Roman,
 Peter's suggestions do not seem to me as ones
by"semi-educated stepper "experts"" in this case.
 I admit, you have practical experience with
steppers, I have almost not. Nevertheless, it is not
absolutely clear for me why linear drivers can not
be succesfully replaced by some digital scheme.
 Look, for example, at
---------------------
http://www.electronicstalk.com/news/pof/pof100.html
The TA3020 is the first 1bit digital amplifier IC using the
patented digital power processing technology (DPPT). This new
audio IC provides a breakthrough level of power and fidelity in a
highly integrated single package. The 2 x 350W Class-TT digital
amplifier is designed for the consumer electronics market. DPPT
incorporates a 1bit sigma-delta digital to analogue convertor
with a sampling frequency of 50MHz together with proprietary
signal processing algorithms to provide uncompromised fidelity
and unsurpassed power efficiency.
----------------------------

 Or take regulated high-frequency PWM PSU, which
Brendan Moran experiment with now. I admit, stepper
motor coils can't be used as high-frequency supply
chokes. But nobody said they should.
 You just replace linear driver with high speed regulated
DC/DC converter for energy saving purpose. Usually
they work at 300-400 khz, if I'm not mistaken.

   Mike.

Roman Black wrote:
> Peter L. Peres wrote:
> > A chopped current drive is much much much better
> > and can be achieved using an opamp to sense the
> > current at the bottom of the transistor switch. This
> > is used to turn on and off a high side switch that powers
> > that phase (or pair of phases).
.
.
.
> No it is NOT "much much much" better! Your attitude is
> typical of semi-educated stepper "experts" who have read
> some datasheets and manufacturers design notes, or done
> "steppers 101" through some institution in *recent* times.
> Chopper drivers DO NOT offer the same quality of performance
> of a good linear constant current system. They will give
> OK results at low speeds, and the ONE way they are better
> than linear drivers is the reduced power needs.
>
> They have significant problems with magnetic ripple,
> recirculating currents, needing to switch between fast/slow
> decay modes, etc etc. Chopper designs are a real tradeoff
> of performance, where the only real winner is total power
> dissipation at the cost of many performance issues.
>
> Have you built a microstepping driver good for 200,000
> microsteps per second? My new linear board will do that.
> How do you propose to control the exact current of 200,000
> individual steps/sec with a chopper driven PWM freq of
> 20,000Hz and 5% magnetic ripple? You only have ONE PWM
> PULSE for TEN actual steps?
.
.
.

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