TMC260 and TMC261 DATASHEET (Rev. 2.05 / 2012-NOV-05)
35
Parameter
Description
Setting
Comment
HDEC
Hysteresis decrement setting. This setting 0… 3
determines the slope of the hysteresis during on
time and during fast decay time. It sets the
number of system clocks for each decrement.
0: fast decrement
3: very slow decrement
%00: 16
%01: 32
%10: 48
%11: 64
Example:
In the example above, a hysteresis start of 7 has been chosen. The hysteresis end is set to
about half of this value, 3. The resulting configuration register values are:
HEND=6 (sets an effective end value of 3)
HSTRT=3 (sets an effective start value of hysteresis end +4)
HDEC=0 (Hysteresis decrement becomes used)
9.2 Constant Off-Time Mode
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
fast 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 9.3 Constant off-time chopper with offset showing the coil current during two cycles,
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 9.4). 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 9.4 Zero crossing with correction using sine wave offset.
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