Switching between battery and usb power.

[Kaisha]

I need a circuit that will switch between USB and battery power (USB if available, battery otherwise).  The thing is that in this case the battery will be a higher voltage (6.5V-9.5V) than the USB, so the 'normal' method show online doesn't work.  I also only have a SPST switch to turn the entire thing on/off (which is connected to VBAT).

I think the circuit below will work, but I'd love another set of eyes, and any input/ideas you guys might have.

[ledtester]

Here's a similar EE StackExchange discussion:

https://electronics.stackexchange.com/a/96070/95488

[vu2nan]

A 5V electromagnetic relay may be used to switch between USB and battery.



Nandu.

[Zero999]

A 5V electromagnetic relay may be used to switch between USB and battery.



Nandu.

That will work, but there needs to be a fairly big capacitor between +V Out and 0V, otherwise there will be a huge voltage sag, when it switches. You probably need to limit the voltage drop below 0.5V.

The minimum capacitance depends on the switching speed of the relay, the maximum allowable voltage drop and maximum current drawn by the device.

C = Δt/ΔV×I

Also note the voltage drop due to the ESR of the capacitor.

Make the capacitor a little larger, than calculated by the formula above.

[Peabody]

Would the battery voltage ever be lower than VUSB?  If it's ever more than one diode drop below VUSB, then USB current would flow through the body diode of the P-channel back to the battery.  You might not want that to happen.

Would you ever need to operate the device from USB with no battery connected?  If so, then the P-channel gate would be floating.  I don't know if that matters if there's no battery.

It's not clear to me what purpose Nchan3 serves.  It seems the diode should be enough.