Design007 Magazine

PCBD-Jan2014

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column beyond design PDN Planning and Capacitor Selection, Part 2 by Barry Olney In-Circuit Design Pty Ltd | Australia In last month's column, PDN Planning and Capacitor Selection Part 1, we looked closely at how to choose the right capacitor to lower the AC impedance of the power distribution network (PDN) at a particular frequency. We also examined capacitor properties and types of capacitors that are readily available and touched on the target frequency approach for analyzing a PDN. This month we will continue on from there looking at the one-capacitor-value-per-decade and optimized value approaches. Figure 4 shows the effect of using the onevalue-per-decade approach where capacitors from each decade are added in parallel. Now to Figure 4: One value capacitor per decade approach. 34 The PCB Design Magazine • January 2014 be fair, I have added three of each value from 100uF to 1pF to total 27 capacitors as the target frequency approach, in Figure 2, had a total of 29 capacitors. In this case, the impedance is below the target impedance from 10KHz to 110MHz. Notice how the combination of capacitors causes anti-resonant (parallel resonant) peaks where the higher frequency capacitor goes capacitive while the lower frequency capacitor is inductive. This occurs as the LC network produced by the combination is effectively a tank circuit that has parallel resonance at the crossing frequency. This happens each time a different value of capacitor is added. These peaks exceed

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