Allegro A3979 Stepper driver tuning

[jkpugh]

Greetings,

I am designing a 3 axis CNC motor driver board with PC interface using your A3979 chip and have a few questions. Data sheet is http://www.allegromicro.com/datafile/3979.pdf

* PFD, page 3, pin 5; (percent fast-decay). I'm not what to hook it to. After reading the application notes, it sounds like it needs to go to a resistor/pot network so that turning the pot gives 0 to 1 volt to control the decay. So at <.21V gives fast decay; .21-.6mV gives mixed decay, and, over .6mV gives slow decay.
+ Is this right? Can I just pull it high to 5VDD or low, to gnd? Is this just for tuning? How would I use this?

* Fixed off-time, page 9 seem easy to calculate but what values should I start with? It looks like these various RC combination gives a MOSFET off-time of 5.6uS to 150uS.
+ Is this a one-time tuning value or should I put in a pot? Is there a "magic-bullet" off-time that will work for any situation? How can I tune it?

Last question, I promise...

* RC blanking using RC1 & RC2 says Tblank = 1400CT, page 9. I understand that the comparator needs blanking time until voltages stabilize but don't understand the formula. Same question as above:
+ Is this a one-time tuning value or should I put in a pot? Is there a "magic-bullet" blanking-time that will work for any situation? How can I tune it?

If you have any other documents on tuning my prototype board, I would like to see them, or if you know of any forums where I could post driver questions, that would be awesome. Thanks in advance for your help!!

Jeff Pugh
Field Service Engineer
www.Matrical.com
Spokane, Wa
509 998-8608
LAT 47-40-58.1452N
LON 117-19-21.2103W