Patent Publication Number: US-6667732-B1

Title: Method of driving liquid crystal device, liquid crystal device, and electronic instrument

Description:
TECHNICAL FIELD 
     The present invention relates to a method of driving a liquid crystal device using a simple matrix panel. The present invention also relates to a liquid crystal device and an electronic instrument using the liquid crystal device, such as OA equipment and measuring instruments. 
     BACKGROUND OF ART 
     In a liquid crystal device using a simple matrix panel, a method of changing a bias ratio according to the power supply voltage or a method of changing a bias ratio when changing a display duty has been employed. The display duty must be changed when changing from a full screen display to a partial display, for example. 
     In conventional liquid crystal devices, a maximum voltage in a liquid crystal driving voltage generated by raising the power supply voltage is divided using a resistance dividing circuit, thereby generating various levels of liquid crystal driving voltages. 
     The bias ratio must be changed when changing the display duty in order to maximize an operation margin. Conventionally, the resistance value of a resistance element in the resistance dividing circuit is designed to be variable. An electric current flowing through the resistance dividing circuit changes when changing the resistance value, whereby the levels of each liquid crystal driving voltage change. Therefore, conventional technology has a problem in that contrast must be adjusted each time the display duty is changed. 
     Accordingly, an object of the present invention is to provide a method of driving a liquid crystal device which can eliminate the need for adjustment of contrast by the user when changing the display duty, a liquid crystal device, and an electronic instrument. 
     Another object of the present invention is to provide a method of driving a liquid crystal device which can easily display a partial display and which enables a partial display with low power consumption, a liquid crystal device, and an electronic instrument. 
     DISCLOSURE OF THE INVENTION 
     One aspect of the present invention provides, a method of driving a liquid crystal device comprising a first substrate on which a plurality of electrodes are formed, a second substrate on which a plurality of segment electrodes are formed, and a liquid crystal interposed between the first substrate and the second substrate, and applying a voltage which changes into at least an ON voltage and an OFF voltage to a pixel formed at each intersection point of the common electrodes and the segment electrodes, the method comprising: 
     a first driving step of driving the liquid crystal device under a condition of a first duty and a first bias ratio; and 
     a second driving step of driving the liquid crystal device under a condition of a second duty and a second bias ratio, 
     wherein the first duty and the second duty and the first bias ratio and the second bias ratio are set so that a root-mean-square voltage applied to the pixel when the intermediate voltage between the ON voltage and the OFF voltage is applied to the pixel in the first driving step equals a root-mean-square voltage applied to the pixel when the intermediate voltage between the ON voltage and the OFF voltage is applied to the pixel in the second driving step. 
     According to this aspect of the present invention, the bias ratio is changed when changing the display duty so that the intermediate values between the ON voltage and the OFF voltage are almost equal. This allows the medium concentration to be almost constant before and after changing the duty. Therefore, the user does not have to adjust the contrast each time the duty is changed. 
     This aspect of the present invention can be applied to both one-line selection driving and multi-line driving. 
     Another aspect of the present invention provides, a method of driving a liquid crystal device comprising a first substrate on which a plurality of electrodes are formed, a second substrate on which a plurality of segment electrodes are formed, and a liquid crystal interposed between the first substrate and the second substrate, and applying a voltage which changes into at least an ON voltage and an OFF voltage to a pixel formed at each intersection point of the common electrodes and the segment electrodes, the method comprising: 
     a first driving step of driving the liquid crystal device under a condition of a first duty n 1  and a first bias ratio c 1 ; and 
     a second driving step of driving the liquid crystal device under a condition of a second duty n 2  and a second bias ratio c 2 , 
     wherein the first duty and the second duty and the first bias ratio and the second bias ratio are set to satisfy n 1 −c 1   2 =n 2 ·c 2   2 . 
     According to this other aspect, the bias ratio is changed from c 1  to c 2  when changing the display duty from n 1  to n 2  so that the intermediate values between the ON voltage and the OFF voltage are almost equal. The condition required for this is to satisfy the relation n 1 ·c 1   2 =n 2 ·c 2   2  according to a Ruckmongathan&#39;s equation as described later. This aspect can be applied to both one-line selection driving and multi-line driving. 
     The first driving step may comprise a step of raising a maximum signal potential supplied to the segment electrodes to generate a selection potential to be supplied to the common electrodes. The second driving step may comprise a step of stopping the raising step and supplying the maximum signal potential supplied to the segment electrodes to the common electrodes as the selection potential. 
     This configuration allows the raising operation can be stopped in the second driving step, thereby reducing power consumption. Moreover, since the potential for the segment electrodes is supplied to the common electrodes, there is no need to generate other liquid crystal drive potentials. 
     When a voltage-raising multiplying factor is “k” in the raising step performed in the first driving step, the relation n 2 =n 1 ·(1/k) 2  may be realized. This is because the bias ratios n 1  and n 2  and the voltage-raising multiplying factor “k” in the raising step satisfy the relation c 1 /c 2 =1/k. 
     A further aspect of the present invention provides, a method of driving a liquid crystal device comprising a first substrate on which a plurality of electrodes are formed, a second substrate on which a plurality of segment electrodes are formed, and a liquid crystal interposed between the first substrate and the second substrate, and applying a voltage which changes into at least an ON voltage and an OFF voltage to a pixel formed at each intersection point of the common electrodes and the segment electrodes, the method comprising: 
     a first driving step of driving the liquid crystal device under a condition of a first duty and a first bias ratio; and 
     a second driving step of driving the liquid crystal device under a condition of a second duty lower than the first duty and a second bias ratio, 
     wherein the first duty and the second duty and the first bias ratio and the second bias ratio are set so that a root-mean-square voltage applied to the pixel when the ON voltage is applied to the pixel in the first driving step is equal to or less than a root-mean-square voltage applied to the pixel when the ON voltage is applied to the pixel in the second step, and a root-mean-square voltage applied to the pixel when the OFF voltage is applied to the pixel in the first driving step is equal to or more than a root-mean-square voltage applied to the pixel when the OFF voltage is applied to the pixel in the second step. 
     According to this further aspect of the present invention, the bias ratios are selectively changed so that the range between the ON voltage and the OFF voltage at a high duty (first duty) includes the range between the ON voltage and the OFF voltage at a low duty (second duty). This allows the contrast obtained in the low-duty driving to be higher than the contrast obtained in the high-duty driving Therefore, the user does not have to adjust the contrast each time the display duty is changed. This aspect can be applied to both one-line selection driving and multi-line driving. 
     A liquid crystal device according to a still further aspect of the present invention comprises; 
