Patent Publication Number: US-8525761-B2

Title: Display device and method of driving the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2008-0034287, filed on Apr. 14, 2008, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a display device and a method of driving the same. More particularly, the present invention relates to an organic light emitting device and a method of driving the same. 
     2. Discussion of the Background 
     A hole-type flat panel display such as an organic light emitting device displays a fixed picture for a predetermined time period, (e.g., a frame), regardless of whether it is a still or motion picture. As an example, when a continuously moving object is displayed, the object moves from a particular position after being at the position for a certain time period of a frame, and is maintained at a new position of a frame for a certain time period before moving again, i.e., movement of the object is discretely displayed. Because the time period of a frame is a time period in which an afterimage is sustained, even if a picture is displayed in this way, movement of an object is continuously perceived. 
     However, when a continuously moving object is viewed on a screen, because a viewer&#39;s eye moves along with the object&#39;s movement, which conflicts with a discrete display of the display device, the viewer may see blur on a screen. For example, assume that the display device displays an object that stays at a position A in a first frame and at a position B in a second frame. In the first frame, the viewer&#39;s eye moves along an estimated movement path of the object from position A to position B. However, the object is not actually displayed at an intermediate position between position A and position B. 
     Therefore, because luminance that is recognized by a person for the first frame is a value, i.e., an average value of luminance of the object and luminance of a background that is obtained by integrating luminance of pixels at a path between position A and position B, an object is perceived to be blurred. 
     In the hole-type display device, because a degree to which the object is perceived to be blurred is proportional to a time period in which the display device sustains the display of the object, a so-called impulse driving method of displaying an image for only some time period and displaying a black color for the remaining time period within a frame has been suggested. In this method, because the time period in which an image is displayed decreases, luminance decreases, so a method of increasing luminance for a display time period or a method of displaying intermediate luminance using adjacent frames instead of a black color has been suggested. However, this method may increase power consumption and complicate driving. 
     Because a pixel of the organic light emitting device has an organic light emitting element and a thin film transistor (TFT) that drives the organic light emitting element, when the organic light emitting element and the TFT operate for a long time period, estimated luminance may not be displayed due to a change of the TFT&#39;s threshold voltage, and when characteristics of a semiconductor that is included in TFTs are not uniform within the display device, a luminance deviation between pixels may occur. 
     SUMMARY OF THE INVENTION 
     The present invention provides a display device and method of driving a display device. 
     Additional features of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. 
     The present invention discloses a display device including a scan driver that generates a plurality of scanning signals, a data driver that generates a data voltage, and a plurality of pixels that receive the data voltage according to the scanning signal and that display luminance corresponding to the data voltage. Each pixel receives its own data voltage and a data voltage of another pixel while displaying a black color when its own scanning signal is in a first state, and stops reception of the data voltage and displays luminance corresponding to its own data voltage when its own scanning signal is in a second state. 
     The present invention also discloses a display device including a scan driver that generates a plurality of scanning signals and a plurality of compensation signals, a data driver that generates a data voltage, and a plurality of pixels that receive the data voltage according to the scanning signals and that display luminance corresponding to the data voltage. Each pixel may include a light emitting element that emits light with an intensity according to a magnitude of a driving current; a capacitor that is connected between a first contact point and a second contact point; a driving transistor that has an input terminal connected to a first voltage and a control terminal connected to the second contact point, and that outputs the driving current; a first switching unit that connects the data voltage to the first contact point while the scanning signal is in a first state and that connects a second voltage to the first contact point while the scanning signal is in a second state; a second switching unit that switches connection between the second voltage and the second contact point according to the compensation signal; and a third switching unit that connects the second contact point to an output terminal of the driving transistor while the scanning signal is in the first state and that connects the light emitting element to the output terminal of the driving transistor while the scanning signal is in the second state. The data driver may change the data voltage in each one horizontal period, and the scanning signal may sustain the first state for a time period that is longer than one horizontal period. 
