Maybe you should look at the 'powerpath' power switches from
www.linear.com. They do exactly what you need.

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Alan B. Pearce
Sent: 07 April 2004 13:54
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE:] LDO Regulator with two inputs?

>>I can't find it now. I'd like to avoid adding diode drops in
>>front of the regulator to get a bit more battery life. Anyone
>>remember seeing such a regulator?
>
>If you used a PFET in series with the battery you could get a
>very low voltage drop. Bias it on with a resistor gate to ground
>and have the wall-wart supply turn it off as it can provide a
>positive (off) gate bias.

Just been looking at an advert for Linear Technology LTC4411 and 4412.
The
4411 appears to have the FET internal, and the 4412 uses an external
one.
http://www.linear.com They are quoting the 4411 @$1.60 ea in 1k, and the
4412 @$1.30 ea for 1k. For your use it looks like you would need the
4412 to
get the required voltage rating, but it goes up to 28V input, but not
clear
from the advert if this is for both inputs.

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics