Reverse current protection circuit for switch circuit

A reverse current protection circuit for a switch circuit includes a reverse current control circuit and an enable/disable circuit coupled to the reverse current control circuit. The reverse current control circuit is coupled to an input terminal and an output terminal of the switch circuit, and disconnects the output terminal of the switch circuit from the input terminal of the switch circuit when an output voltage of the switch circuit is higher than a first predetermined voltage. The enable/disable circuit disables the reverse current control circuit for a first predetermined time period when the output voltage of the switch circuit becomes lower than the first predetermined voltage after being higher than the first predetermined voltage, and enables the reverse current control circuit after the first predetermined time period.

BACKGROUND

The present invention is directed to a reverse current protection circuit and, more particularly, to a reverse current protection circuit capable of protecting a switch circuit from reverse current damage.

Generally, reverse currents are generated in a device when a voltage at an output terminal of the device is higher than a voltage at an input terminal of the device. The reverse currents flow back from the output terminal to the input terminal, which may decrease performance or even damage the device. Therefore, many devices have reverse current protection circuitry.

In a switch circuit, a conventional reverse current protection circuit sets a voltage reference that is equal to the voltage at the input terminal of the switch circuit. When the voltage at the output terminal of the switch circuit is higher than the voltage reference, the conventional reverse current protection circuit generates a RCP (reverse current protection) signal to disconnect the output terminal of the switch circuit from the input terminal of the switch circuit. However, since the voltage reference generally has a certain fluctuation range, reverse currents may have already occurred when the voltage at the output terminal of the switch circuit is higher than the voltage reference. Thus the conventional reverse current protection circuit does not provide the switch circuit with timely and effective protection from reverse currents.

Setting the voltage reference to be lower than the voltage at the input terminal of the switch circuit causes a different problem. When the switch circuit starts up or a load on the switch circuit changes, an overshoot voltage occurs at the output terminal of the switch circuit. Generally, the overshoot voltage is a ripple voltage, and a level of the ripple voltage gradually decreases. When the conventional reverse current protection circuit disconnects the output terminal of the switch circuit from the input terminal of the switch circuit, the overshoot voltage is pulled down to a very low level. When the output voltage is pulled down to be lower than a predetermined level, the input terminal and the output terminal of the switch circuit reconnect. Then, the ripple voltage occurs again at the output terminal of the switch circuit and rapidly rises up to an original level of the overshoot voltage occurring when the switch circuit starts up or the load of the switch circuit changes, and the reverse current protection circuit causes the overshoot voltage to be pulled down to a very low level again. Thus, the conventional reverse current protection circuit causes the output voltage of the switch circuit to be very unstable during start-up and load changes.

Accordingly, it is desired to have a reverse current protection circuit that can both make the output voltage of the switch circuit more stable and protect the switch circuit from reverse current damage during start-up and load changes.

SUMMARY

A reverse current protection circuit and a switch circuit with the reverse current protection circuit are described herein.

The reverse current protection circuit includes a reverse current control circuit that is coupled to an input terminal and an output terminal of the switch circuit. The reverse current control circuit disconnects the output terminal from the input terminal when an output voltage of the switch circuit is higher than a first predetermined voltage.

The reverse current protection circuit also includes an enable/disable circuit that is coupled to the reverse current control circuit. The enable/disable circuit disables the reverse current control circuit for a first predetermined time period when the output voltage of the switch circuit becomes lower than the first predetermined voltage after being higher than the first predetermined voltage, and enables the reverse current control circuit after the first predetermined time period.

In one embodiment, the switch circuit includes the reverse current protection circuit and a voltage regulator circuit. The voltage regulator circuit is coupled between the input and output terminals of the switch circuit, and coupled to the reverse current control circuit. When the output voltage of the switch circuit is equal to or lower than the first predetermined voltage, the voltage regulator regulates the output voltage of the switch circuit to be equal to or lower than a second predetermined voltage.

The above features, and other features and advantages will be readily apparent from the following detailed description when taken in connection with the accompanying drawings.

