Datasheet

TMC5041 DATASHEET (Rev. 1.13 / 2017-MAY-16) 41
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With V
M
the motor supply voltage and R
COIL
the motor coil resistance.
I
Lower Limit
can be treated as a thumb value for the minimum possible motor current setting.
Example:
A motor has a coil resistance of 5Ω, the supply voltage is 24V. With TBL=%01 and
PWM_FREQ=%00, t
BLANK
is 24 clock cycles, f
PWM
is 2/(1024 clock cycles):












For pwm_autoscale mode, a lower coil current limit applies. This limit can be calculated or measured
using a current probe. Keep the motor run-current setting IRUN well above this lower current limit.
7.2.2 PWM_AMPL for Using stealthChop and spreadCycle
When combining stealthChop with spreadCycle or constant off time classic PWM, a switching velocity
can be chosen using VCOOLTHRS. With this, stealthChop is only active at low velocities. Often, a very
low velocity in the range of 1 to a few 10 RPM fits best. In case a high switching velocity is chosen,
special care should be taken for switching back to stealthChop during deceleration, because the phase
jerk can produce a short time overcurrent. (Refer to chapter 7.4 for more details about combining
stealthChop with other chopper modes.)
To avoid a short time overcurrent and to minimize the jerk, the initial amplitude for switching back to
stealthChop at sinking velocity can be determined using the setting PWM_AMPL. Tune PWM_AMPL to a
value which gives a smooth and safe transition back to stealthChop within the application. As a
thumb rule, ½ to ¾ of the last PWM_SCALE value which was valid after the switching event at rising
velocity can be used. For high resistive steppers as well as for low transfer velocities (as set by
VCOOLTHRS), PWM_AMPL can be set to 255 as most universal setting.
Note
The autoscaling function only starts up regulation during motor standstill. After enabling stealthChop
and setting all parameters, be sure to wait until PWM_SCALE has reached a stable state before starting
a motion. Failure to do so will result in zero motor current!
In case the automatic scaling regulation is instable at your desired motion velocity, try modifying the
chopper frequency divider PWM_FREQ. Also adapt the blank time TBL and motor current for best
result.
7.2.3 Acceleration
In automatic current regulation mode (pwm_autoscale = 1), the PWM_GRAD setting should be
optimized for the fastest required acceleration ramp. Use a current probe and check the motor current
during (quick) acceleration. A setting of 1 may result in a too slow regulation, while a setting of 15
responds very quickly to velocity changes, but might produce regulation instabilities in some
constellations. A setting of 4 is a good starting value.
Hint
Operate the motor within your application when exploring stealthChop. Motor performance often is
better with a mechanical load, because it prevents the motor from stalling due mechanical oscillations
which can occur without load.
7.3 Fixed Scaling
Non-automatic, fixed scaling scales the stealthChop amplitude based on the user defined value
PWM_AMPL. The stepper motor has a certain coil resistance and thus needs a certain voltage
amplitude to yield a target current based on the basic formula I=U/R. With R being the coil resistance,
U the supply voltage scaled by the PWM value, the current I results. The initial value for PWM_AMPL
at low velocities can be calculated: