Detailed Description

Overvoltage Monitoring

When operating in overvoltage mode, the MAX6495–

MAX6499 feedback path (Figure 1) consists of IN,

OVSET’s internal comparator, the internal gate charge

pump, and the external n-channel MOSFET, resulting in

a switch-on/off function. When the programmed over-

voltage threshold is tripped, the internal fast compara-

tor turns off the external MOSFET, clamping GATE to

OUTFB within 0.5µs and disconnecting the power

source from the load. When IN decreases below the

adjusted overvoltage threshold, the MAX6495–MAX6499

slowly enhance GATE above OUTFB, reconnecting the

load to the power source.

Overvoltage Limiter

(MAX6495/MAX6496/MAX6499)

When operating in overvoltage-limiter mode, the

MAX6495/MAX6496/MAX6499 feedback path (Figure 2)

consists of OUTFB, OVSET’s internal comparator, the

internal gate charge pump, and the external n-channel

MOSFET, resulting in the external MOSFET operating

as a voltage regulator.

During normal operation, GATE is enhanced 10V above

OUTFB. The external MOSFET source voltage is moni-

tored through a resistive divider between OUTFB and

OVSET. When OUTFB rises above the adjusted over-

voltage threshold, an internal comparator sinks the

charge-pump current, discharging the external GATE,

regulating OUTFB at the OVSET overvoltage threshold.

OUTFB remains active during the overvoltage transients

and the MOSFET continues to conduct during the over-

voltage event, operating in switched-linear mode.

As the transient begins decreasing, OUTFB fall time will

depend on the MOSFET’s GATE charge, the internal

charge-pump current, the output load, and the tank

capacitor at OUTFB.

For fast-rising transients and very large-sized MOSFETs,

add an additional bypass capacitor from GATE to GND to

reduce the effect of the fast-rising voltages at IN. The

external capacitor acts as a voltage-divider working

against the MOSFET’s drain-to-gate capacitance. For a

6000pF gate-to-source capacitance, a 0.1µF capacitor at

Caution must be exercised when operating the

MAX6495/MAX6496/MAX6499 in voltage-limiting mode

the MOSFET’s maximum gate voltage, the MOSFET dis-

sipates power continuously. To prevent damage to the

external MOSFET, proper heatsinking should be imple-

mented.

GATE Voltage

The MAX6495–MAX6499 use a high-efficiency charge

ing the 5V (typ) UVLO threshold, GATE enhances 10V

An overvoltage condition occurs when the voltage at

threshold is crossed, GATE falls to OUTFB within 0.5µs

with a 100mA pulldown current. The MAX6495–MAX6499

include an internal clamp to OUTFB that ensures GATE

is limited to 18V (max) above OUTFB to prevent gate-

to-source damage of the external MOSFET.

72V, Overvoltage-Protection Switches/

Limiter Controllers with an External MOSFET

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7

OVSET

GND

GATE

IN

OUTFB

R1

R2

MAX6495–

MAX6499

Figure 1. Overvoltage Threshold (MAX6495–MAX6499)

OVSET

GND

GATE

IN

OUTFB

R1

R2

MAX6495

MAX6496

MAX6499

Figure 2. Overvoltage-Limiter Protection Switch Configuration