October 2006

P2781/82/84

rev 1.6

General Purpose EMI Reduction IC

4 of 11

Notice: The information in this document is subject to change without notice.

Spread Spectrum Selection

The P278xA performs Zero Cycle Slip when set at low percentage spreading. This allows no 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: the center frequency is the frequency of the external reference input on CLKIN, Pin 1.)

The P2781A is designed for PC peripheral, networking, notebook PC, and LCD monitor applications. It is optimized for

operation between 3 to 78MHz range. In the following application schematic example, the P2781A spread percentage

selection is determined by the external LF value specified in the Loop Filter Selection Table. The Input Frequency Selection

Table specifies the input frequency range. The external LF allows the user to fine tune the spread percentage to optimize the

EMI reduction benefits of the spread spectrum.

Note: Both logic input pins FS1 and FS0 have to be connected to either VDD or VSS. Do not leave them floating.

1

2

3

4

8

7

6

5

VDD

FS0

ModOUT

VSS

LF

FS1

XOUT

XIN/CLKIN

P278xA

+3.3V

Modulated 16MHz Clock

To target Chip

16 MHz

0.1µF

C2

C1

R1

0 Ohm

2.2 K