Re: Diodes for Li coin cell protection

From: CBFalconer (cbfalconer_at_yahoo.com)
Date: 02/06/05 

Date: Sat, 05 Feb 2005 23:35:24 GMT

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. 

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