Voltage monitoring circuit

Abstract

To provide a technique of suppressing a wrong determination and suppressing power consumption.SOLUTION: A voltage monitoring circuit includes: a transistor which is turned off when the voltage of a battery is at least a predetermined threshold value and is turned on when the voltage of the battery is smaller than the threshold value; a first pull-up resistor for connecting the drain of the transistor to a higher potential of the battery; a second pull-up resistor in parallel to the first pull-up resistor, the second pull-up resistor connecting the drain of the transistor to the higher potential of the battery, the second pull-up resistor having a lower resistance value than that of the first pull-up resistor; and a switch arranged between the drain of the transistor and the second pull-up resistor, the switch capable of switching between the on-state in which the drain and the second pull-up resistor are connected to each other and the off-state in which the drain and the second pull-up resistor are not connected to each other. A control unit turns off the switch when the transistor is in the on-state and turns on the switch when the transistor is in the off-state.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The technology disclosed in this specification relates to a voltage monitoring circuit for monitoring the voltage of a battery.

Background Art

[0002] A power supply detection circuit is described in Patent Document 1. The power supply detection circuit described in Patent Document 1 includes a power supply means for supplying a power supply voltage, a power supply detection means for detecting the power supply voltage supplied from the power supply means, and a reset means for outputting a reset signal to the power supply means as the power supply voltage detected by the power supply detection means decreases.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the technology of Patent Document 1, it is considered that the current consumption in the power supply detection circuit increases as the power supply voltage decreases. Also, if the resistance value of the pull-up resistor is increased to reduce the current consumption, it is considered that an incorrect determination may be made that the power supply voltage has decreased even though the power supply voltage has not decreased. This specification provides a technology for suppressing current consumption and suppressing incorrect determination.

Means for Solving the Problems

[0005] The voltage monitoring circuit disclosed herein is a voltage monitoring circuit for monitoring the voltage of a battery, comprising: a transistor connected to the battery via a comparator, which is in an off state when the voltage of the battery is above a predetermined threshold and in an on state when the voltage of the battery is below the threshold; a first pull-up resistor connecting the drain or collector of the transistor to the high potential side of the battery; a second pull-up resistor connected in parallel with the first pull-up resistor to the high potential side of the battery, the second pull-up resistor having a lower resistance than the first pull-up resistor; a switch that can be switched between an on state in which the drain or collector of the transistor is connected to the second pull-up resistor and an off state in which the drain or collector is not connected to the second pull-up resistor; and a control unit that determines whether the voltage of the battery is above the threshold based on the potential of the drain or collector of the transistor, wherein the control unit turns the switch to an off state when the transistor is in an on state and turns the switch to an on state when the transistor is in an off state.

[0006] In the above configuration, the battery voltage gradually decreases as the battery is used. For example, the battery voltage decreases when an electrical device equipped with a voltage monitoring circuit is operated. According to the above configuration, when the battery voltage falls below a predetermined threshold due to battery use, the transistor turns on, and the potential of the drain or collector decreases. The control unit determines whether the battery voltage is above the threshold based on the potential of the drain or collector. Here, when the transistor turns on, current flows through the first pull-up resistor, and this current consumption further uses the battery, which may cause the battery voltage to decrease further. That is, when the transistor turns on, the battery voltage will decrease further even though the battery voltage is below the threshold. However, in the above configuration, by increasing the resistance value of the first pull-up resistor, the current flowing through the first pull-up resistor can be reduced even when the transistor turns on. That is, the current consumption can be reduced. Therefore, when the battery voltage is below the threshold, the current consumption due to the configuration for determining this can be suppressed. Note that the switch is in the off state at this time, so no current flows through the second pull-up resistor.

[0007] On the other hand, if the battery voltage is above the threshold, the transistor turns off, but even when the transistor is off, leakage current can still flow through it. Therefore, if there is no second pull-up resistor, even if the transistor is off, current may flow through the first pull-up resistor due to leakage current. Here, if the resistance value of the first pull-up resistor is increased in the absence of the second pull-up resistor, when current flows through the first pull-up resistor, the voltage drop across the first pull-up resistor may cause the potential of the transistor's drain or collector to become excessively low. In that case, even if the battery voltage is above the threshold and the transistor is off, the excessively low potential of the drain or collector may lead to the misjudgment that the battery voltage is below the threshold. However, according to the voltage monitoring circuit described above, by providing a second pull-up resistor with a lower resistance value than the first pull-up resistor and a switch, and turning the switch on when the transistor is off, the leakage current that occurs when the transistor is off can be channeled through the second pull-up resistor. In this case, the combined resistance of the first and second pull-up resistors reduces the voltage drop compared to the case of the first pull-up resistor alone. This prevents the potential of the transistor's drain or collector from becoming excessively low. As a result, it prevents the system from mistakenly determining that the battery voltage is below the threshold when the battery voltage is above the threshold and the transistor is in the off state.

