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LTC3789 failures

We are using the LTC3789 alonside the LTC4000 in an arrangment which is identical to the DC1721A-A Demo board (schematic and layout) for a LiFe battery charger and portable radio PSU application. Unfortunately we have had half a dozen failures of the LTC3789, causing product failure in the field. The 3789 device fails without warning and ends up running very hot, The VCCINT regulator output has been seen at 9V+ on some failed boards, so it looks like the internal 5V5 regulators are cooked. We don't know if the failure coincides with a user operation, e.g. connecting/disconnecting Vin or the load. The input voltage is 12V nominal, and we have plentry of extra protection (surge and polarity) on the board ahead of the LTC3789. The 12V is supplied from a good quality AC-DC adapter block spec'd for an 11.5A output. Our maximum load is approx 6A at 12V, and we sometimes run the load with the 12V input supply connected to the board, regardless of battery charge state.

if anyone can give me some pointers as to what could be causing the failures than it would be much appreciated.