Abstract:
A display device includes a plurality of pixel circuits formed in a matrix; a plurality of first scan lines for transmitting selection signals to select one or more of the pixel circuits; a plurality of second scan lines for transmitting emission control signals to control the duration of one or more emissions of the selected one or more pixel circuits; and a scan driver for sequentially delaying a primary signal. The primary has a pulse at a first level at about a first period for generating a plurality of secondary signals. The plurality of secondary signals are inverted for outputting the emission control signals, and a signal is generated having a pulse at a second level when at least one of the secondary signals and at least one of the emission control signals are at the first level.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0038950 filed on May 31, 2004 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a display device and a driving method thereof, and more particularly, it relates to an organic light emitting diode (also referred to as “OLED,” hereinafter) display device, a display panel, and a driving method thereof.  
         [0004]     2. Description of the Related Art  
         [0005]     In general, an EL display device is a display device that electrically excites phosphorus organic components, and represents an image by voltage-programming or current-programming m x n numbers of organic light emitting pixels. As shown in  FIG. 1 , each of these organic light emitting pixels includes anode (indium tin oxide: ITO), organic thin film, and cathode (metal) layers. The organic thin film layer has a multi-layered structure including an emission layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) so as to balance electrons and holes and thereby enhance efficiency of light emission. Further, the organic thin film includes an electron injection layer (EIL) and a hole injection layer (HIL).  
         [0006]     Methods of driving the organic light emitting pixels can include a passive matrix method and an active matrix method. The active matrix method employs a thin film transistor (TFT). In the passive matrix method, an anode and a cathode are formed crossing each other, and a line is selected to drive the organic light emitting pixels. On the other hand, in the active matrix method, each indium tin oxide (ITO) pixel electrode (or anode) is coupled to the TFT and the light emitting pixel is driven in accordance with a voltage maintained by the capacitance of a capacitor coupled to a gate of the TFT. The active matrix method can also be classified into a voltage programming method and a current programming method depending on a type of signals transmitted to the capacitor so as to distinctively control the voltage applied to the capacitor.  
         [0007]      FIG. 2  is an equivalent circuit diagram of a pixel circuit according to a conventional voltage-programming method. Referring now to  FIG. 2 , a conventional organic EL display device employing the voltage-programming method supplies currents to an organic light emitting pixel or OLED through a transistor M coupled thereto for light emission, and the amount of current supplied to the OLED is adjusted by a data voltage applied through a switching transistor M 2 . Herein, a capacitor C 1  is coupled between a source and a gate of the transistor M 1  to maintain the amount of the data voltage applied during a predetermined time period.  
         [0008]     When the transistor M 2  is turned on, the data voltage is applied to the gate of the transistor M 1 , and a voltage of V GS  between the gate and the source is charged to the capacitor C 1 . A current I OLED  flows corresponding to the voltage of V GS , and the OLED emits light corresponding to the current I OLED .  
         [0009]     Herein, the current flowing to the OLED is given as Equation 1.  
               I   OLED     =         β   2     ⁢       (       V   GS     -          V   TH            )     2       =       β   2     ⁢       (       V   DD     -     V   DATA     -          V   TH            )     2                 [     Equation   ⁢           ⁢   1     ]             
        where I OLED  represents a current flowing to the OLED, V GS  represents a voltage between the gate and the source of the transistor M 1 , V TH  represents a threshold voltage of the transistor M 1 , V DATA  represents a data voltage, and β represents a constant number.        
 
         [0011]     As shown in Equation 1, the current corresponding to the data voltage is supplied to the OLED, and the OLED emits light corresponding to the current supplied thereto. Herein, the data voltage has multi-level values within a predetermined range to express gray scales.  
         [0012]     However, a pixel circuit according to a conventional voltage-programming method has a problem in expressing high-level gray scales due to a deviation of a threshold voltage V TH  at a driving transistor or a TFT and a mobility of a carrier. The deviation can result from a non-uniform manufacturing process of the TFT. For example, when a pixel circuit drives a TFT in a pixel by applying 3V thereto to express 8-bit gray scales (256 gray scales), a voltage should be applied to a gate of the TFT at an interval of less than 12 mV (=3V/256). However, it is difficult to express such a high gray scale in the case that the deviation of the threshold voltage V TH  is 100 mV due to the non-uniform manufacturing process. Moreover, the deviation of the mobility of the carrier causes the value of β to be changed in Equation 1, and thus expressing the high level gray scale becomes even more difficult.  
         [0013]     By contrast, although the amount of current and voltage supplied from a driving transistor to each of the pixels may not be uniform, the circuit of the pixels employing a current-programming method can still have a uniform panel as long as the currents supplied from a current source to the pixel circuit are uniform.  
         [0014]      FIG. 3  shows an equivalent circuit diagram of a pixel circuit according to a conventional current-programming method.  
         [0015]     As shown in  FIG. 3 , a transistor M 1  is coupled to an OLED to supply a current for light emission, and the amount of the current is adjusted by a data current applied through a transistor M 2 .  
