Load drive circuit

A switch electrically connected to a first terminal and a second terminal. A first comparator detects a load open state where the switch is off and a load is not connected to the second terminal. Upon detecting, a clamp circuit clamps the voltage of the second terminal to a clamp voltage higher than a first reference voltage and lower than an input voltage. A second comparator detects an output-to-supply short circuit state where the switch is off and the second terminal is connected to the power supply. A capacitor with one terminal connected to the second terminal and another terminal connected to a third terminal. A bootstrap circuit supplies a charge current to the third terminal at a constant voltage. A clamp voltage rise prevention circuit prevents the clamp voltage of the clamp circuit from rising, when the first comparator detects the load open state.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority based on 35 USC 119 from prior Japanese Patent Application No. 2014-261586 filed on Dec. 25, 2014, entitled “LOAD DRIVE CIRCUIT”, the entire contents of which are hereby incorporated by reference.

BACKGROUND

This disclosure relates to a load drive circuit that drives a load such as a solenoid by using a switching element.

A background technique of the load drive circuit is disclosed in Japanese Patent Application Publication No. 2013-16959 (Patent Document 1). This load drive circuit detects a load open state, where a load is not connected to an output terminal because of disconnection between the output terminal and the load or any other reason.

As illustrated inFIG. 5, the load drive circuit includes switching element Q1including a MOSFET, first comparator CP1, second comparator CP2, and clamp circuit14. First comparator CP1compares first reference voltage V1that is lower than the voltage of power supply E inputted to power supply terminal Vcc with output voltage Vout of output terminal OUT. When output voltage Vout is equal to or higher than first reference voltage V1, first comparator CP1outputs an H level for detection of the load open state.

Second comparator CP2compares second reference voltage V2that is lower than the voltage of power supply E and higher than a clamp voltage Vclamp with output voltage Vout of output terminal OUT. When output voltage Vout is equal to or higher than second reference voltage V2, second comparator CP2outputs an H level for detection of an output-to-supply short circuit state.

When switching element Q1is off and load20is not connected to output terminal OUT, i.e., load20is in the load open state, clamp circuit14clamps the voltage of output terminal OUT to the clamp voltage Vclamp that is higher than first reference voltage V1and lower than the voltage of power supply E.

With the above-described configuration, when the load is in the load open state and switching element Q1is off, output voltage Vout is clamped to the clamp voltage by the clamp circuit.

In the above state, first comparator CP1can detect the load open state since first reference voltage V1of first comparator CP1is set to GND<V1<Vclamp, whereas second comparator CP2does not detect the output-to-supply short circuit since second reference voltage V2of second comparator CP2is set to Vclamp<V2<E.

Meanwhile, in the output-to-supply short circuit state, the voltage of output terminal OUT rises to the voltage of power supply E, even when switching element Q1is off. In this state, first comparator CP1detects the current state as the load open state since first reference voltage V1of first comparator CP1is set to GND<V1<Vclamp, whereas second comparator CP2can detect the output-to-supply short circuit state since second reference voltage V2is set to Vclamp<V2<E.

In addition, when an N-type MOSFET is used as a high-side transistor of switching element Q1for output, bootstrap circuit BS is added as illustrated inFIG. 5.

DETAILED DESCRIPTION

Hereinafter, load drive circuits according to embodiments are described in detail with reference to the drawings. Note that constituents that are the same as those in the conventional load drive circuit illustrated inFIG. 5are denoted by the same reference signs as those used for the conventional technique.

First Embodiment

FIG. 1is a diagram illustrating a circuit configuration of a load drive circuit according to a first embodiment. The load drive circuit illustrated inFIG. 1includes power supply terminal Vcc, switching element Q1, booster circuit11, buffer circuit12, control circuit13, output terminal OUT, clamp circuit14, constant-current circuit15, first comparator CP1, second comparator CP2, bootstrap circuit BS, and charge current stop circuit16, and is configured to drive load20connected to output terminal OUT by using power inputted from power supply E connected to power supply terminal Vcc.

