KR20080034663A - Organic light emitting diode display and driving method thereof - Google Patents

Abstract

An OLED display device and a driving method thereof are provided to enhance the reliability of image quality by preventing the variation of driving currents of an OLED element due to the variation of characteristics of a driving TFT(Thin Film Transistor). An OLED(Organic Light Emitting Diode) display device includes data and gate lines, a high level driving voltage source(VDD), a source-follower circuit(122-1), an OLED element(OLED), an offset capacitor(Cos), and a switch circuit(122-2). The data lines receive data voltages. The gate lines crossing with the data lines receive scan pulses. The high level driving voltage source generates high level driving voltages. The source-follower circuit varies the voltage of a second node according to the voltage of a first node. The OLED element is connected between a second node and a base-voltage source and is turned on by the voltage of the second node. The offset capacitor is connected between first and third nodes. The switch circuit supplies the data voltages to the first node during a first period, stores an offset voltage in the offset capacitor through a short circuit between the second and third nodes, supplies the data voltages to the third node during a second period, increases the voltage level of the first node, equalizes the voltage of the second node and the data voltages, and maintains the voltage of the second node as the data voltages during a third period.

Description

Organic Light Emitting Diode Display And Driving Method Thereof}

1 is a diagram illustrating a light emission principle of a conventional organic light emitting diode display.

2 is a block diagram schematically illustrating a conventional organic light emitting diode display.

3 is a circuit diagram illustrating in detail a pixel illustrated in FIG. 2;

4 is a diagram illustrating an example in which a threshold voltage of a driving TFT increases due to positive gate-bias stress;

FIG. 5 is a diagram illustrating a current decrease of an organic light emitting diode device according to an increase in a threshold voltage of a driving TFT.

6 is a block diagram illustrating an organic light emitting diode display device according to an exemplary embodiment of the present invention.

7 is a timing diagram of a driving signal supplied to the pixels of FIG. 6.

8 is a circuit diagram illustrating any one of the pixels of FIG. 6.

9 is an equivalent circuit diagram of a pixel for the offset voltage storage period A of FIG.

FIG. 10 is an equivalent circuit diagram of a pixel for the compensation period B of FIG. 7; FIG.

FIG. 11 is an equivalent circuit diagram of pixels for the light emission period C of FIG.

FIG. 12 is a diagram illustrating a relationship between an input voltage and an output voltage of the source-follower circuit shown in FIG. 8. FIG.

FIG. 13 is a simulation result diagram showing an amount of change in driving current according to a characteristic change of a driving TFT DT when expressing the same gray scale. FIG.

<Description of Symbols for Main Parts of Drawings>

116: display panel 118: gate driving circuit

120: data driving circuit 122: pixels

122-1: source-follower circuit 122-2: switch circuit

124: Timing Controller S1 [k]: First Scan Pulse

S2 [k]: second scan pulse ST1, ST2, ST3: first, second and third switch TFTs

Vd: Data voltage DT: Driving TFT

Cos: Offset Capacitor Cst: Storage Capacitor

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic light emitting diode display and a driving method thereof, and more particularly, to an organic light emitting diode display and a method of driving the same, which can improve display quality by minimizing variation in driving current.

Recently, various flat panel displays have been developed to reduce weight and volume, which are disadvantages of cathode ray tubes. Such flat panel displays include liquid crystal displays (hereinafter referred to as "LCDs"), field emission displays (FEDs), plasma display panels (hereinafter referred to as "PDPs"). And organic light emitting diode displays.

Among them, PDP is attracting attention as a display device which is light and small and is most advantageous for large screen because of its simple structure and manufacturing process. However, PDP has low luminous efficiency, low luminance and high power consumption. In addition, an active matrix LCD having a thin film transistor (hereinafter referred to as “TFT”) as a switching device has a disadvantage in that large screens are difficult to use due to the semiconductor process, and power consumption is large due to the backlight unit.

In contrast, organic light emitting diode display devices are classified into inorganic light emitting diode display devices and organic light emitting diode display devices according to the material of the light emitting layer. The organic light emitting diode display devices are self-luminous devices that emit light and have high response speed, high luminous efficiency, high luminance, and a wide viewing angle. . In comparison with the organic light emitting diode display, the inorganic light emitting diode display has higher power consumption and cannot obtain high brightness, and cannot emit various colors of R (Red), G (Green), and B (Blue). On the other hand, the organic light emitting diode display is driven at a low DC voltage of several tens of volts, has a fast response speed, obtains high luminance, and emits various colors of R, G, and B, which is suitable for next-generation flat panel display devices. Do.

