Abstract:
A liquid crystal display device including a light source controller for controlling red, green, and blue lights to be sequentially transmitted through a liquid crystal and a pixel formed between a first substrate on which a first electrode is formed and a second substrate on which a second electrode is formed. A first red light emitting diode (LED) has a first terminal coupled to a first terminal of the light source controller; and a second red LED has a first terminal coupled to a second terminal of the first red LED. A green LED has a first terminal coupled to the first terminal of the light source controller; and a blue LED has a first terminal coupled to the first terminal of the light source controller.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of Korean Patent Application No. 10-2004-0040295 filed on Jun. 3, 2004 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a liquid crystal display device, and more particularly, to a field sequential driving method and a liquid crystal display device using the same.  
         [0004]     2. Description of the Related Art  
         [0005]     Recently, personal computers and televisions have become lightweight and flat, and accordingly display devices are being required to be more light weighted and thinner. Thus, for use instead of a cathode ray tube (CRT), flat panel displays including a liquid crystal display (LCD) have been developed.  
         [0006]     An LCD device utilizes two substrates and a liquid crystal material having an anisotropic dielectric constant injected therebetween, in which an electric field is applied to the liquid crystal material. The amount of light from an external light source transmitted through the substrates is controlled by intensity of the electric field to obtain a desired image signal.  
         [0007]     Such an LCD is the most common type of the flat panel displays, and especially, a thin film transistor (TFT)-LCD using a TFT as a switching element is most commonly used.  
         [0008]     Each pixel in the TFT-LCD can be modeled using a capacitor having a liquid crystal as a dielectric material, that is a liquid crystal capacitor. An equivalent circuit diagram of such a pixel is shown in  FIG. 1 .  
         [0009]     As shown in  FIG. 1 , each pixel in an LCD device includes a TFT  10  having a source electrode and a gate electrode respectively coupled to a data line Dm and a scan line Sn, a liquid capacitor Cl coupled between a drain electrode of the TFT  10  and a common voltage source Vcom, and a storage capacitor Cst coupled to the drain electrode of the TFT  10 .  
         [0010]     As can be seen in  FIG. 1 , the TFT  10  is turned on when a scan signal is applied to the scan line, and a data voltage Vd supplied to the data line Dm is applied to each pixel (not shown) through the TFT  10 . Then, an electric field corresponding to a difference between a pixel voltage Vp and the common voltage Vcom is applied to a liquid crystal (equivalently shown as a liquid crystal capacitor Cl in  FIG. 1 ), and light transmittance is determined by intensity of the electric field. Here, the pixel voltage Vp is maintained for one frame scan or one field, and the storage capacitor Cst is auxiliarily used to maintain the pixel voltage Vp applied to the pixel electrode.  
         [0011]     In general, methods of displaying a color image on an LCD device can be classified into a color filter method and a field sequential driving method.  
         [0012]     An LCD device employing the color filter method forms a color filter layer having 3 primary colors (red, green, and blue) on one of substrates, and controls the amount light transmitted to the color filter to express a desired color. An LCD employing the color filter method adjusts the amount of light from a single light source transmitted through red, green, and blue color filters, and combines the red, green, and blue lights to display a desired color.  
         [0013]     Such an LCD device displaying colors using a single-light source and three color filter layer requires three times or more pixels, compared to displaying monochrome, to respectively correspond to red, green, and blue color areas. Accordingly, a sophisticated manufacturing technology is required to obtain a high resolution image.  
         [0014]     Moreover, adding a separate color filter layer on the substrate of the LCD causes the manufacturing of the LCD to be complicated, and light transmittance of the color filter must be considered as well.  
         [0015]     On the other hand, an LCD employing the field sequential driving method periodically and sequentially turns on/off independent red, green, and blue signals, and synchronously applies a corresponding color signal to the pixel in accordance with the turn on/off period to thereby obtain a full-colored image. In other words, the field sequential driving method uses persistence of vision to display a colored image by way of outputting the red, green, and blue (RGB) lights from RGB light sources (i.e., backlights) and time-dividing the red, green, and blue lights, and sequentially displaying the time-divided red, green, and blue lights on a pixel instead of dividing the pixel into three pixels for red, green, and blue colors.  
         [0016]     The field sequential driving method can be classified into an analog driving method or a digital driving method.  
