JPH10176641A - Power supply circuit for on-vehicle equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a downsized power supply circuit for on-vehicle equipment. SOLUTION: A booster IC 32 is adapted to boost up the source voltage by inducing a voltage in a coil L1 connected between a source line lead from a battery BT and its collector through the switchover of a transistor Tr1 and then charging a capacitor C1 connected to its collector by way of a diode D1 up to a voltage higher than a battery voltage +B by the induced voltage in the coil L1. The booster IC 32 has an ST terminal to which a starter signal to drive an engine starter 4 is applied and an EN terminal to which the battery voltage +B is applied. The booster IC 32 boosts up the source line when the starter signal indicates the driven state of the engine starter 4 and the battery voltage +B is smaller than a reference voltage. As a result, the booster IC 32 operates only during a limited period where the engine is being started to thereby suppress heat generation by its circuit, which thus allows its circuit to be constituted of smaller parts of a lower rating.

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 for an on-vehicle device, which compensates for a power supply voltage lowered by the operation of an engine starter.

[0002]

2. Description of the Related Art Conventionally, an on-vehicle device such as an electronic control device mounted on a vehicle and performing various controls is operated by receiving power supply from a battery. In each in-vehicle device, driving of a driver or the like that performs simple on / off operation uses the supplied power as it is, and driving of a CPU or the like that requires driving at a stable voltage uses a constant voltage. The voltage is stabilized by providing a circuit, and then a power supply is used.

In general, a battery also supplies power to a starting device or the like for starting an engine. Since driving of the engine starting device requires a large amount of power, when the engine starting device operates, the battery voltage is reduced. And the voltage drop is greater when the battery is insufficiently charged.

When the battery voltage becomes lower than the lower limit voltage at which the constant voltage circuit can operate, for example, due to this voltage drop, the constant voltage circuit cannot supply stable power, and the CPU is reset. For example, the normal operation of the vehicle-mounted device cannot be guaranteed.

On the other hand, Japanese Patent Laying-Open No. 3-294660 discloses an apparatus provided with a power supply circuit for raising the power supply voltage to a predetermined value or more when the battery voltage falls below a predetermined value.

[0006]

However, in this device, when the battery voltage is lower than a predetermined voltage from the beginning due to an abnormality in the charging system, the power supply circuit continues to perform the boosting operation even when the engine starting device is not operating. become. For this reason, the boosting section of the power supply circuit must be configured using components having a large rated value, that is, a large size that can be continuously energized and withstand heat generation. However, there is a problem that the size of the device increases.

Particularly, in a high-performance on-vehicle apparatus having many components that must be operated at a stable voltage, such a tendency becomes more remarkable because current consumption in a boosting section of a power supply circuit is increased. SUMMARY OF THE INVENTION An object of the present invention is to provide a power supply circuit for an in-vehicle device capable of reducing the size of the device in order to solve the above problems.

[0008]

In order to achieve the above object, in the vehicle power supply circuit according to the present invention, when the supply voltage from the battery is lower than a predetermined reference voltage, the booster means Is raised above the reference voltage,
The step-up prohibiting unit prohibits the operation of the step-up unit except during the engine start period.

That is, the boosting means does not operate even if the supply voltage from the battery is lower than the reference voltage, unless the engine is in the engine start period. When the supply voltage from the battery drops below the reference voltage, the boosting operation is started, and the power supply voltage supplied to each unit of the device equipped with the power supply circuit is maintained at or above the reference voltage. Thereafter, when the engine start period ends, or when the supply voltage from the battery becomes equal to or higher than the reference voltage even during the engine start period, the boosting unit ends the boosting operation. Then, after the end of the engine start period, even if the supply voltage from the battery drops and becomes lower than the reference voltage, the booster does not operate.

As described above, according to the power supply circuit for the vehicle-mounted device of the present invention, the operation of the boosting means is limited to the engine start period, and the boosting means is operated only for a short period. And the generation of heat can be kept small. As a result, the power supply circuit can be configured using electronic components having a relatively small rated value, that is, a small size, and the vehicle-mounted device on which the power supply circuit is mounted can be reduced in size.

By the way, the step-up prohibiting means includes a drive signal detecting means for detecting a drive signal of the engine starting device, so that an output period of the drive signal can be set as an engine starting period. In this case, the boosting operation by the boosting means can be performed only during the operation of the engine starting device which causes the supply voltage from the battery to drop, and it is possible to reliably prevent the boosting means from operating at other times.

