KR101621329B1 - Organic electroluminescent display device and method of driving the same - Google Patents

Abstract

The present invention provides a semiconductor device comprising: a gate wiring and a data wiring crossing each other; A switching transistor connected to the gate line and the data line, a driving transistor receiving a data voltage through the switching transistor, an organic light emitting diode connected to the driving transistor, and a driving transistor for detecting a threshold voltage of the driving transistor. A pixel including a detection transistor connected between a drain electrode and a gate electrode; A drive mode selecting section for counting the number of frames in which the same image is successively input and selecting a compensation mode as a drive mode of the organic electroluminescent device when the count number is less than a threshold value and a compensation mode when the count value reaches a threshold value ; A scan driver for shortening a detection time of the threshold voltage in the compensation mode compared to the normal mode; And a data driver for outputting to the data line a data voltage for compensating for a difference between threshold voltages detected in the normal mode and the compensation mode in the compensation mode.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an organic electroluminescent display device,

The present invention relates to an organic electroluminescent device, and more particularly, to an organic electroluminescent device and a driving method thereof.

2. Description of the Related Art [0002] With the development of an information society, demands for a display device for displaying images have been increasing in various forms. Recently, a liquid crystal display (LCD), a plasma display panel (PDP) Various flat display devices such as an organic electroluminescent display device (OELD) have been utilized.

Of these flat panel display devices, organic electroluminescent devices are capable of being driven at a low voltage, thin, have excellent viewing angles, and have fast response speeds.

As an organic electroluminescent device, an active matrix type organic electroluminescent device in which a plurality of pixels are positioned in a matrix form to display an image is widely used.

1 is a circuit diagram of a pixel of a general organic electroluminescent device.

As shown in the drawing, the organic electroluminescent element is configured to include a gate wiring GL and a data wiring DL which define the pixel P to intersect with each other.

In the pixel P, a switching transistor Tl, a driving transistor Tl and an organic light emitting diode OD are formed.

With respect to the pixel P having such a configuration, when the gate line GL is scanned, the switching transistor Tl is turned on. In synchronism therewith, the data voltage Vdata transferred through the data line DL is applied to the driving transistor T2. Accordingly, the light emission current I _OD passes through the driving transistor T 2 and is applied to the organic light emitting diode OD. Thus, the organic light emitting diode OD emits light to display an image.

The light emission current I _OD supplied to the organic light emitting diode OD is controlled by the data voltage Vdata applied to the gate electrode of the driving transistor T 2. For example, the light emission current I _OD supplied to the organic light emitting diode _OD is obtained by the following equation (1).

_{I OD = k (Vgs - Vth} ) 2 = k (Vdata - Vss - Vth) 2 .... ( Equation 1)

Here, Vgs is a gate-source voltage as a voltage difference between the gate electrode and the source electrode of the driving transistor T2, Vth is a threshold voltage of the driving transistor T2, and k is a constant value.

As described above, the light emission current I _OD depends on the threshold voltage Vth of the driving transistor T 2. However, the threshold voltage Vth of the driving transistor T2 is shifted according to the driving time. As a result, the light emission current I _OD is changed, and the brightness of the displayed image is changed.

In order to solve this problem, a voltage compensation method for compensating a deviation of the threshold voltage (Vth) has been proposed. In the voltage compensation method, the threshold voltage (Vth) is detected before the emission period of the pixel (P), and the voltage is reflected to the gate electrode side in advance. Thus, as shown in Fig. 2, even when the threshold voltage Vth is shifted, the shifted threshold voltage Vth can be detected. As a result, the threshold voltage (Vth) component is canceled in the above equation (1), and the light emission current I _OD does not depend on the threshold voltage Vth. Therefore, it is possible to improve the luminance change of the image.

On the other hand, when the same image is continuously displayed, the same data voltage (Vdata) is continuously applied to the driving transistor T2. As a result, the driving transistor T2 continues to deteriorate over time and the threshold voltage Vth is shifted. Even if the threshold voltage Vth is shifted, as described above, if the shifted threshold voltage Vth is normally detected and reflected on the gate electrode, the image can be normally displayed.

Incidentally, due to factors such as process conditions, driving timing, driving environment, etc., an error DELTA Vth may occur in detecting the threshold voltage Vth. On the other hand, as the degree of deterioration of the driving transistor T2 increases, the degree of the detection error DELTA Vth increases. Therefore, as time elapses, a change in luminance of an image is caused due to a detection error? Vth.

