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LT4356CMS-1-TR Datasheet(PDF) 8 Page - Linear Technology |
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LT4356CMS-1-TR Datasheet(HTML) 8 Page - Linear Technology |
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8 / 20 page  LT4356-1 8 43561fd Some power systems must cope with high voltage surges of short duration such as those in automobiles. Load circuitry must be protected from these transients, yet high availability systems must continue operating during these events. The LT4356-1 is an overvoltage protection regulator that drives an external N-channel MOSFET as the pass transis- tor. It operates from a wide supply voltage range of 4V to 80V. It can also be pulled below ground potential by up to 60V without damage. The low power supply require- ment of 4V allows it to operate even during cold cranking conditions in automotive applications. The internal charge pump turns on the N-channel MOSFET to supply current to the loads with very little power loss. Two MOSFETs can be connected back to back to replace an inline Schottky diode for reverse input protection. This improves the ef- ficiency and increases the available supply voltage level to the load circuitry during cold crank. Normally, the pass transistor is fully on, powering the loads with very little voltage drop. When the supply volt- age surges too high, the voltage amplifier (VA) controls the gate of the MOSFET and regulates the voltage at the source pin to a level that is set by the external resistor divider from the OUT pin to ground and the internal 1.25V reference. A current source starts charging up the capaci- tor connected at the TMR pin to ground. If the voltage at the TMR pin, VTMR, reaches 1.25V, the FLT pin pulls low to indicate impending turn-off due to the overvoltage condition. The pass transistor stays on until the TMR pin reaches 1.35V, at which point the GATE pin pulls low turning off the MOSFET. The potential at the TMR pin starts decreasing as soon as the overvoltage condition disappears. When the voltage at the TMR pin reaches 0.5V the GATE pin begins rising, turning on the MOSFET. The FLT pin will then go to a high impedance state. The fault timer allows the loads to continue functioning during short transient events while protecting the MOSFET from being damaged by a long period of supply overvoltage, such as a load dump in automobiles. The timer period var- ies with the voltage across the MOSFET. A higher voltage corresponds to a shorter fault timer period, ensuring the MOSFET operates within its safe operating area (SOA). The LT4356-1 senses an overcurrent condition by monitor- ing the voltage across an optional sense resistor placed between the VCC and SNS pins. An active current limit circuit (IA) controls the GATE pin to limit the sense volt- age to 50mV. A current is also generated to start charging up the TMR pin. This current is about 5 times the current generated during an overvoltage event. The FLT pin pulls low when the voltage at the TMR pin reaches 1.25V and the MOSFET is turned off when it reaches 1.35V. A spare amplifier (SA) is provided with the negative input connected to an internal 1.25V reference. The output pull down device is capable of sinking up to 2mA of current allowing it to drive an LED or opto coupler. This amplifier can be configured as a linear regulator controller driving an external PNP transistor or a comparator function to monitor voltages. A shutdown pin turns off the pass transistor and reduces the supply current to less than 7μA. OPERATION |
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