TMC2100 DATASHEET (Rev. 1.07 / 2017-MAY-15)
18
mode rises with the supply voltage. A lower blanking time allows a lower current limit. The run
current needs to be kept above the lower limit: In case the PWM scale drops to a too low value, e.g.
because AIN pin scaling was too low, the regulator may not be able to recover. The regulator will
recover once the motor is in standstill. The lower motor coil current limit can be calculated from
motor parameters and chopper settings:
푉푀
퐼퐿표푤푒푟 퐿푖푚푖푡 = ꢀ퐵퐿퐴푁퐾 ∗ 푓
∗
푃푊푀
푅퐶푂ꢁ퐿
With VM the motor supply voltage and RCOIL the motor coil resistance.
ILower Limit can be treated as a thumb value for the minimum possible motor current setting.
EXAMPLE:
A motor has a coil resistance of 5Ω, the supply voltage is 24V. tBLANK setting is 24 clock cycles:
2
24푉 2 ∗ 24 24푉
퐼퐿표푤푒푟 퐿푖푚푖푡 = 24 ꢀ퐶퐿퐾
∗
∗
=
∗
= 337ꢂꢃ
683 ꢀ퐶퐿퐾 5Ω
683
5Ω
For pwm_autoscale mode, a lower coil current limit applies. This limit can be calculated or measured
using a current probe. Keep the motor run-current setting well above this lower current limit.
5.3 Acceleration
The automatic current regulation compensates for the change of back-EMF at different velocities (see
Figure 5.2 and Figure 5.3). It measures the actual current with each fullstep and subsequently does a
limited correction of the PWM voltage. Therefore, at high acceleration or deceleration, the internal
regulation might not react quickly enough to stabilize the motor current within a range near the
target current. Use a current probe and check the motor current during (quick) acceleration. In case
the current regulation cannot follow (see Figure 5.4), motor current will sink in acceleration phases,
and thus lead to reduced motor torque. During a deceleration phase it will rise for a short time. If the
current deviates too much, the motor cannot bring the required torque during the acceleration phase.
In case motor current drops significantly during acceleration phases with stealthChop there are
several ways to improve current regulation:
-
-
-
-
-
Reduce acceleration
Switch from hold current to run current in time before starting the motion
Increase driver supply voltage (at double voltage, the reaction speed is also doubled)
Use motor with higher current coil winding (back-EMF sinks proportionally)
Increase driver clock frequency (this will speed up regulation when accelerating from
standstill current reduction)
-
Switch configuration from stealthChop to spreadCycle before starting a high velocity motion
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