PRODUCT SPECIFICATION

ML4425

REV. 1.0.2 7/2/01

9

The maximum frequency on the VCO pin is found by:

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 phase sense input impedance is 8k

Ω. This requires a

series resistor RES1 from the motor phase lead as shown in

Figure 6 based on the following equation:

The back EMF sampler takes the motor phase voltages

nal) and calculates the neutral point of the motor by the fol-

lowing equation:

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 refer-

ence point must be calculated in any case.

Figure 5. External VCO Component Connections

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 sampled). The sam-

pled 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 out-

put of the error amplifier outputs a charging or discharging

current to SPEED FB, which provides the control voltage to

the VCO.

f

MAX

0.05

N

×

RPM

MAX

×

=

(3)

RES1

670

Ω V

⁄

V

MOTOR

10V

–

()

×

=

(4)

Neutral

PH1

PH2

PH3

++

3

-------------------------------------------------

=

(5)

CVCO

CVCO

4.3V

0V

5V

2.3V

RVCO

SPEED

FB

CVCO

RVCO

VOLTAGE

CONTROLLED

OSCILLATOR

VCO/TACH

VCO/TACH

RESET

(FROM CAT)

FROM

BACK EMF

SAMPLER

& RAMP

GENERATOR

Figure 6. Back EMF Sampler Detailed Block Diagram

NEUTRAL

SIMULATOR

ΦA + ΦB + ΦC

6

MULTIPLEXER

COMMUTATION

STATE MACHINE

gm =

1

8k

Ω

4k

Ω

4k

Ω

4k

Ω

4k

Ω

4k

Ω

4k

Ω

+

–

SIGN

CHANGER

TO

SPEED FB

FB A

RES1

RES2

RES3

FB B

FB C

F/R

MOTOR

ΦC

F/R

MOTOR

ΦB

MOTOR

ΦA