retrolefty:
Vin is an 'official' shield pin name for the 7-12vdc input for external DC power that which then only feeds the on-board +5vdc voltage regulator and the resistor divider feeding the comparator opamp. Vin is the same as the voltage coming from the external DC power connector except for a series polarity protection diode added between the connector and the Vin pin.
Aha! $) Ok, Vin is the unregulated power supply, so I should place the mosfet between the Raw DC in and the regulator. I know the unregulated voltage before my 5V reg is about 22V. Im thinking that is too high for the mosfet they use. I'll just use a beefier mosfet than the one they use, such as an FDN360P.
Is there any advantage to using a comparator, or can I just go with a set of mosfets to determine using the USB VCC vs the 5V?
Also as a side question, should I use reverse diodes on the D+/D- USB signal lines?