Abstract:
In a plasma display apparatus with power consumption control, a control method is provided that eliminates unnaturalness of images during power control and that holds power consumption to within a target value regardless of the type of image pattern displayed. Differences between power consumption P SA  and target value P SET  are summed to calculate power consumption sum value P SUM , and if P SUM  is negative, brightness set value MCBC is set to its maximum value MCBC MAX . If P SUM  is positive, the value calculated by the equation “MCBC MAX −P SUM ×MCBC MAX /P SUM,MAX ” is set as the MCBC.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for controlling the power consumption of a display apparatus, especially a display apparatus having a plasma display panel, and more particularly a display apparatus having an AC-driven plasma display panel, a display system equipped with such a power consumption control apparatus, and a storage medium with a program stored therein for implementing such a power consumption control method. 
     2. Description of the Related Art 
     Usually, power consumption control for a display apparatus, especially a display apparatus having an AC-driven plasma display panel (PDP), is performed by continuously monitoring the power consumption that changes as the total value of display data changes, and by forcefully reducing the brightness of the entire screen when the power consumption has exceeded its upper limit value and increasing the brightness when the power consumption drops below its lower limit value. In performing the control, in order to minimize the unnaturalness perceived by the viewer viewing the display, brightness is reduced gradually when it is necessary to reduce the brightness because power consumption is too large, and is increased quickly when the brightness can be increased because the power consumption is low enough to permit it. 
     In the case of an AC-driven plasma display, the control of brightness is accomplished by varying the number of sustain pulses during one frame period and thereby varying the length of the sustained-discharge period. The brightness of each pixel, based on display data, is achieved by dividing one frame into a plurality of sub-fields with varying sustained-discharge periods and by selectively enabling or disabling the sub-fields in accordance with whether the bits forming the pixel data are on or off. For example, when data of each pixel consists of eight bits, one frame is divided into eight sub-fields the ratio of whose sustained-discharge periods is 2 0 :2 1 :2 2 : . . . 2 7 , and the corresponding sub-fields are enabled or disabled in accordance with the bit pattern of the pixel data. In the case of color display, the above control is performed independently for each of the three kinds of pixels corresponding to R, G, and B. The brightness of the entire screen is achieved by increasing or decreasing the sustained-discharge periods of all the sub-fields while maintaining the above ratio. 
     As described above, in a display apparatus such as a PDP having a power consumption control function, the speed with which the brightness of the entire screen is reduced to control power consumption is set slower than the speed with which the brightness is increased, in order to minimize the unnaturalness perceived by the viewer viewing the display. In other words, power consumption is quick to rise but slow to fall; therefore, when images with rapidly varying load, such as flashing images, are successively displayed, the power consumption rises quickly in the off period, but does not fall readily in the on period because the speed with which the power consumption is lowered is slow. If such patterns are repeated, the average power consumption does not settle down to the set value but exceeds the set value. If the set value is set lower than the actually permitted power consumption value to avoid the above situation, there arises a problem when displaying images with stable load, that is, the brightness and contrast are reduced more than necessary, resulting in degradation of picture quality. 
     SUMMARY OF THE INVENTION 
     It is, accordingly, an object of the present invention to provide a method of power consumption control that can hold average power consumption within a specified value whether images with rapidly varying load continue or whether image load is stable, and can yet maintain as good a picture quality as possible. 
     According to the present invention, there is provided a method of controlling power consumption of a display unit, comprising the steps of: measuring the power consumption of the display unit; increasing display brightness of the display unit, or decreasing the display brightness at a speed different from the speed of increasing, in accordance with the measured value of the power consumption; summing the power consumption; and controlling the display brightness in accordance with the sum value of the power consumption and thereby controlling the power consumption to within a target value. 
     According to the present invention, there is also provided an apparatus for controlling power consumption of a display unit, comprising: means for inputting a measured value of the power consumption of the display unit; means for increasing display brightness of the display unit, or decreasing the display brightness at a speed different from the speed of increasing, in accordance with the measured value of the power consumption; means for summing the power consumption; and means for controlling the display brightness in accordance with the sum value of the power consumption and thereby controlling the power consumption to within a target value. 
