Chopper Transient Suppression

All:

My Axis4 4 axis chopper board is making excellent progress
but I think I still have a problem with turn on transients.

Here's the Axis4 URL for some context:

<http://gramlich.net/projects/cnc/axis4/rev_a/index.html>

In particular, the schematic of interest is:


<http://gramlich.net/projects/cnc/axis4/rev_a/schematics.html#X_Y_Z_A_Axes>

Basically, what have have is a microcontroller that
strobes the L298 on at 25kHz. When the voltage across
the current sense resistor exceeds the set value,
the comparator turns the H-bridge off.

I deliberately designed the circuit so that pulse that
turns on the H-bridge takes precedence over the comparator
that turns it off. This is because there is supposed to
be transient spike when the H-bridge is turned on that
will cause the voltage to temporarily exceed comparator
set value.

Currently, I have the microcontroller holding the H-bridge
on for the minimal amount of .2uS. The behavior I get is
that my stepper motors vibrate even when they are not being
commanded to move (not good!) What I think is happening is
that when I am close to the coil correct current, the turn-on
spike causes me to turn off prematurely. The current decays
further, until it is so far below what the turn-on transient
is not large enough to cause problems. Then H-bridge is left
on until the current gets back to the desired current level.
In short, I suspect I am getting an asymetric pulse stream in
the steady state, rather than a steady stream of symetric
pulses all of the same width.

I suspect that the transient spike that is occuring on my
system is a bit longer than .2uS, but I lack the tools to
actually measure the transient. Does anybody know that
the all-in-one products like the Allegro use for transient
suppression times? I've dug around in the spec. sheets but
I have not quite stumbled on a specific number. Is it 1uS?
2uS? or more? Obviously, I can experiment and just keep
increasing the number until things stablize, but I thought
I would ask here first, since this group has been so helpful
to me in the past.

Any help on this issue would be greatly appreciated.

-Wayne