Patent Publication Number: US-8531362-B2

Title: Organic light emitting display device and method of driving the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 12/489,280, filed Jun. 22, 2009, which claims priority to and the benefit of Korean Patent Application No. 10-2008-0069529, filed Jul. 17, 2008, the entire content of both of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an organic light emitting display device and a method of driving the same. 
     2. Description of Related Art 
     Various types of flat panel display devices have reduced weight and volume than those of cathode ray tube display devices. The flat panel display devices include liquid crystal display devices, field emission display devices, plasma display devices, organic light emitting display devices, and the like. 
     Among these flat panel display devices, the organic light emitting display device displays images by using organic light emitting diodes that emit light through recombination of electrons and holes. The organic light emitting display device has a fast response time and is driven with low power consumption. 
     An organic light emitting display device displays an image by using pixels arranged in a matrix form. Here, each of the pixels includes an organic light emitting diode and a driving transistor that controls an amount of current supplied to the organic light emitting diode. 
     An operation of the organic light emitting display device will be described. A voltage corresponding to a data signal is first charged into a storage capacitor coupled to a driving transistor. The driving transistor controls an amount of current supplied to an organic light emitting diode, corresponding to the voltage charged into the storage capacitor. Then, the organic light emitting diode emits light of red, green or blue having a luminance corresponding to the amount of current supplied from the driving transistor. 
     However, in a conventional organic light emitting display device, the threshold voltages and mobilities of driving transistors for pixels may be unequal due to process deviation. If the threshold voltages and mobilities of the driving transistors for the pixels are unequal, the pixels generate lights having different luminances, corresponding to the same data signal. Accordingly, an image having a desired luminance may not be displayed by the pixels. 
     In order to solve such a problem, a circuit that compensates for the threshold voltage of a driving transistor may be added to each pixel. However, the circuit added to each of the pixels does not compensate for the mobility of the driving transistor. 
     As time elapses, an organic light emitting diode is degraded, and therefore, an image having a desired luminance may not be displayed. Practically, as the organic light emitting diode is degraded, the luminance of light generated by the organic light emitting diode gradually becomes lower corresponding to the same data signal. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an aspect of the present invention to provide an organic light emitting display device capable of compensating for the threshold voltage and mobility of a driving transistor and deterioration of an organic light emitting diode, and a method of driving the same. 
     According to an embodiment of the present invention, a method is provided for driving an organic light emitting display device including a pixel having a pixel circuit for driving an organic light emitting diode included in the pixel, the pixel circuit including a second capacitor having a first terminal coupled to a gate electrode of a driving transistor and a first capacitor coupled between the gate electrode of the driving transistor and a first power source. The method includes: sinking a first current via the driving transistor; supplying a threshold voltage of an organic light emitting diode to a second terminal of the second capacitor during a period when the first current is sunk via the driving transistor; supplying a data signal to the second terminal of the second capacitor after a voltage corresponding to a difference between a voltage applied to the gate electrode of the driving transistor and the threshold voltage of the organic light emitting diode is charged in the second capacitor; and charging a voltage corresponding to a difference between the voltage applied to the gate electrode of the driving transistor and a voltage of the first power source in the first capacitor. 
     According to another embodiment of the present invention, an organic light emitting display device includes: a scan driver for sequentially supplying a scan signal to scan lines and sequentially supplying a light-emitting control signal to light-emitting control lines; a data driver for supplying data signals to the data lines; pixels at crossing regions of the scan lines and the data lines; and a current sinker coupled to feedback lines for sinking a first current from the pixels, wherein each of the pixels on an i-th horizontal line includes: an organic light emitting diode; a fourth transistor for controlling an amount of current that flows into a second power source via the organic light emitting diode from a first power source; a first transistor coupled between a gate electrode of the fourth transistor and the data line, and configured to turn on when a scan signal is supplied to an i-th scan line; a first capacitor coupled between the gate electrode of the fourth transistor and the first power source; a second capacitor coupled between the gate electrode of the fourth transistor and the first transistor; a second transistor coupled between the gate electrode of the fourth transistor and a second electrode of the fourth transistor, and configured to turn on when the scan signal is supplied to an (i−1)-th scan line; a third transistor coupled between the current sinker and the second electrode of the fourth transistor, and configured to turn on when the scan signal is supplied to the (i−1)-th scan line; and a fifth transistor coupled between an anode electrode of the organic light emitting diode and a common terminal of the first transistor and the second capacitor, and configured to turn on when the scan signal is supplied to the (i−1)-th scan line. 
