Abstract:
An organic light emitting display device capable of driving transistor threshold voltage compensation, including: pixels positioned in the intersections of scan lines and data lines, wherein each pixel comprises: a first transistor and a fourth transistor, connected at a common node, disposed between an anode of an OLED and a first power supply; a cathode of the OLED connected to a second power supply; a second transistor connected between a gate of the first transistor and a data line, and turned on when a scan signal is supplied to a scan line; a third transistor connected between the common node and the data line, and turned on when a scan signal is supplied to the scan line; a first capacitor connected between the gate of the first transistor and the anode of the OLED; and a second capacitor connected between the anode of the OLED and a predetermined voltage source.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C §119 from an application entitled ORGANIC LIGHT EMITTING DISPLAY DEVICE AND METHOD OF DRIVING THE SAME earlier filed in the Korean Industrial Property Office on 5 Dec. 2008, which was duly assigned Serial No. 10-2008-0123141 by that Office. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an organic light emitting display device and a method of driving the same, and more particularly to an organic light emitting display device capable of compensating for threshold voltage of a driving transistor and a method of driving the same. 
         [0004]    2. Discussion of Related Art 
         [0005]    Recently, various flat panel display devices capable of reduced in weight and volume over cathode ray tubes have been developed. As the flat panel display device, there are a liquid crystal display device, a field emission display device, a plasma display panel, an organic light emitting display device, etc. 
         [0006]    Among others, the organic light emitting display device displays images by using an organic light emitting diode generating light by means of recombination of electrons and holes. Such an organic light emitting diode has advantages of being driven with low power consumption and having rapid response speed. 
         [0007]      FIG. 1  is a circuit diagram showing a pixel of a general organic light emitting display device. In  FIG. 1 , transistors included in pixels are set as NMOS transistors. 
         [0008]    Referring to  FIG. 1 , a pixel  4  of the conventional organic light emitting display device includes an organic light emitting diode OLED and a pixel circuit  2  coupled to a data line Dm and a scan line Sn to control the organic light emitting diode OLED. 
         [0009]    An anode electrode of the organic light emitting diode OLED is connected to an anode electrode, and a cathode electrode thereof is connected to a second power supply ELVSS. The organic light emitting diode OLED as above generates light having a predetermined brightness, corresponding to current supplied from the pixel circuit. 
         [0010]    The pixel circuit  2  controls the amount of current supplied to the organic light emitting diode OLED by corresponding to a data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn. To this end, the pixel circuit  2  includes a second transistor M 2  (that is, a driving transistor) connected between a first power supply ELVDD and the organic light emitting diode OLED, a first transistor M 1  connected among the second transistor M 2 , the data line Dm and the scan line Sn, and a storage capacitor Cst connected between a gate electrode and a second electrode of the second transistor M 2 . 
         [0011]    A gate electrode of the first transistor M 1  is connected to the scan line Sn, and a first electrode thereof is connected to the data line Dm. A second electrode of the first transistor M 1  is connected to one terminal of the storage capacitor Cst. Here, the first electrode is set as any one of a source electrode and a drain electrode, and the second electrode is set as the other electrode than the first electrode. For example, if the first electrode is set as a drain electrode, the second electrode is set as a source electrode. When the scan signal is supplied from the scan line Sn, the first transistor M 1  connected to the scan line Sn and the data line Dm is turned on to supply the data signal supplied from the data line Dm to the storage capacitor Cst. At this time, the storage capacitor Cst is charged with voltage corresponding to the data signal. 
         [0012]    A gate electrode of the second transistor M 2  is connected to one terminal of the storage capacitor Cst and a first electrode thereof is connected to the first power supply ELVDD. A second electrode of the second transistor M 2  is connected to the other terminal of the storage capacitor Cst and the anode electrode of the organic light emitting diode OLED. The second transistor M 2  as above controls the amount of current flowing onto a second power supply ELVSS from the first power supply ELVDD via the organic light emitting diode OLED, corresponding to the voltage value stored in the storage capacitor Cst. 
