JP2002062519A - Liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a liquid crystal display device having a wider light control width. SOLUTION: This display device is provided with a liquid crystal display panel 1, a light source 2 which is provided at the back side of the liquid crystal display panel 1 and a light source driving means 3 driving the light source 2 and the device is made to have constitution in which a light control means 7 which performs light control by changing the time ratio of a lighting period and an unlighted period in one light turned on-and-off period while controlling a light source driving means 3 so that the light source 2 is periodically turned on and off and also makes one light turned on-and-off period longer while maintaining the lighting period of the light source 2 as it is when it changes the time ratio to be equal to or smaller than a preliminarily set time ratio is provided.

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, and more particularly, to a liquid crystal display device having a backlight control function of periodically turning on a light source and changing the time ratio between a lighting period and a light-off period to control light. Related to the device.

[0002]

2. Description of the Related Art In recent years, liquid crystal display devices have been used for various products. The liquid crystal display device performs display by driving the liquid crystal corresponding to each pixel, but since the liquid crystal does not emit light by itself, the display reflects external light transmitted through the liquid crystal display panel or displays on the back of the liquid crystal display panel. The provided light-emitting body emits light for display.

In general, a luminous body provided on the back of a liquid crystal display panel is called a backlight, and a fluorescent tube is used. The fluorescent tube generates a discharge in the tube by applying a high voltage between electrodes provided at both ends of the tube. Ultraviolet light generated when the mercury vapor in the tube is excited to a high energy level by the discharge energy and returns to the energy level is converted into visible light by the phosphor applied on the inner surface of the tube to emit light. It has become. Since it is necessary to apply a high voltage to the fluorescent tube in order to cause the fluorescent tube to emit light, generally, an inverter converts low-voltage DC power into high-voltage, high-frequency (about 50 kHz or less) AC power, Supply to fluorescent tube.

[0004] As a dimming method of a liquid crystal backlight,
Conventionally, a PWM (Pulse Wide Modulation) light control method has been used. The PWM dimming method is a time-division method in which the light source is periodically turned on and off, and the time ratio between the lighting period and the light-off period is changed to perform light control, and the minimum dimming ratio (on-duty ratio) can be reduced. It has the feature of being able to.

[0005]

The liquid crystal display device using the PWM dimming method can theoretically control the light emission control from 0% to 100%, but the inverter applies the voltage to the fluorescent tube. Since a certain voltage application time is required for the fluorescent tube to be stably lit by the AC voltage, the brightness cannot be actually reduced to several percent or less. Therefore, for example, in a liquid crystal display device of an in-vehicle navigation device, there is a problem that the brightness cannot be sufficiently reduced in a dark room at night, which hinders safe driving.

The present invention has been made to solve such a problem, and an object of the present invention is to provide a liquid crystal display device having a wider light control width.

[0007]

In order to solve the above problems, a liquid crystal display device according to the present invention comprises a liquid crystal display panel, a light source provided on the back side of the liquid crystal display panel, and light source driving means for driving the light source. The light source drive means is controlled so that the light source periodically blinks, and the dimming is performed by changing the time ratio between the lighting period and the extinguishing period within one blinking period, and the time ratio is determined in advance. When the time ratio is changed to be equal to or less than the predetermined time ratio, a light control means for extending the one blinking cycle while maintaining the lighting period of the light source is provided.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A liquid crystal display device according to the present invention will be described with reference to FIGS. Here, FIG. 1 is a block diagram showing the configuration of the liquid crystal display device of the present invention, and FIG. 2 is a diagram showing the period of the voltage applied to the fluorescent tube when the duty is 100% in the liquid crystal display device of the present invention. 3 is a diagram showing the period of the voltage applied to the fluorescent tube when the duty is 50% in the liquid crystal display device of the present invention, and FIG. 4 is the voltage applied to the fluorescent tube when the duty is 2% in the liquid crystal display device of the present invention. FIG. 5 is a diagram showing the period of the voltage applied to the fluorescent tube when the duty is 1% in the liquid crystal display device of the present invention.

