Load-Dump/Reverse-Voltage Protection Circuits
Undervoltage Protection
The MAX16126/MAX16127 monitor the input voltage for
undervoltage conditions. If the input voltage is below the
undervoltage threshold (VIN < VTH - VTH-HYS), GATE
goes low, turning off the external MOSFETs and FLAG
asserts. When the input voltage exceeds the undervolt-
age threshold (VIN > VTH), GATE goes high after a 150Fs
delay (typ).
For the MAX16126/MAX16127, an external resistive
divider connected between TERM, UVSET, and GND
sets the undervoltage threshold (TERM is connected to
IN when SHDN is high).
Thermal Shutdown
The MAX16126/MAX16127 thermal shutdown feature
turns off the MOSFETs if the internal die tempera-
ture exceeds +145NC (TJ). By ensuring good thermal
coupling between the MOSFETs and the MAX16126/
MAX16127, the thermal shutdown can turn off the
MOSFETs if they overheat.
When the junction temperature exceeds TJ = +145NC
(typ), the internal thermal sensor signals the shutdown
logic, pulling the GATE voltage low and allowing the
device to cool. When TJ drops by 15NC (typ), GATE goes
high and the MOSFETs turn back on. Do not exceed the
absolute maximum junction-temperature rating of TJ =
+150NC.
Flag Output (FLAG)
An open-drain FLAG output indicates fault conditions.
During startup, FLAG is initially low and goes high
impedance when VOUT is greater than 90% of VIN if no
fault conditions are present. FLAG asserts low during
shutdown mode, an overvoltage, thermal shutdown, or
undervoltage fault, or when VOUT falls below 90% of VIN.
TERM Connection
The TERM connection has an internal switch to IN. In
shutdown (SHDN = GND), this switch is open. By con-
necting the voltage threshold resistive divider to TERM
instead of directly to IN, power dissipation in the resistive
divider can be eliminated and the shutdown supply cur-
rent reduced.
Reverse-Voltage Protection
The MAX16126/MAX16127 integrate reverse-voltage
protection, preventing damage to the downstream cir-
cuitry caused by battery reversal or negative transients.
The devices can withstand reverse voltage to -30V
without damage to themselves or the load. During a
reverse-voltage condition, the two external n-channel
MOSFETs are turned off, protecting the load. Connect a
0.1µF ceramic capacitor from IN to GND, connect a 10nF
ceramic capacitor from GATE to SRC, connect 10µF from
OUTPUT to GND, and minimize the parasitic capaci-
tance from GATE to GND to have a fast reserve-battery
voltage-transient protection. During normal operation,
both MOSFETs are turned on and have a minimal forward
voltage drop, providing lower power dissipation and a
much lower voltage drop than a reverse-battery protec-
tion diode.
Applications Information
Automotive Electrical Transients
(Load Dump)
Automotive circuits generally require supply voltage
protection from various transient conditions that occur
in automotive systems. Several standards define various
pulses that can occur. Table 1 summarizes the pulses
from the ISO7637-2 specification.
Most of the pulses can be mitigated with capacitors
and zener clamp diodes (see the Typical Operating
Characteristics and also the Increasing the Operating
Voltage Range section). The load dump (pulse 5a and
5b) occurs when the alternator is charging the battery
and a battery terminal gets disconnected. Due to the
sudden change in load, the alternator goes out of regula-
tion and the bus voltage spikes. The pulse has a rise time
of about 10ms and a fall time of about 400ms, but can
extend out to 1s or more depending on the characteris-
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 val-
ues from the ISO7637-2 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 volt-
age sags. Since the MAX16126/MAX16127 can operate
down to 3V, the downstream circuitry can continue to
operate through a cold-crank condition. If desired, the
undervoltage threshold 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.