         [0017]     The analog driving method predetermines a plurality of gradation voltages corresponding to a total number of gradations to be displayed, and selects a gradation voltage corresponding to gradation data from the plurality of gradation voltages to drive a liquid crystal panel to thereby express gradation using the amount of light transmitted corresponding to the gradation voltage applied to the liquid crystal panel.  
         [0018]      FIG. 2  illustrates a driving voltage and the amount of transmitted light of an LCD panel employing a conventional analog driving method. As shown therein, the driving voltage represents a voltage applied to the liquid crystal, and the optical transmittance represents a ratio of the amount of light transmitted through the liquid crystal to the amount of incident light. In other words, the optical transmittance represents the degree of distortion of the liquid crystal so that the light can pass therethrough.  
         [0019]     Referring to  FIG. 2 , a driving voltage at V 11  level is applied to the liquid crystal in an R-field period Tr for displaying a red color and the amount of light transmitted through the liquid crystal corresponds to the driving voltage. In a G-field period Tg for displaying a green color, a driving voltage at V 12 -level is applied and a corresponding amount of light is transmitted through the liquid crystal. Further, in a B-field period Tb for displaying a blue color, a driving voltage at V 13  level is applied and a corresponding amount of light is transmitted through the liquid crystal. By adding the red, green, and blue lights respectively transmitted through the Tr, Tg, and Tb, a desired colored image can be displayed.  
         [0020]     On the other hand, the digital driving method regulates driving voltages applied to the liquid crystal and controls a voltage application time to thereby express gradations (i.e., grayscales). According to the digital driving method, the gradations are expressed by maintaining the regulated driving voltage and adjusting a timing or duration of the voltage application to control an accumulated amount of light transmitted through the liquid crystal.  
         [0021]      FIG. 3  illustrates waveforms that explain a driving method of an LCD device employing a conventional digital driving method. Waveforms of a driving voltage in accordance with a predetermined number of bits of driving data and corresponding optical transmittance of a liquid crystal are illustrated.  
         [0022]     As shown in  FIG. 3 , a 7-bit digital signal is provided as gradation waveform data for each gradation, and a corresponding gradation waveform is applied to the liquid crystal. The optical transmittance of the liquid crystal is determined according to the applied gradation waveform, thereby expressing the gradations.  
         [0023]     The LCD device employing the conventional field sequential method uses a light emitting diode (LED) as the backlight of R, G, and B, and sequentially drives a red LED, a green LED, and a blue LED. In other words, the field sequential method has an R-field period for red color, a G-field period for green color, and a B-field period for blue color, and the red, green, and blue LEDs are sequentially turned on to emit red, green, and blue lights. Each of red, green, and blue data is applied to the liquid crystal and accumulated in the respective field periods, and a colored image can be displayed through the accumulated red, green, and blue lights.  
         [0024]      FIG. 4  shows a relationship between each of conventional LEDs that respectively emit red, green, and blue lights, and a light source controller driving the conventional LEDs.  
         [0025]     As shown in  FIG. 4 , the conventional LEDs include a red LED (RLED), a green LED (GLED), and a blue LED (BLED), and these LEDs are coupled to the light source controller. When gradation data is applied to a pixel, the light source controller immediately turns on the RLED, GLED, and BLED in sequence, and applies a forward voltage Vf to the respective LEDs to thereby emit light providing sufficient luminance. In  FIG. 3 , anodes of the RLED, GLED, and BLED are coupled to a common terminal VLED supplying the forward voltage, and cathodes of the RLED, GLED, and BLED are respectively coupled to selection terminals R_OUT, G_OUT, and B_OUT. Here, each of the selection terminals R_OUT, G_OUT, and B_OUT is sequentially turned on, and at the same time, the forward voltage is sequentially applied to the RLED, GLED, and BLED to thereby turn them on.  
         [0026]     Here, each of the LEDs, namely, RLED, GLED, and BLED, requires different voltage level to be turned on, and different forward voltages Vf result in different forward currents If. Further, the amount of luminance of the red LED RLED, green LED GLED, and blue LED BLED are respectively different according to the forward current If. Here, the forward voltage Vf represents a voltage applied to the LEDs after the LEDs are turned on, and the forward current If represents a current flowing to the LEDs when the forward voltage Vf is applied thereto.  