On the other hand, the shift of the threshold voltage (Vth) is influenced by the displayed image. For example, in a chess pattern image displayed in white and black, the shift of the threshold voltage (Vth) is large at a portion where white is displayed rather than a portion where black is displayed. That is, the shift of the threshold voltage Vth has a gate bias tendency. Accordingly, if the detection error? Vth exists in detecting the threshold voltage Vth, the white portion having a large shift of the threshold voltage Vth will have a larger detection error? Vth than the black portion. Further, the difference between the detection error? Vth of the white part and the black part and the detection error? Vth becomes larger as the time for displaying the chess pattern becomes longer.

In such a case, if a gray image is displayed after displaying an image of the chess pattern for a long time, the detection error DELTA Vth generated in the chess pattern will be reflected in the gray image. Accordingly, a difference in luminance due to the difference in the detection error? Vth will occur between the portion in which white is displayed and the portion in which black is displayed. As a result, in the gray image, image quality degradation such as afterimage occurs.

As described above, in a conventional organic electroluminescent device, deterioration in image quality such as after-image can be caused by a detection error.

An object of the present invention is to provide an organic electroluminescent device capable of improving image quality and a driving method thereof.

In order to achieve the above-described object, the present invention provides a semiconductor device comprising: a gate wiring and a data wiring intersecting with each other; A switching transistor connected to the gate line and the data line, a driving transistor receiving a data voltage through the switching transistor, an organic light emitting diode connected to the driving transistor, and a driving transistor for detecting a threshold voltage of the driving transistor. A pixel including a detection transistor connected between a drain electrode and a gate electrode; A drive mode selecting section for counting the number of frames in which the same image is successively input and selecting a compensation mode as a drive mode of the organic electroluminescent device when the count number is less than a threshold value and a compensation mode when the count value reaches a threshold value ; A scan driver for shortening a detection time of the threshold voltage in the compensation mode compared to the normal mode; And a data driver for outputting to the data line a data voltage for compensating for a difference between threshold voltages detected in the normal mode and the compensation mode in the compensation mode.

Here, the drive mode selection unit may include an image comparison unit for comparing whether the image of the previous frame and the image of the current frame are the same or not; And a counter for incrementing the number of counts by one when the image of the previous frame is identical to the image of the current frame.

Wherein the data driver further includes a DAC circuit that generates gray voltages using gamma reference voltages and outputs one of the gray voltages corresponding to the gray level of the input image data as a data voltage, The device may further include a gamma reference voltage generator for generating different gamma reference voltages in the normal mode and the compensation mode.

Wherein the data driver further includes a DAC circuit that generates gray voltages using gamma reference voltages and outputs one of the gray voltages corresponding to the gray level of the input image data as a data voltage, The device comprises: a gamma reference voltage generator for generating the same gamma reference voltage in the normal mode and the compensation mode; The controller may further include a controller for outputting the image data input in the normal mode to the data driver, converting the gradation level of the image data input in the compensation mode, and outputting the converted gradation level to the data driver.

The pixel includes a capacitor connected between a drain electrode of the switching transistor and a gate electrode of the driving transistor; And an initialization transistor connected to the gate electrode of the driving transistor for transmitting an initialization voltage for initializing a voltage of a gate electrode of the driving transistor.

According to another aspect of the present invention, there is provided an image processing method comprising the steps of: counting the number of consecutive frames of the same image; Selecting a normal mode when the number of counts is less than a threshold value and selecting a compensation mode as a driving mode of the organic electroluminescent device when the count value reaches a threshold value; Detecting a threshold voltage of the driving transistor in accordance with the driving mode; Applying a data voltage to a gate electrode of the driving transistor in which the detected threshold voltage is reflected in accordance with the driving mode; And applying a light emitting current to the organic light emitting diode according to the applied data voltage to emit light to the organic light emitting diode, wherein the detection time of the threshold voltage in the compensation mode is shorter than the normal mode, And the data voltage during the compensation mode is generated so as to compensate for the difference between the threshold voltage detected in the normal mode and the compensation mode.

Here, in the data driver, generating gradation voltages using gamma reference voltages; Outputting one of the gradation voltages corresponding to a gradation level of the input image data as a data voltage in the data driver; And generating different gamma reference voltages in the normal mode and the compensation mode.

Generating, in the data driver, gradation voltages using gamma reference voltages; Outputting one of the gradation voltages corresponding to a gradation level of the input image data as a data voltage in the data driver; Generating a same gamma reference voltage in a normal mode and a compensation mode; Outputting the image data input in the normal mode to the data driver, converting the gradation level of the image data input in the compensation mode, and outputting the converted gradation level to the data driver.

The compensation mode is performed during the compensation setting time after the count number reaches the threshold value, and when the compensation setting time is passed, a step of counting the number of frames may be newly performed.

