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40 DESIGN007 MAGAZINE I JUNE 2020 a third camp of people—mainly the practic- ing system and circuit designers—who may be torn apart between the opposing opinions, suggestions, and recommendations. To add in- sult to injury, designers working on fast-paced projects may often lack the time, resources, and information to come up with the correct answer themselves. Before we start the illustrations, we need to nail down a few things about the nomencla- ture. Over the years and decades, as the com- plexity of our electrical systems have grown, people have been using different names for the same thing. For example, the switching regu- lators of any kind—whether it converted AC to DC, DC to DC, or DC to AC—were all called switching-mode power supplies decades ago. Later, the regulators converting DC to DC were called DC-DC converters, and in recent years, we started to refer to those circuits and devices as a voltage regulator module (VRM). For the purposes of this column, and to dif- ferentiate between the fundamentally two kinds of ways that we can try to keep the power noise within limits, I will call the passive part of the PDN that has only parallel bypass capac- itors "parallel PDN," and I will refer to a circuit that has an intentional series element (an in- ductor, a ferrite bead, a resistor, or a combina- tion of these) for the purposes of blocking the propagation of noise a "PDN filter." Note that on PCBs, the plane shapes, patch- es, traces, vias—which are part of the current- carrying path—will also have series resistance and inductance, and most designs consider them as side effects that have to be minimized. On the other hand, these parasitic series ele- ments can also be part of intentionally created filters around them [1] . These two different kinds of circuits can be combined to create various power distribution trees. Figure 1 shows a small part of the block schematics of a larger system PDN that we may have today, illustrating these two kinds of passive circuits. Here we assume that the PDN filter is placed on the output of a voltage regulator (for instance, to further suppress the switching ripple), but we can also use PDN fil- ters on the input side of a noisy DC-DC con- verter to block noise from spilling out to the upstream distribution network [2] . These block schematics are highly simpli- fied: three capacitors are shown in the paral- lel PDN path, but it can be a mix of any num- ber of same-valued and/or different-valued ca- pacitors. Similarly, the PDN filter can be more complex, having an entire parallel PDN on its output composed of multiple capacitors, and the series path can be more complex—for in- stance, having series and parallel resistors around the inductive component. To illustrate the situation and to help explain what should go into the design decision con- siderations, take a look at the PDN filter circuit suggested for an encapsulated oscillator circuit (Figure 2). The intended use of this filter was feeding an oscillator circuit with a few milliampere current consumption. If we choose to use fil- ter components by Murata, we can select, for instance, a BLM21PG221SN1 0805-size ferrite bead for L1, a GRM219R60J226ME47 22uF 0805-size X5R 6.3V capacitor for C1, and a Figure 1: Block schematics of a system PDN illustrating the definitions of parallel PDN and PDN filter. Figure 2: Suggested schematics of a PDN filter with a series ferrite bead.

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