DocID18343 Rev 3

11/22

AN3327

L9942 stall detection methods

19

3.2

BEMF detection explained

Comparing the coil current and the induced BEMF voltage of one stepper phase during non-

stalled operation, it can be seen that there is a phase difference of 90° in between the

current and the BEMF voltage.

Figure 7. Back EMF and phase current

The BEMF stall detection algorithm takes advantage of this phenomenon. When the motor

current is transitioning from one polarity to the other there is one step that is typically

programmed at zero Amps. As a result checking for BEMF when the step current is zero

does not interfere with the control of the motor.