Organic light emitting display, device for sensing threshold voltage of driving TFT in organic light emitting display, and method for sensing threshold voltage of driving TFT in organic light emitting display

A device for sensing a threshold voltage of a driving TFT in an organic light emitting display includes a data drive circuit and a timing controller. The data drive circuit applies a data voltage to a gate node of the driving TFT during a first programming period, determines a source node voltage of the driving TFT as a first sensing voltage during a first sensing period in which a gate-source voltage is constant and higher than the threshold voltage, applies another data voltage to the gate node during a second programming period, and determines the source node voltage as a second sensing voltage during a second sensing period in which the gate-source voltage is constant and higher than the threshold voltage. The timing controller calculates a ratio between the first and second sensing voltages, and obtains a change in the threshold voltage using a change in the ratio.

This application claims the priority benefit of Korean Patent Application No. 10-2015-0093654 filed on Jun. 30, 2015, which is hereby incorporated herein by reference for all purposes as if fully set forth herein.

BACKGROUND

Field of the Invention

The present invention relates to an organic light emitting display, and more particularly, to an organic light emitting display including a device for sensing the threshold voltage of a driving TFT and a method for sensing the threshold voltage of a driving TFT in an organic light emitting display.

Discussion of the Related Art

An active-matrix organic light emitting display comprises organic light emitting diodes (OLEDs) that are self-luminous (i.e., emit light themselves). An active-matrix organic light emitting display has advantages including fast response time, high luminous efficiency, high luminance, and wide viewing angle. An OLED comprises an anode and a cathode, as well as organic compound layers HIL, HTL, EML, ETL, and EIL formed between the anode and cathode. The organic compound layers comprise a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL. When an operating voltage is applied to the anode and the cathode, a hole passing through the hole transport layer HTL and an electron passing through the electron transport layer ETL move to the emission layer EML, thereby forming an exciton. As a result, the emission layer EML generates visible light.

In an organic light emitting diode display, pixels each comprising an organic light emitting diode are arranged in a matrix, and the luminance of the pixels is adjusted based on the grayscale of video data. Each individual pixel comprises a driving TFT (thin-film transistor) that controls the drive current flowing through the OLED. The electrical characteristic of the driving TFT, such as threshold voltage, mobility, etc., may vary from pixel to pixel because of the process condition, driving environment, etc. Such variation in the electrical characteristics of the driving TFT causes luminance differences between the pixels. As a solution to this problem, a technology that senses the characteristic parameters (threshold voltage, mobility, etc.) of the driving TFT of each pixel and corrects image data based on the sensing results is known.

In the related art, as shown inFIG. 1, a driving TFT DT is operated according to a source follower method, and then the source node voltage Vs of the driving TFT DT is detected as a sensing voltage Vsen at the time to when the gate-source voltage Vgs of the driving TFT DT reaches saturation state by an electric current flowing through the driving TFT DT, thereby sensing a change in the threshold voltage Vth of the driving TFT DT. However, a long period of time is needed for the gate-source voltage Vgs of the driving TFT DT to reach the threshold voltage Vth of the driving TFT DT. Accordingly, in the related art, it is not possible to sense a change in the threshold voltage Vth of the driving TFT DT during real-time operation.

SUMMARY

Accordingly, the present invention is directed to an organic light emitting display, a device for sensing a threshold voltage of a driving TFT in an organic light emitting display, and a method for sensing a threshold voltage of a driving TFT in an organic light emitting display that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.

An object of the present invention is to provide a device and method for sensing the threshold voltage of a driving TFT in an organic light emitting display so that a change in the threshold voltage of the driving TFT is sensed during real-time operation by reducing sensing time.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a device for sensing a threshold voltage of a driving TFT in an organic light emitting display comprises a data drive circuit configured to apply a first data voltage to a gate node of the driving TFT during a first programming period, determine a source node voltage of the driving TFT as a first sensing voltage during a first sensing period in which a gate-source voltage of the driving TFT is held constant at a first value higher than the threshold voltage of the driving TFT, apply a second data voltage to the gate node of the driving TFT during a second programming period, and determine the source node voltage of the driving TFT as a second sensing voltage during a second sensing period in which the gate-source voltage of the driving TFT is held constant at a second value higher than the threshold voltage of the driving TFT; and a timing controller configured to calculate a sensing ratio based on a ratio between the first and second sensing voltages, calculate a change in the sensing ratio by comparing the calculated sensing ratio with a predetermined initial sensing ratio, and then obtain a change in the threshold voltage of the driving TFT based on the change in the sensing ratio.