         [0016]     Accordingly, when transistors M 2  and M 3  are turned on, a voltage corresponding to the data current I DATA  is stored in a capacitor C 1 , and then the amount of current corresponding to the voltage stored in the capacitor C 1  flows to the OLED so that the OLED can emit light. Herein, the current flowing to the OLED is given as Equation 2.  
               I   OLED     =         β   2     ⁢       (       V   GS     -          V   TH            )     2       =     I   DATA               [     Equation   ⁢           ⁢   2     ]             
        where V GS  represents a voltage between a gate and a source of a transistor M 1 , V TH  represents a threshold voltage of the transistor M 1 , and β represents a constant number.        
 
         [0018]     As shown in Equation 2, the current flowing throughout a panel can be uniform since the amount of the current I OLED  flowing to the OLED and the amount of the data current I DATA  are the same according to the conventional current-programming method. However, if a weak current (I DATA ) flows to the OLED, it takes too much time to charge data lines. For instance, assume that the load of capacity in the data line is set to be 30 pF. In this case, it takes several milliseconds to charge the load of the capacity with data currents of several tens of nA to several hundreds of nA. However, line time is inefficient for fully charging the data line since it is limited to several μs.  
         [0019]     On the other hand, if the amount of the current I OLED  flowing to the OLED is increased to reduce time for charging the data line, brightness of all the pixels may be increased, thereby resulting in a decrease of image quality.  
       SUMMARY OF THE INVENTION  
       [0020]     It is an aspect of the present invention to provide a light emission device capable of compensating a threshold voltage or shifting of a transistor and fully charging data lines.  
         [0021]     In one exemplary embodiment of the present invention, a display device includes a plurality of data lines, a plurality of first scan lines, and a plurality of pixel circuits. The plurality of data lines transmits data signals. The plurality of first scan lines transmits selection signals. The plurality of pixel circuits are respectively coupled to the data lines and the first scan lines. At least one of the pixel circuits includes an emission device for displaying an image, a first switch, a transistor, a first storage device, a second storage device, and a second switch. The emission device displays the image corresponding to data currents supplied thereto. The first switch transmits at least one of the data signals transmitted through the data lines in response to at least one of the selection signals of at least one of the first scan lines. The transistor is diode-connected while the at least one data signal is transmitted from the first switch. The first storage device is coupled between a first transistor electrode and a control electrode of the transistor, and stores a first voltage corresponding to the at least one data signal from the first switch. The second storage device is coupled to the control electrode of the transistor and a second scan electrode for transmitting a first control signal, and switches the first voltage of the first storage device into a second voltage by coupling with the first storage device when the first control signal is changed into a second level from a first level. The second switch transmits a current outputted from the transistor to the emission device in response to a second control signal. The first control signal is set to be maintained at the first level during a horizontal period.  
         [0022]     In one exemplary embodiment of the present invention, a display device includes a display panel, a data driver, a first scan driver, and a second scan driver. The display panel includes a plurality of data lines, a plurality of first scan lines, a plurality of second scan lines, and a plurality of pixel circuits. The plurality of data lines transmits data signals. The plurality of first scan lines transmits selection signals. The plurality of second scan lines transmits emission control signals. The plurality of pixel circuits respectively couple to the data lines, the first scan lines, and the second scan lines. The data driver applies the data signals to the data lines. The first scan driver applies the selection signals to the first scan lines. The second scan driver applies the emission control signals to the second scan lines. The first scan driver and the second scan driver include a shift register for sequentially delaying a first signal having a pulse at a first level by a first period to generate a plurality of second signals. The first scan driver includes a first logical operator and a second logical operator. The first logical operator receives two adjacent second signals outputted from the shift register, and outputs a third signal having a pulse at a fourth level when the two second signals are both at a third level. The second logical operator receives the third signal outputted from the first logical operator and a fourth signal having a pulse at the third-level for a part of a horizontal period, and outputs a signal having a pulse at the third-level as at least one of the selection signals when the third signal and the fourth signal both are at the fourth level. The second scan driver receives the two adjacent second signals outputted from the shift register, and outputs a signal having a pulse at the fourth-level as at least one of the emission control signals when one of the two adjacent second signals is at the third level.  
         [0023]     In one exemplary embodiment of the present invention, a display panel has a plurality of data lines for transmitting data signals, a plurality of scan lines for transmitting selection signals, and a plurality of pixel circuits formed on a plurality of pixels respectively defined by the data lines and the scan lines. At least one of the pixel circuits includes an emission device, a first switch, a transistor, a first storage device, a second storage device, and a second switch. The emission device displays an image corresponding to data currents supplied thereto. The first switch transmits at least one of the data signals transmitted through at least one of the data lines in response to at least one of the selection signals of at least one of the scan lines. The transistor supplies a driving current to drive the emission device, and is diode-connected while the data signal is transmitted from the first switch. The first storage device is coupled between a first transistor electrode and a control electrode of the transistor. The second storage device is coupled between the control electrode of the transistor and a signal line for supplying a first control signal. The second switch couples a second transistor electrode of the transistor and the emission device in response to a second control signal. When the at least one selection signal is in an enable period, the enable period is set to be included in a horizontal period, and the second control signal includes a disable period that is set to be an integer-numbered times of the horizontal period.  
