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AUGUST 2023 I DESIGN007 MAGAZINE 31 Power Design Analysis Workflow A key challenge for PCB designers is deliver- ing power to the load. e correct power for both DC (magnitude) and AC (ripple) are cru- cial for transmit (TX) and receive (RX) signals (Figure 3). At DC, resistance matters, and at AC, induc- tance matters. Looking at DC, there is the DC source, the PCB, and the IC load where the current needs to go. e DC source will go through the copper on the PCB, which has a finite resistance and will therefore have an IR drop across it. e voltage coming at the load side will likely lose some amplitude from the original source. e question that needs to be analyzed is whether the voltage at the load is sufficient. Every IC has a requirement for the amount of voltage needed to power it, and the designer must make sure the IC is getting the needed voltage. What is important for the DC analysis is the resistance of the PCB. For the AC analy- sis, the logic and input/output (I/O) circuits are toggling millions of transistors per second, and every time a transistor toggles, it requires immediate current, which is unlikely to come from the DC source. is is because the PCB is acting more like an inductor. At the higher frequencies at which the switching is taking place, the inducted impedance far overshad- ows the resistance impedance. Consequently, the inductance of the PCB begins to matter a great deal. Because the inductance is largely a function of the geometry of the PCB, the PCB layout becomes crucial. e inductance from the voltage regulator module (VRM) will almost certainly be too high, so localized decoupling caps are used to provide the instantaneous current needed by the switching load. What becomes important is how much inductance there is between the capacitor and its load. e higher the induc- tance, the lower the effectiveness of the capac- itor. is is an issue that must be considered by making sure to identify the capacitors with high-inductance connections to the load and reduce that inductance. Power Workflow: Pre-Layout At the beginning of the design flow, the designer has dozens to hundreds of schematic pages, as well as dozens of voltage rails, and the power connectivity needs to be set up correctly. With a large, complex power structure, the abil- ity to visualize the location of the sources, sinks, passive drops, etc., is immensely useful in mak- ing sure there are no problems such as short cir- cuits or unintended destinations for the power. Figure 3: Sufficient and stable power for both DC and AC is required for components transmitting and receiving signals.