Free Trials

Download a free trial to find out which Altium software best suits your needs

How to Buy

Contact your local sales office to get started on improving your design environment

Downloads

Download the latest in PCB design and EDA software

  • PCB DESIGN SOFTWARE
  • Altium Designer

    Complete Environment for Schematic + Layout

  • CircuitStudio

    Entry Level, Professional PCB Design Tool

  • CircuitMaker

    Community Based PCB Design Tool

  • NEXUS

    Agile PCB Design For Teams

  • CLOUD PLATFORM
  • Altium 365

    Connecting PCB Design to the Manufacturing Floor

  • COMPONENT MANAGEMENT
  • Altium Concord Pro

    Complete Solution for Library Management

  • Octopart

    Extensive, Easy-to-Use Component Database

  • PRODUCT EXTENSIONS
  • PDN Analyzer

    Natural and Effortless Power Distribution Network Analysis

  • See All Extensions
  • EMBEDDED
  • TASKING

    World-Renowned Technology for Embedded Systems Development

  • TRAININGS
  • Live Courses

    Learn best practices with instructional training available worldwide

  • On-Demand Courses

    Gain comprehensive knowledge without leaving your home or office

  • ONLINE VIEWER
  • Altium 365 Viewer

    View & Share electronic designs in your browser

  • Altium Designer 20

    The most powerful, modern and easy-to-use PCB design tool for professional use

    ALTIUMLIVE

    Annual PCB Design Summit

    • Forum

      Where Altium users and enthusiasts can interact with each other

    • Blog

      Our blog about things that interest us and hopefully you too

    • Ideas

      Submit ideas and vote for new features you want in Altium tools

    • Bug Crunch

      Help make the software better by submitting bugs and voting on what's important

    • Wall

      A stream of events on AltiumLive you follow by participating in or subscribing to

    • Beta Program

      Information about participating in our Beta program and getting early access to Altium tools

    All Resources

    Explore the latest content from blog posts to social media and technical white papers gathered together for your convenience

    Downloads

    Take a look at what download options are available to best suit your needs

    How to Buy

    Contact your local sales office to get started improving your design environment

    • Documentation

      The documentation area is where you can find extensive, versioned information about our software online, for free.

    • Training & Events

      View the schedule and register for training events all around the world and online

    • Design Content

      Browse our vast library of free design content including components, templates and reference designs

    • Webinars

      Attend a live webinar online or get instant access to our on demand series of webinars

    • Support

      Get your questions answered with our variety of direct support and self-service options

    • Technical Papers

      Stay up to date with the latest technology and industry trends with our complete collection of technical white papers.

    • Video Library

      Quick and to-the-point video tutorials to get you started with Altium Designer

    The Role of a Decoupling Inductor and Resistor in a PDN

    Zachariah Peterson
    |  July 1, 2019

    Passive components on a green PCB

    You might want to add one of these components to your decoupling network

    In a previous article, we looked at the role of decoupling capacitors, as well as the difference between decoupling and bypassing. A decoupling capacitor provides the same functions as a bypass capacitor, but it also provides another important function in that it compensates changes in the ground potential as an IC switches.

    There is another important point involved in designing your PDN to ensure power integrity. This is the role of inductance when designing your PDN. In high speed designs (which is every design nowadays), decoupling capacitors can be used in conjunction with decoupling capacitors or resistors, forming a decoupling network.

    The Goal of a Decoupling Network

    Designing a decoupling network is not a simple task. With lower speed circuits (i.e., analog signals with frequencies of less than ~50 MHz), using a decoupling capacitor was sufficient for decoupling. The self-resonance frequency of many smaller capacitors was still somewhat higher than the knee frequency for many logic families, thus it would be difficult to drive a power bus to resonance during switching. Furthermore, decoupling capacitors would also act as a bypass capacitor to compensate potential changes as ICs switched.

    With faster logic families, knee frequencies can now coincide with the self-resonance frequency of the equivalent circuit formed by the bypass/decoupling capacitor, the power supply bus, any nearby bypass/decoupling capacitors, conductors that connect components, and the components themselves. This creates the potential for ringing in the power bus with high speed circuits as logic gates switch. Under repeated switching, this would cause a driven resonant oscillation in the power bus with high amplitude. Just as was the case with ground bounce, a single switching output on an IC may not have much of an effect, but many outputs switching simultaneously can produce significant ringing in the power bus and large changes in the potential seen by an IC.

