gEDA-user: CR2032 with supercap, was Re: [SOT] suggestion for a SMT switch

[Jason <geda@xxxxxxxxxxxxxx>]

Have I mentioned I feel like a bull in a china shop as I plow through
datasheets, learning as I go?  ;-)

Details are below, but the big question is, can I use a 200 mF supercap
(EDLC) to dump 100 mA into the motor for short periods ( ~ 1 second)?
The reason I ask is because the Panasonic datasheet, found at [4], lists
a "Maximum Operating Current" of 10 uA...  That doesn't make sense to
me.  Nothing I've researched today on supercaps says I can't do it.

Bob Paddock wrote:
> DJ Delorie wrote:
>> Otherwise, look for the smallest MOSFET that will handle your current,
>> as smaller size usually means smaller leakage current.  For example,
>> the Fairchild 2N7002K (SOT-23) leaks 1 uA when off (Vgs == 0) but the
>> 2N7002T (SOT-523) leaks only 1 nA when off.
> 
> Better check the Gate/Source (Vgs) turn on voltage of the 7002, it is
> not the best at 3V (It is 2.5V Max).
> It would be better to use something like the FDV305N that will turn on at 1.5V.
> 

good call.  The output high voltage of an I/O pin on the pic is Vdd -
0.7V, or 2.3V with a fresh battery.

> Also your impedance between the motor and the coil cell are not well matched,
> unless you can put a big low leakage, low ESR cap across the voltage point
> going to your motor/switch combo.   What will happen is you will turn
> on the motor,
> the voltage will sag from the coil cell to the point that your micro
> resets, that then
> turns off the motor, the low ESR cap will minimize this from happening.
> 

I already have a cap in place, I'll double check the value to account
for the new load.  Thankfully, the motor will only run for a second or
so when the device state changes (haptic notification).

That cap is there for in-circuit programming.  I found this [1]
discussion on providing higher current from a CR2032 while avoiding
brownout. This comment [2], on the same page, goes through some math for
calculating cap values.

Assuming I need to draw 100mA for 1 sec, that's 0.100 Coulombs.  The pic
has a minimum operating voltage of 2V.  Worst case, the battery is down
to 2.7V, so I want no more than 0.5V drop when the motor runs.  From
[2], my capacitor should be 200mF.  Yikes.

Some chaotic googling found this [3], which introduced me to the idea of
supercapacitors.  Digikey has one [4] that seems to fit the bill.  It's
datasheet claims a "low ESR" of 200 ohms.  That seems much higher than
the 1 or 2 ohms I've seen in other places.


> Jason, you don't happen to be in the Cleveland area?  I'm having Déjà
> vu  here with a very
> similar discussion I just had with someone from a similar sounding
> project, a couple of weeks ago.
> 

Nope, never been to Cleveland.  We Steelers fans break out in a rash if
we go there.  ;-)

thx,

Jason.

[1] -
http://interactive-matter.org/2009/08/driving-circuits-from-a-cr2032-lithium-coin-cell/
[2] -
http://interactive-matter.org/2009/08/driving-circuits-from-a-cr2032-lithium-coin-cell/#comment-2041
[3] - http://www.computer.org/portal/web/csdl/doi/10.1109/ITNG.2009.259
[4] -
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=P14163CT-ND


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