     Preferably, the display unit includes a plasma display panel and a plasma display panel control circuit capable of increasing or decreasing the brightness by increasing or decreasing the number of sustain pulses applied to the plasma display panel during one frame period. 
     Also preferably, the above control circuit includes an input for setting the number of sustain pulses for the entire display as a display brightness value, and an input for data of each pixel defining the number of sustain pulses for each pixel, the increasing or decreasing of the brightness is achieved by increasing or decreasing the display brightness value and thereby increasing or decreasing the display brightness, and the control of the brightness is achieved by correcting the increasing or decreasing of the display brightness value in accordance with the sum value of the power consumption and thereby controlling the display brightness. 
     Alternatively, the control of the brightness may be achieved by determining a subtrahend based on the sum value of the power consumption, and by subtracting the subtrahend from data of all the pixels and thereby controlling the display brightness. 
     According to the present invention, there is also provided a method of controlling power consumption of a display unit, comprising the steps of: measuring the power consumption of the display unit; summing differences between the power consumption and its target value; determining a display brightness value for the display unit from the sum value of the power consumption; and setting the determined display brightness value in the display unit. 
     According to the present invention, there is also provided an apparatus for controlling power consumption of a display unit, comprising: means for inputting a measured value of the power consumption of the display unit; means for summing differences between the power consumption and its target value; means for determining a display brightness value for the display unit from the sum value of the power consumption; and means for setting the determined display brightness value in the display unit. 
     According to the present invention, there is also provided a display system comprising: the above-described power consumption control apparatus; a plasma display panel; a drive circuit for driving the plasma display panel; and a control apparatus for controlling the drive circuit in accordance with a set value supplied from the power consumption control apparatus. 
     According to the present invention, there is also provided a storage medium readable by a computer, the storage medium storing therein a program for implementing the above-described power consumption control method when connected to the computer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the configuration of a plasma display apparatus where the present invention is applied; 
     FIG. 2 is a diagram showing a sub-frame structure for achieving an intermediate gray-scale level; 
     FIG. 3 is a block diagram showing the hardware configuration of a power consumption control apparatus according to a first embodiment of the present invention; 
     FIG. 4 is a flowchart illustrating a process for decreasing brightness; 
     FIG. 5 is a flowchart illustrating a process for increasing brightness; 
     FIG. 6 is a graph for explaining the increasing/decreasing speeds of power consumption; 
     FIG. 7 is a graph for explaining the problem to be solved by the present invention; 
     FIG. 8 is a flowchart illustrating a process for the calculation of power consumption sum value P sum ; 
     FIG. 9 is a flowchart illustrating a first example of a process for correcting the increasing/decreasing of MCBC; 
     FIG. 10 is a diagram for explaining the effect achieved by the present invention; 
     FIG. 11 is a flowchart illustrating a second example of the process for correcting the increasing/decreasing of MCBC; 
     FIG. 12 is a flowchart illustrating a third example of the process for correcting the increasing/decreasing of MCBC; 
     FIG. 13 is a flowchart illustrating a fourth example of the process for correcting the increasing/decreasing of MCBC; 
     FIG. 14 is a block diagram of a power consumption control apparatus according to a second embodiment of the present invention; 
     FIG. 15 is a diagram for explaining the operation of the apparatus of FIG. 14; 
     FIG. 16 is a flowchart showing a first means for implementing the averaging of power consumption; 
     FIG. 17 is a circuit diagram showing a second means for implementing the averaging of power consumption; 
     FIG. 18 is a flowchart illustrating a process for the calculation of power consumption sum value P SUM according to a third embodiment of the present invention; 
     FIG. 19 is a flowchart illustrating a process for the calculation of MCBC according to the third embodiment; of the present invention; 
     FIG. 20 is a graph illustrating a technique for calculating the value of MCBC from the value of P SUM ; 
     FIG. 21 is a flowchart illustrating a minuscule margin process; 
     FIG. 22 is a graph showing a power consumption control operation according to the third embodiment of the present invention; and 
     FIG. 23 is a graph showing the power consumption control operation according to the third embodiment of the present invention 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the configuration of an AC-driven plasma display apparatus as an example of a display apparatus where the present invention is applied. 
