JP2011078228A - Overcurrent protection circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an overcurrent protection circuit which precisely prevents overheating of a switching element against an overvoltage of a power supply voltage while driving of a load is properly kept with a simple circuit structure. <P>SOLUTION: The overcurrent protection circuit (8) includes a first comparator (CP1) for input of a voltage output from a current detection circuit (7), comparing it with a reference voltage which is set inside, detecting overcurrent and for output of a detection signal for controlling operation of an inverter circuit (3), a first reference voltage setting circuit (9) setting the reference voltage of the first comparator (CP1) to a first reference voltage when the power supply voltage (Vcc) input from a power supply is not more than a prescribed voltage value, and a second reference voltage setting circuit (10) setting the reference voltage of the first comparator (CP1) to second reference voltage lower than the first reference voltage when the power supply voltage (Vcc) exceeds the prescribed voltage value. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an overcurrent protection circuit that protects a circuit by detecting an overcurrent flowing through the inverter circuit in a circuit that drives a load by providing an inverter circuit with a switching element between a power supply and a load.

  Conventionally, in order to drive a load such as a motor mounted on a vehicle or an electronic device, an inverter circuit constituted by a switching element is provided between the power source and the load, and the load is obtained by turning the switching element on / off. A load driving circuit for controlling the driving and stopping of the motor is used.

  In such a load driving circuit, when an overvoltage is applied as a power supply voltage, there is a problem that a large current flows through the switching element, resulting in an overheated state. In particular, a load driving circuit for driving a load such as a fan motor mounted on a vehicle has two batteries connected in series when a cell motor cannot be started in a cold region with respect to a normal battery power supply voltage (for example, 12V). It is required to operate even under abnormal overvoltage conditions (for example, 24V) such as connecting and using, and it is important to prevent overcurrent of the switching element and thus overheating under such overvoltage conditions. It is a difficult task.

  Conventionally, various techniques have been proposed to prevent overheating of the switching element (see, for example, Patent Document 1). Patent Document 1 includes a temperature sensor that detects the temperature of a MOS transistor and a current detection circuit that detects a current flowing through the MOS transistor in a motor driving device that drives a motor (load) by a MOS transistor (switching element). A motor driving device including an overheat detection circuit is disclosed (FIG. 1 of Patent Document 1). In this motor drive device, when the detected temperature or current exceeds a threshold value, the gate resistance of the drive circuit is decreased to shorten the rise time and fall time when switching the MOS transistor, and It is attempted to reduce the switching loss of the MOS transistor by lowering the PWM frequency.

  Patent Document 1 also describes a motor drive device including an overheat detection circuit including a temperature sensor that detects the temperature of a MOS transistor and a power supply voltage detection circuit that detects a power supply voltage supplied from a battery. (FIG. 4 of Patent Document 1), this motor drive device reduces the gate resistance of the drive circuit when the detected temperature or power supply voltage is equal to or higher than the threshold value, and the rise time during switching of the switching element In addition, the switching time of the MOS transistor is reduced by shortening the fall time and lowering the PWM frequency (FIG. 4 of Patent Document 1).

  In Patent Document 1, not only the switching element is restored from the overheated state by the motor driving device as described above, but also the overheated state in the future even if it is not the current overheated state such as an increase in current flowing through the MOS transistor or an increase in the power supply voltage. By detecting a highly probable state, it is possible to prevent the transition to an overheated state.

JP 2004-274911 A (FIGS. 1 and 4)

  However, in the motor drive device described in Patent Document 1, after detecting an overheat state of the switching element, an overvoltage state or an overcurrent state (highly likely to shift to the overheat state), the switching loss is reduced. The means for recovering from the overheated state or preventing overheating reduces the gate resistance of the drive circuit to shorten the rise time and the fall time at the time of switching of the switching element, and further reduce the PWM frequency. Therefore, the circuit required for switching between the gate resistance and the PWM frequency becomes complicated and the cost of parts increases, and such an overheat prevention means has an abnormal overvoltage as described above by taking an in-vehicle fan motor as an example. Under certain conditions, this is not sufficient to prevent overheating of the switching element.

