Re: Diodes for Li coin cell protection

From: Mike Fields (spam_me_not_mr.gadget2_at_comcast.net)
Date: 02/06/05 

Date: Sat, 5 Feb 2005 17:40:59 -0800

"CBFalconer" <cbfalconer@yahoo.com> wrote in message
news:4205447D.9B762A35@yahoo.com...
> rickman wrote:
> >
> > I had to look pretty hard to find a small, inexpensive diode pair to
> > allow an RTC to be powered by both a Li coin cell and the power rail.
> > The RTC I am using is very low current when not being controlled, but
> > starts using a lot more current when controlled over the I2C bus. This
> > would exhaust the cell long before the expected 5 year life, so I
> > needed a way to power it from the Vcc rail when the board is powered.
> > Diodes are the right idea, but primary Li cells have a very low
> > tolerance for reverse current (charging). It seems that reverse
> > current builds up a layer inside that reduces the capacity of the cell.
> >
> > I have looked at a lot of diodes and I finaly found some that have a
> > low enough reverse current. Maybe I have missed a whole family of low
> > reverse current parts somewhere, but I was not finding very many and
> > mostly they were not cheap. Finally I stumbled on some Varactors which
> > seems to be built with very low reverse currents to work optimally in
> > tuning apps.
> >
> > The ON MMBV432LT1 is a dual, common cathode varactor diode in a SOT23
> > package. Digikey sells it for under $0.10 qty 1000. The reverse
> > current at 125C and 5 volts is under 5 nA.
> >
> > So if anyone needs to power an RTC or an MCU from both Li coin cell and
> > other power, this is a good part to isolate the two.
>
> Use a vacuum tube :-) 6AL5 IIRC will do.
>

Nah, too many pins. Use an 80 -- only 4 pins (I think I may actually have
a
couple of them around still)

mikey

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