Datasheet

LT4363
10
4363fa
OPERATION
Some power systems must cope with high voltage surges
of short duration such as those in vehicles. Load circuitry
must be protected from these transients, yet high availability
systems must continue operating during these events.
The LT4363 is an overvoltage protection regulator that
drives an external N-channel MOSFET as the pass transis-
tor. It operates from a wide supply voltage range of 4V to
80V. It can also be pulled below ground potential by up
to 60V without damage. The low power supply require-
ment of 4V allows it to operate even during cold cranking
conditions in automotive applications. The internal charge
pump turns on the N-channel MOSFET to supply current
to the loads with very little power loss. Two MOSFETs can
be connected back to back to replace an inline Schottky
diode for reverse input protection. This improves the ef-
ficiency and increases the available supply voltage level
to the load circuitry during cold crank.
Normally, the pass transistor is fully on, powering the loads
with very little voltage drop. When the supply voltage surges
too high, the voltage amplifier (VA) controls the gate of the
MOSFET and regulates the voltage at the OUT pin to a level
that is set by the external resistive divider from the OUT
pin to ground and the internal 1.275V reference. A current
source starts charging up the capacitor connected at the
TMR pin to ground. If the TMR voltage reaches 1.275V,
the F LT pin pulls low to indicate impending turn-off due
to the overvoltage condition. The pass transistor stays on
until TMR reaches 1.375V, at which point the GATE pin
pulls low turning off the MOSFET.
A current continues to pull the TMR pin up until it reaches
about 4.3V, at which point the current reverses direction
and pulls the TMR pin down. For the LT4363-2 version,
when the voltage at the TMR pin reaches 0.5V the GATE
pin begins rising, turning on the MOSFET. The F LT pin will
then return to a high impedance state. For the latch-off
version, LT4363-1, both the GATE and F LT pins remain
low even after TMR has reached the 0.5V threshold. Allow
sufficient time for TMR to discharge to 0.5V and for the
MOSFET to cool before attempting to reset the part. To
reset, pull the SHDN pin low for at least 100µs, then pull
high with a slew rate of at least 10V/ms.
The fault timer allows the load to continue functioning
during short transient events while protecting the MOSFET
from being damaged by a long period of supply overvoltage,
such as a load dump in vehicles. The timer period varies
with the voltage across the MOSFET. A higher voltage cor-
responds to a shorter fault timer period, helping to keep
the MOSFET within its safe operating area (SOA).
The LT4363 senses an overcurrent condition by monitor-
ing the voltage across an optional sense resistor placed
between the SNS and OUT pins. An active current limit
circuit (IA) controls the GATE pin to limit the sense volt-
age to 50mV, if the OUT pin potential is above 2V. In the
case of a severe output short that brings OUT below 2V,
the servo sense voltage is reduced to 25mV to reduce
the stress on the pass transistor. During current limit, the
current charging the TMR capacitor is about 5 times the
current during an overvoltage event. The F LT pin pulls low
when the TMR voltage reaches 1.275V and the MOSFET
is turned off when it reaches 1.375V. The MOSFET turns
back on and the F LT pin returns to a high impedance state
after TMR has reached the 0.5V threshold for the LT4363-2
version. For the latch-off version, LT4363-1, both the GATE
and F LT pins remain low even after TMR has reached
the 0.5V threshold. Reset the part in the same way as in
overvoltage time-out case.
An accurate undervoltage comparator keeps the GATE
pin low until the voltage at the UV pin is above the
1.275V threshold. An overvoltage comparator prevents
the MOSFET from turning on after fault time-out while
the voltage at the OV pin is still above 1.275V for the
LT4363-2. The SHDN pin turns off the pass transistor
and all the internal circuitry, reducing the supply current
to a mere 7µA.