>    By definition R and C should discharge to 1/e value
> over 1 second. Where did guys get 2*Pi, I don't know.

That is correct.  Note that when the R*C time constants is significantly
longer than the period of the incoming square wave, the result approaches
a triagle wave (the exponential becomes a line because it doesn't have
time to change slope).

>    To estimate the ripple very roughly, we may keep in
> mind the next: over 0.1*RC PWM cycle voltage change
> would be very aprox V/2.7/10 (0.2v when V=5v)

Right idea, but the details are a bit off.  Assuming square wave input,
the output will always be "near" 2.5V.  Also, you have 2 exponentials per
cycle, one for each half of the square wave.  I haven't checked your
calculations, but these two factors should make the ripple 1/4 of what you
computed.


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