JP5488428B2 - Power backup circuit - Google Patents

Description

  The present invention provides a power supply circuit that repeatedly turns on and off power supply for a power supply target circuit including an electronic element that requires power backup, and the power supply by the power supply circuit is turned off. The present invention relates to a power backup circuit that performs power backup for an electronic element.

  An example of a conventional power backup circuit is disclosed in Patent Document 1. The power supply backup circuit described in Patent Document 1 receives a power supply voltage from a power supply and a backup voltage from a backup capacitor, and is a first voltage value or a second voltage value lower than the first voltage value. A regulator that outputs a constant voltage is provided. The output voltage of this regulator is given to an RTC (real time clock) circuit as a backup voltage.

  When the voltage value of the power source is larger than the predetermined voltage value, the output voltage of the regulator is set as the first voltage value. When the voltage value is lower than the predetermined voltage value, the output voltage of the regulator is switched to the second voltage value. Thus, when the power supply is shut down and the voltage value of the power supply falls below a predetermined value, the output voltage of the regulator is lowered to reduce the current consumption, and the RTC circuit is powered by the backup capacitor. The backup can be performed for a long time.

JP 2006-204055 A

  In the power supply backup circuit of Patent Document 1, it is assumed that the state where the power supply is cut off continues and the charging voltage of the backup capacitor is reduced to around 0V. In this case, immediately after the power supply is turned on, the power supply voltage is mainly used for charging the backup capacitor, so that it takes time until the power supply voltage rises. This power supply voltage is supplied not only to a regulator that supplies power to the RTC circuit, but also to a control circuit that reads the time elapsed by the RTC circuit and performs control processing.

  Here, as described above, even if the power is turned on, if it takes time for the power supply voltage to rise, the power supply voltage supplied to the control circuit reaches the operable voltage range of the control circuit. The time until will be longer. For this reason, even if the power is turned on, there is a problem that the system cannot immediately start the operation. In addition, as a system, there is a problem that the power consumption of the system increases as much as there is a waiting time for starting the operation after the power is turned on.

  In order to shorten the time required for charging the backup capacitor in order to accelerate the rise of the power supply voltage, it is difficult to secure the necessary backup time if the capacitance of the backup capacitor is reduced. Problems arise.

  The present invention has been made in view of the above points, and can reduce the time from the start of power supply to the start of operation in the power supply target circuit while ensuring the necessary backup time. An object is to provide a possible power backup circuit.

In order to achieve the above object, the power backup circuit according to claim 1 comprises:
For a power supply target circuit including an electronic element that requires power backup, a power supply circuit that periodically turns on and off the power supply is provided, and when the power supply by the power supply circuit is turned off, the electronic element For power backup,
A capacitor for providing a backup voltage to the electronic device when the power supply is turned off by the power supply circuit;
Switch means for switching between a disconnected state in which the capacitor is electrically disconnected from the power supply circuit and the electronic element and a connection state in which the capacitor is electrically connected to the power supply circuit and the electronic element;
When the backup voltage by the capacitor drops below a predetermined threshold, a disconnecting means for switching the switch means,
And connecting means for bringing the switch means into a connected state after a predetermined time has elapsed from when the power supply is turned on when the power supply by the power supply circuit is turned on when the switch means is in the disconnected state. It is characterized by that.

  According to the above-described configuration, when the power supply by the power supply circuit is turned off and the backup voltage by the capacitor falls below a predetermined threshold value, the capacitor is separated from the power supply circuit and the electronic element. . For this reason, when power supply is started by the power supply circuit, the power supply voltage is not used to charge the capacitor having a reduced charge amount, and can be immediately started up. Therefore, the power supply target circuit can start operation quickly upon receiving power supply from the power supply circuit.

  Then, when a predetermined time has elapsed from the time when the power supply is turned on, charging of the capacitor is started by switching the switch means to the connected state. At this time, the power supply voltage has risen to a steady voltage, and the capacitor can be charged without substantially affecting the operation of the power supply target circuit.

