Patent Publication Number: US-8988329-B2

Title: Organic light emitting diode display device and method for sensing characteristic parameters of pixel driving circuits

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2011-0142040, filed on Dec. 26, 2011, which is hereby incorporated by reference as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an organic light emitting diode (OLED) display device, and, more particularly, to the OLED display device and a method for sensing characteristic parameters of pixel driving circuits, which are capable of correcting non-uniformity of luminance through simple and rapid sensing of the characteristic parameters. 
     2. Discussion of the Related Art 
     An active matrix organic light emitting diode (AMOLED) display device is a self-luminous device in which an organic light emitting layer emits light through re-combination of electrons and holes. Since the AMOLED display device exhibits high luminance, and employs a low driving voltage while having an ultra-slim structure, it is expected to be a next-generation display device. 
     Such an AMOLED display device includes a plurality of pixels, each of which includes an organic light emitting diode (OLED) constituted by an anode, a cathode, and an organic light emitting layer interposed between the anode and the cathode, and a pixel driving circuit for independently driving the OLED. The pixel driving circuit mainly includes a switching thin film transistor (hereinafter, referred to as a “TFT”), a capacitor, and a driving TFT. The switching TFT charges the capacitor with a voltage corresponding to a data signal in response to a scan pulse. The driving TFT controls the amount of current supplied to the OLED in accordance with the level of the voltage charged in the capacitor, to adjust the amount of light emitted from the OLED. The amount of light emitted from the OLED is proportional to the amount of current supplied from the driving TFT to the OLED. 
     In such an AMOLED display device, however, TFT characteristics such as driving TFT threshold voltage Vth and process tolerance factors (e.g., mobility, parasitic capacitance, and channel width/length) are non-uniform among pixels due to process tolerances. For this reason, non-uniformity of luminance may occur in the AMOLED display device. To solve this problem, a data compensation method is employed. In accordance with this data compensation method, the characteristic parameters of the driving TFT in each pixel driving circuit are measured, and input data is adjusted, based on the result of the sensing. 
     The characteristics of the driving TFT may be measured through sensing of amounts of current flowing through the corresponding pixel at different voltages. For an AMOLED display device having an increased size, however, it is more difficult to rapidly measure amounts of current flowing through a number of pixels. For example, U.S. Pat. No. 7,834,825 discloses a method for sensing an amount of current flowing through a power line (a VDD or VSS line) of an OLED panel while turning on pixels one by one. However, this method has a problem in that there is a difficulty in achieving rapid sensing because the current sensing time is delayed due to parasitic capacitors present in parallel on the power line to achieve increased resolution. 
     Furthermore, in conventional cases, the system for sensing the characteristics of the driving TFT is complex. For this reason, after shipment, it is difficult to measure and compensate the characteristics of the driving TFT. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an OLED display device and method for sensing characteristic parameters of pixel driving circuits in an organic light emitting diode display device that substantially obviate one or more problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an OLED display device and method for sensing characteristic parameters of pixel driving circuits, which are capable of correcting non-uniformity of luminance through simple and rapid sensing of the characteristic parameters. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an OLED display device includes a display panel including a plurality of pixels each having a light emitting element and a pixel driving circuit for independently driving the light emitting element, and a characteristic parameter detecting unit for sensing characteristic parameters of the pixel driving circuit in each of the plural pixels, the characteristic parameter detecting unit driving the pixel driving circuit of one of the plural pixels, which is a sensing pixel, sensing a voltage discharged in accordance with characteristics of a driving thin film transistor (TFT) in the pixel driving circuit of the sensing pixel, on a data line connected to the pixel driving circuit of the sensing pixel, among data lines connected to respective pixel driving circuits of the pixels, and detecting a threshold voltage (Vth) of the driving TFT and a deviation of a process characteristic parameter (k-parameter) of the driving TFT, using the measured voltage. 
     The characteristic parameter detecting unit may include a data driver for driving the data line, sensing a voltage on the data line, and outputting the measured voltage, and a timing controller for detecting the threshold voltage (Vth) and the k-parameter deviation, based on the measured voltage from the data driver, calculating an offset value to compensate the detected threshold voltage (Vth) and a gain value to compensate for the detected k-parameter deviation, storing the calculated offset value and the calculated gain value, compensating input data by use of the stored offset value and the stored gain value, and supplying the compensated input data to the data driver. 
     The timing controller may detect the threshold voltage (Vth) by calculating a difference voltage between the measured voltage from the data driver and a reference voltage supplied to the pixel driving circuit of the sensing pixel. 
     The timing controller may detect the k-parameter deviation by detecting a variation in the voltage discharged in accordance with the characteristics of the driving TFT in the sensing pixel, based on the measured voltage from the data driver, and calculating a ratio of the detected voltage variation in the sensing pixel to a predetermined or previously-detected voltage variation in a reference one of the pixels. 
     The pixel driving circuit may include the driving TFT, which drives the light emitting element, a first switching TFT for supplying the voltage on the data line to a first node of the driving TFT in response to a first scan signal from a scan line, a second switching TFT for supplying a reference voltage from a reference voltage line to a second node of the driving TFT in response to a second scan signal from the scan line, and a storage capacitor for charging a voltage between the first node and the second node, and supplying the charged voltage as a driving voltage for the driving TFT. 
     The data driver may supply a pre-charge voltage to the data line, then measure the voltage on the data line at a time when the driving TFT is driven in a saturated state in accordance with the discharge of the pre-charge voltage from the data line through driving of the first and second switching TFTs, and output the measured voltage. The timing controller may detect the threshold voltage (Vth) by calculating a difference voltage between the measured voltage from the data driver and a reference voltage supplied to the pixel driving circuit of the sensing pixel. 
     A first reference voltage may be supplied to the reference voltage line. The data driver may supply a pre-charge voltage to the data line, then measure the voltage on the data line at a plurality of times when the driving TFT is driven in a saturated state in accordance with the discharge of the pre-charge voltage from the data line through driving of the first and second switching TFTs, and output the measured voltages as first measured voltages. A second reference voltage different from the first reference voltage may be supplied to the reference voltage line. The data driver may supply the pre-charge voltage to the data line, then measure the voltage on the data line at the plurality of times when the driving TFT is driven in the saturated state in accordance with the discharge of the pre-charge voltage from the data line through the driving of the first and second switching TFTs, and output the measured voltages as second measured voltages. The timing controller may detect the threshold voltage (Vth) by detecting a time when a difference voltage between corresponding ones of the first and second measured voltages output from the data driver is equal or similar to a difference voltage between the first and second reference voltages, and then calculating a difference voltage between the first measured voltage measured at the detected time and the first reference voltage or a difference voltage between the second measured voltage measured at the detected time and the second reference voltage. 
     In a programming period, the data driver may supply, to the data line, a sum of a data voltage compensated for the detected threshold voltage (Vth) and the reference voltage, and the driving TFT is driven in accordance with the driving of the first and second switching TFTs. In a pre-charging period following the programming period, the data driver may pre-charge the data line with the pre-charge voltage, and the first and second switching TFTs are turned off. In a discharging period following the pre-charging period, the data driver may be disconnected from the data line, and the pre-charge voltage on the data line is discharged through the first switching TFT and the driving TFT. At a sensing time corresponding to the sensing time or each of the sensing times and following the discharging period, the first switching TFT may be turned off, and the data driver may measure the voltage on the data line, and outputs the measured voltage. The timing controller may detect the k-parameter deviation by calculating a difference voltage between the pre-charge voltage and the voltage measured at the sensing time, to detect a voltage variation in the sensing pixel, and calculating a ratio of the voltage variation in the sensing pixel to a voltage variation in reference to one of the pixels. 
