ML4426CS datasheet(9/16 Pages) MICRO-LINEAR

9 / 16 page

ML4426

The maximum frequency on the VCO pin is found by:

fN RPM

MAX

005

(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

ML4426. The phase sense input impedance is 8k

W. This

requires a series resistor RES1 from the motor phase lead

as shown in Figure 6 based on the following equation:

RES

MOTOR

1 670

W /

(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

(5)

This allows the ML4426 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

FUNCTIONAL DESCRIPTION (Continued)

Figure 5. External VCO Component Connections

CVCO

4.3V

2.3V

RVCO

SPEED

CVCO

RVCO

VOLTAGE

CONTROLLED

OSCILLATOR

VCO/TACH

RESET

(FROM CAT)

FROM

BACK EMF

SAMPLER

& RAMP

GENERATOR

NEUTRAL

SIMULATOR

ΦA + ΦB + ΦC

MULTIPLEXER

COMMUTATION

STATE MACHINE

gm =

Ω

–

SIGN

CHANGER

SPEED FB

FB A

RES1

RES2

RES3

FB B

FB C

MOTOR

ΦC

MOTOR

ΦB

MOTOR

ΦA

Figure 6. Back EMF Sampler Detailed Block Diagram

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.

ML4426CS Datasheet(PDF) 9 Page - Micro Linear Corporation

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