Limitations

AN3327

18/22

DocID18343 Rev 3

5

Limitations

These cases compare an unloaded motor with a stalled motor. The differences between

these two states are dramatic and easily detectable. From the above analysis we can see

that a loaded motor causes the detected BEMF thresholds to droop as the BEMF shifts to

be more aligned with current. This drooping must be taken into account when considering

an acceptable stall threshold. For every application there is a maximum expected torque

requirement for an application. This maximum torque requirement must be taken into

account when determining the BEMF stall threshold.

Other things that might limit this method include a loose or spongy transmission or “soft”

stall where the rotor is allowed to "bounce". These limitations are even more difficult to

overcome in the current / duty cycle methodology. Because the BEMF sensing is done

externally to the IC this limitation can be overcome to some extent with a statistical method

discerning the stall threshold.