KR102296403B1 - Electroluminescence display and driving method thereof - Google Patents

Abstract

The present invention relates to an electroluminescent display device and a driving method thereof, wherein data lines and gate lines intersect and a display panel in which a plurality of pixels are disposed, and a data voltage obtained by adding a black offset voltage and a gradation expression voltage to the data lines and a data driver supplying the data, and the gate driver supplying a scan signal synchronized with the data voltage to the gate lines. The black offset voltage changes as the driving time of the pixels elapses within the voltage margin of the black gray level without increasing the luminance.

Description

ELECTROLUMINESCENCE DISPLAY AND DRIVING METHOD THEREOF

The present invention relates to an electroluminescent display having a driving element for driving pixels.

The electroluminescent display is roughly classified into an inorganic light emitting display and an organic light emitting display according to the material of the light emitting layer. An active matrix type organic light emitting diode display includes an organic light emitting diode (hereinafter referred to as "OLED") that emits light by itself, and has a fast response speed and high luminous efficiency, luminance, and viewing angle. There are advantages.

Pixels of the organic light emitting diode display include an OLED and a driving element that drives the OLED by supplying a current to the OLED according to a gate-source voltage. An OLED of an organic light emitting display device includes an anode and a cathode, and an organic compound layer formed between the electrodes. The organic compound layer includes a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL) and an electron injection layer (Electron Injection layer, EIL). When a current flows through the OLED, holes passing through the hole transport layer (HTL) and electrons passing through the electron transport layer (ETL) move to the light emitting layer (EML) to form excitons, and as a result, the light emitting layer (EML) generates visible light. .

The driving device may be implemented as a TFT having a metal oxide semiconductor field effect transistor (MOSFET) structure. Although the driving device should have uniform electrical characteristics among all pixels, there may be differences between pixels due to process variations and device characteristics variations and may change with the lapse of display driving time. In order to compensate for the deviation in the electrical characteristics of the driving element, an internal compensation method and an external compensation method may be applied to the electroluminescent display device. The internal compensation method samples the gate-source voltage (Vgs) of the driving device, which varies according to the electrical characteristics of the driving device, and compensates the data voltage by the gate-source voltage. The external compensation method compensates for variations in electrical characteristics of the driving device between pixels by sensing a voltage of a pixel that changes according to the electrical characteristics of the driving device, and modulating input image data in an external circuit based on the sensed voltage.

As the display driving time increases, gate bias stress of the driving device may be accumulated, so that the threshold voltage of the driving device may be shifted to a negative voltage. The threshold voltage shift of the driving element may cause image quality deterioration such as an afterimage.

The present invention provides an electroluminescent display device capable of suppressing a shift in the threshold voltage of a driving element to negative polarity, and a driving method thereof.

The electroluminescent display device of the present invention includes a display panel in which data lines and gate lines are crossed and a plurality of pixels are disposed, a data driver supplying a data voltage obtained by adding a black offset voltage and a grayscale expression voltage to the data lines; and the gate driver supplying a scan signal synchronized with a data voltage to the gate lines. The black offset voltage changes as the driving time of the pixels elapses within the voltage margin of the black gray level without increasing the luminance.

Each of the pixels includes a plurality of sub-pixels. Each of the sub-pixels includes a driving device for driving the light emitting device.

The electroluminescent display device includes a sensing line connected to the driving element in each of the sub-pixels, and a threshold voltage of the driving element by converting the threshold voltage of the driving element into digital data in each of the sub-pixels by being connected to the sensing line. A predetermined failure management index indicating a threshold voltage of the driving element is calculated based on an analog-to-digital converter sensing A compensator for varying the black offset voltage when exceeding the voltage is further provided.

The failure management indicator is calculated within a power-off sequence.

The electroluminescent display further includes a compensator for counting the driving times of the pixels and varying the black offset voltage when the driving times reach a predetermined elapsed time.

The driving method of the electroluminescent display includes: supplying a data voltage obtained by adding a black offset voltage and a grayscale expression voltage to the data lines; supplying a scan signal synchronized with the data voltage to gate lines; and varying the black offset voltage as the driving time of the devices elapses. The black offset voltage changes as the driving time of the pixels elapses within the voltage margin of the black gray level without increasing the luminance.

According to the present invention, when the threshold voltage of the driving device is excessively shifted to the negative polarity, the black offset voltage (HCT) is varied within the margin of the data voltage without increasing the luminance of the black gray level to obtain the gate-source voltage (Vgs) of the driving device. by reducing the stress of the driving element. The stress relief of the driving element DT suppresses a shift in the negative polarity of the threshold voltage of the driving element DT, thereby preventing image quality deterioration due to changes over time and extending the lifespan of the pixels.

