MAX16128 Datasheet, PDF (10/15 Pages) Maxim Integrated Products – Load-Dump/Reverse-Voltage Protection Circuits

MAX16128/MAX16129
Load-Dump/Reverse-Voltage Protection Circuits
Table 1. Summary of ISO 7637-2 Pulses
NAME
DESCRIPTION
Pulse 1
Pulse 2a
Pulse 3a
Pulse 3b
Inductive load disconnection
Inductive wiring disconnection
Switching transients
Pulse 4
Cold crank
Pulse 5a
Load dump (unsuppressed)
Pulse 5b
Load dump (suppressed)
*Relative to system voltage
PEAK VOLTAGE (V) (max)^*
12V SYSTEM
-100
50
-150
100
-7
-6
87
(Varies, but less than pulse 5a)
DURATION
1 to 2ms
0.05ms
0.2Fs
100ms (initial)
Up to 20s
400ms (single)
tics of the charging system. The magnitude of the pulse
depends on the bus voltage and whether the system is
unsuppressed or uses central load-dump suppression
(generally implemented using very large clamp diodes
built into the alternator). Table 1 lists the worst-case
values from the ISO 7637-2 specification.
Cold crank (pulse 4) occurs when activating the starter
motor in cold weather with a marginal battery. Due to the
large load imposed by the starter motor, the bus voltage
sags. Since the devices can operate down to 3V, the
downstream circuitry can continue to operate through a
cold-crank condition. If desired, the undervoltage thresh-
old can be increased so that the MOSFETs turn off during
a cold crank, disconnecting the downstream circuitry. An
output reservoir capacitor can be connected from OUT
to GND to provide energy to the circuit during the cold-
crank condition.
Refer to the ISO 7637-2 specification for details on pulse
waveforms, test conditions, and test fixtures.
MOSFET Selection
MOSFET selection is critical to design a proper protec-
tion circuit. Several factors must be taken into account:
the gate capacitance, the drain-to-source voltage rating,
the on-resistance (R_DS(ON)), the peak power-dissipation
capability, and the average power-dissipation limit. In
general, both MOSFETs should have the same part num-
ber. For size-constrained applications, a dual MOSFET
can save board area. Select the drain-to-source voltage
so that the MOSFETs can handle the highest voltage that
might be applied to the circuit. Gate capacitance is not
as critical but it does determine the maximum turn-on
and turn-off time. MOSFETs with more gate capacitance
tend to respond more slowly.
MOSFET Power Dissipation
The R_DS(ON) must be low enough to limit the MOSFET
power dissipation during normal operation. Power
dissipation (per MOSFET) during normal operation can
be calculated using this formula:
P = I_LOAD² 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
Maxim Integrated
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