First, the configuration of the liquid crystal display device of the present invention is shown in FIG.
It will be described according to. In FIG. 1, 1 is a liquid crystal display unit, 2 is a fluorescent tube, 3 is an inverter unit, 4 is an inverter power supply unit, 5 is a filter unit, 6 is a video circuit, 7 is a control unit, Reference numeral 8 denotes an operation unit.

The liquid crystal display unit 1 includes, for example, a TFT (Thin F).
A liquid crystal display panel using an ilm transistor (thin film transistor), which is connected to a power supply unit (not shown) via a video circuit 6.

The fluorescent tube 2 is a backlight of the liquid crystal display 1, provided on the back of the liquid crystal display 1, and connected to the inverter 3. The inverter unit 3 is a fluorescent tube 2
The DC power is supplied to the inverter power supply unit 4 and the control unit 7 for converting DC power supplied to the power supply to high-voltage (for example, 2000 V) and high-frequency (for example, 50 kHz) AC power.

An inverter power supply unit 4 is for supplying a DC voltage (for example, 8 to 10 V) to the inverter unit 3 and is connected to a power supply unit for supplying power to the liquid crystal display unit 1 via a filter unit 5. Have been. The filter section 5 is for preventing a ripple component due to intermittent power supply (PWM control) of the inverter section 3 from entering the power supply side, and constitutes a low-pass filter.

The video circuit 6 is connected to, for example, a power supply unit for supplying a DC power of 12 V and the liquid crystal display unit 1.
Although not particularly shown, a video signal is externally input to the video circuit 6. The video circuit 6 controls the liquid crystal display 1 based on the video signal to display an image or video.

The control unit 7 is constituted by a microcomputer.
This is for changing the time during which the fluorescent tube 2 is lit and the cycle at which the fluorescent tube 2 is lit by PWM control of the inverter unit 3.
It is connected to the inverter unit 3 and the operation unit 8, and includes a storage unit (not shown). The storage unit includes software for the control unit 7 to perform PWM control of the inverter unit 3, a cycle in which the inverter unit 3 applies a voltage to the fluorescent tube 2, and a time period in which the inverter unit 3 applies a voltage to the fluorescent tube 2. Is stored in advance.

Here, the frequency of the voltage applied by the inverter unit 3 to the fluorescent tube 2 and the time during which the voltage is applied are determined by changing the PWM frequency in order to prevent flicker by changing the NTSC fV.
120 Hz, which is twice the frequency of 60 Hz, and 1 which is three times the frequency, 60 Hz
The frequency at which the inverter unit 3 applies a voltage to the fluorescent tube 2 is set to 1/150 (sec), ie, 150 Hz which is an intermediate frequency from 80 Hz. Assuming that (FIG. 2), when the user operates the operation unit 8 to adjust the luminance based on the duty of 100%, the inverter unit 3 applies a voltage to the fluorescent tube 2 for the duty of 100%. The application time is adjusted. For example, when the duty is adjusted from 100% to 50%, as shown in FIGS. 2 and 3, the voltage application time is set to a half of the voltage application time when the duty is 100%. Control. In addition, it is necessary to secure a sufficient time for the inverter unit 3 to apply a voltage to the fluorescent tube 2 so that the fluorescent tube 2 is stably lit. In the present embodiment, this time is set with a duty of 2% as a lower limit. I have.

The time during which the inverter unit 3 applies a voltage to the fluorescent tube 2 is adjusted by shortening the time during which the voltage is applied to the fluorescent tube 2 up to the lower limit (duty: 2%). When the duty is changed to a duty lower than the lower limit, the control unit 7 changes the cycle in which the inverter unit 3 applies a voltage to the fluorescent tube 2.

The operation unit 8 is for the user to perform various operations such as brightness adjustment. For example, although not shown, a switch for adjusting the brightness is provided. By performing the operation, the control unit 7 controls the inverter unit 3 based on this operation, and adjusts the cycle of the voltage that the inverter unit 3 supplies to the fluorescent tube 2.

