JP2000075262A - Backlight driving method for liquid crystal display device, and backlight unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a liq. crystal display panel from being damaged by avoiding a surplus temp. raise even if generating abnormality in a temp. sensor in a backlight of a liq. crystal display device used in an environment severely varying mainly in a temp. condition. SOLUTION: When power supply to a cold cathode tube 12 is started, the power corresponding to the temp. is supplied to the cold cathode tube 12 by the temp. raise caused by the heat generation of a lamp by a temp. sensor 14, in the case that the temp. rising cannot be detected even if the power is supplied to the cold cathode tube 12, that the temp. sensor 14 gets out of order or a short circuit with dew condensation is considered. In this case, the power supplied to the cold cathode tube 12 is limited in a preliminarily fixed value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a backlight driving method and a backlight unit for a liquid crystal display device mainly used outdoors, such as a liquid crystal display device of an on-vehicle AV (audio visual) system or an on-vehicle navigation system. About.

[0002]

2. Description of the Related Art FIG. 5 is a schematic diagram showing a configuration of a liquid crystal display device used in an on-vehicle AV system or a navigation system. The liquid crystal display panel 31 includes a pair of transparent substrates, liquid crystal sealed between the transparent substrates, and a pair of polarizing plates disposed with the pair of transparent substrates interposed therebetween. One of the pair of transparent substrates is provided with a transparent common electrode and a color filter, and the other is formed with transparent pixel electrodes arranged in a matrix. When a voltage is applied between the common electrode and the pixel electrode of the liquid crystal display panel 31, the liquid crystal molecules between the common electrode and the pixel electrode are arranged along the electric field, and the light transmittance of that portion of the liquid crystal display panel 31 is changed. Changes. By controlling the applied voltage for each pixel electrode, a desired image can be displayed on the liquid crystal display panel 31.

The liquid crystal display panel 31 is mounted on a box-shaped lamp house 30. In this lamp house 30, a cold cathode tube 32 is arranged as a backlight. A heater 33 is wound around the cold cathode tube 32,
The heater 33 heats the cold cathode tube 32. Further, between the liquid crystal display panel 31 and the cold cathode tube 32, a milky white plate 35 for scattering light output from the cold cathode tube 32 is arranged. Further, a temperature sensor 34 such as a thermistor is provided near the cold cathode tube 32.

[0004] The cold cathode fluorescent lamp 32 has the property that the luminance greatly changes depending on the temperature. For example, when the temperature is 0 ° C,
The brightness of the cold cathode tube 32 is about 10 to 20% of the maximum brightness. Therefore, in the liquid crystal display device, when the temperature of the cold cathode tube 32 is low, the contrast is significantly deteriorated. A liquid crystal display device used outdoors such as a liquid crystal display device of an in-vehicle AV system or a navigation system is exposed to severe temperature changes, and may be used in an environment of 0 ° C. or less. For this reason, in the liquid crystal display device used for the in-vehicle AV system or the navigation system, as described above,
A heater 33 is wound around the cold-cathode tube 32, and the temperature inside the lamp house 30 is detected by a temperature sensor 34. When the detected temperature is lower than a preset temperature, the heater 33 is energized to turn on the cold-cathode tube 32. Is heated to ensure brightness.

However, in the liquid crystal display device having the above-described structure, a step of winding the heater 33 around the cold cathode tube 32 is required. Also, since it is necessary to energize the heater 33,
There is also a disadvantage that power consumption is large. In order to solve such a drawback, in recent years, a so-called self-heating type cold-cathode tube has been developed in which the temperature rises sharply due to heat generated by the lamp itself when energization is started. The use of this type of cold cathode tube eliminates the need for a step of winding a heater around the lamp, thereby simplifying the manufacturing process, reducing product cost, and reducing power consumption.

When a self-heating type cold-cathode tube is used, it is necessary to supply a large current to the lamp in order to raise the temperature. Not only does the luminance drop, but the liquid crystal display panel may be damaged due to the high temperature. Therefore,
In a self-heating type cold cathode tube, it is necessary to reduce the supply current as the temperature rises. Usually, in a liquid crystal display device using a self-heating type cold cathode tube, a temperature sensor such as a thermistor is arranged near a lamp, and a current corresponding to a detected temperature is supplied to the lamp.

