KR101549900B1 - Pixel circuit of organic light emitting display - Google Patents

Abstract

The present invention relates to an organic light emitting display pixel circuit, and more particularly, to an organic light emitting display pixel circuit using a scan line of a basic pixel circuit as a power source voltage (V _DD ) line and removing a V _DD line to increase a light emitting area, The present invention relates to a circuit for realizing a uniform OLED luminance by providing a compensation circuit for a threshold voltage change of a driving thin film transistor, which is a problem of the OLED display.
According to the present invention, the V _DD The circuit is simplified and the conventional manufacturing method can be used as it is, and the threshold voltage change of the driving thin film transistor, which has a higher luminance as the emission region of the organic light emitting diode increases and affects the luminance unevenness of the organic light emitting diode So that the uniform luminance of the OLED can be expressed. In addition, it is possible to provide a pixel circuit of an organic light emitting display device and a driving method thereof that can freely control a power supply voltage and a reference voltage range without depending on a data voltage in setting a power supply voltage and a reference voltage.

Description

PIXEL CIRCUIT OF ORGANIC LIGHT EMITTING DISPLAY [0002]

The present invention relates to an organic light emitting display pixel circuit, and more particularly, to an organic light emitting display pixel circuit using a scan line of a basic pixel circuit as a power source voltage (V _DD ) line and removing a V _DD line to increase a light emitting area, The present invention relates to a circuit for realizing a uniform OLED luminance by providing a compensation circuit for a threshold voltage change of a driving thin film transistor, which is a problem of the OLED display.

2. Description of the Related Art In recent years, interest in flat panel displays (FPDs) has increased, and various types of flat panel display devices have been actively developed. Representative flat panel display devices include a liquid crystal display (LCD), a plasma display panel (PDP), and an organic electroluminescence display device.

Since the organic light emitting display of the flat panel display device does not need a backlight as a self-luminous element, it can be made thinner and lighter than an LCD, can be manufactured more cheaply, and its response speed is as fast as several tens [ns] , Has wide viewing angle and high contrast ratio, and is receiving attention as a next generation display.

The passive matrix driving method and the active matrix driving method may be used for driving the organic light emitting display device. The active matrix driving method refers to a driving method in which, for example, a voltage for displaying a predetermined gradation is stored in a capacitor, and a stored voltage is applied to the pixel during an entire frame time.

In addition, an organic light emitting display uses an organic light emitting diode, which is a current driving device, to emit light by adjusting the luminance according to the amount of driving current, unlike an LCD using a voltage driven liquid crystal. Therefore, the organic light emitting display requires a pixel circuit for generating a driving current.

An active matrix organic light emitting diode (AMOLED) includes a thin-film transistor (TFT) that controls switching between pixels in order to individually control pixels, and an oxide thin film transistor Has been attracting attention as a next-generation display backplane device due to factors that overcome the limitations of amorphous silicon (a-Si) thin film transistor and low-temperature polycrystalline silicon (LTPS) thin film transistor. Oxide TFTs have high mobility (10 ~ cm ² / Vs), are free from low-temperature process and unevenness of threshold voltage, and can utilize existing a-Si TFT-based process line as they are. You can contribute. Despite these advantages, oxide thin film transistors have unstable characteristics such as threshold voltage (V _TH ) nonuniformity due to gate voltage bias stress.

This is because the threshold voltage of the thin film transistor is changed in the pixel circuit so that different current flows even if the same data voltage is applied to each pixel. If the characteristics of the driving thin film transistor are different between neighboring pixels in the display, the amount of current flowing in the OLED differs according to the position, and different brightness is displayed. A basic AMOLED pixel circuit includes two thin film transistors, one capacitor, two signal lines and one power supply voltage (V _DD ). If a circuit is appropriately designed by adding a thin film transistor, a capacitor and a signal line to a basic AMOLED pixel circuit, if the threshold voltage change of the driving thin film transistor, which is an element affecting the luminance unevenness of the above-mentioned OLED, is compensated, Brightness can be realized. Also, by reducing the number of signal lines, thin film transistors, and capacitors, the emission area of the OLED can be increased.

KR 10-2006-0089818 A

SUMMARY OF THE INVENTION The present invention has been made to overcome the above problems, and it is an object of the present invention to reduce the number of signal lines in a basic active matrix organic light emitting diode (AMOLED) pixel circuit, And an object of the present invention is to provide a pixel compensation circuit for an AMOLED capable of expressing a uniform luminance of an OLED by compensating a change in a threshold voltage of the driving thin film transistor.

Another object of the present invention is to provide a pixel circuit of an organic light emitting display device and a driving method thereof that can freely control a power supply voltage and a reference voltage range without depending on a data voltage in setting a power supply voltage and a reference voltage.

