Abstract:
A back-light driving circuit controlling red (R), green (G), and blue (B) back lights providing light to a liquid crystal panel in a field sequential liquid crystal display. The back-light driving circuit includes a driving voltage generator that provides a driving voltage to each of the R, G, and B back-lights to cause them to emit light having a predetermined luminance. The back-light driving circuit also includes a pulse width modulation (PWM) signal generator for providing a PWM signal to each of the R, G, and B back-lights to control the chromaticity of the light emitted from each back-light. The driving voltages and/or PWM signals provided to each of the R, G, and B back-lights are catered to the particular characteristics of the corresponding back-light to cause them to emit color having a desired luminance and/or chromaticity.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 2003-0084780, filed Nov. 27, 2003, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a field sequential liquid crystal display (FS-LCD), and more particularly, to an LCD capable of obtaining desired chromaticity and luminance regardless of a driving current distribution of a light emitting diode (LED). 
     2. Description of Related Art 
     A color LCD generally includes a liquid crystal panel having an upper substrate, a lower substrate, and a liquid crystal injected between the upper and lower substrates. The color LCD further includes a driving circuit for driving the liquid crystal panel, and a back-light for providing white light to the liquid crystal. Such an LCD may be mainly classified into a red (R), green (G), blue (B) color filter type or a color field sequential driving type depending on its driving mechanism. 
     In the color filter type LCD, a single pixel is divided into R, G, and B subpixels, and R, G, and B color filters are respectively arranged in the R, G, and B subpixels. Light is transmitted from a single back-light to the R, G, and B color filters through the liquid crystal allowing a color image to be displayed. 
     On the other hand, a color FS-LCD includes R, G, and B back-lights that are arranged in a single pixel that is not divided into R, G, and B subpixels. The light of the three primary colors is provided from the R, G, and B back-lights to the single pixel through the liquid crystal so that each of the three primary colors are sequentially displayed in a time-sharing, multiplexed manner, allowing the display of a color image using a residual image effect. 
       FIG. 1  is a perspective view of a configuration of a typical color FS-LCD. 
     Referring to  FIG. 1 , the FS-LCD includes a liquid crystal panel  100  having a lower substrate  101  in which a thin film transistor (TFT) array (not shown) for switching is arranged to be connected to a plurality of gate lines, a plurality of data lines, and a plurality of common lines. The liquid crystal panel also includes an upper substrate  103  in which a common electrode (not shown) is formed to provide a common voltage to the common lines. The liquid crystal panel further includes a liquid crystal (not shown) injected between the upper and lower substrates. 
     The FS-LCD further includes a gate line driving circuit  110  for providing scan signals to the plurality of gate lines of the liquid crystal panel  100 , a data line driving circuit  120  for providing R, G, and B data signals to the data lines, and a back-light system  130  for providing light corresponding to three primary colors, namely, R, G, and B colors, to the liquid crystal panel  100 . 
     The back-light system  130  includes three back-lights  131 ,  133 , and  135  respectively providing R, G, and B light, and a light guide plate  137  providing the R, G, and B light respectively emitted from the R, G, and B back-lights  131 ,  133 , and  135 , to the liquid crystal of the liquid crystal panel  100 . 
     Typically, a time interval of a single frame driven at 60 Hz is 16.7 ms ( 1/60 s). When the single frame is divided into three subframes, as is the case for the FS-LCD, each subframe has a time interval of 5.56 ms ( 1/180 s). The time interval of one subframe is short enough to prevent its field change to be perceived by the human eye. Accordingly, the human eye sees the three subframes during the time interval of 16.7 ms as a single frame, resulting in the recognition of a composite color formed by the three primary colors to display the image. 
     Therefore, the field sequential driving mode may achieve about three times more resolution as the color filter mode for a same-sized panel, increase light efficiency because no color filter is used, and achieve the same color reproduction as a color television set and achieve a high speed of moving picture. However, because the field sequential driving mode divides one frame into three sub-frames, it requires fast operating characteristics. That is, the field sequential driving mode requires a driving frequency of about six times the driving frequency of the color filter driving mode. 