     The data driver may include a plurality of digital-analog converters (DACs) for converting input data into analog data voltages by channels, respectively, a plurality of sampling/holder circuits respectively connected to the data lines by channels, each of the sampling/holder circuits sampling a voltage on a corresponding one of the data lines, and holding and outputting the sampled voltage as the measured voltage, an analog to digital converter (ADC) for converting the measured voltage from each of the sampling/holder circuits into digital data, and outputting the digital data, and a plurality of first switches connected between the DACs and the data lines by channels, respectively, to switch respective output voltages from the DACs. 
     The data driver may further include a multiplexer/scaler connected between the sampling/holder circuits and the ADC. The multiplexer/scaler may select and scale a plurality of measured voltages from the sampling/holder circuits by groups, and output the scaled voltages to the ADC, each group including at least one measured voltage. The ADC may be equal, in number, to output channels of the multiplexer/scaler. 
     The data driver may further include second switches that pre-charge voltages to respective output channels of the DACs. 
     In another aspect of the present invention, a method for sensing characteristic parameters of pixel driving circuits in an OLED display device including a plurality of pixels each including a light emitting element and a corresponding one of the pixel driving circuits to independently drive the light emitting element includes the steps of driving the pixel driving circuit of one of the plural pixels, which is a sensing pixel, sensing a voltage discharged in accordance with characteristics of a driving thin film transistor (TFT) in the pixel driving circuit of the sensing pixel, on a data line connected to the pixel driving circuit of the sensing pixel, among data lines connected to respective pixel driving circuits of the pixels, and detecting a threshold voltage (Vth) of the driving TFT, using the measured voltage, and driving the pixel driving circuit of the sensing pixel, using data voltage compensated for the detected threshold voltage (Vth), sensing a voltage discharged in accordance with the characteristics of the driving TFT, on the data line, and detecting a k-parameter deviation of the driving TFT, based on the measured voltage. 
     The step of detecting the threshold voltage (Vth) may include the step of calculating a difference voltage between the measured voltage and a reference voltage supplied to the pixel driving circuit of the sensing pixel, to detect the threshold voltage (Vth). 
     The step of detecting the k-parameter variation may include the step of detecting a variation in the voltage discharged in accordance with the characteristics of the driving TFT in the sensing pixel, based on the measured voltage, and calculating a ratio of the detected voltage variation in the sensing pixel to a predetermined or previously-detected voltage variation in a reference one of the pixels. 
     The pixel driving circuit may include the driving TFT, which drives the light emitting element, a first switching TFT for supplying the voltage on the data line to a first node of the driving TFT in response to a first scan signal from a scan line, a second switching TFT for supplying a reference voltage from a reference voltage line to a second node of the driving TFT in response to a second scan signal from the scan line, and a storage capacitor for charging a voltage between the first node and the second node, and supplying the charged voltage as a driving voltage for the driving TFT. The step of detecting the threshold voltage (Vth) may include the steps of supplying a pre-charge voltage to the data line, and then sensing the voltage on the data line at a time when the driving TFT is driven in a saturated state in accordance with the discharge of the pre-charge voltage from the data line through driving of the first and second switching TFTs, and calculating a difference voltage between the measured voltage and the reference voltage, to detect the threshold voltage (Vth). 
     The pixel driving circuit may include the driving TFT, which drives the light emitting element, a first switching TFT for supplying the voltage on the data line to a first node of the driving TFT in response to a first scan signal from a scan line, a second switching TFT for supplying a reference voltage from a reference voltage line to a second node of the driving TFT in response to a second scan signal from the scan line, and a storage capacitor for charging a voltage between the first node and the second node, and supplying the charged voltage as a driving voltage for the driving TFT. The step of detecting the threshold voltage (Vth) may include the steps of supplying a first reference voltage to the reference voltage line, supplying a pre-charge voltage to the data line, sensing the voltage on the data line at a plurality of times when the driving TFT is driven in a saturated state in accordance with discharge of the pre-charge voltage from the data line through driving of the first and second switching TFTs, and outputting the measured voltages as first measured voltages, supplying a second reference voltage different from the first reference voltage to the reference voltage line, supplying the pre-charge voltage to the data line, sensing the voltage on the data line at the plurality of times when the driving TFT is driven in the saturated state in accordance with the discharge of the pre-charge voltage from the data line through the driving of the first and second switching TFTs, and outputting the measured voltages as second measured voltages, and detecting a time when a difference voltage between corresponding ones of the first and second measured voltages output from the data driver is equal or similar to a difference voltage between the first and second reference voltages, and calculating a difference voltage between the first measured voltage measured at the detected time and the first reference voltage or a difference voltage between the second measured voltage measured at the detected time and the second reference voltage, to detect the threshold voltage (Vth). 
     The step of detecting the k-parameter deviation may include the steps of supplying, in a programming period, a sum of a data voltage compensated for the detected threshold voltage (Vth) and the reference voltage to the data line, and driving the driving TFT in accordance with the driving of the first and second switching TFTs, pre-charging, in a pre-charging period following the programming period, the data line with the pre-charge voltage, and turning off the first and second switching TFTs, floating the data line in a discharging period following the pre-charging period, and discharging the pre-charge voltage on the data line through the first switching TFT and the driving TFT, turning off the first switching TFT at a sensing time, which corresponds to the sensing time or each of the sensing times and follows the discharging period, and sensing the voltage on the data line, calculating a difference voltage between the pre-charge voltage and the voltage measured at the sensing time, to detect a voltage variation in the sensing pixel, and calculating a ratio of the voltage variation in the sensing pixel to a voltage variation in a reference one of the pixels, to detect the k-parameter deviation. 
     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and along with the description serve to explain the principle of the invention. In the drawings: 
         FIG. 1  is a circuit diagram illustrating an active matrix organic light emitting diode (AMOLED) display device having a function of sensing characteristic parameters of pixel driving circuits in accordance with an exemplary embodiment of the present invention; 
         FIGS. 2A and 2B  are circuit diagrams illustrating sequential steps of a method for sensing a threshold voltage Vth of each pixel driving circuit in accordance with a first embodiment of the present invention; 
         FIG. 3  is a graph depicting variation in output voltage on a data line according to passage of time in the case of  FIGS. 2A and 2B ; 
         FIGS. 4A and 4B  are circuit diagrams illustrating sequential steps of a method for sensing a threshold voltage Vth of each pixel driving circuit in accordance with a second embodiment of the present invention; 
         FIG. 5  is a graph depicting variation in the output voltage on the data line according to passage of time in the case of  FIGS. 4A and 4B ; 
         FIGS. 6A to 6C  are circuit diagrams illustrating sequential steps of a method for sensing a k-parameter of each pixel driving circuit in accordance with an embodiment of the present invention; 
         FIG. 7  is a waveform diagram illustrating driving of the pixel driving circuit shown in  FIGS. 6A to 6C ; 
         FIG. 8  is a graph depicting voltage variations of plural pixels in a pre-charging period and a discharging period in  FIG. 7 ; and 
         FIG. 9  is a circuit diagram illustrating a detailed configuration of a data driver according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     Hereinafter, an OLED display device and method for sensing characteristic parameters of pixel driving circuits in accordance with the present invention will be described in detail. 
     The current, Ids, of a driving thin film transistor (TFT) determines the amount of light emitted from an organic light emitting diode (OLED) of each pixel in an AMOLED display device is determined by characteristic parameters of the driving TFT such as a threshold voltage Vth of the driving TFT and a k-parameter of the driving TFT, as well as a driving voltage Vgs of the driving TFT, as expressed in the following Equation 1: 
     
