AN 958: Board Design Guidelines

ID 683073
Date 1/28/2022
Document Table of Contents

1.1.2. Determining the ZTARGET

To calculate ZTARGET for a power rail, you must know the following information:

  • Maximum transient current requirements for all devices in the system that are powered by the power rail under consideration. You can get this information the from manufacturers of the respective devices.
    Note: You can download the Early Power Estimator (EPE) tool to estimate power consumption for all its FPGAs and CPLDs.
  • Maximum allowable AC ripple on the power rail as a percentage of the supply voltage. You can get this information from the power supply tolerance specifications of the devices being supplied by the power rail under consideration.
  • ZTARGET= [VoltageRail (%Ripple/100)/MaxTransientCurrent]
  • ZTARGET= [(1.1)(0.05)/1.5]=36.7mΩ

If this information is available, then ZTARGET can be calculated as:

  • ZTARGET= [VoltageRail (%Ripple/100)/MaxTransientCurrent]

For example, to reliably decouple a 1.1-volt power rail that allows 5 percent of AC ripple and is expected to supply a maximum transient current of 1.5 A, the target impedance is:

  • ZTARGET= [(1.1)(0.05)/1.5]=36.7mΩ

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