Another method for reducing noise and EM radiation sources is to add a passive Pi filter. A Pi filter allows emission reduction for noise coming from both external as well as internal sources. Figure 3-9 shows an input signal with high-frequency noise superimposed to mimic a VIN with an emission source.

Figure 3-9 Clock Signal with Ringing and FFT Plot

A C-L-C pi filter presents shunt capacitance on both sides of a series impedance. The filter can attenuate noise traveling into or out of a subsystem, but the performance of the filter depends on source and load impedance, component parasitics, DC bias, and damping. An ideal lossless network can develop a high-Q resonance that amplifies noise near cutoff.

The filter has a natural resonant region determined by its effective inductance and capacitance. Source and load impedances, losses, and parasitics determine whether the response is well damped or exhibits peaking.

• Choose the inductor or ferrite for current rating, saturation margin, DCR, and impedance at the disturbance frequency.
• Place the load-side high-frequency capacitor close to the protected device and minimize the return loop.
• Add damping through ESR, a lossy ferrite, or an explicit damping network when the source/load combination produces peaking.
• Verify regulator stability. An input filter can interact with a converter's negative incremental input impedance.
• Measure insertion loss in the actual impedance environment; a 50Ω bench result can not predict a low-impedance power rail.

Figure 3-10 Bi-Directional EMI Filter Using a Pi Filter for High Frequency Noise

The typical passive pi filter shown in Figure 3-11 consists of two capacitors and an inductor connected between them and in series with the main power supply line. This is added in addition to the normal supply capacitors in between the existing capacitors and the power supply. The operating principle can be partitioned into:

1. The Inductor Acts as a Current Source Opposing the Drop: When the MCU suddenly demands more current, the supply voltage at the input of the inductor momentarily dips, causing a negative dI/dt. The collapsing magnetic field induces a forward voltage (V = L × dI / dt) across the inductor. As a result, this induced voltage acts in series with the power supply, pushing stored current forward into the output side of the filter.
2. Teamwork with the Output Capacitor Immediate Response: The output capacitor responds first, instantly dumping its stored charge into the MCU to cover the microsecond-level spike. Sustained Response: As the capacitor discharges, the collapsing magnetic field of the inductor sustains the current flow, recharging the output capacitor and stabilizing the rail. The inductor cannot do this indefinitely; it only supplies current until its stored magnetic energy is completely depleted.
3. Without this action, quick digital switching inside the MCU can cause the supply voltage to drop below the minimum operating threshold. By integrating this type of filter, the filter essentially bridges the tiny time gap between the peak demand of the MCU and the ability of the main power supply to react, preventing accidental processor resets.

Figure 3-11 Pi-Filter Noise Filtering Performance