     In order for the liquid crystal display to obtain the fast operating characteristics, a response speed of the liquid crystal should be fast and a corresponding switching speed for turning the R, G, and B back-lights on and off should also be relatively fast. 
       FIG. 2  is a schematic diagram of a back-light driving circuit used in the FS-LCD of  FIG. 1 . 
     Referring to  FIG. 2 , a conventional back-light driving circuit includes a back-light  200  including R, G, and B back-lights  201 ,  203 , and  205 , for sequentially emitting R, G, and B light, and a driving voltage generator  210  for providing a driving voltage VLED of a same level to the R, G, and B back-lights  201 ,  203  and  205 . 
     The R back-light  201  includes two R light emitting diodes (RLED 1  and RLED 2 ) serially connected for emitting R light. The G back-light  203  includes one G light emitting diode (GLED 1 ) for emitting G light. The B back-light  205  includes two B light emitting diodes (BLED 1  and BLED 2 ) connected in parallel for emitting B light. 
     The driving voltage generator  210  provides the driving voltage (VLED) of the same level to all of the R, G, and B back-lights  201 ,  203  and  205  forming the back-light  200 . The driving voltage (VLED) is provided to an anode electrode of the R light emitting diode (RLED 1 ) in the R back-light  201 , to an anode electrode of the G light emitting diode (GLED 1 ) in the G back-light  203 , and to anode electrodes of the two B light emitting diodes (BLED 1 , BLED 2 ) in the B back-light  205 . 
     The conventional back-light driving circuit further includes a luminance adjuster  208  serially connected between the back-lights  201 ,  203 , and  205  and the ground for adjusting luminance of light emitted from the back-light  200 . The luminance adjuster  208  includes a first variable resistor (RVR) connected between a cathode electrode of the R light emitting diode (RLED 2 ) in the R back-light  201  and the ground for adjusting luminance of light emitted from the R back-light  201 , a second variable resistor (GVR) connected between a cathode electrode of the G light emitting diode (GLED 1 ) in the G back-light  203  and the ground for adjusting luminance of light emitted from the G back-light  203 , and a third variable resistor (BVR) connected between the cathode electrodes of the two B light emitting diodes (BLED 1 , BLED 2 ) in the B back-light  205  and the ground for adjusting luminance of light emitted from the B back-light  205 . 
     Conventionally, although forward driving voltages (RVf, GVf and BVf) of the light emitting diodes (RLED, GLED and BLED) in the R, G and B back-lights  201 ,  203  and  205  are different from one another, the same driving voltage, for example, 4V is provided to the R, G and B back-lights  201 ,  203  and  205  from the driving voltage generator  210 . For example, the R light emitting diode (RLED) requires a forward driving voltage (RVf) of 2.2 V. The G light emitting diode (GLED) requires a forward driving voltage (GVf) of 3.3 V. The B light emitting diode (BLED) requires a forward driving voltage (BVf) of 3.4 V. 
     Conventionally, since all the R, G and B back-lights  201 ,  203  and  205  are provided with the same driving voltage (VLED) of 4V, when trying to drive the R light emitting diodes (RLED 1  and RLED 2 ), the R light emitting diodes (RLED 1  and RLED 2 ) are applied with a forward driving voltage (RVf) of 2.2V through the first variable resistor (RVR) to adjust luminance of light emitted from the R back-light  201 . 
     When trying to drive the G light emitting diode (GLED 1 ), the G light emitting diode (GLED 1 ) is applied with a forward driving voltage (GVf) of 3.3V through the second variable resistor (GVR) to adjust luminance of light emitted from the G back-light  203 . Furthermore, when trying to drive the B light emitting diodes (BLED 1  and BLED 2 ), the B light emitting diodes (BLED 1  and BLED 2 ) are provided with a forward driving voltage (BVf) of 3.4V through the third variable resistor (BVR) to adjust luminance of light emitted from the B back-light  205 . 
