MAX6397/MAX6398
When OUT exceeds the adjusted overvoltage threshold,
an internal GATE pulldown current is enabled until OUT
drops by 5%. The capacitance at OUT is discharged by
the internal current sink and the external OUT load cur-
rent. The discharge time (∆t1) is approximately:
where V
OV
is the adjusted overvoltage threshold, I
OUT
is the external load current and I
GATEPD
is the GATE’s
internal 100mA (typ) pulldown current.
When OUT falls 5% below the overvoltage threshold
point, the internal current sink is disabled and the
MAX6397/MAX6398’s internal charge pump begins
recharging the external GATE voltage. The OUT volt-
age continues to drop due to the external OUT load
current until the MOSFET gate is recharged. The time
needed to recharge GATE and re-enhance the external
nFET is approximately:
where C
ISS
is the MOSFET’s input capacitance, V
GS(TH)
is the MOSFET’s gate-to-source threshold voltage, V
F
is
the internal clamp diode forward voltage (V
F
= 1.5V typ),
and I
GATE
is the MAX6397/MAX6398 charge-pump cur-
rent (75µA typ).
During ∆t2, C
OUT
loses charge through the output load.
The voltage across C
OUT
(∆V2) decreases until the
MOSFET reaches its V
GS(TH)
threshold and can be
approximated using the following formula:
Once the MOSFET V
GS
(
TH
) is obtained, the slope of the
output voltage rise is determined by the MOSFET Q
G
charge through the internal charge pump with respect
to the drain potential. The time for the OUT voltage to
rise again to the overvoltage threshold can be approxi-
mated using the following formula:
where ∆V
OUT
= ( V
OV
x 0.05) + ∆V2.
The total period of the overvoltage waveform can be
summed up as follows:
t
OVP
= ∆t1 + ∆t2 + ∆t3
The MAX6397/MAX6398 dissipate the most power dur-
ing an overvoltage event when I
OUT
= 0 (C
OUT
is dis-
charged only by the internal current sink). The maximum
power dissipation can be approximated using the follow-
ing equation:
The die temperature (T
J
) increase is related to θ
JC
(8.3°C/W and 8.5°C/W for the MAX6397 and MAX6398,
respectively) of the package when mounted correctly
with a strong thermal contact to the circuit board. The
MAX6397/MAX6398 thermal shutdown is governed by
the equation:
T
J
= T
A
+ P
DISS
x (θ
JC
+ θ
CA
)
< 170°C
(typical thermal-shutdown temperature)
For the MAX6397, the power dissipation of the internal
linear regulator must be added to the overvoltage pro-
tection circuit power dissipation to calculate the die
temperature. The linear regulator power dissipation is
calculated using the following equation:
P
REG
= (V
IN
– V
REG
) (I
REG
)
For example, using an IRFR3410 100V n-channel
MOSFET, Figure 12 illustrates the junction temperature
vs. output capacitor with I
OUT
= 0, T
A
= +125°C,
V
OV
< 16V,V
F
= 1.5V, I
GATE
= 75mA, and I
GATEPD
=
100mA. Figure 12 shows the relationship between output
capacitance versus die temperature for the conditions
listed above.
