MAX6397/MAX6398
As the transient begins decreasing, OUT fall time will
depend on the MOSFET’s GATE charge, the internal
charge-pump current, the output load, and the tank
capacitor at OUT.
For fast-rising transients and very large-sized MOSFETs,
add an additional external bypass capacitor from GATE
to GND to reduce the effect of the fast-rising voltages at
IN. The external capacitor acts as a voltage-divider
working against the MOSFETs’ drain-to-gate capaci-
tance. For a 6000pF C
gd
, a 0.1µF capacitor at GATE will
reduce the impact of the fast-rising V
IN
input.
Caution must be exercised when operating the
MAX6397/MAX6398 in voltage-limiting mode for long
durations. If the V
IN
is a DC voltage greater than the
MOSFET’s maximum gate voltage, the FET will dissipate
power continuously. To prevent damage to the external
MOSFET, proper heatsinking should be implemented.
Applications Information
Load Dump
Most automotive applications run off a multicell, 12V
lead-acid battery with a nominal voltage that swings
between 9V and 16V (depending on load current,
charging status, temperature, battery age, etc.). The
battery voltage is distributed throughout the automobile
and is locally regulated down to voltages required by
the different system modules. Load dump occurs when
the alternator is charging the battery and the battery
becomes disconnected. Power in the alternator (essen-
tially an inductor) flows into the distributed power sys-
tem and elevates the voltage seen at each module. The
voltage spikes have rise times typically greater than
5ms and decays within several hundred milliseconds
but can extend out to 1s or more depending on the
characteristics of the charging system (Figure 5).
These transients are capable of destroying semicon-
ductors on the first ‘fault event.’
Setting Overvoltage Thresholds
SET provides an accurate means to set the overvoltage
level for the MAX6397/MAX6398. Use a resistor-divider to
set the desired overvoltage condition (Figure 6). SET has
a rising 1.215V threshold with a 5% falling hysteresis.
Begin by selecting the total end-to-end resistance,
R
TOTAL
= R1 + R2. Choose R
TOTAL
to yield a total cur-
rent equivalent to a minimum 100 x I
SET
(SET’s input
bias current) at the desired overvoltage threshold.
For example:
With an overvoltage threshold set to 20V:
R
TOTAL
< 20V/(100 x I
SET
)
where I
SET
is SET’s 50nA input bias current.
R
TOTAL
< 4MΩ
Use the following formula to calculate R2:
where V
TH
is the 1.215V SET rising threshold and V
OV
is the overvoltage threshold.
R2 = 243kΩ, use a 240kΩ standard resistor.
R
TOTAL
= R2 + R1, where R1 = 3.76MΩ.
Use a 3.79MΩ standard resistor.
A lower value for total resistance dissipates more
power but provides slightly better accuracy.
