Datasheet LT4356-3 (Analog Devices) - 9

LT4356-3
OPERATION
Some power systems must cope with high voltage surges the part for more than 100µs before powering it back up, of short duration such as those in automobiles. Load or second, pull the SHDN below 0.4V for more than 100µs circuitry must be protected from these transients, yet then pull SHDN high with a slew rate higher than 10V/ms. high availability systems must continue operating during The potential at the TMR pin starts decreasing as soon these events. as the output voltage is not being servoed, indicating the The LT4356-3 is an overvoltage protection regulator that overvoltage condition has disappeared, but the GATE drives an external N-channel MOSFET as the pass transis- pin remains low even when the voltage at the TMR pin tor. It operates from a wide supply voltage range of 4V to reaches 0.5V. 80V. It can also be pulled below ground potential by up The fault timer allows the load to continue functioning to 60V without damage. The low power supply require- during short transient events while protecting the MOSFET ment of 4V allows it to operate even during cold cranking from being damaged by a long period of supply overvolt- conditions in automotive applications. The internal charge age, such as a load dump in automobiles. The timer period pump turns on the N-channel MOSFET to supply current varies with the voltage across the MOSFET. A higher voltage to the loads with very little power loss. Two MOSFETs can corresponds to a shorter fault timer period, ensuring the be connected back to back to replace an inline Schottky MOSFET operates within its safe operating area (SOA). diode for reverse input protection. This improves the ef- ficiency and increases the available supply voltage level The LT4356-3 senses an overcurrent condition by monitor- to the load circuitry during cold crank. ing the voltage across an optional sense resistor placed between the V Normally, the pass transistor is fully on, powering the CC and SNS pins. An active current limit circuit (IA) controls the GATE pin to limit the sense volt- loads with very little voltage drop. When the supply volt- age to 50mV. A current is also generated to start charging age surges too high, the voltage amplifier (VA) controls up the TMR pin. This current is about 5 times the current the gate of the MOSFET and regulates the voltage at the generated during an overvoltage event. The FLT pin pulls source pin to a level that is set by the external resistive low when the voltage at the TMR pin reaches 1.25V and divider from the OUT pin to ground and the internal 1.25V the MOSFET is turned off when it reaches 1.35V. reference. A current source starts charging up the capaci- tor connected at the TMR pin to ground. If the voltage at An auxiliary amplifier is provided with the negative input the TMR pin, VTMR, reaches 1.25V, the FLT pin pulls low connected to an internal 1.25V reference. The output pull to indicate impending turn-off due to the overvoltage down device is capable of sinking up to 2mA of current condition. The pass transistor stays on until the TMR allowing it to drive an LED or opto coupler. This amplifier pin reaches 1.35V, at which point the GATE pin pulls low can be configured as a linear regulator controller driving turning off the MOSFET. The GATE pin stays latched off an external PNP transistor or a comparator function to until it is cleared by one of two ways. First, power down monitor voltages. The SHDN pin turns off the pass transistor and reduces the supply current to less than 7µA. Rev D For more information www.analog.com 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Revision History Typical Application .18702 Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related parts