AR# 58694


Design Advisory for Zynq-7000 SoC - Updated UG933 may require additional decoupling capacitors in some cases


A re-evaluation of the Zynq-7000 decoupling capacitor requirements in December 2013 resulted in a dramatic decrease in the number of overall capacitors required, but it was noted that VCCINT in the SB or FF packages could require one or two 4.7F capacitors versus the prior recommendation of zero (only under certain conditions).

The updated decoupling capacitor requirements are included in version 1.6 of the Zynq-7000 SoC PCB Design and Pin Planning Guide (UG933)

How do I know if additional capacitors are required for an existing design?


The addition of the 4.7F capacitor(s) on VCCINT will not be a requirement in a large number of designs, and will not be a requirement at all if VCCINT and VCCBRAM share the same power supply. It should also be noted that Xilinx has reported no problems with designs using the prior guidelines.

The addition of the 4.7F capacitor(s) will only be required for existing designs if there is any dynamic/step current demand of 1A on VCCINT between 3.3s and 0.25s (300kHz and 4MHz) for devices in the SB or FF packages, and if VCCINT and VCCBRAM do not share the same power supply. If VCCINT and VCCBRAM share one supply, then the 4.7 F capacitor requirements for VCCBRAM will suffice for VCCINT as well, so no extra capacitors will need to be added.

An example of step current at low frequency is a design that switches between TX and RX modes at 1KHz, with one of the modes requiring a significant amount of extra current. This periodic requirement of significant current could cause the impedance of the VCCINT supply to drop out of specification if the decoupling network cannot provide the required current.

Low frequency energy can best be detected with the use of a spectrum analyzer. Performing a spectrum analysis on output signals and clocks will clearly show any low frequency energy components. If there are any frequency components between 300kHz and 4MHz, those areas should be checked for their current demands on VCCINT. If the step current demand on VCCINT is 1A in these ranges, and VCCINT does not share a single supply with VCCBRAM, adding the 4.7F capacitor(s) would be recommended. Step current demands outside of this range will be handled by the power supply, bulk capacitors, higher frequency capacitors, and any on-chip capacitance.

AR# 58694
Date 05/21/2018
Status Active
Type Design Advisory
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