JPH11338556A - Power circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely switch between normal operation and backup operation and to surely execute the backup operation, in a power circuit equipped with a backup function. SOLUTION: The power circuit equipped with a VCC circuit 1 which supplies electric power to a microcomputer 10 in the ignition switch ON state and a VDD circuit 2 which supplies electric power to the microcomputer 10 in the ignition switch OFF state is equipped with a switch BUSW for power source switching which is connected to the power line between the VCC circuit 1 and microcomputer 10 and connects and disconnects the power line, a voltage drop detecting circuit 3 which monitors the output voltage of the VCC circuit 1 and turns off the switch BUSW for power source switching when the voltage of the VCC circuit 1 drops and turns ON when the voltage rises, and a voltage control means which raises the voltage of the VCC circuit 2 when the voltage of VCC circuit 1 falls and lowers the voltage of the VCC circuit 2 when the voltage of the VCC circuit 1 rises in response to the detection output of the voltage drop detecting circuit 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit suitable for on-vehicle equipment, and more particularly, to a power supply circuit having a backup function for supplying power even when an ignition switch is turned off in order to retain data stored in a RAM. Power supply circuit.

[0002]

2. Description of the Related Art Electronic devices mounted on automobiles are increasingly computerized, and electronic devices that perform advanced control are increasing. For example, many learning controls are performed in which various data during operation are stored, and these data are used for subsequent control, thereby performing appropriate control in response to variations in actuators, aging, and the like. . In such learning control and the like, data must be retained even when the ignition switch is turned off. Therefore, the power supply circuit of the vehicle-mounted electronic device requires a microcomputer (microcomputer) or the like in the power supply circuit even when the ignition switch is turned off. A power supply circuit having a backup power supply function for supplying power to the power supply is used.

FIG. 7 is a configuration diagram showing a configuration of a conventional on-vehicle electronic device. A power supply circuit (VDD circuit) 102 for supplying a backup power supply for supplying power to a necessary portion such as a microcomputer even when the ignition switch is turned off is directly connected to a battery BATT of the vehicle. An operation power supply circuit (VCC circuit) 101 for supplying power to each part of an electronic control unit (ECU) that controls an engine and the like of a vehicle only when an ignition switch is turned on is connected to a vehicle battery BATT via an ignition switch IGSW. It is connected. The VCC circuit 101 supplies power to the microcomputer 100 via the power supply changeover switch BUSW, and also supplies power to various circuits inside the ECU. And VD
The D circuit 102 directly supplies power to the microcomputer 100. In order to prevent a current from flowing from the VCC circuit 101 to the VDD circuit 102, an element such as a diode is provided at an output portion of the VDD circuit 102.

A changeover switch BUSW for changing over a power supply is a switch for selecting a power supply for supplying power to the microcomputer 100.
During operation of the ECU, the ECU is turned on so that power is supplied from the VCC circuit 101 to the microcomputer 100.
Is shut off when not operating, and power is supplied from the VDD circuit 102 to the microcomputer 100. In addition, VC
The output voltage of the C circuit 101 is a voltage suitable for the operation of the microcomputer 100, for example, 5 V, and the output voltage of the VDD circuit 102 is a voltage required for holding the contents stored in the RAM of the microcomputer 100, for example, 3 V (generally, (Lower than the operating voltage of the microcomputer).

The VCC circuit 101 supplies power from a power supply output terminal of the ECU to a sensor 105 outside the ECU, and the sensor 105 outputs a detection output to an analog / digital (A / D) converter 104 of the ECU. Then, the A / D converter 104 outputs a digital conversion value of the output of the sensor 105 to the microcomputer 100, and the microcomputer 100 controls various actuators and the like according to the digital value. VCC
The circuit 101 or the reduced voltage detection circuit 103 for detecting a decrease in the input voltage of the circuit 101 outputs to the microcomputer 100 a reduced voltage signal HALT indicating a decrease in the output voltage of the VCC circuit 101 due to an operation of shutting off the ignition switch IGSW or the like. When the microcomputer 100 receives the reduced voltage signal HALT from the reduced voltage detection circuit 103, the microcomputer 100 stops the oscillation of the microcomputer operating clock signal and stops the microcomputer driving, and
The power consumption of U is reduced. At this time, the changeover switch BUSW is turned off, and power is supplied to the microcomputer 100 from the VDD circuit 102.

The undervoltage detection circuit 103 is configured to compare the output voltage Vcc of the VCC circuit 101 with the reference voltage Vr by a comparator (CMP) as shown in FIG. 8, and prevents hunting near the inversion region of the detection output. Therefore, a hysteresis comparator is used in CMP. That is,
CMP outputs H when voltage Vcc exceeds voltage VTH.
When the voltage Vcc falls below the voltage VTL, the output is inverted to the L (low voltage) level. These voltage values are set in a relationship of voltage VTH> voltage Vr> voltage VTL. Then, when the CMP output voltage is at the H level, the changeover switch BUSW is turned on, and the microcomputer 100 is turned on (HALT signal = H). Also, C
When the MP output voltage is at the L level, the changeover switch BUSW is in the cutoff state, and the microcomputer 100 is in the non-operation state (HA
LT signal = L).