  The present invention has been made in view of the above problems, and it is possible to accurately prevent overheating of a switching element against an overvoltage of a power supply voltage while appropriately driving a load while having a simple circuit configuration. It is an object of the present invention to provide an overcurrent protection circuit capable of achieving the above.

  The following aspects of the present invention exemplify the configuration of the present invention, and will be described separately for easy understanding of various configurations of the present invention. Each section does not limit the technical scope of the present invention, and some of the components of each section are replaced, deleted, or further, while referring to the best mode for carrying out the invention. Those to which the above components are added can also be included in the technical scope of the present invention.

(1) Power supply, load (2), inverter circuit (3) for driving the load (2), and current detection circuit (7) for detecting the current flowing through the inverter circuit (3) and outputting it as a voltage A voltage output from the current detection circuit (7) in an overcurrent protection circuit (8,8a) for protecting the inverter circuit (3) from an overcurrent of a load driving device (1,1a) comprising: First, the overcurrent is detected by comparing the voltage with a reference voltage (Vcp1) set internally, and a detection signal for controlling the operation of the inverter circuit (3) is output. Comparator (CP1) and the power supply voltage (Vcc) input from the power supply is equal to or lower than a predetermined voltage value, the reference voltage (Vcp1) of the first comparator (CP1) is set to the first reference voltage. When the first reference voltage setting circuit (9, 9a) to be set and the power supply voltage (Vcc) exceed the predetermined voltage value, A second reference voltage setting circuit (10, 10a) for setting the reference voltage (Vcp1) of the comparator (CP1) to a second reference voltage lower than the first reference voltage. An overcurrent protection circuit (claim 1).

(2) In the overcurrent protection circuit according to item (1), the first reference voltage setting circuit (9a) is a first circuit for supplying the reference voltage (Vcp1) of the first comparator (CP1). An overcurrent protection circuit comprising a constant voltage source (13) for generating a reference value (Vref1).

(3) In the overcurrent protection circuit according to (1) or (2), the second reference voltage setting circuit (10, 10a) is a second input from the outside with the power supply voltage (Vcc). An overvoltage detection circuit (11, 11a) that compares a reference value (Vref2) and outputs an overvoltage detection signal when the power supply voltage (Vcc) exceeds the predetermined voltage value, and the overvoltage detection circuit (11 , 11a) when the overvoltage detection signal is input, the reference voltage switching circuit (12, 12a) that operates to set the reference voltage (Vcp1) of the first comparator (CP1) to the second reference voltage And an overcurrent protection circuit (claim 3).

(4) In the overcurrent protection circuit according to any one of (1) to (3), the second reference voltage (Vcp12) is a constant value regardless of the value of the power supply voltage (Vcc). An overcurrent protection circuit according to claim 4 (claim 4).

(5) In the overcurrent protection circuit according to (4), the first reference voltage setting circuit (9a) divides the first reference value (Vref1) into the first reference voltage (Vcp11). A plurality of resistance elements (R8, R9), and the overvoltage detection circuit (11a) divides and inputs the power supply voltage (Vcc) and resistance-divides the second reference value (Vref2). The reference voltage switching circuit (12a) includes a switching element (Q5) that is turned on when the overvoltage detection signal is input, and the first reference voltage (Vcp11). ) To the second reference voltage (Vcp12), and an overcurrent protection circuit (Claim 5).

(6) In the overcurrent protection circuit according to any one of (1) to (3), the value of the second reference voltage decreases as the value of the power supply voltage (Vcc) increases. An overcurrent protection circuit (claim 6).

  The present invention is configured as described above. In the load driving device including the inverter circuit for driving the load, the inverter circuit has a simple circuit configuration and appropriately maintains the driving of the load when the power supply voltage is overvoltage. Can be protected from overcurrent, thereby providing an overcurrent protection circuit capable of accurately preventing overheating of the switching elements constituting the inverter circuit.