     A plasma display panel (PDP)  10  includes a large number of Y electrodes (scan electrodes)  12  arranged parallel to each other, a large number of address electrodes  14  arranged parallel to each other and intersecting at right angles to the Y electrodes  12 , and an equal number of X electrodes (common electrodes)  16  to the number of Y electrodes and also arranged parallel to the Y electrodes. Display cells  18  are formed where each address electrode  14  intersects with the electrodes  12  and  16 . 
     A drive circuit  20  for the PDP  10  comprises a Y scan driver  22  for driving the Y electrodes  12  independently of each other, a Y driver  24  for driving all the Y electrodes  12  simultaneously via the Y scan driver  22 , a common driver  26  for driving all the X electrodes  16  simultaneously, and an address driver  28  for controlling the address electrodes  14  independently of each other. The Y scan driver  22 , the Y driver  24 , and the common driver  26  are supplied with a sustain supply voltage V S , while the address driver  28  is supplied with an address supply voltage V A . 
     As is well known, in the AC-driven PDP, during an address period, a write pulse is selectively applied between a Y electrode  12  and an address electrode  14  to selectively store a charge in each of the corresponding display cells, and during a sustained-discharge period following the address period, AC pulses (sustain pulses) are applied between all the Y electrodes  12  and all the X electrodes  16 , and only display cells, where the charge is stored during the address period, are caused to illuminate. Accordingly, when one Y electrode  12  as a scan line is active, the pattern of the address electrodes  14  set active at that time corresponds to the on/off pattern of the display cells along that scan line, and the length of the subsequent sustained-discharge period, that is, the number of sustain pulses, corresponds to the brightness of the illuminating display cells. 
     A control circuit  30  for the PDP  10  includes a scan driver controller  34  for sequentially scanning the Y electrodes  12  via the scan driver  22 , a display data controller  32  for supplying a display pattern on each scan line to the address electrodes  14  via the address driver  28  in synchronism with the scanning by the scan driver controller  34 , and a common driver controller  36  for applying sustain pulses between the Y electrodes  12  and X electrodes  16  via the Y driver  24  and common driver  26 . The scan driver controller  34  and the common driver controller  36  together constitute a panel drive controller  38 . Display data (DATA) is input to the display data controller  32  in synchronism with a display clock (CLOCK), and temporarily stored in a frame memory  40 . A vertical synchronizing signal (V SYNC ) and a horizontal synchronizing signal (H SYNC ) are supplied to the panel drive controller  38 , while the number of sustain pulses and control codes are input to the common driver controller  36 . 
     FIG. 2 is a diagram for explaining a technique for achieving an intermediate gray-scale level in the AC-driven PDP. One frame (corresponding to one picture) is divided, for example, into eight sub-fields. Each sub-field includes an address period during which a charge is selectively stored or not stored in each display cell in accordance with the display data, and a sustained-discharge period during which the display cells where the charge is stored are caused to illuminate. The ratio of the sustained-discharge periods of the sub-field  1 , sub-field 2, . . . , sub-field  8 , that is, the ratio in terms of the number of sustain pulses, is set to 2 0 :2 1 . . . 2 7 . During the address period of the sub-field  1  the ratio of whose sustained-discharge period is 2 0 , charge is stored only on display cells for which the least significant bit 0 of 8-bit gray-scale data is 1, and during the subsequent sustained-discharge period, these display cells are caused to illuminate. Likewise, during the address period of the sub-field i+1 (i=1 to 7) the ratio of whose sustained-discharge period is 2 i , charge is stored only on display cells for which bit i of the gray-scale data is 1, and during the subsequent sustained-discharge period, these display cells are caused to illuminate. In this way, the gray scale of each pixel can be set in 256 levels. 
     The brightness of the entire screen is set by increasing or decreasing the number of sustain pulses in accordance with a brightness set value (hereinafter called MCBC), while maintaining the sustain pulse count ratio of each sub-field at the above-set value. The number of sustain pulses determined for each sub-field based on MCBC is supplied to the common driver controller  36 . 
