[continued from previous message]   
      
   adjust the duty cycle as needed to maintain the target voltage.   
      
   MOSFET lets power in, which goes through the coil, and charges the   
   capacitor (in parallel with the load). When the MOSFET turns off, there   
   is a voltage kick from the inductor (it goes negative), pulling power   
   from the ground plane.   
      
      
   It is possible to use an opamp for this (rather than a microcontoller),   
   but an opamp would generate very crude PWM, thus, noisier.   
      
   Possible noise reduction approaches:   
      Big capacitor;   
      Secondary inductor, diode, and capacitor.   
   Assuming a constant load, a second inductor could smooth the PWM noise   
   by maintaining closer to a constant current; but is more likely to see   
   voltage ripples if there are sudden changes in the load (if compared   
   with using a bigger capacitor).   
      
   Comparably a microcontroller can generate an higher-frequency PWM   
   signal, and keep the initial noise lower.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)