     Therefore, the conventional back-light driving circuit as described above is provided with the same driving voltage of 4V, regardless of whether the R, G and B light emitting diodes are driven with driving voltages that differ from one another. Since the R, G, and B light emitting diodes are provided with the same driving voltage during the three sub-frames of a single frame used to drive the R, G and B light emitting diodes, power consumption is increased. Furthermore, the driving voltage generating circuit according to conventional mechanisms needs to generate a driving voltage that generally corresponds to the largest voltage of the driving voltages required for the R, G and B light emitting diodes. 
     Another problem is that the forward driving voltages provided to the R, G and B light emitting diodes in each sub-frame need to be manually adjusted using the variable resistors. When distribution of driving currents of the light emitting diodes is large, it is difficult to provide the forward driving voltages suitable for the respective R, G and B light emitting diodes by only manually adjusting them using the variable resistors. 
     SUMMARY OF THE INVENTION 
     The various embodiments of the present invention provide a back-light driving circuit providing driving voltages suitable for each light emitting diode regardless of the distribution of driving currents of the light emitting diodes. The catering of the driving voltages to the particular characteristic of the corresponding light emitting diode helps decrease power consumption and helps maximize efficiency of the driving circuit. 
     The various embodiments present invention also provide for a back-light driving circuit capable of optimizing color purity using PWM values that are catered to the particular light emitting diodes. 
     According to one embodiment of the invention, the back-light driving circuit includes a driving voltage generator providing a driving voltage to each of a plurality of back-lights for causing each of the plurality of back-lights to emit light having a predetermined luminance. At least two of the driving voltages have different driving voltage values. The back-light driving circuit also includes a pulse width modulation (PWM) signal generator providing a PWM signal to each of the plurality of back-lights for controlling chromaticity of light emitted from each of the plurality of back-lights. At least two of the PWM signals are associated with different PWM values. 
     According to one embodiment, at least one of the back-lights includes at least two light emitting diodes. 
     According to one embodiment, a single frame is divided into four sub-frames, and the plurality of back-lights respectively include red (R), green (G), and blue (B) light emitting diodes that are respectively driven in three of the four sub-frames. The R, G and B light emitting diodes are simultaneously driven in a fourth sub-frame or at least one of the R, G and B light emitting diodes is driven in the fourth sub-frame. 
     The R, G, and B light emitting diodes may be driven in an arbitrary order in the three sub-frames, and the fourth sub-frame may be arbitrarily selected from among the four sub-frames. 
     The R, G and B light emitting diodes may each be provided with a different driving voltage from the driving voltage generator, or at least one of the driving voltages provided to the R, G, and B light emitting diodes may be different from two of the driving voltages provided from the driving voltage generator. 
     The plurality of backlights may be driven during a single frame including four sub-frames, the plurality of back-lights respectively including R, G, and B light emitting diodes that are driven in three of the four sub-frames, and a white (W) light emitting diode that is driven in a remaining sub-frame of the four sub-frames. 
     According to one embodiment, the R, G, B, and W light emitting diodes may be driven in an arbitrary order within the four sub-frames. 
     According to one embodiment, the plurality of back-lights include at least one of each of R, G, and B back-lights. 
     The plurality of back-lights may include at least two back-lights associated with the same color. The two back-lights associated with the same color may receive different driving voltages from the driving voltage generator. The two back-lights associated with the same color may also receive different PWM signals from the PWM signal generator. 
     According to one embodiment, the plurality of back-lights include red, green, and blue back-lights, and the driving voltage generator includes a register with prestored driving voltages corresponding to the red, green, and blue back-lights. The PWM signal generator may further include a register with prestored PWM values corresponding to the red, green, and blue back-lights. 
