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ML4425CS

Sensorless BLDC Motor Controller

Fairchild Semiconductor 

ML4425

PRODUCT SPECIFICATION

Functional Description

General

The ML4425 provides all the circuitry for sensorless speed

control of 3-phase Brushless DC (BLDC) motors. Controller

functions include start-up circuitry, back-EMF commutation

control, Pulse Width Modulation (PWM) speed control,

fixed OFF-time current limiting, braking, and undervoltage

protection.

The start-up circuitry aligns the motor to a known position,

then ramps up the motor speed to generate a back-EMF

signal. A back-EMF sampling circuit controls commutation

timing by forming a Phase Locked Loop (PLL). The commu-

tation control circuitry also outputs a speed feedback

(SPEED FB) signal used in the speed control loop. The

speed control loop consists of an error amplifier and PWM

comparator that produce a PWM duty cycle for speed regula-

tion. Motor current is limited by a fixed OFF-time PWM

shutdown comparator that is controlled by an external sense

resistor. Commutation control, PWM speed control, and

current limiting are combined to produce the output driver

signals. Six output drivers are used to provide gating signals

to an external 3 phase bridge power stage sized for the

BLDC motor voltage and current requirements. Additional

functions include a braking function and undervoltage

protection circuit to shut down the output drivers in the event

of a low voltage condition on VDD of the ML4425.

Component Selection

Selecting external components for the ML4425 requires

calculations based on the motor’s electrical and mechanical

parameters. The following is a list of the motor parameters

needed for these calculations :

• DC motor supply voltage – VMOTOR (V)

• Maximum operating current – IMAX (A)

• Number of magnetic poles – N

• Back EMF constant – Ke (V-s/Rad)

• Motor torque constant – Kt (Nm/A) (Kt = Ke in SI units)

• Maximum speed of operation RPMMAX (RPM)

• Moment of inertia of the motor and load – J (Kg-m2)

• Viscous damping factor of the motor and load – ζ

If one or more of the above values is not known, it is still

possible to pick components for the ML4425, but some

experimentation may be necessary to determine the optimal

values. All quantities are in SI units unless otherwise speci-

fied. The following formulas should be considered as a start-

ing point for optimization. All calculations for capacitors and

resistors should be used as the first approximation for select-

ing the closest standard value.

Power Supply and Reference

The supply voltage (VDD) is nominally 12V ±10%. A 100nF

bypass capacitor to ground should be placed as close as pos-

sible to VDD. A 6.9V voltage reference output (VREF) is pro-

vided to set the speed command and current limit of the

ML4425. A 137kΩ from RREF to GND is required to set up a

reference current for internal functions.

Output Drivers

The output drivers LA, LB, LC, HA, HB, and HC provide

totem pole output drive signals for a 3 phase bridge power

stage. All control functions in the ML4425 translate to out-

puts at these pins. LA, LB, and LC provide the low-side

drive signals for phases A, B, and C of the 3 phase power

stage and are 12V active high signals. HA, HB, and HC

provide the high-side signals and are 12V active low signals.

VMOTOR

12V

DC SUPPLY

CAPACITOR

HA

HB

MOTOR

PHASE A

LA

LB

HC

MOTOR

PHASE B

LC

MOTOR

PHASE C

RSENSE

Figure 1. Using RSENSE in a 3-Phase 12V Power Stage

6

REV. 1.0.2 7/2/01

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