         [0027]      FIGS. 5A and 5B  illustrate a relationship between a forward voltage Vf and a forward current If in typical red, green, and blue LEDs, and relative luminance corresponding thereof.  FIG. 5A  shows a relationship between the forward voltage and a corresponding forward current, and  FIG. 5B  shows the forward current and corresponding relative luminance or luminance.  
         [0028]     As shown in  FIG. 5B , relative luminance of the red, green, and blue LEDs are substantially the same when the forward current If applied thereto is set to be 20 mA. For white balancing, the green LED and the blue LED respectively require 3.4V and 3.25V of forward voltages but the red LED requires only 2.1V of forward voltage which is relatively lower than the forward voltages of the green and blue LEDs. The forward voltage is supplied from a terminal VLED of the light source controller, and the light source controller respectively applies associated forward voltages to the red, green, and blue LEDs in sequence. Here, the forward voltage 3.4V of the green LED and the forward voltage 3.25V of the blue LED have a similar voltage value, but the forward voltage 2.1V of the red LED is comparatively lower than the forward voltages of the green and blue LEDs, thereby generating a voltage ripple in the light source controller. In other words, variation of forward voltages produces the voltage ripple, thereby resulting in many problems in controlling the amount of light emitted from the respective LEDs.  
       SUMMARY OF THE INVENTION  
       [0029]     Accordingly, in exemplary embodiments of the present invention, an LCD device that supplies almost the same or similar forward voltages applied to each of LEDs, is provided to thereby solve the forgoing problems.  
         [0030]     In addition, the LCD device according to the exemplary embodiments of the present invention may be designed to consume less power.  
         [0031]     To achieve the foregoing and/or other aspects of the present invention, in an exemplary embodiment of the present invention, an LCD device including a pixel formed by a liquid crystal disposed between a first substrate on which a first electrode is formed and a second substrate on which a second electrode is formed, and a light source controller having a first terminal, is provided. The light source controller controls red, green, and blue lights to be sequentially transmitted through the pixel. The LCD device also includes first and second red LEDs, a first green LED, and a blue LED. The first red LED has a first terminal and a second terminal, the first terminal being coupled to the first terminal of the light source controller. The second red LED has a first terminal coupled to the second terminal of the first red LED. The first green LED has a first terminal coupled to the first terminal of the light source controller. The blue LED has a first terminal coupled to the first terminal of the light source controller.  
         [0032]     The LCD device may further include a second green LED having a first terminal coupled to the first terminal of the light source controller, and coupled to the first green LED in parallel.  
         [0033]     A first voltage applied by the light source controller to emit the first and second red LEDs, a second voltage applied by the light source controller to emit the first and second green LEDs, and a third voltage applied by the light source controller to emit the blue LED may have almost the same or similar voltage levels.  
         [0034]     A combined luminance of the first and second red LEDs, a combined luminance of the first and second green LEDs, and luminance of the blue LED may substantially correspond to each other.  
         [0035]     In another exemplary embodiment according to the present invention, an LCD device including a pixel formed by a liquid crystal disposed between a first substrate on which a first electrode is formed and a second substrate on which a second electrode is formed, and a light source controller having first, second, third and fourth terminals, is provided. The light source controller controls red, green, and blue lights to be sequentially transmitted through the pixel. The LCD device also includes a pair of red LEDs, a green LED and a blue LED. The pair of red LEDs are coupled in series between the first terminal and the second terminal of the light source controller. The green LED is coupled between the first terminal and the third terminal of the light source controller. The blue LED is coupled between the first terminal and the fourth terminal of the light source controller.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]     The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention.  
         [0037]      FIG. 1  shows a pixel in a conventional TFT-LCD.  
         [0038]      FIG. 2  is a waveform illustrating a driving method of an LCD device employing a conventional analog method.  
         [0039]      FIG. 3  is a waveform illustrating a driving method of an LCD device employing a conventional digital method.  
         [0040]      FIG. 4  shows a relationship between LEDs respectively emitting red, green, and blue lights and a light source controller for driving the LEDs.  
         [0041]      FIGS. 5A and 5B  show a relationship between a forward voltage and a forward current, and a corresponding relative luminance in typical red, green, and blue LEDs.  
         [0042]      FIG. 6  illustrates an LCD device according to exemplary embodiments of the present invention.  
         [0043]      FIG. 7  shows a configuration of LEDs coupled to a light source controller according to a first exemplary embodiment of the present invention.  
