JP2973231B2 - Power circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a power supply circuit that outputs a plurality of output voltages having different voltage values to a load, such as a power supply circuit used in a liquid crystal display device, and in particular, measures against a decrease in the power supply voltage. About.

BACKGROUND OF THE INVENTION For example, a conventional power supply circuit of a liquid crystal display circuit includes a constant voltage circuit that sends out a constant output voltage regardless of the level of the power supply voltage, and a plurality of outputs having different voltage values by stepping up and down the output voltage of the constant voltage circuit. It is composed of a step-up / step-down circuit for transmitting a voltage, and the plurality of output voltages are supplied as driving voltages to a liquid crystal display panel to be driven.

However, the power supply circuit of the conventional liquid crystal display circuit cannot maintain the liquid crystal display quality with low power consumption over a wide power supply voltage range.

For example, consider the case of driving a liquid crystal display panel that requires five values of voltages of 0V, 1V, 2V, 3V, and 4V. When a constant voltage circuit generates a 2V liquid crystal drive voltage, and a buck-boost circuit generates a 1V, 3V, 4V liquid crystal drive voltage based on this 2V liquid crystal drive voltage, the power supply voltage is lower than 2V At this time, the constant voltage circuit cannot generate the liquid crystal driving voltage of 2 V, and as a result, the step-up / step-down circuit cannot generate the liquid crystal driving voltage. Therefore,
There is a problem that the liquid crystal driving voltage decreases as the power supply voltage decreases, and the contrast of the liquid crystal display deteriorates.

If the constant voltage circuit generates a 1V liquid crystal drive voltage and the buck / boost circuit generates a 2V, 3V, or 4V liquid crystal drive voltage, the display on the liquid crystal panel will remain until the power supply voltage drops to 1V. Although quality is ensured, there is a problem that charge loss due to charge and discharge of the capacitor is large, current consumption is increased, and the life of a battery constituting a power supply is shortened.

DISCLOSURE OF THE INVENTION It is an object of the present invention to provide a power supply circuit capable of appropriately coping with fluctuations in power supply voltage, particularly a decrease in power supply voltage, and preventing an increase in current consumption.

The power supply circuit according to the present invention has a constant voltage circuit that outputs a voltage corresponding to a predetermined mode control signal, and an output voltage of the constant voltage circuit at a rate based on the predetermined mode control signal. A step-up / step-down circuit for transmitting a plurality of output voltages. The output of the constant voltage circuit and / or the step-up / step-down circuit is supplied as a liquid crystal driving voltage to a load, for example, a liquid crystal driving circuit for driving a liquid crystal display panel.

Here, the means for generating the predetermined mode control signal includes a power supply voltage determination circuit or a heavy load detection circuit. The power supply voltage determination circuit compares the power supply voltage with a predetermined reference voltage and outputs a mode control signal corresponding to the comparison result.

For example, when it is determined that the power supply voltage is equal to or higher than the reference voltage, a corresponding mode control signal is sent to the constant voltage circuit and the step-up / step-down circuit. The constant voltage circuit outputs a high voltage corresponding to the mode control signal, and the step-up / step-down circuit boosts or steps down the high output voltage at a predetermined ratio to output a plurality of voltages.

When it is determined that the power supply voltage is low, the constant voltage circuit outputs a low voltage corresponding to the mode control signal at that time, and the step-up / step-down circuit outputs the low output voltage at a different rate from the above case. To output a plurality of voltages. Then, the outputs of the whole of the constant voltage circuit and the step-up / down circuit at this time are the same as when it is determined that the power supply voltage is the same or higher.

Further, since the load connected to the power supply is known in advance, when the load corresponding to the heavy load is driven, the heavy load detection circuit outputs a mode control signal corresponding thereto. In other words, when a heavy load is driven, the current consumption is large, and it is essential that the voltage drop due to the internal resistance of the power supply, that is, the battery, increases and the power supply voltage decreases. Instead of detecting after the power supply voltage has dropped, the same processing as when the power supply voltage drops before the power supply voltage actually drops is performed.

Therefore, in the present invention, the output of the constant voltage circuit is increased when the power supply voltage is high, and the output of the constant voltage circuit is decreased when the power supply voltage is low or heavy load is driven. Since the ratio of the step-up / step-down of the voltage circuit is made different from the above case, and the voltage supplied to the load is made the same,
The load can be driven stably irrespective of fluctuations in the power supply voltage. The constant voltage circuit outputs a low voltage only when the power supply voltage is lower than the reference voltage or when a heavy load is driven. In other cases, the constant voltage circuit outputs a high voltage. The load can be driven by electric power, and the life can be extended when a battery is used as a power supply.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an example in which a power supply circuit according to one embodiment of the present invention is used as a power supply for driving a liquid crystal display panel.

 FIG. 2 is a circuit diagram of the constant voltage circuit of the embodiment.

 FIG. 3 is a circuit diagram of the step-up / step-down circuit of the embodiment.

 FIG. 4 is an explanatory diagram of the operation of the step-up / step-down circuit of FIG.