In another aspect, a method for sensing threshold voltage of a driving TFT in organic light emitting display comprises applying a first data voltage for sensing to a gate node of the driving TFT during a first programming period; determining a source node voltage of the driving TFT as a first sensing voltage during a first sensing period in which a gate-source voltage of the driving TFT is held constant at a first value higher than the threshold voltage of the driving TFT; applying a second data voltage to the gate node of the driving TFT during a second programming period; determining the source node voltage of the driving TFT as a second sensing voltage during a second sensing period in which the gate-source voltage of the driving TFT is held constant at a second value higher than the threshold voltage of the driving TFT; calculating a sensing ratio based on a ratio between the first and second sensing voltages; calculating a change in sensing ratio by comparing the calculated sensing ratio with a predetermined initial sensing ratio; obtaining a change in the threshold voltage of the driving TFT based on the change in sensing ratio; and adjusting image data output from the data drive circuit to a pixel driven by the driving TFT in the organic light emitting display device based on the change in the threshold voltage to correct an amount of light emitted by the pixel.

In another aspect, an organic light emitting display comprises a display panel including a plurality of pixels, each pixel having an organic light emitting diode (OLED) to emit light and a driving TFT to control an amount of light emitted by the OLED; a data drive circuit configured to apply a first data voltage to a gate node of the driving TFT during a first programming period, determine a source node voltage of the driving TFT as a first sensing voltage during a first sensing period in which a gate-source voltage of the driving TFT is held constant at a first value higher than the threshold voltage of the driving TFT, apply a second data voltage to the gate node of the driving TFT during a second programming period, and determine a source node voltage of the driving TFT as a second sensing voltage during a second sensing period in which the gate-source voltage of the driving TFT is held constant at a second value higher than the threshold voltage of the driving TFT; and a timing controller configured to calculate a sensing ratio based on the ratio between the first and second sensing voltages, calculate a change in the sensing ratio by comparing the calculated sensing ratio with a predetermined initial sensing ratio, and then obtain a change in the threshold voltage of the driving TFT based on the change in the sensing ratio, wherein the data drive circuit is configured to adjust image data output from the data drive circuit to a pixel driven by the driving TFT based on the change in the threshold voltage to correct an amount of light emitted by the pixel.

DETAILED DESCRIPTION

Hereinafter, example embodiments of the present invention will be described in detail with reference to the accompanying drawings. Throughout the specification, like numbers refer to like elements. In describing the present invention, when it is deemed that a detailed description of known functions or configurations may unnecessarily obscure the subject matter of the present invention, the detailed description will be omitted

FIG. 2is a view schematically showing an organic light emitting display according to an example embodiment of the present invention.FIG. 3is a view showing an example of the configuration of a pixel array and a data driver IC.FIG. 4is a view showing the principle for deriving a change in the threshold voltage of the driving TFT based on a sensing ratio.

As shown inFIGS. 2 and 3, an organic light emitting display according to an example embodiment of the present invention may comprise a display panel10, a timing controller11, a data drive circuit12, a gate drive circuit13, and a memory16. A plurality of data lines and sensing lines14A and14B and a plurality of gate lines15intersect each other on the display panel10, and pixels P are arranged in a matrix at the intersections. The gate lines15comprise a plurality of first gate lines15A sequentially supplied with a scan control signal (SCAN ofFIG. 5) and a plurality of second gate lines15B sequentially supplied with a sensing control signal (SEN ofFIG. 5).

Each pixel P may be connected to any one of the data lines14A, any one of the sensing lines14B, any one of the first gate lines15A, and any one of the second gate lines15B. Each pixel P may be connected to a data line14A in response to a scan control signal SCAN input through a first gate line15A, and may be connected to a sensing line14B in response to a sensing control signal SEN input through a second gate line15B.

Each pixel P is supplied with a high-level operating voltage ELVD and a low-level operating voltage ELVSS from a power generator (not shown). Each pixel P may comprise an OLED and a driving TFT that drives the OLED. The driving TFT may be implemented as p-type or n-type. Also, a semiconductor layer of the driving TFT may comprise amorphous silicon, polysilicon, or oxide.