         [0044]      FIG. 8  shows a configuration of LEDs coupled to a light source controller according to a second exemplary embodiment of the present invention.  
         [0045]      FIG. 9  illustrates a conceptual diagram of a pixel of a TFT-LCD.  
     
    
     DETAILED DESCRIPTION  
       [0046]     In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive. There may be parts shown in the drawings, or parts not shown in the drawings, that are not discussed in the specification as they are not essential to a complete understanding of the invention. Like reference numerals designate like elements.  
         [0047]     Hereinafter, an LCD device according to exemplary embodiments of the present invention will be described with reference to  FIG. 6  to  FIG. 9 . The LCD device according to the exemplary embodiments of the present invention has a sufficient number of LEDs for emitting red, green, and blue lights, that have a suitable relationship with each other, and with other components of the LCD device.  
         [0048]     As shown in  FIG. 6 , the LCD device according to the exemplary embodiments of the present invention includes an LCD panel  100 , a scan driver  200 , a data driver  300 , a gradation voltage generator  500 , a timing controller  400 , LEDs RLED  600   a , GLED  600   b , and BLED  600   c  respectively emitting red, green, and blue lights, and a light source controller  700 .  
         [0049]     The LCD panel  100  has a plurality of scan lines for transmitting gate-on signals, and a plurality of data lines dielectrically crossing the plurality of scan lines and for transmitting a gradation data voltage and a reset voltage. A plurality of pixels  110  arranged in a matrix format are surrounded by the scan lines and the data lines. Each pixel includes a thin film transistor TFT (not shown) having a gate electrode and a source electrode respectively coupled to the scan line and the data line, a pixel capacitor (not shown) coupled to a drain electrode of the TFT, and a storage capacitor (not shown).  
         [0050]     The scan driver  200  sequentially applies scan signals to the scan lines and turns on the TFT having the gate electrode coupled to the scan line to which the scan signal is applied.  
         [0051]     The timing controller  400  receives the gradation data signal RGB DATA, a horizontal synchronization (Hsync) signal, and a vertical synchronization (Vsync) signal from an external device or a graphic controller (not shown), and provides necessary control signals Sg, Sd, and Sb to the scan driver  200 , the data driver  300 , and the light source controller  700 , and provides the gradation data signal RGB DATA to the gradation voltage generator  500 .  
         [0052]     The gradation voltage generator  500  generates a gradation voltage corresponding to the gradation data and supplies the gradation voltage to the data driver  300 . The data driver  300  applies the gradation voltage outputted from the gradation voltage generator  500  to a corresponding data line.  
         [0053]     The LEDs  600   a ,  600   b , and  600   c  respectively output red, green, and blue lights, and the light source controller  700  controls turn-on timing of the LEDs  600   a ,  600   b , and  600   c  using control signals Cr, Cg and Cb, respectively. The LEDs  600   a ,  600   b , and  600   c  according to the exemplary embodiments of the present invention are configured to substantially prevent voltage ripple generated due to a difference between forward voltages Vf respectively applied to the LEDs  600   a ,  600   b , and  600   c . Further, LEDs  600   a ,  600   b , and  600   c  according to one of the exemplary embodiments of the present invention are configured to consume less power.  
         [0054]     In the LCD device of  FIG. 6 , the timing for the data driver  300  for applying an associated gradation voltage to the data line and the timing for the light source controller  800  to turn on the LEDs RLED, GLED, and BLED can be synchronized by a control signal provided from the timing controller  500 .  
         [0055]      FIG. 7  illustrates a configuration of the LEDs coupled to the light source controller  700  according to a first exemplary embodiment of the present invention. In the configuration of the LEDs according to the first exemplary embodiment of the present invention, two red LEDs, namely, RLED 1  and RLED 2 , are coupled in series to reduce a difference between forward voltages applied to the respective LEDs, thereby substantially preventing the voltage ripple. The light source controller  700  of  FIGS. 7 and 8  is substantially the same as the light source controller  700  of  FIG. 6 .  
