User Manual

AND8399/D

http://onsemi.com

Transparency Mode

It’s not 100% correct to say that the voltage on the

SLA−pin will represent the real Bemf voltage of the stepper

motor if a microstep position is located at the “coil current

zero crossing”. At the moment the current less state is

entered, the coil voltage is measured. Because at that

moment the coil current will not yet be zero (it takes time to

get all the current out of the coil), the coil voltage will clamp

to the power supply voltage (VBB) + 0.6 V. Once the coil

current is zero, the coil voltage will decay (transient

behavior). The coil voltage will decay to the Bemf voltage

of the stepper motor. It’s only at that moment that the coil

voltage will be equal to the Bemf voltage of the motor!

Figure 11. Coil Voltage Behavior During Current Less State

coil

Coil Current

Zero Crossing

Microstep

coil

+0.6V

BEMF

Voltage Transient

Current Decay

Next step

If the speed of the stepper motor is set too high, it’s

possible that the next (micro)step is set before the transient

behavior has ended. If this is the case, the voltage on the

SLA−pin will never represent the real Bemf voltage.

To help determining if this is the case, AMIS−305xx has

a so called ‘transparent’ mode. In this mode the coil voltage

during the current less state can be measured on the

SLA−pin. This makes it possible to monitor the transient

behavior of the coil voltage during this stage and to verify if

the real Bemf voltage of the stepper motor can be sampled.