JP2001231183A - Power supply backup circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a power supply backup circuit for backing up a volatile memory circuit (RAM) at the time of turn off power in which voltage loss in a reverse current preventing circuit is reduced at the time of charging the volatile memory circuit through the reverse current preventing circuit or during normal operation. SOLUTION: When a volatile memory circuit (RAM) 1 having an electric circuit 4 is backed up by normally charging it from a power supply circuit 5 through a capacitor 2 and discharging the capacitor 2 upon turning the power supply circuit 5 off, a PNP transistor 6 and an NPN transistor 7 are employed in a reverse current preventing circuit 8 thus reducing voltage loss at the time of charging.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply backup circuit for a volatile memory circuit, and more particularly to a power supply backup circuit for reducing voltage loss when charging a backup capacitor and turning off a power supply circuit of a volatile memory circuit. Circuit.

[0002]

2. Description of the Related Art Conventionally, when a voltage of a power supply circuit is turned off, RA
Various power supply backup circuits have been proposed so that the storage contents of volatile storage circuits (backup memory circuits) such as M (random access memory) are not erased.

For example, in Japanese Utility Model Laid-Open No. 4-121357, when a backup memory circuit is backed up in the event of a power failure by using a primary power source for backup and a large-capacity capacitor, the main power source 10 as shown in FIG. Large-capacity capacitor 1 in power supply path to backup memory circuit 14
8 and the large-capacity capacitor 1 of this feed line.
8 shows a backup circuit having a configuration in which a backflow prevention diode 21 is provided between the connection point 8 and the backup memory 14, and a backflow prevention diode 17 of the backup primary battery 15 is connected in series to the cathode side of the diode 21. I have.

In FIG. 3, reference numeral 20 denotes a diode for flowing a charging current from the main power supply 10 to the large-capacity capacitor 18 in the forward direction and for preventing backflow to the power supply circuit. 14 is a diode to be supplied.

Japanese Patent Application Laid-Open No. 5-15086 also discloses a backup circuit using a backflow prevention diode for preventing backflow to a large-capacity backup capacitor.

FIG. 4 shows a backup circuit disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 5-15086.
Is a power supply internal resistance, and 23 is a microcomputer (hereinafter C)
A switch for operating 24) and a CPU 14
Reference numeral 24 denotes a backup memory circuit such as a RAM included in the RAM 24; 25, a data address bus connected between the CPU 24 and the backup memory circuit 14; 18, a large-capacity capacitor for backup; and 20, a charge / power circuit for charging the capacitor 18. Are diodes for preventing backflow, and 21 and 26 are diodes for preventing backflow to the capacitor 18 and the CPU 24.

If the large-capacity capacitor 18 is charged from the main power supply 10 via the backflow preventing diode 20 in the above-described configuration, the large-capacity capacitor 18 discharges even if the main power supply 10 is turned off. The backup memory circuit 14 is supplied with the voltage charged in the large capacity capacitor 18. Here, even if the switch 23 is turned on before the large-capacity capacitor 18 is charged, the diodes 21 and 2
6, the power supply voltage is supplied to the backup memory circuit 14, and the backflow from the backup memory circuit 14 to the capacitor 18 and the CPU 24 can be prevented.

[Problems to be solved by the invention]

In the configuration of the conventional backup circuit described above, the large-capacity capacitor 1
Backflow prevention diodes 21, 17 and 26 are used to prevent backflow to the secondary battery 15 or the primary battery 15 or the CPU 24.

As described above, when the backflow preventing diode is used, the voltage from the main power supply 10 drops by 0.6 V to 0.7 V in the forward direction due to the internal resistance of the backflow preventing diode. No countermeasure against such a voltage drop is taken at all in the above-mentioned publications.

For example, in the circuit configuration shown in FIG. 3, the output voltage of the backflow prevention diode 20 is 0.6 V to 0.2 V with respect to the specified voltage of the main power supply 10 due to a voltage drop due to the internal resistance of the backflow prevention diode 20. It drops by 7V.

