The Effect of PDN Elements on Impedance

In digital systems, we demand that the impedance of the PCB’s power delivery network (PDN) has a low value over a broad frequency range in order to ensure low voltage fluctuations as digital components operate. PDN impedance is determined by much more than the capacitors used for power stability in digital processors. In advanced systems operating in the GHz range, the PDN impedance will be affected by much more than capacitors, there is a plurality of elements that determine PDN impedance up to very high frequencies.

SMD Capacitors (Coverage Up to 10-100 MHz)

The mainstay elements determining PDN impedance and ensuring stable power delivery are capacitors. Most commercially available capacitors are capable of ensuring low PDN impedance, but the frequency range they reach depends on:

• The capacitance value
• ESR and ESL values (determining self-resonant frequency)
• SMD capacitor case size

The ultimate frequency range where a capacitor can ensure low PDN impedance is the capacitor’s self-resonant frequency. In general, smaller case sizes with lower capacitance provide higher self-resonant frequency.

Some capacitors are specifically designed to target very high frequencies, such as capacitors designed for use in RF circuits. An example of the frequency range where these capacitors start to turn inductive is show below. As we can see, the self-resonant frequency of these capacitors varies by case size and can reach very high values (in theory). In practice, not all capacitors can reach extremely high resonant frequencies as suggested below. Read more about these capacitors in this article.

Self-resonant frequency trends for high-frequency capacitors shown by case size.

Plane Pairs (Coverage Near 100 MHz-1 GHz)

As the PDN impedance curve nears the 100 MHz range, the capacitors stop supplying power and instead the PDN impedance curve is determined by the plane capacitance. Power-ground plane pairs are used when high currents are needed in digital systems, as well as to provide high capacitance that maintains its capacitive impedance up to very high resonant frequencies. The ability of planes to deliver power is determined by the spreading inductance in the power-ground plane pair.

Eventually, at very high frequencies (10’s of GHz), the planes will start resonating and emitting radiation from their edges. This is where material selection and the thickness of the dielectric in the plane pair becomes a factor dominating the PDN impedance and noise emission.

Packaging (Coverage From 100 MHz to 1 GHz)

Packaging for advanced digital components can contain its own capacitance that targets low-impedance power delivery into the GHz range. These components compensate for plane pairs on the PCB, which can turn inductive at GHz frequencies. These packages include chip capacitors and possibly embedded capacitance to ensure stable power delivery and low PDN impedance directly to the semiconductor die in the component package. By having this capacitance placed directly on the package, it avoids the inductance from pads and vias on the PCB, as well as pin inductance in BGA/LGA packaging.

Packages for CPUs can contain their own PDN elements to aid power delivery at low impedance.

Die Capacitance (Coverage Above ~1 GHz)

The semiconductor die can provide its own capacitance, often totaling pF worth of capacitance directly on a die. The benefit of this capacitance is that it is located directly on the die and has minimal inductance to supply power to digital interfaces. This is the best situation for providing capacitance at or above 1 GHz in order to support the fastest digital interfaces.

PCB Materials (All Frequencies)

All PCB materials will impact electromagnetic wave propagation and thus the PDN impedance curve. As was mentioned in the above section on power-ground plane pairs, the plane region can start resonating as a parallel plate resonator when excited at 10’s of GHz frequencies.

To ensure power-ground plane pairs are most effective, the material separating them should have some important properties:

• High Dk values are preferred as these provide higher capacitance
• Moderate to high loss tangent is preferred to dampen noise
• Thin layers are preferred for higher capacitance and higher resonance cutoff

With very thin high-Dk dielectrics, such as embedded capacitance materials, radiated emissions from an oscillating PDN can be reduced and the total plan capacitance can be increased. These materials come at a premium cost, but they are often needed in digital systems with high power requirements at high bandwidths.

Summary

To summarize the various portions of the PDN and their effects on frequency, see the data in the table below.

The final wildcard in power integrity has nothing to do with the PCB at all, it is the frequency response capability of the power regulator or VRM. These circuits maintain output voltage stable via a control loop, and the frequency response of the VRM needs to be fast enough to help dampen noise that may be present on the output. It should also not enter sustained oscillation if driven with a fast transient on the power rail, something which should be qualified in measurement when designing a high-current/low-voltage power regulator for large digital processors.

PDN impedance engineering is not for the faint of heart, but understanding the PDN impedance frequency range can help designers make the right engineering choices to reach low PDN impedance. Multi-disciplined design teams rely on the best set of PCB design features in Allegro PCB Designer from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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