JPH10111487A - Lcd back light driving circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an LCD back light driving circuit, by which the brightness of a back light is constant regardless of the fluctuation in the output voltage of a battery, and which consumes less electricity, by controlling the duty of an output pulse based on the output of a voltage monitoring means. SOLUTION: The output of a voltage monitoring means 6 is connected to a pulse width modulation control means (hereinafter called 'PWM control means') 7, while the output of the PWM control means 7 is connected to a back light driving means 8. The battery voltage 100 is inputted in the PWM control means 7 through an operational amplifier. The PWM control means 7 changes the duty in accordance with the fluctuation of the voltage 100. With the duty of the output pulse changed in accordance with the fluctuation of the output voltage of the battery, the brightness of the back light is kept constant regardless of such fluctuation; since the brightness of the back light is adjustable through the control of the duty, reduced power consumption is possible at the time when the back light is unnecessary.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an LCD backlight driving circuit for a liquid crystal display (hereinafter, referred to as an LCD) used in a battery-operated device, and more particularly, to a method for controlling the brightness of a backlight regardless of a change in battery voltage. And an LCD backlight driving circuit capable of maintaining a constant.

[0002]

2. Description of the Related Art A conventional LCD backlight drive circuit turns on an LCD or the like by driving an LED (Light Emitting Diode) or the like as a backlight with an output voltage of a battery.

FIG. 8 is a circuit diagram showing an example of a conventional LCD backlight drive circuit. 8, 1 is a battery, 2 is a resistor, 3 is an LED, and 4 is a switch circuit.

One end of a battery 1 is connected to one end of a resistor 2, the other end of the resistor 2 is connected to the anode of an LED 3,
The cathode of ED3 is connected to one end of switch circuit 4. The other end of the switch circuit 4 is connected to the other end of the battery 1.

Here, the operation of the conventional example shown in FIG. 8 will be described. When the switch circuit 4 is turned “ON”, the current from the battery 1 flows into the LED 3 via the current limiting resistor 2. Therefore, the LED 3 is turned on to illuminate the LCD (not shown).

FIG. 9 is a circuit diagram showing another example of a conventional LCD backlight drive circuit, wherein the same reference numerals as in FIG. In FIG. 9, reference numeral 5 denotes a constant voltage circuit.

[0007] One end of the battery 1 is connected to the constant voltage circuit 5, the output voltage of the constant voltage circuit 5 is connected to one end of the resistor 2, and the other end of the resistor 2 is connected to the anode of the LED3.
The cathode of the LED 3 is connected to one end of the switch circuit 4, and the other end of the switch circuit 4 is connected to the other end of the battery 1.

Here, the operation of the conventional example shown in FIG. 9 will be described. When the switch circuit 4 is turned “ON”, the output voltage of the battery 1 is controlled to a constant voltage by the constant voltage circuit 5 and output.

Since this constant voltage is applied to the series circuit of the resistor 2 and the LED 3, a constant current always flows through the LED 3. Therefore, the LED 3 is turned on to illuminate the LCD (not shown).

[0010]

However, in the conventional LCD backlight driving circuit shown in FIG. 8, since the LED 3 is directly driven by the output voltage of the battery 1, the output voltage of the battery 1 drops due to the decrease of the output voltage of the battery 1 with the elapse of use time. There is a problem that the brightness becomes dark.

Further, in the conventional LCD backlight drive circuit shown in FIG. 9, the brightness of the LED 3 becomes constant by the operation of the constant voltage circuit 5, but the power of the voltage drop in the constant voltage circuit 5 is wasted. There was the problem I mentioned. Therefore, an object of the present invention is to realize an LCD backlight driving circuit with constant backlight brightness and low power consumption irrespective of fluctuations in battery output voltage.

[0012]

According to a first aspect of the present invention, there is provided an LCD backlight driving circuit for driving an LED light source which is a backlight of a liquid crystal display. Voltage monitoring means for taking in the voltage, pulse width modulation control means for controlling the duty of the output pulse based on the output of the voltage monitoring means, and backlight driving means for driving the backlight based on the output of the pulse width modulation control means. It is characterized by having.

