On-Chip Power Delivery
A power delivery network distributes stable supply voltage across a chip despite rapidly changing current demand from switching logic.
Feeding Billions of Transistors
Every gate on a chip draws current from the supply when it switches. A power delivery network (PDN) is the grid of metal wires and connections that carries that current from the package pins to each transistor. Its job sounds mundane but is demanding: it must hold the voltage steady across the whole die even as billions of transistors switch at gigahertz rates, creating enormous and fast-changing current draw.
IR Drop and di/dt Noise
Two effects threaten a stable supply. IR drop is the static voltage loss across the resistance of the power wires; regions far from the supply pins see a lower voltage, which can slow or corrupt logic. di/dt noise (also called Ldi/dt) comes from the inductance of the supply path: when many gates switch at once, the sudden change in current induces a transient voltage droop or overshoot. Both eat into the timing margin that static timing analysis assumes.
- IR drop: resistive voltage loss, worst far from supply pins
- di/dt noise: inductive transients from sudden current swings
- Both reduce the effective voltage and thus the speed of logic
Decoupling Capacitance
The main defense against fast transients is decoupling capacitance: capacitors placed close to the logic that act as local charge reservoirs, supplying the burst of current a switching region needs before the distant supply can respond. On-chip decoupling capacitors, along with package and board capacitors covering different frequency ranges, together keep the voltage within tolerance across all timescales.
Design and Verification
The PDN is designed as a dense mesh on the upper, thicker metal layers to minimize resistance, and it is verified with dedicated tools that check IR drop and electromigration (the slow erosion of metal by high current density). Power integrity interacts with clock and power gating, since abruptly waking a gated block causes an inrush that the network must absorb. Reliable power delivery is a first-order constraint that shapes floorplanning and packaging alongside timing and area.