Patent Publication Number: US-9892705-B2

Title: Data driver and method of driving the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Korean Patent Application No. 10-2014-0105357, filed on Aug. 13, 2014, and entitled, “Data Driver and Method of Driving the Same,” is incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     One or more embodiments described herein relate to a data driver and a method for driving a data driver. 
     2. Description of the Related Art 
     A display apparatus generally includes switching devices connected to pixel electrodes, gate lines, and data lines. An AC/DC converter may also be included to generate various types of voltages. For example, the AC/DC converter may convert an alternating current power source to a direct current power source. An analog circuit may also be included to convert direct current power to an analog driving voltage. 
     The analog driving voltage may be generated, for example, by controlling the level of a reference power source using a power source regulator. The voltage output from the reference power source voltage may be increased using a booster circuit, e.g., an electric charge pump. A data driver generates data voltages based on the analog driving voltage and outputs the data voltages to respective data lines of the display apparatus, for example, through buffers. In operation, power consumption may increase when the data driver outputs the data voltages. 
     SUMMARY 
     In accordance with one embodiment, a data driver includes a plurality of buffers to respectively output data voltages corresponding to pixel image data; a plurality of bias circuits provided in one-to-one correspondence with the buffers, the bias circuits to generate bias currents independent of each other and to apply the bias currents to the buffers, respectively; and a bias signal generator to generate a plurality of bias signals, wherein each of the bias circuits include: a selector to select one bias signal among the bias signals based on corresponding pixel image data among the pixel image data and to output the selected bias signal as a final bias signal; and a bias current generator to generate a corresponding bias current among the bias currents based on the final bias signal. 
     The data driver may include a sampling latch to receive input image data and to sample the pixel image data from the input image data based on a sampling signal; and a digital-to-analog converter to convert the pixel image data to the data voltages and to apply the data voltages to the buffers in one-to-one correspondence, wherein the selector is to receive the corresponding pixel image data from the sampling latch among the pixel image data. 
     The selector may include a variation detector, and a signal multiplexer, wherein the variation detector is to receive the corresponding pixel image data among the pixel image data and to generate a selection signal based on the corresponding pixel image data, and wherein the signal multiplexer is to select one of the bias signals based on the selection signal. 
     The corresponding pixel image data among the pixel image data may include a previous pixel image data provided in an (L−1)th horizontal period and a present pixel image data provided in an L-th horizontal period, and the variation detector may include a pixel memory to store the previous pixel image data; and a comparator to calculate an absolute value of a difference between a previous grayscale value of the previous pixel image data and a present grayscale value of the present pixel image data, and to generate the selection signal based on the calculated absolute value. 
     The comparator may compare upper i (“i” is a natural number) bits of the previous pixel image data and upper i bits of the present pixel image data to generate the selection signal, and wherein a number of the bias signals is 2×i. The value of i may be 1 and the comparator may receive the previous pixel image data and the present pixel image data and may perform an exclusive-OR calculation on the previous pixel image data and the present pixel image data. 
     The bias signals may include a first bias signal, and a second bias signal different from the first bias signal, the first bias signal may include a first transition period and a first control period which are defined in each horizontal period, wherein the second bias signal may include a second transition period and a second control period which are defined in each horizontal period, wherein the first bias signal may have a first transition level in the first transition period and has a first control level lower than the first transition level in the first control period, and wherein the second bias signal may have a second transition level in the second transition period and has a second control level lower than the second transition level in the second control period. 
     The first control level may be different from the second control level. The first transition level may be different from the second transition level. At least a portion of the first control period may not overlap the second control period. 
     The bias signal generator may include a bias signal generator including first and second sub-bias signal generators to respectively generate the first and second bias signals, wherein: the first sub-bias signal generator may generate the first bias signal based on a first transition level value determining the first transition level, a first control level value determining the first control level, and a first activation signal determining the first control period, and the second sub-bias signal generator may generate the second bias signal based on a second transition level value determining the second transition level, a second control level value determining the second control level, and a second activation signal determining the second control period. 
     The first sub-bias signal generator may include: first level value multiplexer to select one value of the first transition level value or the first control level value based on the first activation signal, and to output the selected value as a first intermediate bias signal; and a first bias signal generating circuit to generate the first bias signal based on the first intermediate bias signal and a reference bias current, and the second sub-bias signal generator may include: a second level value multiplexer to select one value of the second transition level value or the second control level value based on the second activation signal, and to output the selected value as a second intermediate bias signal; and a second bias signal generating circuit to generate the second bias signal based on the second intermediate bias signal and the reference bias current. 
     The bias signal generator may subtract the first bias different value from the first transition level value to generate the first control level value, and may subtract the second bias different value from the first transition level value to generate the second control level value, the first bias difference value may include information indicative of a difference between the first transition level and the first control level, and the second bias difference value may include information indicative of a difference between the second transition level and the second control level. 
     The bias signal generator may include a counter to generate the first control activation signal based on a first control start time point corresponding to a start point of the first control period and a first control end time point corresponding to an end point of the first control period, and to generate the second control activation signal based on a second control start time point corresponding to a start point of the second control period and a second control end time point corresponding to an end point of the second control period. 
     The bias signal generator may include: an image controller to receive the input image data, analyze the input image data, and generate at least one of the transition level value, the first and second bias difference values, the first and second control start time points, and the first and second control end time points based on the analyzed result. The image controller may analyze the input image data every horizontal period. 
     In accordance with another embodiment, a method for driving a data driver comprising generating a plurality of data voltages based on pixel image data; outputting the data voltages through a plurality of buffers, respectively; generating bias currents; applying the bias currents to the buffers, respectively; and generating a plurality of bias signals, wherein applying the bias currents to the buffers includes selecting one of the bias signals with respect to each of the buffers based on the pixel image data and generating the bias currents in accordance with the selected bias signal. 
     Each of the pixel image data may include a previous pixel image data provided in an (L−1)th horizontal period and a present pixel image data provided in an L-th horizontal period, and selecting one of the bias signals may include: calculating an absolute value of a difference between a previous grayscale value of the previous pixel image data and a present grayscale value of the present pixel image data; and selecting one of the bias signals in accordance with the calculated absolute value. 
     Calculating the absolute value of the difference between the previous grayscale value of the previous pixel image data and the present grayscale value of the present pixel image data may include comparing upper i (i is a natural number) bits of the previous pixel image data and upper i bits of the present pixel image data. Comparing the upper bits may include: receiving the previous pixel image data and the present pixel image data; and performing an exclusive-OR calculation on previous pixel image data and the present pixel image data. 
     In accordance with another embodiment, a data driver includes a plurality of buffers to respectively output data voltages; and a plurality of bias circuits to respectively output bias currents based on variation in an amount of a corresponding data voltage among the data voltages in each horizontal period, wherein the bias circuits are provided in one-to-one correspondence to the buffers and are to apply the bias currents to the buffers, respectively. 
     The data driver may include a bias signal generator to generate a plurality of bias signals, wherein each of the bias circuits include: a selector to select one of the bias signals and to outputs the selected bias signal as a final bias signal; and a bias current generator to generate the bias current based on the bias signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features will become apparent to those of skill in the art by describing in detail exemplary embodiments with reference to the attached drawings in which: 
         FIG. 1  illustrates an embodiment of a display apparatus; 
         FIG. 2  illustrates an embodiment of a data driver; 
         FIG. 3  illustrates an embodiment of a bias signal generating unit; 
         FIGS. 4A and 4B  illustrate examples of control signals for the unit in  FIG. 3 ; 
         FIG. 5  illustrates an embodiment of a first sub-bias signal generator; 
         FIG. 6  illustrates an embodiment of a bias signal generating circuit in  FIG. 5 ; 
         FIGS. 7A and 7B  illustrate embodiments of first and second bias units in  FIG. 2 ; 
         FIG. 8  illustrates examples of control signals for the units in  FIGS. 7A and 7B ; 
         FIG. 9  illustrates additional examples of control signals for the unit of  FIG. 3 ; 
         FIG. 10  illustrates additional examples of control signals for the units in  FIGS. 7A and 7B ; 
         FIG. 11  illustrates additional examples of control signals for the unit in  FIG. 3 ; 
         FIG. 12  illustrates additional examples of control signals for the units in  FIGS. 7A and 7B ; 
         FIG. 13  illustrates another embodiment of a bias signal generating unit; 
         FIG. 14  illustrates another embodiment of a first bias unit; and 
         FIG. 15  illustrates another embodiment of a bias signal generating unit. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments are described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey exemplary implementations to those skilled in the art. In the drawings, the dimensions of layers and regions may be exaggerated for clarity of illustration. Like reference numerals refer to like elements throughout. 