[0007]

However, since the on-vehicle AV system and the navigation system are exposed to severe temperature changes, the temperature sensor may fail or the wiring of the temperature sensor may be short-circuited due to dew condensation. In such a case, in the conventional backlight unit, since a correct temperature cannot be detected by the temperature sensor, a large current is continuously supplied to the lamp, and the liquid crystal display panel may be damaged.

Accordingly, an object of the present invention is to provide a backlight driving method and a backlight unit for a liquid crystal display device which can prevent an excessive rise in temperature even if an abnormality occurs in a temperature sensor and can prevent damage to the liquid crystal display panel. To provide.

[0009]

SUMMARY OF THE INVENTION The object of the present invention is to provide a method for driving a backlight of a liquid crystal display device, wherein after a lamp is energized, a temperature rise due to heat generation of the lamp is monitored.
The problem is solved by a backlight driving method for a liquid crystal display device, wherein when a temperature rise cannot be detected, the power supplied to the lamp is set to a preset value.

[0010] The above object is achieved by providing a lamp disposed on the back of a liquid crystal display panel, a temperature sensor for detecting a temperature rise due to heat generation of the lamp, and supplying power to the lamp in accordance with an output of the temperature sensor. When the temperature sensor does not detect an increase in temperature, the backlight unit includes power supply means for supplying a preset power to the lamp.

The operation of the present invention will be described below. In the method of the present invention, when energization of the lamp is started, a temperature rise due to heat generation of the lamp is monitored. If the temperature rise is not detected even after the energization of the lamp is started, the power supplied to the lamp is limited to a preset value because the temperature sensor may be faulty. As a result, an excessive rise in temperature can be avoided, and damage to the liquid crystal display panel can be prevented.

Further, the backlight unit of the present invention has a temperature sensor and power supply means, and the power supply means supplies power corresponding to the output of the temperature sensor to the lamp. When the temperature sensor does not detect a rise in temperature, the power supply unit limits the power supplied to the lamp to a preset value. Thus, even if the temperature sensor fails, it is possible to prevent the lamp from being supplied with excessive electric power, thereby preventing the liquid crystal display panel from being damaged.

As the lamp, a self-heating type cold cathode tube can be used. Further, since the brightness of the cold-cathode tube is affected by the temperature of the tube surface, the temperature sensor is preferably arranged on the tube surface. Further, the power supply means includes, for example, a pulse signal generation unit that generates a pulse signal having a duty ratio corresponding to a temperature detected by a temperature sensor, and a pulse signal generation unit that generates a pulse signal only during a period in which the pulse signal is “1”. And a high-voltage oscillation circuit for supplying lamp driving power to the lamp.

[0014]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a schematic diagram showing a liquid crystal display device using a backlight unit according to an embodiment of the present invention. The liquid crystal display panel 11 includes a pair of transparent substrates, a liquid crystal sealed between the transparent substrates, and a pair of polarizing plates disposed with the pair of transparent substrates interposed therebetween. One of the pair of transparent substrates is provided with a transparent common electrode and a color filter, and the other is formed with transparent pixel electrodes arranged in a matrix. When a voltage is applied between the common electrode and the pixel electrode of the liquid crystal display panel 11, the liquid crystal molecules between the common electrode and the pixel electrode are arranged along the electric field, and the light transmittance of that portion of the liquid crystal display panel 11 is changed. Changes. By controlling the applied voltage for each pixel electrode, a desired image can be displayed on the liquid crystal display panel 11.

The liquid crystal display panel 11 is mounted on a box-shaped lamp house 10. A self-heating type cold cathode tube 12 is mounted as a backlight in the lamp house 10, that is, on the back side of the liquid crystal display panel 11. For example, a rod-shaped or U-shaped cold cathode tube 12 is used. A milky white plate 15 for scattering light output from the cold cathode tube 12 is arranged between the liquid crystal display panel 11 and the cold cathode tube 12. A temperature sensor 14 such as a thermistor is mounted on the surface of the cold cathode tube 12.