     a panel including a first substrate on which a plurality of electrodes are formed, a second substrate on which a plurality of segment electrodes are formed, and a liquid crystal interposed between the first substrate and the second substrate; 
     a segment driver which supplies a voltage to the segment electrodes; 
     a common driver which supplies a voltage to the common electrodes; and 
     a power supply circuit which supplies a liquid crystal driving voltage to the common driver and the segment driver, 
     wherein the segment driver includes a circuit of which duty changes between a first duty n 1  and a second duty n 2  (n 2 &lt;n 1 ), 
     wherein the power supply circuit comprises a circuit which sets a bias ratio at a first bias ratio c 1  when the first duty n 1  is set, and sets a bias ratio at a second bias ratio c 2  when the first duty n 2  (c 2 &gt;c 1 ) is set, and 
     wherein the first duty and the second duty and the first bias ratio and the second bias ratio are set to satisfy n 1 ·c 1   2 =n 2 ·c 2   2 . 
     The drive method according the above-described other aspect of the present invention is suitably applied to this liquid crystal device. 
     In addition, the common driver and the power supply circuit may be included in a single-chip IC. 
     An electronic instrument according to still another aspect of the present invention has the above-described liquid crystal device. The liquid crystal device used as a display for the electronic instrument may be driven at a high duty during a normal operation mode, and driven at a low duty when displaying a partial display during a wait mode. In the case of a portable telephone, in particular, power consumption can be reduced by displaying an icon or the like on only part of the display screen in a wait mode without displaying in other areas. The electronic instrument according to the present invention can be applied to any type of electronic instrument which requires partial display in the low-duty driving. The electronic instrument is particularly suitable as a mobile apparatus for which low power consumption is needed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a bias ratio in driving at a high duty using an voltage averaging method. 
     FIG. 2 shows a bias ratio in driving at a low duty using an voltage averaging method. 
     FIG. 3 shows a bias ratio in driving at a high duty using a principle drive method. 
     FIG. 4 shows a bias ratio in driving at a low duty using a principle drive method. 
     FIG. 5 shows a bias ratio in driving at a high duty using a four-line selection method. 
     FIG. 6 shows a bias ratio in driving at a low duty using a four-line selection method. 
     FIG. 7 shows the relation between a voltage and luminance of a liquid crystal panel at each operation point in high-duty driving and low-duty driving in a third embodiment of the present invention. 
     FIG. 8 shows the relation between a voltage and luminance of a liquid crystal panel at operation points in a fourth embodiment using three duties. 
     FIG. 9 is a view schematically showing a liquid crystal device used in each embodiment of the present invention. 
     FIG. 10 is a view showing a liquid crystal waveform using an voltage averaging method. 
     FIG. 11 is a view showing a liquid crystal waveform using a principle drive method. 
     FIG. 12 is a circuit diagram of the segment driver IC shown in FIG.  9 . 
     FIG. 13 is a circuit diagram of the common driver IC shown in FIG.  9 . 
     FIG. 14 is a view showing the power supply circuit in the common driver IC shown in FIG.  13 . 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The present invention will be described below with reference to drawings. 
     Outline of Example Device 
     A liquid crystal device to which a drive method as described later is applied will be described. 
     FIG. 9 shows a simple matrix panel  10 . The panel  10  has a first substrate (not shown) with common electrodes  12  formed thereon, a second substrate (not shown) with segment electrodes  14  formed thereon, and liquid crystals (not shown) interposed between the first substrate and the second substrate. 
     A common driver IC  200  which drives the common electrodes  12 , a segment driver IC  100  which drives the segment electrodes  14 , and an MPU  300  which outputs commands and data to the segment driver IC  100  are also shown in FIG.  9 . This liquid crystal device is installed in a portable telephone, for example. The liquid crystal device displays a full screen display on the panel  10  in a normal operation mode, and displays only part of the panel  10  in a wait mode. Therefore, the liquid crystal device is driven at a high duty in the normal operation mode, and driven at a low duty in the wait mode. 
     In the simple matrix panel  10 , pixels are formed at each intersection point of the common electrodes  12  and the segment electrodes  14 . As a drive waveform supplied to the common electrodes  12  and the segment electrodes  14  of the panel  10 , two types of drive waveforms are conventionally known. One of them is a drive waveform using a voltage averaging method shown in FIG.  10 . The other is a drive waveform using a principle drive method (also called APT method) shown in FIG.  11 . In FIGS. 10 and 11, bold lines indicate drive waveforms of the segment electrodes and thin lines indicate drive waveforms of the common electrodes. The differential voltage between the voltages applied to each electrode is applied to the pixels using either the drive waveform shown in FIG. 10 or the drive waveform shown in FIG.  11 . The same voltage is applied to the liquid crystals to be driven using both methods while only the absolute potential applied to each electrode varies. 
     First Embodiment 
     The root-mean-square voltage of the voltage applied to one pixel of the simple matrix panel  10  is represented by the following equation developed by Ruckmongathan. 
     n: Drive duty 
     L: Simultaneous selection number 
     c: Bias ratio 
     V: Selection voltage              RMS   =           LV   2         n                  nc   2     ±     2      c       +   1                     Λ             (   1   )                         
     The sign “±” for 2c in the root is “+”, in a pixel turned on and is “−” in a pixel turned off. The principle of this equation is described in detail in Ruckuongathan, T. N., “A GENERALIZED ADDRESSING TECHNIQUE FOR RMS RESPONDING MATRIX LCDS” 1988 INTERNATIONAL DISPLAY RESEARCH CONFERENCE, pp. 80 to 85. Therefore, description thereof is omitted. 
     Substituting 1 for the simultaneous selection number L in equation (1) yields the following equation.                RMS        (     L   =   1     )       =       V     n                  nc   2     ±     2      c       +   1                     Λ             (   2   )                         
     Equation (2) represents the root-mean-square voltage at the time of one-line selection driving using the voltage averaging method (Kawakami method) or principle drive method (APT method). 
     The display modes in the liquid crystal device used in a portable telephone include a normal operation made in which the full screen (for example, 100 lines) of the panel  10  shown in FIG. 9 is driven, and a wait mode in which an icon or the like is displayed on part (for example, lines 1 to 25) of the panel  10  shown in FIG.  9 . In the wait mode, lines 26 to 100 are not driven. Therefore, the liquid crystals are driven at a high duty in the normal operation mode, and driven at a low duty in the wait mode. 
     FIG. 1 shows a bias ratio of the power supply when the liquid crystal device is driven at a high duty using the voltage averaging method shown in FIG.  10 . FIG. 2 shows the bias ratio of the power supply when the liquid crystal device is driven at a low duty in the same manner as in FIG.  1 . FIGS. 3 and 4 show the bias ratios of the power supply using the principle drive method (APT method) shown in FIG.  11 . FIG. 3 shows the bias ratio of the power supply when the liquid crystal device is driven at a high duty. FIG. 4 shows the bias ratio of the power supply when the liquid crystal device is driven at a low duty. 
     The bias ratio in equation (1) means a ratio of the half value of a unit signal voltage amplitude dependent on one pixel to the half value of a selection voltage amplitude. The half value S (L=1) of the signal voltage amplitude in one-line selection driving shown in FIGS. 1 to  4  is the unit signal voltage amplitude since L equals 1, which is expressed as follows. 
     