         [0024]     In one exemplary embodiment of the present invention, a method for driving a display device is provided. The display device includes a plurality of data lines, a plurality of first scan lines, a plurality of second scan lines, and a plurality of pixel circuits. The plurality of data lines transmits data signals. The plurality of first scan lines transmit selection signals. The plurality of second scan lines transmit first control signals. The plurality of pixel circuits are respectively coupled to the data lines and the first scan lines, and at least one of the pixel circuits includes a first switch, a transistor, a first storage device, a second storage device, and an emission device. The first switch transmits a data current from at least one of the data lines in response to a pulse at a first level pulse of at least one of the selection signals. The transistor has a first transistor electrode and a control electrode. The first storage device is formed between the first transistor electrode and the control electrode. The second storage device is formed between the control electrode and at least one of the second scan lines. The emission device displays an image corresponding to a current from the transistor. In the method, at least one of the first control signals is changed to a fourth level from a third level and is maintained in the fourth level during a horizontal period. The at least one selection signal is changed from a second level to the first level and a voltage corresponding to the data current is charged to the first storage device during a first period. The at least one first control signal is changed from the fourth level to the third level to change the voltage in the first storage device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention, wherein:  
         [0026]      FIG. 1  illustrates a conceptual organic light emitting pixel or an OLED;  
         [0027]      FIG. 2  shows an equivalent circuit diagram of a pixel according to a conventional voltage-programming method;  
         [0028]      FIG. 3  shows an equivalent circuit diagram of a pixel according to a conventional current-programming method;  
         [0029]      FIG. 4  is a schematic plan view of an OLED according to an embodiment of the present invention;  
         [0030]      FIG. 5  is a pixel circuit diagram according to an embodiment of the present invention;  
         [0031]      FIG. 6  is a driving waveform to drive the pixel circuit of  FIG. 5  according to a first embodiment of the present invention;  
         [0032]      FIG. 7  is a driving waveform to drive the pixel circuit of  FIG. 5  according to a second embodiment of the present invention;  
         [0033]      FIG. 8  is a driving waveform to drive the pixel circuit of  FIG. 5  according to a third embodiment of the present invention;  
         [0034]      FIG. 9  is a driving waveform to drive the pixel circuit of  FIG. 5  according to a fourth embodiment of the present invention;  
         [0035]      FIG. 10  illustrates a scan driver to generate a selection signal and an emission control signal of  FIG. 9  according to an exemplary embodiment of the present invention;  
         [0036]      FIG. 11  shows a drive timings of the scan driver of  FIG. 10 ;  
         [0037]      FIG. 12  is a schematic circuit diagram of a shift register of  FIG. 10 ;  
         [0038]      FIG. 13  illustrates a flip-flop used for the shift register of  FIG. 12 ; and  
         [0039]      FIG. 14  shows a scan driver to generate a selection signal and an emission control signal of  FIG. 9  according to an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0040]     In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive. There may be parts shown in the drawings or parts not shown in the drawings that are not discussed in the specification as they are not essential for a complete understanding of the invention. Like reference numerals designate like elements. Phrases such as “one thing coupled to another” can refer to either “directly coupling a first one to a second one” or “coupling the first one to the second one with a third one provided therebetween.” 
         [0041]      FIG. 4  is a plan view schematically illustrating a light emission device according to an embodiment of the present invention.  
         [0042]     As shown in  FIG. 4 , the light emission device according to the embodiment of the present invention includes an organic EL display panel (hereinafter also referred to as “display panel”)  100 , a data driver  200 , and scan drivers  300  and  400 .  
         [0043]     The display panel  100  includes data lines D 1  to D n  arranged in columns, a plurality of scan lines S 1  to S m , E 1  to E m , and B 1  to B m  arranged in rows, and a plurality of pixel circuits  11 . The data lines D 1  to D n  transmit data currents as image signals to the pixel circuits  11 . The selection scan lines S 1  to S m  transmit a selection signal to the pixel circuits  11 , and emission scan lines E 1  to E m  transmit an emission control signal to the pixel circuits  11 . Further, the boost scan lines B 1  to B m  transmit a boost signal to the pixel circuits  11 . The pixel circuits  11  are formed in areas respectively defined by adjacent data lines and selection signals.  
         [0044]     In operation, the data driver  200  applies the data currents to the data lines D 1  to D n , and the scan driver  300  sequentially applies the selection signals to the selection scan lines S 1  to S m  and the emission scan lines E 1  to E m . Further, the scan driver  400  applies the boost signals to the boost scan lines B 1  to B m .  
         [0045]     Referring to  FIG. 5 , a pixel circuit  11  of  FIG. 4  according to an exemplary embodiment of the present invention will be described hereinafter. As shown,  FIG. 5  illustrates the pixel circuit  11  coupled to the nth data line D n  and the mth scan lines S m , E m , and B m , for exemplary purposes and the invention is not thereby limited.  
