October 2006
rev 1.6
P2781/82/84
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.
+3.3V
16 MHz
XIN/CLKIN VDD
1
2
3
4
8
7
6
5
0.1µF
2.2 K
XOUT
FS0
FS1
Modulated 16MHz Clock
To target Chip
ModOUT
LF
VSS
P278xA
R1
C1
0 Ohm
C2
Note: Both logic input pins FS1 and FS0 have to be connected to either VDD or VSS. Do not leave them floating.
General Purpose EMI Reduction IC
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Notice: The information in this document is subject to change without notice.
PULSECORE [ PulseCore Semiconductor ]