     According to still another embodiment of the present invention, an organic light emitting display device includes: a scan driver for supplying a first scan signal to a first scan line during a first period of a horizontal period, supplying a second scan signal to a second scan line during a second period of the horizontal period, and supplying a light-emitting control signal to light-emitting control lines during the horizontal period; a data driver for supplying data signals to data lines during the second period; a current sinker coupled to the data lines for sinking a first current; and pixels coupled to the first scan line and the second scan line, the light-emitting control lines and the data lines, wherein each of the pixels on an i-th horizontal line includes: an organic light emitting diode; a fourth transistor for controlling an amount of current that flows into a second power source via the organic light emitting diode from a first power source; a first transistor coupled between a gate electrode of the fourth transistor and a data line of the data lines, and configured to turn on when the second scan signal is supplied to the second scan line; a first capacitor coupled between the gate electrode of the fourth transistor and the first power source; a second capacitor coupled between the gate electrode of the fourth transistor and the first transistor; a second transistor coupled between the gate electrode of the fourth transistor and the data line, and configured to turn on when the first scan signal is supplied to the first scan line; a third transistor coupled between a second electrode of the fourth transistor and the data line, and configured to turn on when the first scan signal is supplied to the first scan line; and a fifth transistor coupled between an anode electrode of the organic light emitting diode and a common terminal of the first transistor and the second capacitor, and configured to turn on when the first scan signal is supplied to the first scan line. 
     According to an embodiment of the present invention, a pixel of an organic light emitting display device is provided. The pixel includes: an organic light emitting diode; a pixel circuit for supplying a current to the organic light emitting diode; a current sinker coupled to the pixel circuit for sinking a first current from the pixel circuit. The pixel circuit includes: a fourth transistor for controlling an amount of current that flows into a second power source from a first power source via the organic light emitting diode; a first transistor coupled between a gate electrode of the fourth transistor and the data line and configured to turn on when a scan signal is supplied to a scan line; a first capacitor coupled between the gate electrode of the fourth transistor and the first power source; a second capacitor coupled between the gate electrode of the fourth transistor and the first transistor; a second transistor coupled between the gate electrode of the fourth transistor and a second electrode of the fourth transistor, and configured to turn on when the scan signal is supplied to a previous scan line; a third transistor coupled between the current sinker and the second electrode of the fourth transistor, and configured to turn on when the scan signal is supplied to the previous scan line; and a fifth transistor coupled between an anode electrode of the organic light emitting diode and a common terminal of the first transistor and the second capacitor, and configured to turn on when the scan signal is supplied to the previous scan line. 
     According to an embodiment of the present invention, a pixel of an organic light emitting display device is provided. The pixel includes: an organic light emitting diode; a pixel circuit for supplying a current to the organic light emitting diode, the pixel circuit including: a fourth transistor for controlling an amount of current that flows into a second power source via the organic light emitting diode from a first power source; a first transistor coupled between a gate electrode of the fourth transistor and a data line, and configured to turn on when a second scan signal is supplied to a second scan line coupled to a gate electrode of the first transistor; a first capacitor coupled between the gate electrode of the fourth transistor and the first power source; a second capacitor coupled between the gate electrode of the fourth transistor and the first transistor; a second transistor coupled between the gate electrode of the fourth transistor and the data line, and configured to turn on when a first scan signal is supplied to a first scan line coupled to the gate electrode of the second transistor; a third transistor coupled between a second electrode of the fourth transistor and the data line, and configured to turn on when the first scan signal is supplied to the first scan line; and a fifth transistor coupled between an anode electrode of the organic light emitting diode and a common terminal of the first transistor and the second capacitor, and configured to turn on when the first scan signal is supplied to the first scan line; and a current sinker coupled to the pixel circuit for sinking a first current. 
     In an organic light emitting display device and a method of driving the same according to the embodiments of the present invention, the threshold voltage and mobility of a driving transistor can be compensated for while sinking a first current by a current sinker. Further, in the embodiments of the present invention, current corresponding to a voltage that rises from the threshold voltage of an organic light emitting diode to the voltage of a data signal is supplied to the organic light emitting diode, thereby compensating for degradation of the organic light emitting diode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention. 
         FIG. 1  is a schematic block diagram of an organic light emitting display device according to a first embodiment of the present invention. 
         FIG. 2  is a schematic circuit diagram showing an embodiment of a pixel shown in  FIG. 1 . 
         FIG. 3  is a waveform diagram of driving signals supplied from a scan driver shown in  FIG. 1 . 
         FIG. 4  is a schematic block diagram of an organic light emitting display device according to a second embodiment of the present invention. 
