JP2004213697A - Constant voltage circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a constant voltage circuit capable of preventing reverse-current from a battery for backup and lowering the lowest operation voltage of a VR (voltage regulator). <P>SOLUTION: Bias changeover switches 35, 37 for connecting a substrate with either of the side of a power supply 1 or the side of the battery 7 for backup are connected with the substrate of an output transistor 25. An output terminal of a comparator 39 for comparing input voltage Vbat with output voltage Vout is connected with the changeover switch 35 via the changeover switch 37 and an inverter 41, when the input voltage Vbat is higher than the output voltage Vout, the changeover switch 35 becomes on, the changeover switch 37 becomes off and when the input voltage Vbat is lower than the output voltage Vout, the changeover switch 35 becomes off and the changeover switch 37 becomes on. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a constant voltage circuit for stabilizing power supplied to a load and supplying the same through an output transistor.

FIG. 2 shows an example of a conventional power supply circuit.
For example, a power supply 1 such as a Li-ion battery and a constant voltage circuit (Voltage Regulator, hereinafter abbreviated as VR) for stabilizing the power supply to a backup load 3 such as a RAM when the power supply 1 is turned on. 5) a backup battery 7 for backing up the load 3 when the power supply 1 is off, and a diode 9 for preventing a current from flowing back from the backup battery 7 to the power supply 1 through VR5 when the power supply 1 is off. It is configured.

VR5 is an input terminal (Vbat) 11 to which the power supply 1 is connected, a reference voltage section (Vref) 13, an operational amplifier (OPAMP) 15, an output transistor (DRV) 17 including a P-channel MOS transistor, an output terminal (Vout) 19, and Feedback resistors R1 and R2 are provided.
In the operational amplifier 15, the output terminal is connected to the gate electrode of the output transistor 17, the reference voltage Vref is applied to the inverting input terminal from the reference voltage unit 13, and the output voltage Vout is divided by the resistors R1 and R2 to the non-inverting input terminal. The voltage is applied, and the operational amplifier 15 controls the output voltage Vout such that the voltage divided by the resistors R1 and R2 becomes equal to the reference voltage Vref.
The output terminal 19 is connected to the load 3 via the diode 9, and the backup battery 7 is connected between the diode 9 and the load 3. The backup battery 7 supplies power to the load 3 when the power supply 1 is turned off or when the voltage of the power supply 1 drops. The diode 9 is provided so that the current does not flow backward from the backup battery 7 to the power supply 1.

When a diode is used as a means for preventing a backflow of current, a voltage drop of the VR output voltage Vout occurs in the diode. The voltage drop at the diode is as large as about 0.7 V. In order to secure the required voltage Vbk at the load, the output voltage Vout of VR must be set to Vbk + 0.7 V, and as a result, the minimum operating voltage of VR There was a problem that can not be lowered. Further, when a Zener diode is used as a diode, a large reverse current of about 10 μA flows, and the reverse current becomes about 30 μA at a high temperature.
Therefore, an object of the present invention is to provide a power supply circuit provided with a backup battery, which can prevent a reverse current and reduce the minimum operating voltage of VR.

  A constant voltage circuit according to the present invention supplies a stabilized power to a load via an output transistor and is connected to a backup battery. The first voltage dividing resistor divides a voltage on the power supply side; A second voltage-dividing resistor for dividing the voltage on the load and the backup battery side, and the power supply serving as a driving power source; a divided voltage by the first voltage-dividing resistor and a divided voltage by the second voltage-dividing resistor A comparison circuit that performs a comparison with, based on the output of the comparison circuit, when the voltage on the power supply side is higher than the voltage on the load and the backup battery side, the back bias of the output transistor to the voltage on the power supply side, Switching means for switching the back bias of the output transistor to the voltage on the load side when the voltage on the power supply side is lower than the voltage on the load and the backup battery side. And one in which these units are formed on one chip.

  If the power supply can supply a sufficient voltage, the power supply to the load is performed via VR. When the input voltage of VR is higher than the battery voltage for backup, the comparison circuit controls the switching means to set the back bias of the output transistor to the power supply voltage. When the input voltage of VR is lower than the battery voltage for backup, the comparison circuit controls the switching means to set the back bias of the output transistor to the battery voltage for backup. At this time, the output transistor is off, and a diode is formed between the power supply side terminal (drain) of the output transistor and the substrate by a PN junction from the backup battery to a direction opposite to the power supply side. Therefore, the backflow of current from the backup battery to the power supply can be suppressed.

The comparison circuit compares a divided voltage by the first voltage dividing resistor connected to the power supply side with a divided voltage by the second voltage dividing resistor connected to the backup battery side. Even when the circuit has an offset, the voltage dividing ratio of the first voltage dividing resistor and the voltage dividing ratio of the second voltage dividing resistor are adjusted so that the output is inverted when the input voltage is equal to the backup battery voltage. Is preferred.
As a result, even when an offset exists in the differential input transistor of the comparison circuit, the comparison circuit can be reliably operated.

In the constant voltage circuit according to the present invention, when the voltage on the power supply side is lower than the voltage on the load and the backup battery side, the back bias of the output transistor is switched to the voltage on the load side. Current can be prevented.
Further, since a diode for preventing reverse current is not separately provided, it is not necessary to consider a voltage drop due to the diode, and the minimum operating voltage of VR can be reduced.

FIG. 1 is a circuit diagram showing one embodiment.
A VR 21 is provided to stably supply power from the power supply 1 to the load 3. The power supply 1 is connected to an input terminal (bat) 23 provided on the VR 21, and the input terminal 23 is connected to an output terminal (Vout) 27 via an output transistor (DRV) 25 formed of a P-channel MOS transistor. I have. The output terminal 27 is connected to the load 3, and the backup battery 7 is connected between the output terminal 27 and the load 3. As the backup battery 7, a primary battery such as a button battery, a secondary battery, a super capacitor, or the like can be used.