According to a second aspect of the present invention, in an LCD backlight drive circuit for driving an LED light source, which is a backlight of a liquid crystal display, a voltage monitor for capturing a change in battery voltage, and an output based on the output of the voltage monitor. Pulse width modulation control means for controlling the pulse duty;
The battery voltage is compared with the backlight lighting limit voltage value, and when the voltage falls below the backlight lighting limit voltage value, a voltage drop detection unit that outputs a detection signal, and the backlight driving operation is performed by the detection signal of the voltage drop detection unit. Backlight driving means for stopping and driving the backlight based on the output of the pulse width modulation control means.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration block diagram showing an embodiment of an LCD backlight drive circuit according to the present invention.

In FIG. 1, reference numeral 6 denotes voltage monitoring means, 7 denotes pulse width modulation control means (hereinafter referred to as PWM control means), 8 denotes backlight driving means, 9 denotes voltage drop detecting means, and 10 denotes backlight. LED, 100 is the battery voltage.

The output of the voltage monitoring means 6 is connected to the PWM control means 7, and the output of the PWM control means 7 is connected to the backlight driving means 8.

On the other hand, the output of the voltage drop detecting means 9 is also connected to the backlight driving means 8,
Is connected to the LED 10. The battery voltage 100 is supplied to the voltage monitoring means 6, the backlight driving means 8, and the voltage drop detecting means 9, respectively.

FIG. 2 is a circuit diagram showing a specific example of the embodiment shown in FIG. 1, and 6, 7, 8, 9, 10, and 100 are denoted by the same reference numerals as in FIG. In FIG. 2, 11, 1
2, 14, 15, 16, 17, 20, 21, and 23 are resistors, 13 is an operational amplifier, 18 is a comparator, 19 is an AND circuit, 22 is a transistor, and 101 is a reference voltage.

Reference numerals 11 to 13 designate voltage monitoring means 6,
14 to 18 constitute the voltage drop detecting means 9 and 19 to 23 constitute the backlight driving means 8, respectively.

One end of the resistor 11 is connected to one end of the resistor 12 and the non-inverting input terminal of the operational amplifier 13.
The output of 3 is connected to the inverting input terminal of the operational amplifier 13 and the PWM control means 7. The output of the PWM control means 7 is connected to one input terminal of the AND circuit 19.

On the other hand, one end of the resistor 14 is connected to one end of the resistor 15 and the non-inverting input terminal of the comparator 18, and one end of the resistor 16 is connected to one end of the resistor 17 and the inverting input terminal of the comparator 18. The output of the comparator 18 is output from an AND circuit 19.
Is connected to the other input terminal.

The output of the AND circuit 19 is connected to one end of a resistor 20, and the other end of the resistor 20 is connected to one end of a resistor 21 and the base of a transistor 22. The collector of the transistor 22 is connected to the cathode of the LED 10.
Is connected to one end of the resistor 23.

The battery voltage 100 is equal to the resistances 11, 1
4 and 23, the reference voltage 101 is connected to the resistor 1
6 is connected to the other end. Further, the other ends of the resistors 12, 15, 17 and 21 and the emitter of the transistor 22 are grounded.

The operation of the embodiment shown in FIGS. 1 and 2 will be described with reference to FIG. 3A is a characteristic curve diagram showing a battery voltage 100, FIG. 3B is a characteristic curve diagram showing an output waveform of the backlight driving means 8, and FIG.

The battery voltage 100 is divided by the resistors 11 and 12 and input to the PWM control means 7 via the operational amplifier 13 which is a buffer circuit.

The PWM control means 7 controls the duty so as to decrease when the input voltage is high, and to increase the duty when the input voltage is low. That is, the PWM control means 7 changes the duty in accordance with the voltage fluctuation of the battery voltage 100.

For example, the battery voltage 100 is shown in FIG.
(A) As shown in FIG. 3B, the output pulse of the PWM control means 7 decreases as time elapses, so that when the input voltage is large, the output pulse becomes narrow as shown in FIG. If it is smaller, it becomes wider as shown by "c".

Since the "ON / OFF" of the transistor 22 is controlled by such an output pulse, a current as shown in FIG.

For example, in the waveform shown in "d", since the battery voltage 100 is large, the peak current becomes large as shown in "e". Further, in the waveform shown by "f", the battery current 100 is small, and therefore, the peak current becomes small as shown by "g".

Here, since the average current flowing through the LED 10 becomes constant as shown in FIG.
The brightness of the ED 10 is constant.

The battery voltage 100 is divided by the resistors 14 and 15, and the reference voltage 101 is divided by the resistors 16 and 17 and compared by the comparator 18.