     The present invention also discloses a method of driving a display device, including outputting a data voltage that changes in each horizontal period, applying the data voltage to a pixel while stopping light emission of the pixel by applying a first scanning signal to the pixel for a time period that is longer than the one horizontal period, and allowing the pixel to emit light with luminance corresponding to the data voltage while stopping application of the data voltage to the pixel by applying a second scanning signal to the pixel, the first scanning signal and the second scanning signal having different levels from each other. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention. 
         FIG. 1  is a block diagram of an organic light emitting device according to an exemplary embodiment of the present invention. 
         FIG. 2  is an equivalent circuit diagram of a pixel in an organic light emitting device according to an exemplary embodiment of the present invention. 
         FIG. 3  shows an example of a driving signal waveform that may be applied to pixels of a row in an organic light emitting device according to an exemplary embodiment of the present invention. 
         FIG. 4 ,  FIG. 5 ,  FIG. 6 , and  FIG. 7  are equivalent circuit diagrams of a pixel in each period that is shown in  FIG. 3 . 
         FIG. 8  is a block diagram showing a configuration of a scan driver according to an exemplary embodiment of the present invention. 
         FIG. 9  shows an example of a circuit diagram of a shift register in the scan driver that is shown in  FIG. 8 . 
         FIG. 10  is a signal waveform diagram of an organic light emitting device having the scan driver of  FIG. 9 . 
         FIG. 11  shows another example of a circuit diagram of a shift register in the scan driver that is shown in  FIG. 8 . 
         FIG. 12  is a signal waveform diagram of an organic light emitting device having the scan driver of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Like reference numerals in the drawings denote like elements. 
     It will be understood that when an element or layer is referred to as being “on” or “connected to” another element or layer, it can be directly on or directly connected to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on” or “directly connected to” another element or layer, there are no intervening elements or layers present. 
     First, an organic light emitting device according to an exemplary embodiment of the present invention will be described with reference to  FIG. 1  and  FIG. 2 . 
       FIG. 1  is a block diagram of an organic light emitting device according to an exemplary embodiment of the present invention, and  FIG. 2  is an equivalent circuit diagram of a pixel in an organic light emitting device according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , the organic light emitting device includes a display panel  300 , a scan driver  400 , a data driver  500 , and a signal controller  600 . 
     The display panel  300  includes a plurality of signal lines (G 1 -G n , S 1 -S n , and D 1 -D m ), a plurality of voltage lines (not shown), and a plurality of pixels PX that are connected thereto and that are arranged in approximately a matrix form. 
     The signal lines (G 1 -G n , S 1 -S n , and D 1 -D m ) include a plurality of scanning signal lines (G 1 -G n ) that transmit a scanning signal, a plurality of compensation signal lines (S 1 -S n ) that transmit a compensation signal, and a plurality of data lines (D 1 -D m ) that transmit a data signal. The scanning signal lines (G 1 -G n ) and the compensation signal lines (S 1 -S n ) extend in approximately a row direction and are substantially parallel to each other, and the data lines (D 1 -D m ) extend in approximately a column direction and are substantially parallel to each other. 
     The voltage line includes a driving voltage line (not shown) that transmits a driving voltage. 
     As shown in  FIG. 2 , each pixel PX includes an organic light emitting element LD, a driving transistor Qd, a capacitor Cst, and five switching transistors Qs 1 , Qs 2 , Qs 3 , Qs 4 , and Qs 5 . 
     The driving transistor Qd has an output terminal, an input terminal, and a control terminal. The control terminal of the driving transistor Qd is connected to the capacitor Cst at a contact point N 2 , its input terminal is connected to a driving voltage Vdd, and its output terminal is connected to the switching transistor Qs 5 . 
     A first electrode of the capacitor Cst is connected to the driving transistor Qd at the contact point N 2 , and a second electrode of the capacitor Cst is connected to the switching transistors Qs 1  and Qs 2  at a contact point N 1 . 
     The switching transistors Qs 1 -Qs 5  may be formed in three switching units SU 1 , SU 2 , and SU 3 . 
