Abstract:
An organic light emitting display and a driving method for the display. Pixel circuits of the display, allow an increase in a data current in order to increase speed of charging the data current in a data line. Consequently, writing speed of data in the data line is higher with higher data current. The pixel circuit adjusts drive current passing through organic light emitting diodes to prevent an increase in the drive current due to the increased data current. The driving method divides non-emitting periods of the organic light emitting diode within one frame period. Therefore, the organic light emitting diode emits light at least twice within one frame period resulting in shorter non-emitting periods. Shortening the length of the non-emitting periods prevents flicker and image sticking even when the duty ratio or the overall duration of the emitting period remains constant.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of Korean Patent Application No. 2005-44696, filed on May 26, 2005, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to an organic light emitting display and a driving method for the display and, more particularly, to an organic light emitting display and a driving method, with improved data writing speed and reduced occurrence of flicker and image sticking.  
       BACKGROUND  
       [0003]     Various flat panel displays have been developed as substitutes for a Cathode Ray Tube (CRT) display that is relatively heavy and bulky. Flat panel displays include. Liquid Crystal Displays (LCDs), Field Emission Displays (FEDs), Plasma Display Panels (PDPs), Organic Light Emitting Displays, and the like.  
         [0004]     An organic light emitting display among flat display devices includes an anode electrode, a cathode electrode, and an emission layer disposed between the anode and cathode electrodes. The organic light emitting display is an emissive display that generates light by recombination of electrons and holes in the emission layer. The organic light emitting display has advantages of high response speed and low power consumption when compared with other types of display devices, such as a liquid crystal display device, that require an additional light source.  
         [0005]      FIG. 1  is a circuit diagram showing a conventional current drive pixel. The conventional current drive pixel includes an organic light emitting diode OLED and a pixel circuit. The pixel circuit includes first through fourth transistors T 1 , T 2 , T 3 , T 4  and a capacitor C 1 . The first through fourth transistors T 1 , T 2 , T 3 , T 4  each include a gate, a source, and a drain. The capacitor C 1  includes a first electrode and a second electrode.  
         [0006]     Current through the first transistor T 1  is controlled by a data current Idata applied through the second transistor T 2 . The applied data current Idata is maintained for a predetermined time by the capacitor C 1 . The capacitor C 1  is coupled between a source and a gate of the first transistor T 1 .  
         [0007]     A scan line Sn is coupled to a gate of the second transistor T 2  and a gate of the third transistor T 3 . A data line Dm is coupled to a source of the second transistor T 2 . A source and a drain of the third transistor T 3  are coupled to a drain and the gate of the first transistor T 1 , respectively. A source of the fourth transistor T 4  is coupled to a first power supply ELVdd, a drain of this transistor is coupled to the source of the first transistor T 1 , and a gate of the fourth transistor T 4  is coupled to a light emitting control line En.  
         [0008]     During operation, a scan signal sn applied to the gates of the second and third transistors T 2 , T 3  goes to a low level in order to turn on the second and third transistors T 2 , T 3  that are shown as PMOS field effect transistors in  FIG. 1  and are turned on with a negative gate to source voltage. The first transistor T 1  is diode-connected and a voltage corresponding to the data current Idata is stored in the capacitor C 1 .  
         [0009]     When the scan signal sn changes to a high level to turn off the second and third transistors T 2 , T 3 , and a light emitting control signal en goes to a low level to turn on the fourth transistor T 4 , power is supplied from the first power supply ELVdd and an electric current corresponding to the voltage stored in the capacitor C 1  flows from the first transistor T 1  to the light emitting diode OLED causing it to emit light. The electric current flowing through the light emitting diode OLED I OLED  is expressed by equation 1.  
             Idata   =         β   2     ⁢       (     Vgs   -        Vth          )     2       =     I   OLED               (   1   )             
 
 where, Idata is the data current, Vgs is a voltage between the source and the gate of the first transistor T 1 , Vth is a threshold voltage of the first transistor T, I OLED  is the electronic current flowing through the light emitting diode OLED, and β is a gain factor of the first transistor T 1 . 
 
         [0010]     As indicated in the equation 1, although the threshold voltage Vth and mobility of the first transistor T 1  may not be uniform among different pixel circuits, the electric current I OLED  flowing through the light emitting diode OLED is identical with the data current Idata. Accordingly, when a write current source of a data driver, which supplies the data current Idata to the pixel circuits, remains uniform throughout the panel, uniform display properties may be obtained.  
