Hi, please can someone clarify the purpose of the circuit that's at the bottom of the reference design page for the Mega2560?
It appears to be an LMV358 op-amp (IC7B) driving a FDN340 MOSFET (T1) to switch a 5v supply, "USBVCC".
A 50% potential divider feeds the non-inverting terminal of the op-amp. It halves the voltage of "VIN" to make "CMP". It looks to me as if USBVCC will be switched to the 5v rail, as long as VIN is over 6.6v. Am I right? What's this to achieve, exactly? My guess is that it disables USB comms unless VIN is high enough to run everything.
I'm asking all this as I'm in the process of making a PCB using the minimum necessary components, to further an Arduino based design project. It will have its own constant voltage supply, and only require USB comms occasionally.
Many thanks, Matt.