Abstract:
The invention is an apparatus for driving a liquid crystal display. In the invention, the digital data signals for different pixels are converted to analog signals based on color-specific gray voltages. By providing color-specific gray voltages, the invention corrects any bias stemming from different electro-optical characteristics of different-colored pixels. In one embodiment, the invention includes a signal controller for generating digital data signals for different pixel colors and a gray voltage generator for generating gray voltage signals that are specific to the different pixel colors. A data driver, which is coupled to the gray voltage generator and the signal controller, converts each one of the digital signals to a corresponding analog signal by using one of the gray voltage signals that is associated with the same pixel color as the digital signal that is being converted.

Description:
RELATED APPLICATION  
       [0001]    This application claims priority, under 35 USC § 119, from Korean Patent Aplication No. 2003-0007521 filed on Feb. 6, 2003, the content of which is incorporated herein by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to liquid crystal display and an apparatus and method of driving a liquid crystal display.  
           [0004]    2. Description of the Related Art  
           [0005]    A liquid crystal display (LCD) includes a panel with pixel electrodes, a panel with a common electrode, and a liquid crystal (LC) layer interposed between the two panels. The LC layer contains liquid crystals with dielectric anisotropy. The pixel electrodes are arranged in a matrix of rows and columns and connected to switching elements, such as thin film transistors (TFTs), which supply the pixel electrodes with data voltages. Typically, the data voltages are supplied row by row. The common electrode covers the entire surface of one of the two panels and is supplied with a common voltage. The pixel electrode, the common electrode, and the LC layer form an LC capacitor, which is a basic element of a pixel along with the switching element connected thereto.  
           [0006]    Voltages are applied to the two electrodes to generate an electric field in the LC layer, and the transmittance of light passing through the LC layer is adjusted by controlling the strength of the electric field. Thus, desired images can be displayed by controlling the electric field. In order to prevent image deterioration due to long-time application of the unidirectional electric field, etc., polarity of data voltages with respect to the common voltage is reversed every frame, every row, or every dot.  
           [0007]    The LCD includes a data driver for supplying data voltages to the pixel electrodes, which includes a shift register, a data register, a data latch, a digital-to-analog (D/A) converter, and an output buffer. The data driver latches digital image data for red (R), green (G), and blue (B) colors that are sequentially received from a timing controller in synchronization with a dot clock, converts the digital data into analog voltages, and outputs the analog voltages to the pixels. The D/A conversion of the image data into the analog voltages is performed by a D/A converter in reference to gamma reference voltages received from an external device.  
           [0008]    A conventional LCD utilizes the same gamma reference voltage for the different colors under the assumption that the different-colored pixels have the same electro-optical characteristics. However, the red, green and blue pixels actually have different electro-optical characteristics. Thus, the color conception is non-uniform or biased.  
           [0009]    In an attempt to improve color conception, LCDs that use separate gamma reference voltages for the respective colors have been made. However, this use of separate gamma reference voltages usually increases the number of pins of the data driver, thereby increasing the size of the data driver. Also, additional blocks for generating gamma reference voltages for individual colors are required. The increased number of voltage generating circuits and the larger data driver size undesirably increases the manufacturing cost of the LCD.  
           [0010]    LCDs would benefit from a method of correcting the bias in the color conception without increasing the number of pins or components.  
         SUMMARY OF THE INVENTION  
         [0011]    In one aspect, the invention is an apparatus for driving a liquid crystal display. The apparatus includes a signal controller for generating digital signals for different pixel colors, and a gray voltage generator coupled to the signal controller, wherein the gray voltage generator generates gray voltage signals for each of the different pixel colors. A data driver that is coupled to the gray voltage generator and the signal controller converts each of the digital signals to a corresponding analog signal by identifying one of the gray voltage signals that is associated with the same pixel color as a digital signal that is being converted.  
           [0012]    In another aspect, the invention is a liquid crystal panel assembly. The liquid crystal panel assembly includes a plurality of pixel electrodes, wherein each of the pixel electrodes is associated with a pixel color, a common electrode positioned substantially parallel to the pixel electrodes, and a liquid crystal layer positioned between the pixel electrodes and the common electrode. The panel assembly further includes a data driver for supplying data signals to the pixel electrodes and a gray voltage generator that is coupled to the pixel electrodes. The gray voltage generator generates gray voltages that are each associated with a pixel color, so that the data driver determines a particular data signal for a particular pixel electrode by using one of the gray voltages that is associated with the pixel color of the particular pixel electrode.  