It is a circuit block diagram which shows the motor drive device containing the overcurrent protection circuit in one Embodiment of this invention. FIG. 2 is a circuit configuration diagram showing in detail an example of a configuration of an overcurrent protection circuit in an embodiment of the present invention in the motor drive device shown in FIG. 1. 3 is a graph showing a reference voltage characteristic of a first comparator in the overcurrent protection circuit shown in FIG. 2. 6 is a graph showing another example of the reference voltage characteristic of the first comparator in the overcurrent protection circuit according to the embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 shows a motor drive device 1 using, for example, a motor 2 as a load as an example of a load drive device including an overcurrent protection circuit according to an embodiment of the present invention. The motor drive device 1 includes a DC power supply (not shown) that supplies a power supply voltage Vcc, a motor 2 that is a load in the present embodiment, an inverter circuit 3 that drives the motor 2, and a driver that supplies a drive signal to the inverter circuit 3. A circuit 4, a control circuit 5 that controls the operation of the driver circuit 4, and an overcurrent protection circuit 8 are provided. A PWM signal generation circuit 6 is connected to the control circuit 5.

  The inverter circuit 3 connects a series circuit of a pair of switching elements Q1 and Q2 and a series circuit of another pair of switching elements Q3 and Q4 in parallel between a power supply voltage Vcc terminal and one end of a resistor R1, A so-called full bridge (H bridge) circuit is configured, and the motor 2 is connected between a connection point of the switching elements Q1 and Q2 and a connection point of the switching elements Q3 and Q4. The resistor R1 constitutes a current detection circuit 7 in which one end on the side different from the one connected to the inverter circuit 3 is connected to the GND terminal, and the current flowing through the inverter circuit 3 is detected as a voltage.

  In the inverter circuit 3 shown in FIG. 1, the high-side switching elements Q1 and Q3 are constituted by P-channel MOSFETs, and the low-side switching elements Q2 and Q4 are constituted by N-channel MOSFETs. Needless to say, all the switching elements Q1 to Q4 may be configured by N-channel MOSFETs. Moreover, as these switching elements Q1-Q4, you may use IGBT or a bipolar transistor.

  In the motor drive device 1, a PWM signal generation circuit 6 is connected to the control circuit 5, and the PWM signal generation circuit 6 is, for example, a PWM signal that has been subjected to pulse width modulation in accordance with specifications such as rotational speed control of the motor 2. Is output to the control circuit 5, and the control circuit 5 outputs each of the switching elements Q1 to Q4 based on the PWM signal (and other signals such as a signal indicating the rotational position of the motor 2, for example). Determine the on / off timing. Based on the control signal input from the control circuit 5, the driver circuit 4 generates a drive signal for the inverter circuit 3 (the gate voltage of each switching element Q <b> 1 to Q <b> 4) and supplies it to the inverter circuit 3. The inverter circuit 3 is driven by controlling the energization state of the motor 2 by turning on / off each of the switching elements Q1 to Q4 at a predetermined timing by a drive signal input from the driver circuit 4.

  In the motor drive device 1, the overcurrent protection circuit 8 according to the embodiment of the present invention includes a first comparator CP 1, a first reference voltage setting circuit 9, and a second reference voltage setting circuit 10. The second reference voltage setting circuit 10 includes an overvoltage detection circuit 11 and a reference voltage switching circuit 12.

  In the overcurrent protection circuit 8, a voltage corresponding to the detected current is input from the current detection circuit 7 to one input terminal of the first comparator CP1, and the first reference voltage setting circuit is input to the other input terminal. The reference voltage set by 9 is input. The output terminal of the first comparator CP1 is connected to the control circuit 5, and the first comparator CP1 compares the voltages input to both input terminals, and the voltage input from the current detection circuit 7 uses the reference voltage as a reference voltage. When it exceeds, a detection signal indicating detection of overcurrent is output to the control circuit 5. As a result, the control circuit 5 causes the inverter circuit 3 to overcurrent based on the detection signal input from the overcurrent protection circuit 8, for example, stops the operation of the inverter circuit 3 for a predetermined time after receiving the detection signal. Any suitable control to protect against can be implemented.