     The back-light driving circuit may further include a controller providing signals to the PWM signal generator for controlling selection of a light emitting diode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and exemplary embodiments of the present invention will be described with reference to the attached drawings in which: 
         FIG. 1  is a perspective view of a configuration of a conventional field sequential liquid crystal display; 
         FIG. 2  is a schematic block diagram illustrating a configuration of a back-light driving circuit used in a conventional field sequential liquid crystal display; 
         FIG. 3  is a schematic diagram illustrating a configuration of a back-light driving circuit used in a field sequential liquid crystal display in accordance with an embodiment of the present invention; 
         FIG. 4  is another schematic block diagram illustrating a configuration of a back-light driving circuit used in a field sequential liquid crystal display in accordance with an embodiment of the present invention; 
         FIG. 5  is a signaling diagram of the back-light driving circuit of  FIG. 4 ; and 
         FIG. 6A  is a schematic diagram illustrating a configuration of a back-light driving circuit used in a field sequential liquid crystal display in accordance with an embodiment of the present invention; and 
         FIG. 6B  is a schematic frame division diagram in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  is a schematic diagram illustrating a configuration of a back-light driving circuit used in a field sequential liquid crystal display in accordance with an embodiment of the present invention. 
     The back-light driving circuit according to the embodiment in  FIG. 3  sequentially provides forward driving voltages suitable for respective R, G and B light emitting diodes (RLED, GLED and BLED) to R, G and B back-lights  301 ,  303  and  305 , and drives the respective R, G and B light emitting diodes (RLED, GLED and BLED) by the forward driving voltages so as to achieve a luminance adjusted color. The back-light driving circuit also optimizes chromaticity by controlling different PWM values (RPWM, GPWM and BPWM) suitable for the R, G and B light emitting diodes (RLED, GLED and BLED). According to one embodiment, Pulse Width Modulation (PWM) values for the respective R, G and B light emitting diodes (RLED, GLED and BLED) are different from one another. 
     For example, in the case where one frame includes three sub-frames for sequentially driving the R, G and B light emitting diodes (RLED, GLED and BLED) in each sub-frame, a forward driving voltage (RVf) suitable for the R light emitting diode (RLED) is provided in a first sub-frame to drive the R light emitting diode (RLED). Subsequently, a forward driving voltage (GVf) suitable for the G light emitting diode (GLED) is provided in a second sub-frame to drive G light emitting diode (GLED), and a forward driving voltage (BVf) suitable for the B light emitting diode (BLED) is provided in a third sub-frame to drive the B light emitting diode (BLED). 
     When driving the R light emitting diode (RLED) by generating the driving voltage (RVf) suitable for the R light emitting diode (RLED) in the first sub-frame, a PWM value (RPWM) suitable for the R light emitting diode (RLED) is also provided to adjust chromaticity of the R color. When driving the G light emitting diode (GLED) by generating the driving voltage (GVf) suitable for the G light emitting diode (GLED) in the second sub-frame, a PWM value (GPWM) suitable for the G light emitting diode (GLED) is also provided to adjust chromaticity of G color. When driving the B light emitting diode (BLED) by generating the driving voltage (BVf) suitable for the B light-emitting diode (BLED) in the third sub-frame, a PWM value (BPWM) suitable for the B light emitting diode (BLED) is provided to adjust chromaticity of B color in the third sub-frame. 
     Accordingly, the R, G and B colors having desired luminance is achieved by generating the forward driving voltages suitable for the respective R, G and B light emitting diodes (RLED, GLED and BLED), and chromaticity also adjusted based on the PWM values of the R, G and B light emitting diodes (RLED, GLED and BLED). Therefore, a color having optimized chromaticity at predetermined luminance is provided. 
       FIG. 4  is another schematic block diagram of a configuration of a back-light driving circuit in accordance with an embodiment of the present invention. 
     Referring to  FIG. 4 , the back-light driving circuit includes a back-light  300  for generating R, G and B lights, driving voltage generator  310  for providing driving voltages (VLED 1  and VLED 2 ) to the back-light  300 , LED controller  320  for controlling the drive of the back-light  300  according to first and second control signals (LED_CTRL 0  and LED_CTRL 1 ), and PWM signal generator  330  for generating the PWM signal to the back-light  300  according to output signals provided from the LED controller  320 . 