In the organic light emitting diode display, when a voltage is applied between the anode 100 and the cathode 70 as shown in FIG. 1, electrons generated from the cathode 70 are transferred to the electron injection layer 78A and the electron transport layer ( 78B) is moved toward the organic light emitting layer 78C. Further, holes generated from the anode 100 move toward the organic light emitting layer 78C through the hole injection layer 78e and the hole transport layer 78d. Accordingly, in the organic light emitting layer 78C, light is generated by collision and recombination of electrons and holes supplied from the electron transport layer 78B and the hole transport layer 78d, and the light is emitted to the outside through the anode 100. The image is displayed.

2 is a block diagram schematically illustrating a conventional organic light emitting diode display.

Referring to FIG. 2, a conventional organic light emitting diode display includes a display panel 20 including pixels 28 arranged in regions defined by intersections of a gate line GL and a data line DL, and a display panel. The gate driving circuit 22 driving the gate lines GL of the display 20, the data driving circuit 24 driving the data lines DL of the display panel 20, and the data driving circuit 24. And a timing controller 27 for controlling the data driver circuit 24 and the gate driver circuit 22 to supply a plurality of gamma voltages to the gamma voltage generator 26.

In the display panel 20, pixels 28 are arranged in a matrix. The display panel 20 includes a supply pad 10 that receives a high potential voltage from an external high potential driving voltage source VDD and a base pad 12 that receives a base voltage from an external base voltage source GND. (For example, the high potential driving voltage source VDD and the ground voltage source GND may be supplied from the power supply unit). The high potential driving voltage supplied to the supply pad 10 is supplied to the respective pixels 28. do. The base voltage supplied to the base pad 12 is supplied to the respective pixels 28.

The gate driving circuit 22 sequentially drives the gate lines GL by supplying gate signals to the gate lines GL.

The gamma voltage generator 26 supplies a gamma voltage having various voltage values to the data driving circuit 24.

The data driving circuit 24 converts the digital data signal input from the timing controller 27 into an analog data signal using the gamma voltage from the gamma voltage generator 26. The data driving circuit 24 supplies an analog data signal to the data lines DL whenever the gate signal is supplied.

The timing controller 27 generates a data control signal using a plurality of synchronization signals and controls the data driving circuit 24 through this. The timing controller 27 generates a gate control signal using a plurality of synchronization signals and controls the gate driving circuit through the timing controller 27. In addition, the timing controller 27 supplies a digital data signal supplied from the scaler to the data driving circuit 24.

Each of the pixels 28 receives a data signal from the data line DL when the gate signal is supplied to the gate line GL, and represents a gray level corresponding to the data signal.

3 is an equivalent circuit diagram for any one of the pixels 28.

Referring to FIG. 3, the pixel 28 includes an organic light emitting diode OLED emitting light by a current I _OLED flowing between a high potential driving voltage source VDD and a ground voltage source GND, and a first node n1. A driving TFT DT for controlling the current I _OLED flowing through the organic light emitting diode device OLED according to the voltage of the C, and a capacitor C connected between the first node n1 and the base voltage source GND. And a switch TFT (SW) for controlling the driving TFT (DT) in response to a gate signal and a data signal.

When the gate signal is supplied to the gate line GL, the switch TFT SW is turned on to supply the data signal from the data line DL to the first node n1. The data signal supplied to the first node n1 is charged to the capacitor C and applied to the gate electrode of the driving TFT DT. The driving TFT DT controls the amount of light emitted by the organic light emitting diode _OLED by controlling the driving current I _OLED of the organic light emitting diode OLED in response to a data signal applied to its gate electrode. In addition, the driving TFT DT is controlled according to the data signal stored in the capacitor C to maintain light emission of the organic light emitting diode OLED for one frame even when the switch TFT SW is turned off.

