Patent Publication Number: US-8111236-B2

Title: Backlight driving device and display

Description:
CROSS REFERENCES CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. JP 2007-115953, filed in the Japanese Patent Office on Apr. 25, 2007, the entire content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a backlight driving device for driving a backlight that illuminates a display panel and to a display incorporating this backlight driving device. 
     2. Description of the Related Art 
     A liquid crystal display (LCD) has the advantage over a CRT display that its enclosure can be made thin. Because of this advantage, LCDs are widely used as displays for information technology devices (such as computers) and as TV receivers. Such a liquid crystal display has a backlight as its light source. The backlight illuminates the LCD panel from the rear. 
     In a related-art LCD, the luminance value of an image displayed on the LCD is controlled by supplying a drive signal as shown below to the backlight such that the backlight illuminates the panel. That is, in the related-art LCD, a drive signal is created. The drive signal is pulse width-modulated in such a way that the duty cycle increases with increasing the luminance value of the video signal. A voltage corresponding to the created drive signal is applied to the backlight, thus controlling the luminance value of the displayed image (see, JP-A-2004-252127 (patent reference 1)). 
     In this related-art LCD, in cases where no voltage is applied to the backlight for longer than a given period of time and thus the LCD is under low-temperature conditions, there is the possibility that even if the drive voltage is applied, the backlight is not driven and the panel is not illuminated due to dead band characteristics. Therefore, the backlight is driven according to a drive signal composed of a pulsed signal that is pulse width-modulated, for example, within a range where the duty cycle is kept at an effective value. 
     SUMMARY OF THE INVENTION 
     In the above-described related-art LCD, however, even where a video signal having a luminance value of 0 is displayed, the backlight may not be lit up due to the dead band characteristics of the backlight when the backlight is attempted to be driven by a pulsed signal having an effective duty cycle after that video signal is displayed. To prevent this, the backlight is driven by a drive signal having an effective duty cycle at every given period of time. Consequently, the power consumption is made larger than where no electric power is supplied to the backlight at all by setting the duty cycle to 0. 
     Accordingly, it is desirable to provide a backlight driving device for reducing the power consumption of a backlight, for example, when a video signal having an ineffective luminance value is outputted and displayed. It is also desirable to provide a liquid crystal display incorporating this backlight driving device. 
     One embodiment of the present invention provides a backlight driving device for driving a backlight that illuminates a display panel, the backlight driving device having: a luminance data input means into which luminance data indicated by a video signal to be displayed in the display panel is entered; a drive signal-generating means for creating a drive signal made of a signal which is pulse width-modulated so as to have a duty cycle that is effective over a whole period, based on values of the luminance data entered into the luminance data input means; a power supply means for supplying electric power to the backlight according to the drive signal created by the drive signal-generating means so as to drive the backlight; a video signal decision means for making a decision as to whether each value of the luminance data entered into the luminance data input means is smaller than a given value; and a thinning-out means for thinning out components of the drive signal created by the drive signal-generating means when the video signal decision means has determined that the value of the luminance data entered into the luminance data input means is smaller than the given value and supplying the thinned drive signal to the power supply means. 
     Another embodiment of the present invention provides a display device for displaying a video signal on a display panel, the display device having: a video signal-accepting means for accepting the video signal; a video processing means for displaying the video signal accepted by the video signal-accepting means onto the display panel; a backlight which illuminates the display panel; and a backlight driving means for driving the backlight. The backlight driving means has: a drive signal-generating means for creating a drive signal made of a signal which is pulse width-modulated so as to have a duty cycle that is effective over a whole period, based on values of the luminance data indicated by the video signal accepted by the video signal-accepting means; a power supply means for supplying electric power to the backlight according to the drive signal created by the drive signal-generating means so as to drive the backlight; a video signal decision means for making a decision as to whether each value of the luminance data accepted by the video data-accepting means is smaller than a given value; and a thinning-out means for thinning out components of the drive signal created by the drive signal-generating means when the video signal decision means has determined that the value of the luminance data accepted by the video signal-accepting means is smaller than the given value and supplying the thinned drive signal to the power supply means. 
