JPH07191298A - Liquid crystal display device with back light - Google Patents

Abstract

PURPOSE:To prevent irregularity in illuminance due to a flicker from being generated by equalizing the frequency of a back light illumination signal to the frequency of a vertical synchronous signal for driving a display part. CONSTITUTION:An integration circuit 10 consists of a diode D11, electric resistors R11-R14, a capacitor C11, and an amplifier A1. The integration circuit 10 generates a saw-tooth wave by inputting the vertical synchrorous signal VSY for module driving as original oscillation to the integration circuit 10. Then the generated saw-tooth wave and a back light illuminance control voltage are inputted to a comparator 20 consisting of an amplifier A2 to generate a rectangular wave synchronized with the signal VSY, and this rectangular wave is inputted to the back light for intermittent illumination. Thus, the irregularity in illuminance, etc., due to optical interference between a flicker component generated by the intermittent illumination of the back light and a flicker component of liquid crystal display driving can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device such as a liquid crystal television or a liquid crystal display to which a backlight (fluorescent lamp) is added.

[0002]

2. Description of the Related Art A conventional liquid crystal display device with a backlight function is shown in FIG. A waveform diagram is shown in FIG. The circuit i is a free-running multivibrator that oscillates a square wave, and a square wave is obtained at the output. The circuit ii is mono-multi, and by inputting an output, an arbitrary pulse width is output. The mono-multi output is input to the circuit iii integration circuit of the next stage and outputs a sawtooth wave to the output. The circuit iv is a comparator, which receives a sawtooth wave signal and a DC control voltage from the output and performs a comparison operation. Therefore DC
By raising and lowering the control voltage, the output DC
The pulse PWM (P
pulse width modulation) signal waveform is output.

In this conventional example, when the output pulse is "Hi", the backlight is turned on as shown in the lamp lighting period, the "Hi" period of the pulse is changed, and the lighting time of the backlight is changed. It is possible to dimming. Therefore, the illuminance level in this case shows a waveform approximate to a sine wave as shown in the illuminance level.

On the other hand, writing to the liquid crystal is as follows in the case of the active matrix. Considering an arbitrary picture element, the writing time for NTSC is about 50 μs,
Display data is written in the liquid crystal layer during this period. The data once stored is held (1.66 ms to 50 μs) by the liquid crystal layer until the next writing. However, in the normally white mode liquid crystal, the illuminance level is as shown in FIG. 8B because the data held little by little is lost during the holding period.

With the above technique, the display cycle of the liquid crystal and the lighting time cycle of the backlight are performed independently of each other.

[0006]

Since data is written to the liquid crystal layer at regular intervals in each display of the liquid crystal display section for displaying characters, numbers, etc., the human being is required to use the ratio of the writing time to the holding period. The flicker that I can't feel was generated. Further, by performing the intermittent lighting by the PWM method which is widely used as a method for dimming the backlight, flicker due to the backlight occurs. The situation where the flicker generated by the liquid crystal display device and the flicker generated by the backlight itself interfere with each other will be described below. Considering each of the above flicker by sine wave, it becomes as follows.

Liquid crystal display flicker: v _p = V _p sin2πf _p
t Backlight part flicker: v ₁ = V ₁ sin 2πf ₁
Considering as t AM modulation, an AM wave: v _m = V _m sin2πf _m t = (V ₁ + V _p sin2πf _p t) · sin2πf ₁ t = V ₁ (1 + msin2πf _p t) · sin 2πf ₁ t (1 · 1) where , M = V _p / V ₁ Therefore, optical interference occurs in the form shown by the equation (1.1). Therefore, irregular flicker occurs due to a mismatch between the flicker period of the liquid crystal display section and the flicker period of the backlight due to intermittent lighting, which causes uneven brightness, and uneven brightness that moves irregularly on the screen. It was recognized by the human eye.

An object of the present invention is to provide a liquid crystal display device in which uneven brightness is prevented.

[0009]

SUMMARY OF THE INVENTION The present invention comprises a display section,
A liquid crystal display device with a backlight, comprising: a vertical synchronization signal for driving the display unit; a backlight for generating light passing through the display unit; and a backlight lighting signal for intermittently lighting the backlight. The frequency of the backlight lighting signal is set to be the same as the frequency of the vertical synchronizing signal.

Further, the display unit, a vertical synchronizing signal for driving the display unit, a backlight for generating light passing through the display unit, and a backlight lighting signal for intermittently lighting the backlight are provided. In the liquid crystal display device with a backlight, the frequency of the backlight lighting signal is set sufficiently higher than the frequency of the vertical synchronizing signal.