Here, the data voltage Vd for controlling the driving current I _OLED of the organic light emitting diode _OLED is different from the voltage V _OLED at both ends of the organic light emitting diode _OLED . This is because the driving current I _OLED is a function of the data voltage Vd (I _OLED = f (Vd)), and this function f depends on the characteristics of the driving TFT DT. Therefore, even if the same data voltage Vd is applied, the driving current I _OLED may have a variation in luminance due to the characteristics of the driving TFT DT. The characteristics of the driving TFT DT include a constant value K that is determined by the mobility and parasitic capacitance of the driving TFT DT, the threshold voltage Vth of the driving TFT DT, and the like.

4 is a diagram illustrating an example in which the threshold voltage Vth of the driving TFT increases due to positive gate-bias stress among the characteristics of the driving TFT DT. In Fig. 4, the horizontal axis represents the gate voltage [V] of the sample A-Si: H TFT, and the vertical axis represents the current [A] between the source electrode terminal and the drain electrode terminal of the sample A-Si: H TFT. The index in the box represents the gate voltage application time [sec] for each graph color.

Referring to FIG. 4, a positive gate-bias stress is applied to a hydrogenated amorphous silicon TFT (A-Si: H TFT) for a sample having a channel width / channel length (W / L) of 120 μm / 6 μm. When applied, the characteristics of the sample A-Si: H TFT change. That is, as can be seen in FIG. 4, when a voltage of +30 V is applied to the gate terminal of the sample A-Si: H TFT, the transfer characteristic curve of the TFT shifts to the right as the voltage application time increases. And the threshold voltage of the A-Si: H TFT rises. (Threshold voltage rises from Vth ₁ to Vth ₄ )

When the threshold voltage Vth of the driving TFT DT rises as described above, the operation of the driving TFT DT becomes unstable, so that even though the same data voltage Vd is applied, the current I flowing through the organic light emitting diode OLED is applied. _OLED ) is reduced.

FIG. 5 is a diagram illustrating an example in which the driving current I _OLED is reduced due to the increase in the threshold voltage Vth of the driving TFT T2. In FIG. 5, the horizontal axis represents the threshold voltage Vth of the driving TFT DT, and the vertical axis represents the driving current I _OLED .

As shown in FIG. 5, when the threshold voltage Vth of the driving TFT DT rises from 1V to 5V, the driving current I _OLED gradually decreases even when the same data voltage Vd is applied. Accordingly, the amount of change in the driving current (I _OLED) in accordance with the threshold voltage (Vth) increases the driving _{TFT (DT) (△ I OLED} ) is shown larger to about 74%.

As a result, in the conventional organic light emitting diode display, even if the same data voltage Vd is applied, luminance unevenness may occur due to the deviation of the driving current I _OLED depending on the characteristics of the driving TFT DT. There is a problem of this deterioration.

Accordingly, an object of the present invention is to provide an organic light emitting diode display device and a method of driving the same, which can improve display quality by minimizing deviation of driving current due to a change in characteristics of a driving device.

In order to achieve the above object, the organic light emitting diode display according to the embodiment of the present invention comprises a plurality of data lines supplied with a data voltage; A plurality of gate lines intersecting the data lines and supplied with scan pulses; A high potential drive voltage source for generating a high potential drive voltage; A source-follower circuit that changes the voltage of the second node according to the voltage of the first node; An organic light emitting diode element connected between the second node and a base voltage source and emitting light by a voltage of the second node; An offset capacitor connected between the first node and a third node; And supplying the data voltage to the first node during the first period and shorting the second and third nodes to store an offset voltage in the offset capacitor, and then storing the data voltage in the third node during the second period. Is supplied to increase the potential of the first node to the sum of the offset voltage and the data voltage so that the potential of the second node is equal to the data voltage, and then the potential of the second node for a third period of time. And a switch circuit for holding at the data voltage.

The source-follower circuit includes: a drive element having a gate electrode connected to the first node, a drain electrode connected to the high potential driving voltage source, and a source electrode connected to the second node; And a constant current element connected between the second node and the low potential driving voltage source.

The switch circuit may include: a first switch element forming a current path between the data line and the first node in response to the first scan pulse; A second switch element configured to form a current path between the second node and the third node in response to the first scan pulse; the data line and the response in response to a second scan pulse generated after the first scan pulse; And a third switch element forming a current path between the third nodes.

