5

ML4411/ML4411A

maximum voltage at any PH input does not exceed VCC.

NEUTRAL

0

60

120

180

240

0

300

Figure 2. Typical motor phase waveform with Back-EMF

superimposed (Ideal Commutation)

VCO AND PHASE DETECTOR CALCULATIONS

The VCO should be set so that at the maximum frequency

of operation (the running speed of the motor) the VCO

VCO maximum frequency will be:

F

POLES RPM

×

005

.

where POLES is the number of poles on the motor and

RPM is the maximum motor speed in Revolutions Per

FUNCTIONAL DESCRIPTION

The ML4411 provides closed-loop commutation for

3-phase brushless motors. To accomplish this task, a VCO,

integrating Back-EMF Sampling error amplifier and

sequencer form a phase-locked loop, locking the VCO to

the back-EMF of the motor. The IC also contains circuitry

to control motor current with either linear or constant off-

time PWM modes. Braking and power fail detection

functions are also provided on chip. The ML4411 is

designed to drive external power transistors (N-channel

sinking transistors and PNP sourcing transistors) directly.

Start-up sequencing and motor speed control are

accomplished by a microcontroller. Speed sensing is

accomplished by monitoring the output of the VCO,

which will be a signal which is phased-locked to the

commutation frequency of the motor.

BACK-EMF SENSING AND COMMUTATOR

The ML4411 contains a patented back-EMF sensing

circuit which samples the phase which is not energized

(Shaded area in figure 2) to determine whether to increase

or decrease the commutator (VCO) frequency. A late

commutation causes the error amplifier to charge the

filter (RC) on pin 20, increasing the VCO input while

early

commutation causes pin 20 discharge. Analog speed

control loops can use pin 20 as a speed feedback voltage.

The input impedance of the three PH inputs is about 8Ký

to GND. When operating with a higher voltage motor, the

PH inputs should be divided down in voltage so that the

FIGURE 1. BACK EMKF sensing block diagram

NEUTRAL

SIMULATOR

ΦA + ΦB + ΦC

6

ΦA

ΦB

ΦC

MULTIPLEXER

R

C1

C2

VCO

COMMUTATION

LOGIC

SIGN

CHANGER

b

a

–

+

I(PIN 21)

+

LOOP FILTER

Va – Vb

8K

RC

VCO /TACH OUT

+

–

DIS PWR

8K

8K

ROTATION

SENSE