> Isn't your time constant (1.5 mS ?) too short?
>
> Joe W
>

I'd say it is! In these cases, I like to look at things in the frequency
domain instead of time domain. The PWM has a DC component, a fundamental
frequency component, and a bunch of harmonic components. You are following
this with a low pass filter to attenuate everything but the DC. You will
never get rid of all the AC, but you can attenuate it substantially. When
you change the duty cycle, the LPF output will ramp to the new level due
to the LPF. There will always be ripple in the output, it's just a matter
of how much. I like using LTSPICE (
http://www.linear.com/designtools/software/#LTspice ) to get a good idea
of what's going to happen.

Harold



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