     In the embodiments of the present invention, when it is determined that the value given by the entered luminance data is smaller than the given value, the pulses forming the drive signal are thinned out. The thinned drive signal is supplied to the power supply means and, therefore, it is possible to reduce the electric power consumed by the backlight when a video signal having an ineffective luminance value is outputted, even if such a circuit that produces a drive signal capable of modifying the period of the pulse width modulation and that will become a factor causing an increase in the scale of the driving circuit is not used. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing one example of configuration of a liquid crystal display to which one embodiment of the present invention is applied; 
         FIG. 2  is a block diagram showing a configuration for driving a backlight, the configuration being equipped in a liquid crystal display to which one embodiment of the invention is applied; and 
         FIGS. 3A to 3C  are diagrams illustrating a drive signal used for driving a backlight. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention are hereinafter described in detail with reference to the drawings. 
     A liquid crystal display (LCD) to which an embodiment of the present invention is applied is one example of display device which receives an external video signal and displays the signal on an LCD. Specifically, the LCD is configured as shown in  FIG. 1 . 
     As shown in  FIG. 1 , the liquid crystal display, generally indicated by reference numeral  1 , has a signal input portion  10  for entering a video signal and an audio signal or TV signals from the outside, a data input portion  20  for entering control information about the display  1 , an image processing portion  30  for receiving the video signal mainly from the signal input portion  10  and processing the video signal in a given manner, a display portion  40  for displaying the video signal, and a signal output portion  50  for outputting the video signal and audio signal to the outside. 
     The LCD  1  to which one embodiment of the present invention is applied achieves a reduction in electric power consumed by a backlight  45  equipped in the display portion  40  (described later). First, in the present embodiment, prior to description of the operation of the backlight  45 , processing performed by the whole LCD  1  is described. 
     The signal input portion  10  has an RF input terminal  11  for receiving a signal of terrestrial digital broadcast airwaves received by an antenna (not shown), a BS/CS input terminal  12  for receiving the signal of satellite broadcast airwaves received by another antenna (not shown), a tuner  13  for receiving signals entered from the RF input terminal  11  and from the BS/CS input terminal  12 , an HDMI input terminal  14  for entering HDMI (High-Definition Multimedia Interface) signals, plural analog signal input terminals  15   a - 15   c  for entering analog video signals, and a bus  16  to which analog signals entered from the analog input terminals  15   a - 15   c  are supplied. 
     The RF input terminal  11  is a connector terminal for receiving a terrestrial digital broadcast signal received by the antenna (not shown). The terminal  11  supplies the entered signal to the tuner  13 . 
     The BS/CS input terminal  12  is a connector terminal for receiving satellite broadcast airwave signals received by the antenna (not shown). The input terminal  12  supplies the received signal to the tuner  13 . 
     The tuner  13  receives signals entered from the RF input terminal  11  and from the BS/CS input terminal  12 , demodulates the received signals, and supplies digital TV signal, analog video signal, and analog audio signal to the image processing portion  30 . The tuner  13  may demodulate the received analog airwaves. Consequently, the analog video signal and analog audio signal are outputted to the tuner  13  when the received signals such as analog terrestrial broadcast waves and analog satellite broadcast waves are demodulated. 
     The HDMI input terminal  14  receives an HDMI signal in which a digital video signal, a digital audio signal, and a control signal have been multiplied. The input terminal  14  supplies the entered signal to the image processing portion  30 . 
     The analog signal input terminals  15   a - 15   c  receive analog video signals each made up of an analog video signal and an analog audio signal. For example, the analog video signal entered into the analog signal input terminals  15   a - 15   c  is supplied to the image processing portion  30  via the bus  16 . The analog audio signal is supplied to the signal output portion  50 . The analog video signal entered from the analog signal input terminal  15   a  is directly supplied to the image processing portion  30  without via the bus  16 . 
     The data input portion  20  has a B-CAS card  21 , a B-CAS interface  22  connected with the B-CAS card  21 , a modem interface  23  connected with a modem (not shown), and an Ethernet interface  24  connected with a local area network. Encryptic key information for decrypting encrypted information contained in the received broadcast airwaves encrypted by a conditional access system (such as a B-CAS system) is recorded on the B-CAS card  21 . 