Next, the operation of the liquid crystal display device of the present invention will be described. First, when the user operates the brightness adjustment button provided on the operation unit 8 to start brightness adjustment, the control unit 7
Controls the inverter unit 3 based on this brightness adjustment operation, and changes the time during which the inverter unit 3 applies a voltage to the fluorescent tube 2 in a stepwise manner. For example, assuming that the user operates the brightness adjustment button to reduce the duty from 100% to 2%, the control unit 7 controls the inverter unit 3 so that the 2 reduces the time during which the voltage is applied to 2%.

If the duty desired by the user is lower than the lower limit, that is, if the lower limit of the duty is reduced from 2% to a lower duty, for example, 1%, the control unit 7 sets Control is performed such that the period for supplying the voltage is lengthened while maintaining the time for the inverter unit 3 to supply the voltage to the fluorescent tube 2. For example, FIG.
As shown in (2), when the cycle of the voltage applied by the inverter unit 3 to the fluorescent tube 2 is 1/150 (sec), the control unit 7 controls the inverter unit 3 to apply the voltage to the fluorescent tube 2. While maintaining the time when the duty is 2%, the period of the voltage applied to the fluorescent tube 2 by the inverter unit 3 is changed so as to be gradually increased. In FIG.
By changing the period from 1/150 (sec) to 1/75 (sec), the fluorescent tube 2 emits light at a duty of 1%. If the duty is further reduced and controlled so as to be increased to the upper limit (for example, 1/70 (sec)) of a period determined by the afterglow characteristic of the fluorescent tube 2 and the afterimage characteristic of the eyes, the flicker cannot be visually recognized. % Can be reduced. By doing as described above, the duty ratio (the ratio between the maximum luminance and the minimum luminance) can be doubled or more.

Further, since the cycle is extended after the duty is lowered, the low-order component in the harmonic components of the PWM frequency to be removed by the low-pass filter of the filter section decreases when the duty is low. Ripple countermeasures become easy, for example, a small and inexpensive product can be used.

[0021]

As described above, the liquid crystal display device of the present invention comprises a liquid crystal display panel, a light source provided on the back side of the liquid crystal display panel, and light source driving means for driving the light source.
The light source driving means is controlled so that the light source periodically blinks, and the light control is performed by changing the time ratio between the lighting period and the light-off period within one blinking period, and the time ratio is equal to or less than a predetermined time ratio. In the case of changing to the above, the liquid crystal display device having a wide dimming width can be provided by providing the dimming means for extending the one blinking period while maintaining the lighting period of the light source.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a liquid crystal display device of the present invention.

FIG. 2 shows a duty 10 in the liquid crystal display device of the present invention.
It is a figure which shows the period of the voltage applied to a fluorescent tube at 0%.

FIG. 3 shows a duty 50 in the liquid crystal display device of the present invention.
It is a figure which shows the period of the voltage applied to a fluorescent tube at%.

FIG. 4 shows a duty of 2% in the liquid crystal display device of the present invention.
FIG. 5 is a diagram showing a cycle of a voltage applied to the fluorescent tube at the time of FIG.

FIG. 5 shows a duty of 1% in the liquid crystal display device of the present invention.
FIG. 5 is a diagram showing a cycle of a voltage applied to the fluorescent tube at the time of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Liquid crystal display part 2 Fluorescent tube 3 Inverter part 4 Inverter power supply part 5 Filter part 6 Video circuit 7 Control part 8 Operation part

Claims (1)

[Claims] 1. A liquid crystal display panel, comprising: a light source provided on the back side of the liquid crystal display panel; and light source driving means for driving the light source, wherein the light source driving means is configured to periodically blink the light source. And dimming by changing the time ratio between the lighting period and the light-off period within one blink cycle.
When the time ratio is changed to a predetermined time ratio or less, a dimming unit for increasing the one blinking cycle while maintaining the lighting period of the light source is provided.