[0008] Therefore, the above voltage monitoring circuit can suppress current consumption when the battery voltage is below the threshold, and can also prevent the battery voltage from being mistakenly judged as below the threshold when it is above the threshold.

[0009] The resistance value of the second pull-up resistor may be less than 1 / 10 of the resistance value of the first pull-up resistor.

[0010] This configuration allows for a further reduction in the voltage drop across the combined resistance of the first and second pull-up resistors. This prevents the potential of the transistor's drain or collector from becoming excessively low when the transistor is in the off state. [Brief explanation of the drawing]

[0011] [Figure 1] A schematic diagram showing the voltage monitoring circuit of the embodiment. [Figure 2] A flowchart of the switching process in the example. [Modes for carrying out the invention]

[0012] The voltage monitoring circuit of the embodiment will be described with reference to the drawings. Figure 1 is a schematic diagram of the voltage monitoring circuit 2 of the embodiment. The voltage monitoring circuit 2 shown in Figure 1 is a circuit for monitoring the voltage of a battery 4 (for example, a primary battery or a secondary battery). The voltage monitoring circuit 2 is installed in electrical equipment that operates using the power of the battery 4 (for example, a gas meter, water meter, flow meter, etc.). The voltage monitoring circuit 2 includes an LDO (Low Drop Out) 60, a voltage detector 20, and a microcontroller 10.

[0013] The LDO60 is connected to the battery 4 and the microcontroller 10. The LDO60 is connected to the high-potential side of the battery 4 and steps down the voltage of the battery 4 to output to the microcontroller 10.

[0014] The voltage detector 20 is connected to battery 4 in parallel with the LDO 60. The voltage detector 20 is sometimes called a power supply monitoring IC or a reset IC. The voltage detector 20 mainly consists of a comparator 22 and a transistor 30. The voltage detector 20 has a so-called N-channel open-drain output configuration.

[0015] Comparator 22 has its inverting input terminal 26 (V-) connected to the high potential side of battery 4, and its non-inverting input terminal 24 (V+) connected to the reference potential side. Comparator 22 also has an output terminal 28 (V OUT The comparator 22 is connected to the input terminal (gate 32) of the transistor 30. The comparator 22 outputs a Low signal when the voltage of the battery 4 is above a predetermined threshold Th, and outputs a Hi signal when the voltage of the battery 4 is below the predetermined threshold Th.

[0016] Transistor 30 is, for example, an N-channel enhancement type MOSFET (Metal Oxide Semiconductor Field Effect Transistor). Transistor 30 has its gate 32 connected to the output terminal 28 of comparator 22, its source 34 connected to ground, and its drain 36 connected to the microcontroller 10 as an external output terminal. Drain 36 is also connected to the first pull-up resistor 40 and the second pull-up resistor 42, which will be described later. Transistor 30 is in the off state when comparator 22 outputs a low signal and is in the on state when comparator 22 outputs a high signal. In other words, transistor 30 is in the off state when the voltage of battery 4 is above a predetermined threshold Th and is in the on state when the voltage of battery 4 is below the predetermined threshold Th.

[0017] The first pull-up resistor 40 is connected to the drain 36 of transistor 30 and the output side of LDO 60. The first pull-up resistor 40 is connected to the high-potential side of battery 4 via LDO 60. Therefore, the first pull-up resistor 40 pulls up the drain 36 of transistor 30 by connecting it to the high-potential side of battery 4.

[0018] The microcontroller 10 is connected to the LDO 60 and the voltage detector 20. The microcontroller 10 operates on power supplied from the battery 4 via the LDO 60. The microcontroller 10 may also control the operation of an electrical device (not shown) equipped with a voltage monitoring circuit 2. The microcontroller 10 includes a second pull-up resistor 42, a switch 44, and a control unit 50.

[0019] The second pull-up resistor 42 is connected in parallel with the first pull-up resistor 40 to the drain 36 of the transistor 30 and the output side of the LDO 60. The second pull-up resistor 42 is connected to the high-potential side of the battery 4 via the LDO 60. Therefore, the second pull-up resistor 42 pulls up the drain 36 of the transistor 30 to the high-potential side of the battery 4. The resistance value of the second pull-up resistor 42 is set to a lower value than the resistance value of the first pull-up resistor 40. For example, the resistance value of the second pull-up resistor 42 is less than 1 / 10, less than 1 / 100, or less than 1 / 1000 of the resistance value of the first pull-up resistor 40.