FIG. 2 is a block diagram showing a circuit configuration of the backlight unit according to the present embodiment. The backlight unit 20 of the present embodiment includes a control unit (pulse signal generation unit) 21 including a microcomputer, a switching element 22, a high-voltage oscillation circuit, in addition to the cold cathode tube 12 and the temperature sensor 14 described above. 23, a sensor amplifier 24, and an A / D (analog / digital) converter 25.

The sensor amplifier 24 is a sensor 1 based on temperature.
4 is converted into an electric signal. The output of the sensor amplifier 24 is input to an A / D converter 25, converted into a digital signal, and input to the control unit 21. Control unit 2
1 generates a pulse signal having a duty ratio of 100% (DC) to 70% according to the temperature detected by the temperature sensor 14 and outputs the pulse signal to the switching element 22.

The switching element 22 is connected to a vehicle-mounted battery (power supply) 29, and is turned on during a period in which the pulse signal given from the control section 21 is "1" to turn on the power supply voltage (about 1).
2V) is transmitted to the high-voltage oscillation circuit 23, and is turned off during the period of "0". The high-voltage oscillation circuit 23 oscillates at a predetermined frequency during a period in which the power supply voltage is supplied through the switching element 22 to generate a high-frequency high-voltage pulse for driving the lamp,
This high-frequency high-voltage pulse is supplied to the cold cathode tube 12.

FIG. 3 is a diagram showing the relationship between the temperature measured by the temperature sensor 14 and the duty ratio of the pulse signal output from the control unit 21. When the temperature is 15 ° C. or less, the duty ratio of the pulse signal is set to 100% (switch element 22
Is always on), and when the temperature is 50 ° C. or higher, the duty ratio is set to 70%. When the temperature is between 15 ° C. and 50 ° C., the duty ratio of the pulse signal is controlled to decrease as the temperature increases, as shown in FIG.

In this embodiment, the minimum value of the duty ratio of the pulse signal is set to 70%, and when an abnormality occurs in the temperature sensor 14 as described later, the control unit 21 sets the duty ratio of the pulse signal to 70%. I do. In this case, the minimum value of the duty ratio can ensure the brightness of the cold cathode tube 22 necessary for obtaining good contrast, and increase the temperature of the cold cathode tube 12 to a temperature at which the liquid crystal display panel 11 may be damaged. It is necessary to set under the condition that it is not done. This is because the cold cathode tube 12 and lamp house 10
Therefore, it is necessary to determine the optimum value by conducting experiments and the like in advance.

FIG. 4 is a flowchart for explaining the operation of the driving circuit of the backlight unit according to the present embodiment. When the power of the liquid crystal display device is turned on, first, in step S11, the control unit 21 initializes the value of the variable i (i = 0). Thereafter, the process proceeds to step S12, where the control unit 21 detects the current temperature T0 from the output of the temperature sensor 14. Then, the process proceeds to step S13, where the temperature T0
And outputs a pulse signal (see FIG. 3) having a duty ratio corresponding to the switching element 22 to turn the switching element 22 on and off.

Next, the process proceeds to step S14, where the control section 21 waits for a predetermined time (for example, 2 seconds) to elapse. Then, after a predetermined time has elapsed, the process proceeds to step S15, and the current temperature T1 is detected from the output of the temperature sensor 14. Thereafter, the process proceeds to step S16, where the control unit 21 calculates a temperature difference W between the previous detected temperature T0 and the present detected temperature T1. Then, in step S17, the temperature difference W
Is determined to be within a predetermined range (for example, ± 1 ° C.), and if it is larger than the predetermined range (YES), step S
The program proceeds to 18 to set the current detected temperature T1 to the detected temperature T0, and then returns to step S13.

On the other hand, if the temperature difference W is within the predetermined range in step S17, the process proceeds to step S19, where the control unit 21 increments the variable i. Thereafter, the process proceeds to step S20, where the variable i is set to the predetermined value n.
(For example, n = 10) or more is determined. When i is smaller than n (NO), the process proceeds to step S18, where the current detected temperature T1 is set to T0, and then the process returns to step S13.

