Circuit disconnects load from low-voltage supply
Edited by Bill Travis
Steve Caldwell, Maxim Integrated Products, Chandler, AZ -- EDN, April 24, 2003
Power supplies often include a circuit that disconnects the load when the supply voltage drops too low, such as when a battery is nearly discharged. A p-channel MOSFET connected between supply and load is the typical approach. However, a 1.5V single-cell battery or other low-voltage supply is insufficient to fully turn on the MOSFET. For such low-voltage systems, consider the circuit of Figure 1. A small inverting charge pump, IC1, generates a negative voltage approximately equal to the input supply, VCC. That voltage connects to the ground terminal of a microprocessor supervisor, IC2, which monitors the voltage difference between its own VCC and ground pins. As long as this difference is greater than the supervisor's internal trip-voltage threshold, the reset output voltage assumes the charge-pump output voltage of approximately –2VCC, which provides a gate-source voltage adequate to keep the MOSFET on. When the monitored voltage drops below the threshold of the supervisor, its reset output goes up to VCC and turns off the MOSFET. The supervisor has a threshold of 2.6V. Because the voltage it detects is twice the supply voltage, this circuit disconnects the load when the supply voltage drops below 1.3V, making it suitable for use with a typical 1.5V battery. Other internal threshold voltages are available.
Is this the best Design Idea in this issue? Select at www.edn.com.