[0020] The switch 44 is positioned between the second pull-up resistor 42 and the drain 36 of the transistor 30. The switch 44 is configured to switch between an ON state, where the drain 36 of the transistor 30 is connected to the second pull-up resistor 42, and an OFF state, where the drain 36 of the transistor 30 is not connected to the second pull-up resistor 42. The second pull-up resistor 42 and the switch 44 may be composed of, for example, P-channel FETs. The embodiment shown in Figure 1 is shown as an equivalent circuit.

[0021] The control unit 50 monitors the voltage of the battery 4 based on the signal input from the voltage detector 20. Specifically, when the voltage of the battery 4 is equal to or higher than the threshold value, a Low signal is output from the comparator 22, and the transistor 30 is turned off, so the potential of the drain 36 of the transistor 30 becomes high. On the other hand, when the voltage of the battery 4 is lower than the threshold value, a Hi signal is output from the comparator 22, and the transistor 30 is turned on, so the potential of the drain 36 of the transistor 30 becomes low. The control unit 50 determines whether the voltage of the battery 4 is equal to or higher than the threshold value based on the potential of the drain 36 of the transistor 30. Also, when the control unit 50 determines that the voltage of the battery 4 is lower than the threshold value, it may output information indicating that the voltage of the battery 4 is lower than the threshold value. For example, the control unit 50 may display a screen indicating that the voltage of the battery 4 is lower than the threshold value on a display unit (not shown) such as a monitor.

[0022] Also, the control unit 50 can switch the switch 44 between the on state and the off state. The control unit 50 turns off the switch 44 when the transistor �0 is in the on state (i.e., when the voltage of the battery 4 is lower than the threshold value), and turns on the switch 44 when the transistor 30 is in the off state (i.e., when the voltage of the battery 4 is equal to or higher than the threshold value). When the transistor 30 is in the on state, the potential of the drain 36 of the transistor 30 becomes relatively low. When the transistor 30 is in the off state, the potential of the drain 36 of the transistor 30 becomes relatively high. The control unit 50 may switch the on / off state of the switch 44 based on the potential of the drain 36 of the transistor 30.

[0023] (Switching process; Figure 2) FIG. 2 is a flowchart of a switching process for switching the on / off state of switch 44. The switching process shown in FIG. 2 is started, for example, when the power supply of an electrical device equipped with voltage monitoring circuit 2 is turned on. In S2 of the switching process, control unit 50 turns switch 44 on. Subsequently, if transistor 30 is in the on state (YES in S4), control unit 50 turns switch 44 off (S6), and if transistor 30 is in the off state (NO in S4), control unit 50 turns switch 44 on (S2). Control unit 50 switches the on / off state of switch 44 according to the state of transistor 30. The switching process of the embodiment ends, for example, when the power supply of the electrical device equipped with voltage monitoring circuit 2 is turned off.

[0024] The voltage monitoring circuit 2 of the embodiment has been described above. In the above configuration, the voltage of battery 4 gradually decreases as battery 4 is used. For example, the voltage of battery 4 decreases as the electrical device equipped with voltage monitoring circuit 2 operates. According to the above configuration, when the voltage of battery 4 becomes less than a predetermined threshold Th due to the use of battery 4, transistor 30 turns on and the potential of drain 36 decreases. Control unit 50 determines whether the voltage of battery 4 is greater than or equal to threshold Th based on the potential of drain 36. Here, when transistor 30 turns on, current flows through first pull-up resistor 40, so battery 4 will be further used due to this consumption current, and the voltage of battery 4 may further decrease. That is, when transistor 30 turns on, even though the voltage of battery 4 is less than threshold Th, the voltage of battery 4 will further decrease. However, in the above configuration, by increasing the resistance value of first pull-up resistor 40, even when transistor 30 is in the on state, the current flowing through first pull-up resistor 40 can be reduced. That is, the consumption current can be reduced. Therefore, when the voltage of battery 4 is less than the threshold, the consumption current of the configuration for determining this can be suppressed. At this time, switch 44 is in the off state, so no current is flowing through second pull-up resistor 42.

