Datasheet

TMC2041 DATASHEET (Rev. 1.02 / 2017-MAY-16) 39
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Parameter
Description
Setting
Comment
TFD
(fd3 &
HSTRT)
Fast decay time setting. With CHM=1, these bits
control the portion of fast decay for each chopper
cycle.
0
slow decay only
1…15
duration of fast decay
phase
OFFSET
(HEND)
Sine wave offset. With CHM=1, these bits control
the sine wave offset. A positive offset corrects for
zero crossing error.
0…2
negative offset: -3…-1
3
no offset: 0
4…15
positive offset 1…12
disfdcc
Selects usage of the current comparator for
termination of the fast decay cycle. If current
comparator is enabled, it terminates the fast decay
cycle in case the current reaches a higher negative
value than the actual positive value.
0
enable comparator
termination of fast decay
cycle
1
end by time only
8.3 Random Off Time
In the constant off-time chopper mode, both coil choppers run freely without synchronization. The
frequency of each chopper mainly depends on the coil current and the motor coil inductance. The
inductance varies with the microstep position. With some motors, a slightly audible beat can occur
between the chopper frequencies when they are close together. This typically occurs at a few
microstep positions within each quarter wave. This effect is usually not audible when compared to
mechanical noise generated by ball bearings, etc. Another factor which can cause a similar effect is a
poor layout of the sense resistor GND connections.
Hint
A common factor, which can cause motor noise, is a bad PCB layout causing coupling of both sense
resistor voltages (please refer layouts hint in chapter 19).
To minimize the effect of a beat between both chopper frequencies, an internal random generator is
provided. It modulates the slow decay time setting when switched on by the rndtf bit. The rndtf
feature further spreads the chopper spectrum, reducing electromagnetic emission on single
frequencies.
Parameter
Description
Setting
Comment
rndtf
This bit switches on a random off time generator,
which slightly modulates the off time TOFF using
a random polynomial.
0
disable
1
random modulation
enable