Datasheet LT4363 (Analog Devices) - 10

LT4363
operaTion
Some power systems must cope with high voltage surges sufficient time for TMR to discharge to 0.5V and for the of short duration such as those in vehicles. Load circuitry MOSFET to cool before attempting to reset the part. To must be protected from these transients, yet high availability reset, pull the SHDN pin low for at least 100µs, then pull systems must continue operating during these events. high with a slew rate of at least 10V/ms. The LT4363 is an overvoltage protection regulator that The fault timer allows the load to continue functioning drives an external N-channel MOSFET as the pass transis- during short transient events while protecting the MOSFET tor. It operates from a wide supply voltage range of 4V to from being damaged by a long period of supply overvolt- 80V. It can also be pulled below ground potential by up age, such as a load dump in vehicles. The timer period to 60V without damage. The low power supply require- varies with the voltage across the MOSFET. A higher volt- ment of 4V allows it to operate even during cold cranking age corresponds to a shorter fault timer period, helping conditions in automotive applications. The internal charge to keep the MOSFET within its safe operating area (SOA). pump turns on the N-channel MOSFET to supply current The LT4363 senses an overcurrent condition by monitor- to the loads with very little power loss. Two MOSFETs can ing the voltage across an optional sense resistor placed be connected back to back to replace an inline Schottky between the SNS and OUT pins. An active current limit diode for reverse input protection. This improves the ef- circuit (IA) controls the GATE pin to limit the sense volt- ficiency and increases the available supply voltage level age to 50mV, if the OUT pin potential is above 2V. In the to the load circuitry during cold crank. case of a severe output short that brings OUT below 2V, Normally, the pass transistor is fully on, powering the loads the servo sense voltage is reduced to 25mV to reduce with very little voltage drop. When the supply voltage surges the stress on the pass transistor. During current limit, the too high, the voltage amplifier (VA) controls the gate of the current charging the TMR capacitor is about 5 times the MOSFET and regulates the voltage at the OUT pin to a level current during an overvoltage event. The FLT pin pulls low that is set by the external resistive divider from the OUT when the TMR voltage reaches 1.275V and the MOSFET pin to ground and the internal 1.275V reference. A current is turned off when it reaches 1.375V. The MOSFET turns source starts charging up the capacitor connected at the back on and the FLT pin returns to a high impedance state TMR pin to ground. If the TMR voltage reaches 1.275V, after TMR has reached the 0.5V threshold for the LT4363-2 the FLT pin pulls low to indicate impending turn-off due version. For the latch-off version, LT4363-1, both the GATE to the overvoltage condition. The pass transistor stays on and FLT pins remain low even after TMR has reached until TMR reaches 1.375V, at which point the GATE pin the 0.5V threshold. Reset the part in the same way as in pulls low turning off the MOSFET. overvoltage time-out case. A current continues to pull the TMR pin up until it reaches An accurate undervoltage comparator keeps the GATE about 4.3V, at which point the current reverses direction pin low until the voltage at the UV pin is above the and pulls the TMR pin down. For the LT4363-2 version, 1.275V threshold. An overvoltage comparator prevents when the voltage at the TMR pin reaches 0.5V the GATE the MOSFET from turning on after fault time-out while pin begins rising, turning on the MOSFET. The FLT pin will the voltage at the OV pin is still above 1.275V for the then return to a high impedance state. For the latch-off LT4363-2. The SHDN pin turns off the pass transistor version, LT4363-1, both the GATE and FLT pins remain and all the internal circuitry, reducing the supply current low even after TMR has reached the 0.5V threshold. Allow to a mere 7µA. 4363fb 10 For more information www.linear.com/LT4363 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts