PWM stepper driver with external bridge

Background:
I am designing a microstepping stepper drive system for a home cnc router
table (4 axis). The design involves a pic16f84 driving 2 Allegro a3955sb
motor driver chips (one for each phase). The controller ic is only rated
for
1.5A max phase current and I need 3-5amps for the motors I have. I would
like to use a full bridge consisting of irf9530 and irf530 fets to act as
external amplifiers for the controller. I contacted Allegro tech support to
see if they had any application notes or other comments on using external
bridges with the 3955 and their rather short response was "the 3955 is not
suitable for driving capacitive loads". OK... if the chip cannot drive a
1600pf gate load of a couple of fets, I would even consider using discrete
buffer transistors in the external bridge cct.
Question:
does anyone have any recommendations or suggestions on using external fets
with any of the stepper controller IC's? Pitfalls? Example bridge ccts?
Examples of bullet proof bridge ccts suitable for pwm at 25khz, 5a, 60v?
Thanks
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Lawrence Glaister VE7IT email: lg@...
1462 Madrona Drive http://jfm.bc.ca
Nanoose Bay BC Canada
V9P 9C9
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