Xilinx Spartan-6 FPGA Series Design And Pin Planning Manual

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Xilinx Spartan-6 FPGA Series Design And Pin Planning Manual Manual pdf 22 page image

22 www.xilinx.com Spartan-6 FPGA PCB Design and Pin PlanningUG393 (v1.1) April 29, 2010Chapter 2: Power Distribution SystemFigure 2-4 shows a simplified PDS circuit with all reactive components represented by afrequency-dependent resistor.Role of InductanceInductance is the property of the capacitors and the PCB current paths that slows downchanges in current flow. Inductance is the reason why capacitors cannot respondinstantaneously to transient currents or to changes that occur at frequencies higher thantheir effective range.Inductance can be thought of as the momentum of charge. Charge moving through aconductor represents some amount of current. If the level of current changes, the chargemoves at a different rate. Because momentum (stored magnetic-field energy) is associatedwith this charge, some amount of time and energy is required to slow down or speed upthe charge flow. The greater the inductance, the greater the resistance to change, and thelonger the time required for the current level to change. A voltage develops across theinductance as this change occurs.The PDS, made up of a regulator and multiple stages of decoupling capacitors,accommodates the device current demand and responds to current transients as quickly aspossible. When these current demands are not met, the voltage across the device's powersupply changes. This is observed as noise. Inductance in the current path of the capacitorsshould be minimized, because it retards the ability of decoupling capacitors to quicklyrespond to changing current demands.Inductances occur between the FPGA device and capacitors and between the capacitorsand the voltage regulator (see Figure 2-3). These inductances occur as parasitics in thecapacitors and in all PCB current paths. It is important that each of these parasitics beminimized.Capacitor Parasitic InductanceThe capacitance value is often considered the bypass capacitors’s most importantcharacteristic. In power system applications, the parasitic inductance (ESL) has the same orgreater importance. Capacitor package dimensions (body size) determine the amount ofparasitic inductance. Physically small capacitors usually have lower parasitic inductancethan physically large capacitors.X-Ref Target - Figure 2-4Figure 2-4: Further Simplified PDS CircuitlTRANSIENTFPGAug393_c2_04_091809Z P(f) VRIPPLE+−+V