         [0013]    One terminal of the storage capacitor Cst is connected to the gate electrode of the second transistor M 2 , and the other terminal thereof is connected to the anode electrode of the organic light emitting diode OLED. The storage capacitor Cst as above is charged with voltage corresponding to the data signal. 
         [0014]    The conventional pixel  4  as above supplies the current corresponding to the voltage charged in the storage capacitor Cst to the organic light emitting diode OLED, thereby displaying images having a predetermined brightness. However, the conventional organic light emitting display device as above has a problem that images having even brightness cannot be displayed due to deviation of threshold voltage. 
         [0015]    When the threshold voltage of the second transistor M 2  is actually set to be different for each pixel  4 , each pixel  4  generates light having different brightness by corresponding to the same data signal so that images having even brightness cannot be displayed. 
       SUMMARY OF THE INVENTION 
       [0016]    Therefore, it is an object of the present invention to provide an organic light emitting display device capable of compensating for threshold voltage of a driving transistor, and a method of driving the same. 
         [0017]    In order to accomplish the above object, according to an embodiment of the present invention, there is provided an organic light emitting display device including: a scan driver supplying scan signals to scan lines; a data driver supplying reference power to data lines during a primary period of a period while the scan signals are supplied, and supplying data signals during a secondary period of the period other than the primary period; and pixels positioned in the intersections of the scan lines and the data lines, wherein a pixel positioned in an i th  (i is a natural number) horizontal line comprises: an organic light emitting diode whose cathode electrode is connected to a second power supply; a first transistor and a fourth transistor connected between an anode electrode of an organic light emitting diode and a first power supply; a second transistor connected between a gate electrode of the first transistor and a data line, and turned on when a scan signal is supplied to an i th  scan line; a third transistor connected between a common node of the first transistor and the fourth transistor and the data line, and turned on when a scan signal is supplied to the i th  scan line; a first capacitor connected between the gate electrode of the first transistor and the anode electrode of the organic light emitting diode; and a second capacitor connected between the anode electrode of the organic light emitting diode and a predetermined voltage source. 
         [0018]    Preferably, a gate electrode of the fourth transistor is connected to the gate electrode of the first transistor. A voltage value obtained by subtracting a threshold voltage of the first transistor from a voltage of the reference power supply is set to be lower than a threshold voltage of the organic light emitting diode. The second capacitor is set to have a lower capacity than the first capacitor. The secondary period is set so that a voltage of the anode electrode of the organic light emitting diode does not rise to a voltage obtained by subtracting the threshold voltage from a voltage applied to the gate electrode of the first transistor. The organic light emitting diode further includes a fifth transistor connected between the anode electrode of the organic light emitting diode and an initialization power supply, and turned on when the scan signal is supplied to an i-1 st  scan line. 
         [0019]    According to an embodiment of the present invention, there is provided a method of driving an organic light emitting display device comprising: setting a voltage of an anode electrode of an organic light emitting diode to be lower than a threshold voltage of the organic light emitting diode; supplying a voltage of a reference power supply to a gate electrode and a first electrode of a driving transistor connected to the organic light emitting diode to raise the voltage of the anode electrode of the organic light emitting diode to a voltage obtained by subtracting a threshold voltage of the driving transistor from the reference power; supplying data signals to a gate electrode of the driving transistor; raising the voltage of the anode electrode of the organic light emitting diode to voltage lower than rising voltage of the gate electrode of the driving transistor using a first capacitor and a second capacitor connected in series between the gate electrode of the driving transistor and a cathode electrode of the organic light emitting diode; and implementing a gray scale by stopping the supply of the data signals before the voltage of the anode electrode of the organic light emitting diode rises to the values obtained by subtracting the threshold voltage of the driving transistor from the voltage of the gate electrode of the driving transistor. 
         [0020]    Preferably, the voltage value of the reference power supply is set so that the voltage obtained by subtracting the threshold voltage of the driving transistor from the reference power is set to be lower than the threshold voltage of the organic light emitting diode. 