     The back-light  300  includes R back-lights  301  and  302  for emitting light of R color, G back-lights  303  and  304  for emitting light of G color, and B back-lights  305  and  306  for emitting light of B color. 
     In the illustrated embodiment, each of the R back-lights  301  and  302  includes two serially connected R light emitting diodes (RLED 1  and RLDE 2 ) and (RLED 3  and RLED 4 ), respectively, wherein anode electrodes of the light emitting diodes (RLED 1 ) and (RLED 3 ) are provided with the forward driving voltages (RVf 1  and RVf 2 ), respectively, for driving the R light emitting diodes from output terminals (VLED 1 ) and (VLED 2 ) of the driving voltage generator  310 . 
     Each of the G back-lights  303  and  304  includes one G light emitting diode (GLED 1 ) and (GLDE 2 ) respectively, wherein anode electrodes of the light emitting diodes (GLED 1 ) and (GLED 2 ) are provided with the forward driving voltages (GVf 1  and GVf 2 ), respectively, for driving the G light emitting diode from the output terminals (VLED 1 ) and (VLED 2 ) of the driving voltage generator  310 . 
     Each of the B back-lights  305  and  306  includes two B light emitting diodes (BLED 1  and BLDE 2 ) and (BLED 3  and BLED 4 ) where the B light emitting diodes in each B back-light are connected in parallel. Anode electrodes of the light emitting diodes (BLED 1 , BLED 2 ) and (BLED 3 , BLED 4 ) are respectively provided with the forward driving voltages (BVf 1  and BVf 2 ), for driving the B light emitting diodes from the output terminals (VLED 1 ) and (VLED 2 ) of the driving voltage generator  310 . 
     In an embodiment of the present invention, the back-light  300  includes only R, G and B light emitting diodes, but a backlight  300 ′ in another embodiment (see, for example,  FIG. 6A ) includes the R, G and B light emitting diodes in R, G, B back-lights  301 ′,  303 ′,  305 ′ and a W light emitting diode  309  for emitting a W (white) color in a W back-light  307  coupled between WV f  and WPWM signals. Also, in the illustrated embodiment, each of the R, G and B back-lights include two back-lights. However, each back light but may include one or a plurality number of light emitting diodes. 
     The driving voltage generator  310  sequentially generates the respective forward driving voltages (RVf 1  and RVf 2 ), (GVf 1  and GVf 2 ) and (BVf 1  and BVf 2 ) suitable for the R, G and B back-lights  301  and  302 ,  303  and  304 , and  305  and  306  constituting the back-light  300 . According to one embodiment, the driving voltage generator  310  includes a register  1000  for storing the forward driving voltages (RVf), (GVf) and (BVf) of the R, G and B back-lights. 
     Accordingly, the driving voltage generator  310  provides the respective driving voltages (RVf 1  and RVf 2 ) suitable for the R light emitting diodes to the anode electrodes of the R light emitting diodes (RLED 1  and RLED 3 ) in the R sub-frame to drive the R light emitting diodes, the respective driving voltages (GVf 1  and GVf 2 ) suitable for the G light emitting diodes to the anode electrodes of the G light emitting diodes (GLED 1  and GLED 2 ) in the G sub-frame to drive the G light emitting diodes, and the respective driving voltages (BVf 1  and BVf 2 ) suitable for the B light emitting diodes to the anode electrodes of the B light emitting diodes (BLED 1 , BLED 2 ) and (BLED 3 , BLED 4 ) in the B sub-frame to drive the B light emitting diodes. 
     According to one embodiment, the driving voltage generator  310  provides the same driving voltages (RVf 1  and RVf 2 ) to the R back-lights  301  and  302 , respectively, the same driving voltages (GVf 1  and GVf 2 ) to the G back-lights  303  and  304 , respectively, and the same driving voltages (BVf 1  and BVf 2 ) to the B back-lights  305  and  306 , respectively. 
     According to another embodiment, when the distribution of the driving currents of the respective light emitting diodes is not uniform, the driving voltage generator provides different driving voltages (RVf 1  and RVf 2 ) suitable for the R back-lights  301  and  302 , different driving voltages (GVf 1  and GVf 2 ) suitable for the G back-lights  303  and  304 , and the different driving voltages (BVf 1  and BVf 2 ) suitable for the B back-lights  305  and  306 . 
