When DC Jack is not connected on the UNO R4 and USB C is plugged in. 5V from USB may be seen at the input point (Anode of D1) of on board DC-DC converter.
should this R36 0ohms be replaced with diode? So, that power from USB does not flows back into Buck converter when the UNO R4 is not powered using DC Jack.
From the datasheet on the Buck Converter:
This would also explain why you still have 5v at the Anode, although the reverse current of the Schottky is only 150uA
Negative current Limit will be functional only when DC-DC is powered on. The case we are seeing is input voltage is not provided for DC-DC and Output side we are forcing a potential.
Like you mentioned the power is conducted through body diode of internal Mosfet.
This may cause unwanted loading on the USB power.
It has been a while since I have looked at this level but from my understanding MOSFETs with back to back sources will prevent reverse current flow by allowing current flow through the top FET until the current reaches the positive limit, which should not happen without being powered on.
What I am not sure of is if the the current sensing logic could operate with reverse current as the VIN and EN are tied together with a 100k resistor. If it can, then the lower FET could be switched on and off to attempt to dissipate the excess through PGND. I would be curious to see what a scope would look like at VIN with only USB.
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