  Therefore, in the power backup circuit according to the first aspect, the time from the start of power supply by the power supply circuit to the start of the operation of the power supply target circuit while ensuring the necessary backup time without reducing the capacitance of the capacitor. Can be shortened.

  According to a second aspect of the present invention, the capacitor is connected between a power supply line extending from the power supply circuit to the electronic element and the ground, and the switch means is inserted in a connection line connecting the capacitor and the power supply line. A resistor may be connected in series with the capacitor between the switch means and the power supply line. As a result, when the switch means is switched from the disconnected state to the connected state, the inrush current to the capacitor can be suppressed by the resistance, and an instantaneous decrease in the power supply voltage can be suppressed. In addition, since the capacitor only supplies a backup voltage to an electronic element or the like with a small current consumption, the influence of the voltage drop due to the resistance is slight.

  As described in claim 3, an auxiliary capacitor having a smaller capacity than the capacitor is connected to the power supply line so that the power supply line is in parallel with a series circuit including a resistor, a switching means, and a capacitor. Also good. Since the auxiliary capacitor plays a role of assisting charging of the capacitor when the switch means is switched from the disconnected state to the connected state, it is possible to further suppress a decrease in power supply voltage. The auxiliary capacitor also plays a role of continuously providing a backup voltage to the electronic element when the switch means is switched from the connected state to the disconnected state. Since the auxiliary capacitor has a lower capacity than the capacitor, the influence on the rise time of the voltage after the power supply is started by the power supply circuit can be suppressed small.

  The power backup circuit according to claim 3 uses an auxiliary capacitor together with a resistor in order to suppress a decrease in power supply voltage when the switch means is switched from the disconnected state to the connected state. In addition, only an auxiliary capacitor may be used alone.

  6. The voltage monitoring circuit according to claim 5, wherein the disconnecting means and the connecting means monitor the backup voltage of the capacitor and the power supply voltage by the power supply circuit, and switch the switch means between the disconnected state and the connected state. Can be configured. As described above, the voltage monitoring circuit has two roles of the disconnecting unit and the connecting unit, so that the configuration can be simplified. However, the disconnecting means and the connecting means may be configured by separate circuits.

  The power supply monitoring circuit described above is connected to a power supply line extending from the power supply circuit to the electronic element as described in claim 6, and can receive power supply by a capacitor after the power supply by the power supply circuit is turned off. preferable. This is because the power supply monitoring circuit needs to continue operation even after the power supply by the power supply circuit is turned off to monitor the backup voltage of the capacitor.

  An example of the electronic element is a volatile memory. This is because, as is well known, the volatile memory needs to be continuously supplied with power in order to hold the stored information.

1 is a configuration diagram showing the overall configuration of a power supply 10, power supply target circuits 12, 14 and a power backup circuit 20. FIG. 3 is a block diagram showing a configuration of a voltage monitoring circuit 22 in the power backup circuit 20. FIG. It is a wave form diagram for demonstrating operation | movement of the power supply backup circuit 20 by this embodiment. It is a block diagram which shows the structure of a comparative example. It is a wave form diagram which shows operation | movement of a comparative example.

  Hereinafter, a power backup circuit according to an embodiment of the present invention will be described with reference to the drawings.

  The power backup circuit according to the present embodiment is provided with a power supply circuit that periodically turns on and off the power supply target circuit including electronic elements that require power backup, and the power supply by the power supply circuit is provided. When it is turned off, the electronic device is backed up.

  An example of such a power supply target circuit is a receiver circuit of a receiver in an electronic key system used in a vehicle.

  The electronic key system switches and controls the lock / unlock status of each door based on the result of collating the ID code by bidirectional communication between the portable electronic key and the in-vehicle device, and the electronic key holder holds the vehicle When in the room, the steering lock can be released or the engine can be started.