     Encryptic key information for decrypting encrypted information contained in the received broadcast airwaves encrypted by a conditional access system (such as a B-CAS system) is recorded on the B-CAS card  21 . 
     The B-CAS interface  22  is an interface for entering encryptic key information recorded on the B-CAS card  21 . The interface  22  supplies the entered encryptic key information to the image processing portion  30 . 
     The modem interface  23  is an interface connected, for example, with an ADSL modem and sends and receives given data to and from the Internet network. 
     The Ethernet interface  24  is an interface for performing communications with other information processor, for example, via a local area network. 
     The image processing portion  30  has a control portion  31  for controlling the whole processing about the LCD  1 , a video signal processing portion  32  for performing given processing on a video signal and outputting the processed signal to the display portion  40 , a digital-to-analog converter (DAC)  33  for converting a digital audio signal outputted from the control portion  31  into an analog audio signal, a switch  34  for distributing plural video signals from the signal input portion  10  to various processing portions, a sub-chroma decoder  35  for performing given processing on analog color-difference video signals and converting the video signals into digital video signals, an analog-to-digital converter (ADC)  36  for converting an analog video signal into a digital video signal, an HDMI receiver  37  for receiving an HDMI signal, separating it into a digital video signal and a digital audio signal, and outputting these signals, and a backlight driving portion  38  for controlling the operation of a backlight driving circuit  46  of the display portion  40  (described later). 
     The control portion  31  is a processing portion for controlling the overall processing of the LCD  1 . In particular, the control portion is realized by a processor having the following functions. 
     As a first function, the control portion  31  is equipped with a decoder  31   a  for separating a digital TV signal supplied from the tuner  13  into a digital video signal and a digital audio signal. The decoder  31   a  supplies the demodulated digital video signal to the video signal-processing portion  32 . The decoder also supplies the demodulated digital audio signal to the digital-to-analog converter (DAC)  33 . The decoder  31   a  converts the demodulated digital video signal into an analog video signal and supplies the resulting analog video signal to the switch  34 . 
     When the digital TV signal is encrypted with a B-CAS system, the decoder  31   a  decrypts the signal based on the encryptic key information supplied from the B-CAS interface  22 , thus demodulating the signal into descrambled digital video and audio signals. 
     As a second function, the control portion  31  controls the operation of the backlight driving portion  38 . This processing will be described later. 
     The control portion  31  implements functions other than the first and second functions. In particular, the control portion performs processing for making reservations for recordings of TV programs. Furthermore, the control portion performs processing for supplying given electric power to various processing portions. 
     The video signal processing portion  32  converts the digital video signal demodulated by the decoder  31   a  of the control portion  31  into three color signals synchronized among the three primary colors of light, and supplies the color signals to the display portion  40 . The video signal-processing portion  32  performs processing for converting an interlaced video signal into a progressive video signal. Furthermore, the video signal-processing portion  32  supplies luminance information included in the video signal to the control portion  31 . The video signal processing portion  32  performs conversion processing regarding the color signals on digital video signal supplied from the sub-chroma decoder  35  (described later) and HDMI receiver  37  and on the analog video signal supplied from the switch  34 , as well as on color signals supplied from the decoder  31   a . The converted color signals are supplied to the display portion  40 . 
     The digital-to-analog converter  33  converts the digital audio signal demodulated by the decoder  31   a  of the control portion  31  into an analog audio signal, and supplies the converted analog audio signal to the signal output portion  50 . 
     The switch  34  receives the analog video signals supplied from both of the signal input portion  10  and the decoder  31   a  of the control portion  31  and switches the connection of signal lines to supply the entered signals to the video signal processing portion  32 , sub-chroma decoder  35 , analog-to-digital converter  36 , and signal output portion  50 . 
     The sub-chroma decoder  35  receives the video signals from the switch  34 , selects those video signals in which the color-difference signals of Cr and Cb are quadrature-encoded out of the received video signals, quadrature-demodulates the selected video signals to digitize the analog signals quadrature-demodulated with respect to the color-difference signals, and supplies the digitized video signals to the video signal processing portion  32 . 
