TMC246 / TMC246A DATA SHEET (V2.05 / 2009-Jul-30)
15
Adapting the sine wave for smooth motor operation
After reaching the target current in each chopper cycle, both, the slow decay and the fast decay cycle
reduce the current by some amount. Especially the fast decay cycle has a larger impact. Thus, the
medium coil current always is a bit lower than the target current. This leads to a flat line in the current
shape flowing through the motor. It can be corrected, by applying an offset to the sine shape. In mixed
decay operation via SPI, an offset of 1 does the job for most motors.
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
Blank Time
The TMC246 uses a digital blanking pulse for the current chopper comparators. This prevents current
spikes, which can occur during switching action due to capacitive loading, from terminating the
chopper cycle. The lowest possible blanking time gives the best results for microstepping: A long blank
time leads to a long minimum turn-on time, thus giving an increased lower limit for the current. Please
remark, that the blank time should cover both, switch-off time of the lower side transistors and turn-on
time of the upper side transistors plus some time for the current to settle. Thus the complete switching
duration should never exceed 1.5µs.
The TMC246 allows adapting the blank time to the load conditions and to the selected slope in four
steps (the effective resulting blank times are about 200ns shorter in the non-A-type):
Blank time settings
BL2
GND
BL1
Typical blank time
GND 0.6 µs
VCC 0.9 µs
GND 1.2 µs
VCC 1.5 µs
GND
VCC
VCC
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TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
