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Design007-Apr2021

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44 DESIGN007 MAGAZINE I APRIL 2021 In signal integrity, for high-speed signaling, high-frequency loss is usually considered a bad side effect that we want to minimize. e DC loss, on the other hand, matters much less, because in many high-speed signaling schemes we intentionally block the DC content of the signal. Another description of the column title could be, "Loss may be your friend, but inductance is your enemy." is is clearly just an eye-catching generalization; we could always argue that there are cases in signal integrity, too, when minimiz- ing losses could backfire, or at least would have its negative consequences. In power integrity it is almost the opposite: To deliver DC power, we want to minimize the DC losses, but at the same time we don't want high-frequency noise to travel along the power distribution network. erefore, AC losses in power distribution are usually helpful. Inductance is different, though; while it is present in all conductive structures where current flows or can flow, in power integrity, the only situation when we can con- sider it helpful is when the induc- tance is in the series path as part of an intentional (or accidental) low-pass filtering where we want to block the noise. In applications where we don't need or don't care for blocking power noise from propagating along the PDN struc- ture, increased inductance comes with the downside that we need more capacitance to balance it. In this brief article we show you a few simulation results to illustrate these points. As a reminder, the simplified block sche- matic (Figure 1) illustrates the difference between the Parallel PDN, where we do not have intentional series elements in the power distribution network and the PDN filter where the series element is placed (or taken into account) intentionally to create the filtering. is block schematic is highly simplified: three capacitors are shown in the parallel PDN path, but it can be a mix of any number of same-val- ued and/or different-valued capacitors. Simi- larly, the PDN filter can be more complex, hav- ing an entire parallel PDN on its output, com- posed of multiple capacitors. e series path can be more complex, too, for instance having series and parallel resistors around the induc- Quiet Power by Istvan Novak, SAMTEC Friends and Enemies in Power Distribution Figure 1: Block schematics of a system PDN illustrating the definitions of parallel PDN and PDN filter.

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