers the data signal to the node N1 in response to the scan signal from the current scan line SCAN [n].

A power supply voltage applying transistor M4 is connected between the power supply voltage line ELVDD and the node N1, and applies a power supply voltage ELVDD to the node N1 in response to a scan signal from the previous scan line SCAN [n-1].

The first capacitor Cst is connected between the power supply voltage line ELVDD and the node N1 and stores a data voltage transferred from the switching transistor M3.

The second capacitor Cvth is connected between the first capacitor Cst and the gate terminal of the driving transistor M1 and stores the threshold voltage Vth of the driving transistor M1 for a predetermined time.

In FIG. 1, all of the transistors M1-M5 use P-type metal oxide semiconductor (PMOS) transistors, but it is apparent that the pixel circuit can be designed as an NMOS transistor at the level of those skilled in the art.

FIG. 2 is a timing diagram for driving the pixel illustrated in FIG. 1.

1 and 2, first, if the previous scan signal SCAN [n-1] is at a low level, and the current scan signal SCAN [n] and the emission control signal EMI [n] are at a high level, the pixel of FIG. The threshold voltage compensating transistor M2 and the power supply voltage applying transistor M4 are turned on. Accordingly, the power supply voltage ELVDD is applied to the node N1, and the driving transistor M1 is diode-connected to the node N2 so that the voltage ELVDD- | Vth | corresponding to the difference between the power supply voltage ELVDD and the Munter voltage Vth is applied. As a result, the voltage corresponding to the threshold voltage Vth of the driving transistor M1 is stored in the second capacitor Cvth.

Next, if the previous scan signal SCAN [n-1] is high level, the current scan signal SCAN [n] is low level, and the emission control signal EMI [n] is high level, the switching transistor ( M3 is turned on, and the threshold voltage compensation transistor M2 and the power supply voltage applying transistor M4 are turned off. Therefore, the data signal DATA [m] is applied to the node N1. At this time, the voltage of the node N2 changes to ELVDD- | Vth | -ΔV because of the characteristics of the second capacitor Cvth to maintain the threshold voltage Vth. Here, the variable voltage ΔV is ELVDD-DATA [m]. Therefore, the voltage of the node N2 becomes ELVDD- | Vth |-(ELVDD-DATA [m]).

Finally, when the previous scan signal SCAN [n-1] and the current scan signal SCAN [n] are high level and the light emission control signal EMI [n] is low level, the light emission control transistor M5 is turned on, and The driving current I _OLED generated by the driving transistor M1 is supplied to the organic EL element OLED. At this time, the driving current (I _OLED ) applied to the organic EL device OLED is defined by Equation 1 below.

I _OLED = K (Vsg-| Vth |) ²

= K [ELVDD- {ELVDD- | Vth |-(ELVDD-DATA [m])}-| Vth |] ²

= K (ELVDD-DATA [m]) ²

As shown in Equation 1, the driving current compensated for the threshold voltage is supplied to solve the luminance non-uniformity caused by the threshold voltage deviation.

However, the conventional pixel circuit as shown in FIG. 1 requires three different scanning lines SCAN [n-1], SCAN [n], and EMI [n], resulting in complicated pixel driving circuits and an organic light emitting display device. There was a problem that the aperture ratio of was lowered.

An object of the present invention is to provide a pixel circuit of an organic light emitting display device and a driving method thereof capable of compensating for variation in threshold voltage Vth of a driving transistor included in a pixel circuit and increasing an aperture ratio.

The pixel circuit of the organic electroluminescent display device of the present invention for achieving the above object is an organic EL element connected to the cathode electrode to the reference voltage line, and emits light at a predetermined brightness according to the driving current supplied to the anode electrode; A pixel driver connected to a data line, a previous scan line, a current scan line, and a power supply voltage line and applying the driving current corresponding to the data voltage applied from the data line to the organic EL element; And a diode connected between the pixel driver and the organic EL element to allow the driving current to flow or block in response to a change in the level state of the reference voltage.

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

Example 1

3 is a circuit diagram illustrating a pixel circuit of an organic light emitting display device according to a first embodiment of the present invention.

Referring to FIG. 3, in the pixel circuit according to the first exemplary embodiment of the present invention, four transistors M1, M2, M3, and M4 and two capacitors Cst and Cvth may be electrically connected to each other. It has a connected pixel driver 10 and an organic EL element OLED.

The pixel driver 10 is connected to the data line DATA [m], the previous scan line SCAN [n-1], the current scan line SCAN [n], and the power supply voltage line ELVDD, and generates a driving current to be supplied to the organic EL device OLED. do. The pixel driver 10 will be described later.

The organic EL device OLED is connected between the pixel driver 10 and the reference voltage line ELVSS, and receives the driving current supplied from the pixel driver 10 to emit light with a corresponding luminance.

