Linear Technology LT4363-1 Demo Board: High Voltage Surge Stopper with Current Limit, Latch-Off Version DC1935A-A DC1935A-A Data Sheet

Product codes

DC1935A-A

LT4363

4363fb

For more information

www.linear.com/LT4363

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 FLT 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 FLT pin will

then return to a high impedance state. For the latch-off

version, LT4363-1, both the GATE and FLT 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 overvolt-

age, such as a load dump in vehicles. The timer period

varies with the voltage across the MOSFET. A higher volt-

age corresponds 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 FLT 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 FLT 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 FLT 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.