8904

3-PHASE BRUSHLESS DC

MOTOR CONTROLLER/DRIVER

115 Northeast Cutoff, Box 15036

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

8

Functional Description

Overview of operation. Each electrical revolution

contains six states that control the three half-bridge outputs.

Optimized switching from state to state is achieved through the

adaptive commutation circuitry. During any state, one output is

high, one is low and the other is high impedance. The back-

EMF at the high-impedance output is sensed and compared to

the voltage of the centertap and when the two signals are

equivalent, the FCOM signal toggles. A controlled delay is then

introduced before the sequencer commutates into the next state.

Linear current-mode control is employed to provide

precision control of the motor speed while maintaining ex-

tremely low electrical noise emissions. The speed control is

realized through a frequency-locked loop that processes the

sensed back-EMF signals from the stator phases to eventually

produce a TACH signal. The TACH signal is then compared to

the desired programmed speed, to produce an error. The error

signal is then used to linearly control the current through the

low-side DMOS power devices to obtain the correct speed.

Alternative control schemes can be introduced, giving the

user maximum flexibility and optimization for each application.

An external tachometer signal applied to the SECTOR DATA

input, along with the internal speed reference can be used for

high-precision speed control. As another alternative, the user can

introduce external speed control by driving the FILTER terminal

directly.

Start-up routines are inherent in the solution to guarantee

reliable start-up. During start-up, a YANK feature allows rapid

transition to the nominal operating condition on the FILTER

terminal. This feature is also available when the external speed

control is used.

Dynamic braking can be introduced by either the external

BRAKE terminal or through the brake bit in the serial port.

A serial port allows the user to program various features and

modes of operation, such as motor speed, internal or external

speed control, internal or external speed reference, current limit,

sleep mode, direction, charge current (for blanking pulse), motor

poles, transconductance gain, and various diagnostic outputs.

Full device protection is incorporated, including program-

mable overcurrent limit, thermal shutdown, and undervoltage

shutdown on the logic supply.

Power outputs. The power outputs of the A8904 are n-

channel DMOS transistors with a total source plus sink r

typically 1

Ω. An internal charge pump provides a voltage rail

above the load supply for driving the high-side DMOS gates.

Intrinsic ground clamp and flyback diodes provide protection

when switching inductive loads. These diodes will also rectify

the motor back-EMF during power-down conditions. If neces-

sary, a transient voltage supply can be provided, by connecting

an external Schottky power diode or pass FET in series, between

the power source and the load supply (V

effectively isolates the low impedance path through the power

source. A filter capacitor is also required to ‘hold up’ the

rectified signal, and is connected between the load supply and

ground.

Back-EMF sensing motor startup and running

algorithm. The A8904 provides a complete self-contained

back-EMF sensing, startup and running commutation scheme. A

state machine with six states, (shown in the tables below for

both forward and reverse direction) controls the three half-

bridge outputs. 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 output that

is OFF.

Sequencer State

(forward direction)

OUT

A

OUT

B

OUT

C

1

High

Z

Low

2

High

Low

Z

3

Z

Low

High

4

Low

Z

High

5

Low

High

Z

6

Z

High

Low

Sequencer State

(reverse direction)

OUT

A

OUT

B

OUT

C

1

High

Z

Low

6

Z

High

Low

5

Low

High

Z

4

Low

Z

High

3

Z

Low

High

2

High

Low

Z

At start-up, the outputs are always enabled in state 1. 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 does not move during the initial start-up state,

the outputs are commutated automatically by the start-up

oscillator. When suitable back-EMF signals are detected, the

start-up oscillator is overridden and the corresponding timing

clock is generated, providing synchronous back-EMF commuta-

tion. The start-up oscillator period is determined by

t

where C