         [0046]     The pixel circuit  11  according to the embodiment of the present invention includes an OLED, a driving transistor M 1 , switching transistors M 2  to M 4 , and capacitors C 1  and C 2 .  
         [0047]     The switching transistor M 2  is coupled between the data line Dn and a gate of the driving transistor M 1 . When the switching transistor M 2  is turned on, in response to a selection signal transmitted from the selection scan line S m , a data current I DATA  flows from the driving transistor M 1  to the data line D n . The switching transistor M 3  is coupled between a drain and the gate of the driving transistor M 1 , and diode-connects the driving transistor M 1  in response to the selection signal from the selection scan line S m .  
         [0048]     A source of the driving transistor M 1  is coupled to a power voltage VDD and the drain of the driving transistor M 1  is coupled to the switching transistor M 4 . The gate-source voltage of the driving transistor M 1  is determined corresponding to the data current I DATA , and the capacitor C 1  is coupled between the gate and the source of the driving transistor M 1  so as to maintain the gate-source voltage of the driving transistor M 1  during a predetermined time period. The capacitor C 2  is coupled between the boost scan line B m  and the gate of the driving transistor M 1  so as to adjust a voltage at the gate of the driving transistor M 1 .  
         [0049]     The switching transistor M 4  supplies a current flowing to the driving transistor M 1  to the OLED in response to the emission control signal from the emission scan line E m . The OLED is coupled between the switching transistor M 4  and a power voltage VSS and emits light corresponding to the amount of the current flowing from the driving transistor M 1 .  
         [0050]     In  FIG. 5 , each of the switching transistors M 2  to M 4  is shown as a P-channel transistor, but each or at least one of these switching transistors can be provided as an N-channel transistor in other embodiments of the present invention. Also, these transistors M 2  to M 4  can be replaced with other devices capable of switching both ends thereof in response to application of a control signal. Further, the driving transistor M 1  can be replaced with an N-channel transistor. The detail for modifying a circuit structure when using the one or more N-channel transistors is known to those skilled in the art and is therefore not provided in more detail. In addition, the transistors M 1  to M 4  can be thin-film transistors respectively having a gate electrode, a drain electrode, and a source electrode that respectively function as a control electrode and two main electrodes.  
         [0051]     FIGS.  6  to  9  illustrate a driving method of a pixel circuit according to first, second, third, and fourth embodiments of the present invention.  
         [0052]      FIG. 6  shows the driving waveform to drive the pixel circuit in  FIG. 5  according to the first embodiment of the present invention.  
         [0053]     In  FIG. 6 , a selection signal select[m] applied to the selection scan line Sm becomes a low-level signal, the transistors M 2  and M 3  are turned on and the driving transistor M 1  is diode-connected while allowing the data current I DATA  to flow to the driving transistor M 1  from the data line Dn.  
         [0054]     In addition, when the boost signal boost[m] applied to the boost scan line Bm becomes low, a low-level voltage is applied to the boost scan line Bm of the capacitor C 2 .  
         [0055]     The emission control signal emit[m] applied to the emission scan line Em is maintained at a high level (disable level), and thus the transistor M 4  is turned off and the driving transistor M 1  and the OLED are electrically decoupled.  
         [0056]     As such, a relationship between an absolute voltage value (hereinafter, also referred to as “gate-source voltage”) VGS between the gate and the source of the driving transistor M 1  and the current data I DATA  flowing to the driving transistor M 1  can be given as Equation 3, and the gate-source voltage V GS  of the driving transistor M 1  can be given as Equation 4.  
               I   DATA     =       β   2     ⁢       (       V   GS     -          V   TH            )     2               [     Equation   ⁢           ⁢   3     ]             
        where β represents a constant value and V TH  represents an absolute value of a threshold voltage of the driving transistor M 1 .  
               V   GS     =         V   DD     -     V   G       =           2   ⁢     I   DATA       β       +          V   TH                      [     Equation   ⁢           ⁢   4     ]             
    where V G  represents a gate voltage of the driving transistor M 1 , and V DD  represents a voltage supplied to the driving transistor M 1  by the power voltage V DD .        
 
         [0059]     Next, the transistors M 2  and M 3  are turned off and the transistor M 4  is turned on when the selection signal select[m] becomes a high-level (disable-level) signal and the emission control signal emit[m] becomes a low-level (enable-level) signal.  
         [0060]     Further, when the boost signal boost[m] is changed from the low-level signal into the high level, a voltage at a point where the capacitor C 2  and the boost scan line Bm meet each other can be increased to as much as the amount ΔV B  of the boost signal is increased. Accordingly, the gate voltage V G  of the driving transistor M 1  can be increased by ΔV B  by the coupling of the capacitor C 2  with the boost scan line Bm as given in Equation 5.  
