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Design007-Jan2022

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JANUARY 2022 I DESIGN007 MAGAZINE 61 to position them far enough so that routing power through them sequentially would leave too much uncompensated voltage drop, a sym- metric fork may be our best option. Such a case 2 is shown in Figure 9. A single sense point symmetrically between the two loads will eliminate most of the uncompensated voltage drop. In these days, professional tools can do a good job to simulate the DC voltage drop on power planes, vias and traces, so aer complet- ing the layout, it is always a good idea to check the DC drop to make sure that our design meets the requirements. When it comes to the very fine details, sev- eral other factors may need to be considered. For instance, how do we deal with the volt- age drop across a large pin field connecting a power-hungry chip? Do we need to consider the micro detail of the voltage drop across large pads of power connections? How would the simulated DC drop change if we take the non- vertical sidewalls of copper etching of printed circuit boards into account? ose could be important aspects in a high-end design and may be the subject of future articles. And a final closing thought. For sake of sim- plicity, the voltage diagram of Figure 3 does not include margin; however, in a real design it is always a good idea to add the typical 10–20% margin for any unaccounted contributor. DESIGN007 References 1. "Impact of Regulator Sense-point Location on PDN Response," DesignCon 2015. 2. "How Spatial Variation of Voltage Regulator Output Impedance Depends on Sense Point Loca- tion," DesignCon 2018. Istvan Novak is the principal sig- nal and power integrity engineer at Samtec with over 30 years of experience in high-speed digital, RF, and analog circuit and sys- tem design. He is a Life Fellow of the IEEE, author of two books on power integrity, and an instructor of signal and power integrity courses. He also provides a web- site that focuses on SI and PI techniques. To read past columns or contact Novak, click here. Figure 9: Suggested layout if the voltage regulator has to feed two identical loads.

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