W532 Datasheet, PDF (4/8 Pages) Cypress Semiconductor – Frequency Multiplying, Peak Reducing EMI Solution

W532
Spread Spectrum Frequency Timing
Generation
The benefits of using Spread Spectrum Frequency Timing
Generation are depicted in Figure 2. An EMI emission profile
of a clock harmonic is shown.
Contrast the typical clock EMI with the Cypress Spread Spec-
trum Frequency Timing Generation EMI. Notice the spike in
the typical clock. This spike can make systems fail quasi-peak
EMI testing. The FCC and other regulatory agencies test for
peak emissions. With spread spectrum enabled, the peak en-
ergy is much lower (at least 8 dB) because the energy is
spread out across a wider bandwidth.
Modulating Waveform
The shape of the modulating waveform is critical to EMI reduc-
tion. The modulation scheme used to accomplish the maxi-
mum reduction in EMI is shown in Figure 3. The period of the
modulation is shown as a percentage of the period length
along the X axis. The amount that the frequency is varied is
shown along the Y axis, also shown as a percentage of the
total frequency spread.
Cypress frequency selection tables express the modulation
percentage in two ways. The first method displays the spread-
ing frequency band as a percent of the programmed average
output frequency, symmetric about the programmed average
frequency. This method is always shown using the expression
f_Center ± X_MOD% in the frequency spread selection table.
The second approach is to specify the maximum operating
frequency and the spreading band as a percentage of this fre-
quency. The output signal is swept from the lower edge of the
band to the maximum frequency. The expression for this ap-
proach is f_MAX – X_MOD%. Whenever this expression is used,
Cypress has taken care to ensure that f_MAX will never be ex-
ceeded. This is important in applications where the clock
drives components with tight maximum clock speed specifica-
tions.
SSON# Pin
An internal pull-down resistor defaults the chip into spread
spectrum mode. When the SSON# pin is asserted (active
LOW) the spreading feature is enabled. Spreading feature is
disabled when SSON# is set HIGH (V_DD).
5dB/div
SSFTG
Typical Clock
100%
80%
60%
40%
20%
0%
–20%
–40%
–60%
–80%
–100%
Figure 2. Typical Clock and SSFTG Comparison
Time
Figure 3. Modulation Waveform Profile
Document #: 38-07253 Rev. *A
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