     FIG. 3 is a block diagram showing the configuration of a power consumption control apparatus  42  according to a first embodiment of the present invention. A V s  voltage detection circuit  44  and an I s  current detection circuit  46 , respectively, detect the voltage and current of the sustain power supply being supplied from a V s  power source  48  to the Y scan driver  22 , Y driver  24 , and common driver  26  (FIG.  1 ). A/D converters  50  and  52 , respectively, convert the voltages detected by the V s  voltage detection circuit  44  and I s  current detection circuit  46  into corresponding digital values. A V A  voltage detection circuit  54  and an I A  current detection circuit  56 , respectively, detect the voltage and current of the address power supply being supplied from a V A  power source  58  to the address driver  28  (FIG.  1 ). A/D converters  60  and  62 , respectively, convert the voltages detected by the V A  voltage detection circuit  54  and I A  current detection circuit  56  into corresponding digital values. An MPU  64 , based on the output values of the A/D converters  50 ,  52 ,  60 , and  62 , determines appropriate MCBC in accordance with the flow hereinafter described, converts it to the number of sustain pulses for each sub-field, and supplies the converted values to the common driver controller  36  (FIG. 1) to control the power consumption within a target value. For conversion from MCBC to the number of sustain pulses, it is desirable to use a ROM in which sustain pulse counts are stored in memory areas addressable by corresponding MCBC values. 
     FIG. 4 is a flowchart illustrating the processing performed by the MPU  64  to determine whether the power consumption is greater than its upper limit value and to control the power consumption within a target value by decreasing the MCBC if the power consumption is greater than the upper limit value. The processing of FIG. 4 is invoked by an interrupt that occurs in synchronism with the vertical synchronizing signal V SYNC , that is, for every frame. First, CAP is incremented by 1 (step 1000), and it is determined whether CAP has reached a processing cycle n 1  (step 1002). If CAP has reached n 1 , CAP is cleared to 0 (step 1004), and it is determined whether the average power consumption P AV  has exceeded the upper limit value P SET  (step 1006). The average power consumption P AV  is obtained by calculating power consumption P SA  from V S , I S , V A , and I A  input from the respective A/D converters  50 ,  52 ,  60 , and  62 , using the equation below, and by averaging the obtained values over several frame periods for reasons to be explained later. 
     
       
         
           P 
           SA 
           =I 
           S 
           ×V 
           S 
           +I 
           A 
           ×V 
           A 
         
       
     
     If P AV  is greater than P SET , then it is determined whether the MCBC value has reached its lower limit value (step 1008); if it has not yet reached the lower limit value, the MCBC is decreased by a decrease step width m 1  (step 1010). 
     In the above processing flow, the MCBC decreasing speed a per frame time when P AV  is greater than P SET  is m 1 /n 1 . 
     FIG. 5 is a flowchart illustrating the processing performed by the MPU  64  to determine whether the power consumption is smaller than its lower limit value and to secure the necessary screen brightness and contrast by increasing the MCBC when the power consumption is smaller than the lower limit value. The processing of FIG. 5 is also invoked by the interrupt that occurs in synchronism with the vertical synchronizing signal V SYNC , that is, for every frame. First, CAP is incremented by 1 (step 1100), and it is determined whether CAP has reached a processing cycle n 2  (step 1102). If CAP has reached n 2 , CAP is cleared to 0 (step 1104), and it is determined whether the average power consumption P AV  has fallen below the lower limit value P SET −ΔP 1  (step 1106). ΔP 1  is a control margin for preventing display flicker when P AV  is close to P SET . If P AV  is smaller than P SET −ΔP 1 , then it is determined whether the MCBC value has reached its upper limit value (step 1108); if it has not yet reached the upper limit value, the MCBC is increased by an increase step width m 2  (step 1110). 
     In the above processing flow, the MCBC increasing speed b per frame time when P AV  is smaller than P SET −ΔP 1  is m 2 /n 2 . 
