Datasheet
TMC5041 DATASHEET (Rev. 1.13 / 2017-MAY-16) 42
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With V
M
the motor supply voltage and I
COIL
the target RMS current
The effective PWM voltage U
PWM
(1/SQRT(2) x peak value) results considering the 8 bit resolution and
248 sine wave peak for the actual PWM amplitude shown as PWM_SCALE:
With rising motor velocity, the motor generates an increasing back EMF voltage. The back EMF voltage
is proportional to the motor velocity. It reduces the PWM voltage effective at the coil resistance and
thus current decreases. A higher scale value is necessary to compensate for this. When a higher value
is chosen, it should be made sure, that the maximum driver current is not exceeded during the
acceleration phase. This can be checked with the above formula. A short time excess current will not
do harm to the motor.
Hint
The setting for PWM_AMPL can easily be optimized by tracing the motor current with a current probe
on the oscilloscope. It is not even necessary to calculate the formulas if you carefully start with a low
setting for both.