Power Planning & IR Drop

VLSI Concept · Physical Design

Even copper has resistance — voltage sags on its way across the chip.

Power is delivered through a grid of VDD/GND straps and rings across the die. Because metal wires have real resistance, voltage "droops" (IR drop) as current travels farther from the power pads — cells far from a strap can see a lower effective voltage, which slows them down and can cause timing or even functional failures.

RuleDenser power grid + more straps = lower IR drop, at the cost of routing area for signals.

Why it mattersA cell that works fine in isolation can fail once the whole chip switches at once and voltage sags.

Common mistakeUnder-provisioning power straps in the middle of large blocks far from the chip's power pads.

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