     As previously described, basically a is set smaller than b to reduce the unnaturalness perceived by the viewer viewing the display when the power consumption control is on. FIG. 6 shows how the power consumption changes when the display changes from OFF (all pixel values are zero) to ALL ON (all pixels are at maximum values) and then to OFF again. In the OFF state up to time t 0 , MCBC is at its maximum value. When the state changes from OFF to ALL ON at time t 0 , the power consumption reaches its maximum value; thereafter, MCBC is gradually lowered, and the power consumption gradually decreases until reaching the target value at time t 1 . Thereafter, when the state changes to OFF at time t 2 , MCBC quickly rises to its maximum value, and the power consumption also quickly rises and settles at a constant value. 
     FIG. 7 shows how the power consumption changes when the ALL ON/OFF change is repeated in a short cycle. As can be seen from FIG. 7, when the MCBC decreasing speed is set slower than the MCBC increasing speed, there arises the problem that, in the case of FIG. 7, the average power consumption settles at a level higher than the target value. To address this problem, in the first embodiment of the present invention, differences between the power consumption and its target value are summed, and, based on the sum value, correction is made to the increase/decrease of MCBC. 
     FIG. 8 shows a flow for the calculation of the sum, value P sum  representing the sum of the differences between the power consumption and its target value. In FIG. 8, the processing flow is invoked by the V SYNC  interrupt, and (P SA −P SET ) is added to P SUM  (step 1200). 
     FIG. 9 show a first example of MCBC increase/decrease correction based on P SUM . Processing from step  1306  onward is repeated for every n 3  frame, as in the previously described processing. First, it is determined whether P sum  is positive or not (step 1306). If P sum  is positive, it is determined whether the average power consumption P AV  exceeded the target value P SET  in the previous processing (step  1308 ), and if P AV &gt;P SET  in the previous processing, then it is determined whether P AV  is greater than P SET  in the current processing (step 1310); if P AV &gt;P SET , the current MCBC value is stored in memory MR (step 1312). On the other hand, if, in step  1308 , P AV &lt;P SET  in the previous processing, it is determined whether P AV  is greater than P SET +ΔP 2  in the current processing (step  1314 ). If P AV &gt;P SET +ΔP 2 , the value stored in memory MR is taken as the MCBC value (step 1316). 
     That is, in the processing of FIG. 9, if P sum &gt;0, and if P AV  is greater than P SET  two times in succession, then the current MCBC value is stored in the memory. Further, if P sum &gt;0, and if P AV  has increased from a level lower than P SET  to a level substantially greater than P SET , then the value stored in the memory is taken as the MCBC value. Here ΔP 2  is a control margin for preventing display flicker. 
     In the first example of MCBC increase/decrease correction shown in FIG. 9, when P SUM &gt;0, the MCBC value when P AV &gt;P SET , for example, during the ALL ON period, is stored in the memory, the value stored in the memory then being updated as the MCBC gradually decreases; during the next OFF period, for example, if P AV &lt;P SET , the final value in the ALL ON period is retained in the memory, and when the state changes again to ALL ON, the final value retained in the memory is used as the MCBC value. Accordingly, even when the ALL ON/OFF change is repeated in a short cycle, control is achieved so that the power consumption during the ALL ON period gradually approaches the target value, as shown in FIG.  10 . Instead of using the memory-retained value as the MCBC value, a value obtained by subtracting a constant not smaller than 1 from the memory-retained value may be used as the MCBC value. 
     FIG. 11 is a flowchart showing a second example of MCBC increase/decrease correction based on P sum . In the flow of FIG. 11, it is determined whether P sum  has exceeded a predetermined value α (step  1400 ), and if P sum &gt;α, a sufficiently low fixed value is set as the MCBC (step  1402 ). That is, the value of α serves as an upper limit on the sum value P sum  that adds up excess power values; if this upper limit is exceeded, then the value is determined to be abnormal, and the MCBC is fixed to a low value, regardless of the display brightness value, to protect the power supplies, etc. and to recover the power by an amount proportional to the excess value and thereby control the power within the set value. 
