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42 DESIGN007 MAGAZINE I FEBRUARY 2020 ics of the fundamental tend to radiate if im- pedance peaks appear at the harmonics (Fig- ure 2). Inductance pushes the impedance up and shifts the resonant frequency of capacitors down. The addition of more capacitors to the PDN lowers the impedance at particular fre- quencies, while the combined parallel resis- tance determines the minimum impedance at the resonant frequency (dips). The PDN has four major functions: 1. Provides a low-impedance, high-current supply path to the ICs. 2. Reduces noise on the power supply at the IC die. 3. Minimizes ground bounce and common-mode potential between the IC die and the return path. 4. Reduces electromagnetic radiation from the board fringing fields. The typical PDN topology (Figure 3) consists of numerous connections from the voltage reg- ulator module (VRM) to the capacitors, vias, wide traces, copper pours, power and ground planes, solder balls, and interconnects on the IC silicon itself. The PDN must provide a con- stant supply voltage within a tolerance of 5% at the power pins of each IC. This voltage must be stable from DC up to the maximum band- width, which is typically five times the funda- ment frequency. At the same time, these con- nections that provide the power are also tasked with providing the return signal path. This is where high-current, simultaneous switching noise from the power supply can interfere with the signal return currents. When the return current flows through the impedance of a cavity between two planes, it generates voltage. Although quite small, the accumulated noise from simultaneous switch- ing devices can become significant. Unfortu- nately, as core voltages drop, noise margins become tighter. This voltage, emanating from the vicinity of the signal via, injects a prop- agating wave into the cavity, which can ex- cite the cavity resonances or any other paral- lel structure (e.g., between copper pours over planes). Other signal vias passing through Figure 2: Odd harmonics of the fundamental tend to radiate at high frequencies (iCD PDN Planner).