ML4428CS datasheet(8/16 Pages) MICRO-LINEAR

8 / 16 page

ML4428

CVCO

As given in the section on the VCO and phase detector:

N RPM

VCO

MAX

−

2931 10

Where N is the number of poles in the motor, and RPM is

the motor’s maximum operating speed in revolutions per

minute.

CPWM

This capacitor sets the PWM ramp oscillator frequency.

This is the PWM “switching frequency”. If this value is too

low, <20kHz, then magnetostriction effects in the motor

may cause audible noise. If this frequency is too high,

>30kHz, then the switching losses in the output drivers

may become a problem. 25kHz should be a good

compromise for this value, which can be obtained by

using a 1nF capacitor.

RVCO AND RREF

RVCO should be 10k and RREF should be 120k for normal

operation.

VCO FILTER

See the section on the VCO and Phase detector for

information on these components.

VCO AND PHASE DETECTOR CALCULATIONS

The VCO should be set so that at the maximum frequency

of operation (the running speed of the motor) the VCO

control voltage will be no higher than VREF, or 6V. The

VCO maximum frequency will be:

N RPM

MAX

=× ×

005

where N is the number of poles on the motor and

RPMMAX is the maximum motor speed in Revolutions Per

Minute.

The minimum VCO gain derived from the specification

table (using the minimum FVCO at VVCO = 6V) is:

VCO MIN

VCO

()

× −

2 665 10

Assuming that the VVCO(MAX) = 5.5V, then

VCO

MAX

×× −

5 5 2 665 10

N RPM

VCO

MAX

−

2931 10

Figure 4. Back-EMF Phase Locked Loop Components.

VCO

–

ZRC

RCVCO

FOUT

KVCO(Hz/sec/V)

Gm = 0.23m

SAMPLED

PHASE

BEMF

SAMPLER

ROTOR

PHASE

A/RADIAN

gm = 0.23mA/V

× ω × Atten

× π

PHASE DETECTOR

× C2 × s + 1)

× (C2 + R × C1 × s × C2 + C1)

LOOP FILTER

2.665

× 10–5

CVCO × s

× 2 × π

VCO

RADIAN/sec/V

V/A

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