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Maxim > Design Support > Technical Documents > Application Notes > Automotive > APP 1050
Maxim > Design Support > Technical Documents > Application Notes > Microprocessor Supervisor Circuits > APP 1050
Keywords: reset circuit, power-up, por, power on reset, reset timeout, sequencer, active-high, push-pull,
SOT23, protection circuit
APPLICATION NOTE 1050
Reset Circuit Ensures Valid Supply Voltage
Apr 11, 2002
Abstract: This article focuses on using an external MOSFET switch to ensure a valid system voltage is
present before applying it to the load. This simple circuit uses a reset circuit and the MOSFET to provide
a protection circuit during power-up.
It is often necessary to prevent a power supply's voltage from being applied to a load until the voltage
has reached a certain minimum value. The circuit of Figure 1, consisting of two compact SOT23
packages, protects the load by blocking the supply voltage (nominally 5V) when it is below a pre-set
threshold. IC1's threshold is 4.63V; four other versions of this IC provide thresholds of 4.38V, 3.08V,
2.93V, and 2.63V.
Figure 1. This protection circuit applies power to the load only when V
CC
is above a pre-set threshold
voltage.
IC1's active-high Reset output holds the p-channel MOSFET off by remaining high during power-up. It
continues to remain high through the end of the reset timeout period (140ms minimum), which is initiated
by IC1 as V
CC
passes through the 4.63V threshold. If V
CC
remains above 4.63V after the timeout has
expired, Reset goes low, turns on Q1, and applies V
CC
to the load. Q1's on-resistance (0.2Ω for V
GS
=
4.5V) can deliver 1A of supply current with a drop of only 200mV.
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