     The switching unit SU 1 , which includes switching transistors Qs 1  and Qs 2 , selects one of a data voltage Vdat and a sustain voltage Vsus in response to a scanning signal g i  (i=1, 2, . . . , N) and connects the selected voltage to the contact point N 1 . The switching transistor Qs 1  is connected between the contact point N 1  and the data voltage Vdat, and the switching transistor Qs 2  is connected between the contact point N 1  and the sustain voltage Vsus. 
     The switching unit SU 2 , which includes switching transistor Qs 3 , switches connection between the sustain voltage Vsus and the contact point N 2  in response to a compensation signal s i . Switching transistor Qs 3  is connected between the sustain voltage Vsus and the contact point N 2 . 
     The switching unit SU 3 , which includes switching transistors Qs 4  and Qs 5 , selects one of the contact point N 2  and the light emitting element LD in response to the scanning signal g i  and connects the selected one to the output terminal of the driving transistor Qd. The switching transistor Qs 4  is connected between the output terminal of the driving transistor Qd and the contact point N 2 , and the switching transistor Qs 5  is connected between the output terminal of the driving transistor Qd and the organic light emitting element LD. 
     The switching transistors Qs 1 , Qs 3 , and Qs 4  are n-channel field effect transistors, and the switching transistors Qs 2  and Qs 5  and the driving transistor Qd are p-channel field effect transistors. The field effect transistors may be thin film transistors (TFTs), for example, and they may include polysilicon or amorphous silicon. Channel types of the switching transistors Qs 1 -Q 5  and the driving transistor Qd may change, and in this case, a driving signal waveform for driving the transistors may be inverted. 
     An anode and a cathode of the organic light emitting element LD are connected to the switching transistor Qs 5  and the common voltage Vss, respectively. The organic light emitting element LD emits light with different intensities according to a magnitude of a current I LD  that is supplied by the driving transistor Qd through the switching transistor Qs 5 , thereby displaying an image. A magnitude of the current I LD  depends on a magnitude of a voltage between the control terminal and the input terminal of the driving transistor Qd. 
     Referring again to  FIG. 1 , the scan driver  400  is connected to the scanning signal lines (G 1 -G n ) and the compensation signal lines (S 1 -S n ) of the display panel  300 , and it applies a scanning signal and a compensation signal, which both include a combination of a high voltage Von and a low voltage Voff, to the scanning signal lines (G 1 -G n ) and the compensation signal lines (S 1 -S n ), respectively. 
     The high voltage Von may allow the switching transistors Qs 1 , Qs 3 , and Qs 4  to electrically connect and intercept the switching transistors Qs 2  and Qs 5 , and the low voltage Voff may intercept the switching transistors Qs 1 , Qs 3 , and Qs 4  and allow the switching transistors Qs 2  and Qs 5  to electrically connect. A sustain voltage Vsus is a low voltage, and it may intercept the switching transistors Qs 1 , Qs 3 , and Qs 4  and allow the switching transistors Qs 2  and Qs 5  to electrically connect, like the low voltage Voff. The sustain voltage Vsus and the driving voltage Vdd may be applied through a driving voltage line. 
     The data driver  500  is connected to the data lines (D 1 -D m ) of the display panel  300 , and it applies a data voltage Vdat, which is used to display an image, to the data lines (D 1 -D m ). 
     The signal controller  600  controls an operation of the scan driver  400 , the data driver  500 , a light emission driver, etc. 
     Each driving device  400 ,  500 , and  600  may be directly mounted on the display panel  300  in at least one integrated circuit (IC) chip form, may be mounted on a flexible printed circuit film (not shown) to be attached to the display panel  300  in a tape carrier package (TCP) form, or may be mounted on a separate printed circuit board (PCB) (not shown). Alternatively, the driving devices  400 ,  500 , and  600 , together with the signal lines (G 1 -G n , S 1 -S n , and D 1 -D m ) and the transistors (Qs 1 -Qs 5 , Qd) may be formed on the display panel  300 . Further, the driving devices  400 ,  500 , and  600  may be integrated in a single chip and in this case, at least one of them or at least one circuit element constituting them may be disposed at the outside of the single chip. 