     Regarding the decreasing speed a (=m 1 /n 1 ) in the processing (FIG. 1) in which the MCBC is decreased when the power consumption exceeds the set value, it can be seen that the slower the decreasing speed a is, the more slowly the brightness and contrast decrease and the less the unnaturalness that the viewer viewing the display perceives, but the slow decreasing speed is disadvantageous from the viewpoint of suppressing power consumption. Conversely, as the decreasing speed a increases, the response to excessive power consumption becomes faster, but the unnaturalness increases. To address this problem, in a third example of MCBC increase/decrease correction based on P sum  according to the present invention, the range of values of P sum  from the positive to the negative side is divided, for example, into eight levels, and the decreasing speed is changed according to the value of P sum  so that when the value of P sum  is large in the positive sense, priority is given to power control and the value of a is increased, and when the value of P sum  is large in the negative sense, priority is given to picture quality and the value of a is reduced, as shown in FIG.  12 . 
     Next, when we look at the increasing speed b (=m 2 /n 2 ) in the processing (FIG. 5) in which the MCBC is increased when the power consumption is sufficiently low to permit it, we can see that, contrary to the case of decreasing MCBC, a higher increasing speed b and, hence, a faster change of brightness and contrast, is advantageous in reducing the unnaturalness perceived by the viewer viewing the display; therefore, when the power consumption is sufficiently low, increasing the increasing speed gives better results. Conversely, if the increasing speed b is reduced, the unnaturalness increases, but reduced increasing speed is advantageous when there is no room for increasing the power consumption. In view of this, in a fourth example of MCBC increase/decrease correction based on P sum  according to the present invention, the range of values of P sum  from the positive to the negative side is divided, for example, into eight levels, and the increasing speed is changed according to the value of P sum  so that when the value of P sum  is large in the negative sense, priority is given to picture quality and the value of b is increased, and when the value of P sum  is large in the positive sense, priority is given to power control and the value of b is reduced, as shown in FIG.  13 . 
     FIG. 14 shows the configuration of a power consumption control apparatus  42  according to a second embodiment of the present invention. As in the first embodiment, in the second embodiment also, the MPU  64  performs control to increase or decrease the MCBC in accordance with the flows of FIGS. 4 and 5. Subtractors  70  subtract the subtrahend given by the MPU  64  from R 0  to R 7 , G 0  to G 7 , and B 0  to B 7  which are data to be supplied to the display data controller  32 , and supplies the resulting values to the display data controller  32 . The subtrahend is determined according to the value of P sum  as shown in FIG.  15 . When the subtrahend for the display data is changed, the number of sustain pulses for the entire screen changes, so that the average power consumption can be prevented from exceeding the set value. 
     Lastly, we will describe the purpose of using P AV  obtained by averaging P SA  over several frame periods rather than directly using P SA  calculated from voltage and current values, and how this can be accomplished. 
     When increasing or decreasing the MCBC by calculating P SA  for every n frames (n is an integer), if an image is displayed that turns ON and OFF in a cycle of n frame times, there arises the case where the MCBC is always controlled on the basis of P SA  in the OFF state, causing the average power consumption to exceed its target value. To address this problem, n successive values of P SA  are averaged, and the resulting average value P AV  is used instead of P SA . 
     FIG. 16 is a flowchart illustrating the processing for computing P AV  by averaging P SA , which is implemented by software of the MPU  64 . In FIG. 16, when CAP has reached n, CAP, P AV , the quotient, and the remainder are cleared (step  1502 ), and the process returns to the branch leading to step  1506 . If CAP has not yet reached n,  1  is added to CAP (step  1504 ), P SA  is read (step  1506 ), and the remainder from the previous processing is read (step  1508 ) and added to P SA  (step  1510 ). P SA  is divided by n to obtain the quotient and the remainder (step  1512 ), and the quotient is added to P AV  (step  1514 ). If CAP is equal to n in step  1516 , then P AV  is determined (step  1518 ). 
     FIG. 17 shows a configuration for implementing the averaging of P SA  in hardware. In FIG. 17, the MPU  64  outputs P SA  which is input to a delay circuit consisting of a resistor  72  and a capacitor  74 . The MPU  64  then takes the output of this circuit as P AV . 