Further, according to the first embodiment of the present invention, a difference from the conventional pixel circuit of Fig. 1 is that instead of the light emission control transistor M5, a diode M5 'is formed between the pixel driver 10 and the organic EL element OLED. It is. In FIG. 3, the diode M5 'is designed by connecting a gate terminal and a drain terminal using a PMOS transistor. That is, the emission control line EMI [n] can be eliminated in the pixel.

Instead of eliminating the emission control line EMI [n], the level of the reference voltage ELVSS is maintained at a high level while the previous scan signal SCAN [n-1] and the current scan signal SCAN [n] are low level. The reference voltage ELVSS is generally a voltage lower than the power supply voltage ELVDD and is a negative voltage or a ground voltage. However, the same voltage as the power supply voltage ELVDD is applied to the high level reference voltage ELVSS. This will be described in detail with a timing chart to be described later.

Although the pixel driver 10 is basically composed of two transistors and one capacitor, the pixel driver 10 forms a compensation circuit to solve the variation in the threshold voltage (Vth) of the driving transistor and removes the variation in the threshold voltage. Shall be.

That is, the pixel driver 10 illustrated in FIG. 3 has four transistors M1, M2, M3, and M4 and two capacitors Cst and Cvth as described above.

The driving transistor M1 is connected between the power supply voltage line ELVDD and the light emission control transistor M5 and receives a driving current corresponding to the voltage difference between the voltage applied to the gate terminal connected to the node N2 and the power supply voltage ELVDD. Output

The threshold voltage compensation transistor M2 is connected between the gate and the drain of the driving transistor M1 and diode-connects the driving transistor M1 in response to a scanning signal from the previous scan line SCAN [n-1]. . Therefore, the threshold voltage of the driving transistor M1 is stored in the second capacitor Cvth.

The switching transistor M3 is connected between the data line DATA [m] and the node N1 and transmits a data signal to the node N1 in response to the scan signal of the current scan line SCAN [n].

A power supply voltage applying transistor M4 is connected between the power supply voltage line ELVDD and the node N1, and applies a power supply voltage ELVDD to the node N1 in response to a scan signal from the previous scan line SCAN [n-1].

The first capacitor Cst is connected between the power supply voltage line ELVDD and the node N1 and stores a data voltage transferred from the switching transistor M3.

The second capacitor Cvth is connected between the first capacitor Cst and the gate terminal of the driving transistor M1 and stores the threshold voltage Vth of the driving transistor M1 for a predetermined time.

In FIG. 3, all of the transistors M1-M5 use P-type metal oxide semiconductor (PMOS) transistors, but it is apparent that the pixel circuit can be designed as an NMOS transistor at the level of those skilled in the art.

Therefore, according to the first embodiment, the aperture ratio of the pixel can be improved by removing the emission control line EMI [n] from the conventional pixel. That is, in the case of the bottom emission type organic electroluminescent display device, the aperture ratio can be increased. In the case of the top emission type organic electroluminescent display device, the size of the lower capacitor can be increased by increasing the lower aperture ratio. It is possible.

Example 2

4 is a circuit diagram illustrating a pixel circuit of an organic light emitting display device according to a second embodiment of the present invention.

Referring to FIG. 4, in the second embodiment of the present invention, the pixel driver 10 and the organic EL element OLED are the same as the first embodiment described with reference to FIG. 3. Therefore, the description of the pixel driver 10 and the organic EL element OLED will be omitted instead of the description of FIG. 3.

However, unlike the second embodiment of FIG. 3, the second embodiment of the present invention uses an NMOS transistor in which a diode M5 ″ formed between the pixel driver 10 and the organic EL element OLED connects a gate terminal and a drain terminal. It is used.

Therefore, the second embodiment of the present invention can also improve the aperture ratio of the pixel by removing the emission control line EMI [n] from the conventional pixel.

Hereinafter, the operation of the pixel according to the first and second embodiments of the present invention will be described.

FIG. 5 is a timing diagram for describing an operation of a pixel according to the first and second embodiments of the present invention illustrated in FIGS. 3 and 4.

3 to 5, the previous scan signal SCAN [n-1] and the current scan signal SCAN [n] are sequentially converted to the low level. According to the first and second embodiments of the present invention, the reference voltage ELVSS [n] is maintained at a high level while the previous scan signal SCAN [n-1] and the current scan signal SCAN [n] are low level.

First, when the previous scan signal SCAN [n-1] is low level and the current scan signal SCAN [n] and the reference voltage ELVSS [n] are high level, the threshold voltage compensation transistor M2 in the pixels of FIGS. And the power supply voltage applying transistor M4 are turned on. Accordingly, the power supply voltage ELVDD is applied to the node N1, and the driving transistor M1 is diode-connected to the node N2 so that the voltage ELVDD- | Vth | corresponding to the difference between the power supply voltage ELVDD and the Munter voltage Vth is applied. As a result, the voltage corresponding to the threshold voltage Vth of the driving transistor M1 is stored in the second capacitor Cvth. However, since the reference voltage ELVSS [n] is high level, the diode-connected transistor M5 'or M5 "is reverse biased and does not allow current to pass through.