     A display operation of the organic light emitting device is described in detail below with reference to  FIG. 1 ,  FIG. 2 ,  FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6 , and  FIG. 7 . 
       FIG. 3  shows an example of a driving signal waveform that may be applied to pixels of a row in an organic light emitting device according to an exemplary embodiment of the present invention, and  FIG. 4 ,  FIG. 5 ,  FIG. 6 , and  FIG. 7  are equivalent circuit diagrams of a pixel in each period that is shown in  FIG. 3 . 
     The signal controller  600  receives an input image signal Din and an input control signal ICON for controlling the display of the input image signal Din from an external graphics controller (not shown). The input image signal Din contains luminance information of each pixel PX, and the luminance has grays of a given quantity, for example, 1024 (=2 10 ), 256 (=2 8 ), or 64 (=2 6 ). The input control signal ICON includes, for example, a vertical synchronization signal, a horizontal synchronizing signal, a main clock signal, and a data enable signal. 
     The signal controller  600  processes the input image signal Din to correspond to an operating condition of the display panel  300  based on the input image signal Din and the input control signal ICON, and generates a scanning control signal CONT 1  and a data control signal CONT 2 . The signal controller  600  then sends the scanning control signal CONT 1  to the scanning driver  400 , and sends the data control signal CONT 2  and an output image signal Dout to the data driver  500 . 
     The scanning control signal CONT 1  may include a scanning start signal STV for instructing the scanning start of the high voltage Von to the scanning signal lines (G 1 -G n ) and the compensation signal lines (S 1 -S n ), at least one clock signal for controlling an output period of the high voltage Von, and an output enable signal OE for limiting a sustain time period of the high voltage Von. 
     The data control signal CONT 2  includes a horizontal synchronization start signal for notifying the transmission start of a digital image signal Dout for one row of pixels PX, and a load signal and a data clock signal HCLK for applying a data signal, such as an analog data voltage, to the data lines (D 1 -D m ). 
     The scan driver  400  sequentially changes a voltage of a scanning signal that is applied to the scanning signal lines (G 1 -G n ) and a compensation signal that is applied to the compensation signal lines (S 1 -S n ) to a high voltage Von, and changes the high voltage Von to a low voltage Voff according to the scan control signal CONT 1  from the signal controller  600 . 
     According to the data control signal CONT 2  from the signal controller  600 , the data driver  500  receives a digital output image signal Dout for each row of pixels PX, converts the digital output image signal Dout to an analog data voltage Vdat, and then applies the analog data voltage Vdat to the data lines (D 1 -D m ). The data driver  500  outputs a data voltage Vdat for pixels PX of one row for one horizontal period 1H, as shown in  FIG. 3 . 
     A specific pixel row, for example an i-th row, is described below. 
     Referring to  FIG. 3 , the scan driver  400  changes a voltage of a scanning signal g i  that is applied to the scanning signal line G i  to a high voltage Von according to a scan control signal CONT 1  from the signal controller  600 . In this case, a compensation signal s i  that is applied to the compensation signal line S i  is in a low voltage Voff state, and a data voltage Vdat that is applied to the data lines (D 1 -D m ) is a data voltage (VD i−1 ) for pixels of a previous row and a data voltage VDi for a pixel of a current row. However, when a voltage of the scanning signal g i  changes from a low voltage Voff to a high voltage Von, a data voltage Vdat that is applied to the data lines (D 1 -D m ) may be a data voltage for pixels of a previous row. 
     Accordingly, as shown in  FIG. 4 , the switching transistors Qs 1  and Qs 4  are electrically connected, the switching transistors Qs 2  and Qs 5  are intercepted, and the switching transistor Qs 3  sustains an interception state. 
     If the switching transistor Qs 5  is intercepted, the organic light emitting element LD does not emit light, and a period from this time point until a voltage of the scanning signal g i  changes to a low voltage Voff and the switching transistor Qs 5  is again electrically connected is a non-light emitting period TR. 