Here, the overcurrent protection circuit 8 according to the present invention uses the first reference voltage setting circuit 9 to set the reference voltage of the first comparator CP1 to the first voltage when the power supply voltage Vcc is equal to or lower than a predetermined voltage value. The reference voltage is set to a reference voltage, and when the power supply voltage Vcc exceeds a predetermined voltage value, the reference voltage of the first comparator CP1 is set to a second reference voltage lower than the first reference voltage. A second reference voltage setting circuit 10 for setting is provided.
In the present embodiment, this feature of the present invention is that the second reference voltage setting circuit 10 is interlocked with the first reference voltage setting circuit 9 in accordance with the voltage value of the power supply voltage Vcc, as will be described later. The reference voltage output from the first reference voltage setting circuit 9 is detected by the reference voltage switching circuit 12 when the overvoltage detection circuit 10 detects the overvoltage state of the power supply voltage Vcc. This is realized by switching from the voltage to the second reference voltage.

  Next, an example of a specific configuration and operation of the overcurrent protection circuit according to the embodiment of the present invention will be described with reference to FIGS. 2, the overcurrent protection circuit 8a, the first reference voltage setting circuit 9a, the second reference voltage setting circuit 10a, the overvoltage detection circuit 11a, and the reference voltage setting circuit 12a are the same as the overcurrent protection circuit 8 shown in FIG. 2 is an example of a circuit that specifically configures the first reference voltage setting circuit 9, the second reference voltage setting circuit 10, the overvoltage detection circuit 11, and the reference voltage setting circuit 12. In FIG. 2, the same components as those in FIG.

  As shown in FIG. 2, the overcurrent protection circuit 8a includes a first comparator CP1, and a voltage corresponding to the detected current is input from the current detection circuit 7 to one input terminal of the first comparator CP1. The reference voltage Vcp1 set by the first reference voltage setting circuit 9 is input to the other input terminal. The output terminal of the first comparator CP1 is connected to the control circuit 5.

  The first reference voltage setting circuit 9a has one end connected to the power supply voltage Vcc terminal to generate the first reference value (voltage) Vref1, the other end of the constant voltage source 13 and the GND terminal. Dividing resistors R8 and R9 connected in series are provided. The connection point of the dividing resistors R8 and R9 is connected to the other input terminal of the first comparator CP1, and the voltage at this connection point becomes the reference voltage Vcp1 of the first comparator CP1.

  The second reference voltage setting circuit 10a includes an overvoltage detection circuit 11a and a reference voltage switching circuit 12a. The overvoltage detection circuit 11a is connected in series between the second comparator CP2, the power supply voltage Vcc terminal and the GND terminal. It includes divided resistors R4 and R5 to be connected, and divided resistors R2 and R3 connected in series between the input terminal of the second reference value (voltage) Vref2 input from the outside and the GND terminal. A voltage obtained by dividing the power supply voltage Vcc by the dividing resistors R4 and R5 is input to one input terminal of the second comparator CP2, and the second reference value Vref2 is divided to the other input terminal. A voltage divided by resistors R2 and R3 is input as the reference voltage Vcp2 of the second comparator CP2. The output terminal of the second comparator CP2 is connected to the reference voltage switching circuit 12a.

  The reference voltage switching circuit 12a includes a switching element Q5 composed of a PNP transistor, and the base terminal of the switching element Q5 is connected to the output terminal of the second comparator CP2 of the overvoltage detection circuit 11a via a resistor R6. Has been. The switching element Q5 has an emitter terminal connected to a connection point of the divided resistors R8 and R9 of the first reference voltage setting circuit 9a, a collector terminal connected to one end of the resistor R7, and the other end of the resistor R7. Are connected to the GND terminal together with one end of the resistor R9 of the first reference voltage setting circuit 9a (on the side different from the side connected to the resistor R8).

