Datasheet LT4356-1, LT4356-2 (Analog Devices) - 10

LT4356-1/LT4356-2
OPERATION
Some power systems must cope with high voltage surges The potential at the TMR pin starts decreasing as soon of short duration such as those in automobiles. Load as the output voltage is not being servoed, indicating the circuitry must be protected from these transients, yet overvoltage condition has disappeared. When the voltage high availability systems must continue operating during at the TMR pin reaches 0.5V the GATE pin begins rising, these events. turning on the MOSFET. The FLT pin will then go to a high The LT4356 is an overvoltage protection regulator that impedance state. drives an external N-channel MOSFET as the pass transistor. The fault timer allows the load to continue functioning It operates from a wide supply voltage range of 4V to 80V. during short transient events while protecting the MOSFET It can also be pulled below ground potential by up to 60V from being damaged by a long period of supply overvolt- without damage. The low power supply requirement of 4V age, such as a load dump in automobiles. The timer period allows it to operate even during cold cranking conditions varies with the voltage across the MOSFET. A higher voltage in automotive applications. The internal charge pump corresponds to a shorter fault timer period, ensuring the turns on the N-channel MOSFET to supply current to the MOSFET operates within its safe operating area (SOA). loads with very little power loss. Two MOSFETs can be The LT4356 senses an overcurrent condition by monitor- connected back to back to replace an inline Schottky diode ing the voltage across an optional sense resistor placed for reverse input protection. This improves the efficiency between the V and increases the available supply voltage level to the load CC and SNS pins. An active current limit circuit (IA) controls the GATE pin to limit the sense voltage circuitry during cold crank. to 50mV. A current is also generated to start charging up Normally, the pass transistor is fully on, powering the loads the TMR pin. This current is about 5 times the current with very little voltage drop. When the supply voltage surges generated during an overvoltage event. The FLT pin pulls too high, the voltage amplifier (VA) controls the gate of low when the voltage at the TMR pin reaches 1.25V and the MOSFET and regulates the voltage at the source pin the MOSFET is turned off when it reaches 1.35V. to a level that is set by the external resistive divider from An auxiliary amplifier is provided with the negative input the OUT pin to ground and the internal 1.25V reference. A connected to an internal 1.25V reference. The output pull current source starts charging up the capacitor connected down device is capable of sinking up to 2mA of current at the TMR pin to ground. If the voltage at the TMR pin, allowing it to drive an LED or opto coupler. This amplifier VTMR, reaches 1.25V, the FLT pin pulls low to indicate can be configured as a linear regulator controller driving impending turn-off due to the overvoltage condition. The an external PNP transistor or a comparator function to pass transistor stays on until the TMR pin reaches 1.35V, at monitor voltages. which point the GATE pin pulls low turning 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. Rev. C 10 For more information www.analog.com Document Outline Features 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