1 is a block diagram showing an electroluminescent display device according to an embodiment of the present invention.
2 is a circuit diagram illustrating a pixel circuit and a sensing path connected to the pixel circuit.
3 is a diagram illustrating a power-on sequence, a display driving period, and a power-off sequence.
4 is a diagram illustrating in detail an active section and a vertical blank section.
5 is a diagram illustrating an example in which a threshold voltage Vth of a driving element is shifted to a negative voltage.
6 is a diagram illustrating a horizontal crosstalk compensation voltage.
7 is a diagram illustrating an HCT range and a grayscale expression range in a data voltage.
8 is a diagram illustrating Vgs of a driving device without a luminance increase of a black gray scale.
9 is a diagram illustrating in detail the voltage for each section of the data voltage.
10 is a diagram illustrating an example of a Vth defect management criterion (Vth_LSL).
11 is a flowchart illustrating a method of driving an electroluminescent display device according to an embodiment of the present invention.
12 is a diagram illustrating an example in which a Vth defect management criterion (Vth_LSL) measured in real time in a power-off sequence is changed according to a driving time of pixels.
13 is a diagram illustrating a time point at which an HCT of a data voltage is varied.
14 is a diagram illustrating an example in which the HCT of the data voltage is varied based on the real-time measurement result of the Vth defect management standard (Vth_LSL).
15 is a flowchart illustrating a method of driving an electroluminescent display device according to another embodiment of the present invention.
16 is a diagram illustrating an example in which the HCT of the data voltage is varied according to the lapse of driving time of pixels.

Advantages and features of the present invention and methods of achieving them will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. The present invention is not limited to the embodiments disclosed below, but will be implemented in various different forms, only the embodiments allow the disclosure of the present invention to be complete, and those of ordinary skill in the art to which the present invention pertains It is provided to fully understand the scope of the invention, and the present invention is only defined by the scope of the claims.

Since the shapes, sizes, proportions, angles, numbers, etc. disclosed in the drawings for explaining the embodiments of the present invention are exemplary, the present invention is not limited to the matters shown in the drawings. Like reference numerals refer to substantially like elements throughout. In addition, in describing the present invention, if it is determined that a detailed description of a related known technology may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

When "includes", "includes", "having", "consisting of", etc. mentioned in this specification are used, other parts may be added unless 'only' is used. When a component is expressed in the singular, it may be construed as the plural unless specifically stated otherwise.

In interpreting the components, it is interpreted as including an error range even if there is no separate explicit description.

In the case of a description of the positional relationship, for example, when the positional relationship between two components is described as 'on One or more other elements may be interposed between those elements in which 'directly' or 'directly' are not used.

1st, 2nd, etc. may be used to distinguish the components, but the functions or structures of these components are not limited to the ordinal number or component name attached to the front of the component. For example, in the pixel circuit of FIG. 4 , ordinal numbers such as first, second, third, and fourth placed in front of the elements are sequentially charged to the data lines through the switch elements S1 to S4 based on the order in which they are sequentially charged. it will be pasted

The following embodiments can be partially or wholly combined or combined with each other, and technically various interlocking and driving are possible. Each of the embodiments may be implemented independently of each other or may be implemented together in a related relationship.

In the electroluminescent display device of the present invention, the pixel circuit includes a driving element and a switch element. The driving element and the switch element may be implemented with one or more transistors of an n-type TFT (NMOS) and a p-type TFT (PMOS). The transistor may be implemented as an oxide TFT having an oxide semiconductor pattern or an LTPS TFT having a low temperature poly-silicon (LTPS) semiconductor pattern. A TFT is a three-electrode device including a gate, a source, and a drain. The source is an electrode that supplies a carrier to the transistor. In the TFT, carriers start flowing from the source. The drain is an electrode through which carriers exit the TFT. In a TFT, the flow of carriers flows from source to drain. In the case of the n-type TFT, the source voltage is lower than the drain voltage so that electrons can flow from the source to the drain because carriers are electrons. In an n-type TFT, the direction of current flows from the drain to the source. In the case of a p-type TFT (PMOS), since a carrier is a hole, the source voltage is higher than the drain voltage so that holes can flow from the source to the drain. In a p-type TFT, since holes flow from the source to the drain, the current flows from the source to the drain. It should be noted that the source and drain of the TFT are not fixed. For example, the source and drain may be changed according to an applied voltage. Therefore, the invention is not limited by the source and drain of the TFT. In the following description, the source and drain of the TFT will be referred to as first and second electrodes.

The gate signal of the TFT used as the switch element swings between a gate on voltage and a gate off voltage. The gate-on voltage is set to a voltage higher than the threshold voltage of the TFT, and the gate-off voltage is set to a voltage lower than the threshold voltage of the TFT. The TFT is turned on in response to the gate-on voltage, while turned-off in response to the gate-off voltage. In the case of NMOS, the gate-on voltage may be a gate high voltage (VGH), and the gate-off voltage may be a gate low voltage (VGL). In the case of the PMOS, the gate-on voltage may be a gate low voltage VGL, and the gate-off voltage may be a gate high voltage VGH.

Hereinafter, various embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following embodiments, the electroluminescent display will be mainly described with respect to the organic light emitting display including the organic light emitting material. The technical spirit of the present invention is not limited to an organic light emitting display device, and may be applied to an inorganic light emitting display device including an inorganic light emitting material.

1 is a block diagram showing an electroluminescent display device according to an embodiment of the present invention. 2 is a circuit diagram illustrating a pixel circuit and a sensing path connected to the pixel circuit.

