JPH04313071A - Battery-voltage monitoring circuit - Google Patents

Abstract

PURPOSE:To detect the isolation of a battery when the battery is isolated in the case wherein the voltage of the battery is compared with the voltage of a reference power supply and the voltage of the battery is monitored. CONSTITUTION:A reference power supply 5, a battery 3 whose voltage is to be monitored, a comparator 2 and an MOSFET 10 are provided. The voltage of the reference power supply 5 and the voltage of the battery 3 are individually inputted into a negative input terminal - and a positive input terminal + of the comparator 2. The MOSFET 10 is provided between the positive input terminal + of the comparator 2 and the ground and turned ON by the isolation of the battery 3. When the voltage of the battery 3 is higher than the voltage of a reference power supply 5, the output of the comparator 2 attains an 'H' level. When the voltage of the battery 3 is lower than the voltage of the reference power supply 5, the output attains an 'L' level. When the battery 3 is isolated, the MOSFET 10 becomes ON, and the positive input terminal + of the comparator 2 becomes the ground voltage. The output of the comparator 2 is kept at the 'L' level, and the signal of the detection of the isolation of the battery 3 is obtained.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a battery voltage monitoring circuit that monitors the voltage of a battery built into, for example, an IC card. 2. Description of the Related Art A conventional battery voltage monitoring circuit of this type is configured as shown in FIG. 3, for example. The external power supply connection terminal 6 to which an external power supply is connected is connected to a semiconductor storage section 7 made of, for example, an SRAM via a Schottky barrier diode 8 having an anode connected thereto. The reference power source 5 is connected to the negative input terminal of the comparator 2. The negative electrode of the battery 3 whose voltage is to be monitored is grounded,
Its positive electrode is connected to the positive input terminal of the comparator 2 via a resistor 4. The positive input terminal + of the comparator 2 is connected to the semiconductor storage section 7 via a backflow prevention diode 1 whose anode is connected to the positive input terminal +. The output terminal of the comparator 2 is connected to the monitoring signal output terminal 9. The IC card A includes this battery voltage monitoring circuit. Next, the operation of this battery voltage monitoring circuit will be explained. When the IC card A is disconnected from an external device (not shown), the voltage of the battery 3 is applied to the semiconductor storage section 7 via the diode 1, and the data stored in the semiconductor storage section 7 is retained. On the other hand, when the IC card A is connected to an external device (not shown), the voltage of the power supply provided in the external device is applied to the semiconductor storage section 7 via the external power supply connection terminal 6 and the Schottky barrier diode 8. , the data stored in the semiconductor memory section 7 is held by the voltage of the external power supply. At this time, the voltage from the external power source is blocked by the diode 1 and is not applied to the battery 3. By the way, when the voltage of the battery 3 applied to the comparator 2 is lower than the voltage of the reference power supply 5, the output of the comparator 2, that is, the voltage of the monitoring signal output terminal 9 becomes "L".
If the level is high, on the other hand, the level is "H". As a result, when the IC card A is connected to an external device, the external device detects the voltage level of the monitoring signal output terminal 9, determines whether the voltage of the battery 3 is lower than the voltage of the reference power supply 5, and When the battery voltage is low, a lamp or buzzer installed on the external device notifies you that the battery voltage is low. In this way, the voltage of the battery 3 is monitored. [0005] By the way, when the IC card A is connected to an external device and it is reported that the voltage of the battery 3 has decreased, in order to replace it with a new normal battery,
When the battery 3 is disconnected, a leakage current flows through the diode 1 and the voltage at the positive input terminal + of the comparator 2 increases. As a result, the output of the comparator 2, that is, the voltage at the supervisory signal output terminal 9 goes to the "H" level, and the same detection operation as when the voltage of the battery 3 does not drop occurs, erroneously detecting a drop in the voltage of the battery 3. It turns out. [0006] Therefore, if the IC card A is disconnected from the external device without realizing that the battery 3 has been disconnected, no voltage will be applied to the semiconductor memory section 7, and the stored data will be lost. There's a problem. SUMMARY OF THE INVENTION In view of such problems, it is an object of the present invention to provide a battery voltage monitoring circuit that can obtain a signal indicating that the battery has been disconnected when the battery whose voltage is to be monitored is disconnected. [Means for Solving the Problems] A battery voltage monitoring circuit according to the present invention inputs the voltage of a battery whose voltage is to be monitored to one input terminal of a comparator, and inputs a predetermined voltage to the other input terminal of the comparator. A switch element is provided between one input terminal of the comparator and the voltage section below the predetermined voltage, and the switch element is configured to conduct when the battery is disconnected. [Operation] When the voltage of the battery whose voltage is to be monitored exceeds a predetermined voltage, the output of the comparator becomes "H" level, and when it becomes below the predetermined voltage, the output of the comparator becomes "L" level. When the battery is disconnected, the switch element becomes conductive, the voltage