Wherein counting the number of frames includes incrementing the number of counts by one if the image of the previous frame is identical to the image of the current frame and if the image of the current frame is different from the image of the current frame, Can be initialized.

And a step of applying an initializing voltage to the gate electrode of the driving transistor before the step of detecting the threshold voltage.

In the present invention, when the same image is continuously inputted during the critical time, it is driven in the compensation mode. In this compensation mode, the driving time for detecting the threshold voltage is pulled forward as compared with the driving in the normal mode. Accordingly, when a detection error with respect to the threshold voltage occurs, such a detection error can be reduced. This makes it possible to improve the image quality such as afterimage due to the detection error when the image is changed after the same image is continuously input.

Furthermore, as the detection time is reduced, a voltage difference that is detected at the time of driving in the normal mode and at the compensation mode is generated, and a luminance difference of an image can be generated. Such a luminance difference of the image can be compensated by adjusting the data voltage.

Through the above-described method, the organic electroluminescent device of the present invention can improve the image quality of a displayed image.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

4 is an equivalent circuit diagram of a pixel of an organic electroluminescent device according to an embodiment of the present invention. FIG. 5 is a cross-sectional view of an organic electroluminescent device according to an embodiment of the present invention. 6 is a diagram illustrating a driving mode selection unit of an organic electroluminescent device according to an exemplary embodiment of the present invention. Referring to FIG.

As shown, the organic electroluminescent device 100 according to the embodiment of the present invention includes a display portion 200 and a driving circuit portion.

In the display portion 100, a plurality of gate lines GL extend in a first direction, for example, a row direction. A plurality of data lines DL extend in a second direction that intersects the first direction, for example, a column direction. The gate lines GL and the data lines DL intersecting with each other define a plurality of pixels P arranged in a matrix form. On the other hand, the detection control wiring SCL and the initialization control wiring ICL extend in the first direction corresponding to the gate wiring GL.

A plurality of transistors T1 to T4, a capacitor C and an organic light emitting diode OD may be formed in each pixel P of the display unit 100. [

The switching transistor Tl, the driving transistor T2, the initializing transistor T3 and the detecting transistor T4 may be used as the plurality of transistors T1 to T4. As these transistors T1 to T4, for example, N type transistors can be used.

The switching transistor Tl is connected to the corresponding gate wiring and data lines GL and DL. The gate electrode and the source electrode of the switching transistor Tl are connected to the gate wiring and the data lines GL and DL, respectively.

Meanwhile, a capacitor C may be connected between the driving transistor T2 and the switching transistor T1. The first and second electrodes of the capacitor C are connected to the drain electrode of the switching transistor T1 and the gate electrode of the driving transistor T2, respectively.

The organic light emitting diode OD is connected to the driving transistor T2. The cathode electrode of the organic light emitting diode OD is connected to the drain electrode of the driving transistor T2. The anode electrode of the organic light emitting diode OD is connected to the first driving line PL1 to receive the first driving voltage Vdd. The source electrode of the driving transistor T2 is connected to the second driving line PL2 and receives the second driving voltage Vss. Here, the first driving voltage Vdd may have a higher potential than the second driving voltage Vss.

The gate electrode of the initializing transistor T3 is connected to the initialization control wiring ICL and is on / off switched. Then, the gate electrode of the detection transistor T4 is connected to the detection control wiring SCL to be turned on / off.

The initialization process, the threshold voltage detection process, the data write process, and the light emission process are repeatedly performed in the frame period of the pixel P having such a configuration, which will be described with reference to FIG.

In the initialization process, the pixel P emitting light in accordance with the image data written in the previous frame is initialized using the initialization voltage Vini. For this, the initialization control wiring ICL connected to the pixel P is selected and scanned. Thus, an initialization control voltage Vic having a turn-on voltage (for example, a high voltage) is applied to the initialization control wiring ICL, whereby the initialization transistor T3 is turned on. When the initializing transistor T3 is turned on, the initializing voltage Vint is applied to the gate electrode of the driving transistor T2. As the initializing voltage Vini, for example, when the driving transistor T2 is N type, a high voltage may be applied. Such an initialization voltage Vini may have a value enough to turn on the driving transistor T2. For example, the initialization voltage Vini may have a value equal to or larger than a value obtained by adding the threshold voltage Vth to the second driving voltage Vss.

