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LTC3890 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3890 Datasheet(HTML) 10 Page - Linear Technology |
10 / 24 page LT4363 10 4363fa OPERATION Some power systems must cope with high voltage surges of short duration such as those in vehicles. Load circuitry mustbeprotectedfromthesetransients,yethighavailability systems must continue operating during these events. The LT4363 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,thepasstransistorisfullyon,poweringtheloads withverylittlevoltagedrop.Whenthesupplyvoltagesurges too high, the voltage amplifier (VA) controls the gate of the MOSFET and regulates the voltage at the OUT pin to a level that is set by the external resistive divider from the OUT pin to ground and the internal 1.275V reference. A current source starts charging up the capacitor connected at the TMR pin to ground. If the TMR voltage reaches 1.275V, the FLT pin pulls low to indicate impending turn-off due to the overvoltage condition. The pass transistor stays on until TMR reaches 1.375V, at which point the GATE pin pulls low turning off the MOSFET. A current continues to pull the TMR pin up until it reaches about 4.3V, at which point the current reverses direction and pulls the TMR pin down. For the LT4363-2 version, when the voltage at the TMR pin reaches 0.5V the GATE pin begins rising, turning on the MOSFET. The FLT pin will then return to a high impedance state. For the latch-off version, LT4363-1, both the GATE and FLT pins remain low even after TMR has reached the 0.5V threshold. Allow sufficient time for TMR to discharge to 0.5V and for the MOSFET to cool before attempting to reset the part. To reset, pull the SHDN pin low for at least 100µs, then pull high with a slew rate of at least 10V/ms. The fault timer allows the load to continue functioning duringshorttransienteventswhileprotectingtheMOSFET frombeingdamagedbyalongperiodofsupplyovervoltage, such as a load dump in vehicles. The timer period varies with the voltage across the MOSFET. A higher voltage cor- responds to a shorter fault timer period, helping to keep the MOSFET within its safe operating area (SOA). The LT4363 senses an overcurrent condition by monitor- ing the voltage across an optional sense resistor placed between the SNS and OUT pins. An active current limit circuit (IA) controls the GATE pin to limit the sense volt- age to 50mV, if the OUT pin potential is above 2V. In the case of a severe output short that brings OUT below 2V, the servo sense voltage is reduced to 25mV to reduce the stress on the pass transistor. During current limit, the current charging the TMR capacitor is about 5 times the current during an overvoltage event. The FLT pin pulls low when the TMR voltage reaches 1.275V and the MOSFET is turned off when it reaches 1.375V. The MOSFET turns back on and the FLT pin returns to a high impedance state afterTMRhasreachedthe0.5VthresholdfortheLT4363-2 version.Forthelatch-offversion,LT4363-1,boththeGATE and FLT pins remain low even after TMR has reached the 0.5V threshold. Reset the part in the same way as in overvoltage time-out case. An accurate undervoltage comparator keeps the GATE pin low until the voltage at the UV pin is above the 1.275V threshold. An overvoltage comparator prevents the MOSFET from turning on after fault time-out while the voltage at the OV pin is still above 1.275V for the LT4363-2. The SHDN pin turns off the pass transistor and all the internal circuitry, reducing the supply current to a mere 7µA. |
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Similar Description - LTC3890 |
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