[0025] On the other hand, if the voltage of battery 4 is above the threshold, transistor 30 will be in the off state. However, even when transistor 30 is in the off state, leakage current may flow through it. Therefore, if the second pull-up resistor 42 were not present, it is conceivable that current would flow through the first pull-up resistor 40 due to leakage current, even when transistor 30 is in the off state. Here, if the resistance value of the first pull-up resistor 40 were increased in the absence of the second pull-up resistor 42, it is conceivable that when current flows through the first pull-up resistor 40, the voltage drop across the first pull-up resistor 40 would cause the potential of the drain 36 of transistor 30 to become excessively low. In that case, even if the voltage of battery 4 is above the threshold Th and transistor 30 is in the off state, the excessively low potential of the drain 36 could lead to a misjudgment that the voltage of battery 4 is below the threshold Th. However, according to the voltage monitoring circuit 2 described above, a second pull-up resistor 42 with a lower resistance value than the first pull-up resistor 40 and a switch 44 are provided. By turning on the switch 44 when the transistor 30 is in the off state, the leakage current that occurs when the transistor 30 is in the off state can be passed through the second pull-up resistor 42. In this case, the combined resistance of the first pull-up resistor 40 and the second pull-up resistor 42 reduces the voltage drop compared to the case of the first pull-up resistor 40 alone. This prevents the potential of the drain 36 of the transistor 30 from becoming excessively low. As a result, when the voltage of the battery 4 is above the threshold Th and the transistor 30 is in the off state, it is possible to prevent the circuit from mistakenly determining that the voltage of the battery 4 is below the threshold Th.

[0026] Based on the above, the voltage monitoring circuit 2 can suppress the current consumption when the voltage of battery 4 is below the threshold Th, and can also prevent the circuit from mistakenly determining that the voltage of battery 4 is below the threshold when its voltage is above the threshold Th. Furthermore, since the current consumption when the voltage of battery 4 is below the threshold Th can be suppressed, the period from when the voltage of battery 4 falls below the threshold Th until battery 4 becomes completely unusable (i.e., the lifespan of battery 4 after its voltage falls below the threshold Th) can be extended. In addition, the voltage monitoring circuit 2 can continuously monitor the voltage of battery 4.

[0027] Furthermore, if the resistance value of the second pull-up resistor 42 is less than 1 / 10 of the resistance value of the first pull-up resistor 40, the voltage drop due to the combined resistance of the first pull-up resistor 40 and the second pull-up resistor 42 can be further reduced. This prevents the potential of the drain 36 of the transistor 30 from becoming excessively low. The lower the resistance value of the second pull-up resistor 42, the more effective this is.

[0028] (modified version) In the above embodiment, a MOSFET was used as transistor 30, but the configuration is not limited to this. In a modified example, an IGBT (Insulated Gate Bipolar Transistor) may be used as transistor 30. In this case, the source 34 and drain 36 of transistor 30 may be read as emitter and collector, respectively. Also in this case, the voltage detector 20 has a so-called N-channel open collector output configuration.

[0029] Although specific examples of the present invention have been described in detail above, these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples illustrated above. The technical elements described in this specification or drawings exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technologies illustrated in this specification or drawings can achieve multiple objectives simultaneously, and achieving even one of these objectives itself constitutes technical usefulness. [Explanation of Symbols]

[0030] 2: Voltage monitoring circuit, 4: Battery, 10: Microcontroller, 20: Voltage detector, 22: Comparator, 30: Transistor, 32: Gate, 34: Source, 36: Drain, 40: First pull-up resistor, 42: Second pull-up resistor, 44: Switch, 50: Control unit

Claims

1. A voltage monitoring circuit that monitors the voltage of a battery, A transistor connected to the battery via a comparator, which is turned off when the battery voltage is above a predetermined threshold and turned on when the battery voltage is below the threshold, A first pull-up resistor connects the drain or collector of the transistor to the high-potential side of the battery, A second pull-up resistor, connected in parallel with the first pull-up resistor, connects the drain or collector of the transistor to the high-potential side of the battery, wherein the second pull-up resistor has a lower resistance value than the first pull-up resistor, A switch that can switch between an ON state in which the drain or collector of the transistor is connected to the second pull-up resistor and an OFF state in which the drain or collector of the transistor is not connected to the second pull-up resistor, The system includes a control unit that determines whether the voltage of the battery is equal to or greater than the threshold based on the potential of the drain or collector of the transistor, The control unit is a voltage monitoring circuit that turns the switch off when the transistor is on, and turns the switch on when the transistor is off.

2. The voltage monitoring circuit according to claim 1, wherein the resistance value of the second pull-up resistor is less than 1 / 10 of the resistance value of the first pull-up resistor.

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