     It will be understood that when an element or layer is referred to as being “on”, “connected to” or “coupled to” another element or layer, it can be directly on, connected or coupled 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,” “directly connected to” or “directly coupled to” another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
       FIG. 1  illustrates an embodiment of a display apparatus  1000  which includes a display panel  100  to display an image, gate and data drivers  200  and  300  to drive the display panel  100 , and a timing controller  400  to control a drive of the gate and data drivers  200  and  300 . The display panel may be a liquid crystal display panel, an organic light emitting display panel, an electrophoretic display panel, an electrowetting display panel, or another type of display device. 
     The timing controller  400  receives image information (e.g., RGB) and control signals, for example, from an external image source. The control signals include, for example, a vertical synchronization signal Vsync as a frame distinction signal, a horizontal synchronization signal Hsync as a row distinction signal, a data enable signal DE that defines a period in which data are input, and a clock signal CLK. The data enable signal DE may maintain a predetermined (e.g., high) level only during a period in which the data area output. 
     The timing controller  400  converts a data format of the image information RGB, to a data format appropriate to an interface between the data driver  300  and the timing controller  400 , to generate an input image data Idata. The input image data Idata is applied to the data driver  300 . In addition, the timing controller  400  generates a data control signal DCS and a gate control signal GCS based on the control signals. The timing controller  400  applies the data control signal DCS to the data driver  300  and applies the gate control signal GCS to the gate driver  200 . 
     The gate control signal GCS includes a scanning start signal to indicate the start of the scanning, the clock signal CLK to control an output period of a gate-on voltage, and an output enable signal to control a maintaining time of the gate-on voltage. 
     The data control signal DCS includes a horizontal start signal STH to indicate a start of transmission of the input image data Idata to the data driver  300 , a load signal MS, an inverting signal POL, and the clock signal CLK. 
     The gate driver  200  sequentially applies gate signals to the display panel  100  based on the gate control signal GCS from the timing controller  400 . 
     The data driver  300  converts the input image data Idata to the data voltages based on the data control signal DCS from the timing controller  400 . The data voltages are applied to the display panel  100 . 
     The display panel  100  includes a plurality of gate lines GL 1  to GLm, a plurality of data lines DL 1  to DLn, and a plurality of pixels PX. The gate lines GL 1  to GLm extend in a first direction D 1  and are arranged substantially in parallel to each other in a second direction D 2  substantially perpendicular to the first direction D 1 . The gate lines GL 1  to GLm are connected to the gate driver  200  to receive the gate signals from the gate driver  200 . The data lines DL 1  to DLn extend in the second direction D 2  and are arranged substantially in parallel to each other in the first direction D 1 . The data lines DL 1  to DLn are connected to the data driver  300  to receive the data voltages from the data driver  300 . 
     In the case where display panel  100  is a liquid crystal display, each pixel PX may include, for example, a switching device SW to output a data signal based on the gate signal and a liquid crystal capacitor Clc charged with the data voltage. Each pixel PX is connected to a corresponding gate line of the gate lines GL 1  to GLm and a corresponding data line of the data lines DL 1  to DLn. For example, each pixel PX is turned on or off based on the gate signal applied through a corresponding gate line. The turned-on pixel PX emits light having a grayscale value corresponding to a data voltage applied through a corresponding data line. 
       FIG. 2  illustrates an embodiment of data driver  300  in  FIG. 1 . Referring to  FIG. 2 , the data driver  300  includes a shift register  310 , a sampling latch  320 , a holding memory  330 , a digital-to-analog converter  340 , and first to n-th buffers BP 1  to BPn. 
     The shift register  310  includes a plurality of stages connected to each other, one after another. Each stage receives the clock signal CLK and a first stage is applied with the horizontal start signal STH. When the first stage starts operation based on the horizontal start signal STH, the stages sequentially output a sampling signal based on o the clock signal CLK. 
     The sampling latch  320  receives the input image data Idata and sequentially samples first to n-th pixel image data PD 1  PDn, which corresponds to one line, among the input image data Idata based on the sampling signal sequentially provided from the stages. The sampling latch  320  outputs the first to n-th pixel image data PD 1  to PDn to the holding memory  330  based on a latch signal. 
     The first to n-th pixel image data PD 1  to PDn respectively correspond to images displayed in the pixels PX (refer to  FIG. 1 ), which correspond to one line addressed during one horizontal period. 
     The holding memory  330  holds the first to n-th pixel image data PD 1  to PDn from the sampling latch  320  during one horizontal period, and applies the first to n-th pixel image data PD 1  to PDn to the digital-to-analog converter  340  during one horizontal period. 
     The digital-to-analog converter  340  converts the first to n-th pixel image data PD 1  to PDn to the data voltages. The digital-to-analog converter  340  applies the data voltages to the first to n-th buffers BP 1  to BPn, respectively. 
     The first to n-th buffers BP 1  to BPn receive the data voltages from the digital-to-analog converter  340  and outputs the data voltages to the data lines DL 1  to DLn at the same time point based on the load signal MS. 
     The data driver  300  further includes a bias signal generating unit  350  and a plurality of bias units. The bias units may include, for example, first to n-th bias units BU 1  to BUn provided in one-to-one correspondence with the first to n-th buffers BP 1  to BPn. 
     The bias signal generating unit  350  generates a plurality of bias signals, which include, for example, first and second bias signals BS 1  and BS 2  which are different from each other. The bias signal generating unit  350  outputs the first and second bias signal BS 1  and BS 2  to each of the first to n-th bias units BU 1  to BUn. 
     The first to n-th bias units BU 1  to BUn respectively generate first to n-th bias currents IB 1  to IBn based on the first to n-th pixel image data PD 1  to PDn, and respectively apply the first to n-th bias currents IB 1  to IBn to the first to n-th buffers BP 1  to BPn. For instance, the first bias unit BU 1  receives the first pixel image data PD 1 , generates the first bias current IB 1  based on the first pixel image data PD 1 , and outputs the generated first bias current IB 1  to the first buffer BP 1 . 
     The first to n-th bias units BU 1  to BUn include first to n-th selecting units SU 1  to SUn and first to n-th bias current generating units BG 1  to BGn. 
     Each of the first to n-th selecting units SU 1  to SUn receives the first and second bias signals BS 1  and BS 2  from the bias signal generating unit  350 . In addition, the first to n-th selecting units SU 1  to SUn receive the first to n-th pixel image data PD 1  to PDn, respectively. The first to n-th selecting units SU 1  to SUn respectively receive, for example, the first to n-th pixel image data PD 1  to PDn from the holding memory  330 . For example, the first to n-th selecting units SU 1  to SUn may respectively receive the first to n-th pixel image data PD 1  to PDn from the sampling latch  320 . 
     The first to n-th selecting units SU 1  to SUn select either the first bias signal BS 1  or the second bias signal BS 2  on the first to n-th pixel image data PD 1  to PDn, and generate first to n-th final bias signals FBS 1  to FBSn. For example, the first to n-th selecting units SU 1  to SUn select one of the first and second bias signals in accordance to a variation in the data voltages output from the first to n-th buffers BP 1  to BPn in each horizontal period. 
