TMC262 DATASHEET (Rev. 2.07 / 2013-FEB-14)
37
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.
(HSTART &
HDEC0)
OFFSET
(HEND)
control the portion of fast decay for each
chopper cycle.
Sine wave offset. With CHM=1, these bits control 0…2
the sine wave offset. A positive offset corrects
for zero crossing error.
Selects usage of the current comparator for 0
termination of the fast decay cycle. If current
comparator is enabled, it terminates the fast
1… 15
Duration of fast decay
phase.
Negative offset: -3… -1
No offset: 0
Positive offset: 1… 12
Enable comparator
termination of fast
decay cycle.
3
4… 15
NCCFD
(HDEC1)
decay cycle in case the current reaches a higher
negative value than the actual positive value.
1
End by time only.
9.2.1 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.
A common cause of motor noise is a bad PCB layout causing coupling of both sense resistor
voltages.
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
Enables a random off-time generator, which 0
Disable.
slightly modulates the off time tOFF using a
random polynomial.
1
Random modulation
enable.
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TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
