Data Sheet

AMIS−30543

http://onsemi.com

Speed and Load Angle Output

The SLA−pin provides an output voltage that indicates the

level of the Back−e.m.f. voltage of the motor. This

Back−e.m.f. voltage is sampled during every so−called ”coil

current zero crossings”. Per coil, two zero−current positions

exist per electrical period, yielding in total four zero−current

observation points per electrical period.

BEMF

ZOOM

COIL

Voltage Transient

Micro−step

Coil Current Zero Crossing

Current Decay

Zero Current

COIL

BEMF

Figure 14. Principle of Bemf Measurement

Because of the relatively high recirculation currents in the

coil during current decay, the coil voltage V

COIL

shows a

transient behavior. As this transient is not always desired in

application software, two operating modes can be selected

by means of the bit <SLAT> (see “SLA−transparency” in

Table 12 SPI Control Parameter Overview). The SLA pin

shows in “transparent mode” full visibility of the voltage

transient behavior. This allows a sanity−check of the

speed−setting versus motor operation and characteristics

and supply voltage levels. If the bit “SLAT” is cleared, then

only the voltage samples at the end of each coil current zero

crossing are visible on the SLA−pin. Because the transient

behavior of the coil voltage is not visible anymore, this mode

generates smoother Back e.m.f. input for post−processing,

e.g. by software.

In order to bring the sampled Back e.m.f. to a descent

output level (0 V to 5 V), the sampled coil voltage V

COIL

divided by 2 or by 4. This divider is set through an SPI bit

<SLAG>. (see Table 12 SPI Control Parameter Overview)

The following drawing illustrates the operation of the

SLA−pin and the transparency−bit. “PWMsh” and “I

COIL

0” are internal signals that define together with SLAT the

sampling and hold moments of the coil voltage.