JPH05335091A - Drive circuit for flat fluorescent tube - Google Patents

Abstract

PURPOSE:To stabilize the brightness of a flat fluorescent tube against the change of ambient temperature by using a temperature element giving a change in resistance value, depending upon the ambient temperature. CONSTITUTION:When an output pulse having the predetermined width synchronized with a horizontal synchronous signal is outputted from the first mono-multi C-MOSIC1, the width of the output pulse is adjusted by the second mono-multi C-MOSIC3, depending upon the time constant of a time constant circuit 4 having a thermister 44 to give a change in the resistance value thereof, depending upon ambient temperature. A flat fluorescent tube 8 is caused to light by the output pulse so adjusted in width.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving circuit for a flat fluorescent tube used as a backlight of a liquid crystal display.

[0002]

2. Description of the Related Art Recently, a liquid crystal television using a liquid crystal display has become widespread as a portable small television.

By the way, in such a liquid crystal television, in order to clearly display the screen even in a dark place such as a room, a flat fluorescent tube incorporated into a liquid crystal display as a backlight is being put to practical use.

Here, in the flat fluorescent tube, a pair of flat plates coated with a fluorescent material are arranged at a slight interval, and
It is configured to provide a pair of parallel electrodes between these plane plates, and when a discharge is generated between these electrodes, the main part of the fluorescence is excited by the ultraviolet radiation due to this discharge to generate light, which is uniform over the entire plane. It is designed to obtain brightness.

In such a flat fluorescent tube, it is said that it is desirable to supply a voltage intermittently in a predetermined cycle in order to stabilize the light emitting state. Therefore, in the conventional flat fluorescent tube driving circuit, the flat fluorescent tube is driven. By supplying a pulsed driving voltage consisting of a rectangular waveform with a predetermined duty ratio between a pair of parallel electrodes of the fluorescent tube, a uniform discharge is generated between the parallel electrodes, and uniform brightness can be obtained over the entire plane of the fluorescent tube. I am trying.

[0006]

By the way, in the flat fluorescent tube driven by such a driving circuit, the relationship between the brightness and the ambient temperature is shown in FIG. 4 (b). On the other hand, it is known that the brightness changes greatly.

Therefore, in a liquid crystal television in which such a flat fluorescent tube is incorporated as a backlight, the brightness of the television screen fluctuates greatly depending on the ambient temperature, and it is used especially in a place where the temperature is relatively low. In such a case, the brightness of the flat fluorescent tube makes it difficult to see the TV screen, and when the temperature drops to about 5 ° C, the flat fluorescent tube may not be able to be lit and is used as a backlight at all. There was a problem of not being added.

The present invention has been made in view of the above circumstances, and it is possible to minimize the luminance change of the flat fluorescent tube with respect to the change of the ambient temperature, and further to realize the flat fluorescent tube even in the low temperature region. An object of the present invention is to provide a drive circuit for a flat fluorescent tube that can be surely turned on.

[0009]

SUMMARY OF THE INVENTION The present invention is directed to an output pulse generating means for generating an output pulse having a predetermined pulse width, a time constant circuit having a temperature sensitive element whose resistance value changes depending on the ambient temperature, and a time constant circuit of the time constant circuit. It comprises pulse width adjusting means for adjusting the pulse width of the output pulse from the output pulse generating means by a time constant, and a flat fluorescent tube which is turned on by the output pulse whose pulse width is adjusted by the pulse width adjusting means.

[0010]

As a result, according to the present invention, the time constant of the time constant circuit is changed according to the ambient temperature change by the temperature sensitive element whose resistance value changes according to the ambient temperature, and the pulse of the output pulse is changed by this time constant. The width is determined, and the flat fluorescent tube is turned on by the output pulse having the adjusted pulse width. Therefore, it is possible to minimize the change in brightness of the flat fluorescent tube with respect to the change in ambient temperature, and it is possible to reliably turn on the flat fluorescent tube even in a low temperature range.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example in which the same embodiment is applied to a drive circuit for a flat fluorescent tube used as a backlight of a liquid crystal display of a liquid crystal television. In the figure, 1 is a first mono-multi C-MOSI for generating a synchronization pulse.
In C, the mono-multi C-MOS IC1 is provided with a switching pulse ON / OFF at the R terminal and a horizontal synchronizing signal HD from the TV body (not shown) at the A terminal.
Power supply + 6V at terminal, capacitor 2 at T1 and T2 terminals
1 and a time constant circuit 2 having a resistor 22 are connected to each other. This first mono-multi C-MOS IC changes the position of the switching pulse ON / OFF given to the R terminal by the time constant of the capacitor 21 and the resistor 22 to switch ON / OFF the switching pulse during the horizontal blanking period.
Set so that the switching of the
An output pulse having a predetermined pulse width synchronized with is generated from the Q terminal.

