JP2001196195A - Video camera - Google Patents

Abstract

(57) [Problem] In a conventional device, it is necessary to set circuit constants in accordance with the characteristics of a cold cathode tube and a boosting transformer of an inverter circuit, even if the circuit type is the same. This is
In designing a variety of backlights in a short period of time, it has become quite complicated and inconvenient. According to one embodiment of the present invention, there is provided a microcomputer that controls a power control circuit, a power control circuit that controls an inverter circuit of a backlight, a constant voltage circuit that is controlled by the power control circuit, An inverter circuit driven by a voltage supplied from a constant voltage circuit, a cold cathode tube lit by a high voltage AC signal of the inverter circuit, and a circuit for detecting a tube current of the cold cathode tube,
This is achieved by configuring the system.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control of a backlight unit having a control function for improving the lighting performance of a backlight unit of a liquid crystal monitor mounted on a video camera, and a system therefor.

[0002]

2. Description of the Related Art Discharge lamps used as backlights
In order to improve the lighting performance of the (cold-cathode tube), a conventional device uses a technique in which the pulse width of a switching power supply at startup and during normal operation is changed by an electronic circuit, as described in JP-A-7-302691. However, despite the complicated circuit configuration, detailed control was difficult.

[0003]

A conventional device is designed to match the characteristics of a cold cathode tube or a boosting transformer of an inverter circuit.
Even if the circuit system is the same, it is necessary to make detailed settings for the circuit constants. This has become quite cumbersome and inconvenient in designing a variety of backlights in a short period of time.

[0004]

According to the present invention, there is provided a microcomputer for controlling a power supply control circuit, a power supply control circuit for controlling an inverter circuit of a backlight, and a power supply control circuit. The system is composed of a constant voltage circuit, an inverter circuit driven by the voltage supplied from the constant voltage circuit, a cold cathode tube lit by a high voltage AC signal of the inverter circuit, and a circuit for detecting a tube current of the cold cathode tube. Is achieved by

[0005]

FIG. 1 is a block diagram showing an embodiment of the present invention.

The output of the battery 4 is supplied to a switch circuit 5
It is input to the power supply control circuit 3 and the microcomputer 2. When the power switch 1 is turned on, the microcomputer 2 instructs the power control circuit 3 to turn on, and enters an operation state. The power supplied from the battery 4 by the ON signal input to the switch circuit 5 at the same time
And is supplied to the constant voltage circuit 6. The output voltage of the constant voltage circuit 6 is input to the inverter circuit 7, converted into a high-voltage AC signal, and then input to the cold cathode tube 8. The feedback current from the cold cathode tube 8 is converted to a DC voltage by the current detection circuit 9 and then fed back to the power control circuit 3. Based on the output signal of the current detection circuit 9, it is detected whether the cold-cathode tube 8 is normally turned on. This result is also sent to the microcomputer 2.

Next, a mechanism for keeping the output voltage of the constant voltage circuit 6 constant will be described. When the voltage of the constant voltage circuit 6 increases, the feedback voltage 13 also increases. The internal voltage comparison circuit 11 of the power supply control circuit 3 compares the feedback voltage 13 with the reference voltage 10 and detects that the feedback voltage 13 has become higher than the reference voltage 10. The voltage comparison circuit 11 acts on the PWM circuit 12 to lower the voltage. The PWM circuit 12 lengthens the HI period of the switching signal (decreases the LO period). As a result, the ON period of the switch circuit inside the constant voltage circuit 6 is shortened, and the output voltage of the constant voltage circuit 6 decreases.

On the contrary, when the voltage of the constant voltage circuit 6 decreases,
The feedback voltage 13 also decreases. The voltage comparison circuit 11 detects that the feedback voltage 13 has become lower than the reference voltage 10. The voltage comparison circuit 11
Acts to increase the voltage. The PWM circuit 12 shortens the HI period of the switching signal (lengthens the LO period). As a result, the ON period of the constant voltage circuit 6 becomes longer, and the output voltage of the constant voltage circuit 6 increases.

Next, a method of successively changing the voltage of the constant voltage circuit 6 using the present invention will be described. FIG. 2 is an example of a graph showing a change in the output voltage of the constant voltage circuit 6. The horizontal axis represents time, and the vertical axis represents the output voltage of the constant voltage circuit 6. In the figure, V1 is a normal voltage, and V2 is a lighting voltage. In order to enhance the lighting performance of the cold cathode tube, the starting voltage is increased, and when discharge starts, control is performed to reduce the tube voltage to save power. Hereinafter, description will be made step by step with reference to FIG. 1 and FIG.

(1) First Step When the microcomputer 2 detects that the power switch 1 is turned on, the microcomputer 2 turns on the power control circuit 3 and simultaneously turns on the switch circuit 5. Subsequently, the microcomputer 2
The output voltage of the constant voltage circuit 6 is supplied to the power control circuit 3.
Control is performed to increase the setting of the reference voltage 10 of the power supply control circuit 3 so as to increase to V2. The output voltage of the constant voltage circuit 6 is increased by the output of the power supply control circuit 3. The output voltage of the constant voltage circuit 6 is fed back to the power supply control circuit 3 as a feedback voltage 13, and is compared with a reference voltage 10 by a voltage comparison circuit 11. Based on the comparison result, the power control circuit 3 controls whether the output voltage of the constant voltage circuit 6 is increased or decreased.

