LTC2913 datasheet(9/24 Pages) AD

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LT4356-3

Rev D

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Some power systems must cope with high voltage surges

of short duration such as those in automobiles. Load

circuitry must be protected from these transients, yet

high availability systems must continue operating during

these events.

The LT4356-3 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 volt-

age surges too high, the voltage amplifier (VA) controls

the gate of the MOSFET and regulates the voltage at the

source pin to a level that is set by the external resistive

divider from the OUT pin to ground and the internal 1.25V

reference. A current source starts charging up the capaci-

tor connected at the TMR pin to ground. If the voltage at

the TMR pin, VTMR, reaches 1.25V, the FLT pin pulls low

to indicate impending turn-off due to the overvoltage

condition. The pass transistor stays on until the TMR

pin reaches 1.35V, at which point the GATE pin pulls low

turning off the MOSFET. The GATE pin stays latched off

until it is cleared by one of two ways. First, power down

the part for more than 100µs before powering it back up,

or second, pull the SHDN below 0.4V for more than 100µs

then pull SHDN high with a slew rate higher than 10V/ms.

The potential at the TMR pin starts decreasing as soon

as the output voltage is not being servoed, indicating the

overvoltage condition has disappeared, but the GATE

pin remains low even when the voltage at the TMR pin

reaches 0.5V.

The fault timer allows the load to continue functioning

duringshorttransienteventswhileprotectingtheMOSFET

from being damaged by a long period of supply overvolt-

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

varieswiththevoltageacrosstheMOSFET.Ahighervoltage

corresponds to a shorter fault timer period, ensuring the

MOSFET operates within its safe operating area (SOA).

TheLT4356-3sensesanovercurrentconditionbymonitor-

ing the voltage across an optional sense resistor placed

between the VCC and SNS pins. An active current limit

circuit (IA) controls the GATE pin to limit the sense volt-

age to 50mV. A current is also generated to start charging

up the TMR pin. This current is about 5 times the current

generated during an overvoltage event. The FLT pin pulls

low when the voltage at the TMR pin reaches 1.25V and

the MOSFET is turned off when it reaches 1.35V.

An auxiliary amplifier is provided with the negative input

connected to an internal 1.25V reference. The output pull

down device is capable of sinking up to 2mA of current

allowing it to drive an LED or opto coupler. This amplifier

can be configured as a linear regulator controller driving

an external PNP transistor or a comparator function to

monitor voltages.

The SHDN pin turns off the pass transistor and reduces

the supply current to less than 7µA.

OPERATION

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