@msfujino Unfortunately that didn’t do that trick.
@daCoder Yes, I did some more investigation into the issue in addition to looking at the schematic. My theory is that the ‘100% Mode’ on the TPS628438 buck is the root of the issue.
Feeding the 5V rail with anything from 3.4V to 5V results in the expected ~3uA. As soon as that drops to 3.3V or lower, it immediately jumps up to ~450uA.
The unfortunate problem is that there seems to be some backfeeding from the output to the input (seems internal to the buck?) which means that if you apply 3.3V to the VSYS_3V3, the VBUS goes to ~3.25V which causes the buck to enter the ‘100% Mode’; hence the current drain.