Abstract:
This invention provides a digital-analog converter circuit capable of appropriately correcting the optical characteristics of the liquid crystals according to the change in design or the preference of the user, and achieving goals of miniaturization, cost-lowering, as well as wide design suitability. The digital-analog converter circuit includes a storage device for storing a voltage characteristic curve, a modulating device for generating a frequency signal in accordance with a data from the voltage characteristic curve stored in the storage device in response to a selected data, a variable resistance device connected between a first power source and a second power source, in which the resistance value of the variable resistance device is changed in accordance with the frequency signal from the modulating device, a holding device for holding a voltage generated at the variable resistance device, and an output device for outputting the voltage to a desired output end.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a digital-analog converter circuit. More particularly, the present invention relates to a digital-analog converter circuit, which is capable of generating a driving voltage according to a γcurve for correcting optical characteristic of liquid crystals. 
         [0003]    2. Description of the Related Art 
         [0004]    The liquid crystals are driven by changing a voltage applied thereupon. The relationship between driving voltages and the optical characteristic is generally non-linear. In order to correct this non-linear relationship, a γvoltage is provided to the liquid crystals. Such γvoltage in a low-temperature polysilicon (LTPS) liquid crystal display is constituted by a plurality of resistors in serial connection made on a glass substrate and a plurality of intermediate nodes between these resistors. A γvoltage corresponding to an input data can be extracted from such a structure. The γvoltages forms a voltage curve determined by the characteristic of the liquid crystals. 
         [0005]    However, such a voltage curve is determined by the resistors in serial connection made with using an exposure mask, the resistors structure is fixed and cannot be changed anymore. Therefore, despite the change in the characteristic of the liquid crystals due to the change of their material under such condition, in order to change the value of the series resistors, the exposure mask must be remade. Furthermore, due to the optical characteristic of the liquid crystals themselves, the picture quality of the liquid crystals can be changed according to the preferences of the user. However, the conventional technique can not fulfill such demand. In addition, in order to fulfill various demands, respectiveγvoltage generators with different properties would become necessary, which is an obstacle for device miniaturization and cost reduction. Furthermore, this would worsen the design suitability derived from the concept that one design is applicable to various devices. The design compatibility for a plurality of devices would be difficult, and ultimately leading to the increase of cost. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is accomplished in order to solve such problem, and the objective is to provide a digital-analog converter circuit, which can appropriately correct the optical characteristic of the liquid crystals according to the change in design or the preference of user, and accomplish the goal of device miniaturization, cost-lowering, as well as wide design portability. 
         [0007]    The present invention provides a digital-analog converter circuit comprising a storage device storing a voltage characteristic curve; a modulating device generating a frequency signal in accordance with a data from the voltage characteristic curve stored in the storage device in response to a selected data; a variable resistance device is connected between a first power source and a second power source, in which the resistance value of the variable resistance device is changed in accordance with the frequency signal from the modulating device; a holding device holding a voltage generated at the variable resistance device; and an output device outputting the voltage to a desired output end. 
         [0008]    According to the present invention, the y voltage value from the pre-stored characteristic curve can be freely determined by the modulating frequency, and thus the optical characteristic of the liquid crystals can be appropriately corrected according to the change of the design and the preference of user. In addition, since the change of the circuit design on the glass substrate corresponding to various γvoltage demands is not necessary, the objective of device miniaturization, cost down, and wide design suitability is achievable. Furthermore, by using a single γvoltage generating circuit, RGB independent driving, common AC driving . . . etc can be realized, which has high applicability. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram showing the outline structure of a conventional liquid crystal display device. 
           [0010]      FIG. 2  is a block diagram showing an LCD module, which is the primary part of the conventional liquid crystal display device, suitable for the present invention. 
           [0011]      FIG. 3  is a circuit diagram of the digital-analog converter circuit of the present invention. 
           [0012]      FIG. 4  is a block diagram for showing the basic operation of switch capacitors of the present invention. 
           [0013]      FIG. 5  is a graph showing the relationship of the change in the modulating frequency and the γweighted output voltage of the present invention. 
           [0014]      FIG. 6  is a graph showing the relationship of the data n and the ratio of the two frequencies fa/fb. 
           [0015]      FIG. 7  is a graph showing the changes of the output voltage Vs in  FIG. 3  when the data n is changed. 
           [0016]      FIG. 8  is a block diagram showing a digital-analog converter circuit in accordance with second example of the present invention, from which the γweighted output voltages for each of the  3  colors are obtained. 
           [0017]      FIG. 9  is a graph showing the operation of the second example of the present invention. 
           [0018]      FIG. 10  is a block diagram showing a digital-analog converter circuit in accordance with third example of the present invention. 
           [0019]      FIG. 11  is a graph showing the operation of the third example of the present invention. 