       
           S ( L= 1)= L·c·V=c·V   Λ(3) 
       
     
     In FIGS. 1 and 3, ±V H  shows a common voltage amplitude in high-duty driving. In FIGS. 2 and 4, ±V L  shows a common voltage amplitude in low-duty driving. In FIGS. 1 to  4 , S shows the half value of the segment voltage amplitude in one-line selection driving. 
     The bias ratio c in equation (2) means the ratio represented by (half value of segment voltage amplitude)/(half value of common voltage amplitude) at the time of one-line selection driving. The bias ratio c H  equals S/V H  in the high-duty driving shown in FIGS. 1 and 3. The bias ratio c L  equals S/V L  in the low-duty driving shown in FIGS. 2 and 4. 
     Equation (1) is also applied to multi-line selection driving, which will be described later. 
     Substituting duty n H , bias ratio c H , and selection voltage V H  in the high-duty driving shown in FIGS. 1 and 3 in equation (2) yields the following equation.                RMS        (       L   =   1     ,     n   H       )       =         V   H         n   H                      n   H     ·       (     c   H     )     2       ±     2        c   H         +   1                     Λ             (   4   )                         
     Substituting duty n L , bias ratio c L , and selection voltage V L  in the low-duty driving shown in FIGS. 2 and 4 in equation (2) yields the following equation.                RMS        (       L   =   1     ,     n   L       )       =         V   L         n   L                      n   L     ·       (     c   L     )     2       ±     2        c   L         +   1                     Λ             (   5   )                         
     Equation (3) is expressed as follows using the signs in the high-duty driving and low-duty driving. 
     
