AJ said:
I have a 3.6V Lithium battery and a 5V supply that power a 3.3V regulator.
I have a diode on the input to the 3.3V regulator and I wanted the battery
to power this regulator when the 5V disappears. I don't want to have to put
a diode in between the battery and the 3.3V regulator because of the voltage
drop but I need something there to stop the battery receiving a charge from
the 5V rail (via the diode) instead of its proper charger circuit. This is
where I was thinking of using PNP transistor with tis base permanently tied
low. If the battery is dead flat, the collector could be higher than the
emitter, thus my original question. I have since thought that I could use a
zener/transistor to detect when the input to the 5V supply drops and switch
a MOSFET accordingly but my first idea would be better if it would work ok?
I will give it all a test in the next couple of days but its 2am here in
Australia and my eye's are the size of pea's, not sure I can cope right now.
Anyway, its always nice to hear what other people think first, and I always
get good answers, Thanks John.
Regards,
AJ
Hi, AJ. I'm assuming you'd also want seamless transfer to avoid
glitching on your regulator.
Doing this well really isn't that simple, but it's obviously a common
problem with consumer electronics. So sometimes the best thing to do
is just cast around and see what the semi manufacturers who play with
these things have come up with.
I'd recommend taking a look at the Linear LTC4411 for your problem.
(Maxim makes some good product too, but their great ideas tend to
disappear after a couple of years). It's only available in a 5-pin
SOT-23 package, but it's otherwise just about perfect for what you're
doing. It's got an internal 140 milliohm Rds(on) P-channel MOSFET that
turns off if the output voltage is more than the input voltage, like
this (view in fixed font or M$ Notepad):
|
| Shottky .-----------.
| + Diode | 3.3V | +
| o-------->|-------------------o--| LDO |---o
| | | |
| | '-----------'
| .-------------. | |
| | LTC4411 | | |
| | | | |
| .-----oVin OUTo--' |
| | | | |
| 5V 3.6V| | | | 3.3V
| +| | | |
| --- .--oCTL Statuso |
| - | | | |
| | | | | |
| | o--oGND | |
| | | | | |
| | | '-------------' |
| - | | | -
| o------o--o----------------------------o---------o
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
http://www.linear.com/pc/productDetail.do?navId=H0,C1,C1003,C1142,C1079,P2430
(This is the product page -- you can download the data sheet from
here).
If you don't mind a "store-bought" one IC solution, and you can live
with an SOT-23 package, it'll git r done.
Good luck
Chris