This first mono-multi C-MOSIC 1
Output pulse from the second C-MOSI for mono-multi.
It is given to the B terminal of C3. Second C-M for mono-multi
The OSIC3 is provided with the switching pulse ON / OFF at the R terminal, the A terminal is grounded, and the time constant circuit 4 is connected to the T1 and T2 terminals. Based on the time constant of the time constant circuit 4, the first The pulse width of the output pulse given from the mono-multi C-MOS IC1 is set. In this case, the time constant circuit 4 includes a capacitor 41, a resistor 42,
In addition to the variable resistor 43 and the variable resistor 44, a thermistor 45 is provided, and the time constant is changed by the thermistor 45 that changes the resistance value with respect to changes in the ambient temperature.
That is, when the ambient temperature is lowered with the time constant determined by the temperature characteristic of the thermistor 45, the pulse width of the output pulse of the second mono-multi C-MOS IC 3 is adjusted to be wide, and conversely, the ambient temperature is reduced. When the temperature rises, the pulse width of the output pulse of the second mono-multi C-MOS IC 3 is adjusted to be narrowed.

The second C-MO for mono-multi
The output pulse whose pulse width is adjusted by SIC3 is M
It is connected to the gate G of the OS transistor 5. MOS
In the transistor 5, the source S is grounded, and the drain D is connected to the power source + 6V through the primary winding 61 of the transformer 6.

In the transformer 6, the capacitor 7 is connected in parallel to the primary winding 61, the flat fluorescent tube 8 is connected in series to the secondary winding 62, and this series circuit is connected between the power source +6 V and the drain of the MOS transistor 5. is doing. Next, the operation of the embodiment configured as described above will be described.

Now, the first mono-multi C-MOS IC
When the switching pulse ON / OFF is applied to the R terminal of 1 and the horizontal synchronizing signal HD from the television body (not shown) is applied to the A terminal, the capacitor 21 and the resistor 22 of the time constant circuit 2 connected to the T1 and T2 terminals are used. Switching pulse ON / OFF switching is set so that it falls within the horizontal blanking period according to the time constant.
Is output from the Q terminal as an output pulse synchronized with.

The first mono-multi C-MOSI
The output pulse from C1 is the second mono-multi C-MO.
Given to SIC3. Second mono-multi C-MOS
In the IC3, the pulse width of the output pulse is set by the time constant of the time constant circuit 4 connected to the T1 and T2 terminals, and the output pulse having the set pulse width is given to the gate G of the MOS transistor 5.

As a result, the on / off time of the MOS transistor 5 is controlled according to the pulse width of the output pulse, and the pulsed voltage is supplied to the primary winding 61 of the transformer 6, so that the secondary winding 62 side thereof is provided. A driving voltage having a rectangular waveform is output and supplied to the flat fluorescent tube 8, so that the flat fluorescent tube 8 is turned on.

In this state, when the ambient temperature is now lowered, the resistance value of the thermistor 45 increases due to its temperature characteristic, and the pulse width of the output pulse is increased due to the time constant of the time constant circuit 4 accompanying this. Is adjusted so that the flat fluorescent tube 8 is expanded, the rectangular waveform of the driving voltage supplied to the flat fluorescent tube 8 is also deformed accordingly, and the brightness of the flat fluorescent tube 8 is adjusted so as to be increased. When the temperature rises, the resistance value of the thermistor 45 decreases due to its temperature characteristic, and the pulse width of the output pulse is adjusted to be narrowed by the time constant of the time constant circuit 4 accompanying this. The rectangular waveform of the drive voltage supplied to the fluorescent tube 8 is also deformed accordingly, and the brightness of the flat fluorescent tube 8 is adjusted to be lowered.