       
           S (L=1,n H )= c   H   ·V   H    Λ(6) 
       
     
     
       
           S (L=1,n 1 )= c   L   ·V   L   Λ(7) 
       
     
     The intermediate voltages of the ON voltage and the OFF voltage will be considered. Removing ±2c H  and ±2c L  from equations (4) and (5) respectively yields the following equations.                  RMS   MID          (       L   =   1     ,     n   H       )       =         V   H         n   H                    n   H     ·       (     c   H     )     2       +   1                     Λ             (   8   )                   RMS   MID          (       L   =   1     ,     n   L       )       =         V   L         n   L                    n   L     ·       (     c   L     )     2       +   1                     Λ             (   9   )                         
     The intermediate voltages RMS MID  of the root-mean-square voltages between the ON voltage and the OFF voltage must be equal in both the high-duty driving and low-duty driving. Therefore, equation (8) equals equation (9). Substituting the relations in equations (6) and (7) therein yields the following equation.                  S       c   H            n   H                      n   H     ·       (     c   H     )     2       +   1         =       S       c   L            n   L                      n   L     ·       (     c   L     )     2       +   1                     Λ             (   10   )                         
     Raising both sides of equation (10) to the second power and simplifying the equation yield the following equation. 
     
       
           c   L   2   ·n   L   =c   H   2   ·n   H   Λ(11) 
       
     
     Equation (11) indicates the following. Specifically, the intermediate values between the ON voltage and the OFF voltage applied to the pixels do not change by maintaining the relation between the display duties and the bias ratios so that the products of the display duties (n L , n H ) and the bias ratios (c L , c H ) raised to the second power do not change (n·c 2 =constant). 
     For example, in the case of driving 100 lines (n H =100) at a bias ratio c H  of {fraction (1/10)} and then driving only 10 lines (n L =10) by an external signal at a bias ratio c L  of 0.316 . . . (1/square root of 10), the half-tone display becomes constant when displaying a partial display by changing the display duty. Therefore, the user does not have to adjust the contrast. 
     Second Embodiment 
     FIG. 5 shows a bias ratio of the power supply at a high duty in multi-line selection driving with the simultaneous selection number L in equation (1) being 4. FIG. 6 shows a bias ratio of the power supply at a low duty for displaying partial display in the same multi-line selection driving as in FIG.  5 . 
     Substituting 4 for the simultaneous selection number L in equation (1) yields the following equation. The simultaneous selection number L may be a number other than 4, which is an example.                RMS        (     L   =   4     )       =         2   ·   V       n                  nc   2     ±     2      c       +   1                     Λ             (   12   )                         
     Equation (12) represents the root-mean-square voltage in a 4-line simultaneous selection drive method. In the 4-line simultaneous selection drive method, five levels of signal voltages (PV 2 , PV 1 , VC, MV 1 , MV 2 ) shown in FIG. 5 are required. The signal voltage amplitude S (L=4) represents voltages between PV 2  and VC and between VC and MV 2  shown in FIG.  5 . Since the bias ratio c means the ratio of the half value of the unit signal voltage amplitude dependent on one pixel to the selection voltage amplitude, the signal voltage amplitude S (L=4) is represented by the following equation. 
     
       
           S (L=4)=L· c·V =4 ·c·V   Λ(13) 
       
     
     As in the case obtained equations (4) to (7), substituting high duty n H , low duty n L , and the like in equations (12) and (13) respectively yields the following equations (14) to (17).                RMS        (       L   =   4     ,     n   H       )       =         2   ·     V   H           n   H                      n   H     ·       (     c   H     )     2       ±     2        c   H         +   1                     Λ             (   14   )                 RMS        (       L   =   4     ,     n   L       )       =         2   ·     V   L           n   L                      n   L     ·       (     c   L     )     2       ±     2        c   L         +   1                     Λ             (   15   )                         S (L=4,n H )=4 ·c   H   ·V   H   Λ(16) 
     
       
           S (L=4,n L )=4 ·c   L   ·V   L   Λ(17) 
       