Each pixel P displays an image, and may operate differently in an image display operation for internally compensating for a change in the mobility of the driving TFT and in a compensation operation for sensing and compensating for a change in the threshold voltage of the driving TFT. The compensation operation may be performed for a predetermined amount of time during power-on or power-off. Particularly, the compensation operation may reduce the time taken to sense a change in the threshold voltage of the driving TFT by a method to be described later. Thus, it is possible to sense a change in the threshold voltage of the driving TFT during vertical blanking intervals of a real-time operation, that is, image display operation.

The image display operation and the compensation operation may be implemented depending on the operation of the data drive circuit12and gate drive circuit13under the control of the timing controller11.

The data drive circuit12comprises at least one data driver IC (integrated circuit) SDIC. The data driver IC (SDIC) may comprise a plurality of digital-to-analog converters (DAC)121connected to data lines14A, a plurality of sensing units122connected to sensing lines14B, a MUX123that selectively connects the sensing units122to an analog-to-digital converter (ADC), and a shift register124that generates a selection control signal and sequentially turns on switches SS1to SSk in the MUX123.

In the compensation operation, the DACs121generate a data voltage for sensing and supply it to the data lines14A, under the control of the timing controller11. In the image display operation, the DACs121generate a data voltage for image display and supply it to the data lines14A, under the control of the timing controller11.

The sensing units SU#1to SU#k may be connected to the sensing lines14B on a one-to-one basis. The sensing units SU#1to SU#k may supply a reference voltage to the sensing lines14B or read a sensing voltage stored in the sensing lines14B and supply it to the ADC, under the control of the timing controller11. The ADC converts a sensing voltage selectively input through the MUX123to a digital value and transmits it to the timing controller11.

The gate drive circuit13may generate a scan control signal corresponding to the image display operation or compensation operation and then supply it to the first gate lines15A line by line, under the control of the timing controller11. The gate drive circuit13generates a sensing control signal corresponding to the image display operation or compensation operation and then supplies it to the second gate lines15B line by line, under the control of the timing controller11.

The timing controller11generates a data control signal DDC for controlling the operation timing of the data drive circuit12and a gate control signal GDC for controlling the operation timing of the gate drive circuit131based on timing signals, such as a vertical synchronization signal Vsync, a horizontal synchronization signal Hsync, a dot clock signal DCLK, and a data enable signal DE. The timing controller11may differentiate between the image display operation and the compensation operation based on a predetermined reference signal (a driving power enable signal, a vertical synchronization signal, a data enable signal, etc.), and generate a data control signal DDC and gate control signal GDC corresponding to each of the image display operation and compensation operation. Moreover, the timing controller11may further generate relevant switching control signals CON (including PRE and SAM ofFIG. 5) to operate internal switches in each sensing unit SU#1to SU#k for the image display operation and compensation operation.

As shown inFIG. 4, the timing controller11obtains a first sensing voltage Vsen1and a second sensing voltage Vsen2by sensing a change in the threshold voltage of the driving TFT twice for each pixel, and obtains a change in the threshold voltage of the driving TFT based on the sensing ratio VSR between the first and second sensing voltages Vsen1and Vsen2. InFIG. 4, Vsen1_init indicates a first initial sensing voltage of when a first data voltage for sensing is applied, and Vsen2_init indicates a second initial sensed voltage of when a second data voltage for sensing is applied. VSRinit is an initial sensing ratio, which is equal to the first initial sensing voltage Vsen1_init divided by the second initial sensing voltage Vsen2_init. The initial sensing ratio VSRinit may vary depending on the product model and specification, and is preset at the time of product release and stored in the internal memory of the display device.