  The VR 21 includes an operational amplifier (OPAMP) 29, and an output terminal of the operational amplifier 29 is connected to a gate electrode of the output transistor 25. A reference voltage Vref from a reference voltage unit (Vref) 31 is applied to the inverting input terminal of the operational amplifier 29, and a voltage obtained by dividing the output voltage Vout of the output transistor 25 by the feedback resistors R1 and R2 is applied to the non-inverting input terminal. You. A bias switch 33 composed of an N-channel MOS transistor is interposed between the ground terminal of the feedback resistor R2 and the ground in order to cut off a through current during non-operation.

To the substrate of the output transistor 25, back bias changeover switches 35 and 37 formed of P-channel MOS transistors are connected. The changeover switch 35 connects the substrate of the output transistor 25 to the power supply 1 side, and the changeover switch 37 connects the substrate of the output transistor 25 to the backup battery 7 side.
The VR 21 is also provided with a comparator (comparison circuit, COMP) 39. One input terminal of the comparator 39 has a divided voltage generated by voltage dividing resistors R3 and R4 connected to the input terminal 23 side (power supply 1 side). Is applied to the other input terminal, and a divided voltage by the voltage dividing resistors R5 and R6 connected to the output terminal 27 side (the backup battery 7 side) is applied. The output terminal of the comparator 39 is connected to the operational amplifier 29, the bias switch 33 and the changeover switch 37, and to the changeover switch 35 via the inverter 41, and these components are turned on by the magnitude of the input voltage and the backup battery voltage.・ Off is controlled.
The power of the operational amplifier 29, the reference voltage unit 31, and the comparator 39 is supplied from the power supply 1.
In this embodiment, the VR 21 surrounded by a broken line is formed on one chip.
The switching means constituting the present invention is constituted by the changeover switches 35 and 37 and the inverter 41.

Next, the operation of this embodiment will be described.
In a normal state in which power is supplied from the power supply 1 to the load 3 via the VR 21, the input voltage Vbat on the input terminal 23 side is equal to or higher than the output voltage Vout on the output terminal 27 side, and the comparator 39 outputs “H” (high level). ) Is output, the operational amplifier 29, the bias switch 33 and the changeover switch 35 are turned on, and the changeover switch 37 is turned off. Thus, the back bias of the output transistor 25 becomes the input voltage Vbat. In this state, the output voltage Vout is controlled to a constant voltage by the operational amplifier 29.

When the input voltage is turned off or when the voltage of the power supply 1 becomes lower than the voltage of the backup battery 7, the output of the comparator 39 becomes "L" (low level), and the operational amplifier 29, the bias switch 33, and the changeover switch 35 It turns off and the changeover switch 37 turns on. As a result, the output transistor 25 is turned off, and the back bias of the output transistor 25 becomes the voltage of the backup battery 7, and the output transistor 25 is backed up by the PN junction between the terminal (drain) on the power supply 1 side and the substrate. The current flowing backward from the backup battery 7 through the output terminal 27 is only the current flowing through the voltage dividing resistors R5 and R6 by the diode in the direction opposite to the power supply 1 side from the battery 7. By using a high resistance as the voltage dividing resistors R5 and R6, almost no current can flow through the voltage dividing resistors R5 and R6.
As described above, the backflow current can be prevented without using a diode, so that the minimum operating voltage of the VR 21 can be reduced.

Since the power supply of the comparator 39 uses the power supply 1, the power of the backup battery 7 is not consumed.
When the comparator 39 has an offset, the voltage dividing ratios of the voltage dividing resistors R3, R4 and R5, R6 are adjusted so as to have a difference, and when the input voltage becomes lower than the backup battery voltage, the comparator 39 becomes "". L "is always output.
In addition, in the case where the diode is included in VR in a single chip according to the conventional technology, the chip area is increased because the diode requires a large area to secure the output current. Since an element for flowing a large current other than a transistor is not required, an increase in chip area can be suppressed and a power supply circuit can be formed.
In the embodiment, a P-channel MOS transistor is used as an output transistor. However, an N-channel MOS transistor or a bipolar transistor can be used.

FIG. 3 is a circuit diagram showing one embodiment. FIG. 9 is a circuit diagram illustrating an example of a conventional power supply circuit.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Power supply 3 Load 7 Backup battery 21 Constant voltage circuit (VR)
REFERENCE SIGNS LIST 23 input terminal 25 output transistor 27 output terminal 29 operational amplifier 31 reference voltage section 33 bias switch 35, 37 back bias switch 39 comparator 41 inverter R1, R2 feedback resistor R3, R4, R5, R6 voltage dividing resistor

Claims (1)

In a constant voltage circuit connected to a backup battery while supplying stabilized power to the load via the output transistor,
A first voltage dividing resistor for dividing the voltage on the power supply side;
A second voltage-dividing resistor for dividing the voltage of the load and the backup battery;
A comparison circuit that uses the power supply as a driving power supply and compares a divided voltage by the first voltage dividing resistor with a divided voltage by the second voltage dividing resistor;
Based on the output of the comparison circuit, when the voltage on the power supply side is higher than the voltage on the load and the backup battery side, the back bias of the output transistor is set to the voltage on the power supply side, and the voltage on the power supply side is the load and Switching means for switching the back bias of the output transistor to the voltage on the load side when the voltage is lower than the voltage on the backup battery side, and wherein each of these parts is formed on one chip. circuit.

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