According to an aspect of the present invention, there is provided a pixel circuit comprising: an organic light emitting diode; A driving transistor (T5) having a gate connected to the B node (B) and controlling a current flowing in the organic light emitting diode; A capacitor connected between the B node (B) and the A node (A); A first transistor (T1) for applying a data voltage (DATA) to the A node (A) in response to a reference scan signal (SCAN1); A second transistor (T2) for applying the data voltage (DATA) to the A node (A) in response to an nth (n is a natural number) scan signal SCAN [n]; (B) which is connected between the gate and the drain of the driving transistor T5 and which applies the current scan signal SCAN1 to the B node B in response to the (n-1) th scan signal SCAN [n-1] 3 transistor T3; (B) to a threshold voltage (V _{TH -} T 5) level of the driving transistor in response to the n th scan signal (SCAN [n]), A fourth transistor T4; And a sixth transistor T6 for causing a current by the driving transistor T5 to flow in the organic light emitting diode in response to the EM signal EM.
The reference scan signal, the (n-1) th scan signal, and the n th scan signal have the HIGH, HIGH and LOW voltage levels in the first section and the LOW and LOW levels in the second section, HIGH ',' HIGH ', and' HIGH ',' LOW ', and' LOW 'in the third section.
Wherein the reference scan signal turns on the first transistor, the n-1 scan signal turns on the third transistor, and the nth scan signal turns off the second transistor and the fourth transistor, And the EM signal may turn off the sixth transistor.
And the reference scan signal may be charged to the gate voltage of the driving transistor through the third transistor.
The nth scan signal may turn on the second and fourth transistors.
(B) through the fourth transistor and the driving transistor to the threshold voltage in the reference scan signal.
The data voltage charged to the node A may be held by the capacitor.
The data voltage becomes 'HIGH' and can be charged to the A node A through the first transistor.
The reference scan signal may be used as a power supply voltage.
The first to fourth transistors, the driving transistor, and the sixth transistor may be N-type TFTs.

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According to the present invention, the V _DD The circuit is simplified and the existing manufacturing method can be used as it is, and the luminance is increased as the emission area of the organic light emitting diode increases. In addition, there is an effect of expressing the uniform luminance of the OLED by compensating a threshold voltage change of the driving thin film transistor which affects the luminance unevenness of the organic light emitting diode. Further, there is an effect of providing a pixel circuit of an organic light emitting display device and a driving method thereof that can freely control a power supply voltage and a reference voltage range without depending on a data voltage in setting a power supply voltage and a reference voltage.

Figure 1 is a pixel circuit structure including a conventional two-transistor and one capacitor for driving the OLED (11), and, V _DD Lt; RTI ID = 0.0 > 12 < / RTI >
2 is a diagram illustrating an organic light emitting display pixel circuit structure and operation timing diagram according to the present invention for compensating a threshold voltage of a driving transistor while removing a V _DD line to increase a light emitting area.
3 is a view showing an operation principle of each pixel according to an operation timing of an organic light emitting display pixel circuit according to the present invention.
4 is a graph showing a simulation result of a gate voltage according to a threshold voltage change (0.5 ± 0.5 V) of a driving transistor for each operation timing in an organic light emitting display pixel circuit according to the present invention.
5 is a graph showing a simulation result of an OLED current according to a data voltage change (4 to 13 V) and a threshold voltage change (0.5 ± 0.5 V) of a driving transistor in an organic light emitting display pixel circuit according to the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms, and the inventor should appropriately interpret the concepts of the terms appropriately It should be interpreted in accordance with the meaning and concept consistent with the technical idea of the present invention based on the principle that it can be defined. Therefore, the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred embodiments of the present invention and do not represent all the technical ideas of the present invention. Therefore, It is to be understood that equivalents and modifications are possible.

2 is V _DD FIG. 2 is a diagram showing an organic light emitting display pixel circuit structure and operation timing diagram according to the present invention for compensating a threshold voltage of a driving transistor while removing a line to increase a light emitting region.

In the present invention, the V _DD line is removed and the V _DD signal is replaced with the scan signal, thereby increasing the light emitting area. Further, by using four thin film transistors and two scan signals, six thin film transistors, one capacitor, A pixel circuit including a signal can be appropriately designed so that a circuit capable of compensating a threshold voltage of the driving thin film transistor can be manufactured.

Referring to FIG. 2, the pixel circuit includes a thin film transistor (TFT) T5 for supplying current to the organic light emitting diode and the organic light emitting diode, a data voltage applied to the driving thin film transistor, A switching thin film transistor (T6) for controlling the current flow through the organic light emitting diode only during a period in which the organic light emitting diode needs to emit light, and a switching thin film transistor And a capacitor (C1) for holding the voltage for a predetermined period. Here, all thin film transistors are n-type MOSFETs (metal oxide semiconductor field effect transistors). And includes a data line DATA and scan signal lines SCAN1, SCAN [n], EM, and V _DD The power supply voltage is not supplied by a separate external power supply but is supplied through the scan line SCAN1.

