TMC248-LA DATASHEET (Rev. 1.01 / 2013-MAR-26)
18
7 Chopper Operation
The currents through both motor coils are controlled using a chopper. The TMC248 uses a quiet fixed
frequency chopper. Both coils are chopped with a phase shift of 180 degrees. The Chopper cycles
through three phases: on, fast decay, and slow decay.
+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
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.
When the polarity is changed on one bridge, the PWM cycle on that bridge becomes restarted at
once.
7.1 Mixed Decay Mode
The mixed decay option is realized as a self stabilizing system, 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.
MIXED DECAY IN APPLICATIONS WITH HIGH RESOLUTION OR LOW INDUCTIVITY MOTORS
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.
USING MIXED DECAY CONTINUOUSLY OR WITH HIGH INDUCTIVITY MOTORS AT LOW SUPPLY VOLTAGE
If mixed decay mode is continuously on or high inductivity motors are used at low supply voltage, it
is advised to raise the chopper frequency to minimum 36 kHz, because the half chopper frequency
could become audible.
With low velocities or during standstill mixed decay should be switched off.
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TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