at one input terminal of the comparator is held below the predetermined voltage, and the output of the comparator is held at the "L" level. As a result, when the battery is disconnected, the output of the comparator is held at the "L" level and continues to notify that the battery has been disconnected. EXAMPLES The present invention will be explained in detail below with reference to drawings showing examples thereof. FIG. 1 is a circuit diagram showing the configuration of a battery voltage monitoring circuit according to the present invention. The external power supply connection terminal 6 to which an external power supply is connected is connected to a semiconductor memory section 7 made of, for example, an SRAM via a Schottky barrier diode 8 having an anode connected thereto. The reference power source 5 is connected to the negative input terminal of the comparator 2. The negative electrode of the battery 3 whose voltage is to be monitored is grounded, and its positive electrode is connected to the positive input terminal of the comparator 2 via a resistor 4. The positive input terminal + of the comparator 2 is connected to the semiconductor memory unit 7 via a backflow prevention diode 1 whose anode is connected to the positive input terminal + of the comparator 2.
is connected to. A MOSFET 10, which is a switching element, is interposed between the positive input terminal + of the comparator 2 and ground. Comparator 2 of MOSFET 10
The end connected to the positive input terminal + of is connected to its gate. The output terminal of the comparator 2 is connected to the monitoring signal output terminal 9. The IC card A includes the battery voltage monitoring circuit configured as described above. Next, the operation of the battery voltage monitoring circuit constructed as described above will be explained. If the IC card A is currently disconnected from an external device (not shown), no voltage will be applied to the external power supply connection terminal 6. Therefore, the voltage of battery 3 is
The voltage is applied to the semiconductor memory section 7 via the diode 1, and the data stored in the semiconductor memory section 7 is held by the voltage. In this case, depending on the voltage of battery 3, the MOSFE
Since T 10 is off and its resistance is extremely high, the power consumption by MOSFET 10 is extremely low. Next, when the IC card A is connected to an external device (not shown), the voltage of the external power supply provided in the external device is applied to the semiconductor storage section 7 via the external power supply connection terminal 6 and the Schottky barrier diode 8. give to On the other hand, the voltage of battery 3 is the positive input terminal + of comparator 2 and MOSFET
ET 10. Then, the voltage of the battery 3 turns off the MOSFET 10, so that no voltage drop occurs due to the resistor 4, and the terminal voltage of the battery 3 is applied to the positive input terminal + of the comparator 2. Further, the voltage of the reference power supply 5 is applied to the negative input terminal of the comparator 2, and the voltage of the reference power source 5 is applied to the negative input terminal of the comparator 2.
The voltage of the reference power source 5 is compared with the voltage of the reference power source 5. When the voltage of the battery 3 is higher than the voltage of the reference power source 5, the output of the comparator 2 is "H" level, and when it is lower than the voltage of the reference power source 5, the output is "H" level.
The voltage level of the supervisory signal output terminal 9 changes in the same way, and an external device (not shown) detects the change in the voltage level of the supervisory signal output terminal 9 and monitors the voltage of the battery 3. Then, when the voltage level of the monitoring signal output terminal 9 reaches the "L" level, the external device notifies the voltage drop of the battery 3 by means of a lamp, a buzzer, or the like. [0014] Here, if the external device reports a drop in the voltage of the battery 3, the battery 3 will be replaced with a new normal battery. Therefore, when the battery 3 is disconnected for replacement, a reverse voltage is applied to the diode 1 due to the voltage applied to the external power connection terminal 6, a leakage current flows through the diode 1, and the voltage at the positive input terminal + of the comparator 2 increases. However, when the battery 3 is disconnected, the voltage at the positive input terminal + of the comparator 2 temporarily decreases, turning on the MOSFET 10. Therefore, the leakage current of diode 1 is
10 and the positive input terminal of comparator 2 +
The voltage is maintained at approximately ground voltage and does not rise. Therefore, the voltage level on the output side of the comparator 2, that is, the voltage at the monitoring signal output terminal 9, is held at the "L" level, and a signal similar to that obtained when a voltage drop in the battery 3 is detected is obtained. As a result, the external device continues to notify that the battery 3 is disconnected. Therefore, it is possible to prevent the IC card A from being disconnected from the external device with the battery 3 disconnected, and there is no risk of erasing the data stored in the semiconductor storage section 7. FIG. 2 is a circuit diagram of an equivalent circuit of the battery voltage monitoring circuit when the battery 3 is disconnected.
The voltage monitoring operation when the battery 3 is disconnected will be explained using this equivalent circuit. The reverse impedance of diode 1 is Z
a, the impedance of MOSFET 10 is Zb
, the input impedance of comparator 2 is Zc, the voltage of reference power supply 5 is VR, and the voltage of external power supply terminal 6 is VCC.
shall be. Now, if battery 3 is disconnected, comparator 2
The voltage at the positive input terminal + of is the reverse impedance Za
The voltage is divided by the impedance Zb of the MOSFET 10 and the input impedance Zc of the comparator 2. Therefore, VR>(VCC・Zb・Zc)/{(Za
・Zb )+Zc ・(Za +Zb)}


