TMC5031 DATASHEET (Rev. 1.11 / 2016-APR-28)
34
7 Chopper Operation
The currents through both motor coils are controlled using choppers. The choppers work
independently of each other. In Figure 7.1 the different chopper phases are shown.
+VM
+VM
+VM
ICOIL
ICOIL
ICOIL
RSENSE
RSENSE
RSENSE
On Phase:
Fast Decay Phase:
current flows in
opposite direction
of target current
Slow Decay Phase:
current re-circulation
current flows in
direction of target
current
Figure 7.1 Chopper phases
Although the current could be regulated using only on phases and fast decay phases, insertion of the
slow decay phase is important to reduce electrical losses and current ripple in the motor. The
duration of the slow decay phase is specified in a control parameter and sets an upper limit on the
chopper frequency. The current comparator can measure coil current during phases when the current
flows through the sense resistor, but not during the slow decay phase, so the slow decay phase is
terminated by a timer. The on phase is terminated by the comparator when the current through the
coil reaches the target current. The fast decay phase may be terminated by either the comparator or
another timer.
When the coil current is switched, spikes at the sense resistors occur due to charging and discharging
parasitic capacitances. During this time, typically one or two microseconds, the current cannot be
measured. Blanking is the time when the input to the comparator is masked to block these spikes.
There are two chopper modes available: a new high-performance chopper algorithm called
spreadCycle and a proven constant off-time chopper mode. The constant off-time mode cycles through
three phases: on, fast decay, and slow decay. The spreadCycle mode cycles through four phases: on,
slow decay, fast decay, and a second slow decay.
The chopper frequency is an important parameter for a chopped motor driver. A too low frequency
might generate audible noise. A higher frequency reduces current ripple in the motor, but with a too
high frequency magnetic losses may rise. Also power dissipation in the driver rises with increasing
frequency due to the increased influence of switching slopes causing dynamic dissipation. Therefore, a
compromise needs to be found. Most motors are optimally working in a frequency range of 20 kHz to
30 kHz. The chopper frequency is influenced by a number of parameter settings as well as by the
motor inductivity and supply voltage.
Hint
A chopper frequency in the range of 16 kHz to 30 kHz gives a good result for most motors. A higher
frequency leads to increased switching losses. It is advised to check the resulting frequency and to
work below 50 kHz.
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TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