     Furthermore, the driving voltages (RVf), (GVf) and (BVf) provided to the R, G and B back-lights may be different from one another. For example, all the driving voltages (RVf), (GVf) and (BVf) provided to the R, G and B back-lights may be different from one another, or different driving voltages may be provided to only one or two of the R, G and B back-lights. 
     The LED controller  320  outputs signals for driving the corresponding back-light of the R, G and B back-lights in the corresponding frame of a plurality of sub-frames constituting one frame according to first and second control signals (LED_CTRL 0 ) and (LED_CTRL 1 ). 
     The PWM signal generator  330  generates the corresponding PWM signals (RPWM 1  and RPWM 2 ), (GPWM 1  and GPWM 2 ) and (BPWM 1  and BPWM 2 ) to the R, G and B back-lights  301  and  302 ,  303  and  304 , and  305  and  306  according to the output signals of the LED controller  320 . According to one embodiment, the LED controller includes a register  1002  for storing the PWM signals of the respective R, G and B back-lights. 
     In the illustrated embodiment, the PWM signal generator  330  provides the respective PWM signals (RPWM 1  and RPWM 2 ) to cathode electrodes of the light emitting diodes (RLED 2  and RLED 4 ) of the R back-lights  301  and  302  in the R sub-frame of a plurality of sub-frames constituting one frame to drive the R back-lights  301  and  302 , respectively. The PWM signal generator  330  provides the respective PWM signals (GPWM 1  and GPWM 2 ) to cathode electrodes of the light emitting diodes (GLED 1  and GLED 2 ) of the G back-lights  303  and  304  in the G sub-frame to drive the G back-lights  303  and  304 , respectively. The PWM signal generator  330  also provides the respective PWM signals (BPWM 1  and BPWM 2 ) to cathode electrodes of the light emitting diodes (BLED 1 , BLED 2 ) and (BLED 3 , BLED 4 ) of the B back-lights  305  and  306  in the B sub-frame to drive the B back-lights  305  and  306 , respectively. 
     According to one exemplary embodiment of the present invention, since each of the R, G and B back-lights includes two back-lights  301  and  302 ,  303  and  304 , and  305  and  306  respectively, the PWM signal generator  330  provides the respective first PWM signals (RPWM 1 ), (GPWM 1 ) and (BPWM 1 ) to R, G and B light emitting diodes (RLED 2 ), (GLED 1 ) and (BLED 1 , BLED 2 ) in the first R, G, and B back-lights  301 ,  303 , and  305 , and the respective second PWM signals (RPWM 2 ), (GPWM 2 ) and (BPWM 2 ) to R, G and B light emitting diodes (RLED 4 ), (GLED 2 ) and (BLED 3 , BLED 4 ) in the second R, G, and B back-lights  302 ,  304 , and  306 . 
     According to one embodiment, the PWM signal generator  330  may provide the same PWM signals (RPWM 1  and RPWM 2 ) to the R light emitting diodes (RLED 2  and RLED 4 ), the same PWM signals (GPWM 1  and GPWM 2 ) to the G light emitting diodes (GLED 1  and GLED 2 ), and the same PWM signals (BPWM 1  and BPWM 2 ) to the B light emitting diodes (BLED 1 , BLED 2 ) and (BLED 3 , BLED 4 ), respectively. 
     According to another embodiment, when distribution of the driving currents of the respective light emitting diodes is not uniform, the PWM signal generator  330  may provide the different PWM signals (RPWM 1  and RPWM 2 ) suitable for the R light emitting diodes (RLED 2  and RLED 4 ), the different PWM signals (GPWM 1  and GPWM 2 ) suitable for the G light emitting diodes (GLED 1  and GLED 2 ), and the different PWM signals (BPWM 1  and BPWM 2 ) suitable for the B light emitting diodes (BLED 1 , BLED 2 ) and (BLED 3 , BLED 4 ), respectively. 
