Power network design (IC)

Power network design in integrated circuits (ICs) refers to the process of planning, designing, and verifying the on-chip power distribution network (PDN) to reliably deliver power to all functional blocks within the IC, while meeting performance, power, and area constraints. The PDN is a crucial component of any IC, directly impacting its performance, reliability, and power consumption. Inadequate PDN design can lead to voltage droop, ground bounce, electromigration, and ultimately, functional failure.

Key Considerations in Power Network Design:

• Voltage Drop (IR Drop): Minimizing voltage variations across the PDN is essential. Excessive IR drop can reduce the supply voltage available to logic gates, impacting their switching speed and noise margin, and potentially causing timing violations or functional errors.
• Ground Bounce (SSN): Simultaneous switching noise (SSN), or ground bounce, occurs due to the inductive impedance of the PDN. When multiple gates switch simultaneously, the current surge can induce voltage fluctuations on the ground rail, affecting the noise immunity of other circuits. Minimizing inductance in the PDN is key to controlling ground bounce.
• Electromigration (EM): Electromigration is the transport of metal atoms in interconnects due to the momentum transfer from conducting electrons. High current density in the PDN can lead to electromigration, causing voids and eventually open circuits, reducing the IC's lifetime. Design techniques must ensure current densities remain within acceptable limits.
• Power Consumption: The PDN itself consumes power through resistive losses. Minimizing the resistance of the PDN is crucial to improving the overall power efficiency of the IC.
• Area Overhead: The PDN occupies significant chip area. Balancing the need for low resistance and inductance with area constraints is a key design challenge.
• Reliability: The PDN must be designed to withstand variations in process, voltage, and temperature (PVT). Robust design techniques are necessary to ensure reliable operation under worst-case conditions.
• Signal Integrity: The PDN can impact signal integrity by injecting noise into the signal paths. Decoupling capacitors are strategically placed to filter noise and provide a stable voltage supply.
• Power Grid Topology: Different topologies, such as mesh, tree, and combinations thereof, are employed depending on the specific requirements of the IC. A mesh network generally offers lower resistance and inductance, but at the cost of increased area.

Design Flow:

The power network design process typically involves the following steps:

1. Power Estimation: Estimating the power consumption of each functional block in the IC is a critical first step. Accurate power estimation allows designers to size the PDN appropriately.
2. Topology Planning: Based on power estimates and area constraints, the topology of the PDN is planned.
3. Sizing and Routing: The width and spacing of power rails and vias are determined to meet IR drop, ground bounce, and electromigration requirements.
4. Decoupling Capacitor Placement: Decoupling capacitors are strategically placed to reduce noise and voltage fluctuations.
5. Verification: The designed PDN is verified through simulation to ensure it meets all performance and reliability specifications. Static and dynamic simulations are performed to analyze IR drop, ground bounce, and electromigration.
6. Layout: The physical layout of the PDN is implemented, taking into account manufacturing constraints.
7. Post-Layout Verification: After layout, the PDN is re-verified to account for parasitics introduced during the layout process.

Tools and Techniques:

Specialized EDA (Electronic Design Automation) tools are used to design and verify power networks. These tools employ various techniques, including:

• Static IR Drop Analysis: Evaluates the voltage drop across the PDN under static conditions.
• Dynamic IR Drop Analysis: Evaluates the voltage drop across the PDN during circuit operation.
• Electromigration Analysis: Checks for violations of current density limits.
• Power Grid Optimization: Optimizes the size and routing of the PDN to minimize IR drop, ground bounce, and electromigration.
• Model Order Reduction (MOR): Reduces the complexity of the PDN model to enable efficient simulation.

Power network design is a complex and iterative process that requires careful planning, simulation, and verification. A well-designed PDN is essential for ensuring the performance, reliability, and power efficiency of modern integrated circuits.