BEST MODE FOR CARRYING OUT THE INVENTION A driving power supply circuit for a liquid crystal display panel shown in FIG. 1 is built in a one-chip semiconductor 50, and a constant voltage circuit 1 has a mode of outputting 1V and a mode of outputting 2V. Have. The step-up / step-down circuit 2 is externally provided with a capacitor 6 for charging and discharging an electric charge in order to step-up / step-down the output 7 of the constant voltage circuit 1.
When the output 7 of the constant voltage circuit 1 is 2V, the step-up / step-down circuit 2 steps down the output 7 of the constant voltage circuit 1 and outputs 1V to the output terminal 8,
The output 7 of the constant voltage circuit 1 is boosted to output 3V and 4V to output terminals 10 and 11, respectively. At this time, 2 V of the same potential as the constant voltage circuit output 7 is output to the output terminal 9.

Here, “1V”, “2V”, “3V” and “4V” indicate absolute values, and for example, indicate a negative value when the positive electrode is set to the ground potential.

When the output 7 of the constant voltage circuit 1 is 1 V, the step-up / step-down circuit 2 boosts the output 7 of the constant voltage circuit 1 to output terminals 9, 10, and 1.
1 outputs 2 V, 3 V, and 4 V, respectively, and an output terminal 8 outputs 1 V having the same potential as the output 7 of the constant voltage circuit 1.

The power supply voltage determination circuit 3 determines whether the power supply voltage is higher or lower than 2V. The power supply voltage determination circuit 3 divides the power supply voltage by resistors R1 and R2 as shown in the figure, compares the divided potential with a reference voltage of a reference voltage generation circuit 31 by a comparison circuit 32, and compares the comparison result. Output.

The heavy load detection circuit 4 detects the operation of a heavy load circuit such as an external buzzer when it operates. Here, the heavy load circuit will be described. The CPU section 15 has a predetermined load, here, the buzzer control register 16 when the buzzer sounds.
"1" is written to the D terminal. This buzzer control register 16
Opens an AND gate 17, from which a buzzer clock signal 18 is sent. The buzzer clock signal 18 has a frequency of usually 2 kHz to 8 kHz, and causes the piezoelectric buzzer 21 to ring through a buzzer driver 19 and a transistor 20. When the power supply voltage (voltage between V _{DD and} V _SS ) is low, the sound pressure of the piezoelectric buzzer 21 decreases because the boost coil 22 connected in parallel to the piezoelectric buzzer 21 uses the back electromotive force of the inductance. The voltage applied to the piezoelectric buzzer 21 is increased to increase the sound pressure of the piezoelectric buzzer 21. When the piezoelectric buzzer 21 rings, a current of about mA flows and when the internal impedance of the battery is high, such as when the battery is exhausted, the output voltage of the battery is reduced due to the voltage drop due to the internal impedance of the battery. Will go down.

Therefore, when a heavy load is driven, such as when a buzzer sounds, "1" is written from the CPU unit 15 to the D terminal of the heavy load mode setting register constituting the heavy load detection circuit 4, and its output is output. Transmit as "1". Of course, when stopping driving of the buzzer, “0” is set in the heavy load mode setting register.
To return to the normal mode.

The liquid crystal power supply control means 5 comprises an OR circuit, obtains an OR logic between the output of the power supply voltage determination circuit 3 and the output of the heavy load detection circuit 4, and outputs a mode control signal to the constant voltage circuit 1 and the step-up / step-down circuit 2.

The liquid crystal power supply control means 5 includes, for example, the power supply voltage determination circuit 3
However, if it is determined that the power supply voltage is higher than 2V, the output of the constant voltage circuit 1 is set to 2V, and the operation of the step-up / step-down circuit 2 is also [1V step-down / 3V, 4
When the power supply voltage is determined to be lower than 2 V, the output of the constant voltage circuit 1 is switched to 1 V and the operation of the step-up / step-down circuit 2 is switched to the [2 V, 3 V, 4 V boost] mode.

Further, the liquid crystal power supply control means 5 sets the output of the constant voltage circuit 1 to 2 V during normal operation, that is, when the load is not heavy, in response to the mode control signal output from the heavy load detection circuit 4, and also operates the step-up / step-down circuit 2. Set to [1V step-down / 3V, 4V step-up] mode,
At the time of heavy load operation, the output of the constant voltage circuit 1 is switched to 1V and the operation of the step-up / step-down circuit 2 is switched to the [2V, 3V, 4V step-up] mode.

The liquid crystal drive circuit 12 receives the liquid crystal drive voltage 1 from the step-up / step-down circuit 2.
V, 2V, 3V, 4V and image information 25 from CPU unit 15
Is input, a liquid crystal driving voltage is appropriately selected based on the image information 25, and a liquid crystal display signal 13 is output to the liquid crystal display panel 14. The liquid crystal display panel 14 displays an image based on the liquid crystal display signal 13.

 FIG. 2 is a circuit diagram showing an example of the constant voltage circuit 1.