         [0021]    With the organic light emitting diode and the method of driving the same according to the present invention, the amount of current flowing onto the organic light emitting diode is determined regardless of the threshold voltage of the transistor, making it possible to display images having an even brightness. Also, image having a desired brightness can be displayed by compensating for characteristic deviation of the transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    A more complete appreciation of the present invention, and many of the attendant advantages thereof, will become readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
           [0023]      FIG. 1  is a circuit diagram showing a pixel of a general organic light emitting display device; 
           [0024]      FIG. 2  shows an organic light emitting display device according to an embodiment of the present invention; 
           [0025]      FIG. 3  shows a first embodiment of the pixel of  FIG. 2 ; 
           [0026]      FIG. 4  is a waveform view showing a method of driving the pixel of  FIG. 3 ; 
           [0027]      FIG. 5  shows a second embodiment of the pixel of  FIG. 2 ; and 
           [0028]      FIG. 6  is a waveform view showing a method of driving the pixel of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]    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 not only directly coupled to the second element but may also be indirectly coupled to the second element via a third element. Further, some of the elements that are not essential to the complete understanding of the invention are omitted for clarity. Also, like reference numerals refer to like elements throughout. 
         [0030]    Hereinafter, the present invention will be described in more detail with reference to  FIGS. 2 to 6  attached with exemplary embodiments so that a person having ordinary skill in the art to which the present invention pertains can readily carry out the present invention. 
         [0031]      FIG. 2  shows an organic light emitting display device according to an embodiment of the present invention. 
         [0032]    Referring to  FIG. 2 , the organic light emitting display device includes pixels  140  positioned to be connected to scan lines S 0  to Sn and data lines D 1  to Dm, a scan driver  110  driving the scan lines S 0  to Sn, a data driver  120  driving the data lines D 1  to Dm, and a timing controller  150  controlling the scan driver  110  and the data driver  120 . 
         [0033]    The scan driver  110  receives scan driving control signals SCS from the timing controller  150 . The scan driver  110  having received the scan driving control signals SCS generates scan signals and supplies the generated scan signals sequentially to the scan lines S 0  to Sn. 
         [0034]    The data driver  120  receives data driving control signals DCS from the timing controller  150 . The data driver  120  having received the data driving control signals DCS supplies reference voltage to the data lines D 1  to Dm during a primary period of a period while the scan signals are supplied, and supplies data signals to the data lines D 1  to Dm during periods other than the primary period. Here, the reference power is set so that the voltage obtained by subtracting a threshold voltage from a voltage of a reference power supply is lower than a threshold voltage of the organic light emitting diode. 
         [0035]    The timing controller  150  generates the data driving control signals DCS and the scan driving control signals SCS by corresponding to synchronization signals supplied from the external. The data driving control signals DCS generated by the timing controller  150  are supplied to the data driver  120 , and the scan driving control signals SCS generated therefrom are supplied to the scan driver  110 . The timing controller supplies the data supplied from the external to the data driver  120 . 
         [0036]    The pixel unit  130  receives first power ELVDD, second power ELVSS and initialization power Vint to supply them to respective pixels  140 . The respective pixels having received the first power ELVDD, the second power ELVSS and the initialization power Vint generate light corresponding to the data signals. 
         [0037]    Here, the first power ELVDD is set to have a higher voltage value than the second power ELVSS to supply a predetermined current to an organic light emitting diode. The initialization voltage Vint, which is voltage supplied to an anode electrode of the organic light emitting diode, is set to be lower than the voltage obtained by subtracting the threshold voltage of a first transistor from a reference power. 
         [0038]    Meanwhile, a pixel  140  positioned on an i th  (i is a natural number) horizontal line is connected to an i th  scan line and an i-1 st  scan line. Such a pixel  140  includes a plurality of NMOS transistors and supplies current compensating for the threshold voltage of a driving transistor to the organic light emitting diode. 