     FIGS. 18 and 19 illustrate the processing performed by the MPU in a power consumption control apparatus according to a third embodiment of the present invention. The hardware configuration of the third embodiment is the same as that of the first embodiment shown in FIG.  3 . 
     The embodiments so far described have employed the technique in which the average power consumption is controlled to within the target value by increasing or decreasing the display brightness set value MCBC in accordance with an instantaneous value of power consumption and further by correcting the increasing or decreasing of MCBC or reducing pixel data in accordance with the sum value of the power consumption. In contrast, in the third embodiment of the present invention, the average power consumption is controlled to within the target value by determining the MCBC directly from the sum value of the power consumption. 
     FIG. 18 illustrates the processing performed by the MPU  64  for the calculation of the sum value P SUM  according to the third embodiment of the present invention. In FIG. 18, the sum value P sum  is calculated (step  1600 ) in the same manner as in step  1200  in FIG. 8, and if the sum value P SUM  exceeds its maximum value P SUM,MAX  (step  1602 ), P SUM,MAX  is substituted for P SUM . If the sum value P SUM  is less than its minimum value P SUM,MIN  (where P SUM,MIN &lt;0) (step  1606 ), P SUM,MIN  is substituted for P SUM . 
     FIG. 19 illustrates the process for determining the MCBC according to the third embodiment of the present invention. First, it is determined whether the sum value P SUM  is positive or negative (step  1700 ). If P SUM  is negative, the brightness set value MCBC is set to its maximum value MCBC MAX  (step  1702 ). If P SUM  is positive, the value calculated by the equation 
     
       
         
           MCBC 
           MAX 
           −P 
           SUM 
           ×MCBC 
           MAX 
           /P 
           SUM,MAX 
         
       
     
     is set as the MCBC (step  1704 ). 
     FIG. 20 shows the relationship between the sum value P SUM  and the brightness set value MCBC determined in steps  1702  and  1704 . As shown in FIG. 20, when the sum value P SUM  is negative, MCBC is set to its maximum value MCBC MAX , and when P SUM  is positive, the value of MCBC linearly decreases with increasing P SUM . Here, as shown by dashed line in FIG. 20, the threshold of P SUM  at which the value of MCBC begins to decrease from its maximum value need not necessarily be set at 0. 
     In the third embodiment of the present invention, since the brightness set value MCBC is determined directly from the sum value P SUM , if the values of V S , I S , V A , and I A  are near the A/D conversion threshold values of the A/D converters  50 ,  52 ,  60 , and  62  (FIG. 3) a situation can occur where wandering of digital values is directly reflected in the value of MCBC, causing image flicker. To prevent this, a minuscule margin process is executed after the MCBC has been calculated from the sum value P SUM . FIG. 21 shows the detail of the minuscule margin process executed in step  1706  in FIG.  19 . 
     FIG. 21 concerns the case where the value of MCBC calculated from P SUM  changes from decreasing to increasing. When the calculated MCBC is decreasing, since, in step  1800 , MCBCP retaining the previous value of MCBC is larger than the current value of MCBC, the process proceeds to step  1802  where MCBC is substituted for MCBC F , and after that, 0 is stored in flag MSTART. That is, when MCBC is decreasing, the calculated value of MCBC is directly used as the MCBC, and the flag MSTART is cleared to 0. 
     When the calculated value of MCBC changes from decreasing to increasing, since MCBC F &lt;MCBC in step  1800 , the process proceeds to step  1806  where it is determined whether the value of the flag MSTART is 0 or not. Since MSTART is 0 immediately after the change from decreasing to increasing, the process proceeds to step  1808  where the value of P SUM  is substituted for P SUM,F  retaining the current value of P SUM ; after that, the flag MSTART is set to 1 (step  1810 ), and MCBC F , retaining the previous value of MCBC, is substituted for the MCBC (step  1812 ). That is, immediately after the value calculated from P SUM  has changed from decreasing to increasing, the MCBC is not updated, and the current value of P SUM  is stored as P SUM,F , while setting the flag MSTART to 1. 
