Datasheet

TMC2590 DATASHEET (V1.0 / 2019-FEB-22) 42
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t
I
Target current
Coil current
t
I
Target current
Coil current
Coil current does not have optimum shape Target current corrected for optimum shape of coil current
Figure 10.5 Zero crossing with correction using sine wave offset
Three parameters control constant off-time mode:
Parameter
Description
Setting
Comment
TFD
(HSTART &
HDEC0)
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
NCCFD
(HDEC1)
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.
10.2.1 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.
Hint
A common cause of motor noise is a bad PCB layout causing coupling of both sense resistor
voltages. Noise caused by an insufficient PCB layout cannot be overcome by chopper settings.
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
slightly modulates the off-time t
OFF
using a
random polynomial.
0
Disable.
1
Random modulation
enabled.