         [0039]      FIG. 3  shows a first embodiment of the pixel  140  of  FIG. 2 . For convenience of explanation, a pixel  140  is positioned in an n th  horizontal line and connected to an m th  data line Dm will be described. 
         [0040]    Referring to  FIG. 3 , the pixel  140 , according to the first embodiment, includes an organic light emitting diode OLED and a pixel circuit  142  connected to a data line Dm and scan lines Sn- 1  and Sn to control the organic light emitting diode OLED. 
         [0041]    An anode electrode of the organic light emitting diode OLED is connected to the pixel circuit  142 , and a cathode electrode thereof is connected to a second power supply ELVSS. The organic light emitting diode OLED as above generates light having a predetermined brightness, corresponding to current supplied from the pixel circuit  142 . 
         [0042]    The pixel circuit  142  is charged with a voltage corresponding to a data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn and voltage corresponding to threshold voltage of a first transistor M 1 , and controls the amount of current supplied to the organic light emitting diode OLED corresponding to the charged voltage. To this end, the pixel circuit  142  includes first to fifth transistors M 1  to M 5 , a first capacitor C 1  and a second capacitor C 2 . 
         [0043]    A gate electrode of the first transistor M 1  is connected to a first node N 1 , and a first electrode thereof is connected to a third node N 3 . A second electrode of the first transistor M 1  is connected to an anode electrode (that is, a second node N 2 ) of the organic light emitting diode OLED. 
         [0044]    A gate electrode of the second transistor M 2  is connected to the scan line Sn, and a first electrode thereof is connected to the data line Dm. A second electrode of the second transistor M 2  is connected to the first node N 1  (that is, the gate electrode of the first transistor M 1 ). When the scan signal is supplied to the scan line Sn, the second transistor M 2  as above is turned on to electrically connect the data line Dm to the first node N 1 . 
         [0045]    A gate electrode of the third transistor M 3  is connected to the scan line Sn, and a first electrode thereof is connected to the data line Dm. A second electrode of the third transistor M 3  is connected to the third node N 3  (that is, the first electrode of the first electrode M 1 ). When the scan signal is supplied to the scan line Sn, the third transistor M 3  as above is turned on to electrically connect the data line Dm to the third node N 3 . 
         [0046]    A gate electrode of the fourth transistor M 4  is connected to the first node N 1 , and a first electrode thereof is connected to the first power supply ELVDD. A second electrode of the fourth transistor M 4  is connected to the third node N 3 . The fourth transistor M 4  keeps a turn-off state by means of voltage applied to the first node N 1  and the third node N 3  while the first capacitor C 1  is charged with a predetermined voltage. The fourth transistor M 4  supplies the third node N 3  current corresponding to the current applied to the first node N 1  after the first capacitor C 1  is charged with a predetermined voltage. Here, threshold voltage of the first transistor M 1  and the fourth transistor M 4  included in the same pixel  140  is set to be approximately the same so that the amount of current supplied to the third node N 3  is controlled, regardless of the threshold voltage. The detailed description thereof will be described later. 
         [0047]    A gate electrode of the fifth transistor M 5  is connected to the n- 1  scan line Sn- 1 , and a first electrode thereof is connected to the second node N 2 . A second electrode of the fifth transistor M 5  is connected to the initialization power supply Vint. When the scan signal is supplied to the n- 1  scan line Sn- 1 , the fifth transistor M 5  is turned on to electrically connect the second node N 2  to the initialization power supply Vint. 
         [0048]    The first capacitor C 1  is connected between the first node N 1  and the second node N 2 . The first capacitor C 1  as above is charged with the threshold voltage of the first transistor M 1  and the voltage corresponding to the data signal supplied to the data line Dm. 
         [0049]    The second capacitor C 2  is connected between the second node N 2  and the second power supply ELVSS. The second capacitor C 2  controls rising voltage of the second node N 2  so that the voltage corresponding to the data signal can be charged in the first capacitor C 1 . 
         [0050]      FIG. 4  is a waveform view showing a method of driving the pixel of  FIG. 3 . 