  An example of the operation of this electronic key system will be briefly described. When the vehicle engine is stopped and each door is locked and parked, a request signal is transmitted at predetermined intervals from the on-vehicle transmitter outside the vehicle compartment. Then, a detection area is set in the vicinity of the vehicle. When the holder of the electronic key approaches the vehicle and enters the detection area, the electronic key returns a response signal in response to the request signal. When the receiver of the in-vehicle device receives the response signal, it further communicates with the electronic key, receives the response signal including the ID code of the electronic key, and acquires the ID code from the electronic key.

  When the acquired ID code matches the registered ID code registered in advance, the in-vehicle device gives a control signal to the door lock control device on the vehicle side so that each door is in the unlock standby state. In this state, when the holder of the electronic key touches the door handle, the door lock control device detects it with a touch sensor or the like and unlocks the door.

  Further, when the holder of the electronic key gets off after stopping the engine of the vehicle, the detection area is set inside and outside the vehicle by transmitting a request signal from the in-vehicle transmitter and the out-of-vehicle transmitter. At this time, when it is confirmed that the holder of the electronic key is outside the vehicle based on the request signal returned from the electronic key, the door lock control device operates, for example, a door lock switch provided near the door handle. Depending on the, lock the door.

  As described above, such an electronic key system operates when the vehicle engine is stopped, and is required to reduce power consumption as much as possible. Therefore, the in-vehicle device intermittently transmits a request signal from the transmitter, and turns on the power supply to the receiver (reception circuit) in synchronization with the transmission of the request signal. Then, the vehicle-mounted device turns off the power supply to the receiver (reception circuit) when the scheduled time for returning the response signal from the electronic key in the detection area has elapsed.

  Then, when a signal is received, the receiving circuit of the receiver executes digital filter processing for removing the noise if the signal includes a noise component. In other words, since the frequency band of the noise component changes according to the environment where the vehicle is placed, the digital filter is used so as to cope with the change.

  In the digital filter processing, a noise component included in the received signal is determined, and a signal pass band or the like by the digital filter is adjusted so that the noise component can be removed. The adjustment data is stored in a volatile memory. Then, when the power supply to the receiving circuit is turned off, and then the receiving circuit is turned on, the adjustment data stored in the volatile memory is read and the digital filter adjusted by the adjustment data is used. Execute digital filter processing. Therefore, once the adjustment data of the digital filter is calculated and stored in the volatile memory, the noise components are removed in the receiving circuit thereafter using the adjustment data stored in the volatile memory. It is possible to obtain a processed signal.

  Thus, the receiver circuit of the receiver in the electronic key system has a volatile memory that needs to be backed up while the power supply by the power supply circuit is periodically turned on and off and the power supply is turned off. Therefore, the power supply backup circuit according to the present embodiment is suitable for application.

  However, the power backup circuit according to the present embodiment is not limited to the application target only to the reception circuit of the receiver of the electronic key system described above. For the power supply target circuit including an electronic element that requires power backup. The present invention can be applied as long as the condition that a power supply circuit that periodically turns on and off the power supply is provided is satisfied.

  Next, the configuration of the power backup circuit according to the present embodiment will be described. FIG. 1 is a configuration diagram showing the overall configuration of the power supply 10, the power supply target circuits 12 and 14, and the power supply backup circuit 20.

  In FIG. 1, a microcomputer 12 forms a main part of a receiving circuit that is a power supply target circuit, and performs the above-described digital filter processing on a received signal. A volatile memory 14 is connected to the microcomputer 12, and digital filter adjustment data calculated by the microcomputer 12 is stored in the volatile memory 14. The adjustment data stored in the volatile memory 14 is read by the microcomputer 12.