When the ignition switch IGSW is turned on, the voltage Vcc gradually increases as shown in FIG. 9 due to the load capacity and the like. When the voltage Vcc reaches the voltage VTH, the power supply to the microcomputer 100 is changed to VCC.
Switch to power supply. However, in the illustrated T1 period, the power supply to the microcomputer 100 is the VDD power supply and the voltage Vdd, but the voltage applied to the sensors and the like is the voltage Vcc and the microcomputer 1
It is higher than the power supply voltage Vdd of 00. Therefore, the A / D converter 104 which is a sensor input unit of the microcomputer 100
Since a voltage higher than the operating voltage of the A / D converter 104 (the supply voltage Vdd to the microcomputer 100 at this time) is input as a voltage to be A / D converted, an abnormal input state occurs, and the microcomputer 100 Problems such as latch-up may occur.

As a reduced voltage detection circuit 103 for solving such a problem, the output voltage Vcc of the VCC circuit 101 and the output voltage of the VDD circuit 102 are determined by CMP as shown in FIG.
There is a configuration that compares Vdd. In this circuit configuration, when the voltage Vcc is higher than the voltage Vdd, the CMP output voltage goes to the H level, the changeover switch BUSW becomes conductive, and the microcomputer 100 operates (HALT signal = H).
Becomes When the voltage Vcc is lower than the voltage Vdd, CM
The P output voltage becomes L level, the changeover switch BUSW is cut off, and the microcomputer 100 is in the non-operation state (HALT).
Signal = L). In this method, the power supply voltage of the microcomputer 100 is always higher than the power supply voltage of the sensor 105, so that problems such as latch-up do not occur.

[0009]

However, due to manufacturing variations and the like, the output of the CMP is not inverted, that is, the output is not inverted when both inputs of the CMP have the same voltage.
It is common for some to shift to the higher one.

FIG. 11 is a waveform diagram showing signal waveforms when the offset voltage is positive (a voltage higher than the voltage at the inverting input terminal by the offset voltage Voff becomes a boundary voltage at which the output is inverted). In this case, when the voltage Vcc becomes the voltage Vdd + Voff, the CMP output S is inverted, but at this time, the changeover switch BUSW is turned on and the current Ico flows to the microcomputer side.
Since the changeover switch BUSW is usually made of a semiconductor, the voltage Vcc is set to Ico
Ron voltage drop occurs. Therefore, the voltage Vcc falls below the voltage Vdd + Voff again, and the output S of the CMP is inverted. Then, the changeover switch BUSW is turned off. Accordingly, this time, the voltage drop due to the conduction resistance Ron of the changeover switch BUSW disappears, and the voltage Vcc exceeds the voltage Vdd + Voff again, and the CMP
The output S is inverted. Due to such an operation, there is a problem that a phenomenon such as oscillation occurs when the CMP output S is inverted.

FIG. 12 is a waveform diagram showing signal waveforms when the offset voltage is negative (a voltage lower than the voltage at the inverting input terminal by the offset voltage Voff becomes a boundary voltage at which the output is inverted). In this case, a problem occurs when the output voltage of the VCC circuit 101 drops. When the output voltage Vcc of the VCC circuit 101 decreases, in a normal operation, when the voltage Vcc becomes the voltage Vdd, the CMP output is inverted and the changeover switch BUSW is turned off. However, even when the voltage Vcc drops to the voltage Vdd or less, the CMP output S is not immediately inverted due to the offset voltage Voff, and the changeover switch BUSW
Is not blocked. Therefore, a current flows from the VDD circuit 102 to the load such as the sensor through the changeover switch BUSW,
The non-inverting input terminal of CMP has a voltage Vdd-I (load current)
Ron voltage is applied. There is no problem if the load current is large, but since recent electronic devices are designed to reduce power, the load current is small (the load resistance is large), so the voltage Vdd-I (load current) • Ron is the voltage It may be larger than Vdd-Voff. In this case, the changeover switch BUSW does not enter the cutoff state, and there is a problem that the current continues to flow to the load despite the ignition switch IGSW being turned off, causing the battery to run down.

The present invention has been made in view of such a problem, and has as its object to realize a power supply circuit that can appropriately supply power to a microcomputer and a load.