         FIG. 5  is a schematic circuit diagram showing an embodiment of a pixel shown in  FIG. 4 . 
         FIG. 6  is a waveform diagram of driving signals supplied from a scan driver shown in  FIG. 4 . 
         FIG. 7  is a graph showing simulation results of an amount of current that flows in a degraded organic light emitting diode. 
         FIG. 8  is a graph showing simulation results of luminance corresponding to degradation of an organic light emitting diode. 
         FIG. 9  is a graph showing simulation results for a gray level error corresponding to a change in threshold voltage and mobility of a driving transistor. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, certain exemplary embodiments according to the present invention will be described with reference to the accompanying drawings. Here, when a first element is described as being coupled to a second element, the first element may be directly coupled to the second element or indirectly coupled to the second element via a third element. Further, some of the elements that are not essential to a complete understanding of the present invention are omitted for clarity. Also, like reference numerals refer to like elements throughout. 
       FIG. 1  is a schematic block diagram of an organic light emitting display device according to a first embodiment of the present invention. 
     Referring to  FIG. 1 , the organic light emitting display device according to the first embodiment of the present invention includes a display unit  130  having a plurality of pixels  140  coupled to scan lines S 0  to Sn, light-emitting control lines E 1  to En and data lines D 1  to Dm; a scan driver  110  for driving the scan lines S 0  to Sn and the light-emitting control lines E 1  to En; a data driver  120  for driving the data lines D 1  to Dm; a timing controller  150  for controlling the scan driver  110  and the data driver  120 ; and a current sinker  160  for sinking a current (e.g., a predetermined current). 
     The display unit  130  includes the pixels  140  positioned at crossing regions of the scan lines S 0  to Sn, the light-emitting control lines E 1  to En and the data lines D 1  to Dm. The pixels  140  receive first power ELVDD and second power ELVSS supplied from the outside of the display unit  130 . 
     Each of the pixels  140  includes an organic light emitting diode. While receiving the first power ELVDD and the second power ELVSS supplied from the outside, a pixel  140  supplies current corresponding to a data signal to an organic light emitting diode. Therefore, light having a luminance corresponding to the data signal is generated from the organic light emitting diode. 
     Here, each of the pixels  140  includes a pixel circuit for supplying current to a corresponding organic light emitting diode so as to compensate for the threshold voltage and mobility of a driving transistor and the degradation of the organic light emitting diode. Detailed description will be provided later in conjunction with an exemplary structure of the pixels  140 . 
     The timing controller  150  generates a data driving control signal DCS and a scan driving control signal SCS, corresponding to synchronization signals supplied from the outside of the organic light emitting display device. The data driving control signal DCS generated from the timing controller  150  is supplied to the data driver  120 , and the scan driving control signal SCS generated from the timing controller  150  is supplied to the scan driver  110 . The timing controller  150  supplies data Data supplied from the outside to the data driver  120 . 
     The scan driver  110  sequentially supplies a scan signal through the scan lines S 0  to Sn and sequentially supplies a light-emitting control signal through the light-emitting control lines E 1  to En. Here, the scan driver  110  supplies a light-emitting control signal through an i-th light-emitting control line Ei to overlap with a scan signal sequentially supplied through an (i−1)-th scan line Si−1 and an i-th scan line Si. 
     When a scan signal is supplied, the data driver  120  supplies data signals to the data lines D 1  to Dm. Then, the data signals are supplied to the pixels  140  selected by the scan signal. 
     The current sinker  160  sinks a predetermined current from the pixels  140  via feedback lines F 1  to Fm. Here, the predetermined current is determined as a value at which a desired voltage can be charged into the pixels  140  while allowing the load of each of the feedback lines F 1  to Fm to be charged. For example, the predetermined current may be determined to be equal to current that flows into the organic light emitting diodes when the pixels  140  emit light of the highest gray level. As shown in  FIG. 2 , the current sinker  160  includes a current source Isink. The current source (sink may be used for each channel, or three current sources Isink may be coupled to the feedback lines F 1  to Fm coupled to red, green and blue pixels, respectively. 
     More specifically, different currents flow into red, green and blue pixels  140  corresponding to the maximum luminances of the red, green and blue pixels  140 , respectively. Therefore, the current sinker  160  includes three or more current sources Isink having red, green and blue current sources Isink respectively coupled to the red, green and blue pixels  140 . 
       FIG. 2  is a schematic circuit diagram showing an embodiment of a pixel shown in  FIG. 1 . For the convenience of illustration, a pixel  140  coupled to an m-th data line Dm and an n-th scan line Sn is illustrated in  FIG. 2 . 