  The microcomputer 12 and the volatile memory 14 are supplied with a power supply voltage Vcc from an intermittent operation power supply 10 that intermittently supplies power. The intermittent operation power supply 10 receives a power control signal from a control device of an in-vehicle device (not shown), and periodically repeats on / off of power supply according to the power control signal. As described above, the intermittent operation power supply 10 is turned on in synchronization with the transmission of the request signal from the transmitter of the in-vehicle device by the power control signal so that the receiving circuit operates for a predetermined period. Be controlled.

  A power supply backup circuit 20 according to the present embodiment is connected to a power supply line 16 extending from the intermittent operation power supply 10 to the volatile memory 14. The power backup circuit 20 includes a backflow prevention diode Di, a backup capacitor C1, a switch SW, a resistor R1, an auxiliary capacitor C2, and a voltage monitoring circuit 22.

  The backflow prevention diode Di is for preventing a current from flowing back from the backup capacitor C1 and the auxiliary capacitor C2, and is inserted in the power supply line 16 extending from the intermittent operation power supply 10 to the volatile memory 14. A power supply line from the intermittent operation power supply 10 to the microcomputer 12 branches from a power supply line 16 between the intermittent operation power supply 10 and the backflow prevention diode Di. Therefore, no current flows into the microcomputer 10 from the backup capacitor C1 or the auxiliary capacitor C2.

  One end of a series circuit including a backup capacitor C1, a switch SW, and a resistor R1 is connected to the power supply line 16 between the backflow prevention diode Di and the volatile memory. The other end of the series circuit is grounded.

  The backup capacitor C1 is for providing a backup voltage to the volatile memory 14 when the power supply by the intermittent operation power supply 10 is turned off. The switch SW is controlled by a voltage monitoring circuit 22 which will be described later, and the backup capacitor C1 is electrically disconnected from the power supply line 16 between the intermittent operation power supply 10 and the volatile memory 14, and the power supply The connection state electrically connected to the line 16 is switched. The resistor R1 suppresses the inrush current from the intermittent operation power supply 10 to the backup capacitor C1 when the switch SW is switched from the disconnected state to the connected state, and instantaneously supplies the power supply voltage Vcc supplied by the intermittent operation power supply 10. This is to suppress the decrease.

  An auxiliary capacitor C2 is connected to the power supply line 16 in parallel with a series circuit including a backup capacitor C1, a switch SW, and a resistor R1. The auxiliary capacitor C2 has a lower capacity than the backup capacitor C1, but has two roles. The first role is to assist the charging of the backup capacitor C1 when the switch SW is switched from the disconnected state to the connected state. Thereby, the fall of the power supply voltage Vcc which the intermittent operation power supply 10 supplies can further be suppressed. The second role is to continuously provide a backup voltage to the volatile memory 14 when the switch SW is switched from the connected state to the disconnected state. When the switch SW is switched to the disconnected state, the backup capacitor C1 can no longer provide the backup voltage to the volatile memory 14. However, by providing the auxiliary capacitor C2, the supply of the backup voltage to the volatile memory 14 can be maintained for a while.

  The voltage monitoring circuit 22 receives the power supply voltage from the intermittent operation power supply 10 from the upstream side of the backflow prevention diode Di, and receives the backup voltage from the backup capacitor C1 from the downstream side of the backflow prevention diode Di. Based on these input voltages, the switch SW is switched between the disconnected state and the connected state.

  The configuration of the voltage monitoring circuit 22 will be described in detail with reference to the block configuration diagram of FIG.

  As illustrated in FIG. 2, the voltage monitoring circuit 22 includes a power supply voltage input unit 24, a capacitor voltage input unit 26, and a voltage determination unit 28. The power supply voltage input unit 24 receives the input of the power supply voltage Vcc of the intermittent operation power supply 14 described above, and the capacitor voltage input unit 26 receives the input of the backup voltage of the backup capacitor. Based on the input received by the power supply voltage input unit 24, the voltage determination unit 28 determines whether the power supply is turned on according to whether the power supply voltage is equal to or higher than the first threshold value. Further, the voltage determination unit 28 determines whether the backup voltage is equal to or higher than the second threshold based on the input received by the capacitor voltage input unit 26.

