Liquid crystal display with back-light control function

• US 5,844,540 A
• Filed: 12/29/1997
• Issued: 12/01/1998
• Est. Priority Date: 05/31/1994
• Status: Expired due to Term

• Current Assignee: Sharp Kabushiki Kaisha (Hon Hai Precision Industry Co., Ltd.)
• Original Assignee: Sharp Kabushiki Kaisha (Hon Hai Precision Industry Co., Ltd.)
• Inventors: Terasaki, Hirohide
• Primary Examiner(s): Hjerpe, Richard A.
• Assistant Examiner(s): CHANG, KENT WU
• Application Number:
  US08/998,829
• Time in Patent Office: 337 Days
• Field of Search: 345/102, 345/87, 345/213, 349/61, 349/69, 349/70, 315/DIG. 4
• US Class Current: 345/102
• CPC Class Codes: G09G 2310/0237 Switching ON and OFF the backlight within one frame
• CPC Class Codes: G09G 2320/0247 Flicker reduction other than flicker reduction circuits used for single beam cathode-ray tubes
• CPC Class Codes: G09G 2320/0606 Manual adjustment
• CPC Class Codes: G09G 2320/0626 for control of overall brightness
• CPC Class Codes: G09G 2320/064 by time modulation of the brightness of the illumination source
• CPC Class Codes: G09G 2330/06 Handling electromagnetic interferences [EMI], covering emitted as well as received electromagnetic radiation
• CPC Class Codes: G09G 2360/144 the light being ambient light
• CPC Class Codes: G09G 2360/145 the light originating from the display screen
• CPC Class Codes: G09G 3/3406 Control of illumination source illumination devices structurally associated with liquid crystal cells G02F1/1336
• CPC Class Codes: H05B 41/3927 by pulse width modulation H05B41/3922 takes precedence

Abstract

A liquid crystal display with a back-light control function is provided with a PWM dimmer driving circuit section for applying a PWM dimming to a fluorescent tube provided on the back surface of a liquid crystal panel by controlling an inverter section. The PWM dimming frequency by the PWM dimmer driving circuit section is set such that m flashes occur (m is an integer of not less than n, and is not a multiple of n) in n screen display periods of the liquid crystal panel (n is an integer of not less than 2), for example, 5 frequency occurs in 2 display periods. Based on a display panel vertical synchronizing signal corresponding to the vertical driving frequency of the liquid crystal panel, the PWM dimmer driving circuit section controls the inverter section while synchronizing a lighting timing of the fluorescent tube with a driving timing of the liquid crystal panel. As a result, an occurrence of flicker and flutter can be prevented effectively. Moreover, a sound noise generated in the inverter section can be reduced. Furthermore, variations in dimming can be reduced.

Claims

1. A liquid crystal display with a back-light control function, comprising:

a liquid crystal panel;

display panel driving means for periodically displaying on said liquid crystal display panel by periodically supplying thereto a driving signal;

a light source formed on a back surface of said liquid crystal display panel;

light source driving means for driving said light source; and

dimmer means for dimming with a varying time ratio between a light-on duration and a light-out duration in one lighting period by controlling said light source driving means so as to flash said light source periodically,

wherein said dimmer means controls said light source driving means so as to set the lighting frequency for flashing said light source such that m flashes occur, where m is an integer larger than n, and not a multiple of n, in n screen display periods of said liquid crystal panel, where n is an integer larger than 2 and where a screen display period is a period for the liquid crystal display to refresh its content once.

2. The liquid crystal display with the back-light control function as set forth in claim 1, wherein:

said dimmer means controls said light source driving means so as to have a lighting frequency such that said light source flashes odd number of at least three times in 2 screen display periods.

3. The liquid crystal display with a back-light control function as set forth in claim 1, wherein:

said light source is a fluorescent tube, and

said light source driving means includes a direct current power supply and an inverter circuit for converting a direct current from said direct current power supply into a high frequency alternate current to be applied to said fluorescent tube.