     For instance, when a level of the data voltage output from the first buffer BP 1  varies by a predetermined (e.g., extreme) amount between a (L−1)th horizontal period and an L-th horizontal period following the (L−1)th horizontal period, the first selecting unit SU 1  selects one of the first or second bias signals BS 1  and BS 2  to relatively largely increase the first bias current IB 1 . 
     When a level of the data voltage output from the second buffer BP 2  varies by an amount less than the predetermined amount (e.g., slightly) between the (L−1)th horizontal period and the L-th horizontal period following the (L−1)th horizontal period, the second selecting unit SU 2  selects one of the first or second bias signals BS 1  and BS 2  to relatively largely increase the second bias current IB 2 . The predetermined amount may be determined, for example, based on a certain type of desired performance, the intended application, or different criteria. 
     The first to n-th bias current generating units BG 1  to BGn receive the first to n-th final bias signals FBS 1  to FBSn from the first to n-th selecting units SU 1  to SUn, respectively, to generate the first to n-th bias currents IB 1  to IBn based on the first to n-th final bias signals FBS 1  to FBSn. The first to n-th bias current generating units BG 1  to BGn apply the first to n-th bias currents IB 1  to IBn to the first to n-th buffers BP 1  to BPn. 
       FIG. 3  illustrates an embodiment of the bias signal generating unit  350  in  FIG. 2 , and  FIGS. 4A and 4B  are timing diagrams including examples of control signals for the bias signal generating unit  350  in  FIG. 3 . Waveforms of the first and second bias signals BS 1  and BS 2  and first and second activation signals ES 1  and ES 2  will be described with reference to  FIGS. 4A and 4B . 
     The first bias signal BS 1  includes a first transition period TP 1 , a first control period CP 1 , and a first dummy period DP 1 , which are defined in each horizontal period. In the present exemplary embodiment, the first transition period TP 1 , the first control period CP 1 , and the first dummy period DP 1  are arranged in order of the first transition period TP 1 , the first control period CP 1 , and the first dummy period DP 1  in each horizontal period. 
     The first transition period TP 1 , the first control period CP 1 , and the first dummy period DP 1  do not overlap each other. As an example, the first transition period TP 1  is defined between a start point of the horizontal period and a start point of the first control period CP 1 . The first dummy period DP 1  is defined between an end point of the first control period CP 1  and an end point of the horizontal period. 
     The first bias signal BS 1  has a first transition level TL 1  during the first transition period TP 1 , has a first control level CL 1  during the first control period CP 1 , and has a first dummy level DL 1  during the first dummy period DP 1 . The first transition level TL 1  is higher than the first control level CL 1 . The first dummy level DL 1  may be substantially the same as the first transition level TL 1 . 
     The second bias signal BS 2  includes a second transition period TP 2 , a second control period CP 2 , and a second dummy period DP 2 , which are defined in each horizontal period. In the present exemplary embodiment, the second transition period TP 2 , the second control period CP 2 , and the second dummy period DP 2  are arranged in order of the second transition period TP 2 , the second control period CP 2 , and the second dummy period DP 2  in each horizontal period. The second transition period TP 2 , the second control period CP 2 , and the second dummy period DP 2  do not overlap each other. 
     As an example, the second transition period TP 2  is defined between a start point of the horizontal period and a start point of the second control period CP 2 . The second dummy period DP 2  is defined between an end point of the second control period CP 2  and an end point of the horizontal period. 
     The second bias signal BS 2  has a second transition level TL 2  during the second transition period TP 1 , a second control level CL 2  during the second control period CP 2 , and a second dummy level DL 2  during the second dummy period DP 2 . In this embodiment, the second transition level TL 2  is higher than the second control level CL 2 . The second dummy level DL 2  may be substantially the same as the second transition level TL 2 . 
     As an example, the second transition level TL 2  and the second dummy level DL 2  are substantially the same as the first transition level TL 1  and the first dummy level DL 1 , respectively, and the second control level CL 2  is higher than the first control level CL 1 . As an example, the second transition period TP 2 , the second control period CP 2 , and the second dummy period DP 2  may respectively correspond to the first transition period TP 1 , the first control period CP 1 , and the first dummy period DP 1 . 
     Referring to  FIG. 3 , the bias signal generating unit  350  includes a memory  351 , a control level value generator  352 , a counter  353 , and a bias signal generator  354 . 
     The memory  351  stores a transition level value TL including information relating to the first and second transition levels TL 1  and TL 2 . In addition, the memory  351  stores first and second bias different values BD 1  and BD 2  respectively including information about differences between the first and second transition levels TL 1  and TL 2  and the first and second control levels CL 1  and CL 2 , first and second control start time points CS 1  and CS 2  including information about the start point of the first and second control periods CP 1  and CP 2 , and first and second control end time points CT 1  and CT 2  including information about the first and second control periods CP 1  and CP 2 . 
     The control level value generator  352  receives the transition level value TL and the first and second bias different values BD 1  and BD 2  from the memory  351 . The control level value generator  352  subtracts the first and second bias different values BD 1  and BD 2  from the transition level value TL, and generates first and second control level values LS 1  and LS 2  to determine the first and second control levels CL 1  and CL 2 . 
     The counter  353  receives the clock signal CLK. The counter  353  generates the first activation signal ES 1  based on the first control start time point CS 1  and the first control end time point CT 1  to determine the first control period CP 1 . 
     For example, the counter  353  counts a time lapse from the start point of the horizontal period to the first control start time point CS 1  using the clock signal CLK to define the first transition period TP 1 . The counter  353  outputs a low level during the first transition period TP 1 . Then, the counter  353  counts a time lapse from the start point of the horizontal period to the first control end time point CT 1  to define the first control period CP 1 . The counter  353  outputs a high level during the first control period CP 1 . Subsequently, the counter  353  outputs the low level during the first dummy period DP 1 . As a result, the first activation signal ES 1  has the low level during the first transition period TP 1  and the first dummy period DP 1 , and has the high level during the first control period CP 1 . 
     The counter  353  generates the second activation signal ES 2  based on the second control start time point CS 2  and the second control end time point CT 2  to determine the second control period CP 2 . 
     For example, the counter  353  counts a time lapse from the start point of the horizontal period to the second control start time point CS 2 , using the clock signal CLK to define the second transition period TP 2 . The counter  353  outputs the low level during the second transition period TP 2 . Then, the counter  353  counts a time lapse from the start point of the horizontal period to the second control end time point CT 2  to define the second control period CP 2 . The counter  353  outputs the high level during the second control period CP 2 . Subsequently, the counter  353  outputs the low level during the second dummy period DP 2 . As a result, the second activation signal ES 2  has the low level during the second transition period TP 2  and the second dummy period DP 2  and has the high level during the second control period CP 2 . 
     As described above, since the second transition period TP 2 , the second control period CP 2 , and the second dummy period DP 2  are defined the same as the first transition period TP 1 , the first control period CP 1 , and the first dummy period DP 1 , respectively, the second control start time point CS 2  and the second control end time point CT 2  are substantially the same as the first control start time point CS 1  and the first control end time point CT 1 , respectively. Accordingly, the first activation signal ES 1  generated based on the first control start time point CS 1  and the first control end time point CT 1  may have substantially the same waveform as that of the second activation signal ES 2  generated based on the second control start time point CS 2  and the second control end time point CT 2 . 
     The bias signal generator  354  includes a first sub-bias signal generator  354   a  that generates the first bias signal BS 1  and a second sub-bias signal generator  354   b  that generates the second bias signal BS 2 . The first sub-bias signal generator  354   a  receives the transition level value TL, the first control level value LS 1 , and the first activation signal ES 1  and generates the first bias signal BS 1  based on the transition level value TL, the first control level value LS 1 , and the first activation signal ES 1 . The second sub-bias signal generator  354   b  receives the transition level value TL, the second control level value LS 2 , and the second activation signal ES 2  and generates the second bias signal BS 2  based on the transition level value TL, the second control level value LS 2 , and the second activation signal ES 2 . 
