9

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

gain is:

Ke

Atten

A Radian

××

ω

π

2

23 10

4

./

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

Zs

Cs

s

s

RC

LEAD

LAG

()

()

()

=

+

+

1

1

ω

ω

Where the lead and lag frequencies are set by:

RC

= 1

2

RC C

=

+

12

12

Requiring the loop to settle in 20 PLL cycles with

C

Atten K

N

e

1

4

7 508 10

=

××

×

−

.

R

Atten K

RPM

e

MAX

=

×

××

889 10

4

.

application note 35 for derivation of the above formulas.

smooth transition from sense mode to closed loop

operation.

Figure 5. Typical Sensed Start-up

t

DRIVE

SENSE

DRIVE

SENSE

DRIVE

DRIVE

SENSE

LOOP CLOSED HERE

(RUN MODE)

t

~200

µs

DRIVE ~100ms

SENSE ~3ms

02

468

10

12

3000

2500

2000

1500

1000

500

0

VVCO (VOLTS)

CVCO = 164nF

CVCO = 82nF