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MAX6495

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

Maxim Integrated 

MAX6495–MAX6499

During an initial overvoltage occurrence, the discharge

time (Δt1) of COUT, caused by IOUT and IGATEPD. The

discharge time is approximately:

∆t 1

=C OUT

VOV × 0.05

(IOUT + IGATEPD )

where VOV is the overvoltage threshold, IOUT is the load

current, and IGATEPD is the GATE’s 100mA pulldown

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:

∆t 2

= C ISS

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 forward voltage (1.5V,

typ) and IGATE is the charge-pump current (100μA typ).

During Δt2, COUT loses charge through the output load.

The voltage across COUT (ΔV2) decreases until the

MOSFET reaches its VGS(TH) threshold and can be

approximated using the following formula:

∆V 2

= IOUT

∆t 2

C OUT

GATE

t2

OUTFB

t1

t3

tOV

Figure 5. MAX6495/MAX6496/MAX6499 Timing

72V, Overvoltage-Protection

Switches/Limiter Controllers

with an External MOSFET

Once the MOSFET VGS(TH) is obtained, the slope of the

output-voltage rise is determined by the MOSFET Qg

charge through the internal charge pump with respect to

the drain potential. The new rise time needed to reach

a new overvoltage event can be calculated using the

following formula:

∆t 3

≅

Q GD

VGS

∆VOUT

IGATE

where QGD is the gate-to-drain charge.

The total period of the overvoltage waveform can be

summed up as follows:

ΔtOV = Δt1 + Δt2 + Δt3

The MAX6495/MAX6496/MAX6499 dissipate the most

power during an overvoltage event when IOUT = 0. The

maximum power dissipation can be approximated using

the following equation:

PDISS

=V OV

×

0.975

×

I GATEPD

×

∆t1

∆t OV

The die-temperature increase is related to θJC (8.3°C/W

and 8.5°C/W for the MAX6495/MAX6496/MAX6499,

respectively) of the package when mounted correctly

with a strong thermal contact to the circuit board.

The MAX6495/MAX6496/MAX6499 thermal shutdown is

governed by the equation:

TJ = TA + PDISS (θJC +θCA) < +170°C

Based on these calculations, the parameters of the

MOSFET, the overvoltage threshold, the output load

current, and the output capacitors are external variables

affecting the junction temperature. If these parameters

are fixed, the junction temperature can also be affected

by increasing Δt3, which is the time the switch is on. By

increasing the capacitance at the GATE pin, Δt3 increases

as it increases the amount of time required to charge

up this additional capacitance (75μA gate current). As

a result, ΔtOV increases, thereby reducing the power

dissipated (PDISS).

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