  The voltage monitoring circuit 22 further includes a switch control unit 30, a switch state storage unit 32, and a timer 34.

  The switch state storage unit 32 stores whether the switch SW is in a disconnected state or a connected state. Note that the initial storage of the switch state storage unit 32 is in a disconnected state. When the switch control unit 30 switches the state of the switch SW, the storage content of the switch state storage unit 32 is updated.

  The timer 34 starts time measurement when the switch control unit 30 is instructed to perform time measurement, and outputs the result to the switch control unit 30 when the time measurement reaches a predetermined time. Note that the switch control unit 30 instructs the timer 34 to perform timing when the switch state storage unit 32 stores the disconnected state and the power supply by the intermittent operation power supply 10 is turned on. As a result, the switch control unit 30 can know when a predetermined time has elapsed since the power supply was turned on by the intermittent operation power supply 10.

  The switch control unit 30 outputs a switch control signal to the switch SW based on the determination result in the voltage determination unit 28 described above, the state of the switch SW stored in the switch state storage unit 32, and the time measured by the timer 34. The switch SW is controlled to be switched between a disconnected state and a connected state.

  Hereinafter, the operation of the voltage monitoring circuit 22 will be described with reference to the waveform diagram of FIG. In describing the operation of the voltage monitoring circuit 22, the configuration is appropriately shown in FIG. 4, and the operation waveform is compared with the comparative example shown in FIG. The comparative example shown in FIGS. 4 and 5 is obtained by removing the configurations of the switch SW, the auxiliary capacitor C2, and the voltage monitoring circuit 22 from the configuration of the power supply backup circuit 20 according to the present embodiment. That is, in the comparative example, the backup capacitor C1 is always connected to the intermittent operation power supply 10 and the volatile memory 14 as in the conventional case.

  In the electronic key system to which the power backup circuit 20 according to the present embodiment is applied, when an occupant gets on the vehicle and the engine is started, it is no longer necessary to detect the approach of the occupant holding the electronic key. The electronic key system stops operating. For this reason, intermittent power supply by the intermittent operation power supply 10 is not performed, and the backup voltage by the backup capacitor C1 gradually decreases. When the backup voltage drops below the second threshold, the voltage monitoring circuit 22 switches the switch SW to the disconnected state. Even after the switch SW is disconnected, the backup voltage of the backup capacitor C1 approaches 0V with time.

  In such a state, when the vehicle is stopped and the engine is turned off, as described above, the position of the electronic key needs to be confirmed inside and outside the vehicle interior, so that the operation of the electronic key system is resumed. The waveform diagram of FIG. 3 shows the operation waveform of each part when such an electronic key system resumes operation.

  Here, as in the comparative example shown in FIG. 4, when the backup capacitor C1 is always connected to the power supply line 16, when the electronic key system resumes operation, the intermittent operation power supply 10 drops to around 0V. The power supply voltage Vcc must be supplied to the microcomputer 12 and the volatile memory 14 which are power supply target circuits while charging the backup capacitor C1. For this reason, it takes time for the power supply voltage Vcc to rise, and in particular, since the microcomputer 12 has a high operable voltage range, the waiting time until the power supply target circuit starts operating becomes long.

  On the other hand, in the present embodiment, as shown in FIG. 3, the switch SW is in a disconnected state when the intermittent operation power supply 10 starts to supply power in accordance with the power supply control signal. For this reason, when the intermittent operation power supply 10 starts to supply power, the backup capacitor C1 is electrically disconnected from the power supply line 16, and it is not necessary to charge the backup capacitor C1. The power supply voltage Vcc supplied to 14 immediately rises as shown in FIG. Therefore, the microcomputer 12 and the volatile memory 14 which are power supply target circuits can start operation promptly upon receiving the supply voltage Vcc from the intermittent operation power supply 10.

