Need help with AC detector/battery switch

[Randroid]

Hey all,

I've attached a (very) basic block diagram of my circuit. I'm working on a project that is powered by AC, but has a battery backup when the AC power drops out which will happen from time to time. My boost chip doesn't have an enable pin, so I need a way to keep it disconnected from the battery when the AC power is present. I want to try to use a pFET as a switch at point A, but I'm not sure what the best approach is going to be.

There is a microprocessor involved, and it's been suggested that I use an optocoupler as an AC detector to trigger an interrupt and switch a FET but I'm not familiar with that type of circuit. If anyone has some ideas or suggestions about the most robust way to accomplish this, I'd appreciate any input!

Please let me know if my post needs any clarification.

[DJ]

Diode-or the power supplies together. Tap off the ac, rectify it, divide it down and feed a comparator driving the enable pin of your boost converter.

Helps if you detail voltages, load currents, etc

[Randroid]

My boost converter does not have an enable pin unfortunately. I haven't been able to find a boost converter with an enable pin that fits my specs - 3.7V input from battery, 9V-12V output, around 12-15 W out of the boost max. I've found plenty of boost controllers, but I'm trying to use a self contained IC for this project instead of designing my own booster.

The AC/DC converter is universal so it takes 85-240V, 50-60Hz and outputs 12V, 25W (though I shouldn't ever be above 12-15W).

[expert_vision]

I have designed this for on of my projects and it works pretty well, the uC doesn't sense when power is being swapped; uC doesn't reset.
Adapt to your needs. Hope it helps.

Couldn't attach the picture so I posted on imgur ...

[expert_vision]

I have designed this for on of my projects and it works pretty well, the uC doesn't sense when power is being swapped; uC doesn't reset.
Adapt to your needs. Hope it helps.

Couldn't attach the picture so I posted on imgur ...