[0013]

In order to solve the above-mentioned problems, a power supply circuit (1) according to the present invention is connected to a power supply via a power supply switch, and supplies power to the microcomputer when the power supply switch is on. And a backup power supply circuit that is connected to the power supply without passing through the power supply switch and supplies power to the microcomputer in the power switch off state. A disconnection switch connected to a power supply line between the microcomputer and the disconnection switch for disconnecting the power supply line, and monitoring an output voltage of the normal power supply circuit; A reduced voltage for turning off the switch and connecting the disconnection switch when the voltage of the normal power supply circuit rises In response to the detection output of the undervoltage detection means, increasing the voltage of the backup power supply circuit when the voltage of the normal power supply circuit decreases, and increasing the voltage of the backup power supply circuit when the voltage of the normal power supply circuit increases. And a voltage control means for lowering the voltage.

According to the power supply circuit (1), when switching from the backup power supply circuit to the normal power supply circuit (when the power switch is turned on), the voltage of the backup power supply circuit is high, so that the The power supply voltage of the microcomputer can be kept higher than the input voltage of the microcomputer to prevent latch-up and the like. Also, when the power switch is off, the voltage of the backup power supply circuit is lower, so that no current flows from the backup power supply circuit to the normal power supply circuit side, and the output of the normal power supply circuit by the reduced voltage detection means is reduced. False detection of voltage can be prevented, whereby the disconnection switch can be properly shut off, and wasteful current flowing from the backup power supply to the load through the disconnection switch can be prevented.

A power supply circuit (2) according to the present invention is connected to a power supply via a power supply switch, and supplies a normal power supply to the microcomputer when the power supply switch is on, and a power supply circuit without the power supply switch. A power supply circuit connected to the power supply and having a backup power supply circuit for supplying power to the microcomputer in the power switch off state, wherein the power supply circuit is connected to a power supply line between the normal power supply circuit and the microcomputer;
First and second disconnection switches connected in parallel, for disconnecting the power supply line, and monitoring the output voltage of the normal power supply circuit; A low-voltage detecting unit that sets a disconnection switch to a cut-off state and sets the first disconnection switch to a connected state when the voltage of the normal power supply circuit rises; And comparing means for setting the second disconnection switch to a connected state when the voltage of the normal power supply circuit is high, and disconnecting the second disconnection switch when the voltage of the normal power supply circuit is low. It is characterized by.

According to the power supply circuit (2), the control of the power supply circuit switching by the voltage reduction detecting means and the control of the power supply circuit switching by the comparing means operate so as to compensate for each other's shortcomings, and the power supply circuit is controlled. Since the switching is performed, reliable and stable power switching can be realized.

The power supply circuit (3) according to the present invention is connected to a power supply via a power supply switch, and supplies a power to the microcomputer when the power supply switch is on, and a power supply circuit without the power supply switch. A power supply circuit connected to the power supply and having a backup power supply circuit for supplying power to the microcomputer in the power switch off state, wherein the power supply circuit is connected to a power supply line between the normal power supply circuit and the microcomputer;
First and second disconnection switches connected in parallel, for disconnecting the power supply line, and monitoring the input power supply voltage of the microcomputer, and the first disconnection switch when the input power supply voltage drops. And a voltage reduction detecting means for connecting the first disconnection switch when the input power supply voltage rises, and comparing the voltage of the normal power supply circuit with the voltage of the backup power supply circuit, And comparing means for setting the second disconnection switch to a connected state when the voltage of the power supply circuit is high, and disconnecting the second disconnection switch when the voltage of the normal power supply circuit is low. .

According to the power supply circuit (3), the control of the power supply circuit switching by the low voltage detecting means and the control of the power supply circuit switching by the comparing means operate so as to compensate for each other's shortcomings, and the power supply circuit is controlled. Since the switching is performed, reliable and stable power switching can be realized.

The power supply circuit (4) according to the present invention is characterized in that, in the power supply circuit (1), (2) or (3), the reduced voltage detecting means has a hysteresis characteristic.

According to the power supply circuit (4), switching hunting near the power supply switching boundary (subtle changes in voltage cause
A phenomenon in which the voltage fluctuates a boundary value in a single cycle and the power supply switching occurs in a single cycle can be prevented, and the power supply switching operation is stabilized.

[0021]

Next, an embodiment of the present invention will be described. FIG. 1 is a circuit configuration diagram of a power supply circuit according to an embodiment of the present invention, and shows a power supply circuit used in an in-vehicle electronic device.

A power supply circuit (VDD circuit) 2 for supplying a backup power supply for supplying power to a necessary portion such as a microcomputer even when the ignition switch is turned off is provided with a battery of an automobile.
Connected directly to BATT. An operation power supply circuit (VCC circuit) 1 for supplying electric power to each part of an electronic control unit (ECU) that controls the engine of the vehicle only when the ignition switch is turned on is provided with an ignition switch IG.
It is connected to the car battery BATT via SW. The VCC circuit 1 supplies power to the microcomputer 10 via the power supply changeover switch BUSW, and also supplies power to various circuits inside the ECU. The VDD circuit 2 directly supplies power to the microcomputer 10. It should be noted that Vcc circuit 1
In order to prevent a current from flowing to the DD circuit 2, an element such as a diode is provided at an output portion of the VDD circuit 2.