     The analog-to-digital converter  36  digitizes the video signal supplied from the switch  34  and supplies the digitized video signal to the HDMI receiver  37 . In the analog video signal supplied to the analog-to-digital converter  36 , color-difference signals are not quadrature-encoded. 
     The HDMI receiver  37  receives the HDMI signal from the signal input portion  10 , separates it into a digital video signal and a digital audio signal, and supplies the separated digital video signal to the video signal processing portion  32 , and supplies the separated digital audio signal to the control portion  31 . The HDMI receiver  37  switches the signal to be supplied to the video signal-processing portion  32  between the separated digital video signal and the digital video signal supplied from the analog-to-digital converter  36 . 
     The backlight driving portion  38  creates a pulse-width modulated drive signal and supplies the drive signal to the backlight driving circuit  46  fitted in the display portion  40  (described later), thus controlling the operation of the backlight driving circuit  46 . 
     The display portion  40  has an LCD panel  41  for displaying a video signal by applying voltages to electrodes disposed in the x- and y-directions of the xy orthogonal coordinate system, an x-electrode driver  42  for supplying a driver voltage to the x-direction electrodes of the LCD panel  41 , a y-electrode driver  43  for supplying a drive voltage to the y-direction electrodes of the LCD panel  41 , a driver control portion  44  for activating the x-electrode driver  42  and y-electrode driver  43  according to the video signal supplied from the image processing portion  30 , the backlight  45  for illuminating the LCD panel  41  from the rear, and the backlight driving circuit  46  for supplying a driver voltage to the backlight  45 . 
     The LCD panel  41  is an active-matrix panel in which electrodes are disposed, for example, in the x- and y-directions. Active elements corresponding to the pixels are formed at the intersections of the x- and y-electrodes. 
     The x-electrode driver  42  applies a drive voltage to the electrodes arrayed in the x-direction of the LCD panel  41 . 
     The y-electrode driver  43  applies a drive voltage to the electrodes arrayed in the y-direction of the LCD panel  41 . 
     The driver control portion  44  operates the x-electrode driver  42  and y-direction driver  43  in response to the video signal supplied from the image processing portion  30 . 
     The backlight  45  is an illumination device disposed below the LCD panel  41 . The backlight includes cold cathode fluorescent lamps (CCFLs) disposed parallel to the LCD panel  41 . 
     The backlight driving circuit  46  supplies a drive voltage to the cold cathode fluorescent lamps of the backlight  45 . 
     The signal output portion  50  has an audio output portion  51  for performing given processing on the analog audio signal entered from the signal input portion  10 , a phono amplifier  52  for amplifying the analog audio signal outputted from the audio output portion  51 , a power amplifier  53  for amplifying the analog audio signal outputted from the audio output portion  51 , a loudspeaker  54  for producing audible sound from the analog audio signal amplified by the power amplifier  53 , a recording output terminal  55  for outputting a video signal including an analog video signal and an analog audio signal to the outside, an analog audio output terminal  56  for outputting the analog audio signal from the audio output portion  51  to the outside, a digital audio signal output terminal  57  for outputting the digital audio signal coming from the audio output portion  51  to the outside, and a phono output terminal  58  for outputting the analog audio signal amplified by the phono amplifier  52  to the outside. 
     The audio output portion  51  receives the analog audio signals supplied from the signal input portion  10  and the digital-to-analog converter  33  of the image processing portion  30  and switches the connection of signal lines for supplying the received signals to the output terminals. In particular, the audio output portion  51  supplies the analog audio signal to the phono amplifier  52 , power amplifier  53 , recording output terminal  55 , and analog audio output terminal  56 , and supplies the digital audio signal to the digital audio output terminal  57 . 
     The phono amplifier  52  amplifies the analog audio signal outputted from the audio output portion  51  and outputs the amplified signal to the outside from the phono output terminal  58 . 
     The power amplifier  53  amplifies the analog audio signal outputted from the audio output portion  51  and supplies the amplified signal to the loudspeaker  54 . 