     If the switching transistor Qs 3  is in an interception state and the switching transistor Qs 4  is electrically connected, the driving transistor Qd that has been flowing a current to the organic light emitting element LD instead flows a current to the contact point N 2  because its output terminal is connected to its control terminal. Thereafter, if a voltage of the contact point N 2 , (i.e., the difference between a voltage of a control terminal of the driving transistor Qd and a voltage of an input terminal thereof), becomes a threshold voltage Vth of the driving transistor Qd, the driving transistor Qd is in an interception state. In this case, because the switching transistor Qs 1  is in an electrical connection state, after a data voltage (VD i−1 ) of a previous pixel row is applied to the contact point N 1 , a data voltage (VD i ) of a current pixel row starts to be applied thereto. 
     In this way, in this period, because most of the data voltage (VD i−1 ) of a previous pixel row is charged to the capacitor Cst, this period is called a precharging period T 1 . 
     Referring to  FIG. 3 , the scan driver  400  changes a voltage of a compensation signal s i  that is applied to the compensation signal line S i  to a high voltage Von, thereby starting a charging period T 2 . 
     Accordingly, as shown in  FIG. 5 , the switching transistor Qs 3  is electrically connected, the switching transistors Qs 1  and Qs 4  sustain an electrical connection state, and the switching transistors Qs 2  and Qs 5  sustain an interception state. 
     In this state, a data voltage VDi of a current pixel row is applied to the contact point N 1 , a sustain voltage Vsus is applied to the contact point N 2 , and a voltage difference between two contact points N 1  and N 2  is stored in the capacitor Cst. Therefore, the driving transistor Qd is electrically connected to flow a current, but because the switching transistor Qs 5  is intercepted, the organic light emitting element LD remains off. 
     Referring to  FIG. 3 , as a voltage of the compensation signal s i  changes to a low voltage Voff, the switching transistor Qs 3  is in an interception state, thereby starting a compensation period T 3 . Because the scanning signal g i  continues to sustain a high voltage Von in the compensation period T 3 , the switching transistors Qs 1  and Qs 4  sustain an electrical connection state, and the switching transistor Qs 2  and Qs 5  sustain an interception state. 
     Accordingly, as shown in  FIG. 6 , the contact point N 2  is separated from a sustain voltage Vsus. However, because the driving transistor Qd sustains an electrical connection state, charges that have been charged in the capacitor Cst are discharged through the driving transistor Qd. The discharge stops after being sustained until a voltage difference between a control terminal and an input terminal of the driving transistor Qd becomes a threshold voltage Vth of the driving transistor Qd. 
     Therefore, a voltage V N2  of the contact point N 2  is converged to the following voltage value.
 
 V   N2   =Vdd+Vth   (Equation 1)
 
     In this case, because a voltage V N1  of the contact point N 1  sustains a data voltage VD i  of a current pixel row, a voltage that is stored in the capacitor Cst is represented by Equation 2.
 
 V   N1   −V   N2   =VDi −( Vdd+Vth )  (Equation 2)
 
     Thereafter, as shown in  FIG. 3 , the scan driver  400  changes a voltage of a scanning signal g i  to a low voltage Voff, thereby intercepting the switching transistors Qs 1  and Qs 4  and electrically connecting the switching transistor Qs 2  and Qs 5 , thereby starting a light emitting period TE. Because the compensation signal s i  continues to sustain a low voltage Voff state in the light emitting period TE, the switching transistor Qs 3  also sustains an interception state. 
     Thus, as shown in  FIG. 7 , the contact point N 1  is separated from the data voltage Vdat and connected to the sustain voltage Vsus, and a control terminal of the driving transistor Qd is floated. 
     Therefore, a voltage V N2  of the contact point N 2  is represented by Equation 3.