A changeover switch BUSW for switching power supply is a switch for selecting a power supply for supplying electric power to the microcomputer 10.
When the CU is operating, it is conductive so that power is supplied from the VCC circuit 1 to the microcomputer 10, and when the ECU is not operating, it is shut off and power is supplied from the VDD circuit 2 to the microcomputer 10. In addition, power switch
BUSW is configured by a switching transistor and the like. The output voltage of the VCC circuit 1 is a voltage suitable for the operation of the microcomputer 10, for example, 5 V, and the output voltage of the VDD circuit 2 is a voltage required for holding the contents stored in the RAM of the microcomputer 10, for example, 3 V (general). Lower than the operating voltage of the microcomputer).

The VCC circuit 1 supplies power from a power supply output terminal of the ECU to a sensor 5 outside the ECU, and the sensor 5 outputs a detection output to an analog / digital (A / D) converter 4 of the ECU. Then, the A / D converter 4 outputs a digital conversion value of the output of the sensor 5 to the microcomputer 10, and the microcomputer 10 controls various actuators and the like according to the digital value. The VCC circuit 1 or a reduced voltage detection circuit 3 for detecting a decrease in the input voltage thereof is provided with a reduced voltage signal HALT (low voltage level at the time of voltage drop) indicating a decrease in the output voltage of the VCC circuit 1 due to an operation such as shutting off an ignition switch IGSW. Is output to the microcomputer 10. Also, a low voltage detection circuit 3
Has a hysteresis and the output voltage of the VCC circuit 1
If Vcc exceeds the high comparison voltage VTH, the reduced voltage signal HALT
Is set to a high voltage level (H signal), and if the output voltage Vcc of the VCC circuit 1 is lower than the low comparison voltage VTL voltage (VTH> VTL), the reduced voltage signal HALT is set to a low voltage level (L signal).

The microcomputer 10 receives the reduced voltage signal HALT from the reduced voltage detection circuit 3. When the reduced voltage signal HALT becomes the L signal, the microcomputer 10 stops the oscillation of the microcomputer operating clock signal, and The drive is stopped to reduce the power consumption of the ECU. Also, at this time,
The changeover switch BUSW is turned off, and power is supplied to the microcomputer 10 from the VDD circuit 2. That is, when the output voltage of the VCC circuit 1 drops, that is, when the ignition switch
For example, when the IGSW is shut off, the driving of the microcomputer 10 is stopped, that is, the ECU is stopped.

The VDD circuit 2 also receives the reduced voltage signal HALT. When the reduced voltage signal HALT is an L signal, the output voltage is set to a high level VB1.
At the time of a signal, the output voltage is set to a low level VA1. As a method of switching the output voltage,
Various voltage switching methods such as a method of switching between two types of regulators and a method of switching a voltage dividing resistor for setting an output voltage can be applied. The relationship between the voltages is VB1>VTH> VTL
> VA1.

Next, the operation of the power supply circuit will be described. FIG. 2 is an operation waveform diagram showing the operation of the power supply circuit. In the figure
typ indicates the set voltage, max and min indicate the maximum and minimum voltages in consideration of an error (variation), and it is desirable that the setting of each voltage value does not overlap including the error.

Before the ignition switch IGSW is turned on,
The voltage Vdd is supplied from the VDD circuit 2 to the microcomputer 10, and the supply voltage is the voltage VB1. When the ignition switch IGSW is turned on, the voltage Vcc gradually increases due to the load capacity and the like. And the voltage Vcc
At time t1 when the voltage reaches the voltage VTH, the HALT signal becomes H level, and the power supply to the microcomputer 10 is switched to the VCC circuit 1. Subsequently, the output voltage of the VDD circuit 2 switches to the voltage VA1.

The voltage Vdd at the time of this switching is VB1, which is higher than the voltage Vcc (= VTH) at this time. Therefore, before and after the ignition switch IGSW becomes conductive, the voltage applied to the microcomputer 10 is always the voltage Vcc.
As described above, the voltage applied to the sensor 5 does not fall below. For this reason, A /
A voltage higher than the operating voltage of the A / D converter 4 (the voltage supplied to the microcomputer 10 at this time) is applied to the D / D converter 4.
There is no input as a voltage to be converted, and no problem such as latch-up of the microcomputer 10 occurs.