The first switch element includes a gate electrode connected to a gate line supplied with the first scan pulse, a drain electrode connected to the data line, and a source electrode connected to the first node.

The second switch element includes a gate electrode connected to a gate line supplied with the first scan pulse, a drain electrode connected to the second node, and a source electrode connected to the third node.

The third switch device includes a gate electrode connected to a gate line supplied with the second scan pulse, a drain electrode connected to the data line, and a source electrode connected to the third node.

The first period is defined as a period between the rising edge of the first scan pulse and the rising edge of the second scan pulse, and the second period is the polling edge of the first scan pulse and the polling of the second scan pulse. The third period is defined as the low logical period of the second scan pulse starting from the falling edge of the second scan pulse.

The sum of the first period and the second period is approximately one horizontal period.

The organic light emitting diode display according to the exemplary embodiment of the present invention further includes a storage capacitor connected between the driving voltage source and the third node to maintain a data voltage applied to the third node during the second period for one frame. do.

The offset voltage is a difference voltage between the data voltage and the voltage applied to the second node during the first period.

According to an embodiment of the present invention, a plurality of data lines supplied with a data voltage, a plurality of gate lines intersecting the data lines and supplied with first and second scan pulses, a high potential driving voltage source generating a high potential driving voltage, A source-follower circuit for changing the voltage of the second node according to the voltage of the first node, an organic light emitting diode element connected between the second node and the base voltage source and emitting light by the voltage of the second node, and the first And a switch circuit operated according to a second scan pulse, the method comprising: connecting an offset capacitor between a third node disposed in the same pixel area as the first node and the first node; step; Supplying the data voltage to the first node and shorting the second and third nodes during a first period to store an offset voltage in the offset capacitor; The potential of the second node is increased by supplying the data voltage to the third node subsequent to the first period to increase the potential of the first node to the sum of the offset voltage and the data voltage. Making it equal to the voltage; And maintaining the potential of the second node at the data voltage for a third period following the second period.

Other objects and features of the present invention in addition to the above object will be apparent from the description of the embodiments with reference to the accompanying drawings.

Hereinafter, exemplary embodiments of the present invention will be described with reference to FIGS. 6 to 13.

FIG. 6 is a block diagram illustrating an organic light emitting diode display according to an exemplary embodiment of the present invention, and FIG. 7 is a diagram showing first and second scan pulses S1 [k supplied to one of the pixels 122 of FIG. 6. ], S2 [k]).

6 and 7, an organic light emitting diode display according to an exemplary embodiment of the present invention includes a display panel 116 on which m × n pixels 122 are formed, and data lines DL [1] to. Data driving circuit 120 for supplying an analog data voltage to DL [m] and first gate lines GL1 [1] to GL1 [that intersect the data lines DL [1] to DL [m]. n]) and the second gate lines GL2 [1] through GL2 [that intersect the data lines DL [1] through DL [m] while supplying the first scan pulse S1 [k] sequentially. n]), the gate driver circuit 118 for sequentially supplying the second scan pulse S2 [k], and the timing controller 124 for controlling the driving timing of the data driver circuit 120 and the gate driver circuit 118. ).

The display panel 116 is a pixel defined by the intersection of the gate lines GL [1] to GL [n] and GL2 [1] to GL2 [n] and the data lines DL [1] to DL [m]. Pixels 122 formed in the regions are provided. The display panel 116 is provided with signal wirings for supplying a high potential driving voltage to each of the pixels 122 and signal wirings for supplying a low potential driving voltage. Although not shown, signal lines for supplying a ground voltage to each of the pixels 122 are formed in the display panel 116.

The data driving circuit 120 converts the digital video data RGB into an analog data voltage in response to the control signal DDC from the timing controller 124, and then converts the analog data voltage (hereinafter, referred to as data voltage) into data. To the lines DL [1] to DL [m]. This data voltage is supplied to the pixels 122 through the data lines DL [1] to DL [m].

The gate driving circuit 118 receives the first and second scan pulses S1 [k] and S2 [k] shown in FIG. 7 in response to the control signal GDC from the timing controller 124. Two gate lines GL1 [1] through GL1 [n] and GL2 [1] through GL2 [n] are sequentially supplied. The first and second scan pulses S1 [k] and S2 [k] are used to separate the first and second gate lines GL1 [1] to GL1 [n] and GL2 [1] to GL2 [n]. Are supplied to the pixels 122 through.

