Category: Hardware
Product Number: LT8311
Quick Context:
I'm designing a power module that is, schematically, a near copy of the circuit used by the DC1994A eval board link here.
Prototype boards are in and during testing I'm seeing ~11.5V Vout instead of the desired 12V output.
Problem:
Based on a few oscilloscope measurements, it appears as though the LT8311's opto driver isn't being enabled properly. Consequently, the OPTO pin isn't driving the optoisolator and feedback isn't getting properly sampled on the LT3752 primary side.
I've done some measurement comparisons between our design and the DC1994A evaluation board and the eval board's OPTO pin oscillates between ~4V and ~5V at steady state. Our design is only outputting ~0.4V to ~0.5V after an initial short spike to ~2V+.
Presumably a fault is being flagged somewhere keeping the OPTO driver from properly enabling.
My current best guess is there's too much noise being introduced somewhere in the OPTO circuit but I'm open to any and all suggestions.
Scope of U3 Optoisolator primary side, pin 3 (kind of looks like a runaway feedback loop):
https://ibb.co/Nnf1hJ8C
https://ibb.co/Nnf1hJ8C
Thanks,