Further, the input supply voltage of the backup memory circuit 14 is also set to 0.6 V by the backflow prevention diode 21.
.About.0.7 V, the charging voltage across the capacitor 18 is also reduced by the backflow prevention diode 20, and the voltage holding time during backup is also shortened.

Similarly, considering the circuit example of FIG.
The interface voltage level of the CPU 24 connected to the same main power supply 10 and the interface voltage level of the backup memory circuit 14 connected to the data address bus 25 are shifted, which is not preferable in controlling the operation of the device.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and can reduce the voltage drop in the backflow prevention circuit, increase the charging voltage of the capacitor, and extend the voltage holding time of the capacitor. It is intended to provide a power backup circuit.

[Means for Solving the Problems]

A first power supply backup circuit according to the present invention comprises a power supply circuit 5, an electric circuit 4 connected to the power supply circuit 5 and having a volatile memory circuit 1, and a circuit between the power supply circuit 4 and the volatile memory circuit 1. And a capacitor 2 whose one end is connected to a connection point between the backflow prevention circuit 8 and the volatile circuit 1.
The backflow prevention circuit 8 includes a PNP transistor 6 having an emitter connected to the power supply circuit 5 and a collector connected to the capacitor, and an NPN for switching the PNP transistor 6.
It is characterized by including a transistor 7.

The second power supply backup circuit according to the present invention is characterized in that the backflow prevention diode 3 is provided in parallel between the emitter and the collector of the PNP transistor 6 of the backflow prevention circuit 8 in the first invention. .

According to the present invention, it is possible to prevent backflow from the volatile storage circuit 1 and the capacitor 2 to the power supply circuit 5 and reduce the power supply voltage drop in the backflow prevention circuit 8 to reduce the backup capacitor 2 and the volatile storage. A power supply backup circuit capable of increasing the supply voltage to the circuit 1 is obtained.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a system diagram of a power supply backup circuit showing one embodiment of the present invention, and FIG. 2 is a system diagram of a power supply backup circuit showing still another embodiment of the present invention.

In FIG. 1, reference numeral 5 denotes a power supply circuit such as a DC power supply. Reference numeral 4 denotes various electric circuits which are constituted by, for example, a CPU and have a volatile memory circuit (volatile memory) 1 to be backed up such as a RAM. are doing.

The electric circuit 4 is connected so as to be driven by the driving voltage of the power supply circuit 5, and the voltage from the power supply circuit 5 is supplied to the backflow prevention circuit 8 and the large-capacity backup capacitor 2 having one end grounded. . The volatile memory 1 is backed up by the voltage charged in the backup capacitor during normal driving when the power supply circuit 5 is turned on.

In the backflow prevention circuit 8 of the present embodiment, a PNP transistor 6 having an emitter connected to the positive terminal of the power supply circuit 5 and a collector connected to one end of the large-capacity capacitor 2, and one end connected to the power supply circuit 5. Ground the end, connect the connection midpoint of the directly connected voltage divider resistors R ₂ and R ₃ to the base,
PN above the collector via a base bias resistor R ₁
The switching NPN transistor 7 is connected to the base of the P transistor 6 and has an emitter grounded.

The above-described backflow prevention circuit 8 can be replaced with the backflow prevention diode 20 described with reference to FIGS.

The operation of the power supply backup circuit will be described. The voltage at the time of turning on and off the PNP transistor 6 is determined by using a divided voltage determined by the resistance values of the resistors R ₂ and R ₃ as a threshold value.

When the normal power supply circuit 5 is on, the voltage from the power supply circuit 5 is divided to a predetermined voltage by the voltage dividing resistors R ₁ and R ₂ in the backflow prevention circuit 8, and the bias voltage is applied to the base of the NPN transistor 7. The base potential is higher than the emitter potential and the NPN transistor 7 is turned on.

[0025] NPN transistor 7 is turned on the base of the PNP transistor 6 is biased through a resistor R _1, the base potential of the PNP transistor 6 becomes lower than the emitter potential, the PNP transistor 6 is turned on. Therefore, the large-capacity capacitor 2 is charged from the power supply circuit 5 and a predetermined voltage is supplied to the volatile memory 1.