[0012] Further, the step-up prohibiting means includes a rotational speed detecting means for detecting the rotational speed of the engine, so that when the engine rotational speed detected by the rotational speed detecting means is within a predetermined range, the engine is started. It may be configured to be a period. In this case, if the present invention is applied to an in-vehicle device which originally has an engine speed detecting means for processing in the device, such as an in-vehicle device for controlling an engine, a driving signal detecting means for detecting a driving signal of an engine starting device. Since it is not necessary to newly provide the device, the device configuration can be simplified.

The lower limit of the predetermined range of the engine speed used for estimating the engine start period may be any value as long as the engine stop state can be reliably excluded from the predetermined range. For example, any condition may be used as long as the engine can be reliably removed from the predetermined range, for example, by setting the number of revolutions during idling.

It is desirable to provide hysteresis at the upper and lower limits of the predetermined range of the engine speed. That is, the hysteresis on the high rotation speed side can prevent the booster circuit from performing an unstable operation of repeatedly operating / stopping due to the fluctuation of the engine rotation speed after the engine is started. This is because, when the engine speed fluctuates at a low speed, such as when starting from a very low temperature, it is possible to prevent the booster circuit from similarly performing an unstable operation.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of a vehicle-mounted electronic control device according to a first embodiment to which the present invention is applied.

As shown in FIG. 1, an on-vehicle electronic control unit (hereinafter referred to as an ECU) 2 of this embodiment operates by receiving power supply from a battery BT, and operates by an electromagnetic type rotation detecting an engine speed. Detection from various sensors, such as a sensor 10, an accelerator sensor 12 for detecting an amount of depression of an accelerator, a water temperature sensor 14 for detecting a temperature of engine coolant, and an intake pressure sensor 16 for detecting an intake pressure of air taken into the engine. A signal is input, and an injector 18 or the like for controlling fuel injection into each cylinder of the engine is driven based on the detection signal to control the operating state of the engine.

The ECU 2 has a CPU, ROM, R
An interface circuit 22 configured around a well-known microcomputer 20 having an AM, which converts an analog signal from the rotation sensor 10 into a digital signal and inputs the digital signal.
And an input buffer 24 for inputting starter signals, which will be described later, in addition to detection signals from the accelerator sensor 12, the water temperature sensor 14, and the intake pressure sensor 16, an output driver 18 for driving various loads such as an injector 18, and a battery BT. Power supply circuit 3 for distributing power supplied from the
0.

The battery BT that supplies power to the ECU 2 also supplies power to an engine starting device 4 that cranks and starts the engine.
While S is ON, power is supplied to the ECU 2 and the engine starter 4. The engine starter 4
Performs the cranking operation for starting the engine only while power is supplied via the relay 6 whose contact is closed when the starter switch SS is turned on. And EC
U2 is configured to take in, as a starter signal, the terminal voltage of the coil that activates the contacts of the relay 6,
That is, the starter signal is at the high level only while the engine starting device 4 is performing the cranking operation.

Next, the power supply circuit 30 which is a main part of the present invention is described.
Is a booster 30a that boosts and supplies a supply voltage (hereinafter referred to as a battery voltage) + B from the battery BT when a predetermined boost condition is satisfied, and a constant voltage Vcc (the output voltage + BB of the booster 30a). And a constant voltage unit 30b that outputs the voltage at + 5V in the embodiment.

The booster 30a has a collector connected to a power supply line from the battery BT via a coil L1,
Transistor T whose emitter is grounded via resistor R1
r1, a diode D1 having an anode connected to the collector of the transistor Tr1 and a cathode grounded via a capacitor C1, and controlling the base of the transistor Tr1 based on the emitter voltage of the transistor Tr1. A well-known booster IC 32 that switches the transistor Tr1 so that the voltage becomes a constant value.
The cathode side of the diode D1 is used as the output of the booster 30a.

The booster IC 32 is connected to the input buffer 24
And a ST terminal to which a starter signal taken in by the ECU 2 is applied via the ECU 2 and E to which a battery voltage + B is applied.
When the input signal to the ST terminal is at a high level (+ B) and the voltage applied to the EN terminal is lower than the reference voltage V1 (10 V in this embodiment), the boost condition is satisfied. As a result, the boosting operation described below is set to be performed.

