Meaning
Decoupling strategies that use capacitors to maintain a stable power supply voltage across a wide frequency range define the power integrity design of a system. Implementing printed circuit board decoupling creates local reservoirs of electrical charge that supply high-frequency currents to switching integrated circuits. This strategy keeps the impedance of the power distribution network below the target limit.
Capacitor Selection
Selecting the correct combination of capacitor values is necessary to cover a broad frequency spectrum. Small ceramic capacitors with low parasitic inductance handle high-frequency transitions, while larger bulk capacitors store energy for low-frequency current steps. Each capacitor has a self-resonant frequency where its impedance is lowest.
Designers combine multiple values to create a low-impedance path across the desired bandwidth.
Placement Guideline
Passive components must be placed as close as possible to the power pins of the active integrated circuits. This close placement minimizes the parasitic loop inductance of the connection traces and vias. Long traces between the capacitor and the pin reduce the high-frequency effectiveness of the decoupling network.
Board designs use short, wide traces to connect the capacitors to the power planes.
Plane Design
Dedicated power and ground planes on adjacent board layers provide additional high-frequency capacitance. This embedded capacitance has extremely low inductance and is highly effective at frequencies above one hundred megahertz. The plane configuration works with the discrete capacitors to maintain power stability.
Solid plane layers also prevent electromagnetic radiation from the switching currents.