In the threshold voltage detection process, the threshold voltage Vth of the driving transistor T2 is detected. To this end, the detection control wiring SCL connected to the pixel P is selected and scanned. Thus, a detection control voltage Vsc having a turn-on voltage (e.g., a high voltage) is applied to the detection control wiring SCL, which turns on the detection transistor T4. Thus, the threshold voltage Vth of the driving transistor T2 is detected and reflected to the gate electrode of the driving transistor T2. Therefore, when the threshold voltage detection process is performed, the voltage Vg of the gate electrode becomes Vg = Vth + Vss. This voltage Vg is stored in the capacitor C. [

When threshold voltage detection is performed, data of an image to be displayed during a current frame is written into the corresponding pixel P through a data write process. To this end, the gate line GL connected to the pixel P is selected and scanned. As a result, a gate voltage Vgl having a turn-on voltage (e.g., a high voltage) is applied to the gate wiring GL, thereby turning on the switching transistor Tl. The data voltage Vdata transferred to the data line DL is applied to the first electrode of the capacitor C through the switching transistor Tl. Accordingly, the voltage of the second electrode of the capacitor C is boosted or raised by the data voltage Vdata. As a result, the voltage Vg at the gate electrode of the driving transistor T2 becomes Vg = Vdata + Vth + Vss.

When data is written to the pixel P through the data writing process, the organic light emitting diode OD of the pixel P emits light of a luminance corresponding to the written data. That is, the light emission current I _OD applied to the organic light emitting diode OD is obtained by the following equation (2).

_{I OD = k (Vgs - Vth} ) 2 = k (Vdata + Vth + Vss - Vth) 2 = k (Vdata + Vss) 2 .... Equation (2)

Here, Vgs is a gate-source voltage as a voltage difference between the gate electrode and the source electrode of the driving transistor T2, and k is a constant value.

Accordingly, when the threshold voltage Vth is detected without detection error, the organic light emitting diode OD emits light of a luminance corresponding to the written data.

Meanwhile, the capacitor C stores the voltage Vg of the gate electrode of the driving transistor T2. Accordingly, during the light emission period, the light emission current I _OD corresponding to the written data can continuously flow.

Meanwhile, in the embodiment of the present invention, the time for detecting the threshold voltage Vth is adjusted according to the driving mode, that is, whether the driving is in the normal mode or the compensation mode, which will be described later.

The driving circuit unit includes a driving mode selection unit 300, a control unit 310, a scan driving unit 320, a data driving unit 330, and a gamma reference voltage generating unit 340.

The control unit 310 supplies the data driving unit 330 with a data control signal DCS for controlling the data driving unit 330. Further, the image data Data supplied from the external system is sampled according to the data control signal SCS and supplied to the data driver 350. The controller 310 supplies a scan control signal SCS for controlling the scan driver 320 to the scan driver circuit 340.

The gamma reference voltage generator 340 generates the gamma reference voltages Vgamma. The gamma reference voltages Vgamma thus generated are supplied to the data driver 330.

The data driver 330 generates the gradation voltages using the gamma reference voltages Vgamma. This can be performed in a digital-to-analog converter (DAC) circuit (see 331 in FIGS. 8 and 9) of the data driver 330. FIG. That is, the DAC circuit 331 receives the gamma reference voltage Vgamma and uses them to generate the gradation voltages. To this end, the DAC circuit 331 may include a voltage dividing resistor circuit. Each of the gradation voltages thus generated corresponds to the gradation levels that the image data Data can have. Accordingly, the data driver 330 can output, as the data voltage Vdata, the gradation voltage corresponding to the gradation level of the inputted image data Data in the digital format. In this manner, the data driver 330 outputs the video data in the digital format as video data in the analog format. The data voltage Vdata thus outputted is applied to the corresponding data line DL and is input to the corresponding pixel P.

The scan driver 320 scans the initialization control wiring ICL, the detection control wiring SCL and the gate wiring GL in response to the scan control signal SCS supplied from the control unit 310. [ For example, the initialization control wiring ICL, the detection control wiring SCL and the gate wiring GL are sequentially scanned every frame along the column direction. On the other hand, in each row line, the initialization control wiring ICL, the detection control wiring SCL and the gate wiring GL are scanned according to the driving timing of the corresponding row line. That is, the initialization control wiring ICL, the detection control wiring SCL, and the gate wiring GL are scanned in accordance with the initialization process, the detection process, and the timing of the data writing process. Meanwhile, the scan driver 320 may sequentially select the initialization line IL and apply the initialization voltage Vini to the initialization line IL.

The drive mode selection unit 300 selects whether to drive the organic electroluminescent device 100 in the normal mode and the compensation mode. To this end, the drive mode selection unit 300 includes an image comparison unit 301 and a counter unit 303, as shown in FIG.

