JP2006039345A - Device for dimming backlight - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make a disturbance from an inverter circuit to a video signal inconspicuous as to control over backlight dimming, particularly, burst dimming used by the inverter for a backlight of a liquid crystal television, and a liquid crystal display, etc. <P>SOLUTION: The backlight dimming device is equipped with a PWM dimming circuit 1 which generates PWM pulses controlling the lightness of a backlight, a user I/F means 2 through which a user adjusts the backlight to arbitrary lightness, a microcomputer 3 which outputs a control signal for controlling the duty of an output panel from the PWM dimming circuit, based upon information from the user I/F, an oscillation circuit 4 which controls the oscillation frequency of the inverter with a detection current from a tube current detecting circuit and also controls the stop of the oscillation frequency with a dimming signal from the PWM dimming circuit, a drive circuit 5 which inputs an inverter control signal as the output from the oscillation circuit and drives a transformer 6, the transformer which drives a cold-cathode tube, the cold-cathode tube 7 which makes the liquid crystal panel illuminate, the tube current detecting circuit 8 which detects a current flowing to the cold-cathode tube, and a delay control circuit 9 which delays a vertical synchronization signal driving the liquid crystal panel with different delay quantities for each frame. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to backlight dimming used in backlight inverters such as liquid crystal televisions and liquid crystal displays, and more particularly to control of burst dimming.

  In general, video equipment using liquid crystal such as a liquid crystal television and a liquid crystal display emits light with a backlight using a cold cathode tube and displays an image. The inverter circuit is a circuit that creates a signal for controlling and driving the backlight. Usually, the brightness of the backlight is controlled by changing the input DC voltage or input current of the inverter circuit to change the tube current flowing in the cold-cathode tube and dimming the current (voltage) with a PWM pulse. There is a burst dimming (PWM dimming) method that controls oscillation / stop of the oscillation frequency. The former has the advantage that the noise (audible sound) from the inverter is small and stable, but has the disadvantage that the dimming range is narrow. On the other hand, the latter has an advantage that the light control range is wide, and is used for the function of allowing the user to adjust the backlight. There exists patent document 1 etc. as an example of the above-mentioned PWM dimming system.

FIG. 3 is a diagram showing an example of a conventional PWM dimming method, and FIG. 4 is a schematic diagram emphasizing the reduction of the influence of noise by the conventional backlight dimming. The PWM dimming circuit 1 calculates the duty of the PWM pulse to be set by the microcomputer 3 based on the backlight adjustment value from the user I / F means 2 and controls the presence or absence of oscillation of the oscillation circuit 4. The current flowing through the cold cathode tube 7 is detected by the tube current detection circuit 8 and the oscillation frequency of the oscillation circuit 4 is controlled. The output of the oscillation circuit 4 drives the transformer 6 through the drive circuit 5 and issues the cold cathode tube 7. At this time, the PWM pulse of the PWM dimming circuit is generated at a frequency synchronized with the vertical synchronization signal for driving the liquid crystal panel. Thus, as described in Patent Document 1, it is possible to emit light from the cold cathode tube without causing interference fringes between the scanning frequency of the liquid crystal panel and the frequency of the dimming signal.
JP 11-126696 A

  However, recent liquid crystal televisions, liquid crystal displays, and the like have increased their circuit scales with increasing functionality and performance, and are becoming more susceptible to noise in conjunction with the high density mounting of components. Among them, as described above, when the scanning frequency (vertical frequency) of the liquid crystal panel and the dimming signal of PWM dimming are synchronized, the vertical frequency is as low as 60 Hz, so the noise generated from the inverter circuit is also low. Become. As a result, as shown in FIG. 4, horizontal line noise synchronized with the display screen of the liquid crystal panel appears on the screen. In general, in order to suppress noise, a filter using a capacitor or a coil (inductor) is configured. If the noise is in a high frequency band, it can be easily suppressed by the above-described filter, but if the noise is in a low frequency, there is a problem that it is difficult to suppress.

  The invention according to claim 1 of the present invention is a PWM dimming type inverter circuit that controls the brightness of the backlight by the length of the ON / OFF period of the PWM pulse, and drives the liquid crystal panel with the frequency of the PWM pulse. In addition to being synchronized with the vertical synchronizing signal, the phase is offset for each frame to make it difficult to see the disturbance from the inverter circuit to the video signal.

  According to a second aspect of the present invention, there is provided a PWM dimming circuit for generating a PWM pulse for controlling the brightness of the backlight, a user I / F means for the user to adjust the backlight to an arbitrary brightness, Based on information from the user I / F, a microcomputer that outputs a control signal to control the duty of the output panel from the PWM dimming circuit, and the oscillation frequency of the inverter are controlled by the detected current from the tube current detection circuit An oscillation circuit for controlling the stop of the oscillation frequency by a dimming signal from the PWM dimming circuit, a drive circuit for driving a transformer with an inverter control signal output from the oscillation circuit as an input, and a cold cathode tube A cold cathode tube that emits light from the liquid crystal panel, a tube current detection circuit that detects a current flowing through the cold cathode tube, and a liquid crystal panel Characterized by comprising a delay control circuit which delays a different delay amount for each frame a vertical synchronizing signal for driving the.

