LTC4357

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 LTC4357 controls an external N-channel MOSFET to

form an ideal diode. The voltage across the source and

drain is monitored by the IN and OUT pins, and the GATE

pin drives the MOSFET to control its operation. In effect

the MOSFET source and drain serve as the anode and

cathode of an ideal diode.

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

body diode of the MOSFET. The resulting high forward

voltage is detected at the IN and OUT pins, and the

LTC4357 drives the GATE pin to servo the forward drop

to 25mV. If the load current causes more than 25mV of

voltage drop when the MOSFET gate is driven fully on,

If the load current is reduced causing the forward drop

to fall below 25mV, the MOSFET gate is driven lower by

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

25mV. If the load current reverses and the voltage across

IN to OUT is more negative than –25mV the LTC4357

responds by pulling the MOSFET gate low with a strong

pull-down.

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

of a fully loaded supply is suddenly shorted to ground,

reverse current temporarily flows through the MOSFET that

is on. This current is sourced from any load capacitance

and from the other supplies. The LTC4357 quickly responds

to this condition turning off the MOSFET in about 500ns,

thus minimizing the disturbance to the output bus.