       FIG. 5  illustrates an embodiment of the first sub-bias signal generator  354   a . In one embodiment, the first and second sub-bias signal generators  354   a  and  354   b  may have the same structure and function. Therefore, only the first sub-bias signal generator  354   a  will be described. 
     Referring to  FIG. 5 , the first sub-bias signal generator  354   a  includes a level value multiplexer L-MUX and a bias signal generating circuit BGC. The level value multiplexer L-MUX receives the transition level value TL, the first control level value LS 1 , and the first activation signal ES 1 . The level value multiplexer L_MUX selects either the transition level value TL or the first control level value LS 1  based on the first activation signal ES 1  to generate an intermediate bias signal IBS. 
     For example, the level value multiplexer L-MUX selects the transition level value TL when the first activation signal ES 1  is at the low level and selects the first control level value LS 1  when the first activation signal ES 1  is at the high level to output the intermediate bias signal IBS. As a result, the intermediate bias signal IBS has the transition level value TL during the first transition period TP 1  and has the first control level value LS 1  during the first control period CP 1 . 
     The bias signal generating circuit BGC receives the intermediate bias signal IBS and a reference bias current Iref and generates the first bias signal BS 1 . 
       FIG. 6  illustrates an embodiment of the bias signal generating circuit in  FIG. 5 . Referring to  FIG. 6 , the bias signal generating circuit BGC includes a reference transistor RT, first to k-th mirror transistors MT 1  to MTk, first to k-th switches S 1  to Sk, and an output transistor OT. 
     The source and drain of the reference transistor RT are respectively connected to first and second power sources Vdd and Vss. A gate of the reference transistor RT is connected to the source of the reference transistor RT. 
     Gates of the first to k-th mirror transistors MT 1  to MTk are connected to the gate of the reference transistor RT. The gates of the first to k-th mirror transistors MT 1  to MTk are also connected to sources of the first to k-th mirror transistors MT 1  to MTk, respectively. The drains of the first to k-th mirror transistors MT 1  to MTk are connected the second power source Vss, and are respectively connected to first ends of the first to k-th switches S 1  to Sk. 
     The drain of the output transistor OT is connected to the first power source Vdd. The gate of the output transistor OT is connected to a source of the output transistor OT. The source of the output transistor OT is connected to second ends of the first to k-th switches S 1  to Sk. The nodes, at which the source of the output transistor OT is connected to the other ends of the first to k-th switches S 1  to Sk, will be referred to as first nodes N 1 . 
     The first to k-th switches S 1  to Sk are switched on or off in accordance of the level of the intermediate bias signal IBS. 
     When the reference bias current Iref is applied to the reference transistor RT, the first to k-th mirror transistors MT 1  to MTk respectively generate first to k-th mirror currents by a current mirroring operation. However, the first to k-th mirror currents flow from the first nodes N 1  through the source and the drain of the first to k-th mirror transistors MT 1  to MTk when the first to k-th switches S 1  to Sk are switched on. For instance, the first mirror current flows from the first node N 1  through the source and the drain of the first mirror transistor MT 1  when the first switch S 1  is switched on. 
     When the switches corresponding to the first to k-th mirror currents are switched on, the mirror currents flowing through the first nodes N 1  are added to each other to form an output current Io. The output current Io flows through the source and the drain of the output transistor OT. 
     The first to k-th mirror currents have different values. For instance, when the first to k-th mirror transistors MT 1  to MTk have different sizes, the first to k-th mirror currents have different values. 
     The output current Io has a value controlled by the combination of the switched-on and off of the first to k-th switches S 1  to Sk due to the intermediate bias signal IBS. For example, the switched-on and off of each of the first to k-th switches are determined to allow the value of the output current Io to correspond to the intermediate bias signal IBS. When the output current Io flows through the output transistor OT, the output transistor OT outputs the first bias signal BS 1  corresponding to the output current Io through the gate thereof. 
     The bias signal generating circuit BGC may further include a current source. The first end of the current source is connected to the first power source Vdd, and the second end of the current source is connected to the reference transistor RT. The current source may apply the reference bias current Iref to the reference transistor RT. In another embodiment, a resistor may be used, instead of the current source, to apply the reference bias current Iref to the reference transistor RT. The resistor may be connected, for example, between the first power source Vdd and the reference transistor RT. In this case, the reference bias current Iref may have a value determined, for example, by a resistance of the resistor. 
       FIGS. 7A and 7B  respectively illustrate embodiments of first and second bias units BU 1  and BU 2  in  FIG. 2 . The first bias unit BU 1  includes the first selecting unit SU 1  and the first bias current generating unit BG 1 . 
     The first selecting unit SU 1  includes a first variation detector TD 1  and a first signal multiplexer S-MUX 1 . The first variation detector TD 1  receives the first pixel image data PD 1  and generates a first selection signal SS 1  based on the first pixel image data PD 1 . The first variation detector TD 1  includes a first pixel memory PM 1  and a first comparator CM 1 . 
     The first pixel image data PD 1  includes a previous first pixel image data PD 1 _ p  provided in the (L−1)th horizontal period and a present first pixel image data PD 1 _ c  provided in the L-th horizontal period. The L-th horizontal period follows the (L−1)th horizontal period. 
     The first pixel memory PM 1  stores the pervious first pixel image data PD 1 _ p  and applies the pervious first pixel image data PD 1 _ p  to the first comparator CM 1 . The first pixel memory PM 1  receives the first pixel image data PD 1 _ p  during the (L−1)th horizontal period and stores the first pixel image data PD 1 _ p  therein. Then, the first pixel memory PM 1  applies the pervious first pixel image data PD 1 _ p  to the first comparator CM 1  during the L-th horizontal period. 
     The first comparator CM 1  compares the previous first pixel image data PD 1 _ p  and the present first pixel image data PD 1 _ c  to generate the first selection signal SS 1 . As an example, the first comparator CM 1  calculates an absolute value of a difference between a previous grayscale value of the previous first pixel image data PD 1 _ p  and a present grayscale value of the present first pixel image data PD 1 _ c  and generates the first selection signal SS 1  based on the absolute value of the difference between the previous grayscale value and the present grayscale value. 
     As an example, the first comparator CM 1  compares an upper 1 bit of the present first pixel image data PD 1 _ c  with an upper 1 bit of the previous first pixel image data PD 1 _ p , in order to calculate the difference between the previous grayscale value of the previous first pixel image data PD 1 _ p  and the present grayscale value of the present first pixel image data PD 1 _ c . The first comparator CM 1  receives the upper 1 bit of the present first pixel image data PD 1 _ c  and the upper 1 bit of the previous first pixel image data PD 1 _ p , and performs an exclusive-OR calculation on the upper 1 bit to output the first selection signal SS 1 . 
     When assuming that the difference between the previous grayscale value and the present grayscale value is large (e.g., the previous grayscale value corresponds to 10 grayscale level among 256 grayscale levels and the present gray scale value corresponds to 255 grayscale level among 256 grayscale levels), the upper 1 bit of the previous first pixel image data PD 1 _ p  has a value of “0” and the upper 1 bit of the present first pixel image data PD 1 _ c  has a value of “1”. Accordingly, the first selection signal SS 1  has the value of “1” when the exclusive-OR calculation is performed. 
     On the contrary, when assuming that the difference between the previous grayscale value and the present grayscale value is small (e.g., the previous grayscale value corresponds to 255 grayscale level among 256 grayscale levels and the present gray scale value corresponds to 255 grayscale level among 256 grayscale levels), the upper 1 bit of the previous first pixel image data PD 1 _ p  has the value of “1” and the upper 1 bit of the present first pixel image data PD 1 _ c  has the value of “0”. Therefore, the first selection signal SS 1  has the value of “0” when the exclusive-OR calculation is performed. 