         [0011]     As described above, because a conventional current write type pixel circuit controls a minute current, it takes a long time to charge a capacitive load of the data line with the data current Idata. For example, assuming that the capacitive load of a data line is 30 pF, it takes several ms to charge the data line with an electric current ranging between several tens nA and several hundreds nA. Accordingly, a line time of several tens μs would not be sufficient for charging the capacitive load of the data line specially for low brightness where the data current Idata is small. The above discussion indicates that the conventional current write type pixel circuit requires a long charging time. Therefore, there is a need for a pixel circuit with a shorter charging time.  
       SUMMARY OF THE INVENTION  
       [0012]     Accordingly, it is an aspect of the present invention to provide an organic light emitting display and a driving method for the display that increase the data current in order to reduce the speed of charging the data current in a data line. Embodiments of the invention also divide a non-emitting period of an organic light emitting diode from one frame period in order to prevent occurrence of flicker and image sticking.  
         [0013]     In one embodiment of the present invention an organic light emitting display is presented that includes a pixel portion having a plurality of pixels for displaying an image, a scan driver for providing a scan signal and a light emitting control signal to the pixel portion, and a data driver for providing a data current to the pixel portion. The pixels emit light corresponding to the data current selected by the scan signal during light emitting time periods that are separated by non-emitting periods that occur at least twice during one frame period. The light emitting periods and non-emitting periods occur according to the light emitting control signal. A drive current that is lower than the data current is used to drive the organic light emitting diodes.  
         [0014]     According to one aspect of the present invention, an organic light emitting display is presented that includes a pixel portion having a plurality of pixels for displaying an image, a scan driver, and a data driver for providing a data current to the pixel portion. The scan driver is for providing a scan signal, a boosting signal, and a light emitting control signal to the pixel portion. The pixels that are selected by the scan signal, emit light in response to the light emitting control signal, during emitting periods that occur at least twice during one frame period. The organic light emitting diodes emit light corresponding to a drive current which is proportional to but lower than the data current provided by the data driver.  
         [0015]     According to one aspect of the present invention, a method for driving an organic light emitting display including pixels that emit light corresponding to a drive current is presented. The method includes conducting a data current to the pixels and generating the drive current by the data current and conducting the drive current to an organic light emitting diode at least twice during one frame period causing the organic light emitting diode to emit light of a lower intensity than an intensity corresponding to the data current.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a circuit diagram showing a conventional current drive pixel.  
         [0017]      FIG. 2  shows an organic light emitting display according to a first embodiment of the present invention.  
         [0018]      FIG. 3  is a wave form chart showing the operation of a scan driver shown in  FIG. 2 .  
         [0019]      FIG. 4  is a circuit diagram showing an example of a pixel used for the organic light emitting display of  FIG. 2 .  
         [0020]      FIG. 5  is a circuit diagram showing another example of a pixel used for the organic light emitting display of  FIG. 2 .  
         [0021]      FIG. 6  shows an organic light emitting display according to a second embodiment of the present invention.  
         [0022]      FIG. 7  is a wave form chart showing the operation of a scan driver shown in  FIG. 6 .  
         [0023]      FIG. 8  is a circuit diagram showing an example of a pixel used for the organic light emitting display of  FIG. 6 .  
         [0024]      FIG. 9  is a circuit diagram showing another example of a pixel used for the organic light emitting display of  FIG. 6 .  
         [0025]      FIG. 10  shows an organic light emitting display according to a third embodiment of the present invention.  
         [0026]      FIG. 11  is a graph showing flicker of the organic light emitting display confirmed by sight. 
     
    
     DETAILED DESCRIPTION  
       [0027]      FIG. 2  shows an organic light emitting display  1000  according to a first embodiment of the present invention. The first embodiment of the organic light emitting display  1000  includes a pixel portion  101 , a data driver  200 , and a scan driver  300 . The pixel portion  101  forms an image. The data driver  200  provides a data current. The scan driver  300  provides a scan signal.  
