A8906CLB データシートの表示（PDF） - Allegro MicroSystems

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A8906CLB

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

Allegro MicroSystems 

8906

BIDIRECTIONAL

3-PHASE BRUSHLESS DC

MOTOR CONTROLLER/DRIVER

FUNCTIONAL DESCRIPTION

Power Outputs. The power outputs of

the A8906CLB are n-channel DMOS transis-

tors with a total source plus sink rDS(on) 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 A8906CLB pro-

vides 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:

Direction

Control

OUTA

High

Z

Low

Low

Z

High

OUTB

Low

Low

Z

High

High

Z

OUTC

Z

High

High

Z

Low

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 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 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.

V OUTA

V OUTB

V OUTC

SOURCE ON

V CTAP

BACK-EMF VOLTAGE

SINK ON

FCOM TOGGLES AT

BACK-EMF ZERO CROSSING

FCOM

Dwg. WP-016-1

Direction Control. Serial port bit D2 controls the direction of

rotation (see sequencer state table). The motor should be at a com-

plete stop before beginning a startup sequence that reverses the

direction of rotation.

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 capacitor at CST where

tCST =

4(VCSTH - VCSTL) x CST

IST(charge) + IST(discharge)

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

by pulses of pulldown current at CST at each commutation, which

prevents CST from reaching its upper threshold and thus completing a

cycle and commutating.

115 Northeast Cutoff, Box 15036

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

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