     The first signal multiplexer S-MUX 1  receives the first and second bias signals BS 1  and BS 2  from the bias signal generating unit  350  and receives the first selection signal SS 1  from the first comparator CM 1 . The first signal multiplexer S-MUX 1  selects one of the first and second bias signals BS 1  and BS 2  based on the first selection signal SS 1  and outputs the selected bias signal of the first and second bias signals BS 1  and BS 2  as the first final bias signal FBS 1 . For instance, when the first selection signal SS 1  has the value of “0”, the first signal multiplexer S-MUX 1  selects the first bias signal BS 1  and when the first selection signal SS 1  has the value of “1”, the first signal multiplexer S-MUX 1  selects the second bias signal BS 2 . 
     The first bias current generating unit BG 1  receives the first final bias signal FBS 1  from the first signal multiplexer S-MUX 1  and generates the first bias current IB 1  based on the first final bias signal FBS 1 . The first bias current generating unit BG 1  applies the first bias current IB 1  to the first buffer BP 1  (e.g., refer to  FIG. 2 ). 
     The first bias current generating unit BG 1  generates the first bias current IB 1  having the same value as that of the output current Io through the current mirroring operation using the transistors shown in  FIG. 6 . 
     The second bias unit BU 2  includes the second selecting unit SU 2  and the second bias current generating unit BG 2 . The second selecting unit SU 2  includes a second variation detector TD 2  and a second signal multiplexer S-MUX 2 . The second variation detector TD 2  receives the second pixel image data PD 2  and generates a second selection signal SS 2  based on the second pixel image data PD 2 . The second variation detector TD 2  includes a second pixel memory PM 2  and a second comparator CM 2 . 
     The second pixel image data PD 2  includes a previous second pixel image data PD 2 _ p  provided in the (L−1)th horizontal period and a present second pixel image data PD 2 _ c  provided in the L-th horizontal period. 
     The second pixel memory PM 2  stores the pervious second pixel image data PD 2 _ p  and applies the pervious second pixel image data PD 2 _ p  to the second comparator CM 2 . The second pixel memory PM 2  receives the second pixel image data PD 2 _ p  during the (L−1)th horizontal period and stores the second pixel image data PD 2 _ p  therein. Then, the second pixel memory PM 2  applies the pervious second pixel image data PD 2 _ p  to the second comparator CM 2  during the L-th horizontal period. 
     The second comparator CM 2  compares the previous second pixel image data PD 2 _ p  and the present second pixel image data PD 2 _ c  to generate the second selection signal SS 2 . As an example, the second comparator CM 2  calculates an absolute value of a difference between a previous grayscale value of the previous second pixel image data PD 2 _ p  and a present grayscale value of the present second pixel image data PD 2 _ c  and generates the selection signal SS 2  based on the absolute value of the difference between the previous grayscale value and the present grayscale value. Operation of the second comparator CM 2  may be substantially the same as that of the first comparator CM 1 , except that the second comparator CM 2  receives the previous second pixel image data PD 2 _ p  and the present second pixel image data PD 2 _ c.    
     The second signal multiplexer S-MUX 2  receives the first and second bias signals BS 1  and BS 2  from the bias signal generating unit  350  and receives the second selection signal SS 2  from the second comparator CM 2 . The second signal multiplexer S-MUX 2  selects one of the first or second bias signals BS 1  and BS 2  based on the second selection signal SS 2 , and outputs the selected bias signal of the first and second bias signals BS 1  and BS 2  as the second final bias signal FBS 2 . For instance, when the second selection signal SS 2  has the value of “0”, the second signal multiplexer S-MUX 2  selects the first bias signal BS 1  and when the second selection signal SS 2  has the value of “1”, the second signal multiplexer S-MUX 2  selects the second bias signal BS 2 . 
     The second bias current generating unit BG 2  receives the second final bias signal FBS 2  from the second signal multiplexer S-MUX 2  and generates the second bias current IB 2  based on the second final bias signal FBS 2 . The second bias current generating unit BG 2  applies the second bias current IB 2  to the second buffer BP 1  (refer to  FIG. 2 ). The second bias current generating unit BG 2  generates the second bias current IB 2  having the same value as that of the output current Io through the current mirroring operation using the transistors shown in  FIG. 6 . 
       FIG. 8  is a timing diagram illustrating examples of control signals for the units in  FIGS. 7A and 7B . In the present exemplary embodiment, the previous grayscale value of the previous first pixel image data PD 1 _ p  corresponds to 250 grayscale level among 256 grayscale levels and the present grayscale value of the present first pixel image data PD 1 _ c  corresponds to 255 grayscale level among 256 grayscale levels. 
     The first buffer BP 1  (e.g., in  FIG. 2 ) outputs a first data voltage DV 1  corresponding to the first pixel image data PD 1 . For example, the first data voltage DV 1  has a first voltage  250 G corresponding to 250 grayscale level during the (L−1)th horizontal period and has a second voltage  255 G during the first control period CP 1  of the L-th horizontal period according to the present grayscale value of the first pixel image data PD 1  corresponding to 255 grayscale level. Thus, a variation (or difference) of the first data voltage DV 1  is small during the horizontal period. 
     The previous grayscale value of the previous second pixel image data PD 2 _ p  corresponds to 10 grayscale level among 256 grayscale levels and the present grayscale value of the present second pixel image data PD 2 _ c  corresponds to 255 grayscale level among 256 grayscale levels. 
     The second buffer BP 2  output a second data voltage DV 2 . The second data voltage DV 2  has a third voltage  10 G corresponding to 10 grayscale level during the (L−1)th horizontal period and has the second voltage  255 G during the second control period CP 2  of the L-th horizontal period. Thus, a variation (or difference) of the second data voltage DV 2  is large during the horizontal period. 
     As described with reference to  FIGS. 4A and 4B , the first and second bias signals BS 1  and BS 2  have substantially the same level, except that the first and second bias signals BS 1  and BS 2  respectively have the first and second control levels CL 1  and CL 2 . For example, the first transition period TP 1 , the first control period CP 1 , and the first dummy period DP 1  are substantially the same as the second transition period TP 2 , the second control period CP 2 , and the second dummy period DP 2 , respectively. Also, the first transition level TL 1  and the first dummy level DL 1  are substantially the same as the second transition level TL 2  and the second dummy level DL 2 , respectively. 
     The first variation detector TD 1  calculates the difference between the previous grayscale value of the previous first pixel image data PD 1 _ p  and the present grayscale value of the present first pixel image data PD 1 _ c , to generate the first selection signal SS 1  having the value of “0”. The first signal multiplexer S-MUX 1  selects the first bias signal BS 1  based on the first selection signal SS 1 . Then, the first selecting unit SU 1  outputs the selected first bias signal BS 1  as the first final bias signal FBS 1  during the L-th horizontal period. 
     The second variation detector TD 2  calculates the difference between the previous grayscale value of the previous second pixel image data PD 2 _ p  and the present grayscale value of the present second pixel image data PD 2 _ c , to generate the second selection signal SS 2  having the value of “1”. The second signal multiplexer S-MUX 2  selects the second bias signal BS 2  having the relatively high level in the second control period CP 2  based on the second selection signal SS 2 . Then, the second selecting unit SU 2  outputs the selected second bias signal BS 2  as the second final bias signal FBS 2  during the L-th horizontal period. 
     The first bias current generating unit BG 1  generates the first bias current IB 1  based on the first final bias signal FBS 1 . The second bias current generating unit BG 2  generates the second bias current IB 2  based on the second final bias signal FBS 2 . Accordingly, the first and second bias currents IB 1  and IB 2  have a transition current TI corresponding to the first transition level TL 1 , which is equal to the second transition level TL 2 , during the first transition period TP 1  and the second transition period TP 2 . In addition, the first and second bias currents IB 1  and IB 2  have a dummy current DI corresponding to the first dummy level DL 1 , which is equal to the second dummy level DL 2 , during the first dummy period DP 1  and the second dummy period DP 2 . 
     However, the first bias current IB 1  has a first control current CI 1  corresponding to the first control level CL 1  during the first control period CP 1  and the second control period CP 2 . The second bias current IB 2  has a second control current CI 2  corresponding to the second control level CL 2  during the first control period CP 1  and the second control period CP 2 . 