DETAILED DESCRIPTION

FIG. 1is a schematic block diagram of a switch circuit100in accordance with an exemplary embodiment of the present invention. The switch circuit100includes a voltage regulator circuit102. The voltage regulator circuit102is coupled between an input terminal and an output terminal of the switch circuit100. When an output voltage Vout at the output terminal of the switch circuit100is equal to or lower than a first predetermined voltage Vref_a, the input terminal of the switch circuit100is connected to the output terminal, and the voltage regulator circuit regulates the output voltage Vout to be equal to or lower than a second predetermined voltage Vref_b.

The switch circuit100also includes a reverse current protection circuit104. The reverse current protection circuit104includes a reverse current control circuit106. The reverse current control circuit106is coupled to the input and output terminals of the switch circuit100. When the output voltage Vout is higher than the first predetermined voltage Vref_a, the reverse current control circuit106disconnects the output terminal from the input terminal to prevent a reverse current from flowing into the input terminal and damaging the switch circuit100.

In an exemplary embodiment, the voltage regulator circuit102includes a transistor (not shown inFIG. 1) coupled to the reverse current control circuit106. When the output voltage Vout is higher than the first predetermined voltage Vref_a, the reverse current control circuit106disconnects the output terminal from the input terminal by controlling the transistor in order to protect the switch circuit100from reverse currents.

The reverse current protection circuit104also includes an enable/disable circuit108, which is coupled to the reverse current control circuit106. The enable/disable circuit108disables the reverse current control circuit106for a first predetermined time period t1from when (or after) the output voltage Vout of the switch circuit100becomes lower than the first predetermined voltage Vref_a after being higher than the first predetermined voltage Vref_a, and enables the reverse current control circuit106after the first predetermined time period t1.

The reverse current protection circuit104also may include one or a plurality of voltage pull-down circuits110(two of which are shown inFIG. 1). In an exemplary embodiment including one voltage pull-down circuit110, the voltage pull-down circuit110is coupled to the enable/disable circuit108and the output terminal of the switch circuit100. The voltage pull-down circuit110pulls down the output voltage Vout in order to accelerate a drop of the output voltage Vout when the reverse current control circuit106disconnects the input and output terminals of the switch circuit100, and stops pulling down the output voltage Vout when the first predetermined time period t1ends.

In an exemplary embodiment including a plurality of voltage pull-down circuits110, the plurality of voltage pull-down circuits110are connected in parallel with each other. Each of the voltage pull-down circuits110is coupled to the enable/disable circuit108and the output terminal of the switch circuit100. Each of voltage pull-down circuits110pulls down the output voltage Vout when the reverse current control circuit106disconnects the input and output terminals, and stops pulling down the output voltage Vout at a certain time interval, in sequence, during a second predetermined time period t2after the first predetermined time period t1.

FIG. 2is a schematic block diagram of the enable/disable circuit108of the reverse current protection circuit104in accordance with an exemplary embodiment. The enable/disable circuit108includes an enable/disable signal controller112that is coupled to the reverse current control circuit106. The enable/disable signal controller112outputs a disable control signal when the output voltage Vout of the switch circuit100becomes lower than the first predetermined voltage Vref_a after being higher than the first predetermined voltage Vref_a.

The enable/disable circuit108also includes an enable/disable signal generator114that is coupled to the enable/disable signal controller112and the reverse current control circuit106. In response to the disable control signal received from the enable/disable signal controller112, the enable/disable signal generator114generates an enable/disable (E/D) signal and outputs the E/D signal to the reverse current control circuit106. The enable/disable signal is used for disabling the reverse current control circuit106for the first predetermined time period t1and enabling the reverse current control circuit106after the first predetermined time period t1.

The enable/disable circuit108also includes a voltage pull-down control signal generator116, which is coupled to the reverse current control circuit106, the enable/disable signal generator114, and the voltage pull-down circuit110. The voltage pull-down signal generator116will be discussed in more detail below with reference toFIGS. 3, 5 and 6.

Hereinafter, operation of the switch circuit100ofFIG. 1will be described in detail with reference toFIGS. 3-6.

