Issue link: https://iconnect007.uberflip.com/i/942744
50 DESIGN007 MAGAZINE I FEBRUARY 2018 (which is most likely the case if they are on different clock domains) then the RMS value of the transient currents can be combined. Using this concept, the target impedance for a single IC can be calculated from knowledge of the circuit blocks drawing current from the voltage rail in question. And at the PCB level, transient current drawn by several chips can be calculated statistically. This makes the target impedance a function of frequency. The circuit designer can combine the target impedances, for several chips on the same PCB power rail, by adding linearly at DC and low frequencies and statistically at higher frequencies. Generally, all circuits on the PCB will draw DC current at the same time and so the voltage regulator module (VRM) must be designed to accommodate the potential surges. But as the frequency approaches the 1GHz band, the prob- ability of drawing transient currents in phase and at the same frequency greatly diminishes. As frequency goes up, peak currents become spatially isolated. The target impedance at the different chip locations on the PCB should reflect this in different frequency bands. The PDN profile should be optimized (Fig- ure 3) such that the effective impedance, of the combined VRM, bypass and decoupling capac- itors and plane resonance, falls just below the estimated target impedance, of the PDN, up to the maximum required bandwidth. Key Points • The goal of robust PDN Planning is to design a stable power source, taking into account the worst-case transient condi- tions. • Smart designers prevent problems before they arise; others waste time and resources trying to fix problems. • Engineers and PCB designers need to visualize and understand how and where the currents flow. • Increasing the plane copper thickness is a good solution to current hot spots, for DC and low frequency, but has little impact at high frequencies due to the skin effect. • Quarter-wavelength or longer structures may become effective electromagnetic radiators. Figure 3: Optimized PDN profile (simulated in iCD Design Integrity).