         [0028]     The pixel portion  101  includes a plurality of pixels  111  each including a light emitting diode and a pixel circuit, a plurality of scan lines S 1 , S 2  . . . Sn- 1 , Sn arranged in a column direction, a plurality of data lines D 1 , D 2  . . . Dm- 1 , Dm arranged in a row direction, a plurality of light emitting control lines E 1 , E 2  . . . En- 1 , En arranged in a column direction, and a plurality of first power lines Vdd (not shown) for supplying power.  
         [0029]     Furthermore, in the pixel portion  101 , when the data current is conducted to the pixel  111  through the data lines D 1 , D 2  . . . Dm- 1 , Dm by the scan signal from the scan lines S 1 , S 2  . . . Sn- 1 , Sn, the pixel  111  generates a drive current corresponding to the data current. In response to a light emitting control signal conducted through the light emitting control lines E 1 , E 2  . . . En- 1 , En, the drive current flows through the pixel  111  causing the pixel  111  to emit light.  
         [0030]     The data driver  200  is coupled to the plurality of data lines D 1 , D 2  . . . Dm- 1 , Dm and provides the data current to the pixels  111  through the data lines D 1 , D 2  . . . Dm- 1 , Dm, causing the pixels  111  to generate the drive current corresponding to the data current. Moreover, the amount of the data current is boosted to a value greater than the current required for driving the pixels  111 . The boosted data current being conducted through the data lines D 1 , D 2  . . . Dm- 1 , Dm allows the data lines D 1 , D 2  . . . Dm- 1 , Dm to be rapidly charged, thereby embodying a high speed data write.  
         [0031]     The scan driver  300  is coupled to the pixel portion  101  through the scan lines S 1 , S 2  . . . Sn- 1 , Sn and the light emitting control lines E 1 , E 2  . . . En- 1 , En, and provides the scan signals and the light emitting control signals to the pixels  111 .  
         [0032]     The scan driver  300  provides the data current to a pixel  111  selected by the scan signal and causes the pixel  111  to emit light for a period of time determined by the drive current. The drive current is generated in the pixel  111  in response to the light emitting control signal. Consequently, the pixel  111  divides every one frame period into an emission period and a non-emission period. The brightness of the organic light emitting display is represented by this division during the one frame period.  
         [0033]     As described above, in a case where the pixel  111  emits light once during one frame period, if a user senses the non-emission period, a flicker may be perceived. In addition, when the pixel  111  emits the light once during one frame period, the data signal input to the pixel  111  may stay longer than the predetermined required time. This leads to the occurrence of image sticking.  
         [0034]     One frame is divided into emitting periods, when light is being emitted, and non-emitting periods. The pixel  111  emits light a plurality of times during one frame period according the light emitting control signal. This causes the non-emitting periods to occur also a plurality of times. This does not allow a user to sense the non-emitting periods. When the pixel  111  emits light for a short time, the maintaining time of the data in the pixel  111  is kept short to prevent image flickering from occurring.  
         [0035]      FIG. 3  is a wave form chart showing the operation of the scan driver  300  of the first embodiment of the organic light emitting display  1000  shown in  FIG. 2 . With reference to  FIG. 3 , the scan driver  300  includes a scan signal generator for generating a scan signal and a light emitting control signal generator for generating a light emitting control signal. The scan driver  300  receives a first start signal  1 SP, a second start signal  2 SP, and a clock signal CLK, and generates and provides the scan signal and the light emitting control signal to the pixel portion  101 .  
         [0036]     The scan signal generator includes a shift register. When the first start signal  1 SP is input to the scan signal generator, it outputs a first scan shift signal  1 SR obtained by shifting the first start signal  1 SP. Also, the scan signal generator outputs a second scan shift signal  2 SR using the first scan shift signal  1 SR, and a third scan shift signal  3 SR using the second scan shift signal  2 SR. By repeating the aforementioned operation, n scan shift signals are sequentially generated and output. A first scan signal s 1  is generated by logically combining the first start signal  1 SP and the first scan shift signal  1 SR. A second scan signal s 2  is output by logically combining the first scan shift signal  1 SR and the second scan shift signal  2 SR. By logically combing the second scan shift signal  2 SR and third scan shift signal  3 SR, a third scan signal s 3  is output. Through repeating the above mentioned operation, the scan signal generator generates n scan signals s 1  . . . sn. Because the scan shift signals are sequentially generated, the corresponding n scan signals are also sequentially generated.  
