How does this circuit work ? (Arduino auto power changeover circuit)

[whatsthegeek]

Hi, I've been trying to understand this circuit:


How does it block the USB 5v when the comparator detects there's more than 3.3v at the power jack ?

I think there's something I'm not getting xD


Hope someone can help ^^

[Nusa]

It's actually detecting more than 6.6V at the power jack. Note the voltage divider reducing it by half before the comparator. The comparator controls the mosfet on the USBVCC line. It's a P-channel mosfet (arrow facing out, not in), so a high output from the comparator will turn it off.

[danadak]

The comparator trips when the power jack reaches 6.6V, that
in turn turns off the PMOS fet that connects the 5V usbvcc buss
to the LP2985 regulator input.


Regards, Dana.

[whatsthegeek]

It's actually detecting more than 6.6V at the power jack. Note the voltage divider reducing it by half before the comparator. The comparator controls the mosfet on the USBVCC line. It's a P-channel mosfet (arrow facing out, not in), so a high output from the comparator will turn it off.

Oops, just noticed that xD

But there's something that I don't get, won't the current flow through the flyback diode of the mosfet ?

[glarsson]

But there's something that I don't get, won't the current flow through the flyback diode of the mosfet ?

Only if the 5V from USB is above 5.6V.

[Nusa]

It's actually detecting more than 6.6V at the power jack. Note the voltage divider reducing it by half before the comparator. The comparator controls the mosfet on the USBVCC line. It's a P-channel mosfet (arrow facing out, not in), so a high output from the comparator will turn it off.

Oops, just noticed that xD

But there's something that I don't get, won't the current flow through the flyback diode of the mosfet ?

What he said about diode voltage drop. It probably won't happen. Even if it does likely no harm done.

I believe the real purpose of the circuit is to keep the 5V voltage regulator from trying to power the USB line.

[whatsthegeek]

Ok, thanks for the explanation

I remade the schematic for my own device, is it correct ? would you change some of the components ?

[Peabody]

That's not quite the same circuit as your first one.  Does Vin of the 3.3V regulator need to be tied to +5V instead of Vcc?

[whatsthegeek]

Yes, it needs to get power from the 5v rail otherwise, the board won't get 3.3v when powered from USB

[danadak]

You should add some hysteresis on comparator -


http://www.analog.com/en/analog-dialogue/articles/curing-comparator-instability-with-hysteresis.html


Regards, Dana.