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MAX6495
(Rev.:2005)

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

Maxim Integrated 

72V, Overvoltage-Protection Switches/Limiter

Controllers with an External MOSFET

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 volt-

age drop at full load to limit the MOSFET power dissi-

pation. 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 dissi-

pates 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 MAX6495–MAX6499 thermal-shutdown feature

turns off GATE if it exceeds the maximum allowable

thermal dissipation. Thermal shutdown also monitors

the PC board temperature of the external n-channel

MOSFET when the devices sit on the same thermal

island. Good thermal contact between the MAX6495–

MAX6499 and the external n-channel MOSFET is essen-

tial for the thermal-shutdown feature to operate effec-

tively. Place the n-channel MOSFET 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-over-

load protection is designed to protect the MAX6495–

MAX6499 and the external MOSFET in the event of cur-

rent-limit fault conditions. For continuous operation, do

not exceed the absolute maximum junction-tempera-

ture rating of TJ = +150°C.

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)

During an initial overvoltage occurrence, the discharge

time (∆t1) of COUT, caused by IOUT and IGATEPD. The

discharge time is approximately:

∆t1

=

COUT

VOV × 0.95

(IOUT + IGATEPD)

where VOV is the overvoltage threshold, IOUT is the

load current, and IGATEPD is the GATE’s 100mA pull-

down current.

Upon OUT falling below the threshold point, the

MAX6495/MAX6496/MAX6499s’ charge-pump current

must recover and begins recharging the external GATE

voltage. The time needed to recharge GATE from -VD

to the MOSFET’s gate threshold voltage is:

∆t2

=

CISS

VGS(TH) +

IGATE

VD

where CISS is the MOSFET’s input capacitance,

VGS(TH) is the MOSFET’s gate threshold voltage, VD is

the internal clamp (from OUTFB to GATE) diode’s for-

ward voltage and IGATE is the charge-pump current

(100µA typ).

GATE

∆t2

OUTFB

∆t1

∆t3

∆tOV

Figure 4. MAX6495/MAX6496/MAX6499 Timing

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