IR Drop and Power Integrity
When the power grid cannot keep up
A chip can be perfectly placed, routed, and timed and still fail because the power network cannot deliver current where and when it is needed. That is the domain of IR drop and power integrity, a signoff concern in its own right.
What IR drop is
Current flowing through the resistance of the power grid causes a voltage drop, by Ohm law: V = I times R. So a cell deep in the chip sees a supply slightly lower than the ideal: its effective voltage is VDD minus the IR drop. Lower voltage makes cells switch slower, so IR drop directly eats into timing.
Static versus dynamic IR drop
- Static IR drop: the average voltage loss across the grid under steady current. The easier one to manage.
- Dynamic IR drop: the momentary sag when many cells switch on the same clock edge and draw a current spike together. This is the dangerous one, because it hits exactly when the logic is switching and timing is tightest.
How it is fixed
- Strengthen the power grid: more or wider straps, more vias down to the rails.
- Add decap (decoupling capacitor) cells near hot spots to supply the instantaneous current locally.
- Spread high-activity logic so not everything switches in the same place at the same time.
- Reduce simultaneous switching where possible.
Signoff
IR drop is verified with rail or power-integrity analysis, often using activity from simulation to find the worst switching moments. Electromigration (EM), the slow wearing-out of wires that carry too much current, is checked at the same time, since both are about the power network carrying current safely.
connect it to timing: dynamic IR drop slows cells right when they switch hardest, so a design can pass timing with an ideal supply and fail once real IR drop is applied. That link between power integrity and timing is what separates a senior answer from a textbook one.
plan the power grid during floorplanning, not after. Discovering IR-drop hot spots late, when cells and routes already fill the area, leaves little room to add straps or decaps. Power integrity is cheapest to fix early.