Switching element Q1switches a power supply path extending from power supply terminal Vcc to output terminal OUT between on and off, and controls a current flowing through load20by the switching between on and off. In switching element Q1, a MOSFET is used as a high-side switch. A drain of switching element Q1is connected to power supply terminal Vcc, and a source of switching element Q1is connected to output terminal OUT. Note that a bipolar transistor may be used as switching element Q1.

In addition, resistor R1and gate protection constant-voltage diode ZD1are connected in parallel between a gate and the source of switching element Q1. Switching element Q1, resistor R1, gate protection constant-voltage diode ZD1, buffer circuit12, and clamp circuit14constitute a high-side drive circuit.

Control circuit13is connected to booster circuit11through buffer circuit12, and outputs a control signal that controls the on/off operation of switching element Q1. Control circuit13causes booster circuit11, which boosts voltage E, to boost the control signal to a voltage higher than voltage E, and applies the control signal to the gate of switching element Q1. In this manner, control circuit13controls the on/off operation of switching element Q1.

Constant-current circuit15is connected between power supply terminal Vcc and output terminal OUT in parallel with switching element Q1, and causes constant current I to flow in a range where load20is not driven by the current (for example, in a range from several tens of microamperes to one hundred microamperes).

Clamp circuit14has an impedance that is 10 or more times, and preferably 100 or more times the impedance of load20, and is connected in parallel with load20. When switching element Q1is off, and the load is the open state, constant current I caused to flow by constant-current circuit15flows through only clamp circuit14.

Accordingly, a clamp voltage is determined by the constant current I caused to flow by constant-current circuit15and the impedance of clamp circuit14, and takes a value that is higher than a ground potential and lower than voltage E.

Note that, when the switching element Q1is off and the load is not in the open state, almost all of constant current I caused to flow by constant-current circuit15flows through load20, and voltage Vout of output terminal OUT is almost equal to the ground potential.

Functions of first comparator CP1and second comparator CP2are the same as those illustrated inFIG. 5, and descriptions thereof are omitted.

Capacitor C1is a bootstrap capacitor with one terminal connected to output terminal OUT and another terminal connected to an amp terminal. Bootstrap circuit BS includes a constant voltage circuit that generates a constant voltage on the basis of voltage E of the power supply of power supply terminal Vcc. The bootstrap circuit BS supplies a charge current to the amp terminal at the constant voltage. Note that, in this embodiment, bootstrap circuit BS and capacitor C1together may constitute the bootstrap circuit. The capacitor C1may be outside of the load drive circuit, which may be implemented in a single chip such as an IC (Integrated Circuit).

In addition, charge current stop circuit16is provided between bootstrap circuit BS and the amp terminal. Charge current stop circuit16includes a switch, and constitutes a clamp voltage rise prevention circuit that prevents the clamp voltage of clamp circuit14from rising, when first comparator CP1detects the load open state.

Next, operations of the thus configured load drive circuit of the first embodiment are described in detail with reference toFIG. 2.

First, in a normal state, i.e., in neither the load open state nor the output-to-supply short circuit state, a current flows through load20, when switching element Q1is on. Here, voltage Vout of output terminal OUT is equal to input voltage E. When switching element Q1is off in the normal state, almost all of constant current I caused to flow by constant-current circuit15flows through load20.

Accordingly, when the switching element Q1is off, voltage Vout of output terminal OUT is almost equal to the ground potential, and is smaller than each of reference voltage V1and reference voltage V2. For this reason, each of the outputs from first comparator CP1and second comparator CP2takes an L level, and neither the load open state nor the output-to-supply short circuit state is detected.

Also in the load open state, when switching element Q1is on, voltage Vout of output terminal OUT is equal to voltage E.

