Abstract:
An organic electroluminescence display capable of reducing a size of a data driving unit by decreasing an area of a D/A converter, and a driving circuit thereof. The data driving unit includes a first decoder to generate a first selection signal to correspond to a data signal; a first switch unit to receive first voltages and second voltages and to select one of the first voltages or the second voltages to correspond to the data signal; a second switch unit to select first and second reference voltages from the selected first or second voltages in response to the first selection signal; a second decoder to generate a second selection signal to correspond to the data signal; and a grey level voltage generating unit to receive and distribute the first and second reference voltages, selected by the switch units, to generate grey level voltages and to select and output one grey level voltage to correspond to the second selection signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 2006-50483, filed on Jun. 5, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Aspects of the present invention relate to a driving circuit and an organic electroluminescence display using the same, more specifically to a driving circuit capable of decreasing a grey level error to improve linearity by preventing a voltage sag generated in an analog switch, and an organic electroluminescence display using the same. 
     2. Description of the Related Art 
     A flat panel display has a plurality of pixels arranged in a matrix type pattern on a substrate as a display area, and a scan line and a data line connected to each pixel to display an image by selectively applying a data signal to the pixels. The flat panel displays are classified into passive matrix-type light-emitting displays and active matrix-type light-emitting displays according to a driving mode of respective pixels. The active matrix-type light-emitting displays which turn on light by individual pixels has been mainly used in terms of a high resolution, good contrast and fast operating speed. 
     Active matrix flat panel displays have been used as displays in such applications as personal computers, portable phones, PDAs, etc., or as monitors of various information appliances. Active matrix flat panel displays have been fabricated of liquid crystal displays (LCDs) using a liquid crystal panel, organic electroluminescence displays using organic electroluminescence devices, plasma display panels (PDPs) using plasma panels, etc. as have been known in the art. Recently, various light-emitting displays having a smaller weight and volume than a cathode ray tube have been developed, and attention has been particularly paid to the organic electroluminescence display which exhibits excellent luminous efficiency, luminance and viewing angle and has a rapid response time. 
       FIG. 1  is a circuit view showing a configuration of a conventional organic electroluminescence display  10 . Referring to  FIG. 1 , the organic electroluminescence display  10  includes a pixel unit  100 , a data driving unit  200  and a scan driving unit  300 . The pixel unit  100  includes a plurality of data lines (D 1 ,D 2  . . . Dm- 1 ,Dm) and a plurality of scan lines (S 1 ,S 2  . . . Sn- 1 ,Sn), and a plurality of pixels  101  formed in a region defined by a plurality of the data lines (D 1 ,D 2  . . . Dm- 1 ,Dm) and a plurality of the scan lines (S 1 ,S 2  . . . Sn- 1 ,Sn). Each pixel  101  includes a pixel circuit and an organic electroluminescence device, and the pixel  101  generates a pixel current in the pixel circuit to flow to the organic electroluminescence device, the pixel current flows in the pixels according to data signals transmitted through a plurality of the data lines (D 1 , D 2  . . . Dm- 1 ,Dm) and scan signals transmitted through a plurality of the scan lines (S 1 ,S 2  . . . Sn- 1 ,Sn). 
     The data driving unit  200  is connected with a plurality of the data lines (D 1 ,D 2  . . . Dm- 1 ,Dm), and generates data signals to sequentially transmit a row of data signals to a plurality of the data lines (D 1 ,D 2  . . . Dm- 1 ,Dm). The data driving unit  200  also has a digital-to-analog (D/A) converter, and generates a grey level voltage which is converted from a digital signal into an analog signal by the D/A converter, thereby to transmit the grey level voltage to the data lines (D 1 ,D 2  . . . Dm- 1 ,Dm). 
     The scan driving unit  300  is connected to a plurality of scan lines (S 1 ,S 2  . . . Sn- 1 ,Sn), and generates a scan signal to transmit the scan signal to a plurality of the scan lines (S 1 ,S 2  . . . Sn- 1 ,Sn). A certain row is selected by the scan signals, and a data signal is transmitted to a pixel  101  arranged in the selected row, such that a current corresponding to the data signal is generated in the pixel. 