1 and 2 , an electroluminescent display device according to an embodiment of the present invention includes a display panel 100 and a display panel driving circuit.

The display panel 100 includes an active area AA for displaying an input image on the screen. A pixel array is disposed in the active area AA. The pixel array includes a plurality of data lines 102 , a plurality of gate lines 104 intersecting the data lines 102 , and pixels arranged in a matrix form.

Each of the pixels may be divided into a red sub-pixel, a green sub-pixel, and a blue sub-pixel to implement color. Each of the pixels may further include a white sub-pixel. Each of the sub-pixels 101 includes a pixel circuit.

Touch sensors may be disposed on the display panel 100 . The touch input may be sensed using separate touch sensors or may be sensed through pixels. The touch sensors may be implemented as in-cell type touch sensors arranged on the screen of a display panel or embedded in a pixel array as an on-cell type or an add-on type. can

The display panel driving circuits 110 , 112 , and 120 include a data driving unit 110 and a gate driving unit 120 . A demultiplexer 112 disposed between the data driver 110 and the data lines 102 may be disposed.

The display panel driving circuits 110 , 112 , and 120 write input image data into pixels of the display panel 100 under the control of a timing controller (TCON) 130 during the display driving period and input the data on the screen. display the image. The display panel driving circuit may further include a touch sensor driver for driving the touch sensors. The touch sensor driver is omitted from FIG. 1 . In a mobile device or a wearable device, the display panel driving circuit, the timing controller 130 and the power circuit may be integrated into one integrated circuit.

As shown in FIG. 2 , the data driver 110 gamma-maizes digital data of an input image received from the timing controller 130 in every frame period using a digital-to-analog converter (hereinafter referred to as DAC). The data voltage is output by converting it into a compensation voltage. A data voltage is applied to the pixels through a demultiplexer 112 and a data line 102 . The demultiplexer 112 is disposed between the data driver 110 and the data lines 102 using a plurality of switch elements to distribute the data voltage output from the data driver 110 to the data lines 102 . Since one channel of the data driver 110 is time-divisionally connected to a plurality of data lines by the demultiplexer 112 , the number of data lines 102 may be reduced.

The gate driver 120 may be implemented as a gate in panel (GIP) circuit formed directly on a bezel region of the display panel 100 together with the TFT array in the active region. The gate driver 120 outputs a gate signal to the gate lines 104 under the control of the timing controller 130 . The gate driver 120 may sequentially supply the gate signals to the gate lines 104 by shifting the gate signals using a shift register. The gate signal includes a scan signal SCAN and a sensing signal SENSE.

The scan signal SCAN is generated in the active period AT every frame period. The scan signal SCAN selects pixels to which the data voltage is applied in synchronization with the data voltage.

The sensing signal SENSE is generated in the sensing mode. The sensing mode is divided into before product shipment and after product shipment. After the threshold voltage of the driving element DT is sensed in each of the sub-pixels through a sensing path connected to the pixels before product shipment, the threshold voltage deviation in all sub-pixels is compensated based on the sensing result. In addition, the mobility of the driving element DT is sensed in each of the sub-pixels to compensate for the mobility deviation.

After shipment, the sensing mode is executed in a power ON sequence, a vertical blank period (VB), and a power OFF sequence. In the power-off sequence, the display panel driving circuit and the sensing path are further driven for a preset delay time after receiving the power-off signal to sense the threshold voltage (Vth) of the driving element in each of the sub-pixels, and then the preset Vth failure management standard (hereinafter referred to as “Vth_LSL”) is calculated. The Vth defect management standard is basically used as one of the defect management indicators of the display panel. In the present invention, the Vth defect management criterion is also used for indirectly estimating the deterioration level of pixels according to the lapse of time, in particular, a phenomenon in which the threshold voltage of the driving element DT is shifted to negative polarity. The Vth failure management reference voltage is the standard deviation (σ) allowed as the Vth failure management reference in the distribution of the threshold voltage (Vth) of the driving elements sensed in all sub-pixels in the screen, for example, 4.5σ as in the example of FIG. 10 . If satisfied, the Vth value may be calculated, but is not limited thereto. For example, the shift of the threshold voltage Vth may be estimated by using other direct or indirect indicators showing an example in which the threshold voltage Vth of the driving element is shifted to a negative polarity.

The sensing signal SECSE selects pixels in which electrical characteristics of the driving element DT formed in the pixels are sensed. Electrical characteristics of the driving device include mobility (μ) and a threshold voltage (Vth). The active period is a time during which data of one frame is written to all pixels on the screen. The vertical blank period VB is allocated for a predetermined time between the N-1 th active period and the N th active period. During the vertical blank period VB, next frame data (N-th frame data) is not received by the timing controller 130 .

The timing controller 130 receives digital video data DATA of an input image and a timing signal synchronized therewith from a host system (not shown). The timing signal includes a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, a clock signal DCLK, and a data enable signal DE. The host system may be any one of a television (Television) system, a set-top box, a navigation system, a personal computer (PC), a home theater system, a mobile device, and a wearable device.