In this case, as a concrete circuit example, the resistor 4
When the resistance value of 2 is 15 KΩ, the resistance value of the resistor 43 is 6.2 KΩ, and the resistance value of the variable resistor 44 is 5 KΩ, the total resistance value of the resistors 42 and 43, the variable resistor 44, and the thermistor 45 in the time constant circuit 4 and the surroundings. The relationship of temperature is as shown in FIG. 2, and it was confirmed that the total resistance value increases as the ambient temperature decreases, and conversely, the total resistance value decreases as the ambient temperature increases. At this time, the relationship between the lighting current flowing in the flat fluorescent tube 8 and the ambient temperature is as shown in FIG. 3, and as the ambient temperature decreases,
It was also confirmed that the lighting current increased, and conversely, the lighting current decreased as the ambient temperature increased.

As a result, the pulse width of the output pulse is adjusted as described above, and the brightness of the flat fluorescent tube 8 is adjusted. The relationship between the brightness and the ambient temperature in this case is as shown in FIG. 4 (a). As compared with the conventional drive circuit shown in FIG. 2B, the change in brightness in the flat fluorescent tube 8 with respect to the change in ambient temperature can be made into a gentle change and can be minimized. Even then, it becomes possible to secure the lighting of the flat fluorescent tube 8.

Therefore, in this way, by utilizing the temperature characteristic of the thermistor to adjust the pulse width of the output pulse according to the ambient temperature change, the luminance change of the flat fluorescent tube with respect to the ambient temperature change. The flat fluorescent tube can be reliably turned on even in a low temperature range, so that the brightness of the TV screen does not fluctuate significantly due to the ambient temperature. Even in a relatively low place, it is possible to suppress the decrease in brightness of the flat fluorescent tube as much as possible, and it is not difficult to see the TV screen. Even if the temperature drops to about 5 ° C, the flat fluorescent tube is stable. Can be turned on.

The present invention is not limited to the above-mentioned embodiment, and can be carried out by appropriately modifying it without changing the gist.
For example, in the above-described embodiment, the output pulse for driving the flat fluorescent tube 8 is synchronized with the horizontal synchronizing signal, but it may be synchronized with the vertical synchronizing signal. Further, in the above description, the driving circuit of the flat fluorescent tube used as the backlight of the liquid crystal display of the liquid crystal television has been described, but it can be applied to the driving circuit of the flat fluorescent tube used as the backlight of the liquid crystal display of a word processor or the like. ..

[0024]

According to the present invention, the time constant of the time constant circuit is changed according to the ambient temperature change by the temperature sensitive element whose resistance value changes according to the ambient temperature, and the pulse of the output pulse is changed by this time constant. Since the width is determined and the flat fluorescent tube is turned on by the output pulse whose pulse width is adjusted, it is possible to minimize the change in the brightness of the flat fluorescent tube even when the ambient temperature changes. Furthermore, the flat fluorescent tube can be reliably turned on even in a low temperature range.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a relationship between a total resistance value of a time constant circuit and an ambient temperature according to an embodiment.

FIG. 3 is a diagram showing a relationship between a lighting current and an ambient temperature of the flat fluorescent tube of the embodiment.

FIG. 4 is a diagram showing a relationship between luminance and ambient temperature in the example.

[Explanation of symbols]

1, 3 ... Mono-multi C-MOS IC, 2, 4 ... Time constant circuit, 21, 41 ... Capacitor, 22, 42, 43, 4
4 ... Resistor, 45 ... Thermistor, 5 ... MOS transistor, 6 ... Transformer, 7 ... Capacitor, 8 ... Flat fluorescent tube.

Claims (1)

[Claims] 1. An output pulse generating means for generating an output pulse having a predetermined pulse width, a time constant circuit having a temperature sensitive element whose resistance value changes depending on the ambient temperature, and the output pulse depending on the time constant of the time constant circuit. A flat fluorescent lamp characterized by comprising pulse width adjusting means for adjusting the pulse width of the output pulse from the generating means, and a flat fluorescent tube lit by the output pulse whose pulse width is adjusted by the pulse width adjusting means. Tube drive circuit.