Next, when switching element Q1is off in the load open state, constant current I caused to flow by constant-current circuit15flows through clamp circuit14, and voltage Vout of output terminal OUT is clamped to a clamp voltage by clamp circuit14. Accordingly, when switching element Q1is off, voltage Vout of output terminal OUT exceeds reference voltage V1. As a result, the output of first comparator CP1takes an H level, and the load open state is detected.

When a load open state detection signal is outputted from first comparator CP1to charge current stop circuit16, the switch of charge current stop circuit16is turned off. Hence, the supply of the charge current from bootstrap circuit10to capacitor C1is stopped. For this reason, the charge current from bootstrap circuit10does not raise clamp voltage Vclamp of clamp circuit14.

Accordingly, clamp voltage Vclamp does not exceed second reference voltage V2, and second comparator CP2outputs an L level. This makes it possible to prevent false detection of the output-to-supply short circuit state.

In addition, in the output-to-supply short circuit state, the clamp voltage is equal to voltage E. Hence, voltage E is inputted to a non-inverting terminal of second comparator CP2, and voltage Vclamp is inputted to an inverting terminal of comparator CP2. Accordingly, the second comparator CP2outputs an H level. This enables the detection of the output-to-supply short circuit state.

FIG. 2is a diagram illustrating operation waveforms of components in the load drive circuit according to the first embodiment. In general, as illustrated inFIG. 2, the charge current to capacitor C1is stopped from time t0to time t1(in charge current stop period T2), and the charge current is caused to flow through capacitor C1from time t1to time t2.

However, when the load is in the open state in period T1including the period from time t3to time t4where the charge current should be caused to flow through capacitor C1, the switch of charge current stop circuit16is turned off in the period from time t3to time t4to stop the charge current from bootstrap circuit10. It can be understood that this makes it possible to keep constant the voltage of output terminal OUT, i.e., the clamp voltage.

Second Embodiment

FIG. 3is a diagram illustrating a circuit configuration of a load drive circuit according to a second embodiment. The load drive circuit according to the second embodiment illustrated inFIG. 3includes electric discharge stop circuit17instead of charge current stop circuit16in the load drive circuit according to the first embodiment illustrated inFIG. 1.

Note that the other constituents are the same as those of the load drive circuit of the first embodiment, and hence descriptions thereof are omitted.

Electric discharge stop circuit17includes a switch connected between bootstrap circuit BS and the ground, and is configured to turnoff the switch to stop the electric discharge by a current flowing from capacitor C1through bootstrap circuit BS to the ground, when first comparator CP1detects the load open state.

Operations of the thus configured load drive circuit of the second embodiment are described. The operation in the normal state and the operation in the load open state are the same as those of the load drive circuit of the first embodiment. Accordingly, an operation in a case where switching element Q1is off and first comparator CP1detects the load open state is described here.

First, the electric discharge from capacitor C1to the ground is started, and the output from the amp terminal is lowered. Then, the constant voltage circuit of bootstrap circuit BS tries to cause a current to flow through the amp terminal to output the constant voltage.

When, however, a current flows through the amp terminal and through capacitor C1, the voltage of clamp circuit14rises. For this reason, the voltage of clamp circuit14becomes equal to or higher than first reference voltage V1, and first comparator CP1outputs an H level to the switch of electric discharge stop circuit17.

For this reason, the switch of electric discharge stop circuit17is turned off, and the electric discharge current is stopped from flowing from capacitor C1through bootstrap circuit BS to the ground.

As a result, the voltage of clamp circuit14does not rise any more, and clamp voltage Vclamp does not exceeds second reference voltage V2. Hence, second comparator CP2outputs an L level. This makes it possible to prevent false detection of the output-to-supply short circuit state.

In addition, the clamp voltage is equal to voltage E in the output-to-supply short circuit state. Hence, voltage E is inputted to the non-inverting terminal of second comparator CP2, and voltage Vclamp is inputted to the inverting terminal of comparator CP2. Hence, second comparator CP2outputs an H level. This enables detection of the output-to-supply short circuit state.