If the divided voltage value of the battery voltage 100 becomes smaller than the divided voltage value of the reference voltage 101, the comparator 18 outputs a low level signal as a detection signal, so that the other input terminal of the AND circuit 19 becomes low level. LED10
Stop driving.

As a result, the brightness of the backlight is constant irrespective of the change in the output voltage of the battery by changing the duty of the output pulse in accordance with the change in the output voltage of the battery. Since the brightness can be controlled, power can be saved when the backlight is unnecessary.

Further, by directly driving with the output voltage of the battery, there is no voltage drop in the LCD backlight drive circuit, so that the power consumption is reduced.

FIG. 4 is a block diagram showing another specific example of the voltage monitoring means 6. As shown in FIG. In FIG. 4, 11, 1
2 and 100 have the same reference numerals as in FIG.
/ D converter, 25 is a control circuit, 26 is a D / A converter, 2
7 is a setting means.

One end of the resistor 11 is connected to one end of the resistor 12 and A /
The output of the A / D converter 24 is connected to the control circuit 25. The output of the setting means 27 is connected to the control circuit 25, and the output of the control circuit 25 is D /
Connected to A converter 26. Furthermore, the battery voltage 10
0 is connected to the other end of the resistor 11 and the other end of the resistor 12 is grounded.

Here, the operation of the voltage monitoring means shown in FIG. 4 will be described. The battery voltage 100 is divided by the resistors 11 and 12, and is converted into a digital signal by the A / D converter 24.

The control circuit 25 takes in the digital signal, performs necessary arithmetic processing, converts the digital signal into an analog signal again by the D / A converter 26, and outputs the analog signal to the PWM control means 7 (not shown).

At this time, the control circuit 25 controls the brightness of the LED and the “ON / OFF” of the LED based on the output of the setting means 27.
Can be easily set.

Further, the battery voltage 100 taken in the control circuit 25 is compared with a preset LED lighting limit voltage value, and if it is smaller than the LED lighting limit voltage value, the driving of the LED can be stopped. The voltage drop detecting means 9 becomes unnecessary.

FIG. 5 is a block diagram showing another specific example of the voltage monitoring means and the PWM control means. FIG.
The odors 11, 12, 24, 25, 27 and 100 are shown in FIG.
Reference numeral 28 is a pulse generator.

One end of the resistor 11 is connected to one end of the resistor 12 and A /
The output of the A / D converter 24 is connected to the control circuit 25.

The output of the setting means 27 is connected to the control circuit 25, and the output of the control circuit 25 is connected to the pulse generator 28.
And the output of the pulse generator 28 is connected to the backlight driving means 8. Further, the battery voltage 100 is connected to the other end of the resistor 11 and the other end of the resistor 12 is connected.

Here, the voltage monitoring means and P shown in FIG.
The operation of the WM control circuit will be described. The battery voltage 100 is divided by the resistors 11 and 12, and is converted into a digital signal by the A / D converter 24.

The control circuit 25 takes in the digital signal, performs necessary arithmetic processing, and outputs it to the pulse generator 28. The pulse generator 28 outputs a pulse as shown in FIG. 3B in which the duty is changed based on the input digital signal. The output pulse of the pulse generator 28 is directly input to one end of the resistor 20 shown in FIG.

At this time, the control circuit 25 sets the brightness of the LED and the “ON / OFF” of the LED based on the output of the setting means 27.
Can be easily set.

Further, the battery voltage 100 taken in the control circuit 25 is compared with the LED lighting limit voltage value preset in the control circuit 25. If the LED lighting limit voltage value is smaller than the LED lighting limit voltage value, the driving of the LED is stopped. Therefore, the voltage drop detecting means 9 becomes unnecessary.

FIGS. 6 and 7 show the voltage drop detecting means 9.
FIG. 14 is a circuit diagram showing another specific example of the present invention.
And 101 have the same reference numerals as in FIG.

In FIG. 6, reference numeral 29 denotes a voltage monitoring IC.
One end of the resistor 14 is connected to the end of the resistor 15 and the voltage monitoring IC 29
2 is connected to the input terminal of FIG.
Is connected to the other input terminal of the AND circuit 19 shown in FIG.
Further, the other end of the resistor 14 is connected to the battery voltage 100, and the other end of the resistor 15 and the ground terminal of the voltage monitoring IC 29 are grounded.