The timing controller 124 supplies digital video data RGB to the data driving circuit 120 and operates timings of the gate driving circuit 118 and the data driving circuit 120 using vertical / horizontal synchronization signals and clock signals. It generates a control signal (DDC, GDC) to control.

In the timing diagram of FIG. 7, A is an offset voltage storage period, B is a compensation period, and C is a light emission period. The offset voltage storage period A is defined as a period between the rising edge of the first scan pulse S1 [k] and the rising edge of the second scan pulse S2 [k]. The compensation period B is defined as the period between the falling edge of the first scan pulse S1 [k] and the falling edge of the second scan pulse S2 [k]. The light emission period C is defined as the low logic period of the second scan pulse S2 [k] starting from the falling edge of the second scan pulse S2 [k]. Here, the sum of the offset voltage storage period A and the compensation period B is approximately one horizontal period (1 H). The operation of the pixels 122 in A, B, and C will be described in detail with reference to FIGS. 10 and 11.

Meanwhile, the display panel 116 supplies a high potential driving voltage source VDD for supplying a high potential driving voltage to the pixels 122, a low potential driving voltage source VSS for supplying a low potential driving voltage, and a ground voltage. The ground voltage source GND is connected.

As illustrated in FIG. 8, each of the pixels 122 includes an organic light emitting diode OLED, one driving TFT DT, three switch TFTs ST1 to ST3, one constant current TFT, and two capacitors. Cos, Cst). In particular, the pixel 122 is configured of a source-follower circuit. In the source-follower circuit, the source voltage of the driving TFT DT has a constant offset voltage and follows the gate voltage of the driving TFT DT. By using this, the present invention can transfer the data voltage Vd to the organic light emitting diode OLED without change.

8 is a circuit diagram illustrating a pixel 122 included in an organic light emitting diode display according to an exemplary embodiment of the present invention.

Referring to FIG. 8, in the pixel 122 according to an exemplary embodiment, a driving TFT DT and a constant current device IT are connected in series between a high potential driving voltage source VDD and a low potential voltage source VSS. A source-follower circuit 122-1 which allows the output voltage Vo to follow the input voltage Vi, and is connected between the driving TFT DT and the base voltage source GND, and emits light according to the output voltage Vo. An organic light emitting diode (OLED) and a switch circuit 122-2 which is switched in accordance with a drive signal so that the output voltage Vo is equal to the data voltage Vd.

The anode of the organic light emitting diode OLED is connected to the second node n2 and the cathode is connected to the ground voltage source GND. The organic light emitting diode OLED has a structure as shown in FIG. 1, and the amount of light emitted is controlled according to the output voltage Vo, which is a voltage applied to the second node n2.

The source-follower circuit 122-1 includes a driving TFT DT and a constant current device IT connected in series between the high potential driving voltage source VDD and the low potential voltage source VSS. The gate electrode G of the driving TFT DT is connected to the first node n1, the drain electrode D is connected to the high potential driving voltage source VDD, and the source electrode S is connected to the second node n2. . The gate electrode G of the constant current device IT is connected to the constant voltage source Vs, the drain electrode D is connected to the second node n2, and the source electrode S is connected to the low potential voltage source VSS. The constant current device (IT) operates in a saturation region and maintains a constant voltage difference between the gate and the source by the constant voltage source (Vs) and the low potential voltage source (VSS) to serve as a constant current source. In this case, the constant current Ic generated by the constant current device IT is preferably larger than the driving current Io flowing through the organic light emitting diode OLED. Further, the constant value Kn of the driving TFT DT, which is determined by mobility and parasitic capacitance, is preferably made larger than the constant value Ko of the organic light emitting diode element OLED. This is to prevent the driving current Io from reacting sensitively according to the characteristics Vth and Kn of the driving TFT DT, thereby improving image quality uniformity. Therefore, if In is the current flowing into the second node n2 through the driving TFT DT, the relationship as shown in Equation 1 below is established.

Here, Vth denotes a threshold voltage of the driving TFT DT, and Vto denotes a threshold voltage of the organic light emitting diode OLED.