The image comparison unit 301 compares, for example, whether or not the video of the previous frame and the video of the current frame are the same. The counter unit 303 increases the number of counts by one when the previous frame and the current frame are the same image according to the image comparison result of the image comparing unit 301. [ On the other hand, when the previous frame and the current frame are not the same, the number of counts can be reset to the initial value.

In accordance with the operation of the image comparison unit 301 and the counter unit 303, the drive mode selection unit 300 can count the number of frames in which the same image is continuously displayed. If the number of counts does not exceed the threshold value, the drive mode selection unit 300 outputs the mode selection signal M corresponding to the normal mode.

On the other hand, when the number of counts reaches the threshold value, the drive mode selection unit 300 outputs the mode selection signal M corresponding to the compensation mode. Furthermore, the mode selection signal M corresponding to the compensation mode can be output for a fixed time, for example, a compensation setting time, after the number of counts reaches the threshold value.

Then, when the compensation setting time passes, the number of counts is reset to the initial value, and the counting process can be performed again.

The organic electroluminescent device 100 can be selectively driven in the normal mode or the compensation mode according to the mode selection signal M output as described above.

Here, the threshold value is set so that, when detecting the threshold voltage Vth of the driving transistor T2 of the pixel P, when the detection error? Vth occurs, Corresponds to a threshold time that can be viewed by the user. That is, when the same image is continuously input, if the image is changed before the threshold time elapses, deterioration of the driving transistor T2 due to the same image input will not be large. Accordingly, even when the detection error? Vth occurs, the detection error? Vth will not be large. In such a case, in displaying the changed image, the error of the gate-source voltage Vgs of the driving transistor T2 during the light emitting period is not large, and the afterimage will not be seen by the user.

On the other hand, if the image is changed after the lapse of the critical time, the deterioration of the driving transistor T2 will be large. Accordingly, when the detection error? Vth occurs, the detection error? Vth becomes large. Also, in such a case, in displaying the changed image, the error of the gate-source voltage Vgs of the driving transistor T2 during the light emitting period becomes large, and the afterimage will be visually recognized by the user.

As described above, the threshold value corresponds to the threshold time that affects the deterioration of the image quality of the image in which the detection error DELTA Vth with respect to the threshold voltage Vth is changed, when the same image continuously input is changed to another image.

Therefore, in the embodiment of the present invention, the organic EL device 100 is driven normally before the time when the same image is successively input reaches the threshold time. Then, when the threshold time is reached, the organic EL device 100 is compensated to be driven for a predetermined time, i.e., a compensation set time, in order to compensate for the detection error DELTA Vth. The normal driving and the compensation driving of the organic electroluminescent device 100 will be described with reference to Figs. 7 to 9. Fig. In the following description, the case where the detection error? Vth has a negative value, that is, a voltage lower than the actual threshold voltage Vth is detected will be described as an example.

7 is a diagram illustrating a process of detecting a threshold voltage of a driving transistor when driving an organic light emitting diode according to an exemplary embodiment of the present invention in a normal mode and a compensation mode. In Fig. 7, the first graph Vgsn shows the actual gate-source voltage of the driving transistor T2 shifted by the threshold voltage Vth according to driving for a long time. The second graph Vgse shows the gate-source voltage of the driving transistor T2 detected during the detection time when the detection error? Vth occurs.

8 and 9 are diagrams illustrating a method of generating a data voltage of an organic electroluminescent device according to an embodiment of the present invention.

The image data Data input to the organic electroluminescent device 100 is input to the corresponding pixel P during the normal mode driving. That is, the image data (Data) in the digital format is transmitted to the data driver 330 through the controller 310. The image data Data thus transmitted is converted into image data of an analog format having a gradation voltage corresponding to the gradation level, that is, the data voltage (Vdata) through the DAC circuit 331 of the data driver 330. [ The data voltage Vdata thus converted is output to the corresponding data line DL and written into the corresponding pixel P. Meanwhile, the process of detecting the threshold voltage Vth of the driving transistor T2 before the data writing process is performed during the detection time set in the normal mode, i.e., the normal detection time TS1.

On the other hand, during driving in the compensation mode, the detection process for detecting the threshold voltage Vth of the driving transistor T2 is performed for a time shorter than the normal detection time TS1 set in the normal mode, that is, during the compensation detection time TS2 .

Assume that the detection process is performed during the normal detection time (TS1) according to the timing in the normal mode at the time of the compensation mode driving. In such a case, at the ending time t1 of the normal detection time TS1, the difference between Vgsn and Vgse is DELTA V1. That is, the detection error? Vth in the normal detection time TS1 corresponds to? V1.

