Allegro 3977SED questions

I'm hoping someone can help me figure all this out. I've figured out
quite a bit on this chip...but I have some questions that I can't
answer myself.

I went ahead and bought myself a few Allegro 3977 SED chips. Most of
the stuff on the diagram I understand. But there are some things
that I need elaboration on.

There is something called Percent Fast Decay or PFD. It shows a
place for Vpfd to be entered...and the info tells you the allowable
voltage range...but the info also makes it sound like this is a
voltage that comes from monitoring something. I just can't figure
out where to connect the Vpfd so it works correctly.

Also Allegro is pretty good at telling you what components you
need...but there are 2 sense circuits that don't show what value of
sense resistor or capacitor you need. Does anyone have a clue on
this?

http://www.allegromicro.com/sf/3977/ has the functional diagram that
I've been looking at.

Any help, clues, suggestions much appreciated!!!

Thanks!