     The loudspeaker  54  produces audible sound in response to the analog audio signal supplied from the power amplifier  53 . 
     The recording output terminal  55  is a connector terminal permitting the analog audio signal supplied from the audio output portion  51  and the analog video signal supplied from the video signal processing portion  30  to be outputted to the outside, for example, as analog video signals for recording. 
     The analog audio output terminal  56  is a connector terminal permitting the analog audio signal supplied from the audio output portion  51  to be outputted to the outside. 
     The digital audio output terminal  57  is a connector terminal permitting the digital audio signal supplied from the audio output portion  51  to be outputted to the outside. For example, the outputted signal is a light signal. 
     The phono output terminal  58  is a connector terminal permitting the audio signal supplied from the phono amplifier  52  to be outputted to the outside. 
     Processing for driving the backlight  45  is next described. 
       FIG. 2  is a schematic block diagram showing the configuration of the processing system of the above-described LCD  1  which is associated especially with the operation for driving the backlight  45 . 
     The control portion  31  of the image processing portion  30  has the above-described decoder  31   a . In addition, the control portion  31  has a luminance data input portion  311  for receiving luminance data included in a video signal, a duty cycle-setting portion  312  for setting the duty cycle of a PWM signal created by the backlight driving portion  38  according to the value indicated by the luminance data entered into the luminance data input portion  311 , a video signal decision portion  313  for making a given decision according to the value indicated by the luminance data entered into the luminance data input portion  311 , and a user setting portion  314  to which setting information about the operation of the duty cycle-setting portion  312  is supplied via a user interface. 
     Luminance data included in the video signal decrypted by the decoder  31   a  and luminance data included in the video signal processed by the video signal processing portion  32  are entered into the luminance data input portion  311 . The input portion  311  supplies the entered luminance data to the duty cycle-setting portion  312  and the video signal decision portion  313 . 
     The duty cycle-setting portion  312  creates a duty cycle-setting signal S 1  for setting the duty cycle of a pulsed signal created by the backlight driving portion  38  according to the luminance data supplied from the luminance data input portion  311 . Specifically, the duty cycle-setting portion  312  creates the duty cycle-setting signal S 1  that sets the duty cycle of the PWM signal created by the backlight driving portion  38  as the luminance value included in the luminance data is increased. 
     That is, the duty cycle-setting portion  312  sets the duty cycle between Dmin and Dmax, where Dmin and Dmax are nonzero effective values. The duty cycle-setting portion  312  sets the duty cycle to Dmin when the value indicated by the luminance data is at minimum and sets the duty cycle to Dmax when the value indicated by the luminance data is at maximum. If the value indicated by the luminance data is small, the duty cycle is not set to 0 by the duty cycle-setting portion  312 , for the following reason. If the backlight  45  is ceased to be applied with a voltage for more than a given time, and if one tries to drive the backlight  45  by subsequently creating a pulsed signal having an effective duty cycle, there is the danger that the backlight  45  is not lit up due to the dead band characteristics. This depends on general temperature characteristics of the backlight. The state in which the backlight is not lit up is prevented taking account of such dead band characteristics. Therefore, a related-art backlight-driving system is so designed that a voltage is typically always applied to the backlight, for example, at every PWM period even when an ineffective video signal having a luminance value of 0 at all times is displayed. 
     The video signal decision portion  313  makes a decision as to whether an effective video signal is entered from the signal input portion  10  to the image processing portion  30  according to whether the value indicated by the luminance data supplied from the luminance data input portion  311  is smaller than a given threshold value. For example, when the average of values indicated by the luminance data about individual frames is smaller than the given threshold value, the video signal decision portion  313  determines that the video signal is not effective. When the average of values indicated by the luminance data about individual frames taken over plural frames is smaller than a given threshold value, the video signal decision portion  313  may determine that the video signal is not effective. The video signal decision portion  313  supplies a decision signal S 2  according to the result of the decision to the backlight driving portion  38 . 
