For more information www.analog.com
Blocking diodes are commonly placed in series with supply
inputs for the purpose of ORing redundant power sources
and protecting against supply reversal. The LTC4359
replaces diodes in these applications with a MOSFET to
reduce both the voltage drop and power loss associated
with a passive solution. The curve shown on page 1 illus-
trates the dramatic improvement in power loss achieved in
a practical application. This represents significant savings
in board area by greatly reducing power dissipation in the
pass device. At low input voltages, the improvement in
forward voltage loss is readily appreciated where head-
room is tight, as shown in Figure2.
The LTC4359 operates from 4V to 80V and withstands
an absolute maximum range of –40V to 100V without
damage. In automotive applications the LTC4359 operates
through load dump, cold crank and two-battery jumps,
and it survives reverse battery connections while also
protecting the load.
A 12V/20A ideal diode application is shown in Figure1.
Several external components are included in addition to
the MOSFET, Q1. Ideal diodes, like their nonideal coun-
terparts, exhibit a behavior known as reverse recovery.
In combination with parasitic or intentionally introduced
inductances, reverse recovery spikes may be generated by
an ideal diode during commutation. D1, D2 and R1 protect
against these spikes which might otherwise exceed the
LTC4359’s –40V to 100V survival rating. COUT also plays
a role in absorbing reverse recovery energy. Spikes and
protection schemes are discussed in detail in the Input
Short-Circuit Faults section.
It is important to note that the SHDN pin, while disabling
the LTC4359 and reducing its current consumption to
9µA, does not disconnect the load from the input since
Q1’s body diode is ever-present. A second MOSFET is
required for load switching applications.
All load current passes through an external MOSFET, Q1.
The important characteristics of the MOSFET are on-
resistance, RDS(ON), the maximum drain-source voltage,
BVDSS, and the gate threshold voltage VGS(TH).
Gate drive is compatible with 4.5V logic-level MOSFETs
over the entire operating range of 4V to 80V. In applications
above 8V, standard 10V threshold MOSFETs may be used.
An internal clamp limits the gate drive to 15V maximum
between the GATE and SOURCE pins. For 24V and higher
applications, an external Zener clamp (D4) must be added
between GATE and SOURCE to not exceed the MOSFET’s
VGS(MAX) during input shorts.
The maximum allowable drain-source voltage, BVDSS, must
be higher than the power supply voltage. If the input is
grounded, the full supply voltage will appear across the
MOSFET. If the input is reversed, and the output is held
up by a charged capacitor, battery or power supply, the
sum of the input and output voltages will appear across
the MOSFET and BVDSS > OUT + |VIN |.
Figure1. 12V/20A Ideal Diode with Reverse Input Protection
Figure2. Forward Voltage Drop Comparison
Between MOSFET and Schottky Diode
00.20.1 0.3 0.4