Abstract:
One frame of a color image data is resolved into three frames of R, G, and B colors, which are displayed in sequence. In synchronism with the R, G, B frames, a power supply voltage for displaying each of the three frames is altered to predetermined levels set for the respective frames. Output voltage data for determining the voltage levels for red-, green-, and blue-frame are stored in rewritable forms, which are provided to the power supply circuit so that its output voltage is equilibrated to a reference voltage based on that output voltage data provided. The invention enables suppression of color temperature deviation due to variations in transmissivity of color filters while setting a common gradation level for the R, G, an B frames.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a display apparatus, especially a liquid crystal display, for displaying a picture using three-primary color image data. The invention also relates to a semiconductor device for use in such display apparatus.  
       BACKGROUND OF THE INVENTION  
       [0002]     In recent years lightweight thin display apparatuses are in strong demand along with the popularization of lightweight thin personal computers and TV sets. To meet such demands in different fields, in place of cathode-ray tubes, various flat panel display apparatuses such as a liquid crystal display (LCD) have been developed.  
         [0003]     An LCD has two substrates sandwiching between them a liquid crystal material, to which an electric field is applied. A desired image can be displayed on the LCD by controlling the intensity of the electric field at each pixel to vary the degree of polarization of, and hence the amount of transmitting light through, the liquid crystal at that pixel. In a color LCD, there are provided red (R), green (G), and blue (B) pixels, which are provided with a red (R), a green (G), and a blue (B) color filter, respectively, arranged in such a way that white is obtained when R, G, and B pixels have their maximum luminance.  
         [0004]     However, white color can become reddish or bluish for example. That is, the color temperature can be deviated, mainly due to variations in transmissivity of each color the color filters.  
         [0005]     Such color transmissivity deviation can be suppressed by setting gradation levels of R, G, and B color images independently (patent JPA Laid Open 2003-29724).  
         [0006]     However, gradation of R, G, and B color images is normally performed using one common IC chip. In order to set independent gradation levels for R, G, and B images, three IC chips are required, which results in an increase in cost.  
       SUMMARY OF THE INVENTION  
       [0007]     It is, therefore, an object of the present invention to provide an RGB color display apparatus capable of suppressing color temperature deviation or shift caused by variations in transmissivity of R, G, and B color filters while setting a common gradation level for R, G, and B color images.  
         [0008]     It is another object of the invention to provide a semiconductor device for enabling the gradation setting of such RGB color display apparatus.  
         [0009]     In accordance with one aspect of the invention, there is provided a display apparatus comprising: 
        a color display means;     a source driver and a gate driver for providing red (R), green (G), and blue (B) image data for displaying input image data on the color display means; and     a power supply circuit for supplying a power supply voltage to the source driver and/or gate driver, wherein the power supply voltage to be supplied to the source driver and/or gate driver is made variable at every predetermined output timing of the R, G, and B image data.        
 
         [0013]     In accordance with another aspect of the invention, there is provided a display apparatus, comprising: 
        a color display means  10 ;     a gate driver  20  for supplying a gate voltage to the color display means;     a source driver  30  for supplying a source voltage to the color display means;     a control block ( 60  and  70 ) for supplying, upon receipt of a frame of color image data Dc that contains a synchronization signal, a predetermined timing signal for controlling the gate driver (the timing signal hereinafter referred to as gate driver controlling timing signal) tg to the gate driver, and supplying a predetermined timing signal for controlling the source driver (the timing signal hereinafter referred to as source driver controlling timing signal) ts and image data that include R, G, and B color image data to the source driver;     a gradation block  40  for generating and supplying a gradation voltage to the source driver; and     a power supply block ( 80  and  100 ) for supplying a power supply voltage to the source driver  30  and gradation block  40 , wherein     the control block is adapted to: 
            resolve one frame of color image data into a red-frame that consists of red image data Dr, a green-frame that consists of green image data Dg, and a blue-frame that consists of blue image data Db (the respective frames referred to as R, G, and B frame);     sequentially supply the R, G, and B image data Dr, Dg, and Db, respectively, to the source driver at a predetermined time interval T 2 : and     supply to the power supply block ( 80  and  100 ) a voltage controlling timing signal tp in synchronism with R, G, and B frames, and wherein    
            the power supply block generates, upon receipt of the voltage controlling timing signal tp, an R frame voltage Vr, a G frame voltage Vg, and a B frame voltage Vb for use as the power supply voltage for the respective R, G, and B frames.        
 
         [0025]     The control block may have 
        a controller  60  for controlling image data and various kinds of timing signals, and     a buffer memory  70  for separately storing R image data Dr, G image data Dg, and B image data Db that constitute a color image data in such a way that the R image data Dr, G image data Dg, and B image data Db can be separately retrieved. The control block may be adapted to start supplying the R image data, G image data, and B image data to the source driver a predetermined time Tb after sending out the voltage controlling timing signal tp.        