     Referring to  FIG. 2 , the pixel  140  according to an embodiment of the present invention includes an organic light emitting diode OLED and a pixel circuit  142  for supplying current to the organic light emitting diode OLED. 
     The organic light emitting diode OLED emits light having a luminance corresponding to an amount of current supplied from the pixel circuit  142 . 
     The pixel circuit  142  supplies current corresponding to a data signal to the organic light emitting diode OLED. To this end, the pixel circuit  142  includes first to sixth transistors M 1  to M 6 , a first capacitor C 1  and a second capacitor C 2 . 
     A first electrode of the first transistor M 1  is coupled to the data line Dm, and a second electrode of the first transistor M 1  is coupled to a first node N 1 . A gate electrode of the first transistor M 1  is coupled to the n-th scan line Sn. When a scan signal (e.g., a low level signal) is supplied to the n-th scan line Sn, the first transistor M 1  is turned on to electrically couple the data line Dm to the first node N 1 . Here, the first electrode is one of drain and source electrodes, and the second electrode is the other electrode. For example, if the first electrode is a source electrode, the second electrode is a drain electrode. 
     A first electrode of the second transistor M 2  is coupled to a second electrode of the fourth transistor M 4 , and a second electrode of the second transistor M 2  is coupled to a second node N 2 . A gate electrode of the second transistor M 2  is coupled to an (n−1)-th scan line Sn−1. When a scan signal (e.g., a low level signal) is supplied to the (n−1)-th scan line Sn−1, the second transistor M 2  is turned on to diode-couple the fourth transistor M 4 . 
     A second electrode of the third transistor M 3  is coupled to the current sinker  160 , and a first electrode of the third transistor M 3  is coupled to the second electrode of the fourth transistor M 4 . A gate electrode of the third transistor M 3  is coupled to the (n−1)-th scan line Sn−1. When a scan signal is supplied to the (n−1)-th scan line Sn−1, the third transistor M 3  is turned on to electrically couple the current source Isink to the second electrode of the fourth transistor M 4 . 
     A first electrode of the fourth transistor M 4  (or driving transistor) is coupled to a first power source ELVDD, and the second electrode of the fourth transistor M 4  is coupled to a first electrode of the sixth transistor M 6 . A gate electrode of the fourth transistor M 4  is coupled to the second node N 2 . The fourth transistor M 4  supplies current to the first electrode of the sixth transistor M 6 . Here, the current corresponds to a voltage applied to the second node N 2 , i.e., a voltage charged in the first and second capacitors C 1  and C 2 . 
     A first electrode of the fifth transistor M 5  is coupled to the first node N 1 , and a second electrode of the fifth transistor M 5  is coupled to an anode electrode of the organic light emitting diode OLED. A gate electrode of the fifth transistor M 5  is coupled to the (n−1)-th scan line Sn−1. When a scan signal is supplied to the (n−1)-th scan line Sn−1, the fifth transistor M 5  is turned on to supply a voltage applied to the anode electrode of the organic light emitting diode OLED to the first node N 1 . 
     The first electrode of the sixth transistor M 6  is coupled to the second electrode of the fourth transistor M 4 , and a second electrode of the sixth transistor M 6  is coupled to the anode electrode of the organic light emitting diode OLED. A gate electrode of the sixth transistor M 6  is coupled to an n-th light-emitting control line En. When a light-emitting control signal (e.g., a high level signal) is supplied to the n-th light-emitting control line En, the sixth transistor M 6  is turned off, and when the light-emitting control signal is not supplied to the n-th light-emitting control line En, the sixth transistor M 6  is turned on. 
       FIG. 3  is a waveform diagram illustrating a driving method of the pixel shown in  FIG. 2 . 
     An operation of the pixel  140  will be described in detail with reference to  FIGS. 2 and 3 . A light-emitting control signal is first supplied to the light-emitting control line En so that the sixth transistor M 6  is turned off. 
     Thereafter, a scan signal is supplied to the (n−1)-th scan line Sn−1. When the scan signal is supplied to the (n−1)-th scan line Sn−1, the second, third and fifth transistors M 2 , M 3  and M 5  are turned on. 
     When the fifth transistor M 5  is turned on, a voltage applied to the anode electrode of the organic light emitting diode OLED is supplied to the first node N 1 . Here, the voltage applied to the anode electrode of the organic light emitting diode OLED is a threshold voltage of the organic light emitting diode OLED and increases as the organic light emitting diode OLED degrades. 