Differentiate both sides by using the output voltage (Vo) as a variable for Equation 1 and input the conditions Ic >> Io and Kn >> Ko, and then the difference between the input voltage (Vi) and the output voltage (Vo) (Vi-Vo) The offset voltage defined by) is expressed by Equation 2 below.

The source-follower circuit 122-1 causes the output voltage Vo to follow the input voltage Vi using the same principle as in Equations 1 and 2.

The switch circuit 122-2 includes the first and second switch TFTs ST1 and ST2 switched according to the first scan pulse S1 [k] and 1 <k <n, and the second scan pulse S2 [k. A third switch TFT ST3 switched according to the &quot;), an offset capacitor Cos for storing the offset voltage for a predetermined period, and a storage capacitor Cst for storing the data voltage Vd for a predetermined period.

The gate electrode G of the first switch TFT ST1 is connected to the first gate line GL1 [k], and the drain electrode D is connected to any one of the data lines DL [1] to DL [m]. The source electrode S is connected to the first node n1. The gate electrode G of the second switch TFT ST2 is at the first gate line GL1 [k], the drain electrode D is at the second node n2, and the source electrode S is at the third node ( n3). The gate electrode G of the third switch TFT ST3 is connected to the second gate line GL2 [k], and the drain electrode D is connected to any one of the data lines DL [1] to DL [m]. The source electrode S is connected to the third node n3. The offset capacitor Cos is connected between the first node n1 and the third node n3. The storage capacitor Cst is connected between the high potential driving voltage source VDD and the third node n3.

The switch circuit 122-2 is switched according to the driving signal to store the offset voltage in the offset capacitor Cos. Subsequently, the switch circuit 122-2 causes the sum of the offset voltage and the data voltage Vd to be input to the gate electrode of the driving TFT DT so that the output voltage Vo is equal to the data voltage Vd. do.

The operation of the pixels 122 will be described below with reference to FIGS. 9 and 10.

FIG. 9 is an equivalent circuit diagram of the pixel 122 for the offset voltage storage period A of FIG. 7.

Referring to FIG. 9, during the offset voltage storage period A, the first scan pulse S1 [k] is generated at a high logic voltage to turn on the first switch TFT and the second switch TFTs ST1 and ST2. The second scan pulse S2 [k] is generated at a low logic voltage to turn off the third switch TFT ST3. Accordingly, the data voltage Vd is applied to the first node n1 connected to the gate electrode G of the driving TFT DT. In addition, since the third node n3 positioned on the opposite side of the first node n1 with the offset capacitor Cos interposed therebetween is shorted with the second node n2, the third node n3 has a first output voltage. (Vo1) is charged. As a result, the offset capacitor Cos stores the offset voltage Vos as shown in Equation 3 below during the offset voltage storage period A.

The offset voltage Vos stored in the offset capacitor Cos is maintained at this magnitude even during the compensation period B and the light emission period C.

FIG. 10 is an equivalent circuit diagram of the pixel 122 with respect to the compensation section B of FIG. 7.

Referring to FIG. 10, during the compensation period B, the first scan pulse S1 [k] is inverted to a low logic voltage to turn off the first and second switch TFTs ST1 and ST2. The second scan pulse S2 [k] is inverted to a high logic voltage to turn on the third switch TFT ST3. Accordingly, since the data voltage Vd is applied to the third node n3, the potential Vi2 of the first node n1 connected to the gate electrode G of the driving TFT DT is represented by Equation 4 below. As shown by-(1), the sum of the data voltage Vd and the offset voltage Vos is Vd + Vos. Since the offset voltage Vos is defined as Equation 4- (2) below, the second output voltage Vo2 applied to the second node n2 is a data voltage as shown in Equation 4- (3) below. Compensated for (Vd). As a result, the voltage V _OLED applied to the both ends of the organic light emitting diode _OLED , that is, the second output voltage Vo2 becomes equal to the data voltage Vd, and thus the dependence on the driving TFT DT characteristics of the driving current. Is greatly reduced, resulting in an improvement in image quality uniformity.

The compensated second output voltage Vo2 = Vd is maintained at this magnitude even during the light emission period C. To this end, the storage capacitor Cst maintains the potential of the third node n3 as the data voltage Vd for one frame even after the third switch TFT ST3 is turned off.

FIG. 11 is an equivalent circuit diagram of the pixel 122 for the light emission period C of FIG. 7.

