What Is PDN impedance And Why It Matters

May 21, 2020 Cadence PCB Solutions

Key Takeaways

  • Define PDN impedance and its importance for your designs

  • Determine when and how to consider impedance

  • Understand optimization or reduction of impedance in your circuits

PDN impedance affects how power is delivered and susceptibility to noise

I am a well-trained black belt martial artist. Yet my most humbling moment is when I’m incapacitated by an elderly tai-chi master, who fences off my advanced with ease and shoves me around like a puppet. Based on the principle of tai-chi, the harder you strike the quicker you’ll fall. 

There won’t be any tai-chi maneuvering in PCB design, but a similar concept of how an increase in aggression, in the form of signal rate, can be detrimental to the circuit. To deal with such an issue, PCB designers need to understand what PDN impedance is all about and how it plays an important role in ensuring signal integrity. 

What Is PDN Impedance

The term PDN stands for the power delivery network. It’s basically every element that is connected to the voltage and ground rail. The PDN starts from the voltage regulator module and extends to the connecting traces, vias, and pads. It also includes any capacitance and inductance elements and ICs that are powered by the supply rail. 

In simple, low-speed design, the power supply rail is often viewed as a resistive element, where capacitors are open circuits and inductors are short circuits. However, the characteristic of PDN changes when the frequency of the components increases. At higher frequencies, the value of inductance and capacitance starts bearing weight on how current passes through the supply rail. 

Each element on the PCB contributes to the total impedance of the PDN. As impedance is a function of a frequency, the value varies depending on where it’s measured on the frequency spectrum. Impedance introduced by components is beyond a designer’s control, but impedance derived from PCB traces, planes and layout can be manipulated during design. 

When Should You Care About PDN Impedance

As mentioned, PDN impedance is barely a subject of concern in low-speed design. However, if you’re designing a high-speed circuit, you’ll need to start caring about how PDN impedance affects the functionality of the circuit. 

It all bears down to this equation: 

V ripple = I transient x PDN Impedance.

When components are sending out high-speed clocks or data signals, or oscillating at a high data rate, the current is drawn from the supply rail quickly and continuously. The transient current will then cause ripple noise on the supply rail with the magnitude determined by the PDN impedance.

Chips and microcontrollers on a printed circuit board

PDN impedance becomes an important parameter in high-speed designs.

Naturally, noise introduced into the power rail could affect other components or signals. The amplitude of the voltage ripple may also exceed the tolerance of ICs if the PDN impedance has a large value as transient current passes through. In such a condition, the components are pushed to operate beyond its specified limits. 

How To Reduce PDN Impedance

There’s little you can do on the transient current nor the impedance of the respective components. This leaves designers the next best option, which is to keep the PDN impedance sufficiently low so that the ripple doesn’t exceed the voltage rail tolerance. 

In other words, designers will need to achieve the target PDN impedance, which is given by the equation:

Z target = ( V supply x Tolerance %) / I transient (Max)

This brings us to the actual process of reducing PDN impedance. Often, it revolves in reducing loop impedance and maximizing plane capacitance. 

Loop inductance occurs along decoupling capacitors and their connection to the power and ground plane. To minimize the impedance, the vias that connect the capacitor need to be placed as close as possible. Also, using shorter and thicker traces will result in a lower impedance value.  

Sprawling and densely packed circuit board with capacitors

Capacitor placement and layout has a pronounced effect on PDN impedance.

A secondary loop also occurs between the capacitor and the IC it connects to. Place the capacitor as close to the IC to keep the loop impedance low. In the case of a BGA component, place the capacitors underneath the component to keep the PDN impedance low.

A PCB with a power and ground plane is basically a capacitor. Therefore, reducing the substrate thickness between power and ground will directly increase the capacitance. A higher capacitance value will result in an overall PDN impedance.

Of course, there’s still a chance of getting it wrong with PDN impedance as there are many variables involved. Don’t fret too much, though, and instead, simply use a more-equipped software suite to handle your problems like the layout and analysis tools from Cadence. It’s prudent to use a PDN analysis tool that works along with PCB design software like PSpice Simulator.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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