               Δ   ⁢           ⁢     V   G       =       Δ   ⁢           ⁢     V   B     ⁢     C   2           C   1     +     C   2                 [     Equation   ⁢           ⁢   5     ]             
        where C 1  and C 2  respectively represent capacitance of the capacitors C 1  and C 2 .        
 
         [0062]     Since the gate voltage V G  of the driving transistor M 1  is increased by ΔV G , the current I OLED  flowing to the driving transistor M 1  is given as Equation 6. In other words, the drain current I OLED  of the driving transistor M 1  can be set to be lower than the data current I DATA  because the gate-source voltage V GS  of the driving transistor M 1  is decreased in proportion to the increase of the gate voltage V G  of the driving transistor M 1 . Accordingly, charging time for the data lines can be sufficiently prepared (or reduced) while still controlling (or allowing) weak currents to flow to the OLED.  
         [0063]     Further, the transistor M 4  is turned on by the emission control signal of the emission scan line Em, and therefore the current I OLED  of the driving transistor M 1  is supplied to the OLED which thereby emits light.  
               I   OLED     =         β   2     ⁢       (       V   GS     -     Δ   ⁢           ⁢     V   G       -          V   TH            )     2       =       β   2     ⁢       (           2   ⁢     I   DATA       β       -     Δ   ⁢           ⁢     V   G         )     2                 [     Equation   ⁢           ⁢   6     ]             
 
         [0064]     Further, the data current I DATA  can be given as Equation 7 that is derived from Equation 6.  
               I   DATA     =       I   OLED     +     Δ   ⁢           ⁢     V   G     ⁢       2   ⁢   β   ⁢           ⁢     I   OLED           -       β   2     ⁢       (     Δ   ⁢           ⁢     V   G       )     2                 [     Equation   ⁢           ⁢   7     ]             
 
         [0065]     In  FIG. 6 , timing of each of the selection signal select[m], the emission control signal emit[m], and the boost signal boost[m] is described to be the same, but it is not restricted thereto.  
         [0066]      FIG. 7  describes the driving waveform according to the second embodiment of the present invention.  
         [0067]     In  FIG. 7 , the transistor M 4  should be turned off while the transistors M 2  and M 3  are turned on by the selection signal select[m] applied to the selection scan line Sm so as to allow the data current I DATA  to flow to the driving transistor M 1 . However, when the transistor M 4  is turned on to allow the data current I DATA  to flow to the OLED while the data current I DATA  flows to the driving transistor M 1 , the data current I DATA  and the current I OLED  flowing to the OLED are added together and flow to the drain of the driving transistor M 1 , and a voltage corresponding to this current is programmed to the capacitor C 1 . Meanwhile, delay and rising timing of the selection signal select[m] can differ from delay and falling timing of the emission control signal emit[m] due to a load difference between the selection scan line Sm and the emission scan line Em, or characteristics of the transistor(s) in the circuit (or butter). As such, the transistor M 4  can be properly turned off while the transistor M 2  is turned on by adjusting the off-level pulse of the emission control signal emit[m] to be ended in a period after the on-level pulse of the selection signal select[m]ends, as shown in  FIG. 7 .  
         [0068]     The end of the low pulse of the boost signal boost[m] from the boost scan line Bm should not be prior to the end of the on-level pulse of the selection signal select[m], otherwise the data current I DATA  is programmed after the node voltage of the capacitor C 2  is increased, thereby resulting in the purpose of increasing the node voltage of the capacitor C 2  to become useless. Therefore, the on-level pulse of the selection signal select[m] transmitted to the selection scan line Sm should be adjusted to end in a period prior to the end of the low pulse of the boost signal boost[m] in order to prevent the node voltage of the capacitor C 2  from being increased prior to the completion of the data current I DATA  programming, as shown in  FIG. 7 .  
         [0069]     Further, the voltage at the capacitor C 1  can be changed due to falling of the node voltage of the capacitor C 2  while the voltage is programmed to the capacitor C 1  in the case that the start of the low pulse of the boost signal boost[m] starts before the start of the on-level pulse of the selection signal select[m] starts. Once the voltage at the capacitor C 1  is changed, the voltage programming process should be started over again thereby resulting in a lack of time for programming the voltage to the capacitor C 1 . Therefore, the start of the pulse of the selection signal select[m] should be prior to the start of the low pulse of the boost signal boost[m] so as to program the data current I DATA  after the node voltage of the capacitor C 2  falls, as shown in  FIG. 7 .  
         [0070]      FIG. 8  illustrates the driving waveform according to the third embodiment of the present invention.  