 
 V   N2   =Vdd+Vth−VDi+Vsus   (Equation 3)
 
     Due to electrical connection of the switching transistor Qs 5 , an output terminal of the driving transistor Qd is connected to the light emitting element LD, and the driving transistor Qd flows an output current I LD  that is controlled by a voltage difference Vgs between its control terminal and input terminal. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           I 
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                               ( 
                               
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     Here, K is a constant according to characteristics of the driving transistor Qd. Specifically, K=μ·Ci·W/L, where μ is electric field effect mobility, Ci is capacity of a gate insulating layer, W is a channel width of the driving transistor Qd, and L is a channel length of the driving transistor Qd. 
     According to Equation 4, an output current I LD  of the light emitting period TE is determined by only the constant K, a data voltage Vdat (i.e., VDi), and a fixed sustain voltage Vsus. Therefore, the output current I LD  is not influenced by a threshold voltage Vth of the driving transistor Qd. 
     The output current I LD  is supplied to the organic light emitting element LD, and the organic light emitting element LD emits light with different intensities according to a magnitude of the output current I LD , thereby displaying an image. 
     Therefore, even if a deviation exists in a threshold voltage Vth between the driving transistors Qd or a magnitude of a threshold voltage Vth of each driving transistor Qd sequentially changes, a uniform image can be displayed. 
     By advancing a time point for forming a voltage of the scanning signal g i  at a high voltage Von by a necessary time period, light emission of the light emitting element LD is prevented for a desired time period, whereby a time period in which the pixel PX is in a black color state can be extended. 
     A scan driver for forming such a scanning signal and a compensation signal is described in detail below with reference to  FIG. 8 ,  FIG. 9 ,  FIG. 10 ,  FIG. 11 , and  FIG. 12 . 
       FIG. 8  is a block diagram showing a configuration of a scan driver according to an exemplary embodiment of the present invention,  FIG. 9  shows an example of a circuit diagram of a shift register in the scan driver of  FIG. 8 ,  FIG. 10  is a signal waveform diagram of an organic light emitting device having the scan driver of  FIG. 9 ,  FIG. 11  shows another example of a circuit diagram of a shift register in the scan driver of  FIG. 8 , and  FIG. 12  is a signal waveform diagram of an organic light emitting device having the scan driver of  FIG. 11 . 
     Referring to  FIG. 8 , a scan driver  400  according to an exemplary embodiment of the present invention includes a shift register  410 , a level shifter  460 , and a buffer  470  that are sequentially connected. 
     The shift register  410  includes a plurality of stages that are sequentially connected, and a scanning start signal STV, a plurality of clock signals (CK 1 , CKB 1 , CK 2 , and CKB 2 ), and an output enable signal OE are input thereto. 
     Each stage generates and outputs scanning signals (g 1 -g n ) and compensation signals (s 1 -s n ). 
     The level shifter  460  adjusts and outputs a voltage value of scanning signals (g 1 -g n ) and compensation signals (s 1 -s n ) that are output from the shift register  410 , and the buffer  470  performs a function of sustaining the scanning signals (g 1 -g n ) and the compensation signals (s 1 -s n ) that are output from the level shifter  460 . 
     In the shift register  420  that is shown in  FIG. 9 , each stage (ST i , ST i+1 ) includes a latch  422 , a waveform cutter  424 , and an output definer  426 . 
     The latch  422  delays carry output signals (C i−1 , C i ) (a scanning start signal STV in a first stage) of a previous stage and outputs the carry output signals (C i−1 , C i ) as its own carry output signals (C i , C i+1 ). The latch  422  includes two clocked inverters and one regular inverter. One clocked inverter inverts carry output signals (C i−1 , C i ) of a previous stage and sends inverted the carry output signals (C i−1 , C i ) to a regular inverter according to the first/second clock signal (CK 1 /CK 2 ), and the regular inverter inverts and outputs an input signal. Another clocked inverter inverts the output of the regular inverter and sends the inverted output to the regular inverter according to first/second inversion clock signals (CKB 1 /CKB 2 ). 