Referring to FIG. 11, during the light emitting period C, the first scan pulse S1 [k] is maintained at a low logic voltage so that the first switch TFT and the second switch TFTs ST1 and ST2 are kept turned off. The second scan pulse S2 [k] is inverted to a low logic voltage to turn off the third switch TFT ST3. At this time, since the potential of the third node n3 is maintained as the data voltage Vd by the storage capacitor Cst and the offset voltage Vos is stored in the offset capacitor Cos, the potential of the first node n1 is maintained. Is maintained at the sum of the data voltage Vd and the offset voltage Vos (Vd + Vos). As a result, the voltage of the second node n2 is maintained at the compensated second output voltage Vo2 = Vd, thereby greatly reducing the dependence on the driving TFT DT characteristic of the driving current Io, thereby increasing the image quality uniformity. Is improved.

FIG. 12 is a diagram illustrating a relationship between the input voltage Vi (gate voltage of the driving TFT) and the output voltage Vo (source voltage of the driving TFT) of the source-follower circuit 122-1 shown in FIG. 8. In Fig. 12, the horizontal axis represents the input voltage Vi, and the vertical axis represents the output voltage Vo.

As described above, in the source-follower circuit 122-1, the output voltage Vo follows the input voltage Vi with a constant offset voltage Vos. Therefore, in order to accurately compensate the output voltage Vo with the data voltage Vd, it is necessary to keep the offset voltage Vos constant. In practice, the offset voltage Vos in the source-follower circuit, unlike the offset voltage Vos in the ideal source-follower circuit, maintains some constant only within a limited driving region. For example, the offset voltage Vos is kept constant to some extent within the data voltage Vd driving region of FIG. 12. The driving range of the data voltage Vd is a region in which linearity with respect to the change of the output voltage Vo is ensured compared to the input voltage Vi. In a practically implemented source-follower circuit, a narrow range is compared with an ideal source-follower circuit. Have Further, in the source-follower circuit actually implemented, linearity is somewhat distorted by various causes even in this data voltage Vd driving region. Accordingly, the amount of change in the drive current is somewhat present in accordance with the change in the characteristics of the drive TFT DT at the same gradation. This will be described with reference to FIG. 13.

13 is a simulation result showing the amount of change in driving current according to the characteristic change of the driving TFT DT when the same gray scale is expressed. In Fig. 13, the horizontal axis represents the threshold voltage Vtn of the driving TFT DT, and the vertical axis represents the driving current I _OLED .

As shown in Equation 4, since the output voltage Vo is compensated by the data voltage Vd, the change amount of the driving current ΔI _OLED is ideally 0% due to the increase of the threshold voltage Vtn of the driving TFT DT. Should be indicated. However, in actual cases, the change amount of the driving current ΔI _OLED according to the increase of the threshold voltage Vtn of the driving TFT DT is affected by the non-uniformity of the offset voltage Vos due to the linearity distortion described in FIG. 12. Values from% to 31% are shown. For example, as shown in FIG. 13, when the threshold voltage Vtn of the driving TFT DT rises from 1V to 5V, the change amount of the driving current ΔI _OLED is equal to the data voltage Vd of 6V. 31% when applied, 16% when 7V data voltage Vd is applied, 10% when 8V data voltage Vd is applied, 9% when 9V data voltage Vd is applied % And 17% when the data voltage Vd of 10V is applied.

As described above, even if the old coins change to some extent (9% to 31%) due to the increase of the threshold voltage Vtn of the driving TFT DT when the same gray scale is implemented, the change amount of the driving current ΔI _OLED according to the present invention. Is within the maximum 31%, it can be seen that the variation is significantly reduced compared to the conventional 74% (see Fig. 5). Accordingly, the organic light emitting diode display according to the present invention can significantly reduce the variation of the driving current due to the change of the characteristics of the driving TFT DT, and thus can realize a significantly better display quality.

As described above, the organic light emitting diode display and the driving method thereof according to the embodiment of the present invention drive the driving current flowing through the organic light emitting diode element by directly causing the voltage applied to the organic light emitting diode element to become a data voltage. It is prevented from being sensitively changed by the change of TFT characteristics. Accordingly, the organic light emitting diode display and the driving method thereof according to the embodiment of the present invention can improve the display quality and improve the reliability of the image quality by minimizing the variation of the driving current caused by the characteristic change of the driving TFT in the same gray scale. have.

