TMC5031 DATASHEET (Rev. 1.07 / 2013-APR-30)
36
7.2 Classic 2-Phase Motor Constant Off Time Chopper
The classic constant off-time chopper uses a fixed-time fast decay following each on phase. While the
duration of the on phase is determined by the chopper comparator, the fast decay time needs to be
long enough for the driver to follow the falling slope of the sine wave, but it should not be so long
that it causes excess motor current ripple and power dissipation. This can be tuned using an
oscilloscope or evaluating motor smoothness at different velocities. A good starting value is a fast
decay time setting similar to the slow decay time setting.
I
target current + offset
mean value = target current
on
on
sd
fd
sd
fd
t
Figure 7.3 Classic const. off time chopper with offset showing coil current
After tuning the fast decay time, the offset should be tuned for a smooth zero crossing. This is
necessary because the fast decay phase makes the absolute value of the motor current lower than the
target current (see Figure 7.4). If the zero offset is too low, the motor stands still for a short moment
during current zero crossing. If it is set too high, it makes a larger microstep. Typically, a positive
offset setting is required for smoothest operation.
Target current
Coil current
Target current
Coil current
I
I
t
t
Coil current does not have optimum shape
Target current corrected for optimum shape of coil current
Figure 7.4 Zero crossing with classic chopper and correction using sine wave offset
Three parameters control constant off-time mode:
Parameter
Description
Setting Comment
TFD
Fast decay time setting. With CHM=1, these bits 0
slow decay only
(fd3
HSTRT)
& control the portion of fast decay for each chopper
cycle.
1…15
duration of fast decay
phase
OFFSET
(HEND)
Sine wave offset. With CHM=1, these bits control 0…2
negative offset: -3…-1
no offset: 0
the sine wave offset. A positive offset corrects for
zero crossing error.
3
4…15
positive offset 1…12
disfdcc
Selects usage of the current comparator for 0
termination of the fast decay cycle. If current
comparator is enabled, it terminates the fast decay
enable comparator
termination of fast decay
cycle
cycle in case the current reaches a higher negative
value than the actual positive value.
1
end by time only
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