A8905CLB データシートの表示（PDF） - Allegro MicroSystems

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A8905CLB

3-PHASE BRUSHLESS DC MOTOR CONTROLLER/DRIVER WITH BACK-EMF SENSING

Allegro MicroSystems 

8905

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 commutation

time for efficient synchronous operation. This

circuit commutates the outputs, delayed from the

last zero crossing, using two external timing

capacitors, CD1 and CD2, to measure the time

between crossings.

t FCOM

FCOM

VCWD

VCD1

t CD1

VCD2

t CD2

capacitors, the voltage excursion should be 1.5 V to 2.5 V at rated speed.

Solving for C in the equation I = Cdv/dt, where dv = 2.5 V, I = 22 µA, and

dt = tFCOM =

20/RPM

#motor poles

Use of a capacitor slightly greater than this value will ensure that the commuta-

tion delay capacitors never charge to the high rail.

Blanking and Watchdog Timing Functions. The blanking and watchdog

timing functions are derived from one timing capacitor, CWD.

where

t = BLANK

VTL x CWD

ICWD

and

tWD =

VTH x CWD

ICWD

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.

V CWD

BLANK

V TL

t BLANK

Dwg. WP-016-2

where

[ ] tCD = tFCOM x ICD(charge),

ICD(discharge)

CD1 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, CD1 is

discharged at approximately twice the charging

current. When CD1 reaches the CD threshold, a

commutation occurs. CD2 operates similarly

except on the opposite phase of FCOM . Thus

the commutations occur approximately halfway

between zero crossings. The actual delay is

slightly less than halfway to compensate for

electrical delays in the motor, which improves

efficiency.

Because the commutation-delay capacitors

are adaptive in nature, the absolute value and

tolerance is not critical. In choosing these

NORMAL COMMUTATION

Dwg. WP-022

VTH

V CWD

V TL

t BLANK

BLANK

t WD

Dwg. WP-021

WATCHDOG-TRIGGERED COMMUTATION

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