Datasheet LT4356MP-1, LT4356MP-2 (Analog Devices) - 9

LT4356MP-1/LT4356MP-2
OPERATION
Some power systems must cope with high voltage surges The potential at the TMR pin starts decreasing as soon as of short duration such as those in automobiles. Load cir- the overvoltage condition disappears. When the voltage cuitry must be protected from these transients, yet high at the TMR pin reaches 0.5V the GATE pin begins rising, availability systems must continue operating during these turning on the MOSFET. The FLT pin will then go to a high events. impedance state. The LT4356 is an overvoltage protection regulator that The fault timer al ows the load to continue function- drives an external N-channel MOSFET as the pass transis- ing during short transient events while protecting the tor. It operates from a wide supply voltage range of 4V to MOSFET from being damaged by a long period of supply 80V. It can also be pulled below ground potential by up overvoltage, such as a load dump in automobiles. The to 60V without damage. The low power supply require- timer period varies with the voltage across the MOSFET. A ment of 4V allows it to operate even during cold cranking higher voltage corresponds to a shorter fault timer period, conditions in automotive applications. The internal charge ensuring the MOSFET operates within its safe operating pump turns on the N-channel MOSFET to supply current area (SOA). to the loads with very little power loss. Two MOSFETs can The LT4356 senses an overcurrent condition by monitor- be connected back to back to replace an inline Schottky ing the voltage across an optional sense resistor placed diode for reverse input protection. This improves the effi- between the V ciency and increases the available supply voltage level to CC and SNS pins. An active current limit circuit (IA) controls the GATE pin to limit the sense volt- the load circuitry during cold crank. age to 50mV. A current is also generated to start charging Normally, the pass transistor is fully on, powering the up the TMR pin. This current is about 5 times the current loads with very little voltage drop. When the supply volt- generated during an overvoltage event. The FLT pin pulls age surges too high, the voltage amplifier (VA) controls low when the voltage at the TMR pin reaches 1.25V and the gate of the MOSFET and regulates the voltage at the the MOSFET is turned off when it reaches 1.35V. source pin to a level that is set by the external resistive An auxiliary amplifier is provided with the negative input divider from the OUT pin to ground and the internal 1.25V connected to an internal 1.25V reference. The output pull- reference. A current source starts charging up the capaci- down device is capable of sinking up to 2mA of current tor connected at the TMR pin to ground. If the voltage allowing it to drive an LED or opto coupler. This amplifier at the TMR pin, VTMR, reaches 1.25V, the FLT pin pulls can be configured as a linear regulator controller driving low to indicate impending turn-off due to the overvoltage an external PNP transistor or a comparator function to condition. The pass transistor stays on until the TMR pin monitor voltages. reaches 1.35V, at which point the GATE pin pulls low turn- ing off the MOSFET. A shutdown pin turns off the pass transistor and reduces the supply current to less than 7µA for the LT4356-1. The supply current drops down to 60µA while keeping the internal reference and the auxiliary amplifier active for the LT4356-2 version during shutdown. 4356mp12fc For more information www.linear.com/LT4356MP-1 9