Patent Publication Number: US-6989808-B2

Title: Driving of a liquid crystal display device

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to liquid crystal display devices and more particularly to the driving of an active-matrix liquid crystal display device in which representation of images is achieved by applying a driving voltage to a liquid crystal layer via a thin-film transistor (TFT). 
     Liquid crystal display devices have various advantageous features such as compact size, light weight, low power consumption, and the like. Thus, liquid crystal display devices are used extensively in portable information processing apparatuses such as lap-top computers or palm-top computers. Further, liquid crystal display devises are used also in desktop computers in these days. 
     A typical liquid crystal display device includes a liquid crystal layer confined between a pair of glass substrates and achieves representation of images by inducing a change in the orientation of liquid crystal molecules in the liquid crystal layer by applying a driving voltage to the liquid crystal layer. Such a change in the orientation of the liquid crystal molecules causes a change in the optical property of the liquid crystal layer. 
     In the case of using such a liquid crystal display device in a high-resolution color representation apparatus, there is a need of driving the individual pixels or liquid crystal cells defined in the liquid crystal layer at a high speed. In order to meet this requirement, it is generally practiced to provide a thin-film transistor in correspondence to each of the pixels in the liquid crystal layer and to drive the liquid crystal cells by way of such thin-film transistors. 
       FIG. 1  shows the construction of a liquid crystal panel  10  used in such an active matrix liquid crystal display device of a related art in a plan view, while  FIG. 2  shows the part circled in  FIG. 1  in a cross-sectional view. 
     Referring to  FIG. 2 , the liquid crystal panel  10  generally includes a pair of glass substrates  10 A and  10 B, and a liquid crystal layer  10 C is confined between the substrates  10 A and  10 B. 
     As represented in the plan view of  FIG. 1 , the glass substrate  10 A carries thereon a number of thin-film transistors  11   1 - 11   4  corresponding to the pixels in a row and column formation, wherein the thin-film transistors  11   1  and  11   2  aligned in the row direction are connected commonly to a gate bus line G 1  provided directly on the glass substrate  10 A. Similarly, the thin-film transistors  11   3  and  11   4  are connected commonly to a gate bus line G 2  provided directly on the glass substrate  10 A. Further, the glass substrate  10 A carries thereon a number of generally H-shaped auxiliary electrodes Cs at the level of the gate bus lines G 1  and G 2 , wherein the auxiliary electrode Cs is covered by an insulation film  12  as represented in the cross-sectional view of  FIG. 2 , and data bus lines D 1  and D 2  are formed on the insulation film  12  so as to extend in the column direction as represented in the plan view of FIG.  1 . 
     It should be noted that the data bus lines D 1  and D 2  are covered by another insulation film  13  as represented in the cross-sectional view of  FIG. 2 , and the data bus line D 1  is connected to the respective source regions of the thin-film transistors  11   1  and  11   2  via a conductor pattern branched from the data bus line D 1 . Similarly, the data bus line D 2  is connected to the respective source regions of the thin-film transistors  11   2  and  11   4  via a conductor pattern branched from the data bus line D 2 . 
     Further, there is provided a rectangular pixel electrode of a transparent conductor such as ITO on the insulation film  13  in correspondence to the drain region of each of the thin-film transistors. For example, the drain region of the thin-film transistor  11   1  is connected to a transparent pixel electrode P 1  provided on the insulation film  13  via a contact hole formed in the insulation film  13 . As can be seen from  FIGS. 1 and 2 , the auxiliary electrode Cs is disposed at both sides of the data bus line D 1  or D 2  when viewed in the direction perpendicular to the substrate  10 A, such that the electrode Cs overlaps the edge part of the transparent pixel electrode P 1  or P 2 . Thereby, the auxiliary electrode Cs forms an auxiliary capacitor together with the transparent pixel electrode P 1  or P 2 . 
     Further, each of the transparent pixel electrodes P 1  and P 2  is covered by a molecular alignment film  14 , wherein the molecular alignment film  14 , contacting directly with the liquid crystal layer  10 C, induces an alignment of the liquid crystal molecules in the liquid crystal layer  10 C in a predetermined direction. 
