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Link to the schematic on arduino's website.From my understanding there is a comparator circuit in the arduino Uno schematic which turn off USB supply when there is a Vin present of greater than 7V by switching the PFET off. What I don't understand is why there is the need for the comparator circuit? Is it just to ensure that the Vin voltage is never below 7V? To ease with cost could you not simply plug in the vin into the gate of the pfet. That way if it goes low the pfet would turn on and vice versa? Also the LDO's connected to the Vin won't work anyway when the voltage is below 7V.

Is this to prevent damaging the design if you put loads of volts across the vin and burn the fet?

enter image description here

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    \$\begingroup\$ Low resolution pictures are hard to read. \$\endgroup\$ – Andy aka Jul 12 '18 at 13:53
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The comparator circuit serves one simple purpose.

If the board is connected to external power supply of greater than 6.6 V, it receives the power from internal +5V LDO, and disconnects the +5V rail from USB connector.

If the external power is not connected or is too low, the board becomes "bus powered" and receives +5V from USB VBUS.

The purpose of PFET is to prevent voltage backdriving current into the USB source on USB VBUS pin in self-powered mode, which is prohibited by USB specifications.

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  • \$\begingroup\$ Would it not achieve the same function with just the PFET and the external power directly into the gate of the PFET? \$\endgroup\$ – Hassan Nasir Jul 12 '18 at 14:23
  • \$\begingroup\$ @HassanNasir, no, the function won't be the same. If a customer connect a 6-V supply, the FET will be OFF, and internal LDO will be out of range, and the whole thing will fail. \$\endgroup\$ – Ale..chenski Jul 12 '18 at 15:02

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