  The operation of the overcurrent protection circuit 8a configured as described above will be described as follows. First, in the present embodiment, a predetermined voltage value (Vcc1 shown in FIG. 3) for determining the power supply voltage Vcc as an overvoltage state is set in advance. Then, the value of the second reference value Vref2 in the second reference voltage setting circuit 10a and the resistance values of the divided resistors R2, R3, R4, and R5 are determined so that the reference voltage Vcp2 of the second comparator CP2 is a predetermined value. The voltage value Vcc1 is adjusted to match the voltage divided by the dividing resistors R4 and R5, and the second comparator CP2 compares the power supply voltage Vcc and the second reference value Vref2 by this setting. Thus, an overvoltage detection signal is output when the power supply voltage Vcc exceeds a predetermined voltage value Vcc1. Specifically, the second comparator CP2 in the present embodiment outputs a high level voltage signal when the power supply voltage Vcc is equal to or lower than a predetermined voltage value Vcc1, and the power supply voltage Vcc exceeds the predetermined voltage value Vcc1. Sometimes, a LOW level voltage signal is output as an overvoltage detection signal.

  Therefore, when the power supply voltage Vcc is equal to or lower than the predetermined voltage value Vcc1, the switching element Q5 of the reference voltage switching circuit 12a is applied with the high level base voltage and is turned off (non-conducting state). The voltage setting circuit 9a uses, as the reference voltage Vcp1 of the first comparator CP1, a first reference voltage (Vcp11 shown in FIG. 3) that is a voltage obtained by dividing the first reference value Vref1 by the divided resistors R8 and R9. Output.

  In this case, the first comparator CP1 compares the voltage corresponding to the detected current from the current detection circuit 7 with the first reference voltage Vcp11 set by the first reference voltage setting circuit 9a to detect the current. When the voltage input from the circuit 7 exceeds the first reference voltage Vcp11, a detection signal indicating detection of overcurrent is output to the control circuit 5. In the first comparator CP1 in this embodiment, whether the detection signal indicating the detection of overcurrent is a high level or low level voltage signal can be arbitrarily set according to the specification of the control circuit 5. It is.

  Thereby, the control circuit 5 can implement arbitrary appropriate control for protecting the inverter circuit 3 from overcurrent. Although the present invention is not limited by the operation of the inverter circuit 3 controlled by the control circuit 5, for example, the control circuit 5 determines the detection signal based on the detection signal input from the first comparator CP1. The operation of the inverter circuit 3 may be stopped for a predetermined time after reception, and the operation may be restored after the predetermined time has elapsed. In this case, in the current detection circuit 7, the current corresponding to the detection voltage that coincides with the first reference voltage Vcp <b> 11 becomes a limit value of the current flowing through the inverter circuit 3, and a current equal to or greater than this limit value does not flow through the inverter circuit 3. As a result, the inverter circuit 3 can be protected from overcurrent while maintaining the rotation speed of the motor 2 within an appropriate range.

  On the other hand, when the power supply voltage Vcc exceeds a predetermined voltage value Vcc1, an overvoltage detection signal is input from the overvoltage detection circuit 11a to the reference voltage switching circuit 12a, so that a low level base voltage is applied to the switching element Q5. Turns on (conducting). As a result, in the first reference voltage setting circuit 9a, the first reference value Vref1 is divided by the divided resistor R9 and the parallel circuit of the resistor R7 of the reference voltage switching circuit 12a and the divided resistor R8. Thus, the second reference voltage (Vcp12 shown in FIG. 3) lower than the first reference voltage Vcp11 is output as the reference voltage Vcp1 of the first comparator CP1.

  As described above, the second reference voltage setting circuit 10a in the present embodiment is connected to the first reference voltage setting circuit 9a when the reference voltage switching circuit 12a receives the overvoltage detection signal from the overvoltage detection circuit 11a. The reference voltage Vcp1 of the first comparator CP1 is set to the second reference voltage Vcp12 by operating so as to switch the reference voltage output from the reference voltage to a relatively low reference voltage.

