W166G

W166G Features

• Maximized EMI suppression using Cypress’s SpreadSpectrum technology • Generates a spread spectrum copy of the providedinput • Selectable spreading characteristics • Integrated loop filter components • Operates with a 3.3V or 5V supply • SSON# pin enables frequency spreading • Low power CMOS design • Available in 8-pin SOIC (Small Outline Integrated

W166G Functional Description

The W166 uses a Phase-Locked Loop (PLL) to frequencymodulate an input clock. The result is an output clock whosefrequency is slowly swept over a narrow band near the inputsignal. The basic circuit topology is shown in Figure 1. Theinput reference signal is divided by Q and fed to the phasedetector. A signal from the VCO is divided by P and fed backto the phase detector also. The PLL will force the frequency ofthe VCO output signal to change until the divided output signaland the divided reference signal match at the phase detectorinput. The output frequency is then equal to the ratio of P/Qtimes the reference frequency. (Note: For the W166 the outputfrequency is equal to the input frequency.) The unique featureof the Spread Spectrum Frequency Timing Generator is that amodulating 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 timesslower than the fundamental clock, the spread spectrum pro-cess has little impact on system performance.Frequency Selection With SSFTGIn Spread Spectrum Frequency Timing Generation, EMI re-duction depends on the shape, modulation percentage, andfrequency of the modulating waveform. While the shape andfrequency of the modulating waveform are fixed, the modula-tion percentage may be varied.A larger spreading percentage improves EMI reduction. How-ever, large spread percentages may either exceed systemmaximum frequency ratings or lower the average frequency toa point where performance is affected. For these reasons, nar-row and wide modulation selections are provided.