P2779A

http://onsemi.com

3

Loop Filter Selection Table VDD 3.3 V

C1

C2

R1

Pin 4 LF

Input

(MHz)

FS1

FS0

BW = +0.50%

BW = +0.75%

BW = +1.00%

BW = +1.25%

C1

(pF)

C2

(pF)

R1

(W)

C1

(pF)

C2

(pF)

R1

(W)

C1

(pF)

C2

(pF)

R1

(W)

C1

(pF)

C2

(pF)

R1

(W)

20

1

0

270

100,000

330

270

100,000

560

270

100,000

750

560

100,000

910

21−22

1

0

270

100,000

390

270

100,000

620

270

100,000

866(1%)

560

100,000

1,100

23−24

1

0

270

100,000

510

270

100,000

750

270

10,000

1,000

680

6,800

1,200

25−26

1

0

270

100,000

560

270

100,000

820

270

12,000

1,200

470

4,700

1,200

27−28

1

0

270

100,000

620

270

100,000

1,000

270

6,800

1,200

330

3,300

1,200

29−30

1

0

270

100,000

750

270

100,000

1,100

270

3,900

1,200

330

3,300

1,500

31−32

1

0

270

100,000

820

270

100,000

1,200

270

12,000

2,200

680

6,800

2,200

33−34

1

0

270

100,000

910

270

100,000

1,300

270

10,000

2,200

390

3,900

2,200

35−36

1

0

270

100,000

1,000

270

100,000

1,500

270

5,600

2,200

270

2,700

2,200

37−38

1

0

270

100,000

1,200

270

100,000

1,600

270

3,300

2,200

270

2,700

2,700

2. Please contact factory for loop filter values if desired spread settings are not listed.

Spread Spectrum Selection

The P2779A performs zero cycle slip when set at low

percentage spreading. This prevents any occurrence of

system timing error. The optimal setting should minimize

system EMI to the fullest without affecting system

performance. The spreading is described as a percentage

deviation of the center frequency. (Note that the center

frequency is the frequency of the external reference input on

XIN / CLKIN, pin 1.)

The P2779A is designed for PC peripheral, networking,

notebook PC, and LCD monitor applications. It is optimized

for operation from 20 MHz to 38 MHz. The P2779A’s spread

percentage selection is determined by the external LF value

specified in the Loop Filter Selection Table. The external LF

allows the user to fine tune the spread percentage to optimize

the EMI reduction benefits of the spread spectrum.

Figure 2. Application Schematic

PD selection: Power dissipation can be

reduced by completely turning off the IC.

Modulated 27 MHz

+3.3 V

27 MHz

C1

C2

R1

VDD

VDD

REF

ModOUT

VSS

P2779A

8

7

6

5

0.1 mF

LF

PD

1

2

3

4

XOUT

XIN / CLKIN