     When the third transistor M 3  is turned on, the current source Isink is electrically coupled to the second electrode of the fourth transistor M 4 . When the second transistor M 2  is turned on, the fourth transistor M 4  is diode-coupled. When the fourth transistor M 4  is diode-coupled, the current sunk by the current source Isink flows via the fourth transistor M 4 . At this time, a voltage corresponding to the current that flows into the fourth transistor M 4  is applied to the second node N 2 . 
     In this case, a voltage corresponding to a voltage difference between the first and second nodes N 1  and N 2  is charged in the second capacitor C 2 . Since the voltage applied to the second node N 2  is determined by the current sunk by the current source Isink, a voltage that compensates for the threshold voltage and mobility of the fourth transistor M 4  is charged in the second capacitor C 2 . 
     Furthermore, a voltage applied to the second node N 2  of each of the pixels  140  is determined by the amount of current that flows into the fourth transistor M 4  included in each of the pixels  140 . Here, the current that flows into the fourth transistor M 4  is substantially the same in all the pixels  140 . Therefore, the voltage applied to the second node N 2  of each of the pixels  140  compensates for the threshold voltage and mobility of the fourth transistor M 4 . 
     After a suitable voltage is charged in the second capacitor C 2 , a scan signal is supplied to the n-th scan line Sn. When the scan signal is supplied to the n-th scan line Sn, the first transistor M 1  is turned on. When the first transistor M 1  is turned on, a data signal supplied to the data line Dm is supplied to the first node N 1 . 
     Therefore, the voltage of the first node N 1  rises from the threshold voltage of the organic light emitting diode OLED to the voltage of the data signal. Accordingly, the voltage of the second node N 2  also changes corresponding to variation in the voltage of the first node N 1 . That is, since the second transistor M 2  is turned off, the second node N 2  is in a floating state. At this time, a voltage charged in a previous period is maintained in the second capacitor C 2 , and a voltage corresponding to the voltage applied to the first node N 1  is charged in the first capacitor C 1 . 
     Meanwhile, since the voltage of the first node N 1  is changed from the threshold voltage of the organic light emitting diode OLED to the voltage of the data signal, a voltage that compensates for degradation of the organic light emitting diode OLED is charged in the first capacitor C 1 . 
     As the organic light emitting diode OLED degrades, the threshold voltage of the organic light emitting diode OLED rises. Therefore, when the same data signal is supplied, increment in the voltage of the first node N 1  is decreased as the organic light emitting diode OLED degrades. If the increment in the voltage of the first node N 1  is decreased, the voltage applied to the second node N 2  is lowered, thereby compensating for the degradation of the organic light emitting diode OLED. In other words, as the organic light emitting diode OLED degrades, the voltage of the second node N 2  is lowered, thereby supplying a higher amount of current to the organic light emitting diode OLED corresponding to the same data signal. 
     As described above, in the present invention, when the scan signal is supplied to the (n−1)-th scan line Sn−1, a voltage that compensates for the threshold voltage and mobility of the fourth transistor M 4  is charged in the second capacitor C 2 . During the period when the scan signal is supplied to the n-th scan line Sn, a voltage that corresponds to a data signal and compensates for the degradation of the organic light emitting diode OLED is charged in the first capacitor C 1 . 
     Thereafter, the supply of the light-emitting control signal to the n-th light-emitting control signal is stopped. When the supply of the light-emitting control signal is stopped, the sixth transistor M 6  is turned on. When the sixth transistor M 6  is turned on, current corresponding to the voltage applied to the second node N 2  is supplied to the organic light emitting diode OLED via the sixth transistor M 6  from the fourth transistor M 4 . Then, light having a luminance corresponding to the current supplied from the sixth transistor M 6  is emitted from the organic light emitting diode OLED. 
       FIG. 4  is a schematic block diagram of an organic light emitting display device according to a second embodiment of the present invention. 
     Referring to  FIG. 4 , the organic light emitting display device according to the second embodiment of the present invention includes a display unit  230  having a plurality of pixels  240  coupled to first scan lines S 11  to S 1   n , second scan lines S 21  to S 2   n , light-emitting control lines E 1  to En, and data lines D 1  to Dm; a scan driver  210  for driving the first scan lines S 11  to S 1   n , the second scan lines S 21  to S 2   n , and the light-emitting control lines E 1  to En; a data driver  220  for driving the data lines D 1  to Dm; a timing controller  250  for controlling the scan driver  210  and the data driver  220 ; and a current sinker  260  for sinking a current (e.g., a predetermined current). 
