A8902CLBA Allegro MicroSystems, A8902CLBA Datasheet - Page 7

• Manufacturer Part Number: A8902CLBA
• Description: 3-PHASE BRUSHLESS DC MOTOR CONTROLLER/DRIVER WITH BACK-EMF SENSING
• Manufacturer: Allegro MicroSystems

adaptive commutation delay circuit uses the
back-EMF zero-crossing indicator signal
(FCOM) to determine an optimal commuta-
tion time for efficient synchronous operation.
This circuit commutates the outputs, delayed
from the last zero crossing, using two
external timing capacitors, C
to measure the time between crossings.
where
its 2.5 V reference while FCOM is high.
When FCOM goes low at the next zero
crossing, C
twice the charging current. When CD
reaches the CD threshold, a commutation
occurs. C
opposite phase of FCOM . Thus the com-
mutations occur approximately halfway
between zero crossings. The actual delay is
slightly less than halfway to compensate for
electrical delays in the motor, which im-
proves efficiency.
V CWD
FCOM
V CD1
V CD2
Adaptive Commutation Delay. The
C
D1
charges up with a fixed current from
D2
D1
t
CD
operates similarly except on the
is discharged at approximately
t CD1
= t
t FCOM
FCOM
x
I
CD(discharge)
I
CD(charge)
D1
and C
t CD2
D2
1
Dwg. WP-016-2
,
watchdog timing functions are derived from one timing capacitor, C
where
and
the BLANK signal. BLANK is an internal signal that inhibits the back-
EMF comparators during the commutation transients, preventing errors
due to inductive recovery and voltage settling transients.
position by checking the back-EMF polarity after each commutation. If
the correct polarity is not observed between t
watchdog timer commutates the outputs to the next state to synchro-
nize the motor. This function is useful in preventing excessive reverse
rotation, and helps in resynchronizing (or starting) with a moving
spindle.
Blanking and Watchdog Timing Functions. The blanking and
The CWD capacitor begins charging at each commutation, initiating
The watchdog timing function allows time to detect correct motor
BLANK
V
CWD
BLANK
V
t
t
BLANK
CWD
WD
MOTOR CONTROLLER/DRIVER
=
=
V
t
V
TH
BLANK
t
TL
BLANK
V
I
x C
TL
CWD
I
x C
CWD
3-PHASE BRUSHLESS DC
WD
WD
V
TL
t
WD
BLANK
and t
Dwg. WP-022
V
WD
Dwg. WP-021
8902–A
TH
, then the
WD
.