When there is a change in the threshold voltage of the driving TFT due to driving stress, different sensing data voltages may be applied to each pixel, and the source node voltage of the driving TFT may be acquired as the first and second sensing voltages while the gate-source voltage of the driving TFT is higher than the threshold voltage of the driving TFT. The first and second sensing voltages comprise a change in the mobility of the driving TFT, as well as a change in the threshold voltage of the driving TFT. Thus, by calculating the sensing ratio between the first and second sensing voltages, the change in the mobility of the driving TFT commonly included in the first and second sensing voltages may be canceled out, and only the change in the threshold voltage of the driving TFT may be obtained. In the related art, the source node voltage of the driving TFT is sensed at the timing when the gate-source voltage of the driving TFT is saturated at the threshold voltage of the driving TFT. This means that the sensing requires a very long time, making it impossible to sense a change in the threshold voltage of the driving TFT during a vertical blanking interval in the image display operation. However, if the sensing is done while the gate-source voltage of the driving TFT is higher than the threshold voltage of the driving TFT, as in example embodiments of the present invention, the total time taken for the sensing is reduced to 1/10 as compared to that of the related art even if the sensing is done twice. Accordingly, a change in the threshold voltage of the driving TFT can be adequately sensed during the vertical blanking interval in the image display operation.

In the compensation operation, the timing controller11calculates an nth sensing ratio (n is a positive integer) based on the ratio between the first and second sensing voltages, calculates a change in sensing ratio by comparing the nth sensing ratio with a preset initial sensing ratio, and then obtains a change in the threshold voltage based on the change in sensing ratio. The timing controller11may properly update an (n−1)th compensation value stored in the memory16based on the obtained threshold voltage change.

In the compensation operation, the timing controller11may transmit first and second compensation data corresponding to the first and second data voltage for sensings to the data drive circuit12. Here, the first and second compensation data reflects the change in the threshold voltage of the driving TFT that was sensed in the previous sensing period. In the image display operation, the timing controller11may transmit image data RGB corresponding to the image display data voltage. Here, the image data RGB may be modulated in such a way as to compensate for the change in the threshold voltage of the driving TFT that was sensed in the previous sensing period.

FIG. 5shows detailed configurations of a pixel and a sensing unit according to an example embodiment of the present invention.FIG. 6shows the compensation of a change in the mobility of the driving TFT according to an example embodiment of the present invention.FIGS. 7A and 7Bshow a process of sensing a change in the threshold voltage of the driving TFT according to an example embodiment of the present invention.FIG. 8shows that the change in the threshold voltage of the driving TFT appears as the difference in slope between the curves in the TFT linear region.

With reference toFIG. 5, a pixel P may comprise an OLED, a driving TFT (thin film transistor) DT, a storage capacitor Cst, a first switching TFT ST1, and a second switching TFT ST2.

The OLED comprises an anode connected to a source node Ns, a cathode connected to an input terminal of a low-level operating voltage EVSS, and an organic compound layer positioned between the anode and the cathode.

The driving TFT DT controls the amount of current input into the OLED based on a gate-source voltage Vgs. The driving TFT DT comprises a gate electrode connected to a gate node Ng, a drain electrode connected to an input terminal of a high-level operating voltage EVDD, and a source electrode connected to the source node Ns. The storage capacitor Cst is connected between the gate node Ng and the source node Ns to maintain the gate-source voltage Vgs of the driving TFT DT. The first switching TFT ST1applies a sensing data voltage Vdata on a data line14A to the gate node Ng in response to a scan control signal SCAN. The first switching TFT ST1comprises a gate electrode connected to the first gate line15A, a drain electrode connected to the data line14A, and a source electrode connected to the gate node Ng. The second switching TFT ST2switches on an electrical connection between the source node Ns and a sensing line14B in response to a sensing control signal SEN. The second switching TFT ST2comprises a gate electrode connected to a second gate line15B, a drain electrode connected to the sensing line14B, and a source node connected to the source node Ns.

Also, a sensing unit SU may comprise a reference voltage control switch SW1, a sampling switch SW2, and a sample and hold circuit S/H. The reference voltage control switch SW1is switched on in response to a reference voltage control signal PRE to connect an input terminal of a reference voltage Vref and the sensing line14B. The sampling switch SW2is switched on in response to a sampling control signal SAM to connect the sensing line14B and the sample and hold circuit S/H. When the sampling switch SW2is turned on, the sample and hold circuit S/H samples and holds the source node voltage Vs of the driving TFT DT stored in a line capacitor LCa of the sensing line14B as a sensing voltage Vsen and then passes it to an ADC. Here, a parasitic capacitor present in the sensing line14B may be substituted for the line capacitor LCa.