That is, in the booster 30a, when the boost condition is not satisfied, the transistor Tr1 is continuously turned off by the boost IC32. In this case, the booster 30a simply passes through the coil L1 and the diode D1.
, Ie, the output voltage of the booster 30a + BB [of
f] is (+ B−Vf) obtained by lowering the battery voltage + B by the forward voltage Vf of the diode D1.

On the other hand, when the boost condition is satisfied,
The transistor Tr1 is switched by the booster IC32. In this case, when the transistor Tr1 is turned on, the collector voltage decreases and power supply to the capacitor is momentarily cut off. However, when the transistor Tr1 changes from on to off, the current change at this time causes the coil L1
, And the capacitor C1 is charged with a voltage (+ B−Vf + VL) higher than the output voltage + BB [off] when the transistor Tr1 is off by the induced voltage VL. Therefore, by appropriately controlling the switching of the transistor Tr1, the output voltage + BB of the booster 30a is obtained.
[on] can be set arbitrarily, and here, the output voltage + BB [on] is set to be a predetermined boosted voltage Vup (≧ V1).

Next, the constant voltage section 30b has an emitter connected to the output of the boosting section 30a and a collector connected to the capacitor C2.
A well-known constant voltage IC 34 (for example, made of NEC: μPC78L) for controlling a base current such that a collector voltage becomes a constant predetermined voltage Vcc based on a transistor Tr2 grounded through the transistor Tr2 and an emitter voltage of the transistor Tr2.
05), and the collector output of the transistor Tr2 is used as the output of the constant voltage section 30b.

However, the constant voltage IC 34 cannot maintain the predetermined voltage Vcc as the output voltage unless the input voltage, that is, the output voltage + BB of the booster 30a is not lower than the lower limit voltage V2 (5.6V in this embodiment). It is configured as follows.
From the power supply circuit 30 configured as described above, the booster 30
a (+ BB) and the output (Vc
c) is output and E is output together with the output (+ B) of the battery BT.
It is supplied to each part of CU2. Specifically, for example, in the output driver 26, a battery B
The output (+ B) of the booster 30a is applied to the output of T (+ B) and the special FET which needs to be driven at 10 V or more,
The output (Vcc) of the constant voltage unit 30b is supplied to a device that performs high-precision processing such as the microcomputer 20.

Next, a detailed operation of the booster 30a will be described with reference to a time chart shown in FIG. As shown in FIG. 2, when the starter switch SS is operated and the engine starter 4 starts operating, the battery voltage +
B falls to a value smaller than the reference voltage V1. At the same time, the ST terminal to which the starter signal has been input becomes High level. As a result, the above-described boosting condition is satisfied, and the boosting IC
The boosting operation by 32 is started.

After that, when the starter switch SS is opened and the ST terminal goes to the low level, the boost condition is not satisfied at that time, so that the battery voltage + B has reached the reference voltage V1 as shown in FIG. If not, the boost operation ends. Note that such a state is likely to occur, for example, when the charge amount of the battery BT is insufficient, or when the battery BT is started at an extremely low temperature.

On the other hand, as shown in FIG. 2, when the battery voltage + B recovers to the reference voltage V1 by operating the engine while the starter switch SS is being operated, the boost condition is not satisfied at that time. , The boosting operation ends regardless of the signal level of the ST terminal.

As described above, according to this embodiment, while the starter signal is being input, that is, while the starter switch SS is being operated, and the battery voltage +
Battery voltage + B only when B is smaller than reference voltage V1
, The transistor Tr1
The power supply to the power supply does not continue beyond the operation period of the starter switch SS, and the heat generation in the booster 30a can be reduced. As a result, as the electronic components constituting the boosting section 30a, those having a small rating and therefore a small size can be used, and the circuit of the boosting section 30a (and, consequently, the ECU 2) can be downsized.

Next, a second embodiment will be described. This embodiment has substantially the same configuration as the ECU 2 of the first embodiment, and differs in that the configuration of the power supply circuit 30 is partially different, that a microcomputer 20 is provided with a control port for controlling the power supply circuit 30, And the starter signal is not input to the power supply circuit 30 except for the point that the starter signal is not input to the power supply circuit 30.

FIG. 3 is an electric circuit diagram showing the configuration of the power supply circuit 30 in this embodiment. As shown in FIG. 3, the power supply circuit 30 according to the present embodiment includes a boosting unit 30a and a constant voltage unit 30b configured exactly the same as in the first embodiment, and a strobe signal applied to the ST terminal of the boosting IC 32. A strobe signal generator 30c for generating the strobe signal is provided.
In FIG. 3, the constant voltage unit 30b is omitted.