           [0013]    The invention also includes a method of driving a liquid crystal display. The method entails generating gray voltages for different pixel colors, receiving digital data for pixels having different pixel colors, and converting the digital data to a corresponding analog data by using one of the gray voltages that is associated with a same pixel color as the digital data being converted. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The present invention will become more apparent by describing embodiments thereof in detail with reference to the accompanying drawing in which:  
         [0015]    [0015]FIG. 1 is a block diagram of electronic components of an LCD according to an embodiment of the present invention;  
         [0016]    [0016]FIG. 2 is a schematic diagram of a pixel of an LCD according to an embodiment of the present invention;  
         [0017]    [0017]FIG. 3 is another block diagram of electronic components of an LCD according to an embodiment of the present invention; and  
         [0018]    [0018]FIG. 4 is yet another block diagram of electronic components of an LCD according to an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0019]    The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numerals refer to like elements throughout.  
         [0020]    Embodiments of the invention are described herein in the context of color pixels and more specifically in the context of red, green, and blue pixels. However, it is to be understood that the embodiments provided herein are exemplary embodiments, and the scope of the invention is not limited to the applications or the embodiments disclosed herein. For example, the invention may be used for any pixels that have different electro-optical characteristics, regardless of whether this difference stems from a difference in color.  
         [0021]    It will be understood that when an element, such as a layer, a region or a substrate, is referred to as being “on” (e.g., positioned on, provided on, located on) another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present.  
         [0022]    Referring to FIG. 1, an LCD according to an embodiment of the present invention includes a liquid crystal panel assembly (not shown), a gate driver  400  and the data driver  500  connected to the panel assembly, a digital gray voltage generator  800  connected to the data driver  500 , and a signal controller  600  controlling the above-described elements.  
         [0023]    The display signal lines include a plurality of gate lines G 1 -G n  for transmitting gate signals (also referred to as “scanning signals”) and a plurality of data lines D 1 -D m  for transmitting data signals. The gate lines G 1 -G n  are arranged in rows and extend substantially parallel to each other. The data lines D 1 -D m  are arranged substantially perpendicularly to the rows (i.e., in columns) and extend substantially parallel to each other.  
         [0024]    Referring to FIG. 2, a liquid crystal panel assembly  300  includes a first panel  100 , a second panel  200 , and a liquid crystal layer  3  between the two panels  100 ,  200 . The liquid crystal panel assembly  300  also includes a plurality of display signal lines G 1 -G n  and D 1 -D m  connected to a plurality of pixels that are arranged approximately in a matrix, as shown in FIG. 1.  
         [0025]    Each pixel includes a switching element Q connected to the signal lines G 1 -G n  and D 1 -D m , and an LC capacitor C LC  and a storage capacitor C ST  that are connected to the switching element Q. Depending on the embodiment, the storage capacitor C ST  may be omitted.  
         [0026]    The switching element Q is provided on the first panel  100  and has three terminals: a control terminal connected to one of the gate lines G 1 -G n ; an input terminal connected to one of the data lines D 1 -D m ; and an output terminal connected to both the LC capacitor C LC  and the storage capacitor C ST .  
         [0027]    The LC capacitor C LC  includes a pixel electrode  190  provided on the first panel  100  and a common electrode  270  provided on the second panel  200  as two terminals. The LC layer  3  disposed between the two electrodes  190  and  270  functions as the dielectric material of the LC capacitor C LC . The pixel electrode  190  is connected to the switching element Q. The common electrode  270  is connected to the common voltage V com  and covers the entire surface of the second panel  200 . In some embodiments, the common electrode  270  may be provided on the first panel  100  and the electrodes  190  and  270  may have shapes that are different from what is depicted in FIG. 2.  
         [0028]    The storage capacitor C ST  is formed by the pixel electrode  190  and a separate wire (not shown) located on the first panel  100 . The wire is supplied with a predetermined voltage such as the common voltage V com. Alternatively, the storage capacitor C ST  may be formed by the pixel electrode  190  and its previous gate line G i-1  sandwiching an insulating material.  
         [0029]    In a color display, each pixel is a color pixel for passing light through a color filter  230  (e.g., a red, green, or blue color filter). The color filter  230  is located over an area of the LC layer that is controlled by the pixel electrode  190 . Although the color filter  230  is shown to be located in the second panel  200  in FIG. 2, the invention is not so limited. In some embodiments, the color filter  230  is positioned on or under the pixel electrode  190  on the first panel  100 .  