     When providing the different driving voltages to the R, G and B light emitting diodes respectively, all the driving voltages provided to the R, G and B light emitting diodes may be different from one another, or different driving voltages may be provided to only one or two of the R, G and B light emitting diodes. 
       FIG. 5  is a signaling diagram of the back-light driving circuit of  FIG. 4  according to one embodiment of the invention. 
     In the embodiment of the present invention, it is assumed that one frame includes three sub-frames, that is an R sub-frame (RSF) for driving an R back-light, a G sub-frame (GSF) for driving a G back-light and a B sub-frame (BSF) for driving a B back-light, and the R, G and B back-lights are sequentially driven in the order of the R, G and B back-lights for one frame. 
     The driving voltage generator  310  provides driving voltages, for example, forward driving voltages (RVf 1  and RVf 2 ) of 4.4V to the R light emitting diodes (RLED 1  and RLED 3 ) in the R sub-frame. At this time, the LED controller  320  is provided with first and second control signals (LED_CTRL 0 ) and (LED_CTRL 1 ) of a high state or low state for driving the R back-lights  301  and  302 , respectively, as shown in  FIG. 5 . In response, the LED controller  320  provides its output signals to the PWM signal generator  330  for driving the R back-lights  301  and  302  of the R, G and B back-lights  300 . 
     In the embodiment illustrated in  FIGS. 4 and 5 , the R back-lights  301  and  302  include two R light emitting diodes serially connected to receive a voltage of 4.4V from the driving voltage generator  310 . However, a person of skill in the art should recognize that the R back-lights may receive a driving voltage of 2.2V by connecting the two R light emitting diodes in parallel. 
     The PWM signal generator  330  generates the PWM signals (RPWM 1  and RPWM 2 ) used to drive the R back-lights  301  and  302  by the output signals provided from the LED controller  320  through its output terminals (R 1 _OUT and R 2 _OUT). Therefore, the R back-lights  301  and  302  enable the driving currents corresponding to the corresponding forward driving voltages (RVf 1  and RVf 2 ) applied to anodes of the light emitting diodes (RLED 1  and RLED 3 ) and the corresponding PWM signals (RPWM 1  and RPWM 2 ) applied to cathodes of the light emitting diodes (RLED 2  and RLED 4 ) to flow as shown in  FIG. 5 , and thus, emit light of R color having predetermined luminance and chromaticity. 
     Subsequently, the driving voltage generator  310  provides driving voltages, for example, forward driving voltages (GVf 1  and GVf 2 ) of 3.4V to the G back-lights  303  and  304  in the G sub-frame. At this time, the LED controller ( 320 ) is applied with first and second signals (LED_CTRL 0 ) and (LED_CTRL 1 ) of a high state or low state, for driving the G back-lights  303  and  304 , respectively, as shown in  FIG. 5 . In response, the LED controller  320  provides its output signals to the PWM signal generator  330  for driving the G back-lights of the R, G and B back-lights. 
     The PWM signal generator  330  generates the PWM signals (GPWM 1  and GPWM 2 ) used to drive the G back-light  303  and  304  by the output signals provided from the LED controller  320 , through its output terminals (G 1 _OUT and G 2 _OUT). Therefore, the G back-lights  303  and  304  enable the driving currents corresponding to the corresponding forward driving voltages (GVf 1  and GVf 2 ) applied to anodes of the light emitting diodes (GLED 1  and GLED 2 ) and the corresponding PWM signals (GPWM 1  and GPWM 2 ) applied to cathodes of the light emitting diodes (GLED 1  and GLED 2 ) to flow as shown in  FIG. 5 , and thus, emit light of G color having predetermined luminance and chromaticity. 