     The opposing substrate  10 B, on the other hand, carries a color filter CF in correspondence to the foregoing transparent pixel electrode P 1  or P 2 , and a transparent opposing electrode  15  of ITO, and the like, is provided uniformly on the substrate  10 B. It should be noted that the transparent opposing electrode  15  is covered by another molecular alignment film  16 , and the molecular alignment film  16  induces an alignment of the liquid crystal molecules in the liquid crystal layer  10 C in a desired direction. Further, the substrate  10 B carries thereon an opaque mask BM in correspondence to a gap between a color filter CF and an adjacent color filter CF. 
       FIG. 3  shows an example of the driving signal supplied to the data bus line D 1  or D 2  when driving the liquid crystal panel  10  of  FIGS. 1 and 2 . 
     Referring to  FIG. 3 , a bipolar driving pulse signal is supplied to the data bus line from a driving circuit, wherein it should be noted that the bipolar driving pulse signal changes a polarity thereof between a positive peak level of +V D  and a negative peak level −V D  during the black mode of the liquid crystal panel  10  for representing a black image. Further, a predetermined common voltage V Cs  is supplied to the opposing electrode  15  and the auxiliary electrode Cs from another D.C. voltage source during the black mode. In the white mode of the liquid crystal panel  10  for representing a white image, on the other hand, on the other hand, a bipolar drive pulse signal having an amplitude smaller than a predetermined threshold voltage is supplied to the foregoing data bus line D 1  or D 2 . 
     It should be noted that the foregoing D.C. voltage source for supplying the common voltage V Cs  is provided as an independent unit independent from the driving circuit used for driving the data bus line D 1  or D 2 . The D.C. voltage source provides a voltage of ΔVc as the foregoing common voltage V Cs , wherein the common voltage V Cs  thus set is slightly offset from the central voltage Vc of the bipolar driving pulse signal. It should be noted that the liquid crystal panel  10  of  FIG. 1  or  2  uses a low voltage liquid crystal, characterized by the black mode drive voltage V D  of about 5 V or less, for the liquid crystal layer  10 C. 
     In the liquid crystal panel  10  driven as such, it should be noted that the optimum common voltage V Cs  changes slightly between the black representation mode and the white representation mode. More specifically, the optimum common voltage V Cs  coincides substantially with the central voltage Vc of the bipolar driving pulse signal (ΔVc=0) in the black representation mode, while the optimum common voltage deviates from the central voltage Vc (ΔVc≠0) in the half-tone or white representation mode. As the common voltage V Cs  is applied uniformly to the opposing electrode  15 , it is difficult to change the common voltage adaptively depending on the content of the image to be represented. Thus, it has been practiced to fix the common voltage V Cs  to the optimum voltage at the time of the half-tone representation mode. 
     Meanwhile, the inventor of the present invention has noticed, in a liquid crystal panel using a low voltage liquid crystal for the liquid crystal layer  10 C, that there appears a noticeable flicker in the represented images along the edge part of the auxiliary electrode Cs. In the investigation that constitutes the foundation of the present invention, the inventor has studied this phenomenon and discovered that the flicker is caused as a result of variation of the disclination which is induced in the liquid crystal layer  10 C in the region including the data bus line D 1  or D 2  and the auxiliary electrode Cs by a strong lateral electric field. 
       FIGS. 4A and 4B  show the alignment of the liquid crystal molecules in the liquid crystal layer  10 C and the electric flux of the lateral electric field applied to the liquid crystal layer for the case in which the common voltage V Cs  applied to the auxiliary electrode Cs and the opposing electrode  15  is offset from the central voltage of the bipolar driving pulse signal (V Cs ≠Vc, wherein  FIG. 4A  shows the state in which a voltage of +5V is applied to the data bus line D 1  or D 2  (represented as “D”), while  FIG. 4B  shows the state in which a voltage of −5V is applied to the data bus line D. 
     Referring to  FIG. 4A , it can be seen that a very large lateral electric field is created between the data bus line D and the adjacent auxiliary electrode Cs in the state the voltage of +5V is applied to the data bus line D. Associated with this, there occurs a conspicuous disturbance in the molecular orientation or disclination in the liquid crystal layer  10 C in correspondence to the part between the data bus line D and the auxiliary electrode Cs. As a result of the formation of such a disclination, there is induced a domain structure in the liquid crystal layer  10 C, and a leakage of light occurs in correspondence to the boundary of the domains as represented in  FIG. 4A  by arrows. 
