Xilinx Zynq-7000 Design Manual

Also see for Zynq-7000: Quick start guide Getting started guide User guide Application note User guide

Zynq-7000 PCB Design Guide www.xilinx.com 28UG933 (v1.8) November 7, 2014Chapter 3: Power Distribution SystemL MOUNT = 0.8 nH (based on PCB mounting geometry)To determine the effective in-system parasitic inductance (L IS ), add the via parasitics:L IS = L SELF + L MOUNT = 0.9 nH + 0.8 nHL IS = 1.7 nH Equation 3-3The values from the example are used to determine the mounted capacitor resonantfrequency (F RIS ). Using Equation 3-1:Equation 3-4Equation 3-5F RSELF is 53 MHz, but F RIS is lower at 38 MHz. The addition of mounting inductances shiftsthe effective-frequency band down.A decoupling capacitor is most effective at the narrow-frequency band around its resonantfrequency, and thus, the resonant frequency must be reviewed when choosing a capacitorcollection to build up a decoupling network. This being said, capacitors can be effective atfrequencies considerably higher and lower than their resonant frequency. Recall thatcapacitors of differing values in the same package share the same inductance curve. Asshown in Figure 3-6, for any given frequency along the inductive portion of the curve, thecapacitors are equally effective.Capacitor Anti-ResonanceOne problem associated with combinations of capacitors in a PDS of an AP SoC isanti-resonant spikes in the PDS aggregate impedance. The cause for these spikes is a badcombination of energy storage elements in the PDS (intrinsic capacitances, discretecapacitors, parasitic inductances, and power and ground planes).Anti-resonance can arise between any two consecutive stages of a power distributionsystem, such as between the high-frequency PCB capacitors and the PCB plane capacitance.The inter-plane capacitance of the power and ground planes generally has a high-Q factor.If the high-frequency PCB capacitors also are high-Q, the crossover point between thehigh-frequency discrete capacitors and the plane capacitance might exhibit ahigh-impedance anti-resonance peak. If the AP SoC has a high transient current demand atthis frequency (as a stimulus), a large noise voltage can occur.To correct this type of problem, the characteristics of the high-frequency discrete capacitorsor the characteristics of the V CC and ground planes must be changed, or AP SoC activityshifted to a different frequency away from the resonance.F RIS12π L IS C-------------------------=F RIS12π 1.7 9–×10 H( ) 0.01 6–×10 F( )⋅------------------------------------------------------------------------------------- 38 6×10 Hz==Send Feedback