       
         
           
             
               
                 
                   Ids 
                   = 
                   
                     
                       
                         1 
                         2 
                       
                       · 
                       
                         W 
                         L 
                       
                       · 
                       μ 
                       · 
                       Cox 
                       · 
                       
                         
                           ( 
                           
                             Vgs 
                             - 
                             Vth 
                           
                           ) 
                         
                         2 
                       
                     
                     = 
                     
                       k 
                       · 
                       
                         
                           ( 
                           
                             Vgs 
                             - 
                             Vth 
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 1, “k” represents a process characteristic factor and includes process characteristic factor components such as the ratio of channel width (W) to channel length (L), W/L, mobility μ, and parasitic capacitance Cox in the driving TFT. The threshold voltage Vth and k-parameter of the driving TFT may cause the current of the driving TFT to be non-uniform even when the driving voltage Vgs is constant. That is, the threshold voltage Vth and k-parameter are factor components causing non-uniformity of luminance. To this end, in accordance with the present invention, the threshold voltage Vth and k-parameter are measured for each pixel during an inspection process and/or a display operation. 
     In the OLED display device and method for sensing characteristic parameters of pixel driving circuits in accordance with the present invention, the threshold voltage Vth and k-parameter of the driving TFT in each pixel driving circuit are individually measured through a corresponding data line and a data driver under the condition that the driving TFT is driven by constant current. 
       FIG. 1  illustrates an AMOLED display device having a function of sensing characteristic parameters of pixel driving circuits in accordance with an exemplary embodiment of the present invention. 
     The AMOLED display device shown in  FIG. 1  includes a display panel  20  formed with pixel driving circuits, a data driver  10  for driving data lines DL of the display panel  20  and sensing a voltage to be used for sensing of characteristic parameters of each pixel driving circuit such as a threshold voltage Vth and a k-parameter deviation, through a corresponding one of the data lines DL, and a timing controller  30  for detecting the characteristic parameters of each pixel driving circuit, based on the measured voltage from the data driver  10  for the pixel driving circuit, and compensating the detected characteristic parameters. The data driver  10  and timing controller  30  function as characteristic parameter detection means. The display device shown in  FIG. 1  also includes a scan driver (not shown) for driving scan lines SL 1  and SL 2  of the pixel driving circuits, and an emission controller (not shown) for driving emission control lines EL. The AMOLED display device selectively operates in a sensing mode to measure the characteristic parameter of each pixel driving circuit or a display mode to perform general image display. 
     The data driver  10  includes a digital-to-analog converter (hereinafter, referred to as “DAC”)  12  and an analog-to-digital converter (hereinafter, referred to as “ADC”)  16 , which are connected to each data line DL in parallel, a first switch SW 1  connected between the DAC  12  and the data line DL, and a sampling/holder (S/H) circuit  14  connected between the ADC  16  and the data line DL. The data driver  10  further includes an output buffer (not shown) connected between the DAC  12  and the first switch SW 1 . 
     In either the sensing mode or the display mode, the DAC  12  converts input data from the timing controller  30  into analog data voltage Vdata, and supplies the analog data voltage Vdata to the data line DL of the display panel  20  via the first switch SW 1 . In the sensing mode, the S/H circuit  14  measures a voltage on the data line DL, for calculation of the threshold voltage Vth and k-parameter of the pixel driving circuit connected to the data line DL, and outputs the measured voltage. The ADC  16  converts the measured voltage into digital data. 
     Each pixel driving circuit includes first and second switching TFTs ST 1  and ST 2 , a driving TFT DT, an emission control TFT ET, and a storage capacitor Cs, in order to independently drive an OLED. The pixel driving circuit also includes first and second scan lines SL 1  and SL 2  for supplying first and second scan signals SS 1  and SS 2  as control signals for the first and second switching TFTs ST 1  and ST 2 , respectively, and an emission control line EL for supplying an emission control signal EM as a control signal for the emission control TFT ET. The data line DL is also included in the pixel driving circuit. The data line DL supplies a pre-charge voltage Vpre and the data voltage Vdata to the first switching TFT ST 1 . The pixel driving circuit further includes a reference voltage line RL for supplying a reference voltage Vref to the second switching TFT ST 2 , a first power line PL 1  for supplying a high-level voltage VDD to the emission control TFT ET, and a second power line PL 2  for supplying a low-level voltage VSS to a cathode of the OLED. The pixel driving circuit is driven in either the sensing mode for sensing of deviations of the threshold voltage Vth and k-parameter of the driving TFT DT or the display mode for data display. 