     The display unit  230  includes the pixels  240  positioned at crossing regions of the first scan lines S 11  to S 1   n , the second scan lines S 21  to S 2   n , the light-emitting control lines E 1  to En and the data lines D 1  to Dm. The pixels  240  receive first power ELVDD and second power ELVSS supplied from the outside of the display unit  230 . 
     Each of the pixels  240  includes an organic light emitting diode. While receiving the first power ELVDD and the second power ELVSS supplied from the outside, the pixel  240  supplies current corresponding to a data signal to the organic light emitting diode. Then, light having a luminance corresponding to the data signal is generated from the organic light emitting diode. 
     Here, each of the pixels  240  supplies current to the corresponding organic light emitting diode so as to compensate for the threshold voltage and mobility of a driving transistor and the degradation of the organic light emitting diode. Detailed description will be provided later in conjunction with an exemplary structure of the pixels  240 . 
     The timing controller  250  generates a data driving control signal DCS and a scan driving control signal SCS, corresponding to synchronization signals supplied from the outside of the organic light emitting display device. The data driving control signal DCS generated from the timing controller  250  is supplied to the data driver  220 , and the scan driving control signal SCS generated from the timing controller  250  is supplied to the scan driver  210 . The timing controller  250  also supplies data Data supplied from the outside to the data driver  220 . 
     The scan driver  210  sequentially supplies a first scan signal to the first scan lines S 11  to S 1   n  and sequentially supplies a second scan signal to second scan lines S 21  to S 2   n . Here, the first scan signal supplied to an i-th (i is a natural number) first scan line S 1   i  is supplied during a first period in one horizontal period, and the second scan signal supplied to an i-th second scan line S 2   i  is supplied during a second period different from the first period in the one horizontal period. 
     The scan driver  210  sequentially supplies a light-emitting control signal to the light-emitting control lines E 1  to En. Here, the light-emitting control signal supplied to an i-th light-emitting control line Ei is supplied to overlap with the first scan signal supplied to the i-th first scan line S 1   i  and the second scan signal supplied to the i-th second scan line S 2   i.    
     The data driver  220  supplies data signals to the data lines D 1  to Dm during the second period in the one horizontal period. Then, the data signals are supplied to the pixels  240  selected by the second scan signal. 
     The current sinker  260  sinks a current (e.g., a predetermined current) from the pixel  240  selected by the first scan signal during the first period in the one horizontal period. For example, the current has a magnitude that corresponds to the current that flows into the organic light emitting diode when the pixel  240  emits light having the highest gray level. As shown in  FIG. 5 , the current sinker  260  includes a current source Isink. The current source Isink may be used for each channel, or three current sources Isink may be coupled to feedback lines F 1  to Fm coupled to red, green and blue pixels, respectively. 
     More specifically, different currents flow into red, green and blue pixels  240  corresponding to the maximum luminances of the red, green and blue pixels  240 , respectively. Therefore, the current sinker  260  includes three or more current sources Isink having red, green and blue current sources Isink respectively coupled to the red, green and blue pixels  240 . 
     Referring to  FIG. 5 , a switching element SW is turned on during the first period in the one horizontal period. The current source Isink sinks a current (e.g., a predetermined current) from the pixel  240  when the switching element SW is turned on. Here, the switching element SW may be used for each channel, or one switching element SW may be coupled to all the data lines D 1  to Dm. That is, one or more switching elements SW are included in the current sinker  260  and turned on during the first period in the one horizontal period. 
       FIG. 5  is a circuit diagram showing an embodiment of a pixel shown in  FIG. 4 . For the convenience of illustration, a pixel  240  coupled to an m-th data line Dm and an n-th light-emitting control line En is illustrated in  FIG. 5 . 
     Referring to  FIG. 5 , the pixel  240  according to an embodiment of the present invention includes an organic light emitting diode OLED and a pixel circuit  242  for supplying current to the organic light emitting diode OLED. 
     The organic light emitting diode OLED emits light having a luminance corresponding to an amount of current supplied from the pixel circuit  242 . 
     The pixel circuit  242  supplies a current corresponding to a data signal to the organic light emitting diode OLED. To this end, the pixel circuit  242  includes first to sixth transistors M 1  to M 6 , a first capacitor C 1  and a second capacitor C 2 . 
     A first electrode of the first transistor M 1  is coupled to the data line Dm, and a second electrode of the first transistor M 1  is coupled to a first node N 1 . A gate electrode of the first transistor M 1  is coupled to a  2   n -th scan line S 2   n . When a second scan signal is supplied to the  2   n -th scan line S 2   n , the first transistor M 1  is turned on to electrically couple the data line Dm to the first node N 1 . 