The driving transistor T5 is connected between the scan electrode SCAN1 serving as a substitute for the power source voltage V _DD and the OLED, and controls the driving current flowing in the OLED according to the voltage applied to the gate electrode. Specifically, the drain electrode of the driving transistor T5 is connected to the scan electrode SCAN1, and the source electrode thereof is connected to the anode electrode of the OLED through the emission control transistor T6. A source electrode of T3 and a drain electrode of T4 are connected to the gate electrode of the driving transistor T5, and one electrode B of the capacitor C1 is connected to the gate electrode of the driving transistor T5. The other electrode A of the capacitor C1 is connected to the source electrodes of the switching transistors T1 and T2. Hereinafter, the electrode 'B' of both electrodes of the capacitor C1 will be referred to as one electrode and the electrode 'A' as another electrode.

The switching transistor T1 is connected between the other electrode A of the capacitor C1 and the data line DATA electrode. The gate electrode of the switching transistor T1 is connected to the scan signal line SCAN1 and is turned on by a selection signal from the scan signal line SCAN1 to apply the data voltage V _DATA from the data line DATA to the other electrode A of the capacitor C1.

The switching transistor T2 is connected between the other electrode A of the capacitor C1 and the data line DATA electrode. The gate electrode of the switching transistor T2 is connected to the scan signal line SCAN [n] and is turned on by a selection signal from the scan signal line SCAN [n] to apply the data voltage V _DATA from the data line DATA to the other electrode A of the capacitor C1 do.

The switching transistor T3 is connected between the gate electrode and the drain electrode of the driving transistor T5. The gate electrode of the switching transistor T3 is connected to the scan signal line SCAN [n-1] of the scan signal line SCAN [n] and turned on to the select signal from the scan signal line SCAN [n-1].

The switching transistor T4 is connected between the gate electrode and the source electrode of the driving transistor T5. The gate electrode of the switching transistor T4 is connected to the scan signal line SCAN [n] like the gate electrode of the transistor T2 and is turned on to the selection signal from the scan signal line SCAN [n].

The emission control transistor T6 is connected between the source electrode of the driving transistor T5 and the OLED. The gate electrode of the emission control transistor T6 is connected to the emission control line EM and transmits or cuts off the drive current generated in the drive transistor T5 to the OLED by the emission control signal from the emission control line EM.

The cathode electrode of the OLED is connected to the reference voltage V _SS . The reference voltage V _SS is the ground voltage.

Operation of the operation timing interval (precharging interval, V _TH developing interval, and emission interval) of the pixel circuit having the above configuration will be described below.

FIG. 3 is a view illustrating an operation principle of each organic light emitting display pixel circuit according to an operation timing according to the present invention.

3, in the first precharging period (the first period), the scan signal lines SCAN1 and SCAN [n-1] are turned on in order to turn on the switching transistors T1 and T3, do. The voltage at point A is stored as V _{DATA_L} by the capacitor C1 through the switching transistor T1. V _{DATA _L} means V _DATA is 0V. The voltage at the point B is stored in the capacitor C1 through the switching transistor T3 as V _SCAN1 . V _SCAN1 is the programming voltage. The emission control transistor T6 maintains turn-off because the emission control line EM is at a low voltage. The emission control transistor T6 prevents unnecessary OLED current during the precharging period and the V _TH developing period.

The scan signal lines SCAN1 and SCAN [n-1] are at a low voltage to turn off the switching transistors T1 and T3 in the second V _TH developing period (second section). The voltage of the point A is maintained at V _{DATA _L} while maintaining the low voltage V _DATA. At this time, the scan signal line SCAN [n] becomes a high voltage to turn on the switching transistors T2 and T4. Are discharged to V _{TH _ T5} in _SCAN1 V until the off-voltage of the point B, the switching transistor T5 turns through the switching transistors T4 and T5 by the low voltage of the scan signal line SCAN1. V _{TH _ T5} is a threshold voltage of the switching transistor T5.

Finally, in the emission period (third section), the scan signal line SCAN [n-1] and the scan signal line SCAN [n] become low voltage and the switching transistors T2, T3 and T4 are turned off. The scan signal line SCAN1 becomes a high voltage to turn on the switching transistor T1, and plays the role of a power supply voltage instead of V _DD . Then, the DATA voltage becomes a high voltage, and the voltage at point A becomes V _{DATA_L} to V _{DATA_H} by the coupling of C1. V _{DATA_H} varies with the OLED current required. The emission control line EM becomes a high voltage and the emission control transistor T6 turns on. Therefore, the source voltage of the driving transistor T5 becomes V _OLED and the gate voltage of the driving transistor T5 becomes V _{TH_T5} + V _{DATA_H} . Therefore, the drain current of T5 is as follows.

