A4964 datasheet(39/92 Pages) ALLEGRO

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Sensorless Sinusoidal Drive BLDC Controller

A4964

Allegro MicroSystems, LLC

955 Perimeter Road

Manchester, NH 03103-3353 U.S.A.

www.allegromicro.com

To ensure a smooth transition into the full commutation closed-

loop operation, the peak duty cycle applied to the power bridge

should be close to the duty cycle required for the present motor

speed. This avoids large inrush or braking currents as the drive is

re-enabled. The required duty cycle is determined using the mea-

sured motor speed, the speed resolution and the motor constant,

KM, as:

(%)

1023

100

where DPK is the peak duty cycle applied to the bridge, fE is the

motor electrical cycle frequency and KM is the motor constant set

by the value in the KM[3:0] variable and can be between 0.3 and

1.05 in steps of 0.05.

The actual motor motor electrical cycle frequency, fE, is defined

by:

fE = n × SR (Hz)

where SR is the speed resolution defined by the SR[2:0] variable

and n is a positive integer defined by DO[9:0] when RBS[2:0] =

[001].

Calculation of the correct duty cycle is also used during the wind-

mill detection phase if enabled.

In many systems where the motor and load inertia is low and the

friction load is high, it is more effective to go directly from ramp

to closed-loop commutation without the coast period. In this case,

STM can be set to zero to disable the coast function during start-

up. The coast function during startup is beneficial for high inertia

loads such as fans and blowers and helps provide a low audible

noise startup.

Once in commutation closed-loop mode, the A4964 will synchro-

nize the applied field, created by the motor phase angle counter

and the sinusoidal PWM generator, to the motor position and

ramp the speed to match the demand input.

START WITH PRE-ROTATION (WINDMILLING)

In some cases when motor start is required, the motor may still be

coasting from a previously running state or it may be rotating by

the action of the load on the motor. Setting the WIN bit to 1 will

modify the startup sequence by initially monitoring the motor bemf

for windmill bemf zero-crossing events in order to detect any pre-

rotation. A windmill bemf zero-crossing event must be present for

longer than the windmill bemf filter time, tbf, for the motor speed

and direction to be determined. The windmill bemf fiter time is set

through the serial interface using the BF variable. The motor speed

is available from the readback register during this time.

If no windmill bemf zero-crossing events are detected within

300 ms or the motor speed is less than the minimum windmill

detection frequency, fWMF, defined by the WMF[3:0] variable,

then the motor is assumed to be stationary and the normal start

sequence is followed. If WIN is set to 0, then any pre-rotation

will be ignored and the normal start sequence followed.

If the bemf indicates that the motor is running in the opposite direc-

tion to that programmed, then a brake torque is applied by applying

a PWM signal to all low-side MOSFETs until the bemf indicates

that the motor is stationary. The low-side MOSFETs are switched

at the programmed PWM frequency with the window brake duty

cycle, DWB, defined by the contents of the WBD[3:0] variable.

Once the motor speed is less than fWMF, then the motor is assumed

to be stationary and the normal start sequence is followed.

If the bemf indicates that the motor is running in the same direc-

tion to that programmed, then the alignment and ramp functions

are not used and the startup sequence goes directly to coast and

synchronization before switching directly to full commutation

closed-loop operation.

Motor Control Modes

There are three motor control methods included in the A4964.

These are:

1. open-loop speed (voltage) control

2. closed-loop torque (current) control

3. closed-loop speed control

The control mode and all associated parameters are determined

by the contents of the configuration and control registers. These

registers can be changed on-the-fly through the SPI. The user-

defined values in these registers are held in programmable

non-volatile memory, allowing the A4964 to be pre-programmed

with default values for a specific appliction and avoid the need to

program the register contents at each power on. The motor move-

ment can be controlled directly through the SPI or, in standalone

mode, by a PWM applied to the LIN terminal.

In SPI operating mode, selected by setting the OPM variable

to 0, the running state, direction, and output of the motor are

controlled by a combination of commands through the serial

interface. These are a demand input variable, DI[9:0], plus three

control bits, RUN, DIR, and BRK.

In stand-alone operating mode, selected by setting the OPM vari-

able to 1, the output of the motor is controlled by the duty cycle

of a PWM input, which sets the value in the DI[9:0] demand

input variable. The three control bits, RUN, DIR, and BRK can

still control the running state of the motor through the serial inter-

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