Next, when the previous scan signal SCAN [n-1] is high level, the current scan signal SCAN [n] is low level, and the reference voltage ELVSS [n] is high level, the switching transistor M3 is turned on, The threshold voltage compensating transistor M2 and the power supply voltage applying transistor M4 are turned off. Therefore, the data signal DATA [m] is applied to the node N1. At this time, the voltage of the node N2 changes to ELVDD- | Vth | -ΔV because of the characteristics of the second capacitor Cvth to maintain the threshold voltage Vth. Here, the variable voltage ΔV is ELVDD-DATA [m]. Therefore, the voltage of the node N2 becomes ELVDD- | Vth |-(ELVDD-DATA [m]). Even in this section, since the reference voltage ELVSS [n] is high level, the diode-connected transistor M5 'or M5 "is reverse biased to prevent current from passing through.

Finally, when the previous scan signal SCAN [n-1] and the current scan signal SCAN [n] are high level and the reference voltage ELVSS [n] is low level, the diode-connected transistor M5 'or M5 "is turned on. Then, the driving current I _OLED generated by the driving transistor M1 is supplied to the organic EL element OLED, wherein the value of the driving current I _OLED is the same as that of the conventional pixel circuit described above. It is defined as 1] and the driving current compensated for the threshold voltage is supplied to solve the luminance unevenness according to the threshold voltage deviation.

Further, in the first and second embodiments of the present invention, since the emission control line EMI [n] is removed, the aperture ratio of the pixel can be improved. That is, in the case of the bottom emission type organic electroluminescent display device, the aperture ratio can be increased. In the case of the top emission type organic electroluminescent display device, the size of the lower capacitor can be increased by increasing the lower aperture ratio. It is possible.

In the first and second embodiments of the present invention, a diode is designed by connecting a gate and a drain terminal of a transistor (NMOS or PMOS), but a simple diode may be used.

Although the above has been described with reference to the preferred embodiment of the present invention, those skilled in the art will be variously modified and modified within the scope of the present invention without departing from the spirit and scope of the present invention described in the claims below. It will be appreciated that it can be changed.

Since the driving current flowing through the organic EL element is not affected by the threshold voltage of the driving transistor, the present invention not only compensates for the variation of the threshold voltage of the driving transistor existing between the pixel circuits, but also eliminates the emission control line to reduce the aperture ratio. Can improve.

As a result, in the case of the bottom emission type organic electroluminescent display device, the aperture ratio can be increased, and in the case of the top emission type organic electroluminescent display device, the size of the lower capacitor can be increased by increasing the bottom aperture ratio. It is possible.

Claims (9)

An organic EL device having a cathode electrode connected to the reference voltage line and emitting light at a predetermined luminance according to a driving current supplied to the anode electrode; A pixel driver connected to a data line, a previous scan line, a current scan line, and a power supply voltage line and applying the driving current corresponding to the data voltage applied from the data line to the organic EL element; And And a diode connected between the pixel driver and the organic EL element to allow the driving current to flow or block in response to a change in the level state of the reference voltage. The method of claim 1, And the diode is a diode-connected PMOS transistor. The method of claim 1, And the diode is a diode-connected NMOS transistor. The method of claim 1, The pixel driver, A driving transistor connected between the power supply voltage line and the anode electrode of the diode and outputting the driving current according to a voltage applied to a gate terminal; A threshold voltage compensation transistor connected between the gate terminal and the drain terminal of the driving transistor and diode-connected the driving transistor in response to a scan signal of the previous scan line connected to the gate terminal; A switching transistor connected to the data line and a node and transferring a data voltage in response to a scan signal of the current scan line connected to a gate terminal; A power supply voltage applying transistor connected between the power supply voltage line and the node and transferring the power supply voltage to the node in response to a scan signal of the previous scan line connected to a gate terminal; A first capacitor connected between the power supply voltage line and the node and configured to store the data voltage; And And a second capacitor connected between the first capacitor and the gate terminal of the driving transistor, the second capacitor storing a threshold voltage of the driving transistor. The method of claim 4, wherein And the transistors of the pixel driver are PMOS transistors. The method of claim 5, And the reference voltage applies a high level voltage while a low level scan signal is sequentially applied from the previous and current scan lines. The method of claim 6, And the diode is turned off to block the driving current while the previous low level previous scan signal is supplied to the pixel driver. The method of claim 7, wherein And the diode is turned off to block the driving current while the low level current scan signal is supplied to the pixel driver. The method of claim 8, And the diode is turned on to supply the driving current to the organic EL element when the previous and current scan signals reach a high level.

Images