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46 The PCB Design Magazine • January 2016 Dr. Istvan Novak is a distin- guished engineer at oracle, working on signal and power integrity designs of mid-range servers and new technology de- velopments. With 25 patents to his name, novak is co-author of "Frequency- Domain Characterization of Power Distribu- tion networks." To read past columns, or to contact novak, click here. quiet power HoW To DESIgN A PDN FoR THE WoRST-CASE SCENARIo just push the impedance down at certain fre- quencies, and even if we stay within the pre- defined maximum. Lastly, we flip around the impedance profile and use one peak fixed at 100 mOhm maximum value, and we vary the value of low-frequency and high-frequency asymptotes. This essential- ly creates the inverse of impedance profiles we had in Figure 10; it was a band-reject function there, now we look at pass-band functions. Fig- ure 14 shows the corresponding step responses. Finally, Figure 15 shows the percentage penalty as a function of max/min impedance ratio. Note these cases do not intend to represent practical scenarios; they merely serve our bet- ter understanding. In practice it is very unlikely to have multiple impedance peaks or notches with the same extreme values. Nevertheless these examples serve as a guidance for the de- sign process. If we use the target impedance ap- proach and assume that, due to non-flatness, the worst-case noise is approximately three times higher, we can readjust our impedance target to a lower value and we can then do a straightforward design process. Once the design is known, it is always a good idea to recheck our assumptions with the actual PDN component values and placement and iterate as needed. If we lower the target impedance by a factor of three, very seldom will it be necessary to do it- erations. For more information on the subject, please see the reference section, especially [3] and [4] . PCBDESIgN References 1. Systematic Estimation of Worst-Case PDN Noise: Target Impedance and Rogue Waves, QuietPower column December 2015 2. Steve Sandler, "Target Impedance Limita- tions and Rogue Wave Assessments on PDN Per- formance," paper 11-FR2 at DesignCon 2015, January 27 – 30, 2015, Santa Clara, CA. 3. Target Impedance and Rogue Waves, pan- el discussion at DesignCon 2016, January 19 – 21, 2016, Santa Clara, CA. 4. Jae Young Choi, Ethan Koether, Istvan No- vak, "Electrical and Thermal Consequences of Non-Flat Impedance Profiles," DesignCon 2016, January 19 – 21, 2016, Santa Clara, California. Figure 14: Step responses of the impedance pro- files from Figure 13. Figure 15: relative noise increase as a function of relative max/min ratio of impedance profile.