Diodes for Li coin cell protection

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.

Reply to
rickman
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A really low reverse-current diode can be formed by using the base-emitter junction of a small NPN transistor, like the 2N3904. Just tie the collector to the base.

-Robert Scott Ypsilanti, Michigan (Reply through this forum, not by direct e-mail to me, as automatic reply address is fake.)

Reply to
Robert Scott

Use a vacuum tube :-) 6AL5 IIRC will do.

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Reply to
CBFalconer

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

mikey

Reply to
Mike Fields

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Actually, my first approach was to use a 5U4, but the heat was cooking the MCU. So I thought I might give the solid state diode a try. I'm not much for new fangled stuff, but it seemed like maybe it was time to consider catching up with the 20th century. Opps, I guess I am still behind. :)

Reply to
rickman

Do you have any numbers for this very low tolerance?

Reply to
Guy Macon

[snip]

Why not use a few diodes in series? Say, a couple of BAV99's if you can tolerate the forward voltage drop at currents of interest, else schottky, Sprow.

Reply to
Sprow

snipped-for-privacy@dont-mail-me.com (Robert Scott) wrote in news: snipped-for-privacy@news.provide.net:

What reverse current can I expect using this approach?

How about JFET's (e.g. SST201)? Reverse current typical 2pA (100pA worst case).

M.

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Reply to
Matthias Weingart

Panasonic CR/BR types are rated by the capacity of the battery; Within

3% for coin-type batteries and within 1% for cylindrical type batteries.

With my conditions, I get

120 mAHr * 0.03 / (24 * 365.25 * 5) = 82 nA

I have found some very small, very low reverse current parts at Central Semiconductor. They seem to specialize in very compact parts. I can get dual diodes in SOT563 packages which are less than 2 mm square and are rated for under 1 nA at 25C. Or I can get dual transistors with Icbo of 10 nA max at 25C. The advantage of the transistors is that I don't have to stock a separate part. But may I need to do some tests to make sure the Icbo does not increase too much at higher temps.

Rick "rickman" Collins

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Reply to
rickman

"rickman" wrote

Doubles for every 10 centigrade, I'll wager.

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Nicholas O. Lindan

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