    For this reason, a decoupling inductor, resistor, and capacitor are sometimes used together if ringing in the power bus is severe. Ringing in a power bus can reach ~1 V levels near one of the resonance frequencies in the circuit. In a perfect world, you could suppress ringing while minimizing the impedance of the PDN and ensuring the impedance spectrum is flat. Unfortunately, this is not always possible at all frequencies.

    Suppress PDN Ringing With a Decoupling Network

    As discussed in the previous article, the equivalent RLC model for the decoupling capacitor may be underdamped, and you should try to bring this circuit as close to the critically damped case as possible. However, you will need to consider the entire equivalent circuit for the decoupling capacitor and the rest of the system.

    Ideally, you want to suppress this ringing in one of two ways. First, you can critically damp the response at the power bus. This is rather simple as it requires adding a decoupling inductor, resistor, or both to your PDN. Second, you can try to add components that shift the resonance frequency in any portion of the circuit to values that are much higher than the knee frequency for the switching signal.

    Both methods are somewhat mutually exclusive. Adding an inductor in series between the decoupling capacitor and an IC will increase the impedance seen by any high frequency signals (including a ringing signal) propagating towards the load, but it will also decrease the resonance frequency. Additionally, it will decrease the damping constant by a greater level since the resonance frequency is only inversely proportional to the square root of inductance. Therefore, if the response from the decoupling capacitor is already overdamped, adding a series inductor between the decoupling capacitor and the load can bring the response closer to critical damping.

    If the response seen on the power rail is already underdamped, then you need to increase the damping constant and decrease the ringing amplitude. One simple way is to use a capacitor with larger equivalent series resistance (ESR). Note that electrolytic capacitors tend to have larger ESR values. The other option is to add a resistor and inductor before the relevant IC, as shown in the circuit below:

     Equivalent RLC decoupling network

    Full decoupling network with a bypass capacitor

    Note that L in the above model is equal to the inductance of the conductor leading to the load plus the value of the decoupling inductor. The damping constant in the equivalent RLC network formed by the load, decoupling capacitor, L, and R is equal to the usual value for an RLC series circuit. Adding the inductor decreases the natural resonance frequency, while adding a small resistor R can increase damping in the circuit. When R is equal to the critical value shown above, then the transient response in this circuit will be critically damped.

    An Alternative Decoupling Network

    The network shown above will increase the DC voltage drop throughout the PDN, thus there is an alternative decoupling network that provides the critical damping:

    Alternative RLC decoupling network

    Alternative decoupling network with a bypass capacitor

    In this network, the critical resistance is the same as that shown in the earlier network. However, there is also a restriction on the values of the decoupling and bypass capacitors (shown above). Increasing the damping resistor between the limits shown above will cause the response to move into the overdamped regime, thus slowing down the overall response from the decoupling capacitor.

    Final Thoughts

    Note that adding these circuit elements will increase the input impedance at the load, and any current carried by a noise signal will translate into a large voltage across the load if the increased input impedance is not compensated. Placing a bypass capacitor directly between the power and ground pins on the load will bring the load’s input impedance back to the appropriate level and will provide a low impedance path to ground for high frequencies.

    You should also not use a decoupling capacitor with a series inductor/resistor for every single IC on your PCB. In reality, you only need a small number of these capacitors on the power bus such that they can provide sufficient charge if every IC on the board were to switch simultaneously.

    When designing a PDN for your PCB, you’ll need the layout and simulation tools in Altium Designer to ensure your board is free from power integrity and signal integrity problems. Altium Designer’s suite of simulation tools also helps you identify power integrity problems and simulate the behavior of your decoupling network.

    Contact us or download a free trial if you’re interested in learning more about Altium Designer. You’ll have access to the industry’s best routing, layout, and simulation tools. Talk to an Altium expert today to learn more.

    About Author

    About Author

    Zachariah Peterson has an extensive technical background in academia and industry. Prior to working in the PCB industry, he taught at Portland State University. He conducted his Physics M.S. research on chemisorptive gas sensors and his Applied Physics Ph.D. research on random laser theory and stability.His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental systems, and financial analytics. His work has been published in several peer-reviewed journals and conference proceedings, and he has written hundreds of technical blogs on PCB design for a number of companies. Zachariah works with other companies in the PCB industry providing design and research services. He is a member of IEEE Photonics Society and the American Physical Society.

    most recent articles

    Back to Home