ORing MOSFET Controller with

Ultra-Fast 200ns Turn-Off

10

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Detailed Description

The MAX5079 ORing MOSFET controller drives an

external n-channel MOSFET and replaces ORing

diodes in high-reliability redundant power-management

systems or multiple paralleled power supplies. The

device has an internal charge pump to drive the high-

side n-channel ORing MOSFET. Additional features

include an adjustable undervoltage lockout threshold

(UVLO), output overvoltage detector (OVI/OVP), input

power-good detector (PGOOD), and two programma-

ble reverse voltage detectors to detect both fast and

slow rises in the reverse voltage across the ORing

MOSFET. The input power-supply range is from 2.75V

to 13.2V or down to 1V when an auxiliary supply of at

least 2.75V is available.

Operational Description

This section describes a detailed startup sequence and

behavior of the MAX5079 under different conditions of

2.75V.

pump delivers 2mA to charge the gate capacitance of

the external MOSFET connected to GATE. The constant

gate-charge current prevents large inrush currents from

the input supply. During turn-on, the MAX5079 will

ignore the reverse voltage at IN with respect to BUS.

This is necessary to avoid the unintentional turn-off of

the ORing MOSFET as the momentary inrush current

with three parallel power supplies (PS1, PS2, and PS3)

and three MAX5079s (U1, U2, and U3) provided by out-

must be carefully considered to ensure proper function-

ality of the MAX5079 ICs.

All three power supplies are turned ON simulta-

threshold, the MAX5079 (U1) charge pump turns

on, and U1 monitors the positive potential from

This results in a less than 10µs turn-on time for the

FDB7045L used in the MAX5079 evaluation kit. The

fast turn-on is needed to assure that N1 is ON

to the shorting of the 0.7V (forward drop) of N1’s

charge the charge-pump capacitor to 5V (or 7V),

soft-start time of the power supply is large enough

current (using the current-sharing circuit) or delay

the loading of the BUS until all three power supplies

are up and running.

old, the MAX5079 (U2) charge pump turns on and

voltage difference becomes positive, GATE (U2)

begins sourcing 2mA into N2’s gate capacitance.

During turn-on, the reverse voltage turn-off circuit

enough not to cause any overshoot before N2

turns on and starts sharing the current.

to equilibrium and sharing equal currents.

2) PS1 and PS2 are on and sharing the load when

PS3 is hot-inserted. PS3 will take the same

course as discussed in 1b above.

The external load-sharing controller circuit of PS1

current sharing.

tive, GATE (U3) begins sourcing 2mA into N3’s

gate capacitance. Again, the reverse voltage turn-

off circuit is disabled momentarily, as discussed

before. The load-sharing circuit of PS3’s controller