Before the ignition switch IGSW is shut off,
The voltage Vcc is supplied from the VCC circuit 1 to the microcomputer 10. The output voltage of the VDD circuit 2 is the voltage VA1. When the ignition switch IGSW is turned off, the voltage Vcc gradually decreases due to the load capacity and the like.
At time t2 when the voltage Vcc reaches the voltage VTL, HALT
The signal becomes L level, and the power supply to the microcomputer 10 becomes
The mode is switched to the VDD circuit 2. Subsequently, the output voltage of the VDD circuit 2 switches to the voltage VB1.

The voltage Vdd at the time of this switching is VA1, which is lower than the voltage Vcc (= VTL) at this time. Therefore, the voltage Vcc does not fall below the voltage Vdd even before and after the time when the ignition switch IGSW is turned off.
A current flows from the DD circuit 2 to the VCC circuit 1, and the voltage Vcc
Can be prevented from being erroneously detected by the reduced voltage detection circuit 3 caused by maintaining the high voltage, and wasteful consumption of power due to a load or the like in the ECU can be prevented.

Next, another embodiment of the present invention will be described. FIG. 3 is a circuit configuration diagram of a power supply circuit according to the embodiment of the present invention, and shows a power supply circuit used in a vehicle-mounted electronic device.

A power supply circuit (VDD circuit) 2 for supplying a backup power supply for supplying power to a necessary portion such as a microcomputer even when the ignition switch is turned off is provided with a battery of an automobile.
Connected directly to BATT. An operation power supply circuit (VCC circuit) 1 for supplying electric power to each part of an electronic control unit (ECU) that controls the engine of the vehicle only when the ignition switch is turned on is provided with an ignition switch IG.
It is connected to the car battery BATT via SW. The VCC circuit 1 has a power supply changeover switch BUSW1 and
Power is supplied to the microcomputer 10 via BUSW2,
Power is supplied to various circuits inside U. The VDD circuit 2 directly supplies power to the microcomputer 10.

The power supply changeover switches BUSW1 and BUSW
Reference numeral 2 denotes a switching transistor or the like, and its on-resistances R1 and R2 (resistances in a conductive state) have values at which the microcomputer 10 operates normally, and the resistance R1 is a resistance R2.
It is set to about 100 times. In addition, from the VCC circuit 1 to the VD
In order to prevent a current from flowing to the D circuit 2, an element such as a diode is provided at an output portion of the VDD circuit 2.

The output voltage of the VCC circuit 1 is a voltage suitable for the operation of the microcomputer 10, for example, 5 V, and the output voltage of the VDD circuit 2 is a voltage required for holding the contents stored in the RAM of the microcomputer 10, for example, 3 V ( (Generally lower than the operating voltage of the microcomputer).

The VCC circuit 1 supplies power from a power supply output terminal of the ECU to a sensor 5 outside the ECU, and the sensor 5 outputs a detection output to an analog / digital (A / D) converter 4 of the ECU. Then, the A / D converter 4 outputs a digital conversion value of the output of the sensor 5 to the microcomputer 10, and the microcomputer 10 controls various actuators and the like according to the digital value. The VCC circuit 1 or a reduced voltage detection circuit 3 for detecting a decrease in the input voltage thereof is provided with a reduced voltage signal HALT (low voltage level at the time of voltage drop) indicating a decrease in the output voltage of the VCC circuit 1 due to an operation such as shutting off an ignition switch IGSW. To the microcomputer 10 and a power switch
BUSW2 is controlled (cut off when the low voltage signal HALT is L signal, and conductive when H signal is H signal). The undervoltage detection circuit 3 has a hysteresis. When the output voltage Vcc of the VCC circuit 1 exceeds the high comparison voltage VTH, the undervoltage signal HA is output.
When LT is set to a high voltage level (H signal), and the output voltage Vcc of the VCC circuit 1 falls below the low comparison voltage VTL voltage (VTH> VTL), the reduced voltage signal HALT is set to a low voltage level (L signal).

The microcomputer 10 receives the reduced voltage signal HALT from the reduced voltage detection circuit 3. When the reduced voltage signal HALT becomes the L signal, the microcomputer 10 stops the oscillation of the microcomputer operating clock signal, and The drive is stopped to reduce the power consumption of the ECU. Comparator 6 outputs V
The output voltage Vcc of the CC circuit 1 and the output voltage Vd of the VDD circuit 2
In the circuit for comparing d, the power supply switch BUSW1 is turned on when the voltage Vcc is higher than the voltage Vdd, and is turned off when the voltage Vcc is lower than the voltage Vdd. The relationship between the voltages is set to satisfy the relationship of VTH>VTL> Vdd.

Next, the operation of the power supply circuit will be described. FIG. 4 is an operation waveform diagram showing the operation of the power supply circuit. In the figure
typ indicates a set voltage, and max and min indicate maximum and minimum voltages in consideration of an error (variation).