FIG. 3is a schematic circuit diagram of the switch circuit100ofFIG. 1in accordance with an exemplary embodiment. In the exemplary embodiment, the switch circuit100is a power switch circuit. InFIG. 3, the voltage regulator circuit102is implemented using a conventional low dropout regulator (LDO), which includes a voltage reference118, an amplifier120, a buffer122, a charge pump124, a transistor M1, a capacitor C1and a load R1. The transistor M1may be a power MOST having a drain terminal that can endure a high voltage so that it will not be damaged when the output voltage Vout of the switch circuit100is high.

FIG. 4is a graph illustrating changes of the output voltage Vout of the switch circuit100along with a load current I_load during operation of the voltage regulator circuit102. In other words,FIG. 4shows changes of the output voltage Vout of the switch circuit100along with the load current I_load when the output voltage Vout of the switch circuit100is lower than a first predetermined voltage Vref_a. During a light load current condition, the amplifier120controls an output of the buffer122to regulate a gate voltage of the transistor M1to stabilize the output voltage Vout of the switch circuit100at the second predetermined voltage Vref_b provided by the voltage reference118. During a heavy load current condition, the gate voltage of the transistor M1, controlled by the buffer122, reaches a fixed upper limit to fully turn on the transistor M1and to operate in a linear region, and the load R1draws current from the output terminal of the switch circuit100, thereby causing the output voltage Vout of the switch circuit100to drop to be lower than the second predetermined voltage Vref_b. Further, the charge pump124boosts the buffer122to drive the transistor M1to work in an ultra-low ohmic region to reduce power loss and improve power efficiency during heavy load current condition.

The reverse current control circuit106(FIG. 1) includes a voltage reference126, a comparator128and a transistor M2. The voltage reference126is coupled between the input terminal of the switch circuit100and a first input terminal of the comparator128. A second input terminal of the comparator128is coupled to the output terminal of the switch circuit100, and an output terminal of the comparator128is coupled to a gate terminal of the transistor M2. The comparator128thus receives the regulated input voltage (Vref_a) and the output voltage (Vout) and generates a reverse current protection (RCP) signal. A drain terminal of the transistor M2is coupled to the gate terminal of the transistor M1, and a source terminal of the transistor M2is coupled to ground.

When the output voltage Vout is higher than the first predetermined voltage Vref_a of the voltage reference126, the RCP signal output by the comparator128goes from low to high, as shown inFIG. 5. The RCP signal with high level turns on the transistor M2and the gate voltage of the transistor M1is pulled down, which turns off the transistor M1. In this way, a reverse current is prevented from flowing from the output terminal to the input terminal of the switch circuit100through the transistor M1, thereby protecting the switch circuit100from reverse currents.

As shown inFIG. 5, the output voltage Vout of the switch circuit100drops when the RCP signal is high because the capacitor C1discharges to the load R1and the load R1draws currents from the output terminal of the switch circuit100. After the output voltage Vout becomes lower than the first predetermined voltage Vref_a, the RCP signal goes from high to low, which turns off the transistor M2and the gate voltage of the transistor M1rises. The enable/disable signal controller112detects if the RCP signal becomes low from high. The RCP signal goes from high to low when (or after) the output voltage Vout of the switch circuit100becomes lower than the first predetermined voltage Vref_a after being higher than the first predetermined voltage Vref_a. InFIG. 5, the RCP signal does not go from high to low at the time that the output voltage Vout of the switch circuit100becomes lower than the first predetermined voltage Vref_a because the comparator128has a hysteresis design. If the comparator128did not have a hysteresis design, the RCP signal would go low when the output voltage Vout becomes lower than the first predetermined voltage Vref_a. When the enable/disable signal controller112detects the RCP signal goes from high to low, the enable/disable signal controller112outputs the disable control signal to the enable/disable signal generator114. Then the enable/disable signal generator114outputs to the comparator128the E/D signal, which is high for the first predetermined time period t1and goes low after the first predetermined time period t1. A high E/D signal disables the comparator128. That is to say, the comparator128turns off and maintains the RCP signal at the low level for the first predetermined time period t1. When the first predetermined time period t1ends, the E/D signal goes low and the comparator128is enabled. In other words, if the comparator128does not have a hysteresis design, the enable/disable circuit108disables the reverse current control circuit106for the first predetermined time period t1when the output voltage Vout of the switch circuit100becomes lower than the first predetermined voltage Vref_a after being higher than the first predetermined voltage Vref_a. If the comparator128has a hysteresis design, the enable/disable circuit108disables the reverse current control circuit106for the first predetermined time period t1after the output voltage Vout of the switch circuit100becomes lower than the first predetermined voltage Vref_a after being higher than the first predetermined voltage Vref_a.

