ML4428IP datasheet(9/16 Pages) MICRO-LINEAR

9 / 16 page

ML4428

Figure 4 shows the linearized transfer function of the

Phase Locked Loop with the phase detector formed from

the sampled phase through the Gm amplifier with the

loop filtered formed by R, C1, and C2. The Phase detector

gain is:

Atten

A Radian

××

×× −

23 10

Where Ke is the motor back-E.M.F. constant in V/Radian/

sec,

w is the rotor speed in r/s, and Atten is the back-

E.M.F. resistive attenuator, nominally 0.3.

The simplified impedance of the loop filter is

LEAD

LAG

()

Where the lead and lag frequencies are set by:

ωLEAD

= 1

ωLAG CC

RC C

Requiring the loop to settle in 20 PLL cycles with

wLAG = 10 ¥ wLEAD produces the following calculations

for R, C1 and C2:

Atten K

7 508 10

××

−

C2 = 9 ¥ C1

Atten K

RPM

MAX

××

889 10

where Ke is the back-EMF constant in volts per radian per

second, and RPMMAX is the rotor speed. See Micro Linear

application note 35 for derivation of the above formulas.

The 80k resistor to GND from the RCVCO pin assists in a

smooth transition from sense mode to closed loop

operation.

Figure 5. Typical Sensed Start-up

IMOTOR

DRIVE

SENSE

DRIVE

SENSE

DRIVE

SENSE

LOOP CLOSED HERE

(RUN MODE)

IMOTOR

~200

µs

DRIVE ~100ms

SENSE ~3ms

Figure 3. VCO Output Frequency vs. VVCO (Pin 20)

468

3000

2500

2000

1500

1000

500

VVCO (VOLTS)

CVCO = 164nF

CVCO = 82nF

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