TMC262 DATASHEET (Rev. 2.07 / 2013-FEB-14)
33
9 Chopper Operation
The currents through both motor coils are controlled using choppers. The choppers work
independently of each other. Figure 9.1 shows the three chopper phases:
+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 9.1 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.
Three parameters are used for controlling both chopper modes:
Parameter
Description
Setting
Comment
TOFF
Off time. This setting controls the duration of the 0
Chopper off.
slow decay time and limits the maximum
chopper frequency. For most applications an off
time within the range of 5µs to 20µs will fit.
If the value is 0, the MOSFETs are all shut off and
the motor can freewheel.
1… 15
Off time setting.
(1 will work with
minimum blank time
of 24 clocks.)
A value of 1 to 15 sets the number of system
clock cycles in the slow decay phase to:
ꢏ
ꢔ
ꢻꣂꣃ꣄ ꢇꢌꢇ ꢓꣅ ꢰꢦ ꢯ ꢐꢦ
The SD-Time is
ꢀꢇ ꢌꢇꢞ ꢻꣂꣃ꣄
www.trinamic.com
TRINAMIC [ TRINAMIC MOTION CONTROL GMBH & CO. KG. ]