         [0051]    Specifically describing the operation process of the pixel  140  of  FIG. 3  by combining  FIGS. 3 and 4 , the scan signal is first supplied to the n- 1  scan line Sn- 1  so that the fifth transistor M 5  is turned on. If the fifth transistor M 5  is turned on, the second node N 2  is electrically connected to the initialization power supply Vint. At this time, the second node N 2  is initialized as voltage of the initialization power supply Vint. Here, the initialization power Vint is set to have voltage lower than the threshold voltage of the organic light emitting diode OLED and thus, unnecessary light is not generated in the organic light emitting diode OLED. 
         [0052]    Thereafter, the scan signal is supplied to the scan line Sn. If the scan signal is supplied to the scan line Sn, the second transistor M 2  and the third transistor M 3  are turned on. If the second transistor M 2  and the third transistor M 3  are turned on, the reference power Vref supplied to the data line Dm during a primary period of a period while the scan signal is supplied is supplied to the first node N 1  and the third node N 3 . 
         [0053]    Here, the first node N 1  and the third node N 3  are set to have the same voltage (that is, a reference power Vref) so that the fourth transistor M 4  is turned off. The first transistor M 1  is connected in a diode shape so that the voltage of the second node N 2  rises to the voltage obtained by subtracting the threshold voltage Vth of the first transistor M 1  from the reference power Vref (that is, Vref−Vth (M 1 )). Here, the voltage of Vref−Vth (M 1 ) is set to be lower than the threshold voltage of the organic light emitting diode OLED and thus, unnecessary light is not generated in the organic light emitting diode OLED. 
         [0054]    Meanwhile, the voltage of the reference power Vref is applied to the first node N 1  and the voltage obtained by subtracting the threshold voltage Vth of the first transistor M 1  from the reference power Vref is applied to the second node N 2 , such that the voltage corresponding to the threshold voltage Vth of the first transistor M 1  is charged in the first capacitor C 1 . 
         [0055]    Therefore, the data signal DS is supplied to the data line Dm during a secondary period of the period while the scan signal is supplied. If the data signal is supplied to the data line Dm, the voltage of the first node N 1  rises from the reference power Vref to the voltage of the data signal DS. In other words, the voltage of the first node N 1  may be represented using equation 1 below. 
         [0000]        V   N1   =Vdata−Vref    Equation 1 
         [0056]    Here, Vdata represents the voltage of the data signal DS. 
         [0057]    When the voltage of the first node Ni is changed as shown in equation 1, the voltage variation of the second node N 2  may be represented as shown in equation 2 below, by a coupling phenomenon of the first capacitor C 1 . 
         [0000]      Δ V   N2 (1)={ C 1/( C 1 +C 2)}×( Vdata−Vref )   Equation 2 
         [0058]    When the voltage of the second node N 2  is changed as shown in equation 2, the value of the voltage between the gate and source electrode Vgs of the first transistor M 1  rises from its threshold voltage by a predetermined voltage. In this case, a predetermined current flows by the first transistor M 1  so that the voltage of the second node N 2  is changed by the voltage ΔV 1 . 
         [0059]    Here, the voltage ΔV 1  is set to be different for each pixel according to the characteristics (for example, mobility) of the first transistor M 1  and thus, the characteristic deviation of the first transistor M 1  can be compensated. Actually, when the voltage of the second node N 2  is changed by the voltage ΔV 1 , the voltage between the gate and source electrode of the first transistor M 1  may be represented by equation 3 below. 
         [0000]        Vgs ( M 1)=( Vdata−Vref ) {1 −C 1( C 1 +C 2)−Δ V 1 }+Vth ( M 1)   Equation 3 
         [0060]    Through equation 3 it is shown that the current flowing onto the organic light emitting diode OLED may be represented by equation 4 below. 
         [0000]        I   OLED =β( Vgs ( M 1)− Vth ( M 1)) 2 =β {( Vdata−Vref ) {1 −C 1( C 1 +C 2)−Δ V 1}} 2    Equation 4 
         [0000]    Here, β refers to a constant value. 
