Datasheet

MAX16126/MAX16127
Load-Dump/Reverse-Voltage Protection Circuits
13Maxim Integrated
MOSFET Power Dissipation
The R
DS(ON)
must be low enough to limit the MOSFET
power dissipation during normal operation. Power dis-
sipation (per MOSFET) during normal operation can be
calculated using this formula:
P = I
LOAD
2
x R
DS(ON)
where P is the power dissipated in each MOSFET and
I
LOAD
is the average load current.
During a fault condition in switch mode, the MOSFETs
turn off and do not dissipate power. Limiter mode impos-
es the worst-case power dissipation. The average power
can be computed using the following formula:
P = I
LOAD
x (V
IN
- V
OUT
)
where P is the average power dissipated in both
MOSFETs, I
LOAD
is the average load current, V
IN
is the
input voltage, and V
OUT
is the average limited voltage
on the output. In limiter mode, the output voltage is a
sawtooth wave with characteristics determined by the
R
DS(ON)
of the MOSFETs, the output load current, the
output capacitance, the gate charge of the MOSFETs,
and the GATE charge-pump current.
Since limiter mode can involve high switching currents
when the GATE is turning on at the start of a limiting cycle
(especially when the output capacitance is high), it is
important to ensure the circuit does not violate the peak
power rating of the MOSFETs. Check the pulse power
ratings in the MOSFET data sheet.
MOSFET Gate Protection
To protect the gate of the MOSFETs, connect a zener
clamp diode from the gate to the source. The cathode
connects to the gate, and the anode connects to the
source. Choose the zener clamp voltage to be above 10V
and below the MOSFET V
GS
maximum rating.
Increasing the
Input Voltage Protection Range
The MAX16126/MAX16127 can tolerate -36V to +90V.
To increase the positive input voltage range protection,
connect two back-to-back zener diodes from IN to sys-
tem ground, and connect a resistor in series with IN and
the power-supply input to limit the current drawn by the
zener diodes (see Figure 3).
Zener diode D1 clamps positive voltage excursions and
D2 clamps negative voltage excursions. Set the zener
voltages so the worst-case voltages do not exceed the
ratings of the part. Also ensure that the zener diode
power ratings are not exceeded. The combination of
the series resistor and the zener diodes also help snub
pulses on the supply voltage input and can aid in clamp-
ing the low-energy ISO7637-2 pulses.
Figure 2. Overvoltage and Undervoltage Limiter Protection Configuration (MAX16127)
GND
GATE
V
IN
100kI
SRC
IN OUT
DC-DC
CONVERTER
100I
SHDN
IN
OUT
TERM
OVSET
UVSET
GND
R3
R4
R1
R2
MAX16127
10nF
0.1µF
FLAG
10µF