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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?

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wouldnt that then send current through the Vin pin and back to the coin cell battery? Is that what would happen in this instance?

Yep, that's right.

shouldn't there be a design feature to prevent users from damaging it like that? Fool/fail safe?

No. VIN can also be VOUT. You can use it to power other things that want a higher voltage.

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