  In this case, the first comparator CP1 has a voltage corresponding to the detected current from the current detection circuit 7, and a second reference voltage Vcp12 output from the first reference voltage setting circuit 9a that works in conjunction with the reference voltage switching circuit 12a. And when the voltage input from the current detection circuit 7 exceeds the second reference voltage Vcp12, a detection signal indicating detection of overcurrent is output to the control circuit 5, and the control circuit 5 Based on the above, it is possible to implement any appropriate control for protecting the inverter circuit 3 from overcurrent.

  For example, when the control circuit 5 controls the operation based on the detection signal input from the first comparator CP1 to stop the operation of the inverter circuit 3 for a predetermined time after the reception of the detection signal, When the power supply voltage Vcc is in the overvoltage state, the current limit value corresponding to the second reference voltage Vcp12 is smaller than the current limit value when the power supply voltage Vcc is not in the overvoltage state, and thus flows to the inverter circuit 3. The current is precisely controlled in accordance with the state of the power supply voltage Vcc to protect the inverter circuit 3 from overcurrent. As a result, even under abnormal overvoltage conditions, the switching elements Q1 to Q4 constituting the inverter circuit 3 are controlled. Overheating can be effectively prevented.

  Here, the overcurrent protection circuit 8a shown in FIG. 2 is configured such that the second reference voltage Vcp12 is a constant value regardless of the value of the power supply voltage Vcc, as shown in FIG. In one embodiment, as shown in FIG. 4, the overcurrent protection circuit may be configured such that the value of the second reference voltage decreases as the value of the power supply voltage Vcc increases. . Further, the mode of decrease of the second reference voltage is not limited to the linear decrease as shown in FIG. 4, and can be freely designed such as decreasing in multiple steps or decreasing in a curve. It is possible.

  Further, the motor drive devices 1 and 1a shown in FIG. 1 and FIG. 2 may be configured such that all or part of the constituent elements excluding the motor 2 are configured by a one-chip IC. 8, 8a may be included in the one-chip IC, or may be connected to the one-chip IC as an external circuit.

As mentioned above, although this invention was demonstrated by preferable embodiment, this invention is not limited to embodiment mentioned above, A various deformation | transformation and application are possible within the range of the technical idea of this invention.
For example, in the above-described embodiment, the overcurrent protection circuit according to the present invention has been described as being included in the motor drive devices 1 and 1a that drive the motor 2 by the inverter circuit 3 formed of a full bridge circuit. Such an overcurrent protection circuit is not limited by the type of load of the load driving device to which it is applied and the configuration of the inverter circuit. For example, the overcurrent protection circuit according to the present invention can be applied to a motor driving device that drives an arbitrary single-phase or multi-phase motor, and the inverter circuit that drives the motor is also an appropriate inverter circuit according to the type. can do. Furthermore, the overcurrent protection circuit according to the present invention may be applied to a load driving device that drives a lighting device such as a discharge lamp or an LED as a load.

  In the overcurrent protection circuit according to the present invention, an upper limit value and a lower limit value are set for the second reference voltage of the first comparator CP1, and before the detection voltage from the current detection circuit 7 reaches an overcurrent state, The first comparator CP1 compares the detection voltage from the current detection circuit 7 with the upper limit value of the second reference voltage, and after the detection voltage from the current detection circuit 7 reaches an overcurrent state, the first comparator CP1 The first comparator CP1 may have a function of hysteresis operation such that the detection voltage from the current detection circuit 7 is compared with the lower limit value of the second reference voltage in CP1.

  Further, in the overcurrent protection circuit 8a shown in FIG. 2, the switching element Q5 of the reference voltage switching circuit 12a is capable of switching on / off of its conduction state according to the output signal from the second comparator CP2. Any electronic switch can be used. For example, an NPN transistor or an analog switch may be used. Further, the switching element Q5 and the resistor R7 may be connected in parallel to the divided resistor R8 of the first reference voltage setting circuit 9a.