         [0071]     According to the timing of pulses shown in  FIG. 7 , if the load difference between the boost scan line Bm and the emission scan line Em or the characteristic difference between transistors used in the circuit (or buffer) causes the ending timing between the off-level pulse of the emission control signal emit[m] and the low pulse of the boost signal boost[m] to be changed is substantially the same, the node voltage of the capacitor C 2  flows to the OLED between the end of the low pulse of the boost signal boost[m] and the end of the off-level pulse of the emission control signal emit[m] when the off-level pulse of the emission control signal emit[m] is ended before the low pulse of the boost signal boost[m] ends. As a result, the OLED comes to be under much stress. Repetition of this process can cause a lifespan of the OLED to be shortened. To prevent this problem, the low pulse of the boost signal boost[m] transmitted to the boost scan line Bm should end prior to the end of the off-level pulse of the emission control signal emit[m] transmitted to the emission scan line Em so as to control the data current to flow to the OLED after the node voltage of the capacitor C 2  is increased. Further, though the off-level of the emission control signal emit[m] is described in the above embodiment, on-level of the emission control signal emit[m] can also be used instead of the off-level in PMOS typed transistor.  
         [0072]     Meanwhile, when the off-level pulse of the emission control signal emit[m] starts after the low pulse of the boost signal boost[m] starts, the node voltage of the capacitor C 2  falls and the current flows to the OLED during a period between the start of the pulse of the emission control signal emit[m] and the start of the pulse of the boost signal boost[m]. As a result, the OLED comes to be under much stress, and repetition of this process can shorten a lifespan of the OLED. Therefore, the off-level pulse of the emission control signal emit[m] transmitted to the emission scan line Em should start prior to the start of the low pulse the boost signal boost[m] transmitted to the boost scan line Bm so as to control the node voltage of the capacitor C 2  falls after the transistor M 4  is turned off, as shown in  FIG. 8 .  
         [0073]     In other words, the problems that may occur due to the load difference between the scan lines Sm, Em, and Bm, and the characteristic of the circuit (or buffer) can be solved by setting the length of the off-level pulse of the emission control signal emit[m] to be the same as one horizontal period for one scan line, and cutting both ends of the on-level pulse of the selection signal select[m] by t2 so that the length of the on-level pulse of the selection signal select[m] is shorter than the off-level pulse of the emission control signal emit[m]. Further, the length of the boost signal boost[m] is set to be longer than that of the selection signal select[m] by elongating both ends of the low pulse of the boost signal boost[m] by t1 (herein, t1&lt;t2).  
         [0074]     However, adjusting the length of the pulses of these signals causes data programming time to be reduced by twice t2 compared to the one horizontal period, and thus data programming to the pixel circuit may not be fully completed.  
         [0075]     For instance, in a portrait-type of Quarter Video Graphic Array (QVGA) measuring 320 pixels wide by 240 pixels high, a horizontal period is 52 μs. Assume that t2 is set to be 4 μs. In this case, the data programming time is reduced by 15% (twice t2) so that the data may not be completely programmed and thereby degrading image quality. In this case, the higher the resolution, the more severe the problem becomes.  
         [0076]      FIG. 9  shows the driving waveform to drive the pixel circuit in  FIG. 5  according to the fourth embodiment of the present invention.  
         [0077]     In the fourth embodiment of the present invention, the low pulse width of the boost signal boost[m] is set to be the same as the horizontal period, and both ends of the on-level pulse of the selection signal select[m] are shorter than the horizontal period by t1. Sequentially, the data current I DATA  is programmed before the node voltage of the capacitor C 2  is increased and after the node voltage of the capacitor C 2  is decreased.  
         [0078]     Further, the off-level pulse width of the emission control signal emit[m] is set to be greater than n times the horizontal period (herein, n≧2, n is an integer) so as to control the current to be flowed to the OLED after the node voltage of the capacitor C 2  is increased, and to control the node voltage of the capacitor C 2  to be decreased after the current flowing to the OLED is cut off when the transistor M 4  is turned off.  
         [0079]     As such, the time for data programming can be extended by adjusting the margins of the switching timing in the selection scan signal select[m], the emission scan signal emit[m], and the boost scan signal boost[m].  
         [0080]     Hereinafter, configurational and operational aspects of the scan driver  300  for generating the waveform of  FIG. 9  will be described with reference to  FIG. 10  and  FIG. 11 .  
         [0081]      FIG. 10  illustrates a circuit diagram of the scan driver  300  for generating the selection signal and the emission control signal of  FIG. 9 , according to an embodiment of the present invention, and  FIG. 11  illustrates drive timings of the scan driver  300 .  
         [0082]     As shown in  FIG. 10 , the scan driver  300  includes a shift register  310 , first NAND gates NAND 11  to NAND 1m , NOR gates NOR 11  to NOR 1m , and second NAND gates NAND 21  to NAND 2m . Assume that the number of the first and second NAND gates NAND 11  to NAND 1m  and NAND gates NAND 21  to NAND 2m , and the NOR gates NOR 11  to NOR 1m , respectively correspond to the number of select scan lines S 1  to S m .  
         [0083]     The shift register  310  receives a start signal VSP 1  when a clock signal VCLK is high, and outputs an output signal having the same level as the start signal VSP 1  and maintains the output signal SR 1  at the same level until the next high-level clock signal VCLK. Then, the shift register  310  sequentially outputs a plurality of output signals SR 2  to SR m+1  while shifting the output signal SR 1  by a half clock signal VCLK.  