         [0056]     As shown in  FIG. 7 , the configuration of the LEDs according to the first exemplary embodiment of the present invention includes two red LEDs, RLED 1  and RLED 2 , coupled in series, a green LED GLED, a blue LED BLED, and the light source controller  700 . The red LEDs, the green LED, and the blue LED are coupled to the light source controller  700 . All of the LEDs in  FIG. 7  can be embedded in a single integral circuit (IC) chip or a plurality of IC chips. The single IC chip and/or one or more of the plurality of IC chips may have substantially the same configuration as shown in  FIG. 7 . Further, the RLED 1  and RLED 2  coupled in series can be used as the RLED  600   a  of  FIG. 6 , while the GLED and BLED can be respectively used as the GLED  600   b  and the BLED  600   c  of  FIG. 6 .  
         [0057]     Referring back to  FIGS. 5A and 5B , luminance using two red LEDs according to the first exemplary embodiment of the present invention and luminance using one red LED according to the conventional method should be substantially the same for white balancing purposes. In other words, the luminance of the two red LEDs RLED 1  and RLED 2  coupled in series can be made substantially the same as the luminance of one LED through reducing the luminance of the two red LEDs RLED 1  and RLED 2  by substantially one-half (50%).  
         [0058]     In consideration of the luminance problem, 10 mA of forward current can be applied to the red LEDs RLED 1  and RLED 2  coupled in series to reduce relative luminance from 100 to 50, as can be seen in  FIG. 5B . Accordingly, relative luminances of each of the red LEDs RLED 1  and RLED 2  become 50 in the case of respectively applying 10 mA of forward current to the two red LEDs RLED 1  and RLED 2  coupled in series. Thus, a total relative luminance of the two red LEDs RLED 1  and RLED  2  becomes 100. Here, 10 mA of forward current can be respectively applied to the two red LEDs RLED 1  and RLED 2  by respectively applying 1.9V of forward voltage to the red LEDs RLED 1  and RLED 2 , as can be seen in  FIG. 5A . Therefore, 3.8V (1.9V×2=3.8V) of forward voltage is required to respectively apply 10 mA of forward current to the two red LEDs RLED 1  and RLED 2  coupled in series to control the relative luminances of each of the red LEDs RLED 1  and RLED 2  to be 50.  
         [0059]     Accordingly, a forward voltage Vfr outputted from the terminal VLED of the light source controller  700  becomes 3.8V when the red LEDs RLED 1  and RLED 2  are coupled in series. Sequentially, forward voltages Vfg and Vfb respectively outputted for the green and blue LEDs GLED and BLED are 3.4V and 3.25V, respectively, and thus the forward voltages outputted from the light source controller  700  for the respective LEDs become almost the same or similar to each other, thereby substantially preventing the occurrence of the voltage ripple. This way, the relative luminances of the red, green, and blue LEDs become 100, and does not cause any white balance problem.  
         [0060]     Further, the forward voltages applied to the green LED GLED and the blue LED BLED are respectively 3.4V (Vfg) and 3.25 (Vfb) which are the same values as conventional, and thus the forward currents flowing therefrom are 20 mA, respectively.  
         [0061]     A total power consumed by each of the LEDs in the above configuration according to the first exemplary embodiment of the present invention is given as Equations 1-3. First, a total power consumption of the red LEDs RLED 1  and RLED 2  are given as Equation 1. 
 
 P=V×I= 3.8 V×10 mA=38 mW  [Equation 1]
        where a combined forward voltage Vfr of the red LEDs RLED 1  and RLED 2  is set to be 3.8V, and the forward current flowing therethrough is set to be 10 mA.        
 
         [0063]     A total power consumption of the green LED GLED is given as Equation 2. 
 
 P=V×I= 3.4V*20 mA=68 mW  [Equation 2]
        where the forward voltage Vfg applied to the green LED GLED is set to be 3.4V and the forward current flowing therethrough is set to be 20 mA.        
 
         [0065]     Further, a total power consumption of the blue LED BLED is given as Equation 3. 
 
 P=V×I= 3.5 V×20 mA=65 mW  [Equation 3]
        where the forward voltage Vfb applied to the blue LED BLED is set to be 3.25V and the forward current flowing therethrough is set to be 20 mA.        
 
         [0067]     As shown in Equations 1, 2, and 3, the green LED GLED consumes the most power. Hereinafter, a method for reducing the power consumption of the green LED GLED will be described.  
         [0068]      FIG. 8  shows a configuration of the LEDs according to a second exemplary embodiment of the present invention. The configuration of the LEDs according to the second exemplary embodiment of the present invention is designed to reduce power consumption of the green LED GLED.  