An image display operation for internally compensating for a change in the mobility of the driving TFT will be described below in conjunction with an example configuration of such a pixel andFIG. 6. When a compensation value corresponding to a threshold voltage change is obtained in the compensation operation for sensing a change in threshold voltage, the image display operation is performed based on an image display data voltage reflecting the compensation voltage. A change in the mobility of the driving TFT is not compensated for in the compensation operation but compensated for in the image display operation. Accordingly, in the image display operation, an image is displayed, with the compensation of the changes in both the threshold voltage and mobility of the driving TFT.

The image display operation comprises a initial period Ti, a sensing period Ts, and an emission period Te. During the image display operation, the reference voltage control switch SW1remains ON to apply the reference voltage Vref to the sensing line14B, and the sampling switch SW2remains OFF.

In the initial period Ti, both the scan control signal SCAN and the sensing control signal SEN remain ON. The first switching TFT ST1is turned on in response to the scan control signal SCAN of ON state to apply an image display data voltage to the gate electrode of the driving TFT DT, and the second switching TFT ST2is turned on in response to the sensing control signal SEN of ON state and applies a reference voltage Vref to the source electrode of the driving TFT DT.

In the sensing period Ts, the scan control signal SCAN remains ON, and the sensing control signal SEN is inverted to OFF. The first switching TFT ST1remains ON and holds the voltage at the gate node Ng of the driving TFT DT at the image display data voltage. The second switching TFT ST2is turned off, whereupon a current corresponding to a gate-source voltage difference Vgs, which is set in the initial period Ti, flows through the driving TFT DT. Accordingly, the voltage at the source node Ns of the driving TFT DT rises toward the image display data voltage applied to the gate electrode of the driving TFT DT according to a source-follower method so that the gate-source voltage difference Vgs of the driving TFT DT is programmed to a desired gray level.

In the emission period Te, both the scan control signal SCAN and the sensing control signal SEN remain OFF. The voltage at the gate node Ng of the driving TFT DT and the voltage at the source node Ns rise to a voltage level equal to or higher than the threshold voltage of the OLED while maintaining the voltage difference Vgs programmed in the sensing period Ts, and then maintain this voltage level. A drive current corresponding to the programmed gate-source voltage difference Vgs of the driving TFT DT flows through the OLED. As a result, the OLED emits light, thereby representing a desired gray level.

As such, a change in the mobility of the driving TFT DT is compensated for based on the principle that the source voltage Vs of the driving TFT DT is raised by capacitive coupling while the gate voltage Vg of the driving TFT DT is fixed at the image display data voltage during the sensing period Ts. The drive current, which determines the light intensity (luminance) of the pixel, is proportional to the mobility μ of the driving TFT DT and the gate-source voltage difference Vgs of the driving TFT DT programmed in the sensing period Ts. During the sensing period Ts, in the case of a pixel with high mobility μ, the source voltage Vs of the driving TFT DT rises at a first rate of rise toward the higher gate voltage Vg so that the gate-source voltage difference Vgs of the driving TFT DT is programmed to be relatively small. On the contrary, during the sensing period Ts, in the case of a pixel with low mobility μ, the source voltage Vs of the driving TFT DT rises at a second rate of rise (which is slower than the first rate of rise) toward the higher gate voltage Vg so that the gate-source voltage difference Vgs of the driving TFT DT is programmed to be relatively large. That is, the gate-source voltage is automatically programmed to be inversely proportional to the degree of mobility. As a result, luminance variations are compensated for differences in mobility μ between pixels.

A compensation operation for compensating a change in the threshold voltage of the driving TFT will be described below in conjunction with the above-described example configuration of a pixel andFIGS. 7A and 7BandFIG. 8.

A compensation operation comprises a first process for obtaining a first sensing voltage Vsen1during a first compensation period SP1shown inFIG. 7A, and a second process for obtaining a second sensing voltage Vsen2during a second compensation period SP2shown inFIG. 7B. Here, the first compensation period SP1and the second compensation period SP2may be placed consecutively within one vertical blanking interval or separately in different vertical blanking intervals.

As shown inFIG. 7, the first compensation period SP1may comprise a first programming period T2, a first sensing period T4, and a first sampling period T5. The first compensation period SP1may further comprise a first source node initial period T3in order to increase sensing accuracy. InFIG. 7A, “T1” is a first sensing line initial period for resetting the sensing line14B to a reference voltage Vref in advance before the first programming period T2, and may be omitted.

