This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM5170: VRAMP not switching in standby mode

Part Number: LM5170

I have two LM5170's in a 3-phase system operating on 10VCC, with 48V at VIN. My test procedure is as follows:

  1. Power on, set UVLO < 1.25.
  2. Test VINX for 0V.
  3. Raise UVLO to 3V.
  4. Test VINX for 48V and VRAMP for switching.

At this point one of several things happens. Either the chip successfully starts switching the VRAMP, producing a 0~5V triangle wave; the chip grounds the VRAMP (I assume the cap discharge FET is held on, grounding VRAMP); or the chip locks VRAMP at 16V (I assume a zener voltage clamp diode is present to prevent over-voltage damage).

I've disabled fault detection for use of external gate drivers, and nFAULT raises to 5V, telling me that the chip has reached standby. This is true for both chips. I have measured correct switching behavior out of all 3 channels at one point or another, but I can't consistently reproduce that result. Sometimes cycling UVLO low and back high gets the VRAMP switching to resume, but not always.

There do not seem to be many things I could be doing wrong, but I'm stumped. Does anyone have any idea what could cause this?