Diodes for Li coin cell protection

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rickman 21 years ago

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.

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Robert Scott 21 years ago

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.)

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CBFalconer 21 years ago

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

"If you want to post a followup via groups.google.com, don't use the broken "Reply" link at the bottom of the article. Click on "show options" at the top of the article, then click on the "Reply" at the bottom of the article headers." - Keith Thompson

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Mike Fields 21 years ago

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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rickman 21 years ago

to

rail.

but

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powered.

cell.

have a

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which

optimally in

SOT23

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actually have

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. :)

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Guy Macon 21 years ago

Do you have any numbers for this very low tolerance?

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Sprow 21 years ago

[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.

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Matthias Weingart 21 years ago

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.

Bitte auf mwnews2@pentax.boerde.de antworten.

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rickman 21 years ago

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

snipped-for-privacy@XYarius.com Ignore the reply address. To email me use the above address with the XY removed.

Arius - A Signal Processing Solutions Company Specializing in DSP and FPGA design URL

formatting link

4 King Ave 301-682-7772 Voice Frederick, MD 21701-3110 301-682-7666 FAX

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Nicholas O. Lindan 21 years ago

"rickman" wrote

Doubles for every 10 centigrade, I'll wager.

-- Nicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. To reply, remove spaces: n o lindan at ix . netcom . com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/

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Diodes for Li coin cell protection

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