     
     The intermediate voltages between the ON voltage and the OFF voltage will be considered in the same manner as in the first embodiment. Removing ±2c H  and ±2c L  from equations (14) and (15) respectively yields the following equations.                  RMS   MID          (       L   =   4     ,     n   H       )       =         2   ·     V   H           n   H                    n   H     ·       (     c   H     )     2       +   1                     Λ             (   18   )                   RMS   MID          (       L   =   4     ,     n   L       )       =         2   ·     V   L           n   L                    n   L     ·       (     c   L     )     2       +   1                     Λ             (   19   )                         
     As described above, the intermediate voltages RMS MID  of the root-mean-square voltages are equal if equation (18) equals equation (19). Substituting equations (16) and (17) therein yields the following equation.                  S       c   H            n   H                      n   H     ·       (     c   H     )     2       +   1         =       S       c   L            n   L                      n   L     ·       (     c   L     )     2       +   1                     Λ             (   20   )                         
     Raising both sides of equation (20) to the second power and simplifying the equation yields the following equation. 
       c   L   2   ·n   L   =c   H   2   ·n   H   Λ(21) 
     Therefore, the intermediate values RMS MID  between the ON voltage and the OFF voltage applied to the pixels do not change by maintaining the relation n·c 2  =constant in the multi-line selection driving in the second embodiment in the same manner as in the one-line selection driving in the first embodiment. 
     For example, when driving 100 lines (n H =100) at a bias ratio c H  of {fraction (1/10)} with the simultaneous selection number L being 10, and then driving only 10 lines (n L =10) by an external signal at a bias ratio c L  of 0.316 . . . (1/square root of 10), the user does not have to adjust the contrast when changing the display duty to display a partial display. 
     Third Embodiment 
     Examples 1 and 2 take into consideration only the intermediate values between the ON voltage and the OFF voltage. The ratio of the ON voltage to the OFF voltage (hereinafter called “operation margin”) also varies. The third embodiment illustrates a method of setting conditions while taking into consideration the ON voltage and the OFF voltage. 
     Modifying-equation (1) while taking into consideration S=L·c·V yields the following equation.              RMS   =       S     c   ·     n     ·     L                    nc   2     ±     2      c       +   1                     Λ             (   22   )                         
     As shown in FIG. 7, suppose that the root-mean-square voltage when applying the ON voltage to the liquid crystals at a bias ratio of c 1  and a display duty of n 1  is RMS (ON 1 ), and the root-mean-square voltage when applying the OFF voltage to the liquid crystals is RMS (OFF 1 ). Suppose that the root-mean-square voltage when applying the ON voltage to the liquid crystals at a bias ratio of c 2  and a display duty of n 2  is RMS (ON 2 ), and the root-mean-square voltage when applying the OFF voltage to the liquid crystals is RMS (OFF 2 ). 
     FIG. 7 is a characteristic view showing the relation between the voltage and luminance of a liquid crystal panel. The luminance is indicated in practice by a unit such as nit or candela. The unit is omitted in FIG. 7, in which the luminance is indicated by dimensionless numbers. FIG. 7 shows an example in which the luminance increases as the voltage increases. The present invention may be applied to liquid crystal panels in which the luminance decreases as the voltage increases. 
     In the liquid crystal panel having the characteristics shown in FIG. 7, liquid crystals react when the root-mean-square voltage becomes more than 2.0 V, thereby increasing the luminance. The luminance is saturated when the root-mean-square voltage becomes 2.4 V. 
     A contrast of 60 to 30 (contrast ratio=2) is obtained in the driving at a bias ratio of c 1  and a display duty of n 1 . Therefore, a contrast ratio of 2 or more is obtained after changing to a partial display of a bias ratio of c 2  and a display duty of n 2  by maintaining two relations RMS (ON 1 )≦RMS (ON 2 ) and RMS (OFF 1 )≧RMS (OFF 2 ). 
     This will be described in more detail using equations. 
     The root-mean-square voltages RMS (ON 1 ), RMS (ON 2 ), RMS (OFF 1 ), and RMS (OFF 2 ) shown in FIG. 7 are expressed as follows.                RMS        (   ONI   )       =       S       c   1     ·       n   1       ·     L                    n   1          c   1   2       +     2        c   1       +   1                     Λ             (   23   )                 RMS        (   OFF1   )       =       S       c   1     ·       n   1       ·     L                    n   1          c   1   2       -     2        c   1       +   1                     Λ             (   24   )                 RMS        (   ON2   )       =       S       c   2     ·       n   2       ·     L                    n   2          c   2   2       +     2        c   2       +   1                     Λ             (   25   )                 RMS        (   OFF2   )       =       S       c   2     ·       n   2       ·     L                    n   2          c   2   2       -     2        c   2       +   1                     Λ             (   26   )                         
     If the root-mean-square voltages of the ON voltages are equal, equation (23) equals equation (25), thereby yielding the following equation.                  1       c   1     ·       n   1                      n   1          c   1   2       +     2        c   1       +   1         =       1       c   2     ·       n   2                      n   2          c   2   2       +     2        c   2       +   1                     Λ             (   27   )                         
     If the root-mean-square voltages of the OFF voltages are equal, equation (24) equals equation (26), thereby yielding the following equation.                  1       c   1     ·       n   1                      n   1          c   1   2       -     2        c   1       +   1         =       1       c   2     ·       n   2                      n   2          c   2   2       -     2        c   2       +   1                     Λ             (   28   )                         
     Since the simultaneous selection number L is removed in equations (27) and (28), these equations apply to both one-line selection driving and L-line (L≧2) simultaneous selection driving. 
     Simplifying equation (27) yields the following equation which indicates the condition in which the ON voltages are equal.                  n   2       n   1       =           c   1   2     ·     (         +   2          c   2       +   1     )           c   2   2     ·     (         +   2          c   2       +   1     )                       Λ             (   29   )                         
     For example, when changing the bias ratios c 1  and c 2  to ⅛ and ¼, respectively, the relation between the duties n 1  and n 2  in equation (29) is expressed as follows.                  n   2       n   1       =             (     1   8     )     2     ·     (       2        1   4       +   1     )             (         1           4         )     2     ·     (       2        1   8       +   1     )         =     0.3                 Λ               (   30   )                         