At this time, since the collector-emitter voltage V _CE of the PNP transistor 6 is a saturation voltage, it is approximately 0.2 V
As compared with the case where the backflow prevention diode 20 or the like is used, the forward voltage drop is only 1/3, so that the charging voltage to the large-capacity capacitor 2 also increases, and the voltage holding time of the large-capacity capacitor 2 Can be lengthened.

When the power supply circuit 5 is turned off, the base potential of the NPN transistor 7 drops to the ground potential and the base potential and the emitter potential become the same potential, so that the collector-emitter turns off and the PNP transistor 6 turns off. In this state, the voltage charged in the large-capacity capacitor 2 is supplied to the volatile memory 1 to back up the power supply voltage. Since the PNP transistor 6 is turned off, the backflow from the large-capacity capacitor 2 to the power supply circuit 5 is prevented in the same manner as the backflow prevention diode 20.

FIG. 2 shows another embodiment, in which parts corresponding to those in FIG. FIG.
In the power supply backup circuit of FIG.
The diode 3 is connected between the emitter and the collector of the NP transistor 6 in the forward direction.

That is, the anode side of the diode 3 is connected to the emitter side of the PNP transistor 6, the cathode side of the diode 3 is connected to the collector side, and the diode 3 is connected in parallel between the emitter and the collector of the PNP transistor 6.

In the above configuration, when the power supply voltage is supplied from the power supply circuit 5, the power supply voltage of the volatile memory 1 rises with a time constant determined by the internal impedance of the PNP transistor 6 and the capacitance of the large-capacity capacitor 2. Since the capacity of the large-capacity capacitor 2 is large, the rise time when the power supply voltage is turned on also becomes long. As described above, when the diode 3 is used in combination, the rise time can be shortened.

It is to be noted that the backflow prevention circuit 8 of this embodiment can be applied in place of the backflow prevention diode 20 shown in FIGS. 3 and 4 shown in the configuration of the conventional example. Obviously, the invention is not limited to the power supply backup circuit.

【The invention's effect】

According to the power supply backup circuit of the present invention,
When the PNP transistor is in the ON state, the forward voltage drop can be reduced to 1/3 of that when a diode is used. In addition, since the charging voltage of the capacitor increases, the holding time of the capacitor can be made longer. Furthermore, a device which can reduce a deviation between an electric circuit connected to the volatile memory and an interface voltage supplied to the volatile memory can be obtained.

[Brief description of the drawings]

FIG. 1 is a system diagram of a power supply backup circuit showing one embodiment of the present invention.

FIG. 2 is a system diagram of a power backup circuit showing another embodiment of the present invention.

FIG. 3 is a system diagram of a power supply backup circuit showing a conventional example.

FIG. 4 is a system diagram of a power supply backup circuit showing another conventional example.

[Explanation of symbols]

1 volatile storage circuit (RAM), 2 large-capacity capacitor, 3 diode, 4 electrical circuit (CPU),
5 power supply circuit, 6 PNP transistor, 7 N
PN transistor, 8 ° backflow prevention circuit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H02J 1/00 309 G06F 1/00 330J F term (Reference) 5B011 DA02 DA07 DA13 DB23 EB01 JA03 JB01 5G015 FA08 GB05 HA15 JA07 JA34 JA61 5G065 AA01 BA02 DA04 EA01 GA06 KA02 KA05 LA01 NA01 NA02 NA04 NA06

Claims (2)

[Claims] 1. A power supply circuit, an electric circuit connected to the power supply circuit and having a volatile storage circuit, a backflow prevention circuit provided between the power supply circuit and the volatile storage circuit, and one end of the backflow prevention circuit In a power supply backup circuit including a capacitor connected to a connection point between the circuit and the volatile storage circuit and having the other end grounded, the backflow prevention circuit has an emitter connected to the power supply circuit and a collector connected to the capacitor. A power supply backup circuit, comprising: a PNP transistor; and an NPN transistor for switching the PNP transistor. 2. The power supply backup circuit according to claim 1, further comprising a backflow prevention diode in parallel between an emitter and a collector of the PNP transistor of the backflow prevention circuit.