           [0020]      FIG. 12  is a perspective diagram showing another example of a mobile phone device in which the liquid crystal display device containing the digital-analog converter circuit of the present invention is applied. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    In the following section, some of the examples of the present invention will be described and explained with references to the drawings. Before the detailed descriptions of the present invention, the conventional structures is described. 
         [0022]      FIG. 1  shows a structure of a conventional liquid crystal display device  1 , which comprises liquid crystal cells  3  in a matrix form corresponding to pixels to constitute a liquid crystal cell array  2  as a display unit, and a gate line driving circuit  7  and a source line driving circuit  8  for driving the liquid crystal cell array  2 . 
         [0023]    The liquid crystal cell  3  is formed by a liquid crystal element  5  connected between the drain of a thin film transistor  4  and a ground as a capacitor and an auxiliary capacitor  6  connected therewith in parallel. The gate of the thin film transistor  4  is connected to a gate line GL and the source of the thin film transistor  4  is connected to a source line SL. In addition, the gate line driving circuit sequentially drives the gate lines GL respectively connected to the thin film transistors  4  of a row of the liquid crystal cells  3  corresponding thereto. Furthermore, the source line driving circuit provides a voltage signal to the source line SL, which is co-connected to the sources of the thin film transistors of each column of the liquid crystal cells. 
         [0024]      Fig.2  shows a LCD module  10  as a primary part of the conventional liquid display device, for which the present invention is to be applied onto. This LCD module  10  has cell array  20  with liquid crystal cells being formed in a matrix form (corresponding to the portion designated as numeral  2  in  FIG. 1 ). The relationship of the cell array  20 , a plurality of gate lines GL, a plurality of source lines SL, and thin film transistors provided on the intersection of the gate lines GL and the source lines SL as well as liquid crystal elements is as shown in  FIG. 1 . 
         [0025]    For the gate lines GL, the gate line driving circuit (not shown) sequentially provides an analog voltage to each gate line GL. A digital-analog converter DAC is provided at one side (the top side in  Fig.2 ) of each of the source lines SL. A voltage signal from a γvoltage generator  50  described below and a signal from a data latch  40  used for latching digital data from external are respectively inputted to the digital-analog converter DAC. 
         [0026]    In the γvoltage generator  50 , switching circuits  51  and  52  has either of power source voltage Vdd or the ground voltage Vss complementarily connected to, and (n+1) resistors R 0 ˜RN are serially connected in between, n resistively divided γvoltage values are extracted from intermediate connecting nodes between the serially-connected resistors. Theγvoltage values are provided to the digital-analog converter DAC in a bus and are digitized by the DAC according to the selecting data from the data latch, and then are provided to the source lines. The two switching circuits  51  and  52  can invert the polarity of the voltages applied to the resistor row by switch to the respective opposite sides of the switching circuits  51  and  52 , which causes the inversion of the driving polarity of the liquid crystals. 
         [0027]    The γvoltage generator  50  is constituted by the serially-connected resistors and the intermediate connecting nodes between the serially-connected resistors. The γ voltage value obtained therefrom is fixed. 
       EXAMPLE 1  
       [0028]      FIG. 3  is a circuit diagram of the digital-analog converter circuit of the present invention, in which the structures providing the applied voltages to the liquid crystal elements in  FIG. 2  are mostly replaced. 
         [0029]    A modulating device such as a modulator  110  supplied with control signals CTRL generated from a control unit  100  outputs two frequency signals fa and fb referring to a storage device such as a γ look-up table (LUT)  120 , which stores the γ value of the desired γcurve according to the value of the control signal CTRL. These frequency signals fa and fb are provided to the two switch capacitors  131  and  132 , which are serially connected between a power source Vcc and ground. The frequency signals fa and fb are used to control the two switch capacitors  131  and  132 . The two switch capacitors  131  and  132  compose a variable resistance device, wherein the variable resistance device also can be a switch and a fixed resistor connected in series. 
         [0030]    The operation of the switch capacitor will be explained here.  FIG. 4  is a block diagram explaining the basic operation of the switch capacitor  130 , which is a part of the digital-analog converter circuit shown in  FIG. 3 . One terminal is the capacitor Cpa that connected to the ground, the other terminal is replaced by a current source and power source voltage Vcc, and a switch switch-connected by the frequency fa is provided. The current source generates a constant current Ic. The frequency fa is given by the modulator, which modulates by the control signal CTRL. 
         [0031]    An output voltage Vout is taken out from a connecting node between the current source and the switch nearby one side thereof. The value of the output voltage is expressed as: 
         [0000]        V out= Vcc−Ic /( fa*Cpa ) 
         [0032]    The output voltage Vout is changed by changing the frequency fa outputted from the modulator. Moreover, the same operation also can be achieved by fixed resistors instead of the low current source. 
