Well Ms. Newman, you need to discharge the cap to get the voltage to follow the PWM duty cycle - else the cap charges up to 4.4volts even at the lowest PRF and will stay there! The diode will make sure of that. So you need a discharge path thru another resistor (say, 10 times Rin or higher depending on the ripple one can tolerate) connected parallel to the 1uF, don't you think?

-warrier