Sun May 18 20:48:22 EDT 2014

About that PMOS

Because GND-ref it's easier to make a sinking EIDAC.
This makes the sawtooth charge from 3v3 down.
I used a 2N7000 NMOS for discharge, but don't have a PMOS.
Why not use a 2N3906 PNP for discharge?

I used the NMOS because of large peak current.

Can the PNP take peaks?

Or: what PNP can take peaks?

Let's just assume this is not a problem and fix it when things start
failing.  Discharging 45nF can't be that hard.

From [1]:

   BJTs are not components that can tolerate high current densities;
   you're not allowed to average the current over any arbitrary period
   of time, because destruction mechanisms exist that have nothing to
   do with thermal damages.

So, measuring on the scope.  2N7000 NMOS is about 0.5us, also with 1k
resistor.  2N3904 NPN is about 1us.  Much slower with 10k resistor:
NPN 3.5us, NMOS 1.5us.

( EDIT: I missed that 22k pulldown! )

So the PNP at 1us, how much current is it taking?  Slop is about 3V /
750ns or 1V/250ns or 4V/us.  C = q/V = I*DT / V -> I = CV / DT (*
0.045 4.0) -> 180mA.  That's really ok.  So maybe best to pick a
source resistor to keep the rate down.

[1] http://www.diyaudio.com/forums/solid-state/88122-pulse-current-survivability-small-bjts.html