USB to Battery Switching Circuit

[Twistx77]

Hi,

I'm working in a project where I need to have two souces of power being one of them two AA batteries and the other a USB cable.

I wan't to be able to use the batteries and when a USB is connected automatically cut off the drain of the AA batteries and use the USB instead to power my device.

I know I could use a simple diode to do the trick since the USB voltage will be higher than my two double AAs but for me having that diode means wasting precious energy (about 40mW for a 0.2V schottky diode).

So I designed a simple circuit that might be a solution.

I would like to know what you guys think and if you know a better solution for this.

Also, I would like to know if someone knows a pmos better suited for this application.


Attached are the the images of the circuit and the simulation results ( Output voltage and Batteries current)

[Sam__]

Might be worth having a 10k (or higher if power consumption is your worry) pulling the vUSB input to the comparator down when the input is floating.

[Twistx77]

Good idea, thanks.

[ludzinc]

The comparator doesn't work when VUSB isn't present.

So why not just tie VUSB direct to the 'FET?

[Sam__]

Looks like a good choice of mosfet, very low RDSon.

[Twistx77]

The comparator doesn't work when VUSB isn't present.

So why not just tie VUSB direct to the 'FET?

I can't do that, if I connect it directly then the gate and the  3V battery will be shorted when the USB is not connected,  so the mosfet will be working like a resistor. I could use a diode in the 5V rail to avoid that problem but that would mean that if I use an external USB battery pack I will be wasting power in the diode.