Datasheet

TMC2209 DATASHEET (Rev. 1.03 / 2019-JUN-26) 41
www.trinamic.com
Hint
The values for PWM_OFS and PWM_GRAD can easily be optimized by tracing the motor current with a
current probe on the oscilloscope. Alternatively, automatic tuning determines these values and they
can be read out from PWM_OFS_AUTO and PWM_GRAD_AUTO.
UNDERSTANDING THE BACK EMF CONSTANT OF A MOTOR
The back EMF constant is the voltage a motor generates when turned with a certain velocity. Often
motor datasheets do not specify this value, as it can be deducted from motor torque and coil current
rating. Within SI units, the numeric value of the back EMF constant C
BEMF
has the same numeric value
as the numeric value of the torque constant. For example, a motor with a torque constant of 1 Nm/A
would have a C
BEMF
of 1V/rad/s. Turning such a motor with 1 rps (1 rps = 1 revolution per second =
6.28 rad/s) generates a back EMF voltage of 6.28V. Thus, the back EMF constant can be calculated as:
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I
COILNOM
is the motor’s rated phase current for the specified holding torque
HoldingTorque is the motor specific holding torque, i.e. the torque reached at I
COILNOM
on both coils.
The torque unit is [Nm] where 1Nm = 100Ncm = 1000mNm.
The voltage is valid as RMS voltage per coil, thus the nominal current is multiplied by 2 in this
formula, since the nominal current assumes a full step position, with two coils operating.
6.5 Combine StealthChop and SpreadCycle
For applications requiring high velocity motion, SpreadCycle may bring more stable operation in the
upper velocity range. To combine no-noise operation with highest dynamic performance, the TMC2209
allows combining StealthChop and SpreadCycle based on a velocity threshold (Figure 6.7). A velocity
threshold (TPWMTHRS) can be preprogrammed to OTP to support this mode even in standalone
operation. With this, StealthChop is only active at low velocities.
Running low speed
optionoption
Running low speed
Running high speed
motor stand still
motor going to standby
motor in standby motor in standby
v
t
TSTEP < TPWMTHRS*16/16
TSTEP > TPWMTHRS
0
VACTUAL
~1/TSTEP
current
TPOWERDOWN
RMS current
I_HOLD
I_RUN
dI * IHOLDDELAY
stealthChop
spreadCycle
Chopper mode
TRINAMIC, B. Dwersteg, 14.3.14
Figure 6.7 TPWMTHRS for optional switching to SpreadCycle
UART OTP