Before conducting the ignition switch IGSW,
The microcomputer 10 is supplied with a voltage Vdd from the VDD circuit 2. When the ignition switch IGSW is turned on,
The voltage Vcc gradually increases due to the load capacity and the like. At time t3 when the voltage Vcc reaches the voltage Vdd, the comparator 6
Is inverted, the power switch BUSW1 is connected, and the power supply to the microcomputer 10 is changed to the VCC circuit 1
Switch to.

At the time of this switching, the voltage Vcc is equal to the voltage Vdd (there is an offset difference of the comparator 6, but the difference is small and there is not much problem). Therefore, even before and after the ignition switch IGSW is turned on, the voltage applied to the microcomputer 10 is always equal to or higher than the voltage Vcc.
It does not fall below the voltage applied to the sensor 5. For this reason,
A voltage higher than the operating voltage of the A / D converter 4 (the voltage supplied to the microcomputer 10 at this time) is supplied to the A / D converter 4 as a sensor input unit of the microcomputer 10 as a voltage to be A / D converted. There is no input, and no problem such as latch-up of the microcomputer 10 occurs.

Then, the voltage Vcc further rises, and the voltage VT
When the voltage exceeds H (t4), the reduced voltage signal HALT becomes the H signal, the power supply switch BUSW2 is connected, and V
The power supply from the CC circuit 1 to the microcomputer 10 is also performed via the power supply switch BUSW2, and the oscillation of the microcomputer operation clock signal is started, and the microcomputer 10 starts processing. In this state, the voltage Vcc and the voltage Vin
Is theoretically the same, but the output of the comparator 6 is determined by the influence of the ON resistance of the switch and the offset of the comparator 6, and the state of the power supply switch BUSW1 is determined.

When the ignition switch IGSW is turned off, the voltage Vcc gradually decreases due to the load capacity and the like.
At time t5 when the voltage Vcc reaches the voltage VTL, HALT
The signal becomes L level, the power switch BUSW2 is cut off, the oscillation of the clock signal for microcomputer operation stops, and the microcomputer 10 stops. However, power is supplied to the microcomputer 10 via the power switch BUSW1.
This is also performed from the D circuit 1. Further, the voltage Vcc drops and the voltage
At time t6 when the voltage reaches Vdd, the output of the comparator 6 is inverted, the power switch BUSW1 is turned off, and the power supply to the microcomputer 10 is switched to the VDD circuit 2.

As described above, the ignition switch IGSW
When the power supply is cut off, first, the oscillation of the clock signal of the microcomputer 10 is stopped due to the decrease of the output voltage Vcc of the VCC circuit 1,
Also, the power switch BUSW2 is turned off. Then, based on the comparison result of the voltage Vcc and the voltage Vdd, the power supply switch BUSW1 is turned off and the resistance value of the path passing through the power supply switch BUSW1 is increased. The rise of the voltage Vcc due to the flow of the current to the comparator 6 can be suppressed, and the erroneous detection of the comparator 6 can be prevented. Therefore, when the Vcc voltage decreases, V
Power supply switching from the DD circuit 2 side to the microcomputer 10 can be appropriately performed, so that useless current can be prevented from flowing into a load inside the ECU when the microcomputer 10 is stopped, and wasteful consumption of power can be prevented.

Next, still another embodiment of the present invention will be described. FIG. 5 is a circuit configuration diagram of a power supply circuit according to the embodiment of the present invention, and shows a power supply circuit used in an in-vehicle electronic device. In the present embodiment, the voltage detected by the reduced voltage detection circuit 3 in the power supply circuit shown in FIG.
Vcc is changed to the input voltage Vin to the microcomputer 10 via the power supply changeover switch BUSW1 (2) (detected by the reduced voltage detection circuit 7), and the other configuration is the same.

A power supply circuit (VDD circuit) 2 for supplying a backup power supply for supplying power to a necessary portion such as a microcomputer even when the ignition switch is turned off is provided with a battery of an automobile.
Connected directly to BATT. An operation power supply circuit (VCC circuit) 1 for supplying electric power to each part of an electronic control unit (ECU) that controls the engine of the vehicle only when the ignition switch is turned on is provided with an ignition switch IG.
It is connected to the car battery BATT via SW. The VCC circuit 1 has a power supply changeover switch BUSW1 and
Power is supplied to the microcomputer 10 via BUSW2,
Power is supplied to various circuits inside U. The VDD circuit 2 directly supplies power to the microcomputer 10.

The power supply changeover switches BUSW1 and BUSW
Reference numeral 2 denotes a switching transistor or the like, and its on-resistances R1 and R2 (resistances in a conductive state) have values at which the microcomputer 10 operates normally, and the resistance R1 is a resistance R2.
It is set to about 100 times. In order to prevent a current from flowing from the VCC circuit 1 to the VDD circuit 2,
An element such as a diode is provided at an output portion of the circuit 2.