     The OLED is connected to the driving TFT DT in series between the first power line PL 1  and the second power line PL 2 . In addition to the cathode, which is connected to the second power line PL 2 , the OLED includes an anode connected to the driving TFT DT, and a light emitting layer arranged between the anode and the cathode. The light emitting layer includes an electron injection layer, an electron transport layer, an organic light emitting layer, a hole transport layer, and a hole injection layer. In the OLED, electrons from the cathode are supplied to the organic light emitting layer via the electron injection layer and electron transport layer when a positive bias is applied between the anode and the cathode, and holes from the anode to the organic light emitting layer via the hole injection layer and hole transport layer. Accordingly, the organic light emitting layer fluoresces or phosphoresces through re-combination of the supplied electrons and holes. Thus, the OLED generates luminance proportional to the density of current supplied to the OLED. 
     The first switching TFT ST 1  includes a gate electrode connected to the first scan line SL 1 , a first electrode connected to the data line DL, and a second electrode connected to a first node N 1 . The first and second electrodes function as source and drain electrodes or vice versa, respectively, in accordance with the direction of current flowing through the first switching TFT ST 1 . In the sensing mode, the first switching TFT ST 1  supplies the pre-charge voltage Vpre from the data line DL to the first node N 1  in response to the first scan signal SS 1  supplied from the scan driver to the first scan line SL 1 . In either the sensing mode or the display mode, the first switching TFT ST 1  supplies the data voltage Vdata from the data line DL to the first node N 1  in response to the first scan signal SS 1  supplied to the first scan line SL 1 . 
     The second switching TFT ST 2  includes a gate electrode connected to the second scan line SL 2 , a first electrode connected to the reference voltage line RL, and a second electrode connected to a second node N 2  connected to a gate electrode of the driving TFT DT. The first and second electrodes of the second switching TFT ST 2  function as source and drain electrodes or vice versa, respectively, in accordance with the direction of current flowing through the second switching TFT ST 2 . In either the sensing mode or the display mode, the second switching TFT ST 2  supplies the reference voltage Vref from the reference voltage line RL to the second node N 2  in response to the second scan signal SS 2  supplied from the scan driver to the second scan line SL 2 . 
     The storage capacitor Cs is charged with a difference voltage between the pre-charge voltage Vpre supplied to the first node N 1  and the reference voltage Vref supplied to the second node N 2  or a difference voltage between the data voltage Vdata and the reference voltage Vref. The storage capacitor Cs supplies the charged voltage as the driving voltage Vgs of the driving TFT DT. 
     The gate electrode of the driving TFT DT is connected to the second node N 2 . The driving TFT DT also includes a first electrode connected to the first power line PL 1  via the emission control TFT ET, and a second electrode connected to the anode of the OLED. The first and second electrodes of the driving TFT DT function as source and drain electrodes or vice versa, respectively, in accordance with the direction of current flowing through the driving TFT DT. The driving TFT DT supplies an amount of current, which corresponds to the driving voltage supplied from the storage capacitor Cs, to the OLED which, in turn, emits light. 
     The emission control TFT ET includes a gate electrode connected to the emission control line EL, a first electrode connected to the first power line PL 1 , and a second electrode connected to the first node N 1 . The first and second electrodes of the emission control TFT ET function as source and drain electrodes or vice versa, respectively, in accordance with the direction of current flowing through the emission control TFT ET. In response to the emission control signal EM supplied from the emission controller to the emission control line EL, the emission control TFT ET supplies the high-level voltage VDD to the driving TFT DT only in a display period in the display mode. In either the sensing mode or a non-display period in the display mode, the emission control TFT ET prevents supply of the high-level voltage VDD, to avoid an increase of black luminance. 
     In the display mode, the first switch SW 1  is turned on. The DAC  12  converts input data into data voltage Vdata, and supplies the data voltage Vdata to the data line DL via the first switch SW 1 . In this case, when the first and second switching TFTs ST 1  and ST 2  are turned on in response to the first and second scan signals SS 1  and SS 2 , respectively, the storage capacitor Cs is charged with a difference voltage “Vdata−Vref” between the data voltage Vdata and the reference voltage Vref. When the first and second switching TFTs ST 1  and ST 2  are turned off in response to the first and second scan signals SS 1  and SS 2 , respectively, and the emission control TFT ET is turned on in response to the emission control signal EM, the driving TFT DT supplies the driving current according to the voltage charged in the storage capacitor CS to the OLED which, in turn, emits light. 
     In the sensing mode, the data driver  10  drives the driving TFT DT of each pixel driving circuit, using constant current, measures a voltage on the data line DL connected to the pixel driving circuit, for calculation of the threshold voltage Vth and k-parameter of the pixel driving circuit, and outputs the measured voltage. For respective pixel driving circuits, the voltage sensing operation of the data driver  10  is carried out in a sequential manner. Sensing of the threshold voltage Vth and k-parameter will be described in detail later. 
     The timing controller  30  detects characteristic parameters such as a threshold voltage Vth and a k-parameter deviation, through a predetermined equation using the voltage measured for each pixel by the data driver  10 . The timing controller  30  then sets an offset value for compensation of the detected threshold voltage Vth and a gain value for compensation for the detected k-parameter deviation, and stores the set offset value and gain value for each pixel in a memory (not shown). Also, the timing controller  30  compensates input data, using the offset value and gain value stored for each pixel in the memory, and supplies, to the data driver  10 , data compensated for the characteristic parameters of the pixel driving circuit of the pixel. 
