72V, Overvoltage-Protection Switches/Limiter
Controllers with an External MOSFET
During ∆t2, COUT loses charge through the output load.
The voltage across COUT (∆V2) decreases until the
MOSFET reaches its VGS(TH) threshold and can be
approximated using the following formula:
∆V2
= IOUT
∆t2
COUT
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:
∆t3
≅
QGD
VGS
∆VOUT
IGATE
where QGD is the gate-to-drain charge.
The total period of the overvoltage waveform can be
summed up as follows:
tOV = ∆t1 + ∆t2 + ∆t3
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
=
VOV ×
0.975 × IGATEPD ×
∆t1
∆tOV
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 x θJC < +170°C
12V IN
GATE
IN
OUTFB
MAX6495
SHDN
OVSET
GND
DC-DC
CONVERTER
IN
OUT
GND
Typical Application Circuits
DC-DC
CONVERTER
IN
OUT
GND
GATE
12V
IN
OUTFB
MAX6496
SHDN
GATEP
OVSET
GND
Figure 5. Overvoltage Limiter (MAX6495)
Figure 6. Overvoltage Limiter with Low-Voltage-Drop Reverse-
Protection Circuit (MAX6496)
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