4. The liquid crystal display with a back-light control function as set forth in claim 1, further comprising:

vertical synchronizing signal generation means for generating a display panel vertical synchronizing signal corresponding to a vertical driving frequency of said liquid crystal display panel by said display panel driving means,

wherein said dimmer means includes synchronization means for synchronizing the lighting timing of said light source with a driving timing of said liquid display panel, and controls said light source driving means while synchronizing the lighting timing of said light source with the driving timing of said liquid crystal display panel at every predetermined screens.

5. The liquid crystal display with a back-light control function as set forth in claim 4, wherein:

said synchronization means includes an one-half dividing circuit for dividing a frequency of the display panel vertical synchronizing signal into one-half, and

said synchronization mean s synchronizes the lighting timing of said light source with the driving timing of said liquid crystal display panel at every 2-screen period.

6. The liquid crystal display with a back-light control function as set forth in claim 1, further comprising:

horizontal synchronizing signal generating means for generating a display panel horizontal synchronizing signal corresponding to a horizontal driving frequency of said liquid crystal display panel by said display panel driving means,

wherein said dimmer means includes dividing means for dividing a frequency of the display panel horizontal synchronizing signal so as to set the lighting frequency of said light source by dividing the frequency of the display panel horizontal synchronizing signal.

7. The liquid crystal display with a back-light control function as set forth in claim 6 further comprising:

image processing means for converting a television image of a NTSC system in which 2 vertical periods correspond to 525 horizontal periods into a suitable format to be processed in said display panel driving means,

wherein said dimmer means sets the lighting frequency of the light source such that 5 flashes occur in 2 screen display periods by dividing the frequency of the display panel horizontal synchronizing signal by 105.

8. The liquid crystal display with a back-light control function as set forth in claim 6 further comprising:

image processing means for converting a television image of a PAL system in which 2 vertical periods correspond to 625 horizontal periods into a suitable format to be processed in said display panel driving means,

wherein said dimmer means sets the lighting frequency of the light source such that 5 flashes occur in 2 screen display periods by dividing the frequency of the display panel horizontal synchronizing signal by 125.

9. The liquid crystal display with a back-light control function as set forth in claim 6, further comprising:

video processing means for converting the television video signal into a suitable format to be processed in said display panel driving means so as to enable both a television image of the NTSC system in which 2 vertical periods correspond to 525 horizontal periods and a television image of the PAL system in which 2 vertical periods correspond to 625 horizontal periods to be displayed on said liquid crystal display panel,

wherein said image processing means includes judging means for determining whether the television video signal is of the NTSC system or of the PAL system so as to output a discrimination signal indicating a result of determination, and

said dimmer means switches a dividing ratio of said dividing means based on the discrimination signal, and divides the frequency of the display panel horizontal synchronizing signal by 105 when the discrimination signal is of the NTSC system, while divides the frequency of the display panel horizontal synchronizing signal by 125 if the discrimination signal is of the PAL system so as to set the lighting frequency of said light source such that 5 flashes occur in 2 vertical periods irrespectively of a system of the television video signal.

10. The liquid crystal display with a back-light control function as set forth in claim 6, further comprising:

light-on period set means for setting the light-on period in one lighting frequency of said light source,

wherein said dimmer means includes count means for counting a number of pulses of the display panel horizontal synchronizing signal and determines the light-on period set by said light-on period set means based on a count of the number of pulses of the display panel horizontal synchronizing signal.

11. The liquid crystal display with a back-light control function as set forth in claim 10,

wherein when said light-on period set means sets the light-on duration so as to prevent a complete light-on and a complete light-out, said light-on period set means does not set the light-on duration shorter than a lower limit of a light-on duration set beforehand nor longer than a upper limit of the light-on duration set beforehand to limit a minimum light-on duration and a minimum light-out duration in one lightning frequency of said light source.