Here, the strobe signal generator 30c
It consists of a power supply line of the battery BT, a resistor R2 and a Zener diode D2 connected in series between the ST terminals, and becomes conductive when a voltage higher than the lower limit voltage V2 at which the constant voltage IC 34 normally operates is applied to both ends thereof. , A transistor Tr3 whose collector is connected to the ST terminal and whose emitter is grounded, and a resistor R whose one end is connected to the base of the transistor Tr3 and whose other end is grounded.
3 is provided.

The base of the transistor Tr3 is connected to a control port provided in the microcomputer 20, and according to the signal level (High / Low) of the control port set by the microcomputer 20,
The transistor Tr3 is turned on / off.

When the control port is set to the high level, the transistor Tr3 is turned on,
The ST terminal is forcibly held at the Low level, and the booster IC3
2 is prohibited. On the other hand, if the control port is Lo
When the level is set to the w level, the signal level at the ST terminal is determined by the battery voltage + B and the setting of the limiting circuit 36 by turning off the transistor Tr3. That is, if the battery voltage + B is equal to or higher than the lower limit voltage V2, the limiting circuit 3
6 turns on, a high level is applied to the ST terminal. When the battery voltage + B becomes lower than the lower limit voltage V2, the limiting circuit 36 is turned off, so that the ST terminal is turned on.
Low level is applied.

That is, when the control port is set to the low level and the battery voltage + B is between the reference voltage V1 and the lower limit voltage V2, the boosting condition is satisfied and the boosting IC 32 performs the boosting operation. Have been.
Note that the lower limit for permitting the boost operation by the boost IC 32 is as follows:
The lower limit voltage V2 at which the constant voltage IC 34 guarantees the predetermined voltage Vcc.
The reason is that even when the booster 30a performs the boosting operation in a state where the battery voltage + B is lower than the lower limit voltage V2, the constant voltage IC 34 can generate the predetermined voltage Vcc as soon as the boosting is completed. This is because normal operation cannot be continued as the whole ECU 2.

Here, the boost control process executed by the microcomputer 20 to set the control port will be described with reference to the flowchart shown in FIG. This process is repeatedly executed at a predetermined cycle (for example, 8.2 ms) after the microcomputer 20 is activated and a predetermined initialization process is executed. The control port is initially set to the high level and the flag F used in this processing is initialized to 0 by the initialization processing.

In addition to the present processing, the microcomputer 20 executes processing for calculating the engine speed Ne based on the detection signal from the rotation sensor 10, and the like. As shown in FIG. 4, when the present process is started, first, in step (hereinafter simply referred to as “S”) 110, the engine speed Ne calculated in a process executed separately from the present process is fetched. .

In S120, it is determined whether or not the engine speed Ne is equal to or higher than a preset lower limit lower speed n1.
Then, the control port is set to the high level, and a flag F for hysteresis control described later is set to 0, followed by terminating the present process.

If it is determined at S120 that the engine speed Ne is equal to or higher than the lower limit speed n1, the program proceeds to S120.
The process proceeds to 140, and it is determined whether the engine speed Ne is equal to or higher than a preset upper limit lower limit speed n2. If the engine speed Ne is smaller than the upper limit lower limit speed n2, the process proceeds to S150.

In S150, it is determined whether or not the flag F1 is set to 1. If it is set to 1, the process proceeds to S16.
Then, the control port is set to Low level, and the process is terminated. If it is determined in S150 that the flag F1 is not set to 1, the control port is set to High level.
After setting the level, the process ends.

If it is determined in S140 that the engine speed Ne is equal to or higher than the lower limit upper speed n2, the routine proceeds to S140.
The routine proceeds to 180, where it is determined whether or not the engine speed Ne is equal to or greater than a preset lower limit upper speed n3. If the engine speed Ne is smaller than the lower limit upper speed n3, the process proceeds to S190, where the control port is set to the low level and the flag F is set to 1, followed by terminating the present process.

If it is determined in step 180 that the engine speed Ne is equal to or higher than the lower limit engine speed n3, the process proceeds to S200, and the engine speed Ne is set to the preset upper limit engine speed n3. It is determined whether or not this is the case. Then, the engine speed Ne rises and the upper limit engine speed n4
If so, the process proceeds to S210, where the control port is set to High.
After the level is set and the flag F is set to 0, the process is terminated. On the other hand, in S200, the engine speed Ne is set.
Is determined to be smaller than the upper limit rotation speed n4,
Move to S220.