On the other hand, suppose that the detection process is performed during a time shorter than the normal detection time TS1, that is, during the compensation detection time TS2. In such a case, at the end point t2 of the compensation detection time TS2, the difference between Vgsn and Vgse is DELTA V2. That is, the detection error DELTA Vth detected by the compensation detection time TS2 corresponds to DELTA V2.

As described above, if the detection process is performed for a time TS2 shorter than the normal detection time TS1, the detection error DELTA Vth is reduced from DELTA V1 to DELTA V2. Therefore, the error of the voltage reflected on the gate electrode of the driving transistor T2 is also reduced. As a result, the error of the light emission current I _OD is reduced, so that the luminance error in the displayed image can be reduced.

Therefore, in the embodiment of the present invention, when driving in the compensation mode, the detection time is shortened to reduce the detection error DELTA Vth. Thus, even when the image is changed at the time of driving in the compensation mode, deterioration in image quality such as afterimage due to the detection error DELTA Vth can be improved. Particularly, in the embodiment of the present invention, it is preferable to reduce the compensation detection time TS2 to such an extent that the error of the image brightness due to the detection error DELTA Vth is not visibly recognized by the user due to image quality deterioration such as afterimage.

In this regard, an example of driving a gray image in a compensation mode according to an embodiment of the present invention, in the case where an image of a chess pattern of white and black is displayed for a threshold time and then converted into a gray image, will be described as an example. In the compensation mode, the detection time (ΔVth) is reduced by reducing the detection time. Accordingly, both the detection error of the portion in which white is displayed and the detection error in the portion in which black is displayed will be reduced as compared with the conventional art. Further, the difference between the detection error of the portion in which white is displayed and the detection error in the portion in which black is displayed will be reduced compared with the conventional one. This makes it possible to improve the deterioration of image quality such as a residual image when gray images are displayed.

The operation of adjusting the detection time between the normal detection time TS1 and the compensation detection time TS2 is performed by setting the scan control signal SCS of the control unit 310 in accordance with the MOS selection signal M . ≪ / RTI > That is, in response to the adjustment of the scan control signal SCS, the scan driver 320 can adjust the scan timing of the detection control line SCL.

On the other hand, the voltage Vg of the gate electrode of the driving transistor T2 is changed by reducing the detection time to reduce the detection error DELTA Vth at the time of the compensation mode driving. In this regard, referring to FIG. 7, the gate-source voltage Vgs1 at the end time t1 of the normal detection time TS1 is smaller than the gate-source voltage Vgs1 at the end time t2 of the compensation detection time TS2, The source voltage Vgs2 is high. That is, as the detection time is reduced, the detected voltage is increased by? V3. Accordingly, as compared with the case of driving in the normal mode, when driving in the compensation mode, the luminance of the image is increased.

As described above, the luminance difference of the image can be induced before and after the compensation mode. Therefore, in order to compensate for the luminance difference, the data voltage Vdata input to the pixel P is adjusted. For example, assume that when the compensation mode is driven, video data Data having the same gradation level as that in the normal mode driving is input. In such a case, when the compensation mode is driven, the data voltage (Vdata) is lowered at the same time as in the normal mode operation. Thus, it is possible to cancel the detected detection voltage higher than that in the normal mode driving at the time of the compensation mode driving. Accordingly, it is possible to compensate for the change in the voltage detected by the adjustment of the detection time in the compensation mode driving to change the luminance.

Here, as a method of adjusting the data voltage (Vdata) in the compensation mode driving, a method of adjusting the gradation voltage can be used. This will be described with reference to Fig. Referring to FIG. 8, the gamma reference voltage generator 340 adjusts the gamma reference voltage Vgamma according to the mode selection signal M. Thus, using the gamma reference voltage Vgamma, the gradation voltage generated in the DAC circuit 331 is adjusted. For example, for the image data (Data) having the same gradation level, the gradation voltage is adjusted to be lower than that in the normal mode driving upon driving in the compensation mode. Thus, the output data voltage Vdata can be adjusted. As described above, in the present invention, different gamma reference voltages (Vgamma) can be used in compensation mode and normal mode driving. Here, according to the driving mode, the gradation voltages can be adjusted by adjusting the maximum gamma reference voltage having the maximum value among the gamma reference voltages Vgamma.

Meanwhile, as a method of adjusting the data voltage (Vdata) at the time of driving in the compensation mode, a method of adjusting the image data (Data) can be used. This will be described with reference to Fig. Referring to FIG. 9, the control unit 310 includes a data conversion unit 311. The data converter 311 controls the gradation level of the input image data Data_in according to the mode selection signal M. [ For example, when the mode selection signal M has a value corresponding to the compensation mode, the gradation level of the input image data Data_in is adjusted and the output image data Data_out, (330). The DAC circuit 331 of the data driver 330 outputs the data voltage Vdata corresponding to the gradation level of the output video data Data_out. Here, the input gamma reference voltage Vgamma may correspond to the gamma reference voltage in the normal mode. Through this process, the data voltage Vdata can be adjusted. On the other hand, the data conversion unit 311 can use a look-up table to convert the image data Data.

