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

[Igor Lopez <igor.lopez@xxxxxxxxxxxx>]

Hi Jason,

> 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.

I am assuming we are talking about a DC motor here.
If the resistance of the windings are in the range of 25 ohm then you are pretty
well on your calculations since you will operate the motor with a
constant DC voltage.
However if the winding resistance are much less, you will need to PWM
the DC voltage
in order to achieve an approximate 100mA current. The duty-cycle needs
to be controlled
to get the proper current but that is a different issue. However the
requirement on your
boost capacitor will be drastically reduced since the needed charge
will be much less
between each chop (battery on time) and will depend on the time the
FET is actually
connecting the boost to the winding. You will still have current
through the winding
via the free wheeling diode even when the FET is off (but slowly decaying)

/Igor


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