         [0030]    Referring to FIGS. 3 and 4, the digital gray voltage generator  800  generates gray voltages. The gray voltage generator  800  includes a pair of gamma registers  810  and  820  sequentially connected by ten buses and a 10-bit D/A converter  830  connected to one of the gamma registers  810 ,  820  through 10-bit buses. As shown in FIG. 3, the gamma-registers  810  and  820  are connected to the signal controller  600  through multiple (e.g., two) buses and receive and store R, G, and B gamma voltage data separately. The signal controller  600  separately transmits the R, G and B gamma voltages through different buses and the registers  810  and  820  store the gamma voltage data for one of the colors. Although there may be other registers for other colors, registers for only one color is shown for clarity of illustration. The internal structures of reference numerals  800  and  501  shown in FIG. 4 concern only one color. The D/A converter  830  converts the digital data stored in the gamma registers  810 ,  820  to analog signals for every color and every polarity based on reference voltages  840 , and outputs the analog data to the data driver  500  through two bus lines. The signal controller  600  includes a 10-bit register  610  storing a predetermined number of (e.g., 16 for a total of 256 grays) digital gamma data, which will be supplied for the gray voltage generator  800  through the controller  620 . The above-described configuration generates independent RGB gamma curves.  
         [0031]    The gate driver  400  is connected to the gate lines G 1 -G n  on the liquid crystal panel assembly  300 . The gate driver  400  receives gate signals having a gate-on voltage V on  and a gate-off voltage V off  from an external device and applies them to the gate lines G 1 -G n . As shown in FIG. 3, the gate driver  400  may be divided into two gate sub-drivers  401  and  402  mounted on different portions of the liquid crystal panel assembly  300 . The gate sub-drivers  401  and  402  receive voltages from the signal controller  600  through the level shifter  700 .  
         [0032]    The data driver  500  includes a plurality of data driving ICs  501 - 508  mounted on the liquid crystal panel assembly  300  and is connected to the data lines D 1 -D m  on the liquid crystal panel assembly  300 . The data driver  500  receives gray voltages from the gray voltage generator  800 , selects data voltages among the gray voltages, and applies the data voltages to the data lines D 1 -D m .  
         [0033]    The driving ICs  501 - 508  are connected to the D/A converter  830  of the gray voltage generator  800 , and two buses from the gray voltage generator  800  are divided to be connected to the driving ICs  501 - 508 .  
         [0034]    The driving ICs  501 - 508  are also connected to the signal controller  600  and receive image data and data control signal therefrom. An exemplary data control signal is a 2-bit clock signal. A bus from the signal controller  600  is divided to be connected to the data driving ICs  501 - 508 . The image data are supplied from the signal controller  600  to the data driving ICs  501 - 508  by two signal lines separately connecting the data driving ICs  501 - 508  to the signal controller  600 . These connections and configurations are disclosed in detail in SID 01 DIGEST pp. 106-109 (“An Advanced Interconnect Link For TFT Column Driver Data”), which is incorporated herein by reference. This connection configuration significantly reduces electromagnetic interference (EMI).  
         [0035]    The data driver  500  includes a shift register (not shown), a data register (not shown), a data latch (not shown), a D/A converter (not shown), and an output buffer (not shown). The shift register stores R, G, and B data transmitted from the signal controller  600  into the data register. The D/A converter receives the data stored in the data register through the data latch and converts the data into analog data voltages. The output buffer stores the analog data voltages supplied from the D/A converter and applies the data voltages to the data lines in response to a load signal.  
         [0036]    [0036]FIG. 4 shows the driving IC  501  in greater detail. Although driving ICs  502 - 508  are not shown in the figure, they are substantially similar to the driving IC  501 . The D/A converter of the data driving IC  501 - 508  includes a sampling/holding unit  520  and a series of resistors. As shown, resistors  510  in the driving IC  501  connect the  16  sampling circuits.  
         [0037]    The sampling/holding unit  520  includes sixteen sampling/holding circuits for sampling the data from the D/A converter  830  of the gray voltage generator  800 , which includes eight circuits for positive voltages and eight circuits for negative voltages. The sampled voltages are applied to nodes between the resistors  510  as shown in FIG. 4 such that they determine the finalized values of the gray voltages output from the nodes as indicated by the arrows. Each sampling/holding circuit includes a switch SW, a capacitor C 1 , and a buffer B 1 -B 16 . When a switch SW turns on in response to a sampling start signal, reference gamma voltages from the D/A converter are sampled and stored in the capacitors C 1 , and the sampled gamma reference voltages are output from the analog buffers B 1 -B 16 . The output voltages are divided into a plurality of gray voltages, for example,  256  positive analog gray voltages and 256 negative analog gray voltages (2 8 =256).  