         [0061]    Referring to  FIG. 4 , the current flowing onto the organic light emitting diode OLED is determined, regardless of the threshold voltage of the first transistor M 1 . Therefore, the present invention can display images having a desired brightness, regardless of the threshold voltage of the first transistor M 1 . Also, the current flowing onto the organic light emitting diode OLED is affected by the voltage ΔV 1 . Here, the voltage values of ΔV 1  is determined by the deviation of the first transistor M 1  so that the effect by the deviation of the first transistor M 1  can be compensated. 
         [0062]    Meanwhile, if the supply of the scan signal to the scan line Sn is stopped, the second transistor M 2  and the third transistor M 3  are turned off. In this case, the fourth transistor M 4  supplies the current corresponding to the voltage applied to the first node N 1  to the third node N 3 . The particular deviation and the threshold voltage of the fourth transistor M 4 , which is positioned in the same pixel as the first transistor M 1 , are set to be almost the same as those of the first transistor M 1 . Therefore, the current supplied from the fourth transistor M 4  to the third node N 3  is determined as shown in equation 4. 
         [0063]    The first transistor M 1  supplies the current supplied to the third node N 3  to the organic light emitting diode OLED. Then, the organic light emitting diode OLED generates light having a predetermined brightness. 
         [0064]    Meanwhile, when the secondary period of the period while the scan signal is supplied is set to be long, the voltage of the second node N 2  rises to the voltage obtained by subtracting the threshold voltage of the first transistor M 1  from the voltage applied to the first node N 1 . Therefore, the secondary period, that is, the turn-off time point of the scan signal, is set to the voltage value before the voltage of the second node N 2  rises to the value obtained by the threshold voltage from the voltage of the first node N 1 . Actually, the secondary period is experimentally determined in consideration of characteristics, process conditions and design deviation of the transistor. 
         [0065]    In the present invention, the second capacitor C 2  keeps the rising voltage of the second node N 2  smaller than the rising voltage of the first node N 1 , such that gray scale cannot be displayed. More specifically, when the second capacitor C 2  is removed, the first capacitor C 1  is charged with the voltage corresponding to the threshold voltage of the first transistor M 1  regardless of the supply of the data signal, such that the gray scale cannot be displayed. Therefore, the present invention controls the rising voltage of the second node N 2  using the second capacitor C 2 , such that the gray scale can be displayed. To this end, the second capacitor C 2  is set to have a lower capacity than the first capacitor C 1  (that is, C 1 &gt;C 2 ). 
         [0066]      FIG. 5  shows a second embodiment of the pixel of  FIG. 2 . When describing  FIG. 5 , the same reference numerals will be given to the same constitution of  FIG. 3  and the detailed description thereof will be omitted. 
         [0067]    Referring to  FIG. 5 , the pixel according to the second embodiment includes an organic light emitting diode OLED and a pixel circuit  142 ′ connected to a data line Dm and a scan line Sn to control the organic light emitting diode OLED. 
         [0068]    An anode electrode of the organic light emitting diode OLED is connected to the pixel circuit  142 ′ at node N 2 , and a cathode electrode thereof is connected to a second power supply ELVSS. The organic light emitting diode OLED as above generates light having a predetermined brightness, corresponding to current supplied from the pixel circuit  142 ′. 
         [0069]    The pixel circuit  142 ′ is charged with a voltage corresponding to a data signal supplied to the data line Dm when a scan signal is supplied to the scan line Sn and voltage corresponding to threshold voltage of a first transistor, and controls the amount of current supplied to the organic light emitting diode OLED corresponding to the charged voltage. To this end, the pixel circuit  142 ′ includes first to fifth transistors M 1  to M 5 , a first capacitor C 1  and a second capacitor C 2 ′. 