         [0084]     According to an embodiment of the present invention, the scan driver  300  sets the horizontal period to be the same as a half period of the clock signal VCLK so as to decrease frequency of the clock signal VCLK. However, the output signals SR 1  to SR m+1  correspond to an integer multiple of the clock signal VCLK, the shift register  310  of  FIG. 10  is set to sequentially generate output signals while shifting the output signal SR 1  by a half clock signal VCLK, and then generates a series of overlapped signals from each of adjacent output signals using the NOR gates NOR 11  to NOR 1m  and sets the pulse width of the series of overlapped signals Out 1  t Out m  to be the same as the horizontal period.  
         [0085]     In other words, the NOR gate NOR 1 , performs the NOR operation on these two output signals SR i  and SR i+1  that are adjacent to each other among the output signals SR 1  to SR m+1  of the shift register  310  so as to generate the signal Out i . The NOR gate NOR i  generates a high-level signal only when input signals are low, but the output signal SR i  of the shift register  310  is maintained at the low level during one clock signal period. Herein, the output signal SR i+1  is shifted by a half clock signal VCLK, and therefore the signal Out i  of the NOR gate NOR 1i  is maintained at the high level during a half clock signal period.  
         [0086]     The first NAND gate NAND 1i  performs the NAND operation on these two output signals SR i  and SR i+1  that are adjacent to each other among the output signals SR 1  to SR m+1  of the shift register  310  so as to generate an emission control signal emit[i]. The output signal emit[i] of the first NAND gate is maintained at the high-level signal when one of the output signals SR i  and SR i+1  is low according to the NAND operation (herein, 1&lt;I&lt;m, i is an integer).  
         [0087]     That is, the emission control signal emit[i] is maintained at the high level while the output signals SR i  and SR i+1  are outputted, and these output signals SR i  and SR i+1  are respectively maintained at the low level during one clock signal VCLK. Herein, the output signal SR i+1  is generated by shifting the output signal SR i  by a half clock signal VCLK, and therefore the output signal SR i+1  is maintained at the high level during three times the half clock signal period. In other words, the SR i+1  is maintained at the high level during three horizontal periods.  
         [0088]     Further, the second NAND gate NAND 2i  performs the NAND operation on the signal Out i  of the NOR gate NOR 1i  and a clip signal CLIP, and generates a selection signal select[i]. The selection signal select[i] is maintained at the high level when the clip signal CLIP is low in the inverted signals of the signals Out i  to Out m  generated from the NOR gate NOR i .  
         [0089]     Herein, selection signals select[ 1 ] to select[m] of which both ends are shorter than the horizontal period by t1 can be generated in the case that the clip signal CLIP is maintained at the low level during t1 at both ends of the high-level pulse of the output signals Out 1  to Outm.  
         [0090]     Hereinafter, an internal configuration and operation of the shift register according to the embodiment of  FIG. 10  will be described with reference to  FIG. 12  and  FIG. 13 .  
         [0091]      FIG. 12  schematically illustrates the shift register  310 , and  FIG. 13  illustrates flip-flops used for the shift register  310 . A clock signal VCLKb in  FIG. 12  and  FIG. 13  is an inverted signal of the clock signal VCLK.  
         [0092]     As shown in  FIG. 12 , the shift register  310  includes (m+1) flip-flops FF 1  to FF m+1 , and output signals of the respective flip-flops FF 1  to FF m+1  become output signals SR 1  to SR i+1  of the shift register  310 . The start signal VSP 1  is inputted to the first flip-flop FF 1 , and the ith flip-flop FF i  signal becomes an input signal of the (i+1)th flip-flop FF i+1 .  
         [0093]     As described, the output signals SR 1  to SR m+1  of the shift register  310  should be shifted by a half clock signal VCLK, and thus the clock signals VCLK and VCLKb are inverted in the adjacent flip-flops FF i  and FF i+1 .  
         [0094]     In a longitudinal direction in  FIG. 12 , odd numbered flip-flops FF i  receive the clock signals VCLK and VCLKb as internal clock signals clk and clkb, and even numbered flip-flops FF i+1  receive the clock signals VCLKb and VCLK as the internal clock signals clk and clkb.  
         [0095]     The flip-flop FF i  outputs an input signal (in) as it is when the clock signal clk is high, but the flip-flop FF i  latches the input signal (in) to output during the low-level period when the clock signal clk is low. However, the output signal SR i+1  of the flip-flop FF i+1  is shifted by a half clock signal VCLK with respect to the output signal SR i  of the flip-flop FF i  since the output signal SR i  of the filp-flop FF i  becomes an input signal of the filp-flop FF i+1  and the clock signals VCLK and VCLKb are inverted and inputted to the adjacent flip-flops FF i  and FF i+1 .  
         [0096]     Hereinafter, an embodiment of the flip-flop FF i  of  FIG. 12  will described with reference to  FIG. 13 .  