...(1) 0018] The MOS is
If the impedance Zb of the FET 10 is determined, the voltage at the positive input terminal + of the comparator 2 will be lower than the voltage VR of the reference power supply 5, and when the battery 3 is disconnected, the output of the comparator 2 will be at the "L" level, It is possible to notify that the battery 3 is disconnected. In this embodiment, the MOSFET 10 is used to prevent the voltage increase at the positive input terminal + of the comparator 2 due to the leakage current of the diode 1.
The present invention is not limited to MOSFETs, and the same effect can be obtained as long as it is a switch element that can flow a current slightly larger than the leakage current. Although the positive input terminal + of the comparator 2 is set to the ground voltage by turning on the MOSFET 10, it may be set to a voltage lower than the voltage VR of the reference power supply 5. Effects of the Invention As detailed above, according to the present invention,
When the battery whose voltage is to be monitored is disconnected, the switch element becomes conductive and gives a voltage below the reference power supply voltage to the input terminal of the comparator that was supplying the battery voltage.
The output of the comparator will be the same as when the battery voltage is low, and will continue to notify that the battery is disconnected, calling attention to it. Therefore, it is possible to provide a battery voltage monitoring circuit that can prevent the voltage supply from the outside from being erroneously cut off when the battery is disconnected.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the configuration of a battery voltage monitoring circuit according to the present invention.

FIG. 2 is a circuit diagram of an equivalent circuit of the battery voltage monitoring circuit in FIG. 1;

FIG. 3 is a circuit diagram showing the configuration of a conventional battery voltage monitoring circuit.

[Explanation of symbols]

1 Diode 2 Comparator 3 Battery 5 Reference power supply 6 External power connection terminal 9 Monitoring signal output terminal 10 MOSFET

Claims (1)

[Claims] [Claim 1] Inputting the battery voltage to one input terminal of the comparator and a predetermined voltage to the other input terminal of the comparator,
A battery voltage monitoring circuit that monitors battery voltage based on the output of the comparator, further comprising a switch element interposed between one input terminal of the comparator and the voltage section below the predetermined voltage, the switch element , A battery voltage monitoring circuit characterized in that the circuit is configured to conduct when the battery is disconnected.