Hereinafter, a driving method of an organic electroluminescent device according to an embodiment of the present invention will be described with reference to FIG.

First, the image comparison unit 301 compares the input image of the current frame with the image of the previous frame (S1).

If it is not the same image, the organic EL device 100 is driven in the normal mode (S2). In the case of driving in the normal mode, a process of detecting the threshold voltage Vth of the driving transistor T2 during the set normal detection time TS1 is performed. Then, the data voltage Vdata corresponding to the gradation level of the image data Data of the input image is written in the corresponding pixel P. On the other hand, when it is not the same image, the count number CN of the counter unit 303 is reset to the initial value.

On the other hand, if it is the same image, the counter unit 303 increases the count number CN by one and compares the count number CN with the threshold value CR (S3). The count number CN is increased according to a formula such as CN = CN + 1. If the count CN does not reach the threshold CR, that is, CN < CR, the organic EL device 100 is driven in the normal mode.

On the other hand, when the count CN reaches the threshold, that is, when CN = CR, the organic EL device 100 is driven in the compensation mode (S4).

When driven in the compensation mode, a process of detecting the threshold voltage Vth during the compensation detection time TS2 shorter than the normal detection time TS1 is performed. Since the detected voltage changes as the compensation detection time TS2 becomes shorter, the data voltage Vdata is adjusted to compensate for the change. To this end, the gradation voltage is adjusted or the gradation level of the input image data Data is adjusted.

Such compensation mode driving is performed during the compensation setting time. That is, the drive mode selection unit 300 determines whether the time that is driven in the compensation mode is equal to or shorter than the compensation setting time (S5). If the compensation setting time is not exceeded, the compensation mode is continuously operated. On the other hand, when the compensation setting time is exceeded, the compensation mode driving is stopped and the image comparison is performed again. Here, when the compensation setting time is exceeded, the count number CN can be initialized. When the compensation setting time passes, the organic electroluminescent device 100 is driven in the normal mode until the count number CN reaches the threshold value.

Therefore, after driving in the compensation mode, it is driven in the normal mode, so that the detection time returns to the normal detection time TS1. Then, the data voltage Vdata corresponding to the gradation level of the input video data Data is written in the pixel P.

As described above, in the embodiment of the present invention, when the same image is continuously inputted during the critical time, it is driven in the compensation mode. In this compensation mode, the driving time for detecting the threshold voltage is pulled forward as compared with the driving in the normal mode. Accordingly, when a detection error with respect to the threshold voltage occurs, such a detection error can be reduced. This makes it possible to improve the image quality such as afterimage due to the detection error when the image is changed after the same image is continuously input.

Furthermore, as the detection time is reduced, a voltage difference that is detected at the time of driving in the normal mode and at the compensation mode is generated, and a luminance difference of an image can be generated. Such a luminance difference of the image can be compensated by adjusting the data voltage.

Through the above-described method, the organic electroluminescent device according to the embodiment of the present invention can improve the image quality of a displayed image.

The embodiment of the present invention described above is an example of the present invention, and variations are possible within the spirit of the present invention. Accordingly, the invention includes modifications of the invention within the scope of the appended claims and equivalents thereof.

1 is a circuit diagram of a pixel of a conventional organic electroluminescent device.

2 is a diagram showing a phenomenon in which a threshold voltage of a driving transistor is shifted.

3 is a view schematically showing an organic electroluminescent device according to an embodiment of the present invention.

4 is a circuit diagram of a pixel of an organic electroluminescent device according to an embodiment of the present invention.

5 is a diagram illustrating waveforms of signals for driving an organic electroluminescent device according to an embodiment of the present invention.

6 is a diagram illustrating a driving mode selection unit of an organic electroluminescent device according to an embodiment of the present invention.

7 is a diagram illustrating a process of detecting a threshold voltage of a driving transistor when an organic light emitting device according to an exemplary embodiment of the present invention is driven in a normal mode and a compensation mode.

8 and 9 are diagrams illustrating a method of generating a data voltage of an organic electroluminescent device according to an embodiment of the present invention.

10 is a flow chart illustrating a method of driving an organic electroluminescent device according to an embodiment of the present invention.