The timing controller 130 may adjust the frame rate to be higher than the input frame frequency in the normal driving mode. For example, the timing controller 130 multiplies the input frame frequency by i times to set the frame frequency × i (i is a positive integer greater than 0) Hz for the operation timing of the display panel drivers 110 , 112 , and 120 . can be controlled. The frame frequency is 60 Hz in the NTSC (National Television Standards Committee) scheme and 50 Hz in the PAL (Phase-Alternating Line) scheme. The timing controller 130 may lower the frame frequency to a frequency between 1 Hz and 30 Hz in order to lower the refresh rate of pixels in the low power consumption driving mode.

The timing controller 130 controls the operation timing of the demultiplexer 112 and a data timing control signal for controlling the operation timing of the data driver 110 based on the timing signals Vsync, Hsync, DE received from the host system. The operation timing of the display panel driving circuits 110 , 112 , and 120 is controlled by generating a switch control signal for controlling a switch control signal for a sensing path, a switch element control signal for a sensing path, and a gate timing control signal for controlling an operation timing of the gate driver 120 . The voltage level of the gate timing control signal output from the timing controller 130 may be converted into a gate-on voltage and a gate-off voltage through a level shifter (not shown) and supplied to the gate driver 120 . The level shifter converts a low level voltage of the gate timing control signal into a gate low voltage VGL and converts a high level voltage of the gate timing control signal into a gate high voltage VGH. .

Referring to FIG. 2 , a sensing path may be connected to the pixel circuit. The sensing path may include a REF line (or sensing line) 103, an analog to digital converter (hereinafter referred to as “ADC”), and first and second switch elements (M3, M4), and the like. have. The sensing path may sense the source voltage of the driving element DT to sense electrical characteristics of the driving element. The first switch element M3 supplies a predetermined reference voltage Vref to the REF line 103 to initialize the source voltage of the driving element DT to the reference voltage Vref. The second switch element M4 is turned on after the first switch element M3 is turned off to supply the source voltage of the driving element DT to the ADC. The ADC converts the analog sensing voltage into digital sensing data and transmits it to the compensator 131 . The source voltage of the driving element DT may indicate a threshold voltage or mobility of the driving element DT according to a sensing method. A method of sensing the threshold voltage of the driving element DT through the sensing path or a method of sensing the mobility of the driving element DT through the sensing path may use a known sensing method. The ADC may be integrated in an integrated circuit (IC) of the data driver 110 together with the DAC.

The compensation unit 131 stores compensation values for compensating for the threshold voltage Vth and the mobility μ of the driving element in each of the sub-pixels. The compensator 131 selects a preset compensation value according to the digital sensing data received through the ADC and compensates the pixel data by adding or multiplying the compensation value to the pixel data (digital data) of the input image. The compensated pixel data is transmitted to the data driver 110 , converted into a data voltage Vdata by the DAC of the data driver 110 , and supplied to the data line 102 . The driving element DT of the pixel circuit is driven by the data voltage Vdata supplied through the data line 102 to generate a current. The current flowing through the driving element DT to the OLED, which is the light emitting element, is determined according to the gate-source voltage Vgs of the driving element DT. The compensator 131 may be implemented as an arithmetic circuit in the timing controller 130 .

The compensator 131 may add a black offset value set within a black offset margin of the data voltage Vdata to the pixel data. The black offset value is added to the pixel data within a range in which the luminance of the black grayscale is not increased. The black offset value may be varied according to the Vth defect management criterion measured based on the threshold voltage shift amount of the driving element DT sensed in real time in the power-off sequence. Accordingly, the compensator 131 calculates a predetermined Vth failure management criterion indicating the threshold voltage Vth of the driving element DT based on the digital data received from the ADC of the sensing path, and the Vth failure management criterion is When a predetermined compensation range is exceeded, the black offset voltage of the data voltage Vdata output from the data driver 110 may be varied.

Pixel data input to the data driver 110 may be obtained by adding a threshold voltage compensation value and a black offset value of the driving device, and may be multiplied by a mobility compensation value of the driving device. The data driver 110 converts the pixel data received from the timing controller 130 into an analog data voltage Vdata. In this data voltage Vdata, a block offset voltage is a voltage of a black offset value added to pixel data, and a grayscale voltage is a voltage of a grayscale value of pixel data.

As in the example of FIG. 2 , the pixel circuit includes an OLED, a driving element DT connected to the OLED, a plurality of switch elements M1 and M2 , and a capacitor Cst. The driving element and the switch element may be implemented as TFTs having an NMOS or PMOS structure. It should be noted that the pixel circuit is not limited to FIG. 2 .

The OLED emits light with a current controlled by the driving elements DT according to the data voltage Vdata. The OLED includes an organic compound layer formed between an anode and a cathode. The organic compound layer may include, but is not limited to, a hole injection layer (HIL), a hole transport layer (HTL), a light emitting layer (EML), an electron transport layer (ETL), and an electron injection layer (EIL). The anode of the OLED is connected to the driving element DT through the second node n2, and the cathode of the OLED is connected to the VSS electrode to which the low potential power voltage VSS is applied. The storage capacitor Cst is connected between the gate and the source of the driving device DT through the first and second nodes n1 and n2 . In FIG. 2, “Coled” is the parasitic capacitance of the OLED.

