battery secondary power

[RajTS]

Hi,

I am new here. I have a very basic question, maybe bordering on Dumb.. but couldn't see the answer in previous questions.

My board will normally be working on AC power with an AC-DC converter. (i.e, not portable). When power fails, I want a battery backup ONLY to keep the RTC ticking. Planning for a Non-rechargable (2 x AA size battery). What would be the best way to conserve battery? I mean, battery should be drained only when there is no power. Should I just use two diodes ? (I'll try to upload a simple pic.. Thing is I just switched to Mac.. Yet learning to get around) Or is there some other ingenious way to do it?

Thank you for any suggestions you can provide.

-Raj

[RajTS]

MOSFET, not Transistor, you mean? Instead of the second diode.. We may not need a comparator exactly.. Or use the on-chip ADC.. Still there may be a VERY brief instance it could draw from both sources, but I believe that is better than losing power for a brief instance. I need to study about it a bit more.

Thank You

[RajTS]

This is the circuit i am proposing.. (But don't want to do it on my dev kit). Can anyone kindly provide any inputs on this?

As I mentioned, I need the battery only when here is no power, and only to maintain the real time clock. Non-chargeable battery, so it should neither change from the power, nor discharge into it. (Hence this MOSFET where substrate is not shorted to source, and no Source-Drain diode).

Thank You