MAX6495 データシートの表示（PDF） - Maxim Integrated

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MAX6495

72V, Overvoltage-Protection Switches/Limiter Controllers with an External MOSFET

Maxim Integrated 

MAX6495–MAX6499

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 nMOSFET, resulting in a switch-

on/off function. When the programmed overvoltage

threshold is tripped, the internal fast comparator 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 monitored

through a resistive divider between OUTFB and OVSET.

When OUTFB rises above the adjusted overvoltage

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 overvoltage

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

GATE will reduce the impact of the fast-rising VIN input.

Caution must be exercised when operating the MAX6495/

MAX6496/MAX6499 in voltage-limiting mode for long

durations. If the VIN is a DC voltage greater than the

MOSFET’s maximum gate voltage, the MOSFET dissi-

pates power continuously. To prevent damage to the exter-

nal MOSFET, proper heatsinking should be implemented.

72V, Overvoltage-Protection

Switches/Limiter Controllers

with an External MOSFET

VIN

VOUT

GATE

IN

OUTFB

R1

MAX6495–

MAX6499

OVSET

R2

GND

Figure 1. Overvoltage Threshold (MAX6495–MAX6499)

VIN

GATE

IN

OUTFB

MAX6495

MAX6496

MAX6499

OVSET

GND

VOUT

COUT

R1

R2

Figure 2. Overvoltage-Limiter Protection Switch Configuration

GATE Voltage

The MAX6495–MAX6499 use a high-efficiency charge

pump to generate the GATE voltage. Upon VIN exceed-

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

above VIN (for VIN ≥ 14V) with a 100μA pullup current. An

overvoltage condition occurs when the voltage at OVSET

goes above its VTH+ threshold. When the 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 dam-

age of the external MOSFET.

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