     A first electrode of the second transistor M 2  is coupled to the data line Dm, and a second electrode of the second transistor M 2  is coupled to a second node N 2 . A gate electrode of the second transistor M 2  is coupled to a  1   n -th scan line S 1   n . When a first scan signal is supplied to the  1   n -th scan line S 1   n , the second transistor M 2  is turned on to electrically couple the data line Dm to the second node N 2 . 
     A first electrode of the third transistor M 3  is coupled to the data line Dm, and a second electrode of the third transistor M 3  is coupled to a second electrode of the fourth transistor M 4 . A gate electrode of the third transistor M 3  is coupled to the  1   n -th scan line S 1   n . When a first scan signal is supplied to the  1   n -th scan line S 1   n , the third transistor M 3  is turned on to electrically couple the data line Dm to the second electrode of the fourth transistor M 4 . 
     A first electrode of the fourth transistor M 4  (or driving transistor) is coupled to a first power source ELVDD, and the second electrode of the fourth transistor M 4  is coupled to a first electrode of the sixth transistor M 6 . A gate electrode of the fourth transistor M 4  is coupled to the second node N 2 . The fourth transistor M 4  supplies current to the first electrode of the sixth transistor M 6 . Here, the current corresponds to a voltage applied to the second node N 2 , i.e., a voltage charged in the first and second capacitors C 1  and C 2 . 
     A first electrode of the fifth transistor M 5  is coupled to the first node N 1 , and a second electrode of the fifth transistor M 5  is coupled to an anode electrode of the organic light emitting diode OLED. A gate electrode of the fifth transistor M 5  is coupled to the  1   n -th scan line S 1   n . When a first scan signal is supplied to the  1   n -th scan line S 1   n , the fifth transistor M 5  is turned on to supply a voltage applied to the anode electrode of the organic light emitting diode OLED to the first node N 1 . 
     The first electrode of the sixth transistor M 6  is coupled to the second electrode of the fourth transistor M 4 , and a second electrode of the sixth transistor M 6  is coupled to the anode electrode of the organic light emitting diode OLED. A gate electrode of the sixth transistor M 6  is coupled to the n-th light-emitting control line En. When a light-emitting control signal is supplied to the n-th light-emitting control line En, the sixth transistor M 6  is turned off, and when the light-emitting control signal is not supplied to the n-th light-emitting control line En, the sixth transistor M 6  is turned on. 
       FIG. 6  is a waveform diagram for illustrating a driving method of the pixel shown in  FIG. 5 . In  FIG. 6 , one horizontal period  1 H is divided into a first period and a second period. Here, the switching element SW is turned on during the first period. 
     An operation of the pixel  240  will be described in detail with reference to  FIGS. 5 and 6 . A light emitting control signal is first supplied to the light-emitting control line En, and the sixth transistor M 6  is turned off. 
     Thereafter, a first scan signal is supplied to the  1   n -th scan line S 1   n  during the first period of the one horizontal period  1 H. When the first scan signal is supplied to the  1   n -th scan line S 1   n , the second, third and fifth transistors M 2 , M 3  and M 5  are turned on. 
     When the fifth transistor M 5  is turned on, a voltage applied to the anode electrode of the organic light emitting diode OLED is supplied to the first node N 1 . Here, the voltage applied to the anode electrode of the organic light emitting diode OLED is a threshold voltage of the organic light emitting diode OLED and increases as the organic light emitting diode OLED degrades. 
     When the second transistor M 2  is turned on, the data line Dm is electrically coupled to the second node N 2 . When the third transistor M 3  is turned on, a current (e.g., a predetermined current) is sunk by the current source Isink via the first power source ELVDD, the fourth transistor M 4 , the third transistor M 3  and the switching element SW. When the current is sunk by the current source Isink, a voltage corresponding to the current is applied to the second node N 2 . 
     A voltage corresponding to a voltage difference between the first and second nodes N 1  and N 2  is charged in the second capacitor C 2 . Since the voltage applied to the second node N 2  is determined by the current sunk by the current source Isink, a voltage that compensates for the threshold voltage and mobility of the fourth transistor M 4  is charged in the second capacitor C 2 . 
     Furthermore, a voltage applied to the second node N 2  of each of the pixels  240  is determined by the current that flows into the fourth transistor M 4  included in each of the pixels  240 . Here, the current that flows into the fourth transistor M 4  is substantially the same in all the pixels  240 . Therefore, the voltage applied to the second node N 2  of each of the pixels  240  is a voltage that compensates for the threshold voltage and mobility of the fourth transistor M 4 . 
