8902A Datasheet, PDF (7/12 Pages) Allegro MicroSystems – 3-PHASE BRUSHLESS DC MOTOR CONTROLLER/DRIVER WITH BACK-EMF SENSING

8902–A
3-PHASE BRUSHLESS DC
MOTOR CONTROLLER/DRIVER
Adaptive Commutation Delay. The
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_D1 and C_D2,
FCOM
t _FCOM
VCWD
VCD1
t _CD1
Blanking and Watchdog Timing Functions. The blanking and
watchdog timing functions are derived from one timing capacitor, C_WD.
where
t = _BLANK
V_TL x C_WD
I_CWD
V_TH x C_WD
and
t_WD =
I_CWD
The CWD capacitor begins charging at each commutation, initiating
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.
The watchdog timing function allows time to detect correct motor
position by checking the back-EMF polarity after each commutation. If
the correct polarity is not observed between t_BLANK and t_WD, then the
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.
VCD2
t _CD2
Dwg. WP-016-2
to measure the time between crossings.
where
t_CD = t_FCOM x ^ICD(charge)
I_{CD(discharge)}
C_D1 charges up with a fixed current from
its 2.5 V reference while FCOM is high.
When FCOM goes low at the next zero
crossing, C_D1 is discharged at approximately
twice the charging current. When CD₁
reaches the CD threshold, a commutation
occurs. C_D2 operates similarly except on the
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
BLANK
V _TL
t _BLANK
Dwg. WP-022
NORMAL COMMUTATION
V_TH
V _CWD
BLANK
V _TL
t _BLANK
t _WD
WATCHDOG-TRIGGERED
COMMUTATION
Dwg. WP-021