8905

3-PHASE BRUSHLESS DC

MOTOR CONTROLLER/DRIVER

115 Northeast Cutoff, Box 15036

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

Power Outputs. The power outputs of the

A8905CLB are n-channel DMOS transistors with

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 induc-

tive 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 A8905CLB 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 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 cross-

ings 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 indicated 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 the external

where

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

motor will accelerate synchronously. Once normal synchronous back-EMF

commutation occurs, the startup oscillator is defeated by pulses of pulldown

upper threshold and thus completing a 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

VCSTL

V

CSTH

V

CWD

V

CST

CST

Dwg. WP-020

t

FUNCTIONAL DESCRIPTION