You could lower the cost slighty by using two diodes and only one mosfet.

Switch the 5V from the usb with the mosfet.

Use one diode to lift the regulator for the 13V input by the diode
drop, and use the other diode to prevent reverse voltage into the
regulator when powered by the usb and mosfet.

Alternately, run the circuit at 3.3v and use one regulator and two diodes.

-Adam

On 5/23/06, Vitaliy <spam@maksimov.org> wrote:
> While we're on the subject. :)
>
> I'm working on a design which has a PIC, an FT232R chip, and a bunch of
> stuff connected to the IC. The circuit is normally powered from the car's
> 13V supply (through a 7805).
>
> What I'd like to do, is to be able to power the FT232 and the PIC from USB
> when the circuit is not plugged into the car (for diagnostic purposes).
>
> One way to do this is to have a circuit like the one shown in the attached
> picture. The problem is, the voltage drop on the Shottkys brings the +5V
> supply (worst case +4.75V) dangerously close to the BOR voltage (4.52V).
>
> I could also use two MOSFETs, but they would add about $0.40 to the cost of
> the circuit.
>
> Does anybody have another idea?
>
> Best regards,
>
> Vitaliy
>
>
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