AMIS-30521_10 Datasheet, PDF (17/28 Pages) ON Semiconductor – Micro-Stepping Motor Driver

AMIS−30521, NCV70521
I_COIL
V_BEMF
t
Previous
Micro−Step
I_COIL
ZOOM
Coil Current Zero Crossing
Current Decay
Zero Current
Next
Micro−Step
t
V_COIL
V_BB
Voltage Transient
|V_BEMF|
t
Figure 13. Principle of Bemf measurement
Because of the relatively high re−circulation 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). 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 a SPI bit
<SLAG>. (See Table 12)
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.
http://onsemi.com
17