I _OLED = 1/2 · k · ( V GS _ T5 -V TH _ T5) 2

= 1/2 · k · [ V TH _ T5 + V DATA _H -V OLED -V TH _ T5] 2

= 1/2 · k · (V _DATA -V _OLED ) ² (Saturation region)

k is _{μ · C OX · W / L} , μ is the electric field mobility, C _OX is the capacitance of the gate insulating film, W / L is the width and length of the thin film transistor, V _OLED is the voltage across the OLED anode.

Therefore, the OLED current is independent of the threshold voltage of the driving thin film transistor and is affected only by the data voltage and the OLED voltage. As a result, the present pixel circuit can effectively compensate the threshold voltage change.

FIG. 4 is a graph showing a simulation result of a gate voltage according to a threshold voltage change (0.5 ± 0.5 V) of a driving transistor for each operation timing in an organic light emitting display pixel circuit according to the present invention. 5 is a graph showing a simulation result of an OLED current according to a data voltage change (4 to 13 V) and a threshold voltage change (0.5 ± 0.5 V) of a driving transistor in a display pixel circuit. From these two simulation results, it can be seen that the luminance change due to the threshold voltage change of the driving thin film transistor is compensated.

As a result, the pixel circuit of the present invention can implement a uniform brightness by effectively compensating for the threshold voltage change, and the conventional V _DD The emission area can be increased by removing the line and using the SCAN1 line as the supply voltage line.

100: organic light emitting display pixel circuit according to the present invention
11: Pixel circuit structure including two conventional transistors and one capacitor for driving an OLED
12: V _DD Conventional pixel circuit structure in which lines are removed
20: Operation timing diagram of the organic light emitting display pixel circuit according to the present invention
31: precharging section (first section)
32: Vth detecting section (second section)
33: emission section (third section)

Claims (14)

Organic light emitting diodes;
A driving transistor (T5) having a gate connected to the B node (B) and controlling a current flowing in the organic light emitting diode;
A capacitor connected between the B node (B) and the A node (A);
A first transistor (T1) for applying a data voltage (DATA) to the A node (A) in response to a reference scan signal (SCAN1);
A second transistor (T2) for applying the data voltage (DATA) to the A node (A) in response to an nth (n is a natural number) scan signal SCAN [n];
(B) which is connected between the gate and the drain of the driving transistor T5 and which applies the current scan signal SCAN1 to the B node B in response to the (n-1) th scan signal SCAN [n-1] 3 transistor T3;
(B) to a threshold voltage (V _{TH -} T 5) level of the driving transistor in response to the n th scan signal (SCAN [n]), A fourth transistor T4; And
A sixth transistor (T6) for causing a current by the driving transistor (T5) to flow in the organic light emitting diode in response to an EM signal (EM)
And an organic light emitting display pixel circuit. The method according to claim 1,
The reference scan signal, the (n-1) th scan signal, and the (n)
HIGH ',' HIGH 'and' LOW 'in the first section,
LOW ',' LOW 'and' HIGH 'in the second section,
And changes to 'HIGH', 'LOW', and 'LOW' in the third period. The method of claim 2,
Wherein the reference scan signal turns on the first transistor, the n-1 scan signal turns on the third transistor, and the nth scan signal turns off the second transistor and the fourth transistor, And the EM signal turns off the sixth transistor
Wherein the organic light emitting display pixel circuit comprises:
delete The method of claim 2,
And the reference scan signal is charged to the gate voltage of the driving transistor through the third transistor
Wherein the organic light emitting display pixel circuit comprises:
delete The method of claim 2,
And the n-th scan signal is a signal that turns on the second and fourth transistors
Wherein the organic light emitting display pixel circuit comprises: The method of claim 2,
(B) through the fourth transistor and the driving transistor to the threshold voltage in the reference scan signal
Wherein the organic light emitting display pixel circuit comprises: The method of claim 2,
The data voltage charged in the A node (A) is maintained by the capacitor
Wherein the organic light emitting display pixel circuit comprises:
delete The method of claim 9,
The data voltage becomes 'HIGH' and the A node A is charged through the first transistor
Wherein the organic light emitting display pixel circuit comprises:
delete The method according to claim 1,
Wherein the reference scan signal is used as a power supply voltage. The method according to claim 1,
Wherein the first to fourth transistors, the driving transistor, and the sixth transistor are N-type TFTs.

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