Fault Protection
The MAX1777/MAX1977/MAX1999 provide over/undervoltage
fault protection. Drive PRO low to activate fault
protection. Drive PRO high to disable fault protection.
Once activated, the devices continuously monitor for
both undervoltage and overvoltage conditions.
Overvoltage Protection
When the output voltage is 11% above the set voltage,
the overvoltage fault protection activates. The synchronous
rectifier turns on 100% and the high-side MOSFET
turns off. This rapidly discharges the output capacitors,
decreasing the output voltage. The output voltage may
dip below ground. For loads that cannot tolerate a negative
voltage, place a power Schottky diode across the
output to act as a reverse-polarity clamp. In practical
applications, there is a fuse between the power source
(battery) and the external high-side switches. If the
overvoltage condition is caused by a short in the highside
switch, turning the synchronous rectifier on 100%
creates an electrical short between the battery and
GND, blowing the fuse and disconnecting the battery
from the output. Once an overvoltage fault condition is
set, it can only be reset by toggling SHDN, ON_, or
cycling V+ (POR).
Undervoltage Protection
When the output voltage is 30% below the set voltage
for over 22ms (undervoltage shutdown blanking time),
the undervoltage fault protection activates. Both SMPSs
stop switching. The two outputs start to discharge (see
the Discharge Mode (Soft-Stop) section). When the output
voltage drops to 0.3V, the synchronous rectifiers
turn on, clamping the outputs to GND. Toggle SHDN,
ON_, or cycle V+ (POR) to clear the undervoltage fault
latch.
Thermal Protection
The MAX1777/MAX1977/MAX1999 have thermal shutdown
to protect the devices from overheating. Thermal
shutdown occurs when the die temperature exceeds
+160°C. All internal circuitry shuts down during thermal
shutdown. The MAX1777/MAX1977/MAX1999 may trigger
thermal shutdown if LDO_ is not bootstrapped from
OUT_ while applying a high input voltage on V+ and
drawing the maximum current (including short circuit)
from LDO_. Even if LDO_ is bootstrapped from OUT_,
overloading the LDO_ causes large power dissipation
on the bootstrap switches, which may result in thermal
shutdown. Cycling SHDN, ON3, or ON5, or a V+ (POR)
ends the thermal shutdown state.
Discharge Mode (Soft-Stop)
When PRO is low, and a transition to standby or shutdown
mode occurs, or the output undervoltage fault
latch is set, the outputs discharge to GND through an
internal 12Ω switch, until the output voltages decrease
to 0.3V. The reference remains active to provide an
accurate threshold and to provide overvoltage protection.
When both SMPS outputs discharge to 0.3V, the
DL_ synchronous rectifier drivers are forced high. The
synchronous rectifier drivers clamp the SMPS outputs
to GND. When PRO is high, the SMPS outputs do not
discharge, and the DL_ synchronous rectifier drivers
remain low.
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