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Maxim > Design Support > Technical Documents > Application Notes > Battery Management > APP 2237
Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 2237
Keywords: low voltage, battery disconnect, load disconnect, dc-dc converter, over voltage, dc to dc
converters, overvoltage, voltage monitor, microprocessor supervisor
APPLICATION NOTE 2237
Simple Circuit Disconnects Load from Low-Voltage
Supply
Sep 22, 2003
Abstract: The following application note demonstrates a simple circuit using a charge pump and a
microprocessor supervisor to disconnect a low voltage supply from a load. The circuit uses a P-channel
MOSFET to disconnect the load from a 1.5V supply.
Power supplies often include a circuit that disconnects the load when the supply voltage drops too low
(when a battery is nearly discharged, for example). 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 not
sufficient to fully turn on the MOSFET. For such low-voltage systems, consider the circuit of Figure 1.
Figure 1. This low-voltage circuit disconnects the load when the supply voltage drops below a
predetermined threshold (1.3V in this case).
A small inverting charge pump (IC1) generates a negative voltage approximately equal to the input
supply (V
CC
). That voltage connects to the ground terminal of a microprocessor supervisor (IC2), which
monitors the voltage difference between its own V
CC
and GND pins. As long as this difference is greater
than the supervisor's internal trip-voltage threshold, the RESET output voltage is driven to the charge-
pump output (approximately -2V
CC
), 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 V
CC
and turns the MOSFET off.
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