user47842: 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? 

user4143: 
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.

47842: <p>So I understand that the USB VBUS circuitry turns off provided there is a high enough vin voltage using the PFET and comparator.</p>

<p>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.</p>

<p>If I incorrectly plugged in a voltage source of a low voltage like a coin cell battery across the V<sub>IN</sub> pin and also powered the device using the 9&nbsp;V DC jack, wouldn't that then send current through the V<sub>IN</sub> pin and back to the coin cell battery? </p>

<p>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? </p>
 4143: <blockquote>
  <p>wouldnt that then send current through the Vin pin and back to the coin cell battery? Is that what would happen in this instance?</p>
</blockquote>

<p>Yep, that's right.</p>

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

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