The voltage comparison circuit 11 receives the feedback voltage 1
While monitoring 3, the output voltage of the constant voltage circuit 6 is increased to V2 (FIGS. 2A and 2B). The voltage V2 is determined by software of the microcomputer 2 and data setting.
The voltage value of the voltage V2 and the time for rising to V2 can be set freely.

(2) Second stage When the feedback voltage 13 reaches the reference voltage 10, the voltage comparison circuit 11 sends the feedback voltage 1 to the PWM circuit 12.
3 is controlled to be constant (to keep the output voltage of the constant voltage circuit 6 constant). That is, while the PWM circuit 12 changes the HI / LO ratio of the switching signal, the constant voltage circuit 6
The switching circuit inside the constant voltage circuit 6 is controlled so that the voltage of V1 maintains V2 (between FIG. 2B and FIG. 2C). Note that b to c
The time between them can be freely set by software of the microcomputer 2 and data setting.

(3) Third Step The output voltage of the constant voltage circuit 6 is input to the inverter circuit 7, converted into a high-voltage AC signal, and then input to the cold cathode tube 8. The feedback current from the cold cathode tube 8 is converted into a DC voltage by the current detection
Will be fed back. Based on the output signal of the current detection circuit 9, it is detected whether the cold-cathode tube 8 is normally turned on.
This result is also sent to the microcomputer 2.

The microcomputer 2 includes a backlight
If the output signal of the current detection circuit 9 does not return within a preset time after outputting the ON signal, it is determined that the cold cathode fluorescent lamp 8 did not normally turn on, and the power supply circuit of the backlight unit was turned off. After controlling to stop, the operation of (1) is repeated so as to turn on again.

If the light is not turned on, it is determined that there is an abnormality in the circuit, and the lighting operation of the backlight is stopped.

(4) Fourth Step When the cold cathode fluorescent lamp 8 is normally turned on, the output voltage of the constant voltage circuit 6 is reduced from V2 to V1. At this time, the microcomputer 2 controls the power supply control circuit 3 so that the setting of the reference voltage 10 of the power supply control circuit 3 is gradually reduced so that the output voltage of the constant voltage circuit 6 is gradually reduced. The output voltage of the constant voltage circuit 6 drops due to the output of the power supply control circuit 3 at this time. The output voltage of the constant voltage circuit 6 is fed back to the power supply control circuit 3 and compared with the reference voltage 10 by the voltage comparison circuit 11. Based on the comparison result, the power supply control circuit 3 controls whether the output voltage of the constant voltage circuit 6 is further reduced or increased.

The voltage comparison circuit 11 receives the feedback voltage 1
While monitoring 3, the output voltage of the constant voltage circuit 6 is reduced to V1 (between c and d in FIG. 2). The voltage V2 is determined by software of the microcomputer 2 and data setting.
The voltage value of the voltage V1 and the time to fall to V1 can be set freely.

[0018]

As described above, according to the present invention, an inexpensive and highly reliable video camera can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a video camera according to an embodiment of the present invention.

FIG. 2 is a graph showing a voltage change of an output voltage of the constant voltage circuit 6 of FIG.

[Explanation of symbols]

1 ... power switch, 2 ... microcomputer, 3 ... power control circuit, 4 ... battery, 5 ... switch circuit,
6 ... constant voltage circuit, 7 ... inverter circuit, 8 ... cold cathode tube, 9 ...
Detection circuit, 10… Reference voltage, 11… Voltage comparison circuit, 12… PWM
Circuit, 13: Feedback voltage, V1: Normal output voltage, V2
... lighting voltage, a ... circuit ON point, b ... cold cathode tube lighting start point, c ... cold cathode tube lighting operation end point, d ...
Normal lighting start point.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takehisa Tsukada 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Inside Hitachi Image Information Systems Co., Ltd. (72) Masaru Arai 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Hitachi Image Information System Co., Ltd. (72) Inventor Mikito Komatsu 1410 Inada, Hitachinaka-shi, Ibaraki Prefecture Digital Media Products Division, Hitachi, Ltd. F-term (reference) 3K072 CA14 EB04 HA10

Claims (2)

[Claims] 1. A liquid crystal monitor of a video camera having a function of photoelectrically converting an optical image of a subject into a video signal, and a function of recording and reproducing the video signal. A power supply control circuit, a power supply control circuit for controlling the power supply control circuit, a constant voltage circuit controlled by the power supply control circuit, and a constant An inverter circuit that receives a voltage supply from the voltage circuit, a cold cathode tube that is turned on by a high-voltage AC signal of the inverter circuit, and a circuit that detects a tube current of the cold cathode tube; A video camera having a function of improving lighting properties of a tube by controlling a microcomputer. 2. A microcomputer according to claim 1, wherein a temperature sensor for detecting a temperature around the backlight, voltage conversion means for converting a detection result of the temperature sensor into a voltage, and a converted temperature detection signal are inputted to a microcomputer. Providing means,
A video camera having a function of changing lighting control according to an ambient temperature.