         [0037]     The light emitting control signal generator also includes a shift register. When the second start signal  2 SP is input to the light emitting control signal generator, it outputs a first light emitting control shift signal  1 ER obtained by shifting the second start signal  2 SP. Furthermore, the light emitting control signal generator outputs a second light emitting control shift signal  2 ER using the first light emitting control shift signal  1 ER. In addition, the light emitting control signal generator outputs a third light emitting control shift signal  3 ER using the second light emitting control shift signal  2 ER. By repeating this operation, n light emitting control shift signals are sequentially generated and output. Furthermore, the second start signal  2 SP and the first light emitting control shift signal  1 ER are logically combined to generate a first light emitting control signal e 1 , the first light emitting control shift signal  1 ER and the second light emitting control shift signal  2 ER are logically combined to output a second light emitting control signal e 2 . Moreover, the second light emitting control shift signal  2 ER and the third light emitting control shift signal  3 ER are logically combined to output a third light emitting control signal e 3 . Through repeating this operation, n light emitting control signals are generated. Because the light emitting control shift signals are sequentially generated, the corresponding n light emitting control signals are also sequentially generated.  
         [0038]     In the embodiment shown, a second start signal  2 SP includes two pulses within one frame period, so that the first light emitting control shift signal  1 ER also includes two pulses within the one frame period. Operation of the shift register causes each light emitting control shift signal to include two pulses. However, the second start signal  2 SP may include more pluses than two, in which case the light emitting control signal would also include more pulses than two.  
         [0039]     During generation period of a pulse in the light emitting control signal, a drive current flows to a pixel  111  that causes the pixel  111  to emit light. During a non-generation period of the pulse, the drive current does not flow to the pixel  111  and the pixel  111  does not emit light. Consequently, during one frame period, because emitting periods and non-emitting periods of the pixel  111  alternate, the non-emitting periods of the pixel  111  occur for a short period. As a result, a user cannot sense the non-emitting periods of the pixel  111 , thereby preventing a flicker from occurring.  
         [0040]      FIG. 4  is a circuit diagram showing a first example of the pixel  111  used for the organic light emitting display  1000  of  FIG. 2 . The pixel  111  of  FIG. 4  includes an organic light emitting diode OLED and a pixel circuit. The pixel circuit includes first to fourth transistors M 11 , M 21 , M 31 , M 41  and a capacitor C 11 . Each of the first to fourth transistors includes a source, a drain, and a gate. The capacitor C 11  includes a first electrode and a second electrode.  
         [0041]     Each of the first to fourth transistors M 11 , M 21 , M 31 , M 41  is embodied by a PMOS transistor. Because the source and the drain of each transistor have the same properties, they may be called a first electrode and a second electrode, respectively.  
         [0042]     A source of the first transistor M 11  is coupled with a pixel power supply, a drain of this transistor is coupled with a first node A 1 , and its gate is coupled with a second node B 1 . The first transistor M 11  conducts a drive current from the source to a drain side in response to a voltage of the second node B 1 .  
         [0043]     A source of the second transistor M 21  is coupled with a data line Dm, a drain of this transistor is coupled with the second node B 1 , and its gate is coupled with a scan line Sn. The second transistor M 21  conducts a data current to the second node B 1  in response to a scan signal being conducted through the scan line Sn.  
         [0044]     A source of the third transistor M 31  is coupled with the data line Dm, a drain of this transistor is coupled with the first node A 1 , and its gate is coupled with the scan line Sn. The third transistor M 31  conducts the data current to the first node A 1  in response to a scan signal conducted through the scan line Sn.  
         [0045]     The second transistor M 21  and the third transistor M 31  maintain the same state in response to the scan signal. When the second transistor M 21  and the third transistor M 31  are turned on, the source and the drain of the first transistor M 11  have the same voltage causing the first transistor M 11  to be diode-connected.  
         [0046]     A source of the fourth transistor M 41  is coupled with the first node A 1 , a drain of this transistor is coupled with the organic light emitting diode OLED, and its gate is coupled with a light emitting control line En. The fourth transistor M 41  conducts the drive current flowing through the first transistor M 11  to the organic light emitting diode OLED in response to a light emitting control signal conducted through the light emitting control line En. In response to a light emitting control signal to control the fourth transistor M 41 , the fourth transistor M 41  repeats a switching operation to control a light emitting time of the organic light emitting diode OLED.  