     In the state of  FIG. 4B  in which a voltage of −5V is applied to the data bus line D, on the other hand, the lateral electric field applied to the liquid crystal layer  10 C is substantially reduced and there occurs no substantial formation of domain structure or associated problem of leakage of the light. As the state of FIG.  4 A and  FIG. 4B  appears alternately in correspondence to the polarity of the bipolar driving signal pulse, the leakage light appearing only in the state of  FIG. 4A  causes the flicker. 
     Further, the inventor of the present invention has discovered that there occurs a flow of the liquid molecules in the liquid crystal layer  10 C in the rubbing direction of the molecular alignment film when the value of the common voltage V Cs  of the auxiliary electrode Cs is deviated from the central voltage of the bipolar driving pulse signal. When such a flow occurs in the liquid crystal layer  10 C, there occurs an increase in the thickness of the liquid crystal layer  10 C in correspondence to the part where the liquid crystal molecules are accumulated. When there occurs such a change in the thickness of the liquid crystal layer  10 C, the optical property of the liquid crystal panel  10  is modulated also. 
     Further, in the case a low-voltage liquid crystal is used for the liquid crystal layer  10 C, there tends to occur a sticking of images as a result of the accumulation of impurity ions associated with the flow of the liquid crystal molecules. It should be noted that such a low-voltage liquid crystal, characterized by a low driving voltage, is particularly vulnerable to contamination. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful driving method of a liquid crystal display device wherein the foregoing problems are eliminated. 
     Another and more specific object of the present invention is to provide a method of driving a liquid crystal display device, said liquid crystal display device comprising: a first substrate; a second substrate opposing said first substrate with a gap therebetween; a liquid crystal layer confined in said gap; a thin-film transistor formed on said first substrate; a conductor pattern formed on said first substrate in electrical connection with said thin-film transistor, said conductor pattern supplying an alternate-current driving voltage signal to said thin-film transistor; a pixel electrode provided on said first substrate in electrical connection to said thin-film transistor; an auxiliary electrode formed on said first substrate in the vicinity of said conductor pattern so as to form an auxiliary capacitance with said pixel electrode, said auxiliary electrode being disposed so as to induce a lateral electric field between said auxiliary electrode and said conductor pattern; and an opposing electrode formed on said second substrate; 
     said method comprising the step of: 
     applying to said auxiliary electrode a common voltage substantially equal to a central voltage of said alternate-current driving voltage signal. 
     Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the construction of a liquid crystal display panel of a related art in a plan view; 
         FIG. 2  is a diagram showing the construction of the liquid crystal display device of  FIG. 1  in a cross-sectional view; 
         FIG. 3  is a diagram showing the waveform of a driving signal used in the liquid crystal display device of  FIGS. 1 and 2 ; 
         FIGS. 4A and 4B  are diagrams showing the electric flux line and the alignment of the liquid crystal molecules in a liquid crystal layer used in the liquid crystal display panel of  FIGS. 1 and 2 ; 
         FIG. 5  is a diagram showing the construction of a liquid crystal display device according to a first embodiment of the present invention in a block diagram; 
         FIGS. 6A and 6B  are diagrams showing the electric flux line and the alignment of the liquid crystal molecules in a liquid crystal layer used in the liquid crystal display panel of  FIG. 5 ; 
         FIG. 7  is a diagram showing the possible range of an optimum common voltage according to the first embodiment of the present invention; 
         FIG. 8  is a diagram showing the waveform of another driving voltage signal according to a second embodiment of the present invention; and 
         FIG. 9  is a diagram showing the optimum common voltage corresponding to the driving voltage signal of  FIG. 8  according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     [First Embodiment] 
       FIG. 5  shows the construction of a liquid crystal display device  20  according to a first embodiment of the present invention, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to  FIG. 5 , the liquid crystal display device  20  includes, in addition to the liquid crystal panel  10  described previously with reference to  FIGS. 1 and 2 , a scanning-electrode driving circuit  21  for selectively activating the gate bus lines G 1 -G n  and a signal electrode driving circuit  22  for supplying the A.C. driving signal explained with reference to  FIG. 3  to the data bus lines D 1 -D m , and there is further provided a D.C. voltage source  23  supplying the common voltage V Cs  to the opposing electrode  15  and to the auxiliary electrode Cs as a common voltage supply source.  FIG. 5  further indicates a capacitor PIXEL, wherein it should be noted that the capacitor PIXEL represents the capacitance formed between the transparent pixel electrode P 1  or P 2  and the transparent opposing electrode  15 . 