     Specifically, the duty ratio may be set at 30% when the bias ratio is determined as described above. For example, if n 1  equals 100, n 2  equals 30. 
     Simplifying equation (28) yields the following equation which indicates the condition in which the ON voltages are equal.                  n   2       n   1       =           c   1   2     ·     (         -   2          c   2       +   1     )           c   2   2     ·     (         -   2          c   1       +   1     )                       Λ             (   31   )                         
     For example, substituting ⅛ and ¼ for c 1  and c 2  respectively yields the following equation.                  n   2       n   1       =             (     1   8     )     2     ·     (         -   2          1   4       +   1     )             (     1   4     )     2     ·     (         -   2          1   8       +   1     )         =             (     1   8     )     2     ·     (     2   4     )             (     1   4     )     2     ·     (     3   4     )         =       1   6     =     0.166                 Λ                   (   32   )                         
     Specifically, the duty ratio may be set at 17% when the bias ratio is determined as described above. When changing from driving at a display duty n 1  of 100 and a bias ratio of ⅛ to driving at a bias ratio c 2  of ¼, a contrast higher than that before changing can be secured without adjusting the contrast by setting the display duty n 2  between 30 and 17. 
     When the duty ratio is previously determined, the bias ratio is set as follows. For example, a case of changing the duty ratio from n 1 =100 to n 2 =50 is described below. Suppose that the bias ratio c 1  is {fraction (1/10)} when the duty n 1  is 100. The condition in which the ON voltages are equal is represented by the following quadratic equation.                50   100     =             (     1   10     )     2     ·     (         +   2          c   2       +   1     )           c   2   2     ·     (         +   2          1   10       +   1     )                       Λ             (   33   )                         
     When equation (33) is solved, c 2  equals 0.146837. 
     The condition in which the OFF voltages are equal is represented by the following quadratic equation.                50   100     =             (     1   10     )     2     ·     (         -   2          c   2       +   1     )           c   2   2     ·     (         -   2          1   10       +   1     )                       Λ             (   34   )                         
     When equation (34) is solved, c 2  equals 0.135078. 
     As described above, when changing the driving at a display duty n l  of 100 and a bias ratio of {fraction (1/10)} to driving at a duty ratio n 2  of 50, a contrast higher than that before changing can be secured without adjusting the contrast by setting the bias ratio c 2  between 0.146837 and 0.135078. 
     Fourth Embodiment 
     The third embodiment illustrates the case of changing the display driving between two display duties. The case of setting the bias ratio conditions when employing two display duties among three or more duties will be described here. In this case, the user does not have to adjust the contrast by setting the bias ratio in the same manner as in the third embodiment. 
     FIG. 8 shows a root-mean-square voltage RMS (ON 3 ) of the ON voltage and a root-mean-square voltage RMS (OFF 3 ) of the OFF voltage at a bias ratio of c 3  and a display duty of n 3 , in addition to the root-mean-square voltages RMS (ON 1 ), RMS (OFF 1 ), RMS (ON 2 ), and RMS (OFF 2 ) shown in FIG.  7 . 
     The conditions required in the third embodiment are RMS (ON 1 )≦RMS (ON 2 ) and RMS (OFF 1 )≧RMS (OFF 2 ). 
     In the same manner as in the third embodiment, the conditions required between the display at a bias ratio of c 1  and a display duty of n 1  and the display at a bias ratio of c 1  and a display duty of n 1  are RMS (ON 1 )≦RMS (ON 3 ) and RMS (OFF 1 )≧RMS (OFF 3 ). 
     The conditions required between the display at a bias ratio of c 2  and a display duty of n 2  and the display at a bias ratio of c 3  and a display duty of n 3  are EMS (ON 2 )≦RMS (ON 3 ) and RMS (OFF 2 )≧RMS (OFF 3 ). 
     The user does not have to adjust the contrast by setting the relation between two duties among three or more duties in the above manner. 
     EXAMPLE 5 
     Example 5 illustrates a method of driving the liquid crystal device by changing the duty with reference to details of the segment driver IC  100  and the common driver IC  200  shown in FIG.  9 . 
     FIG. 12 shows the segment driver IC  100 . In FIG. 12, an MPU interface  102 , an input-output buffer  104 , and an output buffer  106  are provided as input-output circuits of the IC  100 . A bus holder  112 , a command decoder  114 , a status circuit  116 , an oscillator circuit  118 , and a timing generation circuit  120  are connected to an internal bus  110  which is connected to the input-output circuits  102 ,  104 , and  106 . 
     The contents of commands designating either the normal operation mode or the wait mode from the MPU  300  are input to the input-output buffer  104  as 8-bit data after the signal to an A 0  terminal of the MPU interface  102  has become LOW, and are decoded by a command decoder  114 . The display duty is set by the counting of a reference clock from the oscillator circuit  118  by the display timing generation circuit  120 . 
     Therefore, the display timing generation circuit  120  sets a high duty in the normal operation mode and sets a low duty in the wait mode according to the commands input through the internal bus  110 . Display data from a display data RAM  130  are read out according to the duty set by the display timing generation circuit  120 . In addition, the liquid crystal device may be driven with low power consumption in the low duty driving, in particular, by lowering the frequency of the reference clock from the oscillator circuit  118 . 
     A page address decoder  132  and a column address decoder  134  are provided for the display data RAM  130  to read out the display data, and the read-out address of the display data RAM  130  is designated. An LCD display address control circuit  140  is connected to the page address decoder  132 . A column address control circuit  142  is connected to the column decoder  134 . An MPU page address control circuit  144  connected to the page address decoder  132  is used to read and write the contents of the display data RAM  130  on the basis of the commands from the MPU  300  shown in FIG.  9 . 
     Data is read out from the display data RAM  130  or written therein through an I/O buffer  136  on the basis of the commands from the MPU  300 . The page address at the time of reading and writing is designated by the page address register  146 . 
     The display data read out from the display data RAM  130  is latched by a display data latch circuit  150 , decoded by a decode circuit  152 , and supplied to the segment electrodes  14  shown in FIG. 9 through a liquid crystal drive circuit  154 . The segment driver IC  100  carries out the multi-line selection drive method with the simultaneous selection number L of 4. Therefore, 5 level potentials including PV 1 , PV 2 , VC, HV 1 , and MV 2  shown in FIG. 5 are supplied to the segment electrodes  14  in the normal operation mode. The supply potentials in the wait mode will be described later. 
