Datasheet LT4356MP-1, LT4356MP-2 (Analog Devices) - 10

LT4356MP-1/LT4356MP-2
APPLICATIONS INFORMATION
The LT4356 can limit the voltage and current to the load
Overcurrent Fault
circuitry during supply transients or overcurrent events. The LT4356 features an adjustable current limit that pro- The total fault timer period should be set to ride through tects against short circuits or excessive load current. short overvoltage transients while not causing damage During an overcurrent event, the GATE pin is regulated to to the pass transistor. The selection of this N-channel limit the current sense voltage across the V MOSFET pass transistor is critical for this application. CC and SNS pins to 50mV. It must stay on and provide a low impedance path from the input supply to the load during normal operation and An overcurrent fault occurs when the current limit cir- then dissipate power during overvoltage or overcurrent cuitry has been engaged for longer than the time-out delay conditions. set by the timer capacitor. The GATE pin is then immedi- ately pulled low by a 10mA current to GND turning off the The following sections describe the overcurrent and the MOSFET. After the fault condition has disappeared and a overvoltage faults, and the selection of the timer capacitor cooldown period has transpired, the GATE pin is allowed value based on the required warning time. The selection of to pull back up and turn on the pass transistor. the N-channel MOSFET pass transistor is discussed next. Auxiliary amplifier, reverse input, and the shutdown func-
Fault Timer
tions are covered after the MOSFET selection. External component selection is discussed in detail in the Design The LT4356 includes an adjustable fault timer pin. Example section. Connecting a capacitor from the TMR pin to ground sets the delay timer period before the MOSFET is turned off.
Overvoltage Fault
The same capacitor also sets the cooldown period before the MOSFET is allowed to turn back on after the fault The LT4356 limits the voltage at the OUT pin during an condition has disappeared. overvoltage situation. An internal voltage amplifier regu- lates the GATE pin voltage to maintain a 1.25V threshold at Once a fault condition, either overvoltage or overcurrent, the FB pin. During this period of time, the power MOSFET is detected, a current source charges up the TMR pin. is still on and continues to supply current to the load. This The current level varies depending on the voltage drop allows uninterrupted operation during short overvoltage across the drain and source terminals of the power transient events. MOSFET(VDS), which is typically from the VCC pin to the OUT pin. This scheme takes better advantage of the When the voltage regulation loop is engaged for longer available Safe Operating Area (SOA) of the MOSFET than than the time-out period, set by the timer capacitor con- would a fixed timer current. The timer function operates nected from the TMR pin to ground, an overvoltage fault down to V is detected. The GATE pin is pulled down to the OUT pin CC = 5V across the whole temperature range. by a 150mA current. After the fault condition has disap- peared and a cooldown period has transpired, the GATE pin starts to pull high again. This prevents the power MOSFET from being damaged during a long period of overvoltage, such as during load dump in automobiles. 4356mp12fc 10 For more information www.linear.com/LT4356MP-1