Those skilled in the art will appreciate that various changes and modifications can be made without departing from the technical spirit of the present invention. Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims.

Claims (15)

A plurality of data lines supplied with data voltages; A plurality of gate lines intersecting the data lines and supplied with scan pulses; A high potential drive voltage source for generating a high potential drive voltage; A source-follower circuit that changes the voltage of the second node according to the voltage of the first node; An organic light emitting diode element connected between the second node and a base voltage source and emitting light by a voltage of the second node; An offset capacitor connected between the first node and a third node; And The data voltage is supplied to the first node during a first period and the second and third nodes are shorted to store an offset voltage in the offset capacitor. Then, the data voltage is supplied to the third node during a second period. Supplying the potential of the first node to be equal to the data voltage by raising the potential of the first node to the sum of the offset voltage and the data voltage, and then increasing the potential of the second node for a third period of time. And a switch circuit for maintaining the data voltage. The method of claim 1, The source-follower circuit, A driving element having a gate electrode connected to the first node, a drain electrode connected to the high potential driving voltage source, and a source electrode connected to the second node; And And a constant current element connected between the second node and the low potential driving voltage source. The method of claim 1, The switch circuit, A first switch element forming a current path between the data line and the first node in response to the first scan pulse; A second switch element forming a current path between the second node and the third node in response to the first scan pulse; And a third switch element forming a current path between the data line and the third node in response to a second scan pulse generated after the first scan pulse. The method of claim 3, wherein The first switch device, And a gate electrode connected to the gate line supplied with the first scan pulse, a drain electrode connected to the data line, and a source electrode connected to the first node. The method of claim 3, wherein The second switch device, And a gate electrode connected to the gate line supplied with the first scan pulse, a drain electrode connected to the second node, and a source electrode connected to the third node. The method of claim 3, wherein The third switch device, And a gate electrode connected to the gate line supplied with the second scan pulse, a drain electrode connected to the data line, and a source electrode connected to the third node. The method of claim 3, wherein The first period is defined as a period between the rising edge of the first scan pulse and the rising edge of the second scan pulse, and the second period is the polling edge of the first scan pulse and the polling of the second scan pulse. And a third period is defined as a low logic period of the second scan pulse starting from a falling edge of the second scan pulse. The method of claim 7, wherein And the sum of the first period and the second period is approximately one horizontal period. The method of claim 1, And a storage capacitor connected between the driving voltage source and the third node to maintain a data voltage applied to the third node for one frame during the second period. The method of claim 1, And the offset voltage is a difference voltage between the data voltage and the voltage applied to the second node during the first period. A plurality of data lines supplied with a data voltage, a plurality of gate lines intersecting the data lines and supplied with first and second scan pulses, a high potential driving voltage source generating a high potential driving voltage, and a voltage of a first node A source-follower circuit for changing a voltage of a second node, an organic light emitting diode device connected between the second node and a base voltage source and emitting light by the voltage of the second node, and according to the first and second scan pulses A driving method of an organic light emitting diode display device having a switch circuit operated, Connecting an offset capacitor between the first node and a third node disposed in the same pixel area as the first node; Supplying the data voltage to the first node and shorting the second and third nodes during a first period to store an offset voltage in the offset capacitor; The potential of the second node is increased by supplying the data voltage to the third node subsequent to the first period to increase the potential of the first node to the sum of the offset voltage and the data voltage. Making it equal to the voltage; And And maintaining the potential of the second node at the data voltage for a third period subsequent to the second period. The method of claim 11, The first period is defined as a period between the rising edge of the first scan pulse and the rising edge of the second scan pulse, and the second period is the polling edge of the first scan pulse and the polling of the second scan pulse. And a third period is defined as a low logical period of the second scan pulse starting from a falling edge of the second scan pulse. The method of claim 12, And the sum of the first period and the second period is approximately one horizontal period. The method of claim 11, And connecting a storage capacitor between the driving voltage source and the third node to maintain the data voltage applied to the third node for one frame during the second period. Driving method. The method of claim 11, And the offset voltage is a difference voltage between the data voltage and the voltage applied to the second node during the first period.

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