         [0069]     According to the configuration shown in  FIG. 8 , two green LEDs GLED 1  and GLED 2  are coupled in parallel. A red LED RLED and a blue LED BLED are designed as they were in the first exemplary embodiment of the present invention, and therefore detailed descriptions related thereto will be omitted. The red LEDs RLED 1  and RLED 2  coupled in series can be used as the RLED  600   a  of  FIG. 6 , while the green LEDs GLED 1  and GLED 2  coupled in parallel can be used as the GLED  600   b  of  FIG. 6 . Further, the blue LED  600   c  can be used as the BLED  600   c  of  FIG. 6 .  
         [0070]     The two green LEDs GLED 1  and GLED 2  coupled in parallel should have the same luminance as one green LED GLED of  FIG. 4 , for example, to realize white balance. Accordingly, each of the green LEDs GLED 1  and GLED 2  should have one-half the luminance (50%) of the luminance of one green LED GLED. As shown in  FIG. 5B , 8 mA of forward current If must be applied to reduce the relative luminance of the green LEDs GLED 1  and GLED 2  from 100 to 50. Thus, 3.15V of forward voltage Vf must be applied to the green LEDs GLED 1  and GLED 2  to apply 8 mA of forward current to the green LEDs GLED 1  and GLED 2 . In other words, by applying 3.15V of forward voltage to the green LEDs GLED 1  and GLED 2  to apply 8 mA of forward current thereto, each relative luminance of each green LED becomes 50 and a total relative luminance of the green LEDs GLED 1  and GLED 2  becomes 100.  
         [0071]     Power consumption of these two LEDs GLED 1  and GLED  2  are given as Equation 4. 
 
 P=V×I= 3.15 V×8 mA+3.15 V×8 mA=50.4 mW  [Equation 4]
        where the forward voltage of each of green LEDs GLED 1  and GLED 2  is 3.15V, and 8 mA of current flows therethrough, and therefore the power consumtion of the green LEDs GLED 1  and GLED 2  is given as Equation 4.        
 
         [0073]     In comparison between Equation 2 and Equation 4, the power consumption of using one green LED is calculated to be 68 mW, whereas the green LEDs coupled in parallel as described in the second exemplary embodiment of the present invention consumes the power of 50.4 mW which is less than using one green LED. In addition, a difference between the forward voltage respectively applied to the two green LEDs GLED 1  and GLED 2  (3.15V), the forward voltage applied to the red LED Vfr (3.8V), and the forward voltage applied to the blue LED Vfb (3.25V) is small enough to substantially prevent the voltage ripple, similar to the first exemplary embodiment of the present invention. In other words, the voltage levels of the forward voltages applied to the red, blue and green LEDs are almost the same or similar to each other.  
         [0074]     As described in the second exemplary embodiment of the present invention, the power consumption can be reduced by using two green LEDs which have a characteristic of consuming much power, and coupling these two green LEDs in parallel. Here, two blue LEDs can also be coupled in parallel. However, such use of the blue LEDs in parallel reduces the power consumption less than when using the green LEDs in parallel, and tends to increase manufacturing cost due to an additional element. Accordingly, the green LEDs consuming much power and/or the blue LEDs can be coupled in parallel to efficiently reduce the power consumption, but such uses may result in increased manufacturing cost.  
         [0075]      FIG. 9  illustrates a conceptual diagram of a pixel  800  of a TFT-LCD. The pixel includes a liquid crystal  850  disposed between a first substrate  810  and a second substrate  820 , a first electrode (common electrode)  830  arranged at the first substrate  810 , and a second electrode (pixel electrode)  840  arranged at the second substrate  820 . Exemplary embodiments of the present invention can be applied to the pixel of  FIG. 9 , as well as other suitable pixels. Further, the pixel  800  can represent any of the pixels  110  of  FIG. 6 . In addition, the first and second substrates  810 ,  820  and the liquid crystal  850  may be equivalently represented, for example, as the liquid crystal capacitor Cl in  FIG. 1 .  
         [0076]     Accordingly, by way of coupling red LEDs in series, a forward voltage applied to each LEDs can be made similar or almost the same as each other, thereby substantially preventing a voltage ripple. In addition, power consumption can be reduced by coupling green LEDs, which consume the most power, in parallel and/or by coupling the blue LEDs in parallel.  
         [0077]     While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.