[0011]

When the formula of trigonometric function is used for formula (1.1), v _m = V ₁ sin2πf ₁ t + mV ₁ sin2πf ₁ t · sin 2πf _p t = V ₁ sin 2πf ₁ t + (1/2) mV ₁ cos 2π (f _{1 -f p) t- (1/2)} mV 1 cos2π (f 1 + f p) t f 1 " when the _{f p, = V 1 sin2πf 1} t ... (1 · 2) display of the liquid crystal drive unit driving period f _With respect to _p , optical interference is reduced by setting the intermittent frequency f _{1 of the} backlight, which performs intermittent lighting for illuminating the liquid crystal display unit, to f ₁ >> f _p.
Since the equation (2) is obtained and the image moves on the screen regularly, it is possible to prevent the occurrence of uneven brightness due to flicker.

The optical interference can be synchronized by setting the intermittent frequency f ₁ of the backlight for intermittent lighting for illuminating the liquid crystal display unit to f ₁ = f _p with respect to the display drive cycle f _p of the liquid crystal drive unit. Therefore, the problem of flicker due to the mismatch between the drive cycle of the liquid crystal display section and the intermittent cycle of the backlight is eliminated, and it is possible to prevent the occurrence of luminance unevenness that regularly moves on the screen.

[0013]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of an embodiment of the present invention. In the figure, the integrating circuit 10 includes a diode D _{1 1} , electric resistors R _{1 1} , R _{1 2} , R _{1 3} , R _{1 4} and a capacitor C _{1 1.}
And an amplifier A ₁ . FIG. 3 shows the signal waveform. A sawtooth wave is generated by inputting VSY to the circuit 10 as an original oscillation. VSY is a vertical synchronizing signal for driving the module. The generated sawtooth wave and the backlight brightness control voltage are input to the comparator 20 composed of the amplifier A ₂ to generate the square wave shown as an example synchronized with VSY. By inputting the same waveform to the backlight,
As an example, the illuminance level is indicated by performing intermittent lighting as shown in the lamp lighting period. Writing is performed at the vertical synchronizing frequency as shown in FIG. 7 as an example, and the liquid crystal display section has an illuminance level as shown in FIG. 7 as an example. Since the two illuminance levels optically interfere with each other at a synchronized frequency, it is possible to prevent luminance unevenness due to a discrepancy between the drive cycle of the liquid crystal display unit and the intermittent lighting cycle of the backlight.

FIG. 2 is a circuit diagram of an embodiment of the present invention.
In the figure, an integrating circuit 30 includes a diode D _{3 1} , electric resistors R _{3 1} , R _{3 2} , R _{3 3} , R _{3 4} , and a capacitor C.
_{3 1} and amplifier A ₃ . The circuit 30 receives the square wave output from the comparator 40 and outputs a sawtooth wave (triangular wave). Comparator 40 is the electrical resistor _{R 4 1, R 4 2,} R 4 3, R 4 4 and the amplifier A
It is composed of ₄ . The sawtooth wave output from the integrating circuit 30 is input to the circuit 40, and a square wave is output. By inputting the same circuit diagram 2 to the backlight, intermittent lighting is performed. In the same circuit diagram 2, the frequency can be changed by changing the constants of the components. Therefore, by changing the constants of the parts, the frequency of intermittent lighting of the backlight is changed so that f ₁ >> f _p . As described above, the optical interference due to the driving cycle of the liquid crystal display section and the intermittent lighting cycle of the backlight is as shown by the equation (1.2), and the uneven brightness is prevented.

[0015]

As described above, according to the present invention, the flicker component generated by the intermittent lighting of the backlight and the flicker component for driving the liquid crystal display do not cause uneven brightness due to optical interference.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a backlight drive circuit that is an embodiment of the present invention.

FIG. 2 is a circuit diagram of a backlight drive circuit that is another embodiment of the present invention.

3 is an output waveform diagram of the circuit of FIG.

FIG. 4 is a circuit diagram of a conventional backlight drive circuit.

5 is an output waveform diagram of the circuit of FIG.

FIG. 6 is a backlight luminance-DC control voltage characteristic diagram.

FIG. 7 is a signal waveform diagram of a liquid crystal display section.

[Explanation of symbols]

 10 Integrator circuit 20 Comparator

Claims (2)

[Claims] 1. A display unit, a vertical synchronization signal for driving the display unit, a backlight for generating light transmitted through the display unit, and a backlight lighting signal for intermittently lighting the backlight. In the liquid crystal display device with a backlight, the liquid crystal display device with a backlight is characterized in that the frequency of the backlight lighting signal is the same as the frequency of the vertical synchronizing signal. 2. A display unit, a vertical synchronization signal for driving the display unit, a backlight for generating light passing through the display unit, and a backlight lighting signal for intermittently lighting the backlight. A liquid crystal display device with a backlight, wherein the frequency of the backlight lighting signal is set sufficiently higher than the frequency of the vertical synchronization signal.