Tweaking a stepper controller - SLA7020M

[atferrari]

I got for free a board controlling a stepper motor.

No manual and very few technical notes available.

I managed to reproduce the circuit myself so that is not a problem. Trying to interfere the micro in charge is useless. I will program my own.

Motor driver is a SLA7020M. http://www.datasheetcatalog.com/data...SLA7020M.shtml

The circuit, according to data, generates three different Vref values applied to the above IC: 0.620V, 0.320V and 0.060 V) for three different speeds (1900, 900 and ??? pps). So far so good but the when the motor is iddle, I measure a Vref which is not the highest nor the lowest: 0.157 V.

The very limited functionality I could check shows that Vref changes almost continuously (my guess is acceleration - contsant speed - deceleration stages).

Up to now I believed I understood the basics of controlling current in a stepper but I see some questions are in order:

a) What is the actual reason to change the Vref (thus actual current) when varying speed?

b) Why the Vref used whit motor iddle is not the lowest one (thus lowest current) Puzzling, to say the least.

c) When iddle, that current is the one ensuring the holding torque?

d) I am going to use that stepper at much lower speeds. Lower speed means a lower Vref being applied to the SLA7020M?

e) Honestly I do not want to do a wrong first atempt that could damage the circuit nor the stepper. What is the safest (even coward) approach?

Tried to conclude something from the datasheet but since hardware is already there all I can imagine myself changing is software and (maybe) Vref.

Gracias for any help.