LTC4358

6

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OPERATION

High availability systems often employ parallel-connected

power supplies or battery feeds to achieve redundancy

and enhance system reliability. ORing diodes have been

a popular means of connecting these supplies at the point

of load. The disadvantage of this approach is the forward

voltage drop and resulting efficiency loss. This drop reduces

the available supply voltage and dissipates significant

power. Using an N-channel MOSFET to replace a Schottky

diode reduces the power dissipation and eliminates the

need for costly heat sinks or large thermal layouts in high

power applications.

The LTC4358 is a single positive voltage ideal diode con-

troller that drives an internal N-channel MOSFET as a pass

transistor to replace a Schottky diode. The IN and DRAIN

pins form the anode and cathode of the ideal diode. The

input supply is connected to the IN pins, while the DRAIN

pin serves as the output. The OUT pin is connected directly

is derived from the output either directly or through an

RC hold-up circuit.

At power-up, the load current initially flows through the

body diode of the internal MOSFET. The internal MOSFET

turns on and the amplifier tries to regulate the voltage

drop across the IN and OUT connections to 25mV. If

the load current causes more than 25mV of drop, the

MOSFET is driven fully on and the voltage drop is equal to

If the load current is reduced causing the forward drop to

fall below 25mV, the internal MOSFET is driven lower by

a weak pull-down in an attempt to maintain the drop at

25mV. If the load current reverses the MOSFET is turned

off with a strong pull-down.

In the event of a power supply failure, such as if the sup-

ply that is conducting most or all of the current is shorted

to ground, reverse current temporarily flows through the

LTC4358 ideal diode that is on. This current is sourced

from any load capacitance and from the other supplies.

The ideal diode is turned off within 500ns, preventing

reverse current from slewing up to a damaging level and

minimizing any disturbance on the output.