         [0047]     Operation of the pixel is described by reference to  FIG. 3 . The pixel operates in response to the scan signal sn, the data current, and a light emitting control signal en.  
         [0048]     When the scan signal sn goes to a low level, both of the second transistor M 21  and the third transistor M 31  are turned on causing the first transistor M 11  to be diode-connected. Accordingly, a drive current corresponding to a data current flows from the source to the drain of the first transistor M 11 . Based on the previously presented equation 1, a voltage between the source and the gate of the first transistor M 11  is expressed by a following equation 2.  
             Idata   =       β   2     ⁢       (     Vgs   -        Vth          )     2               (   1   )               Vgs   =           2   ⁢   Idata     β       +   Vth             (   2   )             
 
 where, Idata is an applied data current, Vgs is a voltage between the gate and the source of the first transistor M 11 , Vth is a threshold voltage of the first transistor M 11 , and β is a gain factor of the first transistor M 11 . 
 
         [0049]     Furthermore, when the scan signal sn changes to a high level, and the second transistor M 21  and the third transistor M 31  are turned off, the capacitor C 11  maintains the voltage at the source and the gate of the first transistor M 11  constant. Moreover, when the light emitting control signal en changes to a low level to turn on the fourth transistor M 41 , a drive current flowing through the first transistor M 11  flows into the organic light emitting diode OLED through the fourth transistor M 41 , thus causing the organic light emitting diode OLED to emit light.  
         [0050]     Also, as shown in  FIG. 5 , the pixel may be embodied by NMOS transistors. In that case, inverted signals of the wave form of  FIG. 3  are used as inputs to the pixel  111 N.  
         [0051]      FIG. 6  shows an organic light emitting display  2000  according to a second embodiment of the present invention. This organic light emitting display  2000  includes a pixel portion  102 , a data driver  200 , and a scan driver  302 . The pixel portion  102  forms the image to be presented. The data driver  200  provides a data current. The scan driver  302  provides a scan signal.  
         [0052]     The pixel portion  102  includes a plurality of pixels  112  each including a light emitting diode and a pixel circuit, a plurality of scan lines S 1 , S 2  . . . Sn- 1 , Sn arranged in a column direction, a plurality of data lines D 1 , D 2  . . . Dm- 1 , Dm arranged in a row direction, a plurality of light emitting control lines E 1 , E 2  . . . En- 1 , En arranged in a column direction, and a plurality of first power lines Vdd (not shown) for supplying a pixel with power.  
         [0053]     Furthermore, in the pixel portion  102 , when the data current is conducted to the pixel  112  through the data lines D 1 , D 2  . . . Dm- 1 , Dm in response to the scan signal from the scan lines S 1 , S 2  . . . Sn- 1 , Sn the pixel  112  generates a drive current corresponding to the data current. In response to a light emitting control signal conducted through the light emitting control lines E 1 , E 2  . . . En- 1 , En the drive current flows through the pixel  112  to cause the pixel  112  to emit light.  
         [0054]     The data driver  200  is coupled to the plurality of data lines D 1 , D 2  . . . Dm- 1 , Dm and conducts the data current to the pixels  112  through the data lines D 1 , D 2  . . . Dm- 1 , Dm so that the pixels  112  generate a drive current corresponding to the data current. The amount of the data current is boosted to be greater than that of the drive current. The boosted data current is conducted to the data lines D 1 , D 2  . . . Dm- 1 , Dm so that the data lines D 1 , D 2  . . . Dm- 1 , Dm are rapidly charged, thereby embodying a data write of a high speed.  
         [0055]     The scan driver  302  is coupled to the pixel portion  102  through the scan lines S 1 , S 2  . . . Sn- 1 , Sn and the light emitting control lines E 1 , E 2  . . . En- 1 , En, and conducts the scan signal and the light emitting control signal to the pixels  112 .  
         [0056]     The scan driver  302  provides the data current to a pre-determined pixel  112  in response to the scan signal, and causes the selected pixel  112  to emit light corresponding to the drive current generated in the pixel  112  in response to the light emitting control signal. Consequently, the pixel  112  divides one frame period into an emission period and a non-emission period, thereby causing brightness of the organic light emitting display to be represented during the one frame period.  