The output voltage of the VCC circuit 1 is set to a voltage suitable for the operation of the microcomputer 10, for example, 5 V, and the output voltage of the VDD circuit 2 is set to a voltage required to hold the contents stored in the RAM of the microcomputer 10, for example, 3 V ( (Generally lower than the operating voltage of the microcomputer).

The VCC circuit 1 supplies power from a power supply output terminal of the ECU to a sensor 5 outside the ECU, and the sensor 5 outputs a detection output to an analog / digital (A / D) converter 4 of the ECU. Then, the A / D converter 4 outputs a digital conversion value of the output of the sensor 5 to the microcomputer 10, and the microcomputer 10 controls various actuators and the like according to the digital value. The VCC circuit 1 or a reduced voltage detection circuit 7 for detecting a decrease in the input voltage thereof is provided with an input voltage to the microcomputer 10 by an operation of shutting off an ignition switch IGSW or the like.
A low voltage signal HALT (low voltage level, low voltage level) indicating the drop of Vin is output to the microcomputer 10 and the power supply switch BUSW2 is controlled (the low voltage signal HALT is cut off when the L signal is low, H
(Conduction at the time of signal). The undervoltage detection circuit 7 has a hysteresis. When the input voltage Vin to the microcomputer 10 exceeds the high comparison voltage VTH, the undervoltage signal HALT is set to a high voltage level (H signal), and the voltage Vin
Is lower than the low comparison voltage VTL voltage (VTH> VTL), the reduced voltage signal HALT is set to the low voltage level (L signal).

The microcomputer 10 receives the reduced voltage signal HALT from the reduced voltage detection circuit 7. When the reduced voltage signal HALT becomes the L signal, the microcomputer 10 stops the oscillation of the microcomputer operating clock signal, and The drive is stopped to reduce the power consumption of the ECU. Comparator 6 outputs V
The output voltage Vcc of the CC circuit 1 and the output voltage Vd of the VDD circuit 2
In the circuit for comparing d, the power supply switch BUSW1 is turned on when the voltage Vcc is higher than the voltage Vdd, and is turned off when the voltage Vcc is lower than the voltage Vdd. The relationship between the voltages is set to satisfy the relationship of VTH>VTL> Vdd.

Next, the operation of the power supply circuit will be described. FIG. 6 is an operation waveform diagram showing the operation of the power supply circuit. In the figure
typ indicates a set voltage, and max and min indicate maximum and minimum voltages in consideration of an error (variation).

Before the conduction of the ignition switch IGSW,
The microcomputer 10 is supplied with a voltage Vdd from the VDD circuit 2. When the ignition switch IGSW is turned on,
The voltage Vcc gradually increases due to the load capacity and the like. At time t7 when the voltage Vcc reaches the voltage Vdd, the comparator 6
Is inverted, the power switch BUSW1 is connected, and the power supply to the microcomputer 10 is changed to the VCC circuit 1
Switch to.

At this switching time, the voltage Vcc is equal to the voltage Vdd (there is a difference due to the offset of the comparator 6, but the difference is small and there is no problem). Therefore, the ignition switch
Before and after the IGSW becomes conductive, the microcomputer 1
The voltage applied to 0 is always equal to or higher than the voltage Vcc, and does not fall below the voltage applied to the sensor 5. Therefore, the A / D converter 4 which is a sensor input unit of the microcomputer 10 has an A / D converter
A voltage higher than the operating voltage of the D converter 4 (supply voltage to the microcomputer 10 at this time) is not input as a voltage to be A / D converted, and no problem such as latch-up of the microcomputer 10 occurs. .

Then, the voltage Vcc further rises, and the voltage Vi
When n exceeds the voltage VTH (t8), the reduced voltage signal HALT becomes H
Signal, the power switch BUSW2 is connected, and the power supply from the VCC circuit 1 to the microcomputer 10 is
This is also performed through the power supply changeover switch BUSW2, and the oscillation of the microcomputer operation clock signal is started, and the microcomputer 10 starts processing.

When the ignition switch IGSW is turned off, the voltage Vcc gradually decreases due to the load capacity and the like.
At time t9 when the voltage Vin reaches the voltage VTL, HALT
The signal becomes L level, the power switch BUSW2 is cut off, the oscillation of the clock signal for microcomputer operation stops, and the microcomputer 10 stops. However, power is supplied to the microcomputer 10 via the power switch BUSW1.
This is also performed from the D circuit 2. Further, the voltage Vcc drops and the voltage
At time t10 when Vin reaches the voltage Vdd, the output of the comparator 6 is inverted and the power switch BUSW1 is turned off, and the power supply to the microcomputer 10 is switched to the VDD circuit 2.