  In this embodiment, the auxiliary capacitor C2 is connected to the power supply line 16. However, since the auxiliary capacitor C2 has a lower capacity than the backup capacitor C1, the intermittent operation power supply 10 starts to supply power. The influence on the rise time of the power supply voltage Vcc after being applied is suppressed to a small level.

  In this embodiment, when the power supply voltage Vcc rises to the first threshold value or more, the switch control unit 30 of the voltage monitoring circuit 22 considers that the supply of the power supply voltage Vcc has started, and sets the timer 34 to the predetermined time. Instruct the timing of The switch control unit 30 instructs the timer 34 to measure a predetermined time only when the switch state storage unit 32 stores that the switch SW is in the disconnected state. Then, at the timing when the predetermined time is counted by the timer 34, the switch control unit 30 outputs a switch control signal and switches the switch SW from the disconnected state to the connected state.

  When a predetermined time elapses from the time when power supply is started by the intermittent operation power supply 10, the power supply voltage Vcc rises to a steady voltage as shown in FIG. Furthermore, the receiving circuit composed of the microcomputer 12 and the like is almost ending its receiving operation. For this reason, when a predetermined time elapses from the start of power supply, the backup capacitor C1 can be charged without substantially affecting the operation of the receiving circuit that is the power supply target circuit.

  Therefore, according to the power supply backup circuit 20 of the present embodiment, the power supply target circuit does not decrease the capacity of the backup capacitor C1 and secures a necessary backup time, while the power supply by the intermittent operation power supply 10 is started. It is possible to shorten the time until a certain receiving circuit starts operating.

  In FIG. 3, the rise of the power supply control signal is shown as a measurement start point for a predetermined time, but in actuality, when the power supply voltage Vcc rises to the first threshold voltage or higher in the voltage monitoring circuit 22. From this time, the timer 34 starts timing.

  Further, even after a predetermined time has elapsed since the start of power supply, when the switch SW is switched to the connected state and charging of the backup capacitor C1 is started, the voltage of the backup capacitor C1 is reduced to around 0V. A relatively large inrush current flows, and the power supply voltage Vcc may drop instantaneously.

  When such a momentary drop in the power supply voltage Vcc causes a problem in the operation of the power supply target circuit or the like, it is preferable to connect the resistor R1 in series with the backup capacitor C1 as described above. This series-connected resistor R1 can suppress an inrush current to the backup capacitor C1 when the switch SW is switched from the disconnected state to the connected state, and suppresses an instantaneous decrease in the power supply voltage Vcc. be able to. The backup capacitor C1 does not supply a backup voltage to the microcomputer or the like, but only supplies it to the volatile memory 14 and the voltage monitoring circuit 22. For this reason, the consumption current flowing from the backup capacitor C1 is very small. Therefore, when supplying the backup voltage, the influence of the voltage drop due to the resistor R1 remains slight.

  Further, in the power backup circuit 20 of the present embodiment, an auxiliary capacitor C2 is connected to the power supply line 16, and this auxiliary capacitor C2 is a backup capacitor when the switch SW is switched from the disconnected state to the connected state. Assist in charging C1. Thereby, the fall of power supply voltage Vcc can be suppressed further.

  As shown in FIG. 3, even when the intermittent operation power supply 10 repeats turning on and off of the power supply, when the backup voltage of the backup capacitor C1 falls below the second threshold value, the switch SW is in the disconnected state. Can be switched to. However, in this case, since the next power supply is performed in a state where the voltage of the backup capacitor C1 is substantially maintained at the second threshold level, the power supply of the backup capacitor C1 is turned off. In the meantime, the electric charge that does not decrease to the second threshold value is charged. As a result, the volatile memory 14 can maintain the storage of the adjustment data even while the power supply by the intermittent operation power supply 10 is turned off.