       FIG. 2  is a circuit view showing a resistance unit which generates a grey level voltage in the D/A converter used in the data driving unit  200  of the organic electroluminescence display  10  as shown in  FIG. 1 . Referring to  FIG. 2 , assume that the resistance unit generates 8 grey level voltages for illustration. In order to generate 8 grey level voltages, 8 resistances (R 1 , R 2 , . . . R 8 ) are connected in series, and a first reference voltage having a high voltage (VrefH) and a second reference voltage having a low voltage (VrefL) are respectively transmitted to both ends of the resistances connected in series, and then the first reference voltage (VrefH) and the second reference voltage (VrefL) become a grey level voltage distributed by the 8 resistances. 
     The organic electroluminescence display  10  as configured above is preferably realized by preparing a small size of a driving circuit to manufacture a small size of the organic electroluminescence display  10 . 
     SUMMARY OF THE INVENTION 
     Accordingly, aspects of the present invention are designed to solve such drawbacks of the prior art and/or realize additional advantages, and therefore an aspect of the present invention is to provide a driving circuit capable of reducing a size of a data driving unit by decreasing an area of a D/A converter, and an organic electroluminescence display using the same. 
     Aspects of the present invention provide an organic electroluminescence display including a pixel unit, a data driving unit and a scan driving unit, wherein the data driving unit includes a first decoder to generate a first selection signal to correspond to a data signal; a first switch unit to receive a plurality of first voltages from a first bus and a plurality of second voltages from a second bus and to select the first voltages or the second voltages to correspond to the data signal; a second switch unit to select a first reference voltage and a second reference voltage from the selected first voltages or the selected second voltages in response to the first selection signal and to transmit the selected first reference voltage and selected second reference voltage; a second decoder to generate a second selection signal to correspond to a data signal; and a grey level voltage generating unit to receive and distribute the first selected reference voltage and the second selected reference voltage, selected by the second switch unit, to generate a plurality of grey level voltages and to select and output one grey level voltage out of the plurality of the grey level voltages to correspond to the second selection signal. 
     Aspects of the present invention provide a driving circuit including a first decoder to generate a first selection signal to correspond to a data signal; a first switch unit to receive a plurality of first voltages from a first bus and a plurality of second voltages from a second bus and to select the first voltages or the second voltages to correspond to the data signal; a second switch unit to select a first reference voltage and a second reference voltage from the selected first voltages or the selected second voltages in response to the first selection signal; a second decoder to generate a second selection signal to correspond to a data signal; and a grey level voltage generating unit to receive and distribute the first reference voltage and the second reference voltage, selected by the second switch unit, to generate a plurality of grey level voltages and to select and output one grey level voltage out of the plurality of the grey level voltages to correspond to the second selection signal. 
     Further aspects of the present invention provide a method for driving an organic electroluminescence display, including selecting a plurality of first voltages or a plurality of second voltages to correspond to a data signal and selecting one plurality of voltages out of the plurality of the first voltages and the plurality of the second voltages; selecting a first reference voltage and a second reference voltage out of the selected first voltages or the selected second voltages to correspond to the data signal; distributing the first reference voltage and the second reference voltage using the lower bit of the data signal; generating a grey level voltage from the distributed first and second reference voltages; and transmitting the generated grey level voltage to a data line. 
     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a circuit view showing a configuration of a conventional organic electroluminescence display; 
         FIG. 2  is a circuit view showing a resistance unit which generates a grey level voltage in a conventional D/A converter; 
         FIG. 3  is a schematic view showing a data driving unit used in an organic electroluminescence display according to an embodiment of the present invention; 
         FIG. 4  is a schematic view showing a configuration of the D/A converter according to an embodiment of the present invention; and 
         FIG. 5  is a circuit view showing one example of a pixel used in an organic electroluminescence display according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
     Here, when one element is connected to another element, one element may be not only directly connected to another element but also indirectly connected to another element via another element. Further, irrelevant elements are omitted for clarity. 
       FIG. 3  is a circuit view showing a data driving unit used in an organic electroluminescence display according to an embodiment of the present invention. Referring to  FIG. 3 , the data driving unit  205  includes a shift register  210 , a sampling latch  220 , a holding latch  230 , a level shifter  240 , a D/A converter  250  and a buffer unit  260 . 
