So I understand that the USB VBUS circuitry turns off provided there is a high enough vin voltage using the PFET and comparator.
I can see that the DC jack is connected to the Vin pin via a diode. However, what is to stop someone from plugging a voltage source across the Vin and at the same time putting across a voltage on the DC jack.
If I incorrectly plugged in a voltage source of a low voltage like a coin cell battery across the VIN pin and also powered the device using the 9 V DC jack, wouldn't that then send current through the VIN pin and back to the coin cell battery?
Is that what would happen in this instance? And if so then shouldn't there be a design feature to prevent users from damaging it like that? Fool/fail safe?