DESCRIPTION OF SYMBOLS 1, 1a: Motor drive device, 2: Motor, 3: Inverter circuit, 4: Driver circuit, 5: Control circuit, 6: PWM signal generation circuit, 7: Current detection circuit, 8, 8a: Overcurrent protection circuit, 9 , 9a: first reference voltage setting circuit, 10, 10a: second reference voltage setting circuit, 11, 11a: overvoltage detection circuit, 12, 12a: reference voltage switching circuit, 13: constant voltage source, CP1: first Comparator, CP2: second comparator, Q1 to Q4: switching element (MOSFET), Q5: switching element (PNP transistor), R1: resistor (current detection circuit), R2, R3, R4, R5, R8, R9 : Division resistance, R6, R7: resistance, Vcc: power supply voltage, Vcc1: predetermined voltage value, Vcp11: first reference voltage, Vcp12: second reference voltage, Vref1: first Quasi-value, Vref2: the second reference value

Claims (6)

A power source; a load (2); an inverter circuit (3) that drives the load (2); and a current detection circuit (7) that detects a current flowing through the inverter circuit (3) and outputs the voltage as a voltage. In the overcurrent protection circuit (8, 8a) for protecting the inverter circuit (3) from overcurrent of the load driving device (1, 1a),
The voltage output from the current detection circuit (7) is input, the overcurrent is detected by comparing the voltage with an internally set reference voltage (Vcp1), and the operation of the inverter circuit (3) A first comparator (CP1) that outputs a detection signal for controlling the power supply, and when the power supply voltage (Vcc) input from the power supply is equal to or lower than a predetermined voltage value, the first comparator (CP1) A first reference voltage setting circuit (9, 9a) for setting a reference voltage (Vcp1) to a first reference voltage, and when the power supply voltage (Vcc) exceeds the predetermined voltage value, A second reference voltage setting circuit (10, 10a) for setting a reference voltage (Vcp1) of the comparator (CP1) to a second reference voltage lower than the first reference voltage. Overcurrent protection circuit.   The first reference voltage setting circuit (9a) includes a constant voltage source (13) for generating a first reference value (Vref1) for supplying a reference voltage (Vcp1) of the first comparator (CP1). The overcurrent protection circuit according to claim 1, further comprising:   The second reference voltage setting circuit (10, 10a) compares the power supply voltage (Vcc) with a second reference value (Vref2) input from the outside, and the power supply voltage (Vcc) is When the overvoltage detection circuit (11, 11a) outputs an overvoltage detection signal when the voltage value exceeds the voltage value, and when the overvoltage detection signal is input from the overvoltage detection circuit (11, 11a), the first comparator (CP1 And a reference voltage switching circuit (12, 12a) that operates to set the reference voltage (Vcp1) of the second reference voltage to the overcurrent protection circuit according to claim 1 or 2. .   4. The overcurrent protection circuit according to claim 1, wherein the second reference voltage (Vcp12) is a constant value irrespective of the value of the power supply voltage (Vcc). 5.   The first reference voltage setting circuit (9a) includes a plurality of resistance elements (R8, R9) that divide the first reference value (Vref1) into the first reference voltage (Vcp11), and the overvoltage detection The circuit (11a) includes a second comparator (CP2) that inputs the power supply voltage (Vcc) by dividing the resistance and inputs the second reference value (Vref2) by dividing the resistance. The circuit (12a) includes a switching element (Q5) that is turned on when the overvoltage detection signal is input, and a resistor for switching the first reference voltage (Vcp11) to the second reference voltage (Vcp12). The overcurrent protection circuit according to claim 4, further comprising an element (R7).   4. The overcurrent protection circuit according to claim 1, wherein a value of the second reference voltage decreases with an increase in the value of the power supply voltage (Vcc). 5.

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