8902–A

3-PHASE BRUSHLESS DC

MOTOR CONTROLLER/DRIVER

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

backward, or remain stationary (if in a null-torque position). If the motor

moves, the back-EMF detection circuit waits for the correct polarity

back-EMF zero crossing (output crossing through centertap). True

back-EMF zero crossings are used by the adaptive commutation delay

circuit to advance the state sequencer (commutate) at the proper time

to synchronously run the motor. Back-EMF zero crossings are indi-

cated by FCOM, an internal signal that toggles at every zero crossing.

FCOM is available at the DATA OUT terminal via the programmable

data out multiplexer.

Startup Oscillator. If the motor does not move at the initial startup

state, then it is in a null-torque position. In this case, the outputs are

commutated automatically by the startup oscillator after a period set by

In the next state, the motor will move, back EMF will be detected,

and the motor will accelerate synchronously. Once normal synchro-

nous back-EMF commutation occurs, the startup oscillator is defeated

cycle and commutating.

Dwg. WP-016-1

FCOM

FCOM TOGGLES AT

BACK-EMF ZERO CROSSING

SOURCE ON

SINK ON

BACK-EMF VOLTAGE

V

OUTA

V

OUTB

V

OUTC

V

CTAP

Power Outputs. The power outputs of

the A8902CLBA are n-channel DMOS

of typically 1

Ω. Internal charge pump boost

circuitry provides voltage above supply for

driving the high-side DMOS gates. Intrinsic

ground clamp and flyback diodes provide

protection when switching inductive loads and

may be used to rectify motor back-EMF in

power-down conditions. An external Schottky

power diode or pass FET is required in series

with the load supply to allow motor back-EMF

rectification in power down conditions.

Back-EMF Sensing Motor Startup and

Running Algorithm. The A8902CLBA

provides a complete self-contained back-EMF

sensing startup and running commutation

scheme. The three half-bridge outputs are

controlled by a state machine. There are six

possible combinations. In each state, one

output is high (sourcing current), one low

(sinking current), and one is OFF (high

impedance or ‘Z’). Motor back EMF is sensed

at the OFF output. The truth table for the

output drivers sequencing is:

Sequencer

State

1

High

Low

Z

2

Z

Low

High

3

Low

Z

High

4

Low

High

Z

5

Z

High

Low

6

High

Z

Low

At startup, the outputs are enabled in one

of the sequencer states shown. The back

EMF is examined at the OFF output by

comparing the output voltage to the motor

centertap voltage at CENTERTAP. The

motor will then either step forward, step

FUNCTIONAL DESCRIPTION