The first switch element M1 is turned on in response to the scan signal SCAN to supply the data voltage Vdata to the gate of the driving element DT connected to the first node n1. The first switch element S1 has a gate connected to the first gate line 1041 to which the first scan signal SCAN is applied, a first electrode connected to the data line 102 , and a first node connected to the first node n1 . Includes 2 electrodes.

The second switch element M2 is turned on in response to the sensing signal SENSE to supply the reference voltage Vref to the second node n2 . The second switch element M2 has a gate connected to the second gate line 1042 to which the sensing signal SENSE is applied, a first electrode connected to the REF line 103 to which the reference voltage Vref is applied, and a second node. and a second electrode connected to (n2).

The driving element DT controls the current of the OLED according to Vgs. The driving element DT includes a gate connected to the first node n1 , a first electrode to which the pixel driving voltage VDD is supplied, and a second electrode connected to the anode of the OLED through the second node n2 .

The capacitor Cst is connected between the first node n1 and the second node n2. The voltage of the capacitor Cst is Vgs of the driving element DT.

Fig. 3 is a view showing a power ON sequence, a display driving period, and a power OFF sequence. Fig. 4 is a view showing details of an active section and a vertical blank section.

3 and 4 , the power-on sequence ON is started after the display power is turned on. In the power-on sequence 0N, the display panel driving circuit and driving voltages of the display panel are generated and the display panel driving circuit is initialized. Mobility of the driving element is sensed during the power-on sequence (ON) and the vertical blank period of the display driving period, and the mobility deviation is compensated with a preset mobility compensation value. The mobility of the driving element DT is sensed in the vertical blank section VB of the power-on sequence ON and the display driving period, and a mobility compensation value is updated based on the sensing result. During the display driving period, pixel data written to the pixels is updated every frame period to display an image on the screen.

The power-off sequence (OFF) is started after an off signal of the display power is received, and the threshold voltage (Vth) of each of the sub-pixels is sensed during a delay time during which the display panel driving circuit and the sensing path are additionally driven, and the result is displayed. It ends when the power output of the display panel driving circuit and the sensing path stops after Vth_LSL is calculated. The threshold voltage compensation value is updated based on the sensing result of the threshold voltage sensed in real time in the power-off sequence. Vth_LSL is calculated according to the threshold voltage sensing result of the driving device formed in all sub-pixels sensed in the power-off sequence OFF. When the threshold voltages of the driving elements are shifted to the negative polarity, Vth_LSL is lowered. Calculation of Vth_LSL may be processed by an arithmetic circuit of the timing controller 130 .

The vertical synchronization signal Vsync defines one frame period. The horizontal synchronization signal Hsync defines one horizontal time period. The data enable signal DE defines an effective data section including pixel data to be displayed on the screen.

The data enable signal DE is synchronized with valid data to be displayed on the pixel array of the display panel 100 . One pulse period of the data enable signal DE is one horizontal period, and a high logic period of the data enable signal DE indicates a data input timing of one pixel line. One horizontal period is a time required to write data to pixels of one pixel line in the display panel 100 .

The timing controller 130 receives the data enable signal DE and the data of the input image during the vertical active period AT. There is no data enable signal DE and no data of the input image in the vertical blank section VB. During the active period AT, data corresponding to one frame to be written in all pixels is received by the timing controller 130 . One frame period is the sum of the active periods AT and the vertical blank period VB.

As can be seen from the data enable signal DE, input data is not received by the display device during the vertical blank period VB. The vertical blank section VB includes a vertical sync time (VS), a vertical front porch (FP), and a vertical back porch (BP). The vertical sync time (VS) is a time from a falling edge of Vsync to a rising edge, and represents the start (or end) timing of one screen. The vertical front porch FP is the time from the falling edge of the last DE indicating the last line data timing of one frame data to the start of the vertical blank period VB. The vertical back porch BP is a time from the end of the vertical blank period VB to the rising edge of the first DE indicating the first line data timing of one frame data.

Due to the accumulation of stress in the driving element DT, the threshold voltage Vth may be shifted to a negative voltage as in the example of FIG. 5 . In this case, when the driving element DT is driven for a long time, the luminance of the pixels is changed at the same gray level because the amount of current flowing to the light emitting element is changed due to the shift of the threshold voltage.

When the threshold voltage of the driving element DT is shifted to a negative voltage, the change in the threshold voltage may be compensated by lowering the data voltage Vdata. For example, when the threshold voltage Vth is initially 0V and the data voltage Vdata is 3V, the threshold voltage Vth is shifted to a negative polarity according to a change with time and is reduced to Vth = -1V. ) is lowered to 2V to compensate for the change in threshold voltage. However, since the compensation voltage margin range is fixed in the output voltage range of the data driver 110 , the change in the threshold voltage is not compensated if the compensation voltage margin range is out of this range. For example, when the lowest voltage of the compensation voltage margin range is 2V, when the threshold voltage (Vth) of the driving element is shifted to negative polarity and lowered to Vth = -1.5V, the change in the threshold voltage is not compensated, so the pixels are driven normally. doesn't happen

In the present invention, the threshold voltage of the driving element DT is sensed in each of the sub-pixels, and Vth_LSL is calculated based on the sensing result. Vth_LSL is lowered when the threshold voltage of the driving element DT is shifted to a negative voltage due to a prolonged use of the product. According to the present invention, the Vgs of the driving element DT is adjusted to be small by increasing the black offset voltage (hereinafter referred to as “HCT”) of the data voltage Vdata within a range in which the luminance of the black gray level does not increase when Vth_LSL is changed. The stress of the driving element DT is relieved. The black offset voltage may be selected as a voltage that does not cause horizontal crosstalk (HCT) within a black offset range in which the luminance of the black gray level does not increase in the output voltage range of the data driver, but is not limited thereto.