     Next, the common driver  200  shown in FIG. 9 will be described with reference to FIG.  13 . 
     The common driver IC  200  shown in FIG. 13 has a common drive circuit  210  and a power supply circuit  220 . The common drive circuit  210  has a bidirectional shift register  212 , a decode circuit  214  which decodes the output therefrom, and a liquid crystal drive circuit  216  which supplies a voltage to the common electrodes  12  shown in FIG. 9 in accordance with the decoding results. The bidirectional shift register  212  enables scanning from both the top and the bottom of the screen. The scanning direction is controlled by the output from a shift direction control circuit  218  which inputs commands from the MPU  300  on the scanning direction through the segment driver IC  100 . 
     The power supply circuit  220  generates  7  levels of potentials (PV 3 , PV 2 , PV 1 , VC, MV 1 , MV 2 , MV 3 ) shown in FIG. 5. A first voltage raising auxiliary circuit  222 , a first voltage raising circuit  224 , an electronic volume  226 , a second voltage raising circuit  228 , and third and fourth voltage raising circuits  230  and  232  are provided therefor in the power supply circuit  220  shown in FIG.  13 . The first to fourth voltage raising circuits are formed of charging pumps. A basic timing generation circuit  234  and first to third voltage raising timing generation circuits  236 ,  238 , and  240  are provided in the power supply circuit  220  to generate voltage raising timing in each voltage raising circuit. A potential generation circuit  242 , a potential switching circuit  244 , and a discharge circuit  246  are also provided in the power supply circuit  220 . The potential generation circuit  242  generates the potentials PV 1  and MV 1  by lowering the potentials PV 2  and MV 2  from the second voltage raising circuit  228 . The potential switching circuit  244  switches potentials output from terminals PV 3  and MV 3 . The potential switching circuit  244  outputs the potentials MV 3  and PV 3  from the third and fourth voltage raising circuits  230  and  232  in the normal operation mode, and outputs the potentials PV 2  and MV 2  according to the output from the second voltage raising circuit  228  in the wait mode. The wait mode is designated by the commands from the MPU  300 . Specifically, the commands designating the wait mode are output from the output buffer  106  of the segment driver IC  100  shown in FIG.  12 . This causes the logic of a power saving terminal (/PSAVE) of the common driver IC  200  to be HIGH, for example, thereby entering the wait mode. The signals from the power saving terminal are also input to the third voltage raising timing generation circuit  240 . The operations of the third and fourth voltage raising circuits  230  and  232  are stopped in the wait mode according to the signals from the third voltage raising timing generation circuit  240 . 
     The operation of the power supply circuit  220  shown in FIG. 13 will be described with reference to FIG.  14 . Power supply potentials VDD and VSS are raised by the first voltage raising circuit  224 . The raised potentials are adjusted to an appropriate potential VC by the electronic volume  226 . Since other potentials PV 3 , PV 2 , PV 1 , MV 1 , MV 2 , and MV 3  are generated on the basis of the potential VC, contrast-and luminance can be adjusted by adjusting the potential VC by the electronic volume  226 . In addition, if the contrast has already been adjusted, it is unnecessary to adjust the contrast by operating the electronic volume  226  each time the liquid crystal device is driven while changing the duty. 
     The second voltage raising circuit  228  generates the potential PV 2  by raising the voltage between the potential VC and the power supply potential VSS. The power supply potential VSS is used as the potential MV 2 . 
     The potential generation circuit  242  generates the potential MV 1  by lowering the voltage between the potentials VC and MV 2 . The potential generation circuit  242  also generates the potential PV 1  by lowering the voltage between the potentials PV 2  and VC. In this example, the potential generation circuit  242  is formed of a ½ voltage lowering circuit. 
     The third voltage raising circuit  230  generates the potential MV 3  by raising the voltage between the potentials PV 2  and MV 2 . The fourth voltage raising circuit  232  generates the potential PV 3  by raising the voltage between the potentials MV 3  and VC. 
     As described above, seven levels of potentials (PV 3 , PV 2 , PV 1 , VC, MV 1 , MV 2 , MV 3 ) shown in FIG. 5 which are required for the 4-line simultaneous selection driving in the normal operation mode can be generated. 
     The contrast in the normal operation mode may be adjusted once by operating the electronic volume  226  shown in FIGS. 13 and 14, as described above. The contrast can be easily adjusted at a constant bias ratio (specifically, the first to fourth voltage-raising multiplying factors are fixed). Conventionally, desired potential levels are generated by a resistance dividing circuit by changing PV 3 . This causes a problem in which power consumption is increased because direct current flows through the resistance dividing circuit. Moreover, the bias ratio varies. These conventional problems can be solved by this example. 
     The potential VC has been set equal to the power supply potential VDD. Therefore, when about 3 V is required as the potential VC, the power supply potential VDD must be increased contrary to the demand for decrease in the voltage. In this example, since the potential VC is generated by raising the potential VDD, the power supply potential VDD can be lowered. 
     Next, driving in the wait mode will be described. As examples of the drive methods in the wait mode, a method of changing the potentials PV 3  and MV 3  so that the bias ratio is that shown in FIG. 6 instead of the bias ratio in the normal operation mode shown in FIG. 5 can be given. This can be achieved by changing the voltage-raising multiplying factors in the third voltage raising circuit  230  and the fourth voltage raising circuit  232  shown in FIGS. 13 and 14. 
     In the power supply circuit  220  in the common driver IC  200  shown in FIG. 13, the bias ratio in the wait mode is changed without changing the voltage-raising multiplying factor. 
     Specifically, the operations of the third and fourth voltage raising circuits  230  and  232  which generate the common potentials PV 3  and MV 3  are stopped in the wait mode. In the potential switching circuit  244 , the segment potentials PV 2  and MV 2  are supplied to the common electrodes  12  instead of the common potentials PV 3  and MV 3 . The potential switching circuit  244  shown in FIG. 13 allows the PV 3  and MV 3  terminals shown in FIG. 13 to output the potentials PV 2  and MV 2  when set in the wait mode by power saving signals. 
     Therefore, the liquid crystal device, which is driven by 7-level driving in the normal operation mode, is driven by 5-level driving excluding the potentials PV 3  and MV 3  in the wait mode. 
     The following equation is a modification of equation (21). 
     