If the variable i is greater than or equal to n in step S20 (in the case of YES), it means that the temperature does not rise despite the current being supplied to the cold cathode tube 12, and the temperature sensor 14 It is considered that an abnormality occurred. Therefore, the process proceeds to step S21 and the control unit 21
Is a preset value of the duty ratio of the pulse signal supplied to the switching element 22 (70% in the present embodiment)
To maintain. As a result, the current supplied to the cold cathode tube 12 is limited to 70% of the maximum current, and a rise in the temperature of the cold cathode tube 12 is avoided. Also, in this case, 70% of the power at the maximum is supplied to the cold-cathode tube 12, so that although the temperature rise speed of the cold-cathode tube 12 becomes slow, the temperature gradually rises to obtain good contrast. Required brightness can be secured.

As described above, in the present embodiment, when the temperature sensor 14 does not detect a rise in temperature even when power supply to the cold cathode tube 12 is started,
2 is limited to a preset value. As a result, even if the temperature sensor 14 breaks down or a short circuit occurs due to dew condensation, excessive supply of current to the cold-cathode tube 12 is avoided, and the liquid crystal display panel 11
Can be avoided. In the present embodiment,
Even if an abnormality occurs in the temperature sensor 14, a preset power is supplied to the cold-cathode tube 12, so that it is possible to secure a luminance necessary for obtaining a good contrast.

In the above embodiment, the case where the present invention is applied to a direct type backlight has been described. However, the present invention can also be applied to an edge light type backlight (side light). Further, in the above-described embodiment, a case has been described in which the present invention is applied to a display device of an in-vehicle AV system or a navigation system. However, the scope of the present invention is not limited to in-vehicle electronic devices. Absent.

[0027]

As described above, according to the present invention,
After the lamp has been energized, if the temperature sensor cannot detect a rise in temperature, the power supplied to the lamp is limited to a certain value, so that even if an error occurs in the temperature sensor, the lamp will not become hot. It is possible to prevent the liquid crystal display panel from being damaged. Also in this case, since a certain amount of power is supplied to the lamp, the temperature of the lamp slowly increases, but the temperature of the lamp gradually increases, so that the brightness required to obtain good contrast can be secured. . Therefore, according to the present invention, there is an effect that the reliability of the liquid crystal display device used particularly in an environment where the temperature change is severe is improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a liquid crystal display device using a backlight unit according to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a circuit configuration of a backlight unit according to the embodiment of the present invention.

FIG. 3 is a diagram illustrating a relationship between a temperature measured by a thermistor and a duty ratio of a pulse signal output from a control unit.

FIG. 4 is a flowchart illustrating an operation of the backlight unit driving circuit according to the embodiment of the present invention.

FIG. 5 is a schematic diagram showing a configuration of a liquid crystal display device used for a vehicle-mounted AV system or a navigation system.

[Explanation of symbols]

 10, 30 lamp house, 11, 31 liquid crystal display panel, 12, 32 cold-cathode tube, 14, 34 temperature sensor, 20 backlight unit, 21 control section, 22 switching element, 23 high-voltage oscillation circuit, 24 sensor amplifier, 25 A / D converter, 33 heaters.

Claims (5)

[Claims] In a backlight driving method for a liquid crystal display device, after energization of a lamp is started, a temperature rise due to heat generation of the lamp is monitored, and when a temperature rise cannot be detected, power supplied to the lamp is set in advance. A backlight driving method for a liquid crystal display device, characterized in that: 2. A lamp disposed on a rear surface of a liquid crystal display panel, a temperature sensor for detecting a temperature rise due to heat generation of the lamp, and an electric power corresponding to an output of the temperature sensor is supplied to the lamp. And a power supply means for supplying a predetermined power to the lamp when a rise in temperature is not detected by the backlight unit. 3. The backlight unit according to claim 2, wherein the lamp is a self-heating type cold cathode tube. 4. The backlight unit according to claim 2, wherein the temperature sensor is disposed on a tube surface of the lamp. 5. The power supply unit includes: a pulse signal generation unit configured to generate a pulse signal having a duty ratio corresponding to a temperature detected by the temperature sensor; The backlight unit according to claim 2, further comprising a high-voltage oscillation circuit that supplies electric power.