When the HCT is increased, the Vgs of the driving element DT is reduced in the black grayscale, so that the stress of the driving element DT is relieved. The stress relief of the driving element DT suppresses a shift in the negative polarity of the threshold voltage of the driving element DT, thereby preventing image quality deterioration due to changes over time and extending the lifespan of the pixels.

The black offset range is a voltage range in which the luminance of the black gray scale is set to a voltage range in which the luminance of the black gray scale does not increase in the output voltage range of the data driver 110 so that the data voltage Vdata can be varied without increasing the luminance of the black gray scale. Accordingly, even if the HCT is varied within the black offset range, the luminance of the black gray level does not change.

6 is a diagram illustrating a horizontal crosstalk compensation voltage.

Referring to FIG. 6 , a test image that may cause horizontal crosstalk may include a white background and a black region in the center. The black region may be displayed as a box with a black gray scale.

On the display panel 100 , parasitic capacitances exist in an overlap region where the data line 102 , the REF line 103 , and the gate line 104 intersect. Due to the parasitic capacitance, when the image shown in FIG. 6 is displayed on the screen, horizontal crosstalk in which luminance is lowered in the black area and left and right white areas may be seen. When the voltage difference of the data voltage Vdata between the white region and the black region is reduced, the parasitic capacitance value is reduced. The present invention prevents horizontal crosstalk by raising the HCT in the black region to prevent horizontal crosstalk. In addition, according to the present invention, when the threshold voltage of the driving element is out of a predetermined range, the HCT is increased to relieve the stress by reducing the Vgs of the driving element without increasing the luminance of the black grayscale.

7 is a diagram illustrating an HCT range and a grayscale expression range in a data voltage. 8 is a diagram illustrating Vgs of a driving device without a luminance increase of a black gray scale. 9 is a diagram illustrating in detail the voltage for each section of the data voltage Vdata. 10 is a diagram illustrating an example of Vth_LSL.

As shown in FIG. 7 , the data voltage includes a grayscale expression range and a black offset range lower than the grayscale expression range. Since the HCT is selected within the black offset range, it is a voltage that does not increase the luminance of the black gradation. The maximum rising voltage of the HCT is lower than the minimum gray level expression voltage in the gray level expression range as shown in FIG. 8 . The minimum grayscale expression voltage is a black grayscale voltage corresponding to the minimum grayscale value of pixel data. In FIG. 8 , “Black Vgs” is Vgs of the driving element DT in the black gray scale. In the present invention, when the Vth_LSL measured in the power-off sequence is out of the preset LSL compensation range, the stress of the driving device is relieved by increasing the HCT to reduce the Vgs of the driving device.

As shown in FIG. 9 , the data voltage may include a first compensation section lower than the grayscale expression range and a second compensation section higher than the grayscale expression range.

The first compensation section includes a black offset range that is a variable range of the HCT, a compensation voltage margin range (NBTiS Margin), and an initial Vth compensation range. The initial Vth compensation range is a compensation voltage margin set to compensate for the threshold voltage of the driving device before shipment.

The compensation voltage margin range NBTiS Margin is set as a voltage section between the black offset range and the gray scale expression range. When the threshold voltage Vth of the driving device is shifted to the negative polarity, the data voltage Vdata is reduced within the compensation voltage margin range NBTiS Margin, so that the shift in the threshold voltage of the driving device DT may be compensated. However, when the threshold voltage Vth of the driving element DT is shifted in the negative polarity direction to the extent that it deviates from the compensation voltage margin range NBTiS Margin, the data voltage Vdata cannot be lowered any longer, so the threshold voltage shift is not compensated. . In the present invention, Vth_LSL is measured based on the threshold voltage shift of the driving element in each of the sub-pixels sensed in real time in the power-off sequence, and when Vth_LSL changes larger than a preset compensation range, the threshold voltage (Vth) of the driving element DT The HCT is varied by estimating the time of shift to negative polarity to the extent that it is out of the compensation voltage margin range (NBTiS Margin).

The second compensation section of the data voltage Vdata includes a mobility compensation range, a temporal Vth compensation range, and an OLED afterimage compensation range. The mobility compensation range is a voltage margin set to compensate for a difference in the mobility of the driving device measured in real time in the power-on sequence and the display driving period. The temporal Vth compensation range is a voltage margin set to compensate for the change in the threshold voltage when the threshold voltage of the driving element is shifted to a positive polarity. The OLED afterimage compensation range is a voltage margin for compensating for the afterimage due to parasitic capacitance of the OLED. The present invention provides a compensation method when the threshold voltage of a driving element is shifted to negative polarity. Therefore, it should be noted that the second compensation period of the data voltage is not essential in the present invention.