     The driving voltage generator  310  provides driving voltages, for example, forward driving voltages (BVf 1  and BVf 2 ) of 3.3V to the B back-lights  305  and  306  in the B sub-frame. At this time, the LED controller  320  is provided with first and second signals (LED_CTRL 0 ) and (LED_CTRL 1 ) of a high state or low state, for driving the B back-lights  305  and  306 , respectively, as shown in  FIG. 5 . In response, the LED controller  320  provides its output signals to the PWM signal generator  330  for driving the B back-lights  305  and  306  of the R, G and B back-lights. 
     The PWM signal generator  330  generates the PWM signals (BPWM 1  and BPWM 2 ) used to drive the B back-light  305  and  306  by the output signals provided from the LED controller  320 , through its output terminals (B 1 _OUT and B 2 _OUT). Therefore, the B back-lights  305  and  306  enable the driving currents corresponding to the corresponding forward driving voltages (BVf 1  and BVf 2 ) applied to anodes of the light emitting diodes (BLED 1 , BLED 2 ) and (BLED 3 , BLED 4 ) and the corresponding PWM signals (BPWM 1  and BPWM 2 ) applied to cathodes of the light emitting diodes (BLED 1 , BLED 2 ) and (BLED 3 , BLED 4 ) to is flow as shown in  FIG. 5 , and thus, emit light of B color having predetermined luminance and chromaticity. 
     Therefore, since the forward driving voltages (RVf 1  and RVf 2 ), (GVf 1  and GVf 2 ) and (BVf 1  and BVf 2 ) of the R, G and B back-lights generated from the driving voltage generator  310  and the driving currents corresponding to the PWM signals (RPWM 1  and RPWM 2 ), (GPWM 1  and GPWM 2 ) and (BPWM 1  and BPWM 2 ) of the R, G and B back-lights generated from the PWM signal generator  330  flow during one frame, light having predetermine luminance and chromaticity is emitted. 
     Although the embodiment illustrated in  FIG. 5  divides one frame into three sub-frames and sequentially drives the R, G and B light emitting diodes in each sub-frame, it will be apparent to a person of skill in the art that is also possible to divide one frame into at least four sub-frames (see, for example,  FIG. 6B ), sequentially drive the R, G and B light emitting diodes in the three sub-frames, and drive all the R, G and B light emitting diodes or at least one of the R, G and B light emitting diodes in the remaining one frame. Furthermore, it is also possible that the back-lights of the invention include R, G, B and W light emitting diodes for driving the R, G and B light emitting diodes in the three sub-frames of the four sub-frames and the W light emitting diode in the remaining one sub-frame. 
     Furthermore, although according to one embodiment of the invention the R, G and B light emitting diodes (RLED, GLED and BLED) are controlled to emit in the order of R, G and B in each sub-frame of a single frame, it is possible to arbitrarily change the order for emitting the light emitting diodes in order to obtain optimal luminance and chromaticity. Also, although the embodiment illustrated in  FIG. 5  divides one sub-frame into two intervals (RF 1  and RF 2 ), (GF 1  and GF 2 ) and (BF 1  and BF 2 ) where the first intervals RF 1 , GF 1 , and BF 1  are control intervals for selecting forward driving voltages suitable for the R, G, and B light emitting diodes, and second intervals RF 2 , GF 2 , and BF 2  are used to generate the selected forward driving voltages to drive each of the light emitting diodes, as shown in  FIG. 5 , the present invention is not limited to this embodiment, and other embodiments may be possible where more than two intervals are used, or where the intervals are used differently or in a different order from what is described with respect to  FIG. 5 . 
     The back-light driving circuit according to the above-described exemplary embodiments stores the forward driving voltages suitable for respective R, G and B light emitting diodes in the register  1000  and stores the PWM values suitable for the R, G and B light emitting diodes in the other register  1002 , and generates the forward driving voltages and the PWM signals corresponding to the R, G and B light emitting diodes in each of the sub-frames, thereby emitting light having optimal luminance and chromaticity as well as increasing its efficiency. 
     Although the present invention has been described with reference to certain exemplary embodiments, it will be understood by those skilled in the art that a variety of modifications and variations may be made to the present invention without departing from the spirit or scope of the present invention. Of course, the scope of the invention is to be determined by the appended claims and their equivalents.