Hi,
we operated several DC/DC converters using the LTC3779 in a step-up/down circuit very close to typical design notes from Analog Devices. Input voltage range is about 20-60V at max. 400W, output ca. 50V. After several months of operation we suddenly found some boards non-functional with an obviously defect ExtVCC input (ca. 20 Ohms to Ground) which also affected other parts in our circuit.
In our circuit the ExtVCC input is normally fed with a stablilized 12V source from another board (powered by a separate large battery), any high overvoltage or reverse voltage at this pin can be excluded.
Any idea what could have happened here? Perhaps an internal defect caused by permanent too high temperature of the chip? What would happen if the gate driver currents supplied through the EXTVCC pin exceed certain limits? Is there any internal protection?