     When the calculated value continues to increase, since MSTART is 1, the process proceeds to step  1814  after steps  1800  and  1806 . In step  1814 , the value of (P SUM,F −P SUM ) is compared with a predetermined margin P SUM,MG . The value of (P SUM,F −P SUM ) indicates how much the P SUM  has decreased from the value of P SUM  stored as P SUM,F  when the calculated value of MCBC changed from decreasing to increasing (from FIG. 20, the increase in MCBC corresponds to the decrease in P SUM ) If the value of (P SUM,F −P SUM ) is smaller than the margin P SUM,MG , it is determined that the change is minuscule, and the process proceeds to step  1812  where the MCBC is not updated. If the value of (P SUM,F −P SUM ) is equal to or larger than the margin P SUM,MG , it is determined that the change is significant, and the process proceeds to step  1802  where the MCBC is updated. 
     With the above minuscule margin process, image flicker when the measured value is near the A/D conversion threshold value can be prevented. 
     FIG. 22 shows the power consumption control operation according to the third embodiment of the present invention. It is assumed here that the display ratio (representing the percentage of ON pixels) immediately after power on at time t 0  is at 100% (ALL ON) as shown in part (a). At this time, the sum value P SUM  increases from 0, as shown part (b), but since MCBC decreases with increasing P SUM , instantaneous power consumption P SA  decreases as shown in part (c), and accordingly the rising curve of the sum value P SUM  gradually trails off. The falling curve of the instantaneous power P SA  also gradually trails off until finally settling at the target power P SET . 
     When the display is extinguished at time t 1  with the display ratio dropping to 0%, and the extinguished state continues for a sufficient period of time, the sum value P SUM  drops to its minimum value P SUM,MIN . When the display ratio becomes 100% at time t 2 , the sum value P SUM  begins to increase from P SUM,MIN , but during the period when the sum value P SUM  is negative, MCBC is maintained at its maximum value. As a result, as shown in part (c), the power consumption P SA  during that period is maintained above the target value P SET  to provide a screen brightness that matches the display ratio. In the meantime, the sum value P SUM  increases linearly. When the sum value P SUM  becomes positive, the instantaneous power P SA  begins to decrease, its curve gradually sloping off and finally settling at P SET , as already noted. 
     In this way, in the third embodiment of the present invention, the speed with which the brightness is reduced based on the power consumption control is fast when the screen is bright, and decreases gradually as the screen becomes dark, as shown in FIG.  22 ( c ). Because of the characteristics of the human eye, when the screen is bright, the brightness change is not noticeable even if the brightness decreasing speed is fast, but when the screen is relatively dark, the brightness change becomes visible if the brightness decreasing speed is fast. Thus the above-described technique offers the advantage that the degradation in image quality due to power consumption control is not relatively noticeable, compared with the prior art technique in which the brightness is reduced at a constant speed when the instantaneous power has exceeded a target value (as shown by semi-dashed lines in FIG.  22 ( c )). 
     Further, when the sum value of the power consumption is sufficiently low, as in the period from time t 2  to time t 3 , sufficient brightness commensurate with the display ratio can be obtained. Accordingly, in the case of an image, such as a moving image, that entails rapid changes in display ratio, the degradation in image quality due to power consumption control is not noticeable. More specifically, when the display ratio changes as shown schematically in part (a) of FIG. 23, for example, in the prior art the brightness is controlled so that the instantaneous power is brought to its target value P SET  when it increases above P SET , as shown in part (b), while in the third embodiment of the present invention, the brightness that matches the change of the display ratio as close as possible can be achieved as shown in part (c). 
     The program implementing the processing flows of the MPU  64  thus far described is stored in a ROM (not shown) built into the MPU, but it is also possible to store the program in a separate storage medium such as a ROM and provide the program only. 
     As described above, according to the present invention, since the number of sustain pulses or the display data is controlled based on the sum value P sum  that adds up excess power consumption values, the average value of power consumption does not exceed the set value regardless of the type of image pattern displayed, thus achieving optimum control of the number of sustain pulses or the display data considering picture quality.