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Maxim > Design Support > Technical Documents > Application Notes > Battery Management > APP 2012
Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 2012
Keywords: MOSFET driver, reverse battery protection, mosfet, reverse battery protected, power switch
drivers, diode protection
APPLICATION NOTE 2012
MOSFET Driver Is Reverse-Battery Protected
Jun 20, 2003
Abstract: MOSFET power-switch drivers must be protected from a reverse-battery connection. A small
rectifier diode can protect against reversed batteries, but that approach is generally unacceptable for
systems in which battery life is crucial. For a 6V battery, the diode's forward voltage drop (0.6V to 0.7V
typical) produces a constant power loss of approximately 10%, which increases with dwindling battery
voltage. An improvement over single-diode protection is shown.
MOSFET power-switch drivers must be protected from a reverse-battery connection. A small rectifier
diode can protect against reversed batteries, but that approach is generally unacceptable for systems in
which battery life is crucial. For a 6V battery, the diode's forward voltage drop (0.6V to 0.7V typical)
produces a constant power loss of approximately 10%, which increases with dwindling battery voltage.
Figure 1 illustrates an improvement over single-diode protection. A p-channel MOSFET (Q1) protects
the driver (U1) from reverse-battery insertion. Q1's low on-resistance of 10mΩor less produces a
forward-voltage drop of only a few millivolts (vs. hundreds of millivolts for a diode). Thus, substituting a
MOSFET for the diode provides an immediate improvement in efficiency.
Figure 1. An external p-channel MOSFET (Q1) easily protects this MOSFET driver (U1) against reversed
batteries and reversed load current.
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Connecting a positive battery voltage to the drain of Q1 produces forward bias in its body diode, which
clamps the source voltage one diode drop below the drain voltage. When the source voltage exceeds
Q1's threshold voltage, Q1 turns on. Once the FET is on, the battery is fully connected to the system
and can deliver power to U1 and the load.
An incorrectly inserted battery reverse-biases the body diode. The source remains at ground potential
and the gate is connected to ground, so Q1 remains off, disconnecting the reversed battery from the
system. VDS equals the full negative battery voltage when you insert the battery backwards, and in some
applications Q1 may have to withstand transient levels many times the battery voltage. Q1 must be able
towithstand VGS and VDS voltages at least equal to the maximum battery voltage.
Besides reverse-battery protection, the Figure 1 circuit also protects against reverse load current at
startup. Q3 ensures that no load current flows until U1 enables both n-channel MOSFETs (Q2 and Q3).
Reverse load current can cause Q1 to turn on inadvertently, leaving the circuit unprotected against
reversed-battery connections.
Related Parts
MAX1614
High-Side, n-Channel MOSFET Switch Driver
Free Samples More Information
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APPLICATION NOTE 2012, AN2012, AN 2012, APP2012, Appnote2012, Appnote 2012
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