12. The liquid crystal display with a back-light control function as set forth in claim 10, wherein:

said light-on period set means sets a light-on duration by outputting digital control data corresponding to a number of pulses of the display panel horizontal synchronizing signal to said dimmer means, and

said dimmer means alters a count value of said count means for counting a number of flashes of said light source in one lighting period of said light source based on the digital control data.

13. The liquid crystal display with a back-light control function as set forth in claim 12,

wherein said light-on period set means includes detection means for detecting an intensity of incident light from an outside, and

said light-on period set means is provided with an automatic correcting function for alternating the control data so as to set the light-on duration longer as the intensity of the incident light from the outside increases.

14. The liquid crystal display with a back-light control function as set forth in claim 12,

wherein said light-on period set means monitors a power source voltage of said light source driving means for driving said light source and is provided with an automatic correcting function for alternating the control data so as to set the light-on duration longer when the power source voltage becomes lower than a predetermined level.

15. The liquid crystal display with a back-light control function as set forth in claim 12, wherein:

said light source is a fluorescent tube, and

said light-on period set means includes detection means for detecting a temperature on a surface of the fluorescent tube, and is provided with an automatic adjusting function for alternating the control data according to a detection output from said detection means.

16. The liquid crystal display with a back-light control function as set forth in claim 12, wherein:

said light-on period set means includes detection means for detecting a brightness of said light source and is provided with an automatic adjusting function for alternating the control data based on a detection output from said detection means.

17. The liquid crystal display with a back-light control function as set forth in claim 1, further comprising:

television image processing means for converting a television image into a suitable form to be processed by said display panel driving means;

computer graphic image processing means for converting a computer graphic image into a suitable form to be processed in said display panel driving means; and

image switching means for switching a video signal to be inputted to said display panel driving means either to a video signal processed in said television video processing means or to a video signal processed by said computer graphic image processing means.

18. The liquid crystal display with a back-light control function as set forth in claim 1,

wherein said liquid crystal display panel includes plural display areas respectively having different display periods so as to enable plural images to be displayed simultaneously, and

said display panel driving means controls a display in each display area such that a display frequency of one display area is an integer multiple of a display frequency of another display area.

19. The liquid crystal display with a back-light control function as set forth in claim 1, further comprising:

television image processing means for converting a television image into a suitable form to be processed in said display panel driving means;

computer graphic video processing means for converting the computer graphic image into a suitable form to be processed in said display panel driving means,

wherein said liquid crystal panel includes a first display area for displaying the television image and a second display area for displaying the computer graphic image, and

said display panel driving means controls a display in each display area such that a display frequency of one of the first display area and the second display area is an integer multiple of a display frequency of the second display area and the first display area, respectively.

20. A method for reducing flicker in a liquid crystal display with a back-light control function, comprising the steps of:

periodically displaying on said a crystal display panel by periodically supplying thereto a driving signal;

driving a light source formed on a back surface of said liquid crystal display panel;

dimming with a varying time ratio between a light-on duration and a light-out duration in one lighting period flashing said light source periodically; and

setting the lighting frequency for flashing said light source such that m flashes occur, where m is an integer larger than n, and not a multiple of n, in n screen display periods of said liquid crystal panel, where n is an integer larger than 2 and where a screen display period is a period for the liquid crystal display to refresh its content once.

21. The method as recited in claim 20, further comprising:

generating a display panel vertical synchronizing signal corresponding to a vertical driving frequency of said driving step;

dividing a frequency of the display panel vertical synchronizing signal into one-half; and

synchronizing the lighting timing of said light source with the driving timing of said liquid crystal display panel at every two screen period.

22. The method as recited claim 20, further comprising:

generating a display panel horizontal synchronizing signal corresponding to a horizontal driving frequency of said liquid crystal panel by said driving step; and

dividing a frequency of the display panel horizontal synchronizing signal so as to set the lighting frequency of said light source by dividing the frequency of the display panel horizontal synchronizing signal.