     The shift register  210  comprises a plurality of flip flops, and the shift register  210  controls the sampling latch  220  to correspond to a clock signal (CLK and /CLK) and a synchronizing signal (HSP). The sampling latch  220  sequentially receives a row of data signals according to a control signal of the shift register  210 , and then outputs the data signals in parallel. A mode for sequentially receiving a signal and outputting the signal in parallel is referred to as Serial-In-Parallel-Out (SIPO). The holding latch  230  receives the signal in parallel, and then outputs the signal in parallel. A mode for receiving a signal in parallel and outputting the signal in parallel is referred to as Parallel-In-Parallel-Out (PIPO). The level shifter  240  changes a level of the signal, outputted from the holding latch  230 , into an operating voltage of the system and transmits the operating voltage to the D/A converter  250 . The D/A converter  250  transmits the signal, received as the digital signal, as an analog signal to select a corresponding grey level voltage and transmits the grey level voltage to the buffer unit  260 , and the buffer unit  260  amplifies the grey level voltage, and then transmits the amplified grey level voltage to data lines. 
       FIG. 4  is a schematic view showing a configuration of the D/A converter according to an embodiment of the present invention. Referring to  FIG. 4 , the D/A converter  250  includes a first decoder  251 , a first switch unit  252 , a second switch unit  253 , a second decoder  254  and a grey level voltage generating unit  255 . Assume that the D/A converter  250  displays 256 grey levels, and the data signal transmitted to the D/A converter  250  comprises 8-bit signals. 
     In order to generate  256  grey levels from 8-bit signals, the first decoder  251  uses 3 bits D 1 /D 1 B, D 2 /D 2 B, and D 3 /D 3 B, that is, all bits except an uppermost bit D 0 /D 0 B out of the upper 4 bits of the data signal to generate a first selection signal. The first switch unit  252  comprises 32 transistors, and odd-numbered transistors from a left to a right direction have respective sources connected to first bus lines (v 11 ,v 21  . . . v 91 ), respective gates to receive the uppermost bit signal, and respective drains connected to the second switch unit  253 . Even-numbered transistors have respective sources connected to second bus lines (v 12 ,v 22  . . . v 92 ), respective gates to receive an accessory signal of the uppermost bit signal, and respective drains connected to the second switch unit  253 . At this time, the first bus lines (v 11 ,v 21  . . . v 91 ) and the second bus lines (v 12 , v 22  . . . v 92 ) transmit nine first voltages and nine second voltages, respectively, and one group of voltages out of the voltages transmitted through the first bus lines (v 11 ,v 21  . . . v 91 ) and the voltages transmitted through the second bus lines (v 12 ,v 22  . . . v 92 ) is transmitted to the second switch unit  253  through the first switch unit  252  using the uppermost bit of the data signal. That is to say, the nine voltages, transmitted through the first bus lines (v 11 ,v 21  . . . v 91 ) to correspond to the uppermost bit of the data signal, are sequentially transmitted to the second switch unit  253  through the odd-numbered transistors, and the nine voltages transmitted through the second bus lines (v 12 ,v 22  . . . v 92 ) to correspond to the uppermost bit of the data signal, are sequentially transmitted through the even-numbered transistors, to the second switching unit  253 . The second bus lines (v 12 ,v 22  . . . v 92 ) and the first switch unit  252  are connected in the same manner as in the first bus lines (v 11 ,v 21  . . . v 91 ) and the first switch unit  252 . At this time, the first voltages are higher than the second voltages. The transistors connected to the first bus lines (v 11 ,v 21  . . . v 91 ) and the transistors connected to the second bus lines (v 12 ,v 22  . . . v 92 ) are operated by receiving the uppermost bit signal of the data signal, and the transistors connected to the first bus lines (v 11 ,v 21  . . . v 91 ) and the transistors connected to the second bus lines (v 12 ,v 22  . . . v 92 ) are operated in a reverse manner as follows. When the uppermost bit is set to 1, a grey level of the data signal is estimated to range from 128 to 256 grey levels, and therefore the odd-numbered transistors are in an ON state to select the first bus lines (v 11 ,v 21  . . . v 91 ), and when the uppermost bit is set to 0, the grey level of the data signal is estimated to range from 0 to 127 grey levels, and therefore the even-numbered transistors are in an ON state to select the second bus lines (v 12 ,v 22  . . . v 92 ). That is to say, one group of bus lines out of the first bus lines (v 11 ,v 21  . . . v 91 ) and the second bus lines (v 12 ,v 22  . . . v 92 ) is selected by the transistors. The odd-numbered transistors sequentially transmit the voltage to the second switch unit  253  in order from a highest voltage to a lower voltage, and the even-numbered transistors sequentially transmit the voltage to the second switch unit  253  in order from below the lower voltage to a lowest voltage. 