In a presently preferred embodiment, the enable/disable circuit108also includes the voltage pull-down control signal generator116, which is coupled to the voltage pull-down circuit110. The voltage pull-down circuit110includes a resistor R2and a transistor M3. The resistor R2is coupled between the output terminal of the switch circuit100and a drain terminal of the transistor M3, a gate terminal of the transistor M3is coupled to the voltage pull-down control signal generator116, and a source terminal of the transistor M3is coupled to ground.

The voltage pull-down control signal generator116outputs the voltage pull-down control (Pull_load) signal to the gate terminal of the transistor M3. When the RCP signal goes high, the Pull_load signal goes from low to high, which turns on the transistor M3and current is pulled from the output terminal of the switch circuit100, thereby pulling the output voltage Vout down. The voltage pull-down control signal generator116keeps the Pull_load signal high until or after the first predetermined time period t1ends.

In another exemplary embodiment shown inFIG. 6, the reverse current protection circuit104includes four voltage pull-down circuits110connected in parallel. In this embodiment, the voltage pull-down control signal generator116outputs four Pull_load signals (Pull_load_0, Pull_load_1, Pull_load_2, Pull_load_3) to these four voltage pull-down circuits110. When the RCP signal goes from low to high, the four Pull_load signals go from low to high. The voltage pull-down control signal generator116causes the four Pull_load signals to go from high to low sequentially at a certain time interval during the second predetermined time period t2after the first predetermined time period t1. In this way, the voltage pull-down circuits110stop pulling down the output voltage Vout at a certain time interval in sequence during the second predetermined time period t2. If all of the voltage pull-down circuits110stop pulling down the output voltage Vout at the end of the first predetermined time period t1, currents flowing through the transistor M1would charge the capacitor C1rapidly, causing a voltage of the capacitor C1to rise rapidly, in turn the output voltage Vout of the switch circuit100could exceed the first predetermined voltage Vref_a due to the rapid rise of the voltage of the capacitor C1. However, this issue is avoided by making a plurality of voltage pull-down circuits110stop pulling down the output voltage Vout at a certain time interval in sequence during the second predetermined time period t2.

The enable/disable signal controller112, enable/disable signal generator114, and the voltage pull-down control signal generator116may be implemented with common digital logical circuits, which are known by those of skill in the art. For example, the voltage pull-down control signal generator116may be implemented with an AND gate.

As shown inFIG. 4, the first predetermined voltage Vref_a is lower than the input voltage Vin of the switch circuit100. The reverse current control circuit106can be tweaked to take action in advance by setting the first predetermined voltage Vref_a to be lower than the input voltage Vin, thereby providing better protection for the switch circuit100against reverse currents. The second predetermined voltage Vref_b is lower than the first predetermined voltage Vref_a, causing the switch circuit100to operate at more securely. In an exemplary embodiment, the first predetermined voltage Vref_a is 20 mV lower than the input voltage Vin, and the second predetermined voltage Vref_b is 70 mV lower than Vin.

In the present invention, during the first predetermined time period t1, since the output voltage Vout of the switch circuit100is a ripple voltage and a level of the ripple voltage gradually decreases, the reverse current control circuit106does not disconnect the output terminal from the input terminal of the switch circuit100even if the output voltage Vout is higher than the first predetermined voltage Vref_a, and the voltage regulator circuit102regulates the output voltage Vout. With respect to the above-mentioned issue of the output voltage Vout being unstable when using the conventional reverse current protection circuit, the present invention provides a stable output voltage Vout during start-up and load changes of the switch circuit, and at the same time, the prevent invention provides timely and effective protection for the switch circuit from reverse currents. The prevent invention provides timely and effective protection for the switch circuit from reverse currents.