         [0097]     As shown in  FIG. 13 , the flip-flop FF i  includes an inverter  312  forming a latch on a first three-phase inverter  311  provided in an input terminal of the flip-flop FF i , and a second three-phase inverter  313 . When the clock signal clk is high, the first three-phase inverter  311  inverts the input signal (in) as an output, and the inverter  312  inverts an output signal of the three-phase inverter  311  as an output. When the clock signal clk is low, the first three-phase inverter  311  is blocked and the output signal of the inverter  312  is inputted to the second three-phase inverter  313 , and an output signal of the second three-phase inverter  313  is inputted to the inverter  312 . Further, the output signal of the inverter  312  becomes the signal Out i  of the flip-flop FF i . In other words, the flip-flop FF i  outputs the input signal (in) as it is when the clock signal clk is high, and latches the input signal (in) in the high level when the clock signal clk is low.  
         [0098]      FIG. 14  illustrates the scan driver  300  to generate a selection signal and an emission control signal (or waveform) of  FIG. 9  according to another embodiment of the present invention.  
         [0099]     As shown therein, the scan driver  300  according to the embodiment of  FIG. 14  generates emission control signals emit[ 1 ] to emit[i] using internal signals of the flip-flops FF 1  to FF m+1 , and differing from the embodiment of  FIG. 10 .  
         [0100]     Further, the flip-flop FF 1  receives an inverted signal /VSP 1  of the start signal VSP 1  when the clock signal clk is high, and the inverted signal /VSP 1  is maintained until the next high-level clock signal. The flip-flops FF 2  to FF m+1  sequentially output a plurality of output signals /SR 2  to SR m+1  while shifting the output signal /SR 1  of the flip-flop FF 1  by a half clock signal.  
         [0101]     The odd numbered flip-flops receive the clock signals VCLK and VCLKb as the internal clock signals clk and clkb, and the even numbered flip-flops receive the clock signal VCLKb and VCLK as the internal clock signals clk and clkb in the embodiment of  FIG. 14 .  
         [0102]     Further, the first NAND gate NAND 1i  outputs an emission control signal emit[i] by performing the NAND operation on an internal signal of the ith flip-flop FF i  and the internal signal of the (i+1)th flip-flop FF (i+1) . In other words, the first NAND gate NADN 1i  performs the NAND operation on the input signals of the inverter  312  included in the ith flip-flop FF i  and the (i+1)th flip-flop FF (i+1)  so as to generate the emission control signal emit[i].  
         [0103]     The second NAND gate NADN 2i  outputs an output signal /Out i  by performing the NAND operation on the output signal /SR i  of the ith flip-flop FF i  and the output signal /SR i+1  of the (i+1)th flip-flop FF (i+1) .  
         [0104]     The detail of a circuit for generating the selection signal select[i] by using the output signal /Out i  of the second NAND gate NAND 2i  according to the embodiment of  FIG. 14  is substantially the same as the circuit described in the embodiment of  FIGS. 10, 12 , and/or  13 , and therefore is not provided in more detail. However, since the output signal /Out i  of the second NAND gate NAND 2i  is an inverted output signal Out i , the selection signal select[i] can be generated by coupling the inverter to the output terminal of the second NAND gate NAND 2i  and performing the NAND operation on the output signal of the inverter and the clip signal CLIP.  
         [0105]     In a like manner, an emission control signal can be generated by using the internal signal of the flip-flops FF 1  to FF m+1 , and a driving waveform can be substantially the same as the driving waveform according to the embodiment of  FIG. 10 .  
         [0106]      FIG. 6  to  FIG. 14  is generally focused on the pixel circuit of  FIG. 5 , and the switching transistors M 2  to M 4  are described as the P-channel transistor, but a scan driver of the present invention can be applied with other types of transistors with possible changes to the signal level of the described embodiments as are known to those skilled in the art and the present invention is not thereby limited.  
         [0107]     In addition, the scan driver  300  that generates the selection signals select[ 1 ] to select[m] and the emission control signals emit[ 1 ] to emit[m], and the scan driver  400  that generates the boost signals boost[ 1 ] to boost[m] are shown as two separate drivers, but these scan drivers  300  and  400  can be provided as one driver.  
         [0108]     For example, an inverted signal of the output signals Out 1  to Out m  of the NOR gates NOR 1  to NOR 1m  in the scan driver  300  can be used as the boost signal, or the output signals /Out i  to /Out m  of the second NAND gates NAND 21  to NAND 2m  can be used as the boost signals.  
         [0109]     Also, a structure of the driving circuit can be simplified by replacing these scan drivers  300  and  400  with one driver, and the number of signal lines provided in the display panel  100  can be reduced by using the same clock signal and input signal in the respective scan drivers  300  and  400 .  
         [0110]     Further, the scan driver generating the selection signals select[ 1 ] to select[m] and the emission control signals emit[ 1 ] to emit[m] are described as being provided by the driver  300 , but can also be separately provided.  
         [0111]     In addition, time for data programming can be extended by shifting the boost signal and elongating the width of the pulse by two times.  
         [0112]     While this invention has been described in connection with certain exemplary embodiments, it is to be understood by those skilled in the art that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications included within the spirit and scope of the appended claims and equivalents thereof.