ML4425 查看數據表（PDF） - Micro Linear Corporation

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产品描述 (功能)

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ML4425

Sensorless BLDC Motor Controller

Micro Linear Corporation 

FUNCTIONAL DESCRIPTION (Continued)

The maximum frequency on the VCO pin is found by:

fMAX = 0.05 ™ N ™ RPMMAX

(3)

The voltage at the VCO/TACH pin is equal to the rotor

speed. The voltage at SPEED FB is controlled by the back

EMF sampler.

BACK EMF SAMPLER

The input to the voltage controlled oscillator is the back

EMF sampler. The back EMF sense pins FB A, FB B, and

FB C inputs to the back EMF sampler require a signal

from the motor phase leads that is below the VDD of the

ML4425. The phase sense input impedance is 8kW. This

requires a series resistor RES1 from the motor phase lead

as shown in Figure 6 based on the following equation:

1 6 W RES1= 670 / V ™ VMOTOR - 10V

(4)

The back EMF sampler takes the motor phase voltages

divided down to signals that are less than VDD (12V

nominal) and calculates the neutral point of the motor by

the following equation:

Neutral = PH1+ PH2 + PH3

(5)

3

This allows the ML4425 to compare the back EMF signal

to the motor's neutral point without the need for bringing

out an extra wire on a WYE wound motor. For DELTA

wound motors there is no physical neutral to bring out, so

this reference point must be calculated in any case.

The back EMF sampler measures the motor phase that is

not driven (i.e. if LA and HB are on, then phase A is

driven low, phase B is driven high, and phase C is

ML4425

CVCO

SPEED CVCO

FB

RVCO

RVCO

FROM

BACK EMF

SAMPLER

& RAMP

GENERATOR

RESET

(FROM CAT)

VOLTAGE

CONTROLLED

OSCILLATOR

VCO/TACH

4.3V

CVCO

2.3V

5V

VCO/TACH

0V

Figure 5. External VCO Component Connections

sampled). The sampled phase provides a back EMF signal

that is compared against the neutral of the motor. The

sampler is controlled by the commutation state machine.

The sampled back EMF is compared to the neutral through

an error amplifier. The output of the error amplifier outputs

a charging or discharging current to SPEED FB, which

provides the control voltage to the VCO.

MOTOR ΦA

MOTOR ΦB

MOTOR ΦC

RES1

RES2

RES3

FB A

FB B

FB C

4kΩ 4kΩ 4kΩ

NEUTRAL

SIMULATOR

ΦA + ΦB + ΦC

6

MULTIPLEXER

SIGN

CHANGER

gm =

1

8kΩ

+

–

TO

SPEED FB

4kΩ 4kΩ 4kΩ

F/R

F/R

COMMUTATION

STATE MACHINE

Figure 6. Back EMF Sampler Detailed Block Diagram

9

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