In the first programming period T2, a scan control signal SCAN, sensing control signal SEN, and reference voltage signal PRE are all input as ON. In the first programming period T2, the first switching TFT ST1is turned on to apply a first data voltage for sensing Vdata1′ to the gate node Ng of the driving TFT DT, and the second switching TFT ST2and the reference voltage control switch SW1are turned on to apply the reference voltage Vref to the source node Ns of the driving TFT DT. As a result, the gate-source voltage Vg of the driving TFT DT is programmed to a first level LV1. Here, the first data voltage for sensing Vdata1′ reflects a threshold voltage component Vth(n−1) of the previous sensing period.

In the first source node initial period T3, the scan control signal SCAN is inverted to OFF, and the sensing control signal SEN and the reference voltage control signal PRE remain ON. In the first source node initial period T3, the first switching TFT ST1is turned off to make the gate node Ng of the driving TFT DT float, and the second switching TFT ST2and the reference voltage control switch SW1are turned on to constantly apply the reference voltage Vref to the source node Ns of the driving TFT DT. As a result, the source node Ns of the driving TFT DT is reset for the second time to the reference voltage Vref while the gate-source voltage Vgs of the driving TFT DT is held at the first level LV1. The reason why the source node Ns of the driving TFT DT is reset for the second time to the reference voltage Vref is because sensing accuracy can be increased by making the voltage at the start point of the first sensing period T4equal for all pixels.

In the first sensing period T4, the scan control signal SCAN is held at OFF level, the sensing control signal SEN is held at ON level, and the reference voltage control signal PRE is inverted to OFF level. In the first sensing period T4, the first switching TFT ST1is turned off to keep the gate node Ng of the driving TFT DT floating, and the reference voltage control switch SW1is turned off to disconnect the source node Ns of the driving TFT DT from an input of the reference voltage Vref. In this state, a pixel current flows through the driving TFT DT by the gate-source voltage Vg of the first level LV1, and the source node voltage Vs of the driving TFT DT rises due to this pixel current. The source node voltage Vs of the driving TFT DT is stored in the line capacitor LCa of the sensing line14B by the turned-on second switching TFT ST2.

In the first sampling period T5, the sensing control signal SEN is inverted to OFF level, and the sampling control signal SAM is input as ON level. In the first sampling period T5, the second switching TFT ST2is turned off to release the electrical connection between the source node Ns of the driving TFT DT and the sensing line14B. Also, the sampling control switch SW2is turned on to connect the sensing line14B and the sample and hold circuit S/H, thereby sampling the source node voltage Vs of the driving TFT DT stored in the sensing line14B as the first sensing voltage Vsen1. The first sensing voltage Vsen1is converted to a first digital value by an ADC and then stored in an internal latch in the data drive circuit12.

As shown inFIG. 7B, the second compensation period SP2may comprise a second programming period T2′, a second sensing period T4′, and a second sampling period T5′. The second compensation period SP2may further comprise a second source node initial period T3′ in order to increase sensing accuracy. InFIG. 7B, “T1” is a second sensing line initial period for resetting the sensing line14B to a reference voltage Vref in advance before the second programming period T2′, and may be omitted.

In the second programming period T2′, a scan control signal SCAN, sensing control signal SEN, and reference voltage signal PRE are all input as ON. In the second programming period T2′, the first switching TFT ST1is turned on to apply a second data voltage for sensing Vdata2′ to the gate node Ng of the driving TFT DT, and the second switching TFT ST2and the reference voltage control switch SW1are turned on to apply the reference voltage Vref to the source node Ns of the driving TFT DT. As a result, the gate-source voltage Vg of the driving TFT DT is programmed to a second level LV2. Here, the second data voltage for sensing Vdata2′ reflects a threshold voltage component Vth(n−1) of the previous sensing period.