     Setting information regarding the operation of the duty cycle-setting portion  312  is supplied to the user setting portion  314  via the user interface (not shown). Specifically, a value about a threshold value about the processing performed by the video signal decision portion  313  is supplied as the setting information from the user interface to the user setting portion  314 . The setting information concerning the threshold value is supplied to the video signal decision portion  313 . 
     The backlight driving portion  38  has a PWM signal-creating portion  381  for creating a PWM signal according to the duty cycle-setting signal S 1  supplied from the control portion  31  and a thinning-out portion  382  for thinning out pulses forming the PWM signal created by the PWM signal-creating portion  381  according to the decision signal S 2  supplied from the control portion  31 . 
     The PWM signal-creating portion  381  is a signal-generating circuit having a fixed PWM period, and creates a PWM signal corresponding to the duty cycle-setting signal S 1  supplied from the control portion  31 . That is, the PWM signal-creating portion  381  creates a PWM signal made of pulses having a duty cycle corresponding to the duty cycle-setting signal S 1  and supplies the created PWM signal to the thinning-out portion  382 . Because the duty cycle set by the duty cycle-setting signal is between Dmin to Dmax, the PWM signal-creating portion  381  creates a drive signal made of pulses which are pulse width-modulated so as to have an effective duty cycle over the whole PWM period. In the present embodiment, it is assumed, for example, that the PWM frequency is 150 Hz. 
     The thinning-out portion  382  thins out the pulses forming the PWM signals created by the PWM signal-creating portion  381  according to the decision signal S 2  supplied from the control portion  31 . In particular, if the video signal decision portion  313  of the control portion  31  has determined that the video signal is ineffective and correspondingly the decision signal S 2  is outputted, and if the backlight  45  is attempted to be driven by supplying a signal having an effective duty cycle, then the thinning-out portion  382  supplies the drive signal S 3  to the backlight driving circuit  46 , the signal S 3  being made up of pulses which form the PWM signal and which have been thinned out in such an extent as to prevent the danger that the backlight is not lit up. Meanwhile, if the video signal decision portion  313  of the control portion  31  has determined that the video signal is effective and correspondingly the decision signal S 2  is supplied, the thinning-out portion  382  supplies the drive signal S 3  to the backlight driving circuit  46  without thinning out the pulses forming the PWM signal. 
     The backlight driving circuit  46  applies a drive voltage S 4  to the backlight  45 , the voltage S 4  being pulse number-modulated according to the drive signal S 3  supplied from the backlight driving portion  38 . The frequency of the drive voltage S 4  created by the backlight driving circuit  46  is set much higher than the PWM frequency of the drive signal created by the backlight driving circuit  38 . In the present embodiment, the frequency is set to 50 kHz, for example. 
     The backlight  45  lights up the cold cathode fluorescent lamps in response to the drive voltage S 4  supplied from the backlight driving circuit  46  to illuminate the rear surface of the LCD panel  41 . 
     In the present embodiment, the functions associated with the luminance data input portion  311 , duty cycle-setting portion  312 , video signal decision portion  313 , and user setting portion  314  are incorporated on a processor that realizes the control portion  31 . The present embodiment is not limited to this configuration. The functions may also be incorporated, for example, in the backlight driving portion  38 . 
     The operation of the backlight  45  that becomes different according to the state of the video signal is described by referring to  FIG. 3 , where the relationship between the drive signal S 3  created by the backlight driving portion  38  and the drive voltage S 4  created by the backlight driving circuit  46  is shown. 
       FIG. 3A  is a graph showing the drive signal S 3  and drive voltage S 4  under the condition where a video signal, for example, having luminance data indicating a very high average luminance value is displayed on the LCD panel  41 . 
     At this time, the duty cycle-setting portion  312  creates a duty cycle-setting signal for setting the duty cycle to Dmax, for example, because the average luminance value indicated by the luminance data is very high. The created signal is supplied to the PWM signal-creating portion  381 . Because the average luminance value indicated by the luminance data is very high, the video signal decision portion  313  determines that the video signal is effective. The decision portion  313  supplies the decision signal S 2  corresponding to the result of the decision to the thinning-out portion  382 . 