         [0057]     As described above, in a case where the pixel  112  emits light once during one frame period, a user senses the non-emission period as a flicker.  
         [0058]     One frame is divided into emitting periods and non-emitting periods. Accordingly, the pixel  112  emits light a plurality of times during one frame period according to the light emitting control signal. This causes the non-emitting periods to be displayed a plurality of times but for short periods, so that a user cannot sense the non-emitting periods.  
         [0059]      FIG. 7  is a wave form chart showing the operation of the scan driver  302  shown in  FIG. 6 . The scan driver  302  includes a scan signal generator for generating a scan signal and a light emitting control signal generator for generating a light emitting control signal. The scan driver  302  receives a first start signal  1 SP, a second start signal  2 SP, and a clock signal CLK, and generates and provides the scan signal, the light emitting control signal, and a boosting signal to the pixel portion  102 .  
         [0060]     The scan signal generator includes a shift register. When the first start signal  1 SP is input to the scan signal generator, the scan signal generator outputs a first scan shift signal  1 SR obtained by shifting the first start signal  1 SP. Furthermore, the scan signal generator outputs the second scan shift signal  2 SR using the first scan shift signal  1 SR, and outputs a third scan shift signal  3 SR using the second scan shift signal  2 SR. By repeating this operation, n scan shift signals are sequentially generated and output. A first scan signal s 1  is generated by logically combining the first start signal  1 SP and the first scan shift signal  1 SR. A second scan signal s 2  is output by logically combining the first scan shift signal  1 SR and the second scan shift signal  2 SR. By logically combing the second scan shift signal  2 SR and third scan shift signal  3 SR, a third scan signal s 3  is output. Through repetition of the above operation, the scan signal generator generates n scan signals. Because the scan shift signals are sequentially generated, the corresponding n scan signals are also sequentially generated.  
         [0061]     The light emitting control signal generator also includes a shift register. When the second start signal  2 SP is input to the light emitting scan signal generator, it outputs the first light emitting shift signal obtained by shifting the second start signal  2 SP. The light emitting control signal generator outputs a second light emitting control shift signal  2 ER using the first light emitting control shift signal  1 ER. Further, the light emitting control signal generator outputs a third light emitting control shift signal  3 ER using the second light emitting control shift signal  2 ER. By repeating this operation, n light emitting control shift signals are sequentially generated and output. The second start signal  2 SP and the first light emitting control shift signal are logically combined to generate a first light emitting control signal e 1 . The first light emitting control shift signal  1 ER and the second light emitting control shift signal  2 ER are logically combined to output a second light emitting control signal e 2 . Similarly, the second light emitting control shift signal  2 ER and the third light emitting control shift signal  3 ER are logically combined to output a third light emitting control signal e 3 . Through repeating this operation, n light emitting control signals are generated. Because the light emitting control shift signals are sequentially generated, the corresponding n light emitting control signals are also sequentially generated.  
         [0062]     In addition, a second start signal  2 SP is embodied by two pulses, so that the first light emitting control shift signal  1 ER is embodied by two pulses. Operation of the shift register causes each light emitting control shift signal to be embodied by two pulses. However, the second start signal  2 SP may be embodied by more pluses than two, in which case the corresponding light emitting control signal would include more than two pulses.  
         [0063]     Further, the scan driver  302  generates and provides the boosting signal to the pixel through a boosting line Bn.  
         [0064]      FIG. 8  is a circuit diagram showing an example of the pixel  112  used for the second embodiment of the organic light emitting display  2000  shown in  FIG. 6 . Referring to  FIG. 8 , the pixel  112  includes an organic light emitting diode OLED and a pixel circuit. The pixel circuit includes first to fourth transistors M 12 , M 22 , M 32 , M 42 , a first capacitor C 12 , and a second capacitor C 22 . Each of the first to fourth transistors includes a source, a drain, and a gate. The first capacitor C 12  and the second capacitor C 22  each include a first electrode and a second electrode.  
         [0065]     In the exemplary embodiment shown, each of the first to fourth transistors M 12 , M 22 , M 32 , M 42  is embodied by a PMOS transistor. Because a source and a drain of each transistor have the same properties, they may be called a first electrode and a second electrode, respectively.  