In the second source node initial period T3′, the scan control signal SCAN is inverted to OFF, and the sensing control signal SEN and the reference voltage control signal PRE remain ON. In the second source node initial period T3′, the first switching TFT ST1is turned off to make the gate node Ng of the driving TFT DT float, and the second switching TFT ST2and the reference voltage control switch SW1are turned on to keep applying the reference voltage Vref to the source node Ns of the driving TFT DT. As a result, the source node Ns of the driving TFT DT is reset for the second time to the reference voltage Vref while the gate-source voltage Vgs of the driving TFT DT is held at the second level LV2. The reason why the source node Ns of the driving TFT DT is reset for the second time to the reference voltage Vref is because sensing accuracy can be increased by making the voltage at the start point of the second sensing period T4′ equal for all pixels.

In the second sensing period T4′, the scan control signal SCAN is held at OFF level, the sensing control signal SEN is held at ON level, and the reference voltage control signal PRE is inverted to OFF level. In the second sensing period T4′, the first switching TFT ST1is turned off to keep the gate node Ng of the driving TFT DT floating, and the reference voltage control switch SW1is turned off to disconnect the source node Ns of the driving TFT DT from an input of the reference voltage Vref. In this state, a pixel current flows through the driving TFT DT by the gate-source voltage Vg of the second level LV2, and the source node voltage Vs of the driving TFT DT rises due to this pixel current. The source node voltage Vs of the driving TFT DT is stored in the line capacitor LCa of the sensing line14B by the turned-on second switching TFT ST2.

In the second sampling period T5′, the sensing control signal SEN is inverted to OFF level, and the sampling control signal SAM is input as ON level. In the second sampling period T5′, the second switching TFT ST2is turned off to release the electrical connection between the source node Ns of the driving TFT DT and the sensing line14B. Also, the sampling control switch SW2is turned on to connect the sensing line14B and the sample and hold circuit S/H, thereby sampling the source node voltage Vs of the driving TFT DT stored in the sensing line14B as the first sensing voltage Vsen1. The second sensing voltage Vsen2is converted to a second digital value by an ADC and then stored in an internal latch in the data drive circuit12.

The first and second sensing voltages Vsen1and Vsen2stored as digital values in the internal latch are transmitted to the timing controller11. The timing controller11calculates the sensing ratio VSR between the first and second sensing voltages Vsen1and Vsen2, and reads a change ΔVth in the threshold voltage of the driving TFT DT from a look-up table by using a change in sensing ratio—which is obtained by subtracting the sensing ratio VSR from a preset initial sensing ratio VSRinit)—as a read address.

In accordance with example embodiments of the present invention, a change in the threshold voltage of the driving TFT may be accurately sensed by canceling out a change in the mobility of the driving TFT commonly included in the first and second sensing voltages by using a sensing ratio VSR. Further, a threshold voltage change ΔVth may be determined by a change in sensing ratio VSR. Even for pixels having driving TFTs with the same mobility, a change in the threshold voltage Vth of the driving TFT is represented as a difference in slope between the curves in the TFT linear region in which Vgs is lower than Vth. Also, the voltage values in the TFT linear region may be sensed to reduce the time taken for the sensing.

Moreover, in example embodiments of the present invention, since a change in mobility is linearly and internally compensated for during the image display operation, accurate and fast sensing may be done in the TFT linear region during the compensation operation. In cases where fast sensing is done as discussed above without linearly compensating for a change in mobility, a sensing voltage comprises the change in mobility as well as a change in threshold voltage, and the change in mobility has a much greater effect on the sensing voltage, thereby making it possible to precisely detect a change in threshold voltage.

FIG. 9shows a method for sensing a change in the threshold voltage of the driving TFT according to an example embodiment of the present invention.FIG. 10shows a vertical blanking interval in one frame during which a change in the threshold voltage of the driving TFT is sensed.

With reference toFIG. 9, first and second sensing voltages are obtained by fast sensing in the TFT linear region, and a change in the threshold voltage of the driving TFT is obtained based on the sensing ratio between the sensing voltages. Thus, a number of processes for deducing a change in threshold voltage, such as programming, source node resetting, sensing, and sampling, may be performed during the vertical blanking interval. That is, it is possible to sense a change in the threshold voltage of the driving TFT DT during real-time operation, without the need of arranging a time during power-on or power-off to sense a threshold voltage change, thereby improving compensation performance.

Here, the vertical blanking interval indicates the time between active intervals for image display during which data for image display is not written, as illustrated inFIG. 10. During the vertical blanking interval, a data enable signal DE continues to remain at low logic level L. When the data enable signal DE is at low logic level, data writing is paused.