  As described above, according to the present invention, in the PWM dimming type inverter circuit that controls the luminance of the backlight by the length of the ON / OFF period of the PWM pulse, the frequency of the PWM pulse is driven vertically to drive the liquid crystal panel. By synchronizing with the synchronization signal and offsetting the phase of each frame, it is possible to provide a backlight dimming device having an effect of making it difficult to see the interference from the inverter circuit to the video signal.

(Embodiment 1)
FIG. 1 is an example of a configuration diagram of a backlight dimming device of the present invention, and FIG. 2 is a schematic diagram emphasizing the reduction of the influence of noise due to backlight dimming.

  In FIG. 1, 1 is a PWM dimming circuit for generating a PWM pulse for controlling the brightness of the backlight, 2 is a user I / F means for the user to adjust the backlight to an arbitrary brightness, and 3 is the user A microcomputer that outputs a control signal to control the duty of the output panel from the PWM dimming circuit based on the information from the I / F, 4 controls the oscillation frequency of the inverter by the detected current from the tube current detection circuit An oscillation circuit for controlling the stop of the oscillation frequency by a dimming signal from the PWM dimming circuit, 5 a drive circuit for driving a transformer with an inverter control signal output from the oscillation circuit as an input, and 6 a cooling circuit A transformer for driving the cathode tube; 7 a cold cathode tube for emitting light from the liquid crystal panel; 8 a tube current detecting circuit for detecting a current flowing through the cold cathode tube; A delay control circuit which delays a different delay amount for each frame a vertical synchronizing signal for driving the liquid crystal panel. A specific operation will be described below.

  The PWM dimming circuit 1 calculates the duty of the PWM pulse to be set by the microcomputer 3 based on the backlight adjustment value from the user I / F means 2 and controls the presence or absence of oscillation of the oscillation circuit 4. The current flowing through the cold cathode tube 7 is detected by the tube current detection circuit 8 and the oscillation frequency of the oscillation circuit 4 is controlled. The output of the oscillation circuit 4 drives the transformer 6 through the drive circuit 5 and issues the cold cathode tube 7. At this time, the PWM pulse of the PWM dimming circuit is generated at a frequency synchronized with the vertical synchronizing signal for driving the liquid crystal panel, but the phase is controlled by the delay control circuit 9 with a different delay amount for each frame. By doing so, as shown in FIG. 2, the change point of the inverter control signal is not concentrated on a specific line on the liquid crystal panel screen, but is dispersed for each frame. This makes noise from the inverter circuit inconspicuous.

  The backlight dimming device according to the present invention makes it difficult to see the disturbance to the video signal from the PWM dimming inverter circuit, and is useful in liquid crystal televisions, liquid crystal displays, and the like.

The figure which showed the block diagram of the backlight light control apparatus of this invention Schematic diagram emphasizing the reduction of noise effects due to backlight dimming of the present invention The figure which showed the block diagram of the conventional backlight light control device Schematic diagram that emphasizes the reduction of noise effects by conventional backlight dimming

Explanation of symbols

1 PWM dimming circuit 2 User I / F means 3 Microcomputer 4 Oscillation circuit 5 Drive circuit 6 Transformer 7 Cold cathode tube 8 Tube current detection circuit 9 Delay control circuit

Claims (2)

In a PWM dimming inverter circuit that controls the brightness of the backlight by the length of the ON / OFF period of the PWM pulse, the frequency of the PWM pulse is synchronized with a vertical synchronization signal that drives the liquid crystal panel, and for each frame Backlight dimming device that makes it difficult to see the interference from the inverter circuit to the video signal by offsetting the phase. A PWM dimming circuit for generating a PWM pulse for controlling the brightness of the backlight, a user I / F means for the user to adjust the backlight to an arbitrary brightness, and information from the user I / F A microcomputer that outputs a control signal to control the duty of the output panel from the PWM dimming circuit, and the oscillation frequency of the inverter is controlled by the detected current from the tube current detection circuit, and the dimming from the PWM dimming circuit An oscillation circuit for controlling the stop of the oscillation frequency by a signal, a drive circuit for driving a transformer with an inverter control signal output from the oscillation circuit as an input, a transformer for driving a cold cathode tube, and a liquid crystal panel A cold cathode tube for emitting light, a tube current detection circuit for detecting a current flowing through the cold cathode tube, and a vertical synchronization signal for driving a liquid crystal panel Backlight dimmer apparatus characterized by comprising a delay control circuit which delays a different delay amount for each frame.

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