     Sensing Threshold Voltage Vth and First Compensation Method 
       FIGS. 2A and 2B  are circuit diagrams illustrating sequential steps of a method for sensing a threshold voltage Vth of each pixel driving circuit in accordance with a first embodiment of the present invention.  FIG. 3  is a graph depicting variation in the output voltage on the data line according to passage of time in the case of  FIGS. 2A and 2B . 
     As shown in  FIG. 2A , the DAC  12  supplies the pre-charge voltage Vpre to the data line DL via the turned-on first switch SW 1 . The pre-charge voltage Vpre may be supplied from an external voltage source to the data line DL via the first switch SW 1 . Thereafter, as shown in  FIG. 2B , the first switch SW is turned off, and the first and second switching TFTs ST 1  and ST 2  are turned on. Accordingly, the driving TFT DT is driven in a saturated region by the difference voltage between the pre-charge voltage Vpre and the reference voltage Vref, which is charged in the storage capacitor Cs. As a result, the pre-charge voltage Vpre from the data line DL is discharged through the first switching TFT ST 1 , driving TFT DT, and OLED. When the voltage of the storage capacitor Cs reaches the threshold voltage Vth of the driving TFT DT in accordance with discharge of the pre-charge voltage Vpre, the voltage on the data line DL is saturated, as shown in  FIG. 3 . At a time T 1  when the voltage on the data line DL is saturated, the S/H circuit  14  measures the voltage on the data line DL, namely, a voltage Vsen, and outputs the measured voltage Vsen. The ADC  14  converts the measured voltage Vsen from the S/H circuit  14  into digital data, and outputs the digital data. The timing controller  30  calculates a difference voltage “Vref−Vsen” between the reference voltage Vref and the measured voltage Vsen, to detect the threshold voltage Vth of the driving TFT DT. The timing controller  30  then sets an offset value for compensation of the detected threshold voltage Vth, and stores the offset value. Offset value setting and storage of the timing controller  30  are carried out for each pixel. 
     Sensing Threshold Voltage Vth and Second Compensation Method 
       FIGS. 4A and 4B  are circuit diagrams illustrating sequential steps of a method for sensing a threshold voltage Vth of each pixel driving circuit in accordance with a second embodiment of the present invention.  FIG. 5  is a graph depicting variation in the output voltage on the data line according to passage of time in the case of  FIGS. 4A and 4B . 
     As shown in  FIG. 4A , after supply of the pre-charge voltage Vpre to the data line DL and supply of a first reference voltage Vref 1  to the reference voltage line RL, the first and second switching TFTs ST 1  and ST 2  are turned on. Accordingly, the driving TFT DT is driven. The S/H circuit measures a voltage Vsen 1  on the data line DL at a plurality of times when the voltage Vsen 1  is saturated in accordance with discharge of the pre-charge voltage Vpre from the data line DL through the first switching TFT ST 1 , driving TFT DT, and OLED, as shown in  FIG. 5(   a ). The S/H circuit  14  then outputs the measured voltages. 
     Thereafter, as shown in  FIG. 4B , the pre-charge voltage Vpre is again supplied to the data line DL, and a second reference voltage Vref 2  different from the first reference voltage Vref 1  is supplied to the reference voltage line RL. The first and second switching TFTs ST 1  and ST 2  are then turned on, thereby causing the driving TFT DT to be turned on. The S/H circuit  14  measures a voltage Vsen 2  on the data line DL at a plurality of times when the voltage Vsen 2  is saturated in accordance with discharge of the pre-charge voltage Vpre from the data line DL through the first switching TFT ST 1 , driving TFT DT, and OLED, as shown in  FIG. 5(   b ). The S/H circuit  14  then outputs the measured voltages through the ADC  16 . 
     Meanwhile, the timing controller  30  defines, as a threshold voltage (Vth) sensing time, the time when the difference voltage “Vsen 1 −Vsen 2 ” between the first measured voltage Vsen 1  measured in the case of  FIG. 4A  and the second measured voltage Vsen 2  measured in the case of  FIG. 4B  is equal to the difference voltage “Vref 1 −Vref 2 ” between the first reference voltage Vref 1  and the second reference voltage Vref 2  as shown in  FIG. 5(   c ). The timing controller  30  calculates a difference voltage “Vref 1 −Vsen 1 ” between the first reference voltage Vref 1  and the first measured voltage Vsen 1  measured at the Vth sensing time or a difference voltage “Vref 2 −Vsen 2 ” between the second reference voltage Vref 2  and the second measured voltage Vsen 2 , to detect the threshold voltage Vth of the driving TFT DT. The timing controller  30  then sets an offset value for compensation of the detected threshold voltage Vth, and stores the offset value. Offset value setting and storage of the timing controller  30  are carried out for each pixel. 
     Sensing k-Parameter and Compensation Method 
       FIGS. 6A to 6C  are circuit diagrams illustrating sequential steps of a method for sensing a k-parameter of each pixel driving circuit in accordance with an embodiment of the present invention.  FIG. 7  is a waveform diagram illustrating driving of the pixel driving circuit shown in  FIGS. 6A to 6C . 
     In a programming period in  FIG. 7 , as shown in  FIG. 6A , the DAC  12  applies the threshold voltage Vth detected at a preceding stage to the data line DL via the turned-on first switch SW 1 , and thus supplies a sum of the compensated data voltage Vdata (Vdata=Vimage+Vth) and the reference voltage Vref, namely, a sum voltage “Vimage+Vth+Vref”. In the programming period, the first and second switching TFTs ST 1  and ST 2  are turned on by the first and second scan signals SS 1  and SS 2 , respectively. As a result, the storage capacitor Cs is charged with the data voltage Vdata (Vdata=Vimage+Vth), which has been compensated for the threshold voltage Vth. Accordingly, the data voltage Vdata (Vdata=Vimage+Vth) is supplied as the driving voltage Vgs of the driving TFT DT. Thus, the driving TFT DT supplies current Ids proportional to the k-parameter and data voltage Vimage, as expressed by the following Equation 2:
 