 
         [0028]     The power supply block may have: 
        a power supply circuit  100  for generating the power supply voltage, and     a storage unit  80  for storing, and delivering to the power supply circuit, output voltage data Dvr, Dvg, and Dvb that are respectively defined for the R, G, and B frames (the respective output voltage data referred to as R, G, and B output voltage data).        
 
         [0031]     The storage unit  80  may be a programmable ROM capable of rewritably storing the R, G, and B output voltage data.  
         [0032]     The power supply circuit may have: 
        registers  121 ,  122 , and  123  for respectively storing the R, G, and B output voltage data (the registers respectively referred to as red-, green-, and blue-register, or in short, R, G, and B registers);     a selector  130  for sequentially retrieving from the R, G, and B registers the R, G, and B output voltage data associated with the voltage controlling timing signal tp, respectively, and for outputting the selected voltage as the reference voltage Vref to control the power supply voltage associated with the color frame; and     a voltage regulation circuit adapted to equilibrate the feedback voltage Vfb associated with the power supply voltage to the reference voltage Vref.        
 
         [0036]     The selector  130  may have a counter  131  for counting the voltage controlling timing signal tp, and a logic circuit for outputting either one of the R, G, and B output voltage data in accord with the count of the counter.  
         [0037]     A semiconductor device of the invention is formed to include the elements constituting the color display apparatus except for the color display means.  
         [0038]     According to the invention, one frame of color image is resolved into R, G, and B frames, which are displayed in turn on the display apparatus. The power supply voltage is altered to preset voltage levels in synchronism with the R, G, and B frames. Thus, while setting a common gradation level for R, G, and B frames, deviation in color temperature caused by the variations in transmissivity of R, G, and B color filters can be suppressed.  
         [0039]     As described above, supply of R, G, B image data to a color display means (for example, source driver of an LCD panel) is started when a predetermined delay time Tb has elapsed after sending out a voltage controlling timing signal tp. This circumvents supplying image data to the display means while voltage levels are changing, which adds stability to the image displayed.  
         [0040]     It will be recalled that R output voltage data Dvr, G output voltage data Dvg, and B output voltage data Dvb defined for the R, G, and B frames, respectively, are stored in a storage device such as a programmable ROM, and that these output voltage data are supplied to the power supply circuit to serve as the reference voltages for the respective frames. Thus, color temperature deviation (or shift) can be alleviated by simply altering R, G, and B output voltage data in accord with the characteristic of the LCD panel in use, without changing the structure of the LCD panel. Moreover, given an LCD panel, its color temperature can be set by a user as he likes by simply altering the R, G, and B output voltage data, which is convenient for mass production of LCDs. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]      FIG. 1  shows a structure of an LCD according to the invention.  
         [0042]      FIG. 2  shows a structure of a power supply block according to the invention.  
         [0043]      FIG. 3  shows an arrangement of a selector for use in a power supply circuit according to the invention.  
         [0044]      FIG. 4  is a timing diagram illustrating operation of an LCD of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0045]     Preferred embodiments of a display apparatus and a semiconductor device therefor according to the invention will now be described in detail, with LCD as an example, and with reference to accompanying drawings. Referring to  FIG. 1 , there is shown a circuit arrangement of an LCD according to the invention. Elements of the LCD other than its LCD panel  10  are preferably fabricated in the form of semiconductor devices.  FIG. 2  shows a circuit arrangement of a power supply block.  
         [0046]     In the example shown in  FIG. 1 , the color LCD panel  10  can be, for example, an active-matrix type display apparatus utilizing thin film transistors (TFTs) arranged in a matrix configuration. A gate driver  20  supplies a gate voltage to the gates of the TFTs of the LCD panel  10 . A source driver  30  provides a source voltage in accord with an image data to the sources (or drains) of the TFTs of the LCD panel  10 .  
         [0047]     A gradation block  40  generates a gradation voltage to be supplied to the source driver  30 . A common voltage generation circuit  50  divides the power supply voltage by resistors  51  and  52  and provides one of the divided voltages via a buffer (voltage follower) as a common voltage for use in alternation driving of the LCD panel  10 , and supplies it to the source driver  30 .  
         [0048]     The control block consists of a controller  60  and a buffer memory  70 . This control block receives a frame of color image data Dc that contains a synchronization signal, and supplies a predetermined gate driver controlling timing signal tg to the gate driver  20 , and provides a predetermined source driver controlling timing signal ts and image data Dr, Dg, and Db to the source driver  30 .  
