I've been watching some of Dave Jones videos (EEVblog). In #225 he discusses using a PWM signal to control the voltage of a power supply. It needs to be DC to do this, so he uses a filter. He uses a first-order RC filter, with the output going to the "+" input of an op amp. With a10% duty cycle on the input he shows the output at 10% of the input peak, with some ripple.
What I don't get is why the filter capacitor doesn't (eventually) charge to the input peak voltage. 'Cause the input never goes negative, and the op amp input impedance is too high to have any affect (or is it?). Why doesn't it just keep charging?
If it matters, the input is 5v peak, R is 10k, & C is 100nf.
It's embarrassing that I don't understand something as simple as this, but I was a software engineer, so that might explain it .