     Since the first control current CI 1  is smaller than the second control current CI 2 , a power consumption in the first and second buffers BP 1  and BP 2  when the first control current CI 1  is applied to the first and second buffers BP 1  and BP 2  is smaller than a power consumption in the first and second buffers BP 1  and BP 2  when the second control current CI 2  is applied to the first and second buffers BP 1  and BP 2 . 
     In addition, since the first control current CI 1  is smaller than the second control current CI 2 , a through rate of the first and second buffers BP 1  and BP 2  when the first control current CI 1  is applied to the first and second buffers BP 1  and BP 2  is smaller than a through rate in the first and second buffers BP 1  and BP 2  when the second control current CI 2  is applied to the first and second buffers BP 1  and BP 2 . 
     The first bias current IB 1  is applied to the first buffer BP 1  and the second bias current IB 2  is applied to the second buffer BP 2  that outputs the second data voltage DV 2  extremely varied according to the horizontal period. 
     Since the first control current CI 1  smaller than the second control current CI 2  is applied to the first buffer BP 1  during the first and second control periods CP 1  and CP 2 , the power consumption in the first buffer BP 1  is more reduced than the power consumption in the second buffer BP 2 . 
     Further, since the second control current CI 2  greater than the first control current CI 1  is applied to the second buffer BP 2 , the second buffer BP 2  may secure the through rate enough to output the second data voltage DV 2  that is relatively greatly varied. For example, since the variation in amount of the second data voltage DV 2  is large, the first data voltage DV 1  increases to the second voltage  255 G at the start point of the first control period CP 1 , but the second data voltage DV 2  does not increase to the second voltage  255 G. The second control current CI 2  is applied to the second buffer BP 2  during the first control period CP 1 , and thus the second data voltage DV 2  rapidly increases to the second voltage  255 G. 
     The second buffer BP 2  may increase the second data voltage DV 2  to the second voltage  255 G in the first control period CP 1  using only the through rate corresponding to the transition current TI. 
     As described above, each of the first and second bias units BU 1  and BU 2  selects one of the first or second bias signals BS 1  and BS 2  in accordance with the first and second pixel image data PD 1  and PD 2 , and outputs the bias current corresponding to the selected bias signal of the first and second bias signals BS 1  and BS 2 . Therefore, the first and second buffers BP 1  and BP 2  are respectively applied with the first and second bias currents IB 1  and IB 2 , which respectively correspond to the first and second data voltages DV 1  and DV 2  and which have through rates corresponding to variations in the amount of the first and second data voltages DV 1  and DV 2 . As a result, power consumption in the first and second buffers BP 1  and BP 2  may be reduced. 
     In addition, a layout of the data driver  300  may be simplified since the data driver  300  includes only one bias signal generating unit  350  having a complex circuit configuration. Also, the first to n-th buffers BP 1  to BPn respectively include the first and n-th bias units BU 1  to Bun, each having a simple circuit configuration for selecting one of the first or second bias signals BS 1  and BS 2  generated by the bias signal generating unit  350 . 
     The first and second bias units all and BU 2  have been described as a representative example. In one embodiment, the first to n-th bias units BU 1  to BUn may have the same structure and function. 
       FIG. 9  is a timing diagram illustrating additional examples of control signals for the unit in  FIG. 3 , and  FIG. 10  is a timing diagram illustrating examples of control signals for the units in  FIGS. 7A and 7B . 
     Referring to  FIG. 9 , the first and second control periods CP 1  and CP 2  are defined to be different from each other. For example, at least a portion of the first control period CP 1  does not overlap the second control period CP 2 . In one embodiment, the width of the first control period CP 1  is greater than that of the second control period CP 2 , and the end point of the first control period CP 1  is substantially coincident with the end point of the second control period CP 2 . Thus, the start point of the first control period CP 1  is faster than the start point of the second control period CP 2 . 
     According to another exemplary embodiment, at least a portion of the second control period CP 2  may not overlap the first control period CP 1 . According to another exemplary embodiment, the first and second control periods CP 1  and CP 2  may have the same width, but may start at different start points. 
     Also, in the present exemplary embodiment, the first transition level TL 1 , the first control level CL 1 , and the first dummy level DL 1  may be substantially the same as the second transition level TL 2 , the second control level CL 2 , and the second dummy level DL 2 , respectively. 
     Hereinafter, the operation of the data driver  300  according to another embodiment will be described with reference to  FIGS. 7A, 7B, and 10 . The first and second data voltages DV 1  and DV 2 , the first and second pixel image data PD 1  and PD 2 , and the first and second selection signals SS 1  and SS 2  in  FIG. 10  may correspond to the description relating to  FIGS. 7A and 7B . 
     The first signal multiplexer S-MUX 1  selects the first bias signal BS 1  having the first control period CP 1  with the relatively large width based on the first selection signal SS 1 . Then, the first selecting unit SU 1  outputs the selected first bias signal BS 1  as the first final bias signal FBS 1  in the L-th horizontal period. 
     The second signal multiplexer S-MUX 2  selects the second bias signal BS 2  having the second control period CP 2  with the relatively small width based on the second selection signal SS 2 . Then, the second selecting unit SU 2  outputs the selected second bias signal BS 2  as the second final bias signal FBS 2  in the L-th horizontal period. 
     The first bias current generating unit BG 1  generates the first bias current IB 1  based on the first final bias signal FBS 1 , and the second bias current generating unit BG 2  generates the second bias current IB 2  based on the second final bias signal FBS 2 . The first bias current IB 1  has the transition current TI, the first control current CI 1 , and the dummy current DI respectively during the first transition period TP 1 , the first control period CP 1 , and the first dummy period DP 1 . The second bias current IB 2  has the transition current TI, the first control current CI 1 , and the dummy current DI respectively during the second transition period TP 2 , the second control period CP 2 , and the second dummy period DP 2 . 
     Since the transition current TI is greater than the first control current CI 1 , power consumption in the first and second buffers BP 1  and BP 2 , when the transition current TI is applied to the first and second buffers BP 1  and BP 2 , is greater than the power consumption in the first and second buffers BP 1  and BP 2  when the first control current CI 1  is applied to the first and second buffers BP 1  and BP 2 . 
     In addition, since the transition current TI is greater than the first control current CI 1 , the through rate of the first and second buffers BP 1  and BP 2 , when the transition current TI is applied to the first and second buffers BP 1  and BP 2 , is greater than the through rate in the first and second buffers BP 1  and BP 2  when the first control current CI 1  is applied to the first and second buffers BP 1  and BP 2 . 
     The first bias current IB 1  is applied to the first buffer BP 1  and the second bias current IB 2  is applied to the second buffer BP 2 , that outputs the second data voltage DV 2  which extremely varies according to the horizontal period. 
     Since the transition current TI is applied to the first buffer BP 1  during the first transition period TP 1  having the width smaller than that of the second transition period TP 2 , and the first control current CI 1  is applied to the first buffer BP 1  during the first control period CP 1  having the width greater than that of the second control period CP 2 , power consumption in the first buffer BP 1  is reduced more than power consumption in the second buffer BP 2 . 
     In addition, since the transition current TI is applied to the second buffer BP 2  during the second transition period TP 2  having the width greater than that of the first transition period TP 1 , the second buffer BP 2  may secure the through rate corresponding to the first transition current TI during a time period sufficient enough to output the second data voltage DV 2 , that is relatively greatly varied. 
     Therefore, the first and second buffers BP 1  and BP 2  are respectively applied with the first and second bias currents IB 1  and IB 2  respectively corresponding to the first and second data voltages DV 1  and DV 2 , and have the through rates corresponding to variations in the amounts of the first and second data voltages DV 1  and DV 2 . As a result, power consumption in the first and second buffers BP 1  and BP 2  may be reduced. 