The difference between the threshold voltages of the PMOS-FETs 101 and 102 is output to the connection point 103 as a reference voltage. Here, the PMOS-FET 101 is a depression type FET, and the PMOS-FET 102 is an enhancement type FET. When the difference between the threshold voltages of the PMOS-FETs 101 and 102 is created by the work function difference of the polysilicon gate, it is possible to stably generate about 1V. Note that the reference voltage at the connection point 103 is output as a constant voltage with respect to _VDD . The five MOS-FETs 104, 105, 106, 107, and 108 are differential amplifier circuits of an operational amplifier, and constitute a differential buffer circuit.

The mode control signal HVLD113 is a signal for controlling the output mode of the constant voltage circuit 1. When HVLD is LOW, the reference voltage is amplified by the feedback resistors 109 and 110, and a voltage twice the reference voltage is output as VL2 from the terminal 112. Is done. HVLD is HI
At the time of GH, the voltage of the same potential as the reference voltage
Is output as

Thus, when the reference voltage is set to -1 V with respect to V _DD (zero potential), -2 V is output to VL2 when HVLD is LOW, and -1 V is output to VL1 when HVLD is HIGH .

FIG. 3 is a circuit diagram showing an example of the step-up / step-down circuit 2. 20
F _B of 1 f _A and 202 is a clock signal, the timing chart is as shown in Figure 4. In order to prevent the overlap timing of the charging and discharging, and a time difference Δt between the rising of falling and f _B of the clock signal f _A. Level translators 204, 205, 206, 207, 208, 209, 210 and 2
Reference numeral 11 denotes a level conversion circuit for converting a control signal including the above-described clock signal into a signal having a higher amplitude.

In the step-up / step-down circuit 2, the clock signal f _A
H, the clock signal f _B is LOW timing A and f _A the LOW, f _B
At the timing B of HIGH, the step-up / step-down operation is realized by changing the connection state between the charge transfer capacitors (212, 213, 214 in FIG. 3) and the power supply terminals from _VDD to VL4. When HVLD is LOW, VL2 is reduced by 1/2 to generate VL1, VL2 is increased by 1.5 times to generate VL3, and VL2 is doubled to generate VL4.

 When HVLD is HIGH, VL1 is boosted twice to generate VL2, VL1 is boosted three times to generate VL3, and VL1 is boosted four times to generate VL4.

The connection state of the transfer capacitor in each mode is as shown in FIG.

The liquid crystal power supply control means 5 may control the liquid crystal power supply by directly receiving the output of the power supply voltage determination circuit 3 or the output of the heavy load control circuit 4, or may control the liquid crystal power supply by software such as a microcomputer. Good.

In the above-described embodiment, an example in which a D-type flip-flop circuit is used as the heavy load detection circuit 4 has been described. However, another type of flip-flop circuit may be used, or the buzzer control register 16 of the buzzer circuit may be configured. The flip-flop circuit used may be used as it is.

INDUSTRIAL APPLICABILITY The power supply circuit according to the present invention is not limited to a power supply circuit for a liquid crystal display device, but may be any other power supply circuit that needs to output a multi-level voltage by a combination of a constant voltage circuit and a step-up / down circuit. Applied.

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G05F 1/00-5/00 H02M 3/00-3/44

Claims (8)

(57) [Claims] 1. A constant voltage circuit for outputting a voltage corresponding to a mode control signal, and an output voltage of the constant voltage circuit is input, and a plurality of input voltages are stepped up or down at a rate based on the mode control signal. And a step-up / step-down circuit for transmitting the output voltage of the power supply. 2. A power supply voltage is compared with a predetermined reference voltage.
2. The power supply circuit according to claim 1, further comprising a power supply voltage determination circuit for transmitting a mode control signal corresponding to the comparison result. 3. A heavy load detection circuit according to claim 1, further comprising a heavy load detection circuit for detecting a drive when a predetermined external load is driven and transmitting a mode control signal corresponding to the detection result. Power circuit. 4. A heavy load detecting circuit according to claim 2, further comprising a heavy load detecting circuit for detecting a drive when a predetermined external load is driven and transmitting a mode control signal corresponding to the detected result. Power circuit. 5. A mode control signal from a power supply voltage judgment circuit and an OR logic of a mode control signal from a heavy load detection circuit are obtained and output to a constant voltage circuit and a step-up / step-down circuit as a mode control signal. Power supply circuit described in the paragraph. 6. A CPU for outputting image information, an output voltage of a constant voltage circuit and an output voltage of a step-up / step-down circuit are input as a liquid crystal driving voltage, and image information is input from the CPU to an external liquid crystal display panel. 6. The power supply circuit according to claim 5, further comprising a liquid crystal display drive circuit for transmitting a display signal, wherein the power supply circuit is used as a power supply circuit for a liquid crystal display device. 7. The power supply circuit according to claim 5, wherein each member is constituted by a one-chip semiconductor device. 8. The power supply circuit according to claim 7, wherein a terminal for an external capacitor of the step-up / step-down circuit is provided.