     The second switch unit  253  carries out a switching operation in response to the first selection signal generated in the first decoder  251 , and the 16 transistors selectively output the signals, transmitted from the first switch unit  252 , according to the first selection signal, and then transmit the signals to the grey level voltage generating unit  255 . At this time, two transistors of the second switch unit  253  are connected to respective ends of the grey level voltage generating unit  255 , and the second switch unit  253  outputs two voltages from one group of bus lines, selected by the first switch unit  252 , out of the first bus lines (v 11 ,v 21  . . . v 91 ) and the second bus lines (v 12 ,v 22  . . . v 92 ) to select a first reference voltage and a second reference voltage. The first reference voltage is changed by the first selection signal, and the second reference voltage is reduced to the lowest voltage which is transmitted through the selected bus lines. 
     The second decoder  254  receives a lower 4 bits D 4 /D 4 B, D 5 /D 5 B, D 6 /D 6 B, and D 7 /D 7 B of the data signal to generate a 16-bit second selection signal. Sixteen resistances are connected in series in the grey level voltage generating unit  255 , and the first reference voltage and the second reference voltage, selected by the second switch unit  253 , are transmitted to respective ends of the grey level voltage generating unit  255 , and then 16 grey level voltages are generated by the 16 resistances. The grey level voltage generating unit  255  receives the second selection signal to select and output one grey level voltage out of the 16 grey level voltages. Accordingly, the total 256 grey level voltages are generated by the uppermost bit of the data signal, the first selection signal and the second selection signal. 
       FIG. 5  is a circuit view showing one example of a pixel  101  used in the organic electroluminescence display according to an embodiment of the present invention. Referring to  FIG. 5 , the pixel is connected to the data line (Dm), the scan line (Sn) and the pixel power line (ELVdd) and includes a first transistor (M 1 ), a second transistor (M 2 ), a capacitor (Cst) and an organic electroluminescence device (OLED). 
     The first transistor (M 1 ) has a source connected to the pixel power line (ELVdd); a drain connected to an anode electrode of the organic electroluminescence device (OLED); and a gate connected to a first node (N 1 ). The second transistor (M 2 ) has a source connected to the data line (Dm); a drain connected to the first node (N 1 ); and a gate connected to the scan line (Sn). The capacitor (Cst) is connected between the first node (N 1 ) and the pixel power line (ELVdd) to maintain a voltage between the first node (N 1 ) and the pixel power line (ELVdd) during a predetermined period. The organic electroluminescence device (OLED) includes the anode electrode, a cathode electrode and an emitting layer, wherein if the anode electrode is connected to a drain of the first transistor (M 1 ) and the cathode electrode is connected to the low-potential power source (ELVSS) so as to allow a current to flow from an anode electrode to a cathode electrode of the organic electroluminescence device (OELD) to correspond to the voltage which is applied to the gate of the first transistor (M 1 ), then the light is emitted in the emitting layer and a brightness is adjusted to correspond to a capacity of the current. The capacitor (Cst) is connected between the first node (N 1 ) and the pixel power line (ELVdd) to maintain a voltage between the first node (N 1 ) and the pixel power line (ELVdd) during a predetermined period. The organic electroluminescence device (OLED) includes the anode electrode, a cathode electrode and an emitting layer, wherein if the anode electrode is connected to a drain of the first transistor (M 1 ) and the cathode electrode is connected to the low-potential power resource (ELVSS) so as to allow a current to flow from an anode electrode to a cathode electrode of the organic electroluminescence device (OELD) to correspond to the voltage which is applied to the gate of the first transistor (M 1 ), then the light is emitted in the emitting layer and a brightness is adjusted to correspond to a capacity of the current. 
     According to the driving circuit according to aspects of the present invention and the organic electroluminescence display using the same, a size of the D/A converter may be decreased by selecting a reference voltage using an analog switch since the uppermost bit is connected to the analog switch. 
     Although several embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.