Issue link: https://iconnect007.uberflip.com/i/241303
beyond design pdn planning and capacitor selection, part 2 continues Figure 6: One value capacitor per decade approach including VRM, loop inductance and plane capacitance of 3M ECM. this method can be used to dampen electromagnetic emissions at a particular frequency. Either way, each PDN on the board should be analyzed to give confidence in the final product. Points to Remember • There are three approaches to analyzing the PDN: target frequency, one value capacitor per decade or optimized value. • Poor PDN design can result in unusual, intermittent signal integrity issues including high crosstalk and excessive emission of radiation. • The integrity of the PCB stackup and the PDN are the basis for a stable product. • Decoupling is not the process of placing a capacitor adjacent to the IC, but rather it is the process of placing an L-C network adjacent to the IC to supply the high transient switching current. • Decoupling capacitors supply instantaneous current—at different frequencies—to the drivers until the power supply can respond. • For bulk decoupling at the supply level, 38 The PCB Design Magazine • January 2014 tantalum is usually preferred, due to the availability of high capacitance ratings. • Large capacitance value ceramics provide effective decoupling at about 25% of the nominal capacitance value compared to standard tantalums. • The capacitance of a capacitor is determined by three factors. The number of layers in the part, the dielectric constant and the active area. • The dielectric thickness is inversely related to the capacitance—so the thicker the dielectric, the lower the capacitance value. • The rating compared to working voltage for multilayer ceramic capacitors is 2:1. • The dielectric constant depends on the ceramic material used. NP0 has the lowest dielectric constant, followed by X7R and Y5V. • Dissipation Factor (Df) is the percentage of energy wasted as heat in the capacitor. • While X2Y MLCCs offer superior performance and space wise, they are about five times the price of standard 0402.