JPS61240828A - Power source backup circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a power supply backup circuit, and particularly to a memory IC.
The present invention relates to a power supply backup circuit suitable for semiconductor ICs, which switches to a built-in power supply to retain memory contents when the power supply supplied to the memory circuit of a semiconductor IC or logic IC is cut off or voltage drops.

(Technical Background of the Invention) Various types of power backup circuits are known, but the basic one is a circuit that utilizes the forward characteristics of a diode.

FIG. 3 is a circuit diagram showing the basic configuration of a conventional power backup circuit. As shown in the figure, terminal A is connected to an external power supply, terminal B is connected to a semiconductor IC such as a memory circuit, and power is normally supplied from this external power supply. A backup battery 1 is connected to a diode 2 between the supply line 3 and ground.
connected via. In order to stabilize the potential of the supply line 3, a resistor 4 is connected between the supply line 3 and ground. In this way, power is normally supplied to the semiconductor IC from the external power supply for the drive, but when the potential of the supply line 3 decreases due to the external power supply being cut off or voltage drop, etc., power is supplied from the battery 1 via the diode 2. Electric power is supplied to the supply line 3.

[Problems with background technology]

As mentioned above, conventional power backup circuits utilize the forward direction of diodes, so the voltage applied to semiconductor ICs, etc. to which power is supplied will always drop by the forward direction voltage of the diode. Become. Normally, the forward voltage of this diode is about 0.7V, but the battery voltage must be raised in advance by this drop, and when the supply current increases, the forward voltage There was a drawback that the voltage VF increased and the supply voltage decreased.

[Purpose of the invention]

The present invention has been made to eliminate these drawbacks, and provides a power supply that prevents the supply potential from dropping by the forward voltage valve of the diode during power backup, and that can achieve this with a simple circuit configuration. The purpose is to provide a backup circuit.

[Summary of the invention]

In order to achieve the above object, the present invention provides a power supply backup circuit that switches to a second power supply (built-in battery) as a power supply when a first power supply (external power supply) is cut off, and outputs a first level signal if the voltage is above a predetermined potential, and outputs a second level signal if it is less than a predetermined potential, and the output of the detection circuit is the first level signal. A first switch circuit connects the external power supply to supply power when the signal is at the second level, and cuts off the supply from the built-in battery when the signal is at the second level, and the output of the detection circuit is connected to the second level. This invention provides a power backup circuit comprising a second switch circuit that connects the built-in battery to supply power when the signal is at the first level, and operates to cut off the supply of the built-in battery when the signal is at the first level. be.

[Embodiments of the invention]

Hereinafter, the present invention will be described in detail with reference to FIGS. 1 and 2 of the accompanying drawings. FIG. 1 is a circuit diagram of the same embodiment.

This circuit includes a detection circuit that detects the voltage level of terminal A connected to an external power source, a first switch circuit that electrically connects terminal A and terminal B, and an electrical connection between battery 1 and terminal B. and a second switch circuit connected to. All of these circuits are configured as CMOS circuits, the detection circuit is configured with two CMOS inverter circuits 11.12, and the first and second switch circuits are configured as analog switch circuits 13.14. There is. The output of the detection circuit is the analog switch circuit 13.14
is supplied as a control signal to

First, a case of normal operation in which an external power source (for example, 5V) is connected to terminal A and supplied to terminal B will be described. In this case, the CMOS inverter circuit 11 detects the potential level of the terminal A as "H", its output becomes "L", and the output of the CMOS inverter 12 becomes "H". At this time, the analog switch circuit 13 is turned on, the analog switch circuit 14 is turned off, and 5V from the external power supply is supplied to terminal B, which becomes the IC power supply. That is, the analog switch circuits 13 and 14 operate so as to turn on when the control input, which is the output of the detection circuit, is "H11" and turn off when it is "11 L".

Next, when the external power supply is cut off or the voltage drops, the inverter 11 detects the "L" level, its output becomes "Tobi", and the output of the inverter 12 also becomes L11.Therefore, the analog switch circuit 13 is turned off, and the analog The switch circuit 14 is turned on.The voltage of the battery 1 is now 5■
If this is done, the voltage of 5V from the battery 1 will be supplied to the terminal B as is.

Note that the circuit threshold of the inverter 11 is about half the power supply voltage, but the threshold can be raised to 4.5V, for example.
By setting it to a certain level, it will be able to respond to minute fluctuations in the external power supply.

FIG. 2 is a diagram that does not show the input/output characteristics of the inverter 11 used in this embodiment. In the case of a normal inverter, the threshold value is low as shown by characteristic a shown by a solid line in the figure.

However, as described in this embodiment, if the threshold value is increased and the characteristics are set as shown by the dotted line in the figure, even minute voltage fluctuations can be accommodated.

Note that if an operational amplifier is used instead of an inverter, the analog switch circuit 13.14 will change depending on the voltage value of the external power supply.
You can also set it to turn on or off. In any case, the detection circuit is designed such that a predetermined detection level is set, and if the voltage is higher than this detection level, it outputs a first level signal, and if it is lower, it outputs a second level signal. It is sufficient as long as it has a function.

A pull-down resistor 15 connected between the input of the inverter 11 and ground causes the inverter 11 to
It is used to set the input level to "L", and its resistance value may be as high as several hundred Ω to several + MO. The capacitor 16 connected between the terminal B and the ground is used to prevent chattering during switching of the analog switch circuits 13 and 14, but generally when configured as an IC, a bypass capacitor is included in the periphery. There is no need for special installation.

〔Effect of the invention〕

As described above, the present invention has a circuit configuration that does not involve a voltage drop in the internal IAN battery (second power supply) that operates as a supply source when the external power supply (first power supply) is cut off, so that the battery voltage is reduced during power backup. It is possible to obtain a power supply backup circuit that does not require increasing the voltage in advance.

Further, if this circuit is configured as a CMOS circuit, the current consumption during backup is very small, which has the advantage of reducing power consumption.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing input/output characteristics of an inverter used in the present invention, and FIG.
The figure is a circuit diagram showing the configuration of a conventional circuit. 11.12... Inverter (detection circuit), 13...
Analog switch circuit (first switch circuit), 14.
...Analog switch circuit (second switch circuit), 1
...battery (second ° power source). Applicant's agent Kiyoshi Inomata 121! ]-IN'L! 31!

Claims (1)

[Claims] 1. In a power supply backup circuit that switches to a second power supply as a power supply when a first power supply is cut off, the voltage of the first power supply is detected, and the voltage is at a predetermined potential. a detection circuit that outputs a first level signal if the potential is above a predetermined potential, and outputs a second level signal if the potential is less than a predetermined potential; connected so that the power supply of the second
a first switch circuit that operates to cut off the supply of the first power source when the signal is at the level level, and a second power source that operates to cut off the supply of the first power source when the output of the detection circuit is the signal at the second level; and a second switch circuit which is connected so that the signal is at the first level and operates to cut off the supply of the second power supply when the signal is at the first level. 2. The power backup circuit according to claim 1, wherein the detection circuit and the first and second switch circuits are all constructed from CMOS circuits.