Why not simply use steering diodes. If the dc voltage from the wall wort is
higher than the battery voltage then the diode blocks the current from
flowing out the battery and you run off the power supply. As soon as the
external voltage is less than the battery the current is supplied from the
batteries. No chip needed, simple, reliable, and cheap. All you need are
two diodes that can handle the maximum current.


At 01:12 PM 2/3/98 PST, you wrote:
>Hi
>
>I am looking to power my device from a wall adapter but would like to
>switch over to backup batteries in the event of a power failure or if I
>decide to make the device portable for a few days.  I want this thing to
>switch over automatically.
>
>I looked at the devices by Maxim but they only seem to switch the
>battery for the purpose of powering memory.  Am I missing something?  I
>want to still be able to have a functioning device running on batteries.
>If anyone knows of a solution for me (preferably 1 chip) I would love to
>know about it.  Thanks for your help.
>
>Charles
>
>______________________________________________________
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>
Larry G. Nelson Sr.
mailto:L.Nelson@ieee.org
http://www.ultranet.com/~nr