8902–A

3-PHASE BRUSHLESS DC

MOTOR CONTROLLER/DRIVER

Blanking and Watchdog Timing Functions. The blanking and

where

t

and

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

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.

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

to measure the time between crossings.

where

its 2.5 V reference while FCOM is high.

When FCOM goes low at the next zero

reaches the CD threshold, a commutation

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.

Dwg. WP-016-2

FCOM

t FCOM

tCD1

VCWD

V

CD1

VCD2

t CD2

WATCHDOG-TRIGGERED

COMMUTATION

Dwg. WP-022

BLANK

CWD

V

V

TL

NORMAL COMMUTATION

V

TL

Dwg. WP-021

BLANK

CWD

V