         [0033]    In the structure of  FIG. 3 , the two switch capacitors each driven by different frequencies are serially connected, and an output voltage Vs appears in an intermediate node therebetween having the following value: 
         [0000]        Vs=Vcc·Cpa·fa /( Cpa·fa+Cpa·fb ) 
         [0034]    According to time sequence which is the same as the period of frequency ratio fa/fb outputted by the modulator, the voltage passes though the turned off switches SW 1 ˜SWn sequentially. Then, this voltage passes through one of corresponding holding device that provided between the output sides of the switches and the grounds, and is supplied to the digital-analog converter DAC as source voltages Vo 0 ˜Vo(n−1). The holding device can be a sampling-hold circuits consisted of capacitors and buffers, Moreover, the switches and the sampling-hold circuits are disposed corresponding to the source lines. The digital-analog converter DAC selects the source voltage in accordance with the desired γvalue determined by digital data provided to the digital-analog converter. The source voltage is provided to the source line corresponding thereto. 
         [0035]      FIG. 5  is a graph showing the relationship of the change in the modulating frequency and the γ-weighted output voltage, in which the frequency output fa modulated by the modulator referring to the γlook-up table is shown in solid lines, and the frequency output fb is shown by dashed lines. By representing the γcurve as the value of 64, and making the data n changeable from 0 to 63, the combination of the modulating frequencies fa and fb according to the γ curve of the γ look-up table can be obtained, and it has been shown that the γweighted output voltage Vo 0 ˜Vo (n−1) can be obtained by the foresaid Vs formula. 
         [0036]      FIG. 6  is a graph showing the relationship of the data n and the ratio of two frequencies fa/fb. Modulator chooses a value in table  120  in response to the predetermined value of data n and outputs a ratio of two frequencies fa/fb. The fa decreases and fb increases when the data n increases as shown in  FIG. 5 . Take γ=1.8 as an example, the ratio of the frequency fa/fb is  9  when n=0, and decreases rapidly at the initial time, then the decrease of the ratio slows down, and producing a curve slowly approaching the value of 0. 
         [0037]      FIG. 7  is a graph showing the change of the output voltage Vs of  FIG. 3  when the data n is changed. The obtained data n specific to the respective source line is provided to the source line corresponding thereto. 
       EXAMPLE 2  
       [0038]      FIG. 8  is a block diagram showing the digital-analog converter circuit in accordance with second example of the present invention, in which the γweighted output voltages for the three different colors are obtained.  FIG. 9  is a graph showing its operation. In this example, three holding device are provided, and switches SWR, SWG, SWB are provided and sequentially selected in accordance with the time divisions as shown in  FIG. 9 . 
         [0039]    In the  FIG. 8 , the switches of SW 1  to SWn and capacitors in  FIG. 3  are shown as sampling-hold circuits  151 ˜ 153 . The required voltage outputs corresponding to obtained data for the respective colors pass through the respective buffer  161 ˜ 163  and then are supplied to DAC  140 . 
         [0040]    The operation for one color is totally the same as the operation shown in  FIG. 3 . When observing one pixel, the appropriate y weighted output voltages for the respective three colors are provided to the respective source lines. 
       EXAMPLE 3  
       [0041]      FIG. 10  is a block diagram showing a digital-analog converter circuit in accordance with third example of the present invention, which is suitable as a device displaying by inverting polarity during each frame to eliminate residue image. 
         [0042]    In the  FIG. 10 , the digital-analog converter circuit that compare with the  FIG. 3  has two charge pumps  171 ,  172  instead of the two switch capacitors. A switch device  200  for switching power source voltages between Vss and Vcc and supplying to charge pumps  171 ,  172 , switches SWp and SWn, sampling-hold circuits  181 ,  182  and buffers  191 ,  192  are provided. The switches SWp and SWn select and output the outputs obtained from the charge pumps respectively when the polarity is switched. 
         [0043]      FIG. 11  is a graph showing the operation of the third example shown in  FIG. 10 . At the initial period, Vcc is selected by the switch  200 , and operation as the same as shown in  FIG. 5  is proceeded, the γ weighted output voltage is obtained and outputted through the closed switch SWp, sampling-hold circuit  181  and buffer  191 . In the next period, Vss is selected by the switch  200 , and then the polarity is inverted. The γ weighted output voltage outputted from modulator  11  is increased with the change of data n from the negative voltage. The output voltage is outputted through switch SWn, sampling-hold circuit  181  and buffer  191 . 
         [0044]    The examples mentioned above are merely exemplifications; therefore various modifications are also possible and covered by this invention. For example, it is possible to extract the best γ voltage of transmissive-mode and reflective-mode. 
         [0045]    Although the digital-analog converter circuit has been explained on the premise of being used in liquid display devices above, digital-analog converter circuit of such kind in the present invention is also suitable as a part of the source line driving circuit in the liquid display device. 
         [0046]    In addition, although the liquid display device of such kind is suitable as the display device I in the mobile phone device  50  as shown in  FIG. 12 , it is not limited to mobile phones, but also suitable in electronic devices such as any of digital camera, personal digital assistant (PDA), notebook computer, desktop computer, television, automobile display, or portable DVD player. 
         [0047]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.