       
           n   2   =n   1 ·( c   1   /c   2 ) 2   Λ(35) 
       
     
     In equation (35), the bias ratio c 1  equals (PV 2 −VC)/L/PV 3 , and the bias ratio c 2  equals (PV 2 −VC)/L/PV 2 . Therefore, (c 1 /c 2 ) in equation (35) equals the ratio of the common potential PV 3  in the normal operation mode to the common potential PV 2  in the wait mode PV 2 /PV 3  (MV 2 /MV 3 ). The ratio (MV 2 /MV 3 ) equals a third voltage-raising multiplying factor “k” in the third voltage raising circuit  230  as shown in FIG.  14 . Therefore, (c 1 /c 2 ) in equation (35) equals 1/k. Consequently, equation (35) is expressed as follows, using the third voltage-raising multiplying factor “k”. 
     
       
           n   2   =n   1 ·(1/K) 2   Λ(36) 
       
     
     If (c 1 /c 2 ) in equation (35), that is, the third voltage-raising multiplying factor “k” in equation (36) is either 2 or 3, the relation between the duty n 1  in the normal operation mode and the duty n 2  in the wait mode becomes as shown in the following Table 1. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Wait duty n 2   
                 Wait duty n 2   
               
               
                 Normal 
                 (third voltage- 
                 (third voltage- 
               
               
                 operation 
                 raising multiplying 
                 raising multiplying 
               
               
                 duty n 1   
                 factor = 2) 
                 factor = 3) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 80 
                 20 
                 9 (8) 
               
               
                 100 
                 25 (24) 
                 11 (12) 
               
               
                 120 
                 30 (32) 
                 13 (12) 
               
               
                 140 
                 35 (36) 
                 16 
               
               
                 160 
                 40 
                 18 (16) 
               
               
                   
               
               
                 The values in parentheses indicate the nearest multiples of four.  
               
            
           
         
       
     
     Since n 1  and n 2  in the multi-line selection drive method must be multiples of the simultaneous selection number L, the nearest multiples of four are employed in this example. 
     AS described above, the display duty n 2  in the wait mode is determined uniquely it the third voltage-raising multiplying factor and the normal operation duty n 1  are determined. The driving at the display duty n 2  eliminates the need for adjustment of the contrast.