As described above, the ignition switch IGSW
When the power supply is cut off, first, the oscillation of the clock signal of the microcomputer 10 is stopped due to the decrease of the output voltage Vcc of the VCC circuit 1,
Also, the power switch BUSW2 is turned off. Then, based on the comparison result of the voltage Vcc and the voltage Vdd, the power supply switch BUSW1 is turned off and the resistance value of the path passing through the power supply switch BUSW1 is increased. The rise of the voltage Vcc due to the flow of the current to the comparator 6 can be suppressed, and the erroneous detection of the comparator 6 can be prevented. Therefore, when the Vcc voltage decreases, V
The power supply switching from the DD circuit 2 to the microcomputer 10 can be appropriately performed, so that useless current can be prevented from flowing into the load inside the ECU when the microcomputer 10 is stopped, and wasteful consumption of power can be prevented.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram showing a power supply circuit according to a first embodiment of the present invention.

FIG. 2 is an operation waveform diagram showing an operation of the power supply circuit according to the first embodiment of the present invention.

FIG. 3 is a circuit configuration diagram showing a power supply circuit according to a second embodiment of the present invention.

FIG. 4 is an operation waveform diagram showing an operation of the power supply circuit according to the second embodiment of the present invention.

FIG. 5 is a circuit configuration diagram showing a power supply circuit according to a third embodiment of the present invention.

FIG. 6 is an operation waveform diagram showing an operation of the power supply circuit according to the third embodiment of the present invention.

FIG. 7 is a circuit configuration diagram showing a conventional power supply circuit.

FIG. 8 is an explanatory diagram showing an operation of a hysteresis comparator.

FIG. 9 is an operation waveform diagram showing an operation of a conventional power supply circuit.

FIG. 10 is a circuit configuration diagram showing a conventional power supply circuit.

FIG. 11 is an operation waveform diagram showing an operation of a conventional power supply circuit.

FIG. 12 is an operation waveform diagram showing an operation of a conventional power supply circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... VCC circuit 2 ... VDD circuit 3 ... Low voltage detection circuit 4 ... A / D (analog digital) converter 5 ... Sensor 10 ... Microcomputer (microcomputer)

Claims (4)

[Claims] 1. A normal power supply circuit connected to a power supply via a power switch and supplying power to a microcomputer when the power switch is on, a power supply switch connected to the power supply without the power switch and the power switch off A power supply circuit including a backup power supply circuit that supplies power to the microcomputer in a state, a disconnection switch connected to a power supply line between the normal power supply circuit and the microcomputer, and disconnecting the power supply line; Monitoring the output voltage of the normal power supply circuit, setting the disconnection switch to a cut-off state when the voltage of the normal power supply circuit decreases, and setting the disconnection switch to a connected state when the voltage of the normal power supply circuit increases A voltage reduction detecting means, and a voltage of the normal power supply circuit decreases in response to a detection output of the voltage reduction detecting means. A power control circuit for increasing the voltage of the backup power supply circuit and reducing the voltage of the backup power supply circuit when the voltage of the normal power supply circuit increases. 2. A normal power supply circuit which is connected to a power supply via a power switch and supplies power to the microcomputer when the power switch is on, a power supply circuit which is connected to the power supply without passing through the power switch and the power switch is off. In a power supply circuit having a backup power supply circuit for supplying power to the microcomputer in a state, the power supply circuit is connected to a power supply line between the normal power supply circuit and the microcomputer, and disconnects the power supply line. Connected first and second disconnection switches, and monitoring the output voltage of the normal power supply circuit, when the voltage of the normal power supply circuit decreases, turns off the first disconnection switch, A low voltage detecting means for connecting the first disconnection switch when the voltage rises, and a voltage of the normal power supply circuit; Comparing the voltage of the backup power supply circuit with the second disconnection switch when the voltage of the normal power supply circuit is high, and disconnecting the second disconnection switch when the voltage of the normal power supply circuit is low. A power supply circuit comprising: 3. A normal power supply circuit which is connected to a power supply via a power switch and supplies power to the microcomputer in the power switch on state, and which is connected to the power supply without passing through the power switch, and wherein the power switch is off. In a power supply circuit having a backup power supply circuit for supplying power to the microcomputer in a state, the power supply circuit is connected to a power supply line between the normal power supply circuit and the microcomputer, and disconnects the power supply line. The connected first and second disconnection switches and the input power supply voltage of the microcomputer are monitored, and when the input power supply voltage decreases, the first disconnection switch is turned off, and the input power supply voltage increases. Sometimes the first
A low-voltage detecting means for setting a disconnection switch to a connection state; comparing a voltage of the normal power supply circuit with a voltage of the backup power supply circuit; and when the voltage of the normal power supply circuit is high, connecting the second disconnection switch And a comparing means for turning off the second disconnection switch when the voltage of the normal power supply circuit is low. 4. The power supply circuit according to claim 1, wherein said low voltage detecting means has a hysteresis characteristic.