         [0070]    The second capacitor C 2 ′ is positioned between a second node N 2  and a third power supply V 3 . Here, the third power supply V 3  swings through a high and a low voltage. In other words, as shown in  FIG. 6 , the third power supply V 3  connected to the pixel  140  positioned in the nth horizontal line maintains a low voltage only during a period overlapping with a period while the scan signal is supplied to the scan line Sn, and maintains a high voltage during periods other than the period. 
         [0071]    Comparing the pixel  140  according to the second embodiment of the present invention with the pixel  140  according to the first embodiment of the present invention, the fifth transistor M 5  is omitted and the second capacitor C 2  is connected to the third power supply V 3  in the second embodiment. In other words, the organic light emitting diode OLED is initialized using the third power supply V 3  in the second embodiment. 
         [0072]      FIG. 6  is a waveform view showing a method of driving the pixel of  FIG. 5 . 
         [0073]    Specifically describing the operation process of the pixel  140  of  FIG. 5  by combining  FIGS. 5 and 6 , the voltage of the third power supply V 3  is first fallen to a low voltage. If the voltage of the third power supply V 3  is fallen, the voltage of the second node N 2  is also fallen by a coupling phenomenon of the second capacitor C 2 ′. At this time, the voltage values of the low voltage of the third power supply V 3  are set so that the voltage of the second node N 2  is set to be lower than voltage obtained by subtracting the threshold voltage of the first transistor M 1  from the voltage of the reference power supply Vref. 
         [0074]    Thereafter, the second transistor M 2  and the third transistor M 3  are turned on by the scan signal supplied to the scan line Sn. If the second transistor M 2  and the third transistor M 3  are turned on, the reference power Vref, supplied to the data line Dm during the primary period of the period while the scan signal is supplied, is supplied to the first node N 1  and the third node N 3 . 
         [0075]    Here, the first node N 1  and the third node N 3  are set to have the same voltage (that is, the reference power Vref) so that the fourth transistor M 4  is turned off. The first transistor M 1  is connected in a diode shape so that the voltage of the second node N 2  rises to the voltage obtained by subtracting the threshold voltage of the first transistor M 1  from the reference power Vref (that is, Vref−Vth (M 1 )). Here, the voltage of Vref−Vth (M 1 ) is set to be lower than the threshold voltage of the organic light emitting diode OLED and thus, unnecessary light is not generated in the organic light emitting diode OLED. 
         [0076]    Meanwhile, the voltage of the reference power supply Vref is applied to the first node N 1  and the voltage obtained by subtracting the threshold voltage of the first transistor M 1  from the reference power Vref is applied to the second node N 2 , such that the voltage corresponding to the threshold voltage of the first transistor M 1  is charged in the first capacitor C 1 . 
         [0077]    Therefore, the data signal Ds is supplied to the data line Dm during a secondary period of the period while the scan signal is supplied. If the data signal DS is supplied to the data line Dm, the voltage of the first node N 1  is determined as shown in the equation 1. When the voltage of the first node N 1  is changed as shown in the above equation 1, voltage variation of the second node N 2  is changed as shown in the above equation 2 by a coupling phenomenon of the first capacitor C 1 . 
         [0078]    Here, when the voltage of the second node N 2  is changed into the voltage ΔV 1 , the voltage between the gate and source electrode of the first transistor M 1  may be represented by the above equation 3. The current flowing onto the organic light emitting diode OLED may be represented by the above equation 4. 
         [0079]    Thereafter, the supply of the scan signal to the scan line Sn is stopped, such that the second transistor M 2  and the third transistor M 3  are turned off. The voltage of the third power supply V 3  rises to a high voltage. Here, since the first node N 1  is set to be in a floating state, the voltage charged in the first capacitor C 1  maintains the voltage charged prior to the previous period, although the voltage of the third power supply V 3  rises to the high voltage. In other words, the voltage Vgs of the first transistor M 1  maintains the voltage charged during the previous period regardless of the rising voltage of the third power supply V 3 , thereby making it possible to provide desired current to the organic light emitting diode OLED. 
         [0080]    While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiment, 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.