 Ids=k×V image 2   [Equation 2]
 
     In a pre-charging period in  FIG. 7 , as shown in  FIG. 6B , the DAC  12  charges the data line DL with the pre-charge voltage Vpre via the first switch SW 1 . Also, the first and second switching TFTs ST 1  and ST 2  are turned off by the first and second scan signals SS 1  and SS 2 , respectively. The pre-charge voltage Vpre may be equal to the reference voltage Vref. 
     In a discharging period in  FIG. 7 , as shown in  FIG. 6C , the first switch SW 1  is turned off, thereby causing the data line DL to be floated. The first switching TFT ST 1  is turned on by the first scan signal SS 1 . Accordingly, the driving TFT DT is driven in a saturated state and, as such, the pre-charge voltage Vpre of the data line DL is discharged through the first switching TFT ST 1 , driving TFT DT, and OLED. As a result, the voltage of the data line DL is dropped. Referring to  FIG. 7 , it can be seen that the voltage gradient of a reference pixel, namely, a voltage variation ΔVref, and the voltage gradient of a sensing pixel, a voltage variation ΔV, may be different from each other due to different k-parameter characteristics of driving TFTs DT. 
     At a sensing time Tsen in  FIG. 7 , as shown in  FIG. 6C , the first switching TFT ST 1  is turned off by the first scan signal SS 1 . In this state, the S/H circuit  14  the voltage Vsen on the data line DL, and outputs the measured voltage Vsen via the ADC  16 . As shown in  FIG. 8 , the timing controller  30  calculates a ratio of the difference voltage ΔRef between the pre-charge voltage Vpre and the measured voltage Vsen of the reference pixel at the sensing time Tsen (ΔRef=Vpre−Vsen 0 ) to the difference voltage ΔV between the pre-charge voltage Vpre and the measured voltage Vsen 1  or Vsen 2  of the sensing pixel (ΔV=Vpre−Vsen 1  or Vsen 2 ), to detect a k-parameter ratio between the pixels (namely, the k-parameter ratio between the reference pixel and the sensing pixel). From the detected k-parameter ratio, a gain value for compensation for a k-parameter deviation between the pixels is detected. The detected gain value is then stored. In other words, the timing controller  30  calculates the ratio between the voltage variation ΔRef (ΔRef=Vpre−Vsen 0 ) of the reference pixel generated during the discharging period and the voltage variation ΔV (ΔV=Vpre−Vsen 1  or Vsen 2 ) of the sensing pixel generated during the discharging period, to detect a k-parameter deviation between the pixels, and thus to detect a gain value for compensation for the detected k-parameter deviation. The timing controller  30  then stores the gain value. 
     Using the difference voltage ΔV between the pre-charge voltage Vpre and the measured voltage Vsen shown in  FIG. 8  (ΔV=Vpre−Vsen), it may be possible to calculate the amount of current flowing through the driving TFT DT and to detect the k-parameter ratio between the pixels (namely, the k-parameter ratio between the reference pixel and the sensing pixel. 
     In detail, since the driving TFT DT is driven in a saturated region in the discharging period in  FIG. 7 , it can be seen that “ΔV” is proportional to the current of the driving TFT DT, as expressed by the following Equation 3. In Equation 3, “Cload” represents load applied to the data line DL, namely, the parasitic capacitance of the data line DL. 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     V 
                   
