TMC246A Datasheet, PDF (12/24 Pages) List of Unclassifed Manufacturers – High Current Microstep Stepper Motor Driver with sensorless stall detection, protection / diagnosis and SPI Interface

TMC246 DATA SHEET (V2.01 / Sep. 14th, 2005)
12
Chopper Principle
Chopper cycle / Using the mixed decay feature
The TMC246 uses a quiet fixed frequency chopper. Both coils are chopped with a phase shift of 180
degrees. The mixed decay option is realized as a self stabilizing system (pat. fi.), by shortening the
fast decay phase, if the ON phase becomes longer. It is advised to enable the mixed decay for each
phase during the second half of each microstepping half-wave, when the current is meant to
decrease. This leads to less motor resonance, especially at medium velocities. With low velocities or
during standstill mixed decay should be switched off. In applications requiring high resolution, or using
low inductivity motors, the mixed decay mode can also be enabled continuously, to reduce the
minimum motor current which can be achieved. When mixed decay mode is continuously on or when
using high inductivity motors at low supply voltage, it is advised to raise the chopper frequency to
36kHz, because the half chopper frequency could be audible under these conditions.
target current phase A
actual current phase A
on
slow decay
on
fast decay
slow decay
oscillator clock
resp. external clock
mixed decay disabled
mixed decay enabled
When polarity is changed on one bridge, the PWM cycle on that bridge becomes restarted at once.
Fast decay switches off both upper transistors, while enabling the lower transistor opposite to the
selected polarity. Slow decay always enables both lower side transistors.
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 to adapt 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
GND
VCC
VCC
BL1 Typical blank time
GND 0.6 µs
VCC 0.9 µs
GND 1.2 µs
VCC 1.5 µs
Copyright © 2005, TRINAMIC Motion Control GmbH & Co KG