  The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

  For example, in the above-described embodiment, when the switch SW is switched from the disconnected state to the connected state, the resistor R1 and the auxiliary capacitor C2 are used to suppress a momentary decrease in the power supply voltage Vcc. However, the resistor R1 and the auxiliary capacitor C2 can be omitted if the instantaneous drop in the power supply voltage Vcc does not matter. Further, even when it is necessary to suppress an instantaneous drop in the power supply voltage Vcc, it is not always necessary to provide both the resistor R1 and the auxiliary capacitor C2, and only one of them may be provided.

  In the above-described embodiment, the voltage monitoring circuit 22 monitors both the power supply voltage of the intermittent operation power supply 10 and the backup voltage by the backup capacitor C1, and switches the switch SW to the connected state or to the disconnected state. It was something to switch. However, monitoring of the power supply voltage and switching to the connected state, and monitoring of the backup voltage and switching to the disconnected state may be performed by independent separate circuits.

  In the above-described embodiment, an example of a volatile memory has been described as an electronic element that needs to be backed up. However, other electronic elements, for example, Patent Document 1 are described depending on the configuration of a power supply target circuit. In this way, the RTC circuit can also be targeted.

  Furthermore, in the above-described embodiment, the backflow prevention diode Di is used to prevent the backflow of current from the backup capacitor C1, but the backflow can be prevented or the backflow current can be a very small backflow prevention element. For example, it can be used in place of the backflow prevention diode Di.

10 intermittent power supply 12 microcomputer 14 volatile memory 16 power supply line 20 power backup circuit 22 voltage monitoring circuit R1 resistor C1 backup capacitor C2 auxiliary capacitor

Claims (7)

A power supply circuit that periodically turns on and off the power supply is provided for a power supply target circuit including an electronic element that requires power backup, and when the power supply by the power supply circuit is turned off, the electronic element A power backup circuit for performing power backup for
A capacitor that provides a backup voltage to the electronic device when the power supply is turned off by the power supply circuit;
Switch means for switching between a disconnected state in which the capacitor is electrically disconnected from the power supply circuit and the electronic element and a connection state in which the capacitor is electrically connected to the power supply circuit and the electronic element;
When the backup voltage by the capacitor drops below a predetermined threshold, the switching means for setting the switch means to the disconnected state,
Connection means for bringing the switch means into a connected state after elapse of a predetermined time from when the power supply is turned on when the power supply by the power supply circuit is turned on when the switch means is in a disconnected state; A power supply backup circuit comprising: The capacitor is connected between a power supply line extending from the power supply circuit to the electronic element and ground, and the switch means is inserted in a connection line connecting the capacitor and the power supply line. ,
2. The power backup circuit according to claim 1, wherein a resistor is connected in series with the capacitor between the switch means and the power supply line.   An auxiliary capacitor having a smaller capacity than the capacitor is connected to the power supply line so that the power supply line is in parallel with a series circuit including the resistor, the switch means, and the capacitor. The power supply backup circuit according to claim 2. The capacitor is connected between a power supply line extending from the power supply circuit to the electronic element and a ground,
2. The power backup circuit according to claim 1, wherein an auxiliary capacitor having a smaller capacity than the capacitor is connected to the power supply line in parallel with the capacitor.   The disconnecting means and the connecting means are configured by a voltage monitoring circuit that monitors the backup voltage of the capacitor and the power supply voltage by the power supply circuit and switches the switch means to either the disconnected state or the connected state. The power supply backup circuit according to claim 1, wherein the power supply backup circuit is a power backup circuit.   6. The power supply monitoring circuit is connected to a power supply line extending from the power supply circuit to the electronic element, and is powered by the capacitor after power supply by the power supply circuit is turned off. Power supply backup circuit described in 1.   The power supply backup circuit according to claim 1, wherein the electronic element is a volatile memory.

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