The present invention senses the threshold voltage Vth of the driving element DT in all sub-pixels in the power-off sequence as shown in FIG. Measure Vth_LSL. When the difference between the previously measured value and the currently measured Vth_LSL, that is, the change in Vth_LSL is greater than the preset LSL compensation range, the HCT in the data voltage Vdata is increased, and the Vgs of the driving element is reduced in the black gray scale without increasing the luminance. If the LSL compensation range is set from 0V to -0.5V, the HCT may rise when Vth_LSL changes to a voltage lower than -0.5V.

The data voltage Vdata output from the data driver 110 is NBTiS Margin + (Vth - Vth_LSL) + HCT + grayscale voltage. Here, Vth is the threshold voltage of the driving element DT. Vth - Vth_LSL is a voltage within the compensation voltage margin range (NBTiS Margin) and the initial Vth compensation range. HCT is a voltage that varies within a black offset range according to Vth_LSL. The grayscale voltage is a voltage within the grayscale expression range and is determined according to the grayscale value of the pixel data. When the real-time measured Vth_LSL in the power-off sequence is out of the preset LSL compensation range, the HCT in the data voltage Vdata rises. When the present invention is applied to a field emission display device, since the HCT of the data voltage changes according to an increase in driving time, the black gray voltage without increasing the luminance of the black gray in the data voltage is changed.

The reference voltage Vref may be determined to be 2.5V - LSL. Here, LSL is the initial Vth_LSL measured before product shipment. When LSL is -0.5V, the reference voltage Vref is Vref = 2.5V - LSL = 2.5-(-0.5V) = 3V. In the present invention, the reference voltage Vref is fixed regardless of Vth_LSL measured in the power-off sequence.

11 is a flowchart illustrating a method of driving an electroluminescent display device according to an embodiment of the present invention.

Referring to FIG. 11 , when the power switch of the electroluminescent display device is turned on, driving power for the display panel driving circuit is generated from the power supply circuit in the power-on sequence, so that the pixels are in a display driving state ( S01 and S02 ). In the display driving state, when an input image is received by the timing controller 130 , pixel data of the input image is written into pixels by the display panel driving circuit to display the input image on the screen ( S03 and S04 ).

In the display driving period, the mobility of the driving device DT is sensed in real time in the vertical blank section VB for every frame period, and pixel data is modulated based on the sensing result to compensate for the mobility of the driving device DT. There is (S05).

When the power switch of the electroluminescent display device is turned off, a power-off signal is generated and a power-off sequence is started (S06). In the power-off sequence, the display panel driving circuit and the sensing path are driven for a preset delay time, so that the threshold voltage Vth of the driving element DT is sensed in real time in each of the sub-pixels. In the power-off sequence, the compensator 131 measures Vth_LSL from the threshold voltage distribution of the sensed driving element (S07).

When Vth_LSL is out of the preset LSL compensation range, the compensation unit 131 changes the HCL (S08 and S09). When Vth_LSL is out of the preset LSL compensation range, the HCT of the data voltage Vdata rises to decrease Vgs of the driving element. Accordingly, according to the present invention, when the threshold voltage Vth of the driving element is excessively shifted to the negative polarity, the black gray voltage of the data voltage Vdata in which the luminance of the black gray level in the pixel is not increased is changed.

After Vth_LSL is measured, since power to the display panel driving circuit is not generated, the pixels are changed to the display-off state (S10).

12 is a diagram illustrating an example in which Vth_LSL measured in real time in a power-off sequence varies according to driving times of pixels. 13 is a diagram illustrating a time point at which the HCT of the data voltage Vdata is changed.

12 and 13 , the Vth_LSL measured in the power-off sequence tends to decrease as the driving time of the pixels increases. When Vth_LSL is out of the preset LSL compensation range, the HCT of the data voltage Vdata is changed. When the HCT increases, the black gray voltage changes without increasing the luminance of the black gray scale and Vgs (Black Vgs) of the driving element DT decreases.

14 is a diagram illustrating an example in which HCT of a data voltage is varied based on a real-time measurement result of Vth_LSL.

Referring to FIG. 14 , the initial values of Vth, Vth_LSL, and Black Vgs when the product is shipped are as follows.

(1) Initial value

Vth_LSL = Vth = 0V

Black Vgs - Vth = Vg - Vs - Vth = -0.5V

Vg = HCT - Vth_LSL = 2.5V

Vs = Vref = 3V

Here, Vg is the gate voltage of the driving device in the black gray scale, that is, the voltage of the first node n1 in FIG. 2 . Vs is the source voltage of the driving element DT, that is, the voltage of the second node n2 in FIG. 2 .

When the driving time of the pixels elapses, the threshold voltage of the driving element DT of the sub-pixels is shifted to a negative polarity. When the driving time of the pixels reaches lapse 1, Vth_LSL and Black Vgs change as follows.