     As shown in  FIG. 10 , a period of the first clock signal CK 1  and the second clock signal CK 2  is two times a horizontal period 1H, and a duty ratio thereof is greater than 50%. The first clock signal CK 1  and the second clock signal CK 2  have a phase difference of about 180°, and the first/second inversion clock signal (CKB 1 /CKB 2 ) is an inversion signal of the first/second clock signal (CK 1 /CK 2 ), respectively. The scanning start signal STV and the carry output signals (C i−1 , C i , and C i+1 ) sustain a high voltage Von state for two horizontal periods 2H, and each of the carry output signals (C i , C i+1 ) is delayed by about one horizontal period 1H from front end carry output signals (C i−1 , C i ). 
     The waveform cutter  424  cuts and outputs an output signal of the latch  422  according to the second/first clock signal CK 2 /CK 1 . The waveform cutter  424  includes a NAND gate and an inverter. Thus, it is identical to an AND gate from a logical view. The NAND gate uses the output of the latch  422  and the second/first clock signal CK 2 /CK 1  as two inputs, and the output thereof is input to the inverter. The output signal of the waveform cutter  424  becomes scanning signals (g 1 -g n ), and is in a high voltage state for approximately a high voltage period of the second/first clock signal CK 2 /CK 1 . 
     The output definer  426  cuts and outputs the output signal of the waveform cutter  424  according to the output enable signal OE. The output definer  426  also includes a NAND gate and an inverter. Thus, it is identical to an AND gate from a logical view. The NAND gate uses the output of the waveform cutter  424  and the output enable signal OE as two inputs, and the output thereof is input to the inverter. A period of the output enable signal OE is identical to one horizontal period 1H, and it may have various duty ratios, including about 50% as shown in  FIG. 10 . The output of the output definer  426  becomes compensation signals (s 1 -s n ), which become a high voltage two times while the scanning signals (g 1 -g n ) are at a high voltage. 
     A period in which the scanning signals (g 1 -g n ) are at a high voltage is longer than one horizontal period 1H, and a data voltage (VD 0 -VD n−1 ) (VD 0  is a null data voltage) of a previous pixel row is applied to each pixel PX for a front half period, and data voltages (VD 1 -VD n ) of the corresponding pixel are applied for a rear half period. The compensation signals (s 1 -s n ) become a high voltage one time for a front half period in a period in which the scanning signals (g 1 -g n ) are at a high voltage, and become a high voltage one more time for a rear half period. Thereby, the driving transistor Qd operates according to the data voltages (VD 0 -VD n−1 ) of a previous pixel row, but because the organic light emitting element LD does not operate, each pixel PX does not display the data voltages (VD 0 -VD n−1 ) of a previous pixel row with luminance. 
     Consequently, because each pixel PX displays a black color for a time period that is longer than one horizontal period 1H, an impulse effect may be improved. 
     In the shift register  430  of  FIG. 11 , each stage (ST i , ST i+1 ) includes a latch  432 , a voltage sustainer  434 , a waveform cutter  436 , and an output definer  438 . 
     The latch  432 , which includes two clocked inverters and one regular inverter, delays carry output signals (C i−1 , C i ) (a scanning start signal STV in a first stage) of a previous stage and outputs the carry output signals (C i−1 , C i ) as its own carry output signals (C i , C i+1 ), similar to the latch  422  of  FIG. 9 . One clocked inverter inverts carry output signals (C i−1 , C i ) of a previous stage and sends the inverted carry output signals (C i−1 , C i ) to a regular inverter according to the first/second clock signal CK 1 /CK 2 , and the regular inverter inverts and outputs an input signal. Another clocked inverter inverts the output of the regular inverter and sends the inverted output to the regular inverter according to the first/second inversion clock signal CKB 1 /CKB 2 . 
