Meaning
Target impedance specifications representing the maximum allowable impedance of a power distribution network across a wide frequency range define the power integrity requirements of a system. Maintaining this impedance prevents the transient current demands of the integrated circuit from causing unacceptable voltage fluctuations on the power rails. This value is calculated based on the maximum current step and the allowable voltage ripple.
Calculation Method
The required value is determined by dividing the allowable supply voltage ripple by the maximum dynamic current step of the load. For example, a system with a one-volt rail, a five percent allowable ripple, and a two-ampere current step requires a target of twenty-five milliohms. The power distribution network must remain below this impedance across all operating frequencies.
Meeting this target requires a combination of decoupling capacitors and power planes.
Frequency Response
System impedance varies across the frequency spectrum due to the characteristics of the power planes, packaging, and capacitors. At low frequencies, bulk capacitors dominate the response, while ceramic capacitors handle the mid-frequency region. At high frequencies, the package inductance and on-die capacitance determine the impedance.
The decoupling network must be designed to avoid impedance peaks that exceed the target.
Design Tradeoff
Reducing the impedance requires adding more capacitors, which increases the component count, board area, and cost. High-performance designs must balance the number of capacitors against the thermal and physical space limits. Automated board simulation tools are used to optimize the decoupling network to meet the target with the minimum number of parts.
Achieving the target impedance ensures the stable operation of the high-speed logic.