         [0038]    Now, the operations of the LCD will be described in detail.  
         [0039]    The signal controller  600  is supplied with RGB image signals R, G and B and input control signals controlling the display thereof from an external graphic controller (not shown). The input control signals include a vertical synchronization signal V sync , a horizontal synchronization signal H sync , a main clock MCLK, a data enable signal DE, etc. On the basis of the image data and the input control signals, the signals controller  600  processes the image data to be suitable for the gate and data drivers  400  and  500  and the liquid crystal panel assembly  300 , generates a plurality of gate control signals CONT 1  for the gate driver  400  and the processed image data R′, G′ and B′, and the data control signals CONT 2  for the data driver  500 .  
         [0040]    The gate control signals CONT 1  include a vertical synchronization start signal STV for instructing to start outputting a gate-on voltage V on , a gate clock signal CPV for controlling the output time of the gate-on voltage V on , and an output enable signal OE for defining the duration of the gate-on voltage V on .  
         [0041]    The data control signals CONT 2  include a horizontal synchronization start signal STH for instructing to start inputting the processed image data R′, G′ and B′, a load signal LOAD for instructing to apply data voltages to the data lines D 1 -D m , an inversion signal RVS for reversing the polarity of the data voltages (with respect to the common voltage V com ), and a data clock signal HCLK.  
         [0042]    The control signals CONT 1  and CONT 2  may be transmitted through buses for the transmission of the image data.  
         [0043]    The data driver  500  receives a packet of the image data R′, G′, and B′ for a pixel row from the signal controller  600  and converts the image data R′, G′, and B′ into data voltages selected from the analog gray voltages in response to the data control signals CONT 2  from the signal controller  600 . The data driver  500  then outputs the data voltages to the data lines D 1 -D m .  
         [0044]    Responsive to the gate control signals CONT 1  supplied from the signal controller  600 , the gate driver  400  applies the gate-on voltage V on  to the selected gate line G 1 -G n , thereby turning on the switching elements Q connected thereto. The data voltages applied to the data lines D 1 -D m  are supplied to the pixels through the activated switching elements Q.  
         [0045]    The difference between the data voltage and the common voltage V com  is represented as a voltage across the LC capacitor C LC , i.e., the pixel voltage. The LC molecules in the LC capacitor C LC  have orientations depending on the magnitude of the pixel voltage, and the molecular orientations determine the polarization of light passing through the LC layer  3 . A polarizer or polarizers (not shown) attached to at least one of the panels  100  and  200  convert the light polarization into the light transmittance.  
         [0046]    By repeating this procedure by a unit of a horizontal period (which is indicated by 1H and equal to one period of the horizontal synchronization signal Hsync, the data enable signal DE, and a gate clock signal), all gate lines G 1 -G n  are sequentially supplied with the gate-on voltage V on  during a frame, thereby applying the data voltages to all pixels. When the next frame starts after finishing one frame, the inversion control signal RVS applied to the data driver  500  is controlled such that the polarity of the data voltages is reversed (which is called “frame inversion”). The inversion control signal RVS may be also controlled such that the polarity of the data voltages flowing in a data line in one frame are reversed (which is called “line inversion”), or the polarity of the data voltages in one packet are reversed (which is called “dot inversion”).  
         [0047]    The numbers described in the embodiments herein are based on the assumption that the number of the total gray voltages is 256. A person of ordinary skill in the art will understand that these numbers may vary depending on the number of gray voltages.  
         [0048]    The gate driver and the data driver, instead of having chip types, may be incorporated into the liquid crystal panel assembly by forming the elements of the drivers using the processes for forming the switching elements of the pixels and the display signal lines.  
         [0049]    The provision of separate signal lines between the signal controller and the respective data driving ICs reduces EMI. Furthermore, the individual gamma reference voltages for the red, green and blue colors facilitate to adjust color temperature and color coordinates. The adjustment of the color temperature and the color coordinates enlarges the color representation, which may be limited by the liquid crystal characteristics and color filters, to realize the various color representation.  
         [0050]    Furthermore, the signal controller supplies digital gamma values such that the gamma can be varied depending on the frames, and thus the dynamic luminance ratio for motion picture can be increased to realize dynamic images. It is preferable that the configuration of the signal controller is changed to be suitable for the driving ICs. That is, it is preferable that the signal controller transmits R, G and B gamma values in digital format when it is powered, and it analyzes the input image data and adjusts the gamma values when the dynamic images are required.  
         [0051]    Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.