Understanding Arduino's Automatic Power Source Selection

[Dirk.S]

Hello Everyone

I'm trying to understand the automatic power source selector of the Arduino boards but can't get my head wrapped around it. I've rearranged the circuit and have come up with the schematic below. I understand that IC7B is a opamp used as a comparator to switch the FET (T1) either on or off depending whether USB power is used or external power is used.

The thing is, the positive input for the comparator is the external source applied through a voltage divider and the negative input for the comparator is the 3V3 output from the 3V3 linear voltage regulator as I understand it. This makes sense since if your using USB power, then there will be 3.3V on the negative leg of the comparator and nothing on the positive leg, and hence the FET will be switched on. But the thing is, the 3V3 regulator is situated after the FET, so when the system is first powered up, how does the opamp get the 3V3 input to switch on the FET when connected to USB power only?

The entire circuit can be viewed over here:
https://arduino.cc/en/uploads/Main/arduino-mega2560_R3-sch.pdf

Thanks in advance
Dirk

**broken link removed**

 

[Dirk.S]

Someone on the Arduino forum assited me with this question. The internal diode gets forward biased allowing the 3V3 regulator to be switched on.