October 2003
rev D
General Purpose EMI Reduction IC
Features
FCC approved method of EMI attenuation
Provides up to 15 dB of EMI suppression
Generates a
1 X
or
½ X
low EMI spread
spectrum clock of the input frequency
Output frequency from
6MHz
to
20MHz
Digital spread selections
Spreading ranges from +/-0.4% to +/-5.0%
Ultra low cycle-to-cycle jitter
Zero-cycle slip
3.3V and 5.0V operating voltage range
10 mA output drives
TTL or CMOS compatible outputs
Ultra-low power CMOS design
Available in 8 pin SOIC and TSSOP
Product Description
The P2681A is a versatile spread spectrum
frequency modulator designed specifically for digital
camera and other digital video and imaging
applications. The P2681A reduces electromagnetic
interference (EMI) at the clock source, which
provides system wide reduction of EMI of all clock
dependent signals. The P2681A allows significant
Block Diagram
P2681A
system cost savings by reducing the number of circuit
board layers and shielding that are traditionally
required to pass EMI regulations.
The P2681A uses the most efficient and optimized
modulation profile approved by the FCC.
The P2681A modulates the output of a single PLL in
order to “spread” the bandwidth of a synthesized
clock and, more importantly, decreases the peak
amplitudes of its harmonics. This results in
significantly lower system EMI compared to the
typical narrow band signal produced by oscillators
and most frequency generators. Lowering EMI by
increasing a signal’s bandwidth is called “spread
spectrum clock generation”.
Applications
The P2681A is targeted towards MFP, xDSL, fax
modem, set-top box, USB controller, DSC, and
embedded systems.
Alliance Semiconductor
2575, Augustine Drive
•
Santa Clara, CA
•
Tel: 408.855.4900
•
Fax: 408.855.4999
•
www.alsc.com
Notice: The information in this document is subject to change without notice.
October 2003
rev D
Pin Configuration
P2681A
Pin Description
Pin#
1
2
3
4
5
6
7
8
Pin Name
XIN/CLKIN
XOUT
DIV2
LF
VSS
ModOUT
SR0
VDD
Type
I
I
I
I
I
O
I
P
Description
Connect to crystal or clock input.
Crystal output
Digital logic input used to select normal output mode or divide-by-2 output
mode. When this pin is Low, the frequency of the output clock is the same
as the input clock frequency. When it is tied High, the output frequency is
half the input clock frequency. This pin has an internal pull-low resistor.
External Loop Filter for the PLL. By changing the value of the CRC circuit,
the % spread can be adjusted accordingly. See Table 1.2 for detail value.
Ground Connection. Connect to system ground.
Spread Spectrum Clock Output.
Digital logic input used to select Spreading Range between large or small
for a given LF value (see Table 1.1 and 1.2). When SR0=0, the spreading %
is smaller than SR0=1. This pin has an internal pull-up resistor.
Connect to +3.3V or 5.0V
General Purpose EMI Reduction IC
Notice: The information in this document is subject to change without notice.
2 of 10
October 2003
rev D
Table 1 - Modulation Output and Spreading Selection VDD @ 3.3V
DIV=0
SR0
Xin
ModOut
0
1
6 MHz
6 MHz
TBD
TBD
TBD
8 MHz
8 MHz
TBD
TBD
TBD
10 MHz
10 MHz
+/-0.75%
+/-1.30%
C1=1,000 pF
C2=10,000 pF
R1=1K
12 MHz
12 MHz
+/-0.75%
+/-1.30%
C1=1,000 pF
C2=10,000 pF
R1=1.5K
16 MHz
16 MHz
+/-0.75%
+/-1.30%
C1=390 pF
C2=3,900 pF
R1=2.2K
P2681A
Modulation
Rate
ModOut/256
ModOut/256
Loop filter value*
10 MHz
12 MHz
5 MHz
6 MHz
+/-0.75%
+/-0.75%
SR0
+/-1.30%
+/-1.30%
C1=1,000 pF
C1=1,000 pF
TBD
TBD
C2=10,000 pF
C2=10,000 pF
Loop filter value*
R1=1K
R1=1.5K
*For additional spread % selection please refer to Loop Filter Selection Table
DIV=0
Table 2 - Loop Filter Selection Table VDD @ 3.3V
Xin
ModOut
0
1
6 MHz
3 MHz
TBD
TBD
8 MHz
4 MHz
TBD
TBD
16 MHz
8 MHz
+/-0.75%
+/-1.30%
C1=390 pF
C2=3,900 pF
R1=2.2K
Modulation
Rate
ModOut/128
ModOut/128
Please contact Alliance Semiconductor for more information
General Purpose EMI Reduction IC
Notice: The information in this document is subject to change without notice.
3 of 10
October 2003
rev D
P2681A
Spread Spectrum Selection
The P2681A performs Zero Cycle Slip when sets at low % 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).
Example:
The P2681A is designed for PC peripheral applications. It is not only optimized for operation between
6MHz – 16MHz range, but its output frequency can be extended down to one half of the input clock frequency
using the Divide-by-Two feature. This feature extends low frequency operation to as low as 3MHz. Setting Pin 3
high (DIV2=1; Divide-by-Two mode) sets the output frequency (ModOUT) to half the frequency of the input clock
(CLKIN). This is a simple way to generate a spread spectrum modulated low frequency clock when only a
higher frequency signal is available. If you want the output frequency to be the same as the input, you need to
set
DIV2=0.
The P2681A’s spread % selection is determined by the external LF value specified in Table 2. Table 1 provides
a particular LF value which allows the % spreading to be selected between +/-0.75% or +/-1.30% by setting SR0
to either 0 and 1. At a specified LF value (See Table 2), SR0 pin allows the user to have the flexibility to digitally
select between large or small % spreading by setting SR0=1 or SR0=0 respectively.
P2681A Application Schematic
General Purpose EMI Reduction IC
Notice: The information in this document is subject to change without notice.
4 of 10
October 2003
rev D
P2681A
EMC Software Simulation
By using Alliance Semiconductor’s proprietary EMC simulation software – EMI-lator®, radiated system level EMI
analysis can be made easier to allow a quantitative assessment on Alliance’s EMI reduction products. The
simulation engine of this EMC software has already been characterized to correlate with the electrical
characteristics of Alliance EMI reduction IC’s. The figure below is an example of the simulation result. Please
visit our web site at
www.alsc.comfor
information on how to obtain a free copy and demonstration of EMI-lator®.
Simulation Result from EMI-lator®
General Purpose EMI Reduction IC
Notice: The information in this document is subject to change without notice.
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