     In the above-mentioned description, the first and second bias units BU 1  and BU 2  have been described as a representative example. In one embodiment, the first to n-th bias units BU 1  to BUn may have the same structure and function. 
       FIG. 11  is a timing diagram illustrating additional examples of control signals for the unit in  FIG. 3 , and  FIG. 12  is a timing diagram illustrating additional examples of control signals for the units in  FIGS. 7A and 7B . Referring to  FIG. 11 , the first and second transition levels TL 1  and TL 2  may be defined to be different from each other. Also, in the present exemplary embodiment, the second transition level TL 2  is higher than the first transition level TL 1 . 
     For example, the first dummy level DL 1  may be lower than the first transition level TL 1 . The second control level CL 2  and the second dummy level DL 2  may be substantially the same as the first control level CL 1  and the first dummy level DL 2 , respectively. Also, the first transition period TP 1 , the first control period CP 1 , and the first dummy period DP 1  may be substantially the same as the second transition period TP 2 , the second control period CP 2 , and the second dummy period DP 2 , respectively. 
     Hereinafter, operation of the data driver  300  will be described with reference to  FIGS. 7A, 7B, and 12 . The first and second data voltages DV 1  and DV 2 , the first and second pixel image data PD 1  and PD 2 , and the first and second selection signals SS 1  and SS 2  in  FIG. 12  may correspond to the description relating to  FIGS. 7A and 7B . 
     The first signal multiplexer S-MUX 1  selects the first bias signal BS 1  having the relatively high level in the first transition period TP 1  based on the first selection signal SS 1 . Then, the first selecting unit SU 1  outputs the selected first bias signal BS 1  as the first final bias signal FBS 1  in the L-th horizontal period. 
     The second signal multiplexer S-MUX 2  selects the second bias signal BS 2  having the second control period CP 2  with the relatively small width in the first transition period TP 1  based on the second selection signal SS 2 . Then, the second selecting unit SU 2  outputs the selected second bias signal BS 2  as the second final bias signal FBS 2  in the L-th horizontal period. 
     The first bias current generating unit BG 1  generates the first bias current IB 1  based on the first final bias signal FBS 1 , and the second bias current generating unit BG 2  generates the second bias current IB 2  based on the second final bias signal FBS 2 . Accordingly, the first bias current IB 1  has the first transition current TI 1  corresponding to the first transition level TL 1  during the first transition period TP 1 , the first control current CI 1  during the first control period CP 1 , and the first dummy current DI 1  corresponding to the first dummy level DL 1  during the first dummy period DP 1 . 
     In addition, the second bias current IB 2  has the second transition current TI 2  corresponding to the second transition level TL 2  during the second transition period TP 2 , the first control current CI 1  during the second control period CP 2 , and the first dummy current DI 1  during the first dummy period DP 1 . 
     Since the first transition current TI 1  is smaller than the second transition current TI 2 , power consumption in the first and second buffers BP 1  and BP 2 , when the first transition current TI 1  is applied to the first and second buffers BP 1  and BP 2 , is smaller than power consumption in the first and second buffers BP 1  and BP 2  when the second transition current TI 2  is applied to the first and second buffers BP 1  and BP 2 . 
     In addition, since the first transition current TI 1  is smaller than the second transition current TI 2 , the through rate of the first and second buffers BP 1  and BP 2 , when the first transition current TI 1  is applied to the first and second buffers BP 1  and BP 2 , is smaller than the through rate in the first and second buffers BP 1  and BP 2  when the second transition current T 12  is applied to the first and second buffers BP 1  and BP 2 . 
     The first bias current IB 1  is applied to the first buffer BP 1  that outputs the first data voltage DV 1  slightly varied during the horizontal period, and the second bias current IB 2  is applied to the second buffer BP 2  that outputs the second data voltage DV 2  extremely varied during the horizontal period. 
     Therefore, since the first transition current TI 1  smaller than the second transition current TI 2  is applied to the first buffer BP 1  during the first and second transition periods TP 1  and TP 2 , power consumption in the first buffer BP 1  is reduced to a greater extent than power consumption in the second buffer BP 2 . In addition, since the second transition current TI 2  greater than the first transition current TI 1  is applied to the second buffer BP 2 , the second buffer BP 2  may secure a through rate sufficient enough to output the second data voltage DV 2 , that is relatively greatly varied. 
     As described above, each of the first and second bias units BU 1  and BU 2  selects one of the first and second bias signals BS 1  and BS 2  in accordance with the first and second pixel image data PD 1  and PD 2 , and outputs the bias current corresponding to the selected bias signal of the first and second bias signals BS 1  and BS 2 . 
     Therefore, the first and second buffers BP 1  and BP 2  are respectively applied with the first and second bias currents IB 1  and IB 2 , that respectively correspond to the first and second data voltages DV 1  and DV 2 , and have through rates corresponding to variations in the amount of the first and second data voltages DV 1  and DV 2 . As a result, power consumption in the first and second buffers BP 1  and BP 2  may be reduced. 
     In the above-mentioned description, the first and second bias units BU 1  and BU 2  have been described as a representative example. In one embodiment, the first to n-th bias units BU 1  to BUn may have the same structure and function. 
       FIG. 13  illustrates another embodiment of a bias signal generating unit, and  FIG. 14  illustrates another embodiment of a first bias unit. Referring to  FIG. 13 , the bias signal generating unit  350  generates a plurality of bias signals. The bias signals may include first to fourth bias signals BS 1  to BS 4 , which are different from each other. The first to fourth bias signals BS 1  to BS 4  may have waveforms substantially similar to the first and second bias signals BS 1  and BS 2  in  FIGS. 4A and 4B . 
     For example, the first bias signal BS 1  has a first transition level during a first transition period and a first control level during a first control period. The second bias signal BS 2  has a second transition level during a second transition period and a second control level during a second control period. The third bias signal BS 3  has a third transition level during a third transition period and a third control level during a third control period. The fourth bias signal BS 4  has a fourth transition level during a fourth transition period and a fourth control level during a fourth control period. 
     Among the first to fourth control periods, at least one control period may be different from the other control periods. In addition, among the first to fourth transition levels, at least one transition level may be different from the other transition levels. Also, at least one control level of the first to fourth control levels may be different from the other control levels. Various combinations of periods and levels of the first to fourth bias signals BS 1  to BS 4  may be different from each other in other embodiments. Thus, the first to fourth bias signals BS 1  to BS 4  may have different waveforms. 
     The bias signal generating unit  350  includes the memory  351 , the control level value generator  352 , the counter  353 , and a bias signal generator  554 . 
     The memory  351  stores first to fourth transition level values TV 1  to TV 4 , including information about the first to fourth transition levels. In addition, the memory  351  stores first to fourth bias different values BD 1  to BD 4  respectively including information about differences between the first to fourth transition levels and the first to fourth control levels, first to fourth control start time points CS 1  to CS 4  including information about the start point of the first to fourth control periods, and first to fourth control end time points CT 1  to CT 4  including information about the first to fourth control periods. 
     The control level value generator  352  receives the first to fourth transition level values TV 1  to TV 4  and the first to fourth bias different values BD 1  to BD 4  from the memory  351 . The control level value generator  352  subtracts the first to fourth bias different values BD 1  to BD 4  from the first to fourth transition level values TV 1  to TV 4 , respectively, and generates first to fourth control level values LS 1  to LS 4 , respectively, to determine the first to fourth control levels. 
     The counter  353  receives the clock signal CLK and generates first to fourth activation signals ES 1  to ES 4  based on the first to fourth control start time points CS 1  to CS 4  and the first to fourth control end time points CT 1  to CT 4 , to respectively determine the first to fourth control periods. Operation of the counter  353  may be as described with reference to  FIG. 3 . 
     The bias signal generator  554  includes first to fourth sub-bias signal generators  554   a  to  554   d  that respectively generate the first to fourth bias signals BS 1  to BS 4 . 
