The ESD protection diode D1 (PRTR5V) seems to be powered through +5V when the board is powered via VIN.
If I remove jumper SJ1 in order to power the board directly from an external 3.3V power supply, will this ESD protection diode still function?
Or will there be no ESD protection when powering the board from a 3.3V supply?
Post the schematic where you saw that.
The schematic is at https://content.arduino.cc/assets/MKRWAN1310V3.0_sch.pdf
It is not powered through +5, the group of 4 limits voltage by conducting if D+, or D- exceeds its Vf and + 5V or goes below ground. The singleton protects for reverse polarity. You really do not want any of them to turn on as that there is a fault it is protecting from. It is dependent on the +5 to adsorb the fault.
It's dependent on the +5v to absorb the fault, you said. So does it mean that it won't protect the board from ESD when the board is powered through 3.3V like described in the first post?
Diode D1 (PRTR5V) will give protection to the USB inputs at all times, even if no supply voltage is present
From the datasheet:
PRTR5V0U2F and PRTR5V0U2K from Nexperia integrate two ultra low capacitance rail-to-rail ESD protection channels and one additional ESD protection diode each to ensure signal line protection even if no supply voltage is available.
It is only the USB port that is protected by D1, other parts of the MKRWAN1310 are still vulnerable to ESD.
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