W183 Datasheet, PDF (3/9 Pages) Cypress Semiconductor – Full Feature Peak Reducing EMI Solution

W183
Overview
The W183 product is one of a series of devices in the Cypress
PREMIS family. The PREMIS family incorporates the latest
advances in PLL spread spectrum frequency synthesizer tech-
niques. By frequency modulating the output with a low fre-
quency carrier, peak EMI is greatly reduced. Use of this tech-
nology allows systems to pass increasingly difficult EMI testing
without resorting to costly shielding or redesign.
In a system, not only is EMI reduced in the various clock lines,
but also in all signals which are synchronized to the clock.
Therefore, the benefits of using this technology increase with
the number of address and data lines in the system. The Sim-
plified Block Diagram shows a simple implementation.
Functional Description
The W183 uses a phase-locked loop (PLL) to frequency mod-
ulate an input clock. The result is an output clock whose fre-
quency is slowly swept over a narrow band near the input sig-
nal. The basic circuit topology is shown in Figure 1. The input
reference signal is divided by Q and fed to the phase detector.
A signal from the VCO is divided by P and fed back to the
phase detector also. The PLL will force the frequency of the
VCO output signal to change until the divided output signal
and the divided reference signal match at the phase detector
input. The output frequency is then equal to the ratio of P/Q
times the reference frequency. (Note: For the W183 the output
frequency is equal to the input frequency.) The unique feature
of the Spread Spectrum Frequency Timing Generator is that a
modulating waveform is superimposed at the input to the VCO.
This causes the VCO output to be slowly swept across a pre-
determined frequency band.
Because the modulating frequency is typically 1000 times
slower than the fundamental clock, the spread spectrum pro-
cess has little impact on system performance.
Frequency Selection With SSFTG
In Spread Spectrum Frequency Timing Generation, EMI re-
duction depends on the shape, modulation percentage, and
frequency of the modulating waveform. While the shape and
frequency of the modulating waveform are fixed for a given
frequency, the modulation percentage may be varied.
Using frequency select bits (FS2:1 pins), the frequency range
can be set (see Table 2). Spreading percentage is set with pin
SS% as shown in Table 1.
A larger spreading percentage improves EMI reduction. How-
ever, large spread percentages may either exceed system
maximum frequency ratings or lower the average frequency to
a point where performance is affected. For these reasons,
spreading percentages between 0.5% and 2.5% are most
common.
Clock Input
Reference Input
Freq.
Divider
Q
V_DD
Phase
Detector
Charge
Pump
Σ
VCO
Post
Dividers
Feedback
Divider
P
Modulating
Waveform
PLL
CLKOUT
(EMI suppressed)
GND
Figure 1. Functional Block Diagram
Document #: 38-07158 Rev. *A
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