     The first sub-bias signal generator  554   a  receives the first transition level value TV 1 , the first control level value LS 1 , and the first activation signal ES 1  and generates the first bias signal BS 1  based on the first transition level value TV 1 , the first control level value LS 1 , and the first activation signal ES 1 . 
     The second sub-bias signal generator  554   b  receives the second transition level value TV 2 , the second control level value LS 2 , and the second activation signal ES 2  and generates the second bias signal BS 2  based on the second transition level value TV 2 , the second control level value LS 2 , and the second activation signal ES 2 . 
     The third sub-bias signal generator  554   c  receives the third transition level value TV 3 , the third control level value LS 3 , and the third activation signal ES 3  and generates the third bias signal BS 3  based on the third transition level value TV 3 , the third control level value LS 3 , and the third activation signal ES 3 . 
     The fourth sub-bias signal generator  554   d  receives the fourth transition level value TV 4 , the fourth control level value LS 4 , and the fourth activation signal ES 4  and generates the fourth bias signal BS 4  based on the fourth transition level value TV 4 , the fourth control level value LS 4 , and the fourth activation signal ES 4 . 
     Operation of the first to fourth sub-bias signal generators  554   a  to  554   d  may be substantially the same as the first and second bias signal generators  354   a  and  354   b  in  FIGS. 5 and 6 . 
     Referring to  FIG. 14 , the first bias unit BU 1  includes the first selecting unit TU 1  and the first bias current generating unit BG 1 . In addition, the first selecting unit TU 1  includes a first variation detector UD 1  and a first signal multiplexer T-MUX 1 . The first variation detector UD 1  receives the first pixel image data PD 1  and generates the first selection signal SS 1  in accordance with the first pixel image data PD 1 . The first variation detector UD 1  includes the first pixel memory PM 1  and a first comparator DM 1 . 
     The first comparator DM 1  compares the previous first pixel image data PD 1 _ p  and the present first pixel image data PD 1 _ c  and generates the first selection signal SS 1 . As an example, the first comparator DM 1  calculates an absolute value of a difference between a previous grayscale value of the previous first pixel image data PD 1 _ p  and a present grayscale value of the present first pixel image data PD 1 _ c , and generates the first selection signal SS 1  based on the absolute value of the difference between the previous grayscale value of the previous first pixel image data PD 1 _ p  and the present grayscale value of the present first pixel image data PD 1 _ c.    
     In the present exemplary embodiment, the first comparator DM 1  compares upper 2 bits of the present first pixel image data PD 1 _ c  and upper 2 bits of the previous first pixel image data PD 1 _ p  to generate the first selection signal SS 1 . Accordingly, the first selection signal SS 1  may have four values of “00”, “01”, “10”, and “11”. 
     The first signal multiplexer T-MUX 1  receives the first to fourth bias signals BS 1  to BS 4  from the bias signal generating unit  350 , and receives the first selection signal SS 1  from the first comparator DM 1 . The first signal multiplexer T-MUX 1  selects one bias signal of the first to fourth bias signals BS 1  to BS 4  based on the first selection signal SS 1  and outputs the selected bias signal as the first final bias signal FBS 1 . 
     For instance, when the first selection signal SS 1  has the value of “00”, the first signal multiplexer T-MUX 1  selects the first bias signal BS 1 . When the first selection signal SS 1  has the value of “01”, the first signal multiplexer T-MUX 1  selects the second bias signal BS 2 . In addition, when the first selection signal SS 1  has the value of “10”, the first signal multiplexer T-MUX 1  selects the third bias signal BS 3 . When the first selection signal SS 1  has the value of “11”, the first signal multiplexer T-MUX 1  selects the fourth bias signal BS 4 . 
     The first bias current generating unit BG 1  receives the first final bias signal FBS 1  from the first signal multiplexer T-MUX 1  and generates the first bias current IB 1  based on the first final bias signal FBS 1 . The first bias current generating unit BG 1  applies the first bias current IB 1  to the first buffer BP 1 . 
     In the present exemplary embodiment described with reference to  FIGS. 13 and 14 , the bias signal generating unit  350  generates the four bias signals and the first selecting unit TU 1  selects one of the four bias signals based on the compared results of the upper 2 bits of the first pixel image data PD 1 . 
     In this or another embodiment, the bias signal generating unit  350  may generate 2i (“i” is a natural number) bias signals and the first selecting unit TU 1  may select one of the 2i (“i” is a natural number) bias signals based on the compared results of upper i bits of the first pixel image data PD 1 . 
     As the number of the bias signals selected by the first selecting unit TU 1  increases, the first selecting unit TU 1  selects the bias signal more precisely corresponding to variation in the amount of the first data voltage DV 1 . Therefore, the first buffer BP 1  receives the first bias current IB 1  corresponding to the variation in amount of the first data voltage DV 1 , and has a through rate corresponding to variation in the amount of the first data voltage DV 1 . As a result, power consumption in the first buffer BP 1  may be reduced. 
       FIG. 15  illustrates another embodiment of a bias signal generating unit  350  which includes an image controller  355 . The image controller  355  receives the input image data Idata, analyzes the input image data Idata, generates at least one of the transition level TL, the first and second bias different values BD 1  and BD 2 , the first and second control start time points CS 1  and CS 2 , or the first and second control end time points CT 1  and CT 2  based on the analyzed result, and applies the generated value to the memory  351 . 
     For example, the image controller  355  analyzes the input image data Idata, and calculates an average grayscale value of the input image data Idata, and generates at least one of the transition level TL, the first and second bias different values BD 1  and BD 2 , the first and second control start time points CS 1  and CS 2 , or the first and second control end time points CT 1  and CT 2  based on the average grayscale value. 
     In the present exemplary embodiment, the image controller  355  periodically analyzes the input image data every horizontal period and newly generates at least one of the transition level TL, the first and second bias different values BD 1  and BD 2 , the first and second control start time points CS 1  and CS 2 , or the first and second control end time points CT 1  and CT 2 . 
     As described above, when the bias signal generating unit  350  includes the image controller  355 , the waveforms of the first and second bias signals BS 1  and BS 2  are determined depending on the input image data Idata. Accordingly, the first to n-th bias currents IB 1  to IBn having waveforms corresponding to the input image data Idata may be generated based on the first and second bias signals BS 1  and BS 2 . 
     In the present exemplary embodiment, the image controller  355  serves as a part of the data driver  300 . In another embodiment, the image controller  355  may be included in the timing controller  400 . In addition, the image controller  355  may be provided in a card or board shape without being included in the timing controller  400 . In this case, the image controller  355  may be connected between the image source and the timing controller  400 , or may be in a device connected between the image source and the timing controller  400 . 
     By way of summation and review, one type of data driver drives pixels in a display based on an analog driving voltage. More specifically, this data driver generates a data voltage using the analog driving voltage and outputs the data voltage to the data lines through buffers. Power consumption by the buffers consume a large portion of the total power consumed by the data driver. 
     In accordance with one or more of the aforementioned embodiments, a data driver includes a plurality of buffers to respectively output data voltages corresponding to pixel image data, a plurality of bias units BU 1  to BUn which are provided in one-to-one correspondence to the buffers and which generate bias currents IB 1  to IBn independent to each other and apply the bias currents to the buffers, respectively, and a bias signal generating unit to generate a plurality of bias signals. Each of the bias units includes a selecting unit to select one bias signal among the bias signals based on a corresponding pixel image data among the pixel image data and top output the selected bias signal as a final bias signal; and a bias current generating unit to generate a corresponding bias current among the bias currents in response to the final bias signal. The bias currents may be controlled according to variation in the amount of the data voltage output from the buffers in each horizontal period in the unit of buffer. As a result, the power consumption in the buffers may be reduced. 
     Example embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. In some instances, as would be apparent to one of skill in the art as of the filing of the present application, features, characteristics, and/or elements described in connection with a particular embodiment may be used singly or in combination with features, characteristics, and/or elements described in connection with other embodiments unless otherwise indicated. Accordingly, it will be understood by those of skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.