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MAX6495

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

Maxim Integrated 

MAX6495–MAX6499

MOSFET Selection

Select external MOSFETs according to the application current

level. The MOSFET’s on-resistance (RDS(ON)) should be

chosen low enough to have a minimum voltage drop at

full load to limit the MOSFET power dissipation. Determine

the device power rating to accommodate an overvoltage

fault when operating the MAX6495/MAX6496/MAX6499 in

overvoltage-limit mode.

During normal operation, the external MOSFET dissipates

little power. The power dissipated in the MOSFET during

normal operation is:

P = ILOAD2 x RDS(ON)

where P is the power dissipated in the MOSFET, ILOAD

is the output load current, and RDS(ON) is the drain-to-

source resistance of the MOSFET.

Most power dissipation in the MOSFET occurs during a

prolonged overvoltage event when operating the

MAX6495/MAX6496/MAX6499 in voltage-limiter mode.

The power dissipated across the MOSFET is as follows

(see the Thermal Shutdown in Overvoltage-Limiter Mode

section):

P = VDS x ILOAD

where VDS is the voltage across the MOSFET’s drain

and source.

Thermal Shutdown

The devices’ thermal-shutdown feature turns off GATE

if it exceeds the maximum allowable thermal dissipa-

tion. Thermal shutdown also monitors the PC board

temperature of the external nMOSFET when the devic-

es sit on the same thermal island. Good thermal con-

tact between the MAX6495–MAX6499 and the external

nMOSFET is essential for the thermal-shutdown feature

to operate effectively. Place the nMOSFET as close to

possible to OUTFB.

When the junction temperature exceeds TJ = +160°C, the

thermal sensor signals the shutdown logic, turning off the

GATE output and allowing the device to cool. The thermal

sensor turns the GATE on again after the IC’s junction

temperature cools by 20°C. Thermal-overload protection

is designed to protect the MAX6495–MAX6499 and the

external MOSFET in the event of current-limit fault condi-

tions. For continuous operation, do not exceed the abso-

lute maximum junction temperature rating of TJ = +150°C.

72V, Overvoltage-Protection

Switches/Limiter Controllers

with an External MOSFET

Peak Power-Dissipation Limit

The devices activate an internal 100mA pulldown on

GATE when SHDN goes low, OVSET exceeds its

threshold or UVSET falls below its threshold. Once

the voltage on GATE falls below the OUTFB voltage,

current begins to flow from OUTFB to the 100mA pull-

down through the internal clamp diode, discharging the

output capacitors.

Depending on the output capacitance and the initial volt-

age, a significant amount of energy may be dissipated by

the internal 100mA pulldown. To prevent damage to the

device ensure that for a given overvoltage threshold, the

output capacitance does not exceed the limit provided in

Figure 4. This output capacitance represents the sum of

all capacitors connected to OUTFB, including reservoir

capacitors and DC-DC input filter capacitors.

Thermal Shutdown in Overvoltage-Limiter Mode

When operating the MAX6495/MAX6496/MAX6499 in

overvoltage-limit mode for a prolonged period of time, a

thermal shutdown is possible. The thermal shutdown is

dependent on a number of different factors:

●● The device’s ambient temperature

●● The output capacitor (COUT)

●● The output load current (IOUT)

●● The overvoltage threshold limit (VOV)

●● The overvoltage waveform period (tOV)

●● The power dissipated across the package (PDISS)

100,000

MAXIMUM OUTPUT CAPACITANCE

vs. OVERVOLTAGE THRESHOLD

10,000

1000

SAFE OPERATING AREA

100

10

0 10 20 30 40 50 60 70

OVERVOLTAGE THRESHOLD (V)

Figure 4. Safe Operating Area for 100mA Pulldown

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