AMIS-30542 Datasheet, PDF (17/29 Pages) ON Semiconductor – AMIS-30542 Micro-Stepping Motor Driver

AMIS−30542
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.
I_COIL
V _BEMF
t
ICOIL
Previous
Micro−step
ZOOM
Coil Current Zero Crossing
Current Decay
Zero Current
Next
Micro−step
t
V COIL
VBB
Voltage Transient
|VBEMF |
t
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 14 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 is
divided by 2 or by 4. This divider is set through an SPI bit
<SLAG>. (see Table 14 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.
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