     Therefore, the PMW signal-creating portion  381  creates a pulsed signal having a duty cycle of Dmax and supplies the signal to the thinning-out portion  382 , which in turn supplies the drive signal S 3  ( FIG. 3A ) whose pulses are not thinned out according to the decision signal S 2  to the backlight driving circuit  46 . 
     The backlight driving circuit  46  applies the drive voltage S 4  to the backlight  45 , the voltage S 4  being made of a pulsed signal that is PNM-modulated according to the drive signal S 3  supplied from the backlight driving portion  38 . 
       FIG. 3B  is a diagram showing drive signal S 3  and drive voltage S 4  under the condition, for example, where the video signal having luminance data whose average luminance value is relatively low is displayed on the LCD panel  41 . 
     At this time, the average luminance value of the luminance data is relatively low and so the duty cycle-setting portion  312  creates the duty cycle-setting signal S 1  for setting the duty cycle, for example, to Dmin and supplies the signal S 1  to the PWM signal-creating portion  381 . The video signal decision portion  313  determines that the average luminance value of the luminance data is relatively low but the video signal is effective, and supplies the decision signal S 2  corresponding to the result of the decision to the thinning-out portion  382 . 
     Therefore, the PWM signal-creating portion  381  creates a pulsed signal having a duty cycle of Dmin and supplies the created signal to the thinning-out portion  382 . The thinning-out portion  382  supplies the drive signal S 3  shown in  FIG. 3B , whose pulses are not thinned out according to the decision signal S 2 , to the backlight driving circuit  46 . 
     The backlight driving circuit  46  applies the drive voltage S 4  made of a pulsed signal, which is PNM-modulated according to the drive signal S 3  supplied from the backlight driving portion  38 , to the backlight  45 . 
       FIG. 3C  is a diagram showing the drive signal S 3  and drive voltage S 4  under the state in which the video signal having no effective luminance value is displayed on the LCD panel  41 . 
     Because the average luminance value of the luminance data is relatively low, the duty cycle-setting portion  312  creates the duty cycle-setting signal S 1  for setting the duty cycle, for example, to Dmin and supplies the created signal to the PWM signal-creating portion  381 . The video signal decision portion  313  determines that the video signal is not effective, and supplies the decision signal S 2  corresponding to the result of the decision to the thinning-out portion  382 . 
     Consequently, the PWM signal-creating portion  381  creates a pulsed signal having a duty cycle of Dmin and supplies the signal to the thinning-out portion  382 . The thinning-out portion  382  supplies the drive signal S 3 , whose pulses are thinned out at every PWM period as shown in  FIG. 3C  according to the decision signal S 2 , to the backlight driving circuit  46 . The thinning-out portion  382  thins out the pulses at every PWM period as a specific example. The invention is not limited to this example. The thinning-out portion may thin out more pulses at regular intervals of time to such an extent as to prevent the danger that the backlight is not lit up when the backlight  45  is attempted to be driven by supplying a signal having an effective duty cycle. 
     The backlight driving circuit  46  applies the drive voltage S 4  made of a pulsed signal, which is PNM-modulated according to the drive signal S 3  supplied from the backlight driving portion  38 , to the backlight  45 . 
     In this way, in the LCD  1 , when the video signal decision portion  313  has determined that the value indicated by the entered luminance data is smaller than a given value, the thinning-out portion  382  thins out pulses forming the PWM signal, and the backlight driving circuit  46  applies the drive voltage S 4  corresponding to the drive signal S 3  to the backlight  45 . In consequence, electric power consumed when an ineffective video signal is displayed on the LCD panel  41  can be reduced. 
     The drive signal S 3  as shown in  FIG. 3C  can be created, for example, by doubling the PWM period of the PWM signal-creating portion. In contrast, in the LCD  1 , the PWM period of the PWM signal-creating portion  381  is fixed. Accordingly, the backlight driving portion  38  associated with the present embodiment creates a similar signal without using a PWM signal-creating signal whose PWM period is variable. Hence, increase in circuit scale of the drive circuit that would otherwise be caused by using a PWM signal-creating circuit having a variable PWM period is suppressed. As a result, the power consumption can be reduced as mentioned previously. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.