     As shown in  FIG. 12 , a period of the first clock signal CK 1  and the second clock signal CK 2  is two times a horizontal period 1H, and a duty ratio thereof is 50% or below. The first clock signal CK 1  and the second clock signal CK 2  have a phase difference of about 180°, and the first/second inversion clock signal CKB 1 /CKB 2  is an inversion signal of the first/second clock signal CK 1 /CK 2 , respectively. The scanning start signal STV and the carry output signals (C i−1 , C i , and C i+1 ) sustain a high voltage Von state for two horizontal periods 2H, and each of the carry output signals (C i , C i+1 ) is delayed by about one horizontal period 1H from front end carry output signals (C i−1 , C i ). 
     The voltage sustainer  434  includes two inverters, and an output thereof becomes scanning signals (g i , g i+1 ). The scanning signals (g 1 -g n ) sustain a high voltage Von state for two horizontal periods 2H, and each scanning signal (g 1 -g n ) is delayed by about one horizontal period 1H from a scanning start signal STV or front end scanning signals (g 1 -g n−1 ). The voltage sustainer  434  may be omitted, and the carry output signals (C i−1 , C i , and C i+1 ) may be directly used as scanning signals (g 1 -g n ). 
     The waveform cutter  436  cuts and outputs an output signal of the latch  432  according to the second/first clock signal CK 2 /CK 1 . The waveform cutter  436  includes a NAND gate and an inverter. Thus, it is identical to an AND gate from a logical view. The NAND gate uses an output of the latch  432  and the second/first clock signal CK 2 /CK 1  as two inputs, and an output thereof is input to the inverter. 
     The output definer  438  cuts and outputs an output signal of the waveform cutter  436  according to an output enable signal OE. The output definer  438  also includes a NAND gate and an inverter. Thus, it is identical to an AND gate from a logical view. The NAND gate uses an output of the waveform cutter  436  and an output enable signal OE as two inputs, and the output thereof is input to the inverter. A period of the output enable signal OE is identical to one horizontal period 1H, and it may have various duty ratios, including about 50% as shown in  FIG. 12 . The output of the output definer  438  becomes compensation signals (s i , s i+1 ), which become a high voltage only one time while the scanning signals (g 1 -g n ) are a high voltage, unlike the case of  FIG. 9  and  FIG. 10 . 
     A period in which the scanning signals (g 1 -g n ) are at a high voltage is longer than one horizontal period 1H, and data voltages (VD 0 -VD n−1 ) (VD 0  is a null data voltage) of a previous pixel row are applied to each pixel PX for a front half period and data voltages (VD 1 -VD n ) of the corresponding pixel are applied for a rear half period. A voltage of the compensation signals (S 1 -S n ) becomes a high voltage one time for a rear half period in a period in which the scanning signals (g 1 -g n ) are at a high voltage. 
     Consequently, because each pixel PX displays a black color for a time period that is longer than one horizontal period 1H, an impulse effect may be improved. Particularly, in the present exemplary embodiment, a time period in which the scanning signals (g 1 -g n ) are at a high voltage can be lengthened by a desired time period by extending a high voltage period of a scanning start signal STV, and thus a black color display time period can be freely adjusted, as compared with the exemplary embodiment that is described in  FIG. 9  and  FIG. 10 . 
     The scan driver and the driving method thereof shown in  FIG. 9 ,  FIG. 10 ,  FIG. 11 , and  FIG. 12  can be applied to other pixels besides the pixel PX shown in  FIG. 2 , and they can be applied to other display devices besides an organic light emitting device. For example, the scan driver and the method of driving the same can be applied when a scanning signal is in a high voltage state, each pixel receives a data voltage while displaying a black color, and when a scanning signal is in a low voltage state, each pixel stops reception of a data voltage and displays luminance corresponding to its own data voltage. In  FIG. 2 , even in a case where switching transistors Qs 2 , Qs 3 , and Qs 4 , which compensate a threshold voltage of the driving transistor Qd, are omitted, the scan drivers and the methods of driving the same that are shown in  FIG. 9 ,  FIG. 10 ,  FIG. 11 , and  FIG. 12  can be applied. In this case, portions  426 ,  436 , and  438  that are related to compensation signals may be omitted. 
     By adjusting a high voltage period length of the scanning signal, impulse driving can be realized. 
     It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.