LTC4357



4357fd

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

voltagedropandresultingefficiencyloss.Thisdropreduces

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,

reversecurrenttemporarilyflowsthroughtheMOSFETthat

is on. This current is sourced from any load capacitance

andfromtheothersupplies.TheLTC4357quicklyresponds

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

thus minimizing the disturbance to the output bus.

MOSFET Selection

The LTC4357 drives an N-channel MOSFET to conduct

the load current. The important features of the MOSFET

Gate drive is compatible with 4.5V logic-level MOSFETs

FETs may be used. An internal clamp limits the gate drive

to 15V between the GATE and IN pins. An external Zener

clamp may be added between GATE and IN for MOSFETs

must be higher than the power supply voltage. If an input

is connected to GND, the full supply voltage will appear

across the MOSFET.

ORing Two-Supply Outputs

Where LTC4357s are used to combine the outputs of two

power supplies, the supply with the highest output voltage

sources most or all of the load current. If this supply’s

output is quickly shorted to ground while delivering load

current, the flow of current temporarily reverses and

flows backwards through the LTC4357’s MOSFET. When

the reverse current produces a voltage drop across the

MOSFETofmorethan–25mV,theLTC4357’sfastpull-down

activates and quickly turns off the MOSFET.

If the other, initially lower, supply was not delivering load

current at the time of the fault, the output falls until the

body diode of its ORing MOSFET conducts. Meanwhile,

the LTC4357 charges its MOSFET gate with 20µA until the

forwarddropisreducedto25mV.Ifinsteadthissupplywas

delivering load current at the time of the fault, its associ-

ated ORing MOSFET was already driven at least partially

on, and the LTC4357 will simply drive the MOSFET gate

harder in an effort to maintain a drop of 25mV.

applications inForMation