This configuration eliminates the need for the reference voltage 101, and is effective when the reference voltage cannot be used due to the circuit configuration.

Generally, when the battery voltage 100 falls below a preset LED lighting limit voltage value, the driving of the LED is stopped by the voltage drop detecting means 9 or the like, whereby the load current decreases and the battery voltage 100 rises.

In the configuration shown in FIG. 2, when the rise of the battery voltage 100 exceeds the LED lighting limit voltage value, the LED driving is started again, the load current increases, and the battery voltage 100 is again reduced to the LED lighting limit voltage. Below the value.

For this reason, the LCD backlight driving circuit may repeat "operation" and "stop". FIG. 7 is a circuit diagram showing a specific example of the voltage drop detecting means which has solved such a problem.

In FIG. 7, reference numeral 30 denotes a latch circuit. One end of the resistor 14 is connected to one end of the resistor 15 and the non-inverting input terminal of the comparator 18, and one end of the resistor 16 is connected to one end of the resistor 17 and the inverting input terminal of the comparator 18.

The output of the comparator 18 is connected to the latch circuit 30.
The output of the latch circuit 30 is connected to the other input terminal of the AND circuit 19 shown in FIG.

The other end of the resistor 14 is connected to the battery voltage 1
00, the other end of the resistor 16 and the power supply terminal of the latch circuit 30 are connected to the reference voltage 101, and the resistors 15 and 1
7 and the ground terminal of the latch circuit 30 are grounded.

With such a configuration, once the LCD backlight drive circuit enters the "stop" state, the latch circuit 30 holds this state, so that the LCD backlight drive circuit repeats "operation" and "stop". Can be prevented.

[0058]

As is apparent from the above description,
According to the present invention, the following effects can be obtained. By changing the duty of the output pulse in accordance with the fluctuation of the output voltage of the battery, it is possible to realize an LCD backlight driving circuit with constant backlight brightness and low power consumption regardless of the fluctuation of the output voltage of the battery.

[Brief description of the drawings]

FIG. 1 is a configuration block diagram showing an embodiment of an LCD backlight drive circuit according to the present invention.

FIG. 2 is a circuit diagram showing a specific example of the embodiment shown in FIG.

FIG. 3 is a characteristic curve diagram showing a battery voltage, an output waveform of a backlight driving unit, and a current waveform flowing to an LED.

FIG. 4 is a block diagram showing another specific example of the voltage monitoring means.

FIG. 5 is a configuration block diagram showing another specific example of the voltage monitoring means and the PWM control means.

FIG. 6 is a circuit diagram showing another specific example of the voltage drop detecting means.

FIG. 7 is a circuit diagram showing another specific example of the voltage drop detecting means.

FIG. 8 is a circuit diagram showing an example of a conventional LCD backlight drive circuit.

FIG. 9 is a circuit diagram showing another example of a conventional LCD backlight drive circuit.

[Description of Signs] 1 Battery 2,11,12,14,15,16,17,20,2
1, 23 resistor 3, 10 LED 4 switch circuit 5 constant voltage circuit 6 voltage monitoring means 7 pulse width modulation control means 8 backlight driving means 9 voltage drop detection means 13 operational amplifier 18 comparator 19 AND circuit 22 transistor 24 A / D Converter 25 Control circuit 26 D / A converter 27 Setting means 28 Pulse generator 29 Voltage monitoring IC 30 Latch circuit 100 Battery voltage 101 Reference voltage

Claims (2)

[Claims] An LCD backlight driving circuit for driving an LED light source, which is a backlight of a liquid crystal display, includes a voltage monitor for capturing a change in battery voltage, and a duty of an output pulse is controlled based on an output of the voltage monitor. An LCD backlight drive circuit, comprising: a pulse width modulation control means for controlling a backlight; and a backlight drive means for driving a backlight based on an output of the pulse width modulation control means. 2. An LCD backlight driving circuit for driving an LED light source, which is a backlight of a liquid crystal display, comprising: a voltage monitor for capturing a change in battery voltage; and a duty of an output pulse is controlled based on an output of the voltage monitor. Pulse width modulation control means for comparing the battery voltage with the backlight lighting limit voltage value, and outputting a detection signal when the voltage falls below the backlight lighting limit voltage value; and detecting the voltage drop detection means. A backlight driving means for stopping a driving operation of the backlight by a signal and driving the backlight based on an output of the pulse width modulation control means.