         [0066]     A source of the first transistor M 12  is coupled with a pixel power supply, a drain thereof is coupled with a first node A 2 , and a gate thereof is coupled with a second node B 2 . The first transistor M 12  conducts a drive current from its source to its drain in response to a voltage at the second node B 2 .  
         [0067]     A source of the second transistor M 22  is coupled with a data line Dm, a drain thereof is coupled with a second node B 2 , and a gate thereof is coupled with a scan line Sn. The second transistor M 22  conducts a data current to the second node A 2  in response to a scan signal conducted through the scan line Sn.  
         [0068]     A source of the third transistor M 32  is coupled with the first node A 2 , a drain thereof is coupled with the data line Dm, and a gate thereof is coupled with the scan line Sn. The third transistor M 32  conducts a current flowing from its source to its drain in response to a scan signal conducted through the scan line Sn.  
         [0069]     A first electrode of the first capacitor C 12  is coupled to the pixel power supply ELVdd and a second electrode thereof is coupled to the second node B 2 . The first capacitor C 12  maintains a voltage corresponding to the data signal for a predetermined time.  
         [0070]     A first electrode of the second capacitor C 22  is coupled to the second node B 2  and a second electrode thereof is coupled to a boosting signal line Bn. The second capacitor C 22  changes a gate voltage of the first transistor M 12  according to the boosting signal, causing an electric current flowing from the source to the drain of the first transistor M 12  to be reduced. Consequently, the drive current flowing into the organic light emitting diode OLED will be less than the data current. The pixel circuit  112  allows maximizing the data current without increasing the drive current. Maximizing the amplitude of the data current for charging the data lines allows the time for charging the data line to be shorter.  
         [0071]     A source of the fourth transistor M 42  is coupled with the first node A 2 , a drain thereof is coupled with the organic light emitting diode OLED, and a gate thereof is coupled with a light emitting control line En. The fourth transistor M 42  conducts the drive current flowing from the first transistor M 12 , through the first node A 2 , to the organic light emitting diode OLED in response to a light emitting control signal conducted through the light emitting control line En.  
         [0072]     Operation of the pixel  112  is described by reference to  FIG. 7 . The pixel  112  operates in response to the scan signal sn, the data current, the boosting signal bn, and the light emitting control signal en.  
         [0073]     During a period when the light emitting control signal en has a high state, the boosting signal bn is low and the scan signal sn is also low.  
         [0074]     When the scan signal sn changes to a low level, both of the second transistor M 22  and the third transistor M 32  are turned on causing the data current Idata to flow from the source to the drain of the first transistor M 12  and causing the first transistor M 12  to become diode-connected. A voltage between the gate and the source of the first transistor M 12  is based on the formerly presented equation 1 and is expressed by a following equation 3.  
             Idata   =       β   2     ⁢       (     Vgs   -        Vth          )     2               (   1   )               Vgs   =           2   ⁢   Idata     β       +   Vth             (   3   )             
 
 where, Idata is an applied data current, Vgs is a voltage between the gate and the source of the first transistor M 12 , Vth is a threshold voltage of the first transistor M 12 , and β is a gain factor of the first transistor M 12 . 
 
         [0075]     Furthermore, after the scan signal sn changes to a high level, and the second transistor M 22  and the third transistor M 32  are turned off, the light emitting control signal en changes to a low level to turn on the fourth transistor M 42 . When the fourth transistor M 42  is turned on, an electric current flowing through the first transistor M 12  flows into the organic light emitting diode OLED through the fourth transistor M 42 , thus causing the light emitting diode OLED to emit light.  
         [0076]     In this case, when the second transistor M 22  is turned off, the gate voltage of the first transistor M 12  is increased by coupling of the first capacitor C 12  to the second capacitor C 22 . The increased gate voltage of the first transistor M 12  is expressed by a following equation 4.  
               Δ   ⁢           ⁢   Vg     =       Δ   ⁢           ⁢     Vselect   ·   C     ⁢           ⁢   2         C   ⁢           ⁢   1     +     C   ⁢           ⁢   2                 (   4   )             
 
 where, ΔVg is the amount by which the gate voltage of the first transistor M 12  is increased by the coupling of the first capacitor C 12  and the second capacitor C 22 , and ΔVselect is voltage amplitude of a selection signal. 
 
         [0077]     In addition, the electric current flowing through the organic light emitting diode OLED is expressed by a following equation 5.  