(2) Oversight 1

Vth LSL = Vth = -0.5V

Black Vgs - Vth = Vg - Vs - Vth = 2V - 3V -(-0.5V) = -0.5V

Vg = HCT - Vth_LSL = 2.5V - 0.5V = 2V

Vs = Vref = 3V

When the driving time of the pixels elapses, the threshold voltage of the driving element DT of the sub-pixels is shifted to a negative polarity. When the driving time of the pixels reaches lapse 2, Vth_LSL is out of the LSL compensation range and HCT rises to 2.6V. At this time, Vth_LSL and Black Vgs change as follows. At this time, Black Vgs is reduced to -0.4V.

(3) Oversight 2

Vth_LSL = Vth = -0.6V

Black Vgs - Vth = - Vg - Vs - Vth = 2V - 3V -(-0.6V) = -0.4V

Vg = HCT - Vth_LSL = 2.6V - 0.6V = 2V

Vs = Vref = 3V

15 is a flowchart illustrating a method of driving an electroluminescent display device according to another embodiment of the present invention. 16 is a diagram illustrating an example in which an HCT of a data voltage is varied according to the lapse of driving time of pixels. In this embodiment, the HCT is varied when the display driving time reaches a preset time without sensing the threshold voltage of the driving element.

15 and 16 , when the power switch of the electroluminescent display device is turned on, driving power for the display panel driving circuit is generated from the power supply circuit in the power-on sequence, so that the pixels are in the display driving state (S11 and S12). ). In the display driving state, when an input image is received by the timing controller 130 , pixel data of the input image is written into pixels by the display panel driving circuit to display the input image on the screen. During the display driving period, the compensator 131 counts the driving times of the pixels, accumulates the count values and stores them in the memory (S13).

When the driving time of the pixels reaches the preset lapse 1, the compensator 131 increases the HCT by the preset rising width (S14 and S15). When the driving time of the pixels further elapses to reach the lapse of time 2, the compensator 131 further increases the HCT by a preset rising width.

When the power switch of the electroluminescent display is turned off, a power-off signal is generated and a power-off sequence is started (S16). In the power-off sequence, since power to the display panel driving circuit is not generated, the pixels change to the display-off state (S17).

Those skilled in the art from the above description will be able to see that various changes and modifications can be made without departing from the technical spirit of the present invention. Accordingly, 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.

100: display panel 110: data driver
120: gate driver 130: timing controller
131: compensation part DT: driving element
M1~M4 : switch element

Claims (10)

a display panel in which data lines and gate lines cross and a plurality of pixels are disposed;
a data driver supplying a data voltage obtained by adding a black offset voltage and a gradation expression voltage to the data lines; and
a gate driver supplying a scan signal synchronized with the data voltage to gate lines;
An electroluminescent display device in which the black offset voltage changes as the driving time of the pixels elapses within a margin of the data voltage without increasing the luminance of the black gray scale. The method of claim 1,
Each of the pixels includes a plurality of sub-pixels,
Each of the sub-pixels includes a driving device for driving the light emitting device. 3. The method of claim 2,
a sensing line connected to the driving element in each of the sub-pixels;
an analog-to-digital converter connected to the sensing line to convert the threshold voltage of the driving element into digital data in each of the sub-pixels to sense the threshold voltage of the driving element; and
calculating a predetermined failure management index indicating a threshold voltage of the driving element based on the digital data received from the analog-to-digital converter, and varying the black offset voltage when the failure management index exceeds a predetermined compensation range An electroluminescent display device further comprising a compensator. 4. The method of claim 3,
An electroluminescent display device in which the failure management index is calculated within a power-off sequence. 3. The method of claim 2,
and a compensator for counting the driving time of the pixels and varying the black offset voltage when the driving time reaches a predetermined elapsed time. A method of driving an electroluminescence display including a display panel in which a plurality of pixels intersecting data lines and gate lines are disposed, the method comprising:
supplying a data voltage obtained by adding a black offset voltage and a grayscale expression voltage to the data lines;
supplying a scan signal synchronized with the data voltage to gate lines; and
varying the black offset voltage as the driving time of the pixels elapses;
A method of driving an electroluminescence display in which the black offset voltage is changed as driving time of the pixels elapses within a margin of the data voltage without increasing the luminance of the black grayscale. 7. The method of claim 6,
Each of the pixels includes a plurality of sub-pixels,
A driving method of an electroluminescent display device, wherein each of the sub-pixels includes a driving element for driving a light emitting element. 8. The method of claim 7,
The step of varying the black offset voltage as the driving time of the pixels elapses may include:
sensing a threshold voltage of the driving device through a sensing path connected to the driving device;
calculating a predetermined defect management index from a threshold voltage distribution of driving elements obtained in each of the sub-pixels; and
and a compensator configured to vary the black offset voltage when the defect management index exceeds a predetermined compensation range. 9. The method of claim 8,
A method of driving an electroluminescence display in which the failure management index is calculated within a power-off sequence. 8. The method of claim 7,
The step of varying the black offset voltage as the driving time of the pixels elapses may include:
counting the driving times of the pixels; and
and varying the black offset voltage when the driving time reaches a predetermined elapsed time.

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