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ML4425

Sensorless BLDC Motor Controller

Micro Linear Corporation 

ML4425

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 commutation 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 regulation. 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 – z

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 specified. The following formulas should be

considered as a starting point for optimization. All

calculations for capacitors and resistors should be used as

the first approximation for selecting 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 possible to VDD. A 6.9V voltage reference output

(VREF) is provided to set the speed command and current

limit of the ML4425. A 137kW 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

outputs 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

LA

HB

MOTOR

PHASE A

LB

HC

MOTOR

PHASE B

LC

MOTOR

PHASE C

RSENSE

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

6

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