     The liquid crystal display device  20  of  FIG. 5  is a so-called low-voltage liquid crystal display device and the signal electrode driving circuit supplies a bipolar driving voltage pulse signal similar to the one shown in  FIG. 3  to the data bus lines D 1 -D m  with an amplitude of ±5V. 
     In the present invention, the inventor has discovered that the formation of the disclination becomes substantially the same in the state in which a driving voltage pulse of +5V is applied to the selected data bus line D 1 -D m  and in the state in which a driving voltage pulse of −5V is applied to the selected data bus line D 1 -D m , by setting the common voltage V Cs  supplied from the common voltage source  23 , to be equal to the central voltage (0V) of the bipolar driving voltage pulse signal. As a result, although the leakage of the light itself is not eliminated, the flicker of the leakage light is successfully eliminated. Further, it was discovered that, by setting the voltage V Cs  as set forth above, the sticking of images caused as a result of the flow of the liquid crystal molecules in the liquid crystal layer  11 C, is also suppressed successfully. 
       FIGS. 6A and 6B  show the electric flux in the liquid crystal layer  10 C for the case in which the common voltage V Cs  is set to 0 V. 
     Referring to  FIGS. 6A and 6B , it can be seen that, although the disclination formation in the liquid crystal layer  10 C by the lateral electric field is not avoidable, the degree of the disclination in the liquid crystal layer  10 C is more or the same in the state of  FIG. 6A  in which a driving voltage pulse of +5V is applied to the selected signal electrode D 1 -D m  and in the state of  FIG. 6B  in which a driving voltage pulse of −5V is applied to the selected signal electrode D 1 -D m . As a result, there occurs no substantial flicker in the light that has leaked through the liquid crystal display device. 
     Further, as a result of the reduced disclination formation in the liquid crystal layer  10 C caused by the foregoing setting of the common voltage V Cs , the flow of the liquid crystal molecules is also reduced. As a result, the problem of thickness increase in the liquid crystal layer  10 C and associated problem of local accumulation of the impurity ions in the liquid crystal layer  10 C are effectively reduced. Thus, the present invention reduces the sticking of images in the liquid crystal display device  20  of  FIG. 5  by setting the common voltage V Cs  to be equal to 0V. 
       FIG. 7  shows the flicker formation in the liquid crystal panel  10  having a 12-inch diagonal size for the case in which the common voltage V Cs  is changed variously, wherein  FIG. 7  represents the flicker formation represented in terms of a domain fluctuation DF defined as
   DF =( B   p   −B   n )/ B   p ×100( B   p   &gt;B   n ),  
where B p  represents the leakage of light during the positive frame interval in which a positive drive voltage pulse of +5V is applied, while B n  represents the leakage of light during the negative frame interval in which a negative drive voltage pulse of −5V is applied. Further,  FIG. 7  represents the thickness increase observed for the liquid crystal layer  10 C of the liquid crystal display device  20  of  FIG. 5 , wherein the thickness increase was measured at a point offset from the right upper corner of the 12-inch panel  10  by a distance of 2 cm in the lateral direction and 2 cm in the longitudinal direction. The measurement was made after 20 minutes of operation.
