Small 'UPS' for audio use.

You don't need an IC to do that unless you want intelligent battery charging. Simply use a diode in series with the battery supply and make sure the voltage of the supply from the mains PSU is greater than the battery voltage so the diode is reverse biased when mains is present. This will allow the mains derived supply to power the radio and disconnects the batteries. When the mains supply goes down, the diode is forward biased and the batteries take over. It's completely seamless. A humble 1N400x series diode should be more than ample.

Obviously you'd need to bypass the diode with another diode wired the other way round in series with a resistor to trickle charge the batteries, or you could make a more elaborate constant current design switchable between proper charge (C/10 or whatever) and trickle charge.

Dave

I was hoping for an IC to do all this - UPS - charging and maintence charge and changeover all in one go. I'm sure they must exist. ;-)

Dave Plowman: Why, oh why make it difficult??..... less to go wrong with the series diodes and resistor trickle charge.... just about bullet proof.

-- Best Regards, Daniel Sofie Electronics Supply & Repair

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snipped:

Because it doesn't do what I want.

I beg to differ, unless you have a requirement that you haven't stated in your OP. We're not mind readers. :-)

Dave

I need a rather more sophisticated charger - a reasonably fast recharge then maintenance charge.

Then you use a diode for the "UPS" functionality and look for a sophisticated charger?

My thoughts exactly.

Dave