                   = 
                   
                     
                       Ids 
                       × 
                       t 
                     
                     Cload 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     Since the discharging period and “Cload” are constant, and the threshold voltage Vth has been compensated, it can be seen that the “ΔV” ratio between the reference pixel and the sensing pixel is equal to the current ratio between the reference pixel and the sensing pixel, and is also equal to the k-parameter ratio between the reference pixel and the sensing pixel, as expressed by the following Equation 4. It can also be seen that the “ΔV” ratio between the reference pixel and the sensing pixel is equal to the ratio between the measured voltage of the reference pixel at the specific sensing time Tsen shown in  FIG. 8  and the measured voltage of the sensing pixel at the specific sensing time Tsen. Accordingly, it can be seen that the k-parameter deviation between pixels (that is, the k-parameter ratio between the reference pixel and the sensing pixel) can be easily calculated, using the ratio between the measured voltage Vsen 0  of the reference pixel and the measured voltage Vsen 1  or Vsen 2  of the sensing pixel. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             Vref 
                           
                           
                             Δ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             V 
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                 ( 
                                 
                                   Iref 
                                   × 
                                   t 
                                 
                                 ) 
                               
                               / 
                               Cload 
                             
                             
                               
                                 ( 
                                 
                                   I 
                                   × 
                                   t 
                                 
                                 ) 
                               
                               / 
                               Cload 
                             
                           
                           = 
                           
                             Iref 
                             I 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                 k 
                                 ref 
                               
                               × 
                               
                                 Vimage 
                                 2 
                               
                             
                             
                               k 
                               × 
                               
                                 Vimage 
                                 2 
                               
                             
                           
                           = 
                           
                             
                               k 
                               ref 
                             
                             k 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     Meanwhile, “Vdata” for compensation of the threshold voltage Vth and k-parameter includes the “ΔV” ratio between the reference pixel and the sensing pixel, as expressed by the following Equation 5: 
     
       
         
           
             
               
                 
                   
                     
                       
                         Vdata 
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                 
                                   k 
                                   ref 
                                 
                                 k 
                               
                             
                             × 
                             Vimage 
                           
                           + 
                           Vth 
                           + 
                           Vref 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   Vref 
                                 
                                 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   V 
                                 
                               
                             
                             × 
                             Vimage 
                           
                           + 
                           Vth 
                           + 
                           Vref 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     When “Vdata” calculated through Equation 5 is applied to a current equation as expressed by the following Equation 6, it can be seen that the current Ids of the driving TFT DT is expressed irrespective of the threshold voltage Vth and k-parameter of the driving TFT DT. That is, it can be seen that desired compensation has been made. 
     
       
         
           
             
               
                 
                   
                     
                       
                         Ids 
                         = 
                           
                         ⁢ 
                         
                           
                             k 
                             ⁡ 
                             
                               ( 
                               
                                 Vgs 
                                 - 
                                 Vth 
                               
                               ) 
                             
                           
                           2 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             k 
                             ⁡ 
                             
                               ( 
                               
                                 Vdata 
                                 - 
                                 Vref 
                                 - 
                                 Vth 
                               
                               ) 
                             
                           
                           2 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             k 
                             ( 
                             
                               
                                 
                                   
                                     Δ 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     Vref 
                                   
                                   
                                     Δ 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     V 
                                   
                                 
                               
                               × 
                               Vimage 
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           k 
                           × 
                           
                             Vref 
                             V 
                           
                           × 
                           
                             Vimage 
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             k 
                             ref 
                           
                           × 
                           Vimage 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
             
           
         
       
     
     In other words, since the voltage Vgs to drive the driving TFT DT is a “Vth”-compensated voltage, the current of the driving TFT DT can be calculated through the following Equation 7: 
     
       
         
           
             
               
                 
                   
                     
                       
                         I 
                         = 
                           
                         ⁢ 
                         
                           
                             k 
                             ⁡ 
                             
                               ( 
                               
                                 Vgs 
                                 - 
                                 Vth 
                               
                               ) 
                             
                           
                           2 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             k 
                             ⁡ 
                             
                               ( 
                               
                                 Vdata 
                                 + 
                                 Vth 
                                 - 
                                 Vth 
                               
                               ) 
                             
                           
                           2 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           k 
                           × 
                           
                             Vdata 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ] 
                 
               
             
           
         
       
     
     Since the current of the driving TFT DT in the reference pixel, which has a standard k-parameter, namely, a k′-parameter, and the current of the driving TFT DT in the sensing pixel, which has a k-parameter, should be equal, the driving voltage V′data of the reference pixel and the driving voltage Vdata of the sensing pixel can be expressed, using the ratio between the k′-parameter of the reference pixel and the k-parameter of the sensing pixel, as expressed by the following Equation 8: 
     
