DESCRIPTION

The LT4356 surge stopper protects loads from high voltage transients. It regulates the output during an overvoltage event, such as load dump in automobiles, by controlling the gate of an external N-channel MOSFET. The output is limited to a safe value thereby allowing the loads to continue functioning. The LT4356 also monitors the voltage drop between the VCC and SNS pins to protect against overcurrent faults. An internal amplifier limits the current sense voltage to 50mV. In either fault condition, a timer is started inversely proportional to MOSFET stress. If the timer expires, the FLT pin pulls low to warn of an impending power down. If the condition persists, the MOSFET is turned off. After a cool down period, the GATE pin pulls up turning on the MOSFET again. The auxiliary amplifier may be used as a voltage detection comparator or as a linear regulator controller driving an external PNP pass transistor. Back-to-back FETs can be used in lieu of a Schottky diode for reverse input protection, reducing voltage drop and power loss. A shutdown pin reduces the quiescent current to less than 7µA for the LT4356-1 during shutdown. The LT4356-2 differs from the LT4356-1 during shutdown by reducing the quiescent current to 60µA and keeping alive the auxiliary amplifier for uses such as an undervoltage lockout or always-on regulator.

 

FEATURES

Stops High Voltage Surges

Adjustable Output Clamp Voltage

Overcurrent Protection

Wide Operation Range: 4V to 80V

Reverse Input Protection to –60V

Low 7µA Shutdown Current (LT4356-1)

Auxiliary Amplifier for Level Detection Comparator or Linear Regulator Controller (LT4356-2)

Adjustable Fault Timer

Controls N-channel MOSFET

Shutdown Pin Withstands –60V to 100V

Fault Output Indication

Guaranteed Operation to 125°C

Available in (4mm × 3mm) 12-Pin DFN, 10-Pin MSOP or 16-Pin SO Packages

 

APPLICATIONS

Automotive/Avionic Surge Protection

Hot Swap/Live Insertion

High Side Switch for Battery Powered Systems

Intrinsic Safety Applications

 

APPLICATIONS INFORMATION

The LT4356 can limit the voltage and current to the load circuitry during supply transients or overcurrent events. The total fault timer period should be set to ride through short overvoltage transients while not causing damage to the pass transistor. The selection of this N-channel MOSFET pass transistor is critical for this application. It must stay on and provide a low impedance path from the input supply to the load during normal operation and then dissipate power during overvoltage or overcurrent conditions. The following sections describe the overcurrent and the overvoltage faults, and the selection of the timer capacitor value based on the required warning time. The selection of the N-channel MOSFET pass transistor is discussed next. Auxiliary amplifier, reverse input, and the shutdown functions are covered after the MOSFET selection. External component selection is discussed in detail in the Design Example section.

Overvoltage Fault

The LTC4356 limits the voltage at the OUT pin during an overvoltage situation. An internal voltage amplifier regulates the GATE pin voltage to maintain a 1.25V threshold at the FB pin. During this period of time, the power MOSFET is still on and continues to supply current to the load. This allows uninterrupted operation during short overvoltage transient events. When the voltage regulation loop is engaged for longer than the time-out period, set by the timer capacitor connected from the TMR pin to ground, an overvoltage fault is detected. The GATE pin is pulled down to the OUT pin by a 150mA current. After the fault condition has disappeared and a cool down 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.

Overcurrent Fault

The LT4356 features an adjustable current limit that protects against short circuits or excessive load current. During an overcurrent event, the GATE pin is regulated to limit the current sense voltage across the VCC and SNS pins to 50mV. An overcurrent fault occurs when the current limit circuitry has been engaged for longer than the time-out delay set by the timer capacitor. The GATE pin is then immediately pulled low by a 10mA current to GND turning off the MOSFET. After the fault condition has disappeared and a cool down period has transpired, the GATE pin is allowed to pull back up and turn on the pass transistor.