               I   OLED     =       β   2     ⁢       (     Vgs   -     Δ   ⁢           ⁢   Vg     -        Vth          )     2               (   5   )             
 
 where, I OLED  is an electric current flowing through the organic light emitting diode, Vgs is a voltage between the gate and source of the first transistor M 12  when the data current flows through the first transistor M 12 , ΔVg is the amount by which the gate voltage of the first transistor M 12  is increased by the coupling of the first capacitor C 12  and the second capacitor C 22 , Vth is a threshold voltage of the first transistor M 12 , and β is a gain factor of the first transistor M 12 . An increase in the voltage applied to the gate of the first transistor M 12  causes the drive current to be reduced. This allows the pixel circuit to use a larger data current and to attain a higher speed for writing to the data line without increasing the drive current that passes through the organic light emitting diode OLED. 
 
         [0078]     Also, as shown in  FIG. 9 , the pixel may be embodied by NMOS transistors. In that case, inverted signals of the wave form of  FIG. 7  are used as inputs to the pixel  112 N.  
         [0079]      FIG. 10  shows an organic light emitting display  3000  according to a third embodiment of the present invention. A scan signal generator  310  of the scan driver is formed on one side of the pixel portion  100 , a light emitting control signal generator  320  is formed on another side of the pixel portion  100 , so that the organic light emitting display  3000  is symmetric.  
         [0080]     When the scan signal generator  310  and the light emitting control signal generator  320  are formed within one scan driver, a dummy space is formed on a side opposite to the side where the scan driver is formed in order to form a symmetric organic light emitting display. As the scan signal generator  310  and the light emitting control signal generator  320  are formed within one scan driver, the size of the scan driver formed with be greater than the size of either of the scan signal generator  310  or the light emitting control signal generator  320 . As a result, when the scan signal generator  310  is formed on one side of the pixel portion  100  and the light emitting control signal generator  320  is formed on the other side, the overall size of the organic light emitting display  3000  may be smaller.  
         [0081]      FIG. 11  is a graph showing flicker grades for the organic light emitting displays of the invention. The grading of the flicker is done by sight.  FIG. 11  shows flicker grades in green and blue parts of the images of the organic light emitting displays when the pixel portion emits light once, twice, and four times during one frame period. Table 1 shows a qualitative amount of flicker that each flicker grade shown on the vertical axis of  FIG. 11  signifies.  
                               TABLE 1                       1   2   3   4   5                   No flicker   Small amount   Common   Large   Significant           of flicker   amount of   amount of   amount of               flicker   flicker   flicker                    
         [0082]     G or B notations in  FIG. 11  indicate that the organic light emitting display represents only green or blue, respectively. Pulse  1  denotes light that is emitted only once during one frame period, Pulse  2  denotes light that is emitted twice during one frame period, and Pulse  4  denotes light that is emitted four times during one frame period. Duty Ratio, shown on the horizontal axis, represents a ratio between the emitting period and the non-emitting period during one frame period. For example, the Duty Ratio of 100% indicates that there is no non-emitting period during the frame and the entire frame period corresponds to the light-emitting period. As the Duty Ratio is reduced, the portion corresponding to the non-emitting period is increased. For example a Duty Ratio of 20% indicates that 20% of the frame period corresponds to the emitting period and 80% to the non-emitting period.  
         [0083]     The graph of  FIG. 11  indicates that when the Duty Ratio is low, flicker degree is high for Pulse  1  and low for Pulse  4 . Consequently, for the same Duty Ratio, indicating the same overall duration of emitting period within one frame, the amount of flicker is reduced when the number of pulses are increased indicating that the emitting period is divided into more frequent but shorter emitting intervals.  
         [0084]     In accordance with a light emitting display and a driving method for the display of the present invention, the light emitting period of the organic light emitting diode during one frame is adjusted to adjust the brightness of the organic light emitting display. In order to preserve the same brightness as that of light emitted during an entire one frame period, a larger current should be applied to the organic light emitting diode. As a result, the amount data current provided to the data line is increased resulting in a faster writing speed of data to the data line. In addition, during one frame period, the organic light emitting diode emits light in a manner to divide each non-emitting period into shorter intervals. A shorter non-emitting period prevents flicker and image sticking from occurring.  
         [0085]     Although exemplary embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes might be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.