AMIS-30512 Datasheet, PDF (17/30 Pages) ON Semiconductor – Micro-Stepping Motor Driver

AMIS−30512
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
ICOIL
current zero crossings”. Per coil, two zero−current positions
exist per electrical period, yielding in total four zero−current
observation points per electrical period.
V _BEMF
t
ZOOM
Previous
Micro−step
Coil Current Zero Crossing
Next
Micro−step
I_COIL
Current Decay
Zero Current
t
V_COIL
V_BB
Voltage Transient
V_BEMF
t
Figure 12. 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 15: SPI Control Register 2). 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 any more, 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 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>. (Table 15: SPI Control Register 2)
The following drawing illustrates the operation of the
SLA−pin and the transparency−bit. “PWMsh” and
“Icoil=0” are internal signals that define together with SLAT
the sampling and hold moments of the coil voltage.
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