         [0049]     The controller  60  separates one frame of color image data Dc into R, G, and B frames of R, G, and B image data, respectively. Each of the R, G, B frames of R, G, B image data is stored separately in the buffer memory  70 .  
         [0050]     The buffer memory  70  consists of a memory storage device such as an RAM. This buffer memory  70  is adapted to store color image data Dc in the form of separate R image data Dr, G image data Dg, and B image data Db, that can be retrieved in sequence as R, G, and B frames of R, G, B image data, respectively, at every predetermined time interval T 2 .  
         [0051]     Since one frame of color image data Dc is separated into three R, G, and B frames of image data, which are displayed in sequence, the predetermined period T 2  is ⅓ of the frame period T 1  for one frame of color image data Dc.  
         [0052]     The controller  60  provides the power supply block ( 80  and  100 ) with a voltage controlling timing signals tp synchronized to the R, G, and B frames.  
         [0053]     The power supply block has a power supply circuit  100  for generating a power supply voltage, and a memory storage device  80  for storing predetermined R, G, and B output voltage data Dvr, Dvg, and Dvb, respectively, defined for the R, G, B frames, respectively, and supplies R, G, and B output voltage data to the power supply circuit  100 . The power supply circuit  100  generates R, G, and B output voltages Vr, Vg, and Vb according to R, G, and B output voltage data Dvr, Dvg, and Dvb, respectively. These output voltages are, for example, Vr=9.1 V, Vg=9.0 V, and Vb=9.2 V.  
         [0054]     The memory storage device  80  preferably consists of a programmable ROM, such as an EEPROM, that is capable of rewritably storing R, G, and B output voltage data Dvr, Dvg, and Dvb, respectively. Levels of these output voltage data Dvr, Dvg, and Dvb can be arbitrarily changed in accord with the preferred condition of the LCD panel, or by a user as needed.  
         [0055]     Referring to  FIG. 2 , there is shown an internal structure of the power supply circuit  100 , along with the storage device  80 .  FIG. 3  shows an exemplary circuit arrangement of a selector for use in the power supply circuit  100 .  
         [0056]     In the example shown in  FIG. 2 , the power supply circuit  100  is a step-up switching power supply circuit. A coil Lo and a switching transistor Qo are connected in series between input voltage Vin and the ground. The voltage at the node of the series connection of the coil Lo and the switching transistor Qo is rectified and smoothed by a diode Do and a capacitor Co before it is output as output voltage Vout (Vr, Vg, Vb) from the power supply circuit.  
         [0057]     An interface circuit (hereinafter referred to as I/F circuit)  110  receives R, G, B output voltage data Dvr, Dvg, Dvb, respectively, from the memory storage device  80  via, for example, a 3-way serial communications line. The I/F circuit  110  stores respective R, G, and B output voltage data Dvr, Dvg, and Dvb, respectively, in R, G, and B register  121 ,  122 , and  123 , respectively.  
         [0058]     The selector  130  is fed R, G, B output voltage data Dvr, Dvg, and Dvb, respectively, from the R, G, B register  121 ,  122 , and  123 . One of these output voltage data Dvr, Dvg, and Dvb is selected and output every time a timing pulse of the timing signal tp is input. Output voltage data (Dvr, Dvg, Dvb) read out from the selector  130  corresponds to that (R, G, and B) frame read out from the buffer memory  70 . This output voltage data (Dvr, Dvg, Dvb) is a digital signal.  
         [0059]     A digital analog converter (DAC)  140  converts the output voltage data (Dvr, Dvg, Dvb) received from the selector  130  into an analog voltage. The analog output voltage of the DAC  140  serves as reference voltage Vref to obtain predetermined power supply voltage Vout (Vr, Vg, Vb).  
         [0060]     An exemplary circuit arrangement of the selector  130  is shown in  FIG. 3 . As seen in  FIG. 3 , a counter  131  is a 3-value repeat counter that counts up every time a timing pulse of the timing signal tp is input. The counter is reset to an initial value by, for example, a synchronization signal contained in the color image data Dc. This synchronization signal is easily obtained from the controller  60 . By the synchronization signal, correspondence is established between output voltage data (Dvr, Dvg, and Dvb) and the associated (R, G, and B) frame read out from the buffer memory  70 .  
         [0061]     R, G, and B output voltage data Dvr, Dvg, and Dvb, respectively, are also input into the logic circuits (which are NAND circuits  132 - 137  and NOT circuits  138 - 139  in the example shown in  FIG. 3 ) of the selector circuit  130 . This selector circuit  130  outputs either one of R, G, and B output voltage data Dvr, Dvg, and Dvb, respectively, according to the count of the counter  131 .  