In S220, it is determined whether the flag F is set to 1 or not.
Then, the process is terminated after setting the control port to Low level, and if it is determined that the flag F is not set to 1, the process proceeds to S240 to set the control port to High.
After setting the level, the process ends.

That is, according to the present processing, as shown in FIG. 5B, when the engine speed Ne is increased by starting the engine and becomes equal to or higher than the rising lower limit speed n2, the control port is set to the low level. And the transistor Tr3
Is turned off, the boosting unit 30a can perform the boosting operation in accordance with the battery voltage + B. When the engine speed Ne further increases and becomes equal to or higher than the upper limit engine speed n4, the control port goes high. When the setting is performed and the transistor Tr3 is turned on, the boosting operation by the boosting unit 30a is prohibited.

After the engine speed Ne has reached the upper limit lower limit speed n2, the control port is set to Hi if the engine speed Ne is lower than the lower limit lower limit speed n1 even if the engine speed Ne decreases.
When the voltage is held at the gh level and becomes smaller than the lower limit lower limit rotation speed n1, the control port is set to the high level and the transistor Tr3 is turned on, so that the boosting operation by the booster 30a is prohibited.

After the engine rotation speed Ne reaches the upper limit rotation speed n4, the control port is set to Lo if the engine rotation speed Ne decreases but is equal to or higher than the lower limit rotation speed n3.
w, while the control port is set to the low level when the number of rotations becomes lower than the lower limit upper limit rotation speed n3, and the transistor Tr3 is turned off.
Can perform a boosting operation according to the battery voltage + B.

Therefore, as a whole, the state of the battery voltage + B and the engine speed Ne is as shown in FIG.
In the range indicated by the diagonal lines in (a), the boosting operation is performed assuming that the boosting condition is satisfied, and the upper limit value and the lower limit value of the engine speed Ne have hysteresis within the range in which the boosting condition is satisfied. Will be.

In this embodiment, n1: 30 [rp]
m], n2: 50 [rpm], n3: 400 [rpm]
m] and n4: 500 [rpm]. Next, the operation state of the booster 30a in the present embodiment will be described with reference to FIG.
This will be described along the time chart shown in FIG.

As shown in FIG. 6, the starter switch SS
Is operated, when the engine starting device 4 starts operating, the battery voltage + B decreases to a value smaller than the reference voltage V1. However, as long as the engine speed Ne is smaller than the rising lower limit speed n2, the control port remains set to the high level (the ST terminal input is at the low level). Does not start.

Thereafter, when the engine speed Ne rises and reaches the upper limit lower limit speed n2, the control port is set to the low level. If the battery voltage + B is equal to or higher than the lower limit voltage V2, the ST terminal input becomes high. Level, as a result, the boost condition is satisfied, and the boost operation by the boost IC 32 is started.

Once the step-up operation is started in this way, the engine speed Ne decreases and the upper and lower speed limit n
Even if it becomes smaller than 2, the step-up operation will not be stopped unless it becomes smaller than the lowering lower limit rotation speed n1. When the engine speed Ne increases and reaches the upper limit engine speed n4, the control port is set to the high level. As shown in FIG. 6, the battery voltage + B
Is not restored to the reference voltage V1, the booster 30a
, The boost operation ends.

On the other hand, as shown in FIG. 6, when the battery voltage + B recovers to the reference voltage V1 before the engine speed Ne reaches the upper limit engine speed n4, the booster IC3
With the setting of 2, the boosting operation ends at that point regardless of the signal level of the ST terminal. As described above, when the engine rotation speed Ne exceeds the upper limit rotation speed n4 and the boosting operation is completed, even if the engine rotation speed Ne becomes lower than the upper limit rotation speed n4, unless the engine rotation speed Ne becomes lower than the lower limit upper rotation speed n3, The boost operation is not restarted.