Description of the Related Art

100: organic electroluminescent device 200: display part

300: drive mode selection unit 310:

320: scan driver 330:

340: gamma reference voltage generator

Claims (11)

A gate wiring and a data wiring crossing each other; A switching transistor connected to the gate line and the data line, a driving transistor receiving a data voltage through the switching transistor, an organic light emitting diode connected to the driving transistor, and a driving transistor for detecting a threshold voltage of the driving transistor. A pixel including a detection transistor connected between a drain electrode and a gate electrode; A drive mode selecting section for counting the number of frames in which the same image is successively input and selecting a compensation mode as a drive mode of the organic electroluminescent device when the count number is less than a threshold value and a compensation mode when the count value reaches a threshold value ; A scan driver for shortening a detection time of the threshold voltage in the compensation mode compared to the normal mode; A data driver for outputting to the data line a data voltage for compensating for a difference between threshold voltages detected in the normal mode and the compensation mode in the compensation mode, And an organic electroluminescent device. The method according to claim 1, Wherein the drive mode selection unit comprises: An image comparison unit for comparing whether the image of the previous frame and the image of the current frame are the same; And a counter for incrementing the number of counts by one when the image of the previous frame is identical to the image of the current frame Organic electroluminescent device. The method according to claim 1, Wherein the data driver further includes a DAC circuit for generating gray voltages using gamma reference voltages and outputting one of the gray voltages corresponding to the gray level of the input image data as a data voltage, The organic electroluminescent device further includes a gamma reference voltage generator for generating different gamma reference voltages in the normal mode and the compensation mode Organic electroluminescent device. The method according to claim 1, Wherein the data driver further includes a DAC circuit for generating gray voltages using gamma reference voltages and outputting one of the gray voltages corresponding to the gray level of the input image data as a data voltage, The organic electroluminescent device includes a gamma reference voltage generator for generating the same gamma reference voltage in the normal mode and the compensation mode; And a controller for outputting the image data input in the normal mode to the data driver, converting the gradation level of the image data input in the compensation mode, and outputting the converted gradation level to the data driver Organic electroluminescent device. The method according to claim 1, The pixel includes: A capacitor connected between a drain electrode of the switching transistor and a gate electrode of the driving transistor; And an initialization transistor connected to the gate electrode of the driving transistor for transferring an initialization voltage for initializing a voltage of a gate electrode of the driving transistor Organic electroluminescent device. Counting the number of frames in which the same image is continuously input; Selecting a normal mode when the number of counts is less than a threshold value and selecting a compensation mode as a driving mode of the organic electroluminescent device when the count value reaches a threshold value; Detecting a threshold voltage of the driving transistor in accordance with the driving mode; Applying a data voltage to a gate electrode of the driving transistor in which the detected threshold voltage is reflected according to the driving mode; And applying a light emission current to the organic light emitting diode according to the applied data voltage to emit the organic light emitting diode, The detection time of the threshold voltage in the compensation mode is shorter than that in the normal mode, The data voltage in the compensation mode is generated to compensate for the difference in the threshold voltages detected in the normal mode and the compensation mode A method of driving an organic electroluminescent device. The method according to claim 6, Wherein the step of applying a data voltage to a gate electrode of the driving transistor, in which the detected threshold voltage is reflected, Generating, in the data driver, gradation voltages using gamma reference voltages; Outputting one of the gradation voltages corresponding to a gradation level of the input image data as a data voltage in the data driver; Comprising the steps of: generating, in a normal mode and a compensation mode, different gamma reference voltages A method of driving an organic electroluminescent device. The method according to claim 6, Wherein the step of applying a data voltage to a gate electrode of the driving transistor, in which the detected threshold voltage is reflected, Generating, in the data driver, gradation voltages using gamma reference voltages; Outputting one of the gradation voltages corresponding to a gradation level of the input image data as a data voltage in the data driver; Generating a same gamma reference voltage in a normal mode and a compensation mode; And outputting the image data input in the normal mode to the data driver, converting the gradation level of the image data input in the compensation mode, and outputting the converted gradation level to the data driver A method of driving an organic electroluminescent device. The method according to claim 6, Wherein the compensation mode is performed during a compensation setting time after the count number reaches a threshold, After the compensation set time has elapsed, a step of counting the number of frames is newly performed A method of driving an organic electroluminescent device. The method according to claim 6, Wherein counting the number of frames comprises: If the image of the previous frame and the image of the current frame are the same, the count number is increased by one, When the image of the previous frame and the image of the current frame are different, the count number is initialized A method of driving an organic electroluminescent device. The method according to claim 6, Further comprising the step of applying an initialization voltage to the gate electrode of the driving transistor before the step of detecting the threshold voltage A method of driving an organic electroluminescent device.

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