TMC5130A DATASHEET (Rev. 1.14 / 2017-MAY-15)
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7.2 Automatic Scaling
In stealthChop voltage PWM mode, the autoscaling function (pwm_autoscale = 1) regulates the motor
current to the desired current setting. The driver measures the motor current during the chopper on
time and uses a proportional regulator to regulate the PWM_SCALE in order match the motor current
to the target current. PWM_GRAD is the proportionality coefficient for this regulator. Basically, the
proportionality coefficient should be as small as possible in order to get a stable and soft regulation
behavior, but it must be large enough to allow the driver to quickly react to changes caused by
variation of the motor target current, the motor velocity or effects resulting from changes of the
supply voltage. As the supply voltage level and motor temperature normally change only slowly, a
minimum setting of the regulation gradient often is sufficient (PWM_GRAD=1). If stealthChop
operation is desired for a higher velocity range, variations of the motor back EMF caused by motor
acceleration and deceleration may require a quicker regulation. Therefore, PWM_GRAD setting should
be optimized for the fastest required acceleration and deceleration ramp (see Figure 7.4). The quality
of a given setting can be examined when monitoring PWM_SCALE and motor velocity. Just as in the
acceleration phase, during a deceleration phase the voltage PWM amplitude must be adapted in order
to keep the motor coil current constant. When the upper acceleration and the upper deceleration used
in the application are identical, the value determined for the acceleration phase will already be
optimum for both.
Figure 7.2 Scope shot: good setting for PWM_GRAD
Figure 7.3 Scope shot: too small setting for PWM_GRAD
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