       
         
           
             
               
                 
                   
                     
                       k 
                       × 
                       
                         Vdata 
                         2 
                       
                     
                     = 
                     
                       
                         k 
                         ′ 
                       
                       × 
                       
                         V 
                         ′ 
                       
                       ⁢ 
                       
                         data 
                         
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       
                         V 
                         ′ 
                       
                       ⁢ 
                       data 
                     
                     = 
                     
                       
                         
                           k 
                           
                             k 
                             ′ 
                           
                         
                       
                       × 
                       Vdata 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
     Accordingly, the threshold voltage Vth and k-parameter of the driving TFT in the sensing pixel can be compensated through calculation of the gain value for compensation for the k-parameter ratio between pixels and the offset value for compensation of the threshold voltage Vth with the data voltage Vdata, as expressed by the following Equation 9. It is possible to achieve data compensation by multiplying the data voltage Vdata by the gain value, and then adding the offset value to the value obtained by the multiplication. 
     
       
         
           
             
               
                 
                   
                     
                       
                         V 
                         ′ 
                       
                       ⁢ 
                       data 
                     
                     = 
                     
                       
                         
                           
                             k 
                             
                               k 
                               ′ 
                             
                           
                         
                         × 
                         Vdata 
                       
                       + 
                       Vth 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     gain 
                     = 
                     
                       
                         k 
                         
                           k 
                           ′ 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     offset 
                     = 
                     Vth 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     9 
                   
                   ] 
                 
               
             
           
         
       
     
       FIG. 9  is a circuit diagram illustrating a detailed configuration of the data driver according to an embodiment of the present invention. 
     The data driver  10  shown in  FIG. 9  includes a shift register  40 , a latch  42 , n DAC  12  respectively connected to a plurality of output channels CH 1  to CHn, n sampling/holder (S/H) circuits  14  connected to respective output channels CH 1  to CHn, and n output buffers  44  each connected between a corresponding one of the n DAC  12  and a corresponding one of the n output channels CH 1  to CHn. The data driver  10  also includes n first switches SW 1  each connected between a corresponding one of the n output buffers  44  and a corresponding one of the n output channels CH 1  to CHn, n second switches SW 2  each connected between a corresponding one of the n DAC  12  and a corresponding one of the n output buffers  44 , and a multiplexer (MUX)/scaler  46  connected between the n S/H circuits  14  and the ADC  16 . 
     The shift register  40  outputs sequential sampling signals in response to respective data shift clocks from the timing controller  30  shown in  FIG. 1  in either the display mode or the sensing mode. 
     In response to the sequential sampling signals from the shift register  40 , the latch  43  sequentially samples data from the timing controller  30  and latches the sampled data. When data for one horizontal line is latched, the latch  43  outputs the latched data to the n DAC  12  in a simultaneous manner. 
     Each of the n DAC  12  converts input data into a corresponding data voltage in either the display mode or the sensing mode, and supplies the data voltage to a corresponding one of the n output channels CH 1  to CHn via a corresponding one of the n second switch SW 2 , a corresponding one of n output buffers  44 , and a corresponding one of the n first switches SW 1 . 
     Each of the n second switches SW 2  switches the pre-charge voltage Vpre supplied from outside during the pre-charging period in the sensing mode, and supplies the pre-charge voltage Vpre to a corresponding one of the n output channels CH 1  to CHn via the corresponding output buffer  44  and corresponding first switch SW 2 . Meanwhile, the pre-charge voltage Vpre may be supplied from the timing controller  30  via the latch  42  and each DAC  12 . In this case, the second switches SW 2  to switch the pre-charge voltage Vpre may be dispensed with. 
     Each first switch SW 1  is always turned on in the display mode. In the sensing mode, each first switch SW 1  is turned on during a period, in which the pre-charge voltage Vpre and data voltage Vdata are supplied, while being turned off during a period in which the voltage of the corresponding data line DL supplied through a corresponding one of the output channels CH 1  to CHn is measured. 
     In the sensing mode, each of the n S/H circuits  14  samples a measured voltage supplied through a corresponding one of the n data lines and a corresponding one of the n output channels CH 1  to CHn, and holds the sampled voltage. 
     The MUX/scaler  46  sequentially selects the measured voltages output from the n S/H circuits  14 , scales the selected voltages to match the driving voltage range of the ADC  16 , and outputs the scaled voltages to the ADC  16 . The MUX/scaler  46  may group the n measured voltages such that each group includes one or more measured voltages, to select the measured voltages by groups. This may be determined in various manners by the designer. 
     The ADC  16  converts a measured voltage from the MUX/scaler  46  into digital data, and supplies the digital data to the timing controller  30 . Practically, one or more ADCs  16  may be provided to be equal in number to the number of output channels of the MUX/scaler  46  and, as such, the ADCs  16  may be connected to respective output channels of the MUX/scaler  46 . 
     As apparent from the above description, in accordance with the method and apparatus for sensing characteristic parameters of pixel driving circuits in accordance with the present invention, it is possible to simply and rapidly measure the threshold voltage Vth and k-parameter of the driving TFT in each pixel driving circuit, through driving of the driving TFT by constant current. In accordance with the present invention, therefore, it is possible to measure the threshold voltage Vth and k-parameter of each pixel, not only during an inspection process, but also in a sensing mode intervening between successive display modes. Thus, it is also possible to measure variations of the threshold voltage Vth and k-parameter depending on passage of the use time of the AMOLED display device, and to compensate for the measured variations. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.