     After a voltage (e.g., a predetermined voltage) is charged in the second capacitor C 2 , a second scan signal is supplied to the  2   n -th scan line S 2   n  during the second period. When the second scan signal is supplied to the  2   n -th scan line S 2   n , the first transistor M 1  is turned on. When the first transistor M 1  is turned on, a data signal supplied to the data line Dm is supplied to the first node N 1 . 
     At this time, the voltage of the first node N 1  rises from the threshold voltage of the organic light emitting diode OLED to the voltage of the data signal. Accordingly, the voltage of the second node N 2  is also changed corresponding to variation in the voltage of the first node N 1 . That is, since the second transistor M 2  is turned off, the second node N 2  is in a floating state. At this time, a voltage charged in a previous period is maintained in the second capacitor C 2 , and a voltage corresponding to the voltage applied to the first node N 1  is charged in the first capacitor C 1 . 
     Meanwhile, since the voltage of the first node N 1  is changed from the threshold voltage of the organic light emitting diode OLED to the voltage of the data signal, a voltage that compensates for degradation of the organic light emitting diode OLED is charged in the first capacitor C 1 . 
     Furthermore, as the organic light emitting diode OLED degrades, the threshold voltage of the organic light emitting diode OLED rises. Therefore, when the same data signal is supplied, increment in the voltage of the first node N 1  is decreased as the organic light emitting diode OLED degrades. If the increment in the voltage of the first node N 1  is decreased, the voltage applied to the second node N 2  is lowered, thereby compensating for the degradation of the organic light emitting diode OLED. In other words, as the organic light emitting diode OLED degrades, the voltage of the second node N 2  is lowered, thereby supplying a higher current to the organic light emitting diode OLED corresponding to the same data signal. 
     As described above, in the present invention, a voltage that compensates for the threshold voltage and mobility of the fourth transistor M 4  is charged in the second capacitor C 2  during the first period of the one horizontal period. During the second period of the one horizontal period, a voltage that corresponds to a data signal and compensates for the degradation of the organic light emitting diode OLED is charged in the first capacitor C 1 . 
     Thereafter, the supply of the light-emitting control signal to the n-th light-emitting control signal is stopped. When the supply of the light-emitting control signal is stopped, the sixth transistor M 6  is turned on. When the sixth transistor M 6  is turned on, current corresponding to the voltage applied to the second node N 2  is supplied to the organic light emitting diode OLED via the sixth transistor M 6  from the fourth transistor M 4 . Therefore, light having a luminance corresponding to the current supplied from the transistor M 4  is emitted from the organic light emitting diode OLED. 
       FIG. 7  is a graph showing simulation results of an amount of current that flows in a degraded organic light emitting diode. In  FIG. 7 , the label “reference” refers to the current that flows before the organic light emitting diode is degraded, and the label “related art” shows a result of a pixel (e.g., 5TR 2Cap) generally used to compensate for a threshold voltage. 
     Referring to  FIG. 7 , in a related art pixel, an amount of current that flows into an organic light emitting diode is decreased when the organic light emitting diode degrades. Therefore, in the related art pixel, luminance may be lowered corresponding to the degradation of the organic light emitting diode. However, in a pixel according to the first or second embodiment of the present invention, when an organic light emitting diode is degraded, an amount of current for the same data signal is increased, and thus, luminance is maintained at a predetermined level. That is, in the embodiments of the present invention, the degradation of the organic light emitting diode is compensated for, so that an image having a desired luminance can be displayed as the organic light emitting diodes degrade. 
       FIG. 8  is a graph showing simulation results of luminance corresponding to degradation of an organic light emitting diode. 
     Referring to  FIG. 8 , in the related art pixel, luminance is lowered when the organic light emitting diode degrades. However, in the pixel according to the first or second embodiment of the present invention, a predetermined luminance can be maintained even though the organic light emitting diode is degraded. 
       FIG. 9  is a graph showing simulation results of a gray level error corresponding to a change in threshold voltage and mobility of a driving transistor. In  FIG. 9 , the threshold voltage of the driving transistor is changed by ±0.5V, and the mobility of the driving transistor is changed by ±10. 
     Referring to  FIG. 9 , in the related art pixel, a high gray level error occurs corresponding to a change in threshold voltage and mobility. However, in the embodiments of the present invention, a gray level error is minimized corresponding to a change in threshold voltage and mobility, and accordingly, an image having a desired luminance can be displayed. 
     While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the present invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.