AMIS-30421 Datasheet, PDF (21/41 Pages) ON Semiconductor – Micro-Stepping Stepper Motor Bridge Controller

AMIS−30421
transient behavior. This transient behavior (which is not the
BEMF) can be made visible or invisible on the SLA−pin by
means of SPI bit <SLAT>. When set to transparent
(<SLAT> = ‘1’), the coil voltage is sampled every PWM
cycle and updated on the SLA−pin (see Figure 19). When set
to not−transparent (<SLAT> = ‘0’), only the last sample
(taken right before leaving the “coil current zero crossing”)
will be copied to the SLA−pin (see Figure 20).
Icoil
When working in not−transparent mode (<SLAT> = ‘0’)
keep in mind that there is a delay between applying the NXT
pulse (to leave the “coil current zero crossing”) and the
updated voltage on the SLA−pin (see t_{SLA_DELAY} in
Figure 20 and Table 5).
Icoil
t
t
Previous
Microstep
V _NXT
Coil Current
Zero Crossing
Next
Microstep
t
Icoil
Current Decay
t
Previous
Microstep
V_NXT
Coil Current
Zero Crossing
Next
Microstep
t
Icoil
Current Decay
t
Vcoil V_BB + 0.6V
V _SLA
Transparent
Bemf of previous
zero crossing
V BEMF
t
Vcoil _{VBB + 0.6V}
V_SLA
Not−transparent Bemf of previous
zero crossing
VBEMF
t
Last sample before leaving
zero crossing is retained.
t
Remark: Vcoil is only drawn during the coil current zero crossing
Figure 19. Principle of BEMF Measurement in
Transparent Mode
The relationship between the voltage measured on the
SLA−pin and the coil voltage is:
V_SLA = 0.6 + (0.6 x <SLA_OFFS>) + (V_coil x <SLAG>)
SPI bit <SLA_OFFS> can be used to add an additional
offset of 0.6 V. Five different SLA gain values can be set by
means of SPI bits <SLAG[2:0]>.
AMIS−30421 has the ability to stretch the “coil current
zero crossing”. If NXT pulses are applied too fast it’s
possible that the “coil current zero crossing” is too short
making it impossible to measure the real BEMF (see
Remark: Vcoil is only drawn during the coil current zero crossing
tSLA _DELAY
t
Figure 20. Principle of BEMF Measurement in
Not−Transparent Mode
Figure 21). By using SPI bits <MIN_SLA_TIME[1:0]> one
can stretch the “coil current zero crossing” without changing
the speed of the motor (see Figure 21). AMIS−30421 will
ignore but keep track of the NXT pulses applied during the
“stretched coil current zero crossing” and compensate the
ignored pulses when leaving the “coil current zero
crossing”.
More information on using the SLA−pin can be found in
application note AND8399. Although this application note
refers to AMIS−305xx, it is also valid for AMIS−30421.
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