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60 DESIGN007 MAGAZINE I JANUARY 2022 problem: If we connect the regulator's sense point further away from the regulator, the phase shi along that path may reduce the sta- bility margin of the regulator's feedback loop. In extreme cases this can also happen even if we do not have an additional LC filter. As it was shown 1 , the DC resistance of a power rail with the bulk capacitors may produce notice- able phase shi near the crossover frequency of typical regulators. Figure 7 shows the sketch of the board layout 1 , Figure 8 shows the result- ing difference caused by miniscule layout dif- ferences between the two PDN rails layouts. Note the marked difference in the two impedance profiles. Follow-on tests and analy- sis showed that the two power rails had system- atically different phase shi at the converter's crossover frequency due to very minor layout differences between the two sides. If we need to supply power to more than one load with the same regulator, in lucky cases may have two nominally identical chips, draw- ing approximately the same current. If we have Figure 7: Layout sketch of a large memory board with two PDN rails with mirror-image component placement and layout. Figure 8: Measured impedance profiles of the two PDN rails.

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