         [0062]     In the example shown in  FIG. 3 , R output voltage data Dvr is selected when the output levels of the output ends i and ii of the counter  131  are respectively high (H) and low (L); G output voltage data Dvg is selected when the output levels of the output ends i and ii are low (L); and B output voltage data Dvb is selected when the output levels of the output ends i and ii are respectively low (L) and high (H).  
         [0063]     Referring back to  FIG. 2 , an error amplifier  150  outputs a difference signal FB indicative of the difference between reference voltage Vref and feedback voltage Vfb that is obtained by dividing power supply voltage Vout by resistors R 1  and R 2 . A PWM comparator  170  provides a pulse width modulation signal PWM by comparing the difference signal FB with a triangular wave signal CT received from an oscillating circuit  160 . A driver  180  provides a gate control signal N 1  to a switching transistor Qo based on the pulse width modulation signal PWM and the clock signal CLK received from the oscillating circuit  160 . These feedback control loops are adapted to equilibrate feedback voltage Vfb to reference voltage Vref so as to hold power supply voltage Vout at a predetermined level.  
         [0064]     The section of the power supply circuit  100 , subsequent to the error amplifier  150 , providing output voltage Vout, constitutes a voltage regulation circuit. The voltage regulation circuit can be of any configuration so long as it can provide output voltage Vout in accord with given reference voltage Vref. Thus, it should be understood that the circuit arrangement shown in  FIG. 2  is intended to be illustrative of an inventive voltage regulation circuit.  
         [0065]     Reference numeral  200  of  FIG. 2  represents an IC chip that incorporates a portion of the power supply circuits  100  that serves as the control circuit thereof. It is noted that the operation of the control circuit portion incorporated in the IC chip  200  is independent of the magnitudes of the output current or output voltage, and that it can be manufactured separately from an erasable memory storage such as an EEPROM. Therefore, the IC chip  200  can be used as a general-purpose control IC chip for a variety of LCDs.  
         [0066]     Operation of the LCD of the invention, as configured above, will now be described with reference to the timing diagram shown in  FIG. 4 .  
         [0067]     In the present invention, one frame of full-color image data Dc is resolved into three R, G, and B frames to be displayed in sequence. This implies that, given a frame rate for one full-color image being 60 (or 75) frames/sec, one full-color image is resolved into 180 (or 225) frames of three primary (R, G, and B) color images, that will be displayed at the frame rate of 180 (or 225) frames/sec according to the invention. That is, as shown in  FIG. 4 , one frame period T 2  of the invention for one primary color image is ⅓ of one frame period T 1  for one frame of full-color image.  
         [0068]     It is seen in  FIG. 4  that one frame period for R image (referred to as R frame period) starts when a pulse of the timing signal tp is issued at time t 1 . Time t 1  is the beginning of the frame period T 1  for one (frame of) full-color image. That is, it corresponds to a frame synchronization timing of the power supply circuit  100 , or more particularly, of the selector  130  of  FIG. 2 . Therefore, R image data is read out from the buffer memory  70  at time t 1 , and at the same time R output voltage Vr associated with R output voltage data Dvr is output from the power supply circuit  100 .  
         [0069]     However, the power supply circuit  100  has a certain delay time between reception of output voltage data (Dvr for example) and generation of a predetermined output voltage associated therewith (Vr in this example). Thus, during the delay time, the power supply circuit  100  fails to provide a valid output voltage to properly display the data.  
         [0070]     Therefore, it is preferable for R image data to be read out after power supply voltage Vout has reached predetermined power supply voltage Vr. To do this, a blanking period is provided starting at time t 1  and lasting a predetermined delay time Tb. This blanking period Tb equals a time interval necessary for power supply voltage Vout to grow up to predetermined power supply voltage Vr. Thus, R image data is read out after expiration of the blanking period Tb.  
         [0071]     At time t 2 , G image data can be read out from the buffer memory  70 , when G output voltage Vg is provided from the power supply circuit  100  in accord with G output voltage data Dvg. Similarly, at time t 3 , B image data can be read out from the buffer memory  70  and B output voltage Vb is provided from the power supply circuit  100  in accord with B output voltage data Dvb.  
         [0072]     At time t 4 , the next frame of full-color image is started, for which a similar procedure to the one as described above is repeated.  
         [0073]     The invention enables suppression of color temperature deviation or shift caused by variations in transmissivity of color filters while setting a common gradation level for R, G, and B colors.  
         [0074]     It should be appreciated that color temperature deviation can be alleviated, without changing the structure of the LCD, by simply rewriting R, G, B output voltage data Dvr, Dvg, and Dvb, respectively. Moreover, the color temperature of a given LCD panel can be set by a user as needed.  
         [0075]     Although the power supply circuit has been described with reference to a step-up switching power supply circuit as shown in  FIG. 2 , a step-down switching power supply circuit, a series type power supply circuit, and other types of power supply circuit can be alternatively used.