 
     Referring to  FIG. 7 , it can be seen that the domain fluctuation, and hence the flicker formation, increases with increasing deviation of the common voltage V Cs  from the central voltage of the bipolar driving voltage pulse. Further, it can be seen that there appears a liquid crystal flow in the panel diagonal direction along the rubbing direction of the molecular alignment film  14 , wherein the liquid crystal flow appears particularly conspicuously in the black representation mode in which the amplitude of the driving voltage pulse signal applied to the liquid crystal panel  10  becomes maximum. As a result, the cell thickness of the liquid crystal layer  10 C is also increased. As explained already, such an increase in the liquid crystal cell thickness tends to invite an accumulation of impurity ions contained in the liquid crystal, and the contamination of the liquid crystal by such an accumulation of the impurity ions induces a conspicuous sticking in the represented images. 
     In  FIG. 7 , it can be seen that, in a region A in which the deviation Δ C  of the common voltage V Cs  is less than about 0.025V, in other words in the region A in which the foregoing deviation ΔV C  is less the about 1/20 of the voltage amplitude (5V) of the drive voltage pulse, the domain fluctuation DF is less than about 10% and no substantial sticking of images is recognized. On the contrary, in a region B in which the foregoing deviation ΔV C  exceeds 0.25V but is smaller than about 2V, a linear sticking was recognized. Further, in a region C in which the deviation ΔV C  exceeds about 2V, the domain fluctuation exceeds 50% and a considerable flicker is recognized. Further, the thickness increase of the liquid crystal layer  10 C reaches as much as 0.025 μm. In this case, the liquid crystal molecules are caused to flow in the liquid crystal layer  10 C with a velocity such that the liquid crystal molecules move by a distance of more than 80 μm during the interval of 24 hours. 
     From the foregoing, it is preferable to set the common voltage V Cs  in the region B in which the deviation ΔV C  with respect to the amplitude center of the bipolar driving pulse voltage signal is less than about 50% of the maximum voltage amplitude for the black representation mode, more preferably in the region A in which the deviation ΔV C  is less than about 10%. In the region B, it should be noted that the liquid crystal molecules in the liquid crystal layer  10 C moves over a distance of 80 μm or less during the interval of 24 hours. 
     It should be noted that the foregoing result is not only pertinent to the liquid crystal panel of the 12-inch size but is applicable also to general liquid crystal panels having a diagonal size of 10-13 inches. 
     [Second Embodiment] 
     In the foregoing embodiment, it was assumed that the drive voltage pulse signal supplied to the data bus lines D 1 -D m  is a bipolar voltage pulse having a central voltage of 0V. The present invention, however, is never limited to such a particular driving signal but is applicable to the case in which the driving voltage pulse signal includes a D.C. voltage offset as represented in FIG.  8 . 
     Referring to  FIG. 8 , the driving voltage pulse signal has a voltage amplitude of ±2.5V in the black representation mode, and the driving voltage pulse signal is supplied to the data bus line D 1 -D m  together with a D.C. offset of 2.37V. Thereby, an optimum common voltage V Cs  of 2.37V, which is substantially equal to the foregoing D.C. voltage offset, is applied to the auxiliary electrode Cs and to the opposing electrode  15 . 
     In the driving process noted above, it should be noted that the optimum common voltage V Cs  may be different in the black representation mode and in the white representation mode. In the example of  FIG. 8 , the common voltage V Cs  optimized for the case in which the amplitude of the driving voltage pulse signal is set smaller than the threshold voltage of image representation, does not coincide with the common voltage V Cs  of 2.37 V optimized for the black representation mode. In fact, the optimized common voltage for the foregoing case takes a value of 2.42V rather than 2.37V.  FIG. 9  represents the relationship between the optimum common voltage V Cs  and the gradation level for two different liquid crystal panels A and B. 
     In view of the fact that the common voltage V Cs  is applied to the entirety of the liquid crystal panel, it is difficult to change the optimum common voltage V Cs  adaptively depending on the gradation level to be represented. In the present invention, therefore, the optimum common voltage V Cs  is optimized for the black representation mode in which the flow of the liquid crystal molecules in the liquid crystal layer  10 C appears most significantly. 
     In the description heretofore, the present invention is described with reference to the so-called H-type Cs liquid crystal panel represented in  FIGS. 1 and 2 . However, the present invention is by no means limited to such a specific construction of the liquid crystal panel but is applicable to other liquid crystal panels such as “independent Cs type” or “Cs-on-gate type.” 
     Further, the present invention is not limited to the embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the invention.