Third Embodiment

FIG. 4is a diagram illustrating a circuit configuration of a load drive circuit according to a third embodiment. In the load drive circuit according to the third embodiment, charge current stop circuit16of the load drive circuit according to the first embodiment illustrated inFIG. 1is not provided. Instead, second comparator CP2sets second reference voltage V2to a voltage that is higher than a voltage to be generated by a current including a current flowing from bootstrap circuit BS through the amp terminal and capacitor C1to clamp circuit14and that is lower than voltage E, when first comparator CP1detects the load open state.

Note that the other constituents are the same as those of the load drive circuit of the first embodiment, and hence descriptions thereof are omitted.

Operations of the thus configured load drive circuit according to the third embodiment are described. The operation in the normal state and the operation in the load open state are the same as those of the load drive circuit of the first embodiment. Accordingly, an operation in a case where the switching element Q1is off and first comparator CP1detects the load open state is described here.

First, when first comparator CP1detects the load open state, first comparator CP1transmits a load open state detection signal to second reference voltage V2of second comparator CP2. In response to the load open state detection signal, second reference voltage V2is set to a voltage that is higher than voltage Vclamp2generated by current I2including current I1flowing from bootstrap circuit BS through the amp terminal and capacitor C1to clamp circuit14and that is lower than voltage E. Second reference voltage V2is preferably set according to voltage E.

Here, current I2is a total current of current I1flowing from bootstrap circuit BS through the amp terminal and capacitor C1to clamp circuit14and current I from constant-current circuit15. Voltage Vclamp2generated by current I2is larger than voltage Vclamp generated by current I from constant-current circuit15.

Second reference voltage V2is a voltage that is higher than voltage Vclamp2and lower than voltage E. Hence, clamp voltage Vclamp2does not exceeds second reference voltage V2, when the circuit is not in the output-to-supply short circuit state. Accordingly, second comparator CP2outputs an L level. This makes it possible to prevent false detection of the output-to-supply short circuit state.

In addition, in the output-to-supply short circuit state, the clamp voltage is equal to voltage E. Hence, voltage E is inputted to the non-inverting terminal of second comparator CP2, and a voltage that is higher than voltage Vclamp2and lower than voltage E is inputted to the inverting terminal of comparator CP2. Accordingly, second comparator CP2outputs an H level. This enables detection of the output-to-supply short circuit state.

A problem occurring when the load is in the load open state in the technique described in Patent Document 1, in which bootstrap circuit BS is added, is described with reference toFIG. 6. InFIG. 6, Q1ON/OFF represents an on/off signal of switching element Q1, OUT represents an output voltage, CP1represents an output of first comparator CP1, CP2represents an output of second comparator CP2, and C1irepresents a charge current flowing through capacitor C1.

When the load is in the load open state in period T1, a current flows from bootstrap circuit BS through capacitor C1to a resistor in clamp circuit14. For this reason, the voltage greatly drops because of the resistor in clamp circuit14, and clamp voltage Vclamp, i.e., output voltage OUT rises. At time t14, output voltage OUT equals to second reference voltage V2. Accordingly, second comparator CP2outputs an H level. In other words, false detection of the output-to-supply short circuit state occurs, although the circuit is not in the output-to-supply short circuit state, actually.

In this embodiment, when the first comparator detects the load open state, the clamp voltage rise prevention circuit prevents the clamp voltage of the clamp circuit from rising. In addition, when the first comparator detects the load open state, the second reference voltage is set to a voltage that is higher than the voltage to be generated by the current including the current flowing from the bootstrap circuit through the third terminal and the capacitor to the clamp circuit and that is lower than the input voltage. Accordingly, the above-described embodiment makes it possible to prevent the false detection of the output-to-supply short circuit state, when a bootstrap circuit is connected.