LTC3649: part failing when inductive load switched in and out: which pins to check and then protect?

Dear Power by Linear,

     I've designed the LTC3649 into a medical device whose tip is connected by a cable that could be two meters long (by my estimate - my customer cannot be more exact.) A footswitch enables a relay which connects or disconnects the load, and the load itself is fairly inductive. The output voltage is user adjustable, from near-zero volts to a maximum of 13.0VDC. That's why I chose the LTC3649, in fact - for the near zero Vout capability. Here's what I can share of the schematic:

Two prototypes have failed as I tested switching the load on and off via the relay with Vout set to its maximum of 13V, with a 1A load. This was a 1A CC, using an electronic load. When things go wrong, the system loses the ability to set the output voltage below around 6VDC with no load. If I load the output, then Vout will go down to near-zero. Also, when the relay is off (therefore always no-load for the 3649), after the damage the Vout oscillates if set higher than about 11V.

Ch.1 = ISET pin voltage, Ch.2 = VoPre (output voltage prior to relay)

I expected a lot of transients when turning the relay on and off - not surprisingly it's worse with higher output current. Here's a shot of the relay turning on into the 1A CC load:

Ch.1 = ISET pin voltage, Ch.2 = VoPre

However, I don't see anything potentially fatal in these images. After one prototype died, I added a 30V, 1A schottky from VoPre to GND right at the relay pins to protect against negative transients, but the prototype with that schottky died in the same way as the first.

     Perhaps ISET needs negative transient protection, too? A simple impedance check seems to indicate that the ISET pin is OK. What other pins might be sensitive?

Many thanks,


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