Note that the battery voltage at the time of starting the engine + B
Is smaller than the lower limit voltage V2, the setting of the control port (that is, the engine speed N
e) Regardless of e), the ST terminal is held at the low level,
No boost operation is performed. Here, FIG. 7 shows the operation shown in FIG. 6 converted into the relationship between the battery voltage + B and the engine speed Ne. X in the figure corresponds to the graph in FIG. Y corresponds to

Incidentally, Z1 in the figure indicates a state in which the battery BT has been discharged and the battery voltage + B has dropped as a result of leaving the key switch KS turned on, and Z2 in the figure indicates an engine starting device due to an abnormality in the charging system or the like. 4 indicates a state in which the battery voltage + B is lower than the reference voltage V1 even when the engine 4 is not driven. In this embodiment, the boosting operation is not performed in such a case, and the engine is started from the engine speed Ne. The boost operation is performed only during the period estimated to be medium.

As described above, according to the present embodiment, similarly to the first embodiment, the booster 30a is operated only for a short time, only when the engine is started. Therefore, as the electronic components constituting the boosting section 30a, those having a small rating and therefore a small size can be used, and the circuit of the boosting section 30a (and hence the ECU) can be used.
2) can be downsized.

Further, according to the present embodiment, since the start of the engine is estimated from the engine speed Ne, even if the ECU 2 is configured so as not to take in the starter signal, it can be used without any problem. . Further, according to the present embodiment, since the upper limit value and the lower limit value of the engine speed Ne that determine the setting of the signal level of the control port have hysteresis, the engine speed Ne is close to the upper limit value and the lower limit value. Even if the voltage varies, the booster 30a does not repeatedly start and stop the operation, so that a highly reliable device with stable operation can be obtained.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and can be implemented in various modes. For example, in the above embodiment, the power supply circuit 30 is connected to the ECU 2 that controls the engine.
However, the present invention may be applied to various ECUs and other in-vehicle devices that perform other processes. In particular, when applied to the engine starter 4, the startability of the engine can be improved.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a schematic configuration of an electronic control device according to a first embodiment and a circuit configuration of a power supply circuit as a main part.

FIG. 2 is a time chart illustrating an operation timing of a booster of the power supply circuit.

FIG. 3 is a circuit configuration diagram of a power supply circuit according to a second embodiment.

FIG. 4 is a flowchart illustrating a boost control process executed by the microcomputer.

5A is an explanatory diagram illustrating an operation range in which a booster of a power supply circuit performs boosting, and FIG. 5B is an explanatory diagram illustrating a control state of a transistor Tr3 by the process of FIG. 4;

FIG. 6 is a time chart illustrating operation timing of a booster of the power supply circuit.

FIG. 7 is an explanatory diagram in which the operation shown in FIG. 6 is converted into a relationship between an engine speed and a battery voltage.

[Explanation of symbols]

2 ... Electronic control device (ECU) 4 ... Engine starting device 10 ... Rotation sensor 12 ... Accelerator sensor 14
… Water temperature sensor 16… intake pressure sensor 18… injector 18
... Output driver 20 ... Microcomputer 22 ... Interface circuit 24 ... Input buffer 26 ... Output driver 30
... power supply circuit 30a ... step-up section 30b ... constant voltage section 30c ... strobe signal generation section 32 ... step-up IC 34 ... constant voltage IC 36 ... limiting circuit 38 ... switching circuit Tr1 to Tr3 ... transistor L1 ... coils C1, C2 ... capacitor R
1 to R3: Resistance D1: Diode D2: Zener diode

Claims (4)

[Claims] 1. An apparatus which is mounted on a device which operates by receiving power supply from a battery which supplies power to an engine starting device for starting an engine, wherein when the supply voltage from the battery is lower than a predetermined reference voltage, In a power supply circuit for an in-vehicle device provided with a booster for boosting a voltage to the reference voltage or more, a booster prohibiting unit for inhibiting the operation of the booster is provided except during a start-up period of the engine. Power supply circuit. 2. The engine according to claim 1, wherein the boosting prohibiting unit includes a driving signal detecting unit that detects a driving signal of the engine starting device, and sets an output period of the driving signal as a starting period of the engine. 2. The power supply circuit for an in-vehicle device according to 1. 3. The engine according to claim 1, wherein said step-up prohibiting means includes a number-of-revolutions detecting means for detecting a number of revolutions of said engine, and said engine detects a number of revolutions detected by said number-of-rotations detecting means within a predetermined range. 2. The power supply circuit for an in-vehicle device according to claim 1, wherein the power supply circuit is a start-up period. 4. The vehicle-mounted power supply circuit according to claim 3, wherein a hysteresis is provided at an upper limit and a lower limit of the predetermined range of the engine speed.