Diodes for Li coin cell protection

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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.


Re: Diodes for Li coin cell protection

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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.)

Re: Diodes for Li coin cell protection
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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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Re: Diodes for Li coin cell protection
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Use a vacuum tube :-)  6AL5 IIRC will do.

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Re: Diodes for Li coin cell protection

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

mikey



Re: Diodes for Li coin cell protection
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to
rail.
but
 This
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powered.
cell.
have a
of low
and
which
optimally in
SOT23
cell and
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.  :)


Re: Diodes for Li coin cell protection


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Do you have any numbers for this very low tolerance?


Re: Diodes for Li coin cell protection
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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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removed.

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Re: Diodes for Li coin cell protection

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Doubles for every 10 centigrade, I'll wager.

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer:  Electronics; Informatics; Photonics.
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Re: Diodes for Li coin cell protection
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[snip]

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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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