7.2.3 Step 3: Static Analysis

Delivering reliable power to circuits is always of critical importance because voltage drops (also known as IR drops) can happen at every level within an electronic system, on-chip, within a package, and across the board. Robust system performance can only be ensured by understanding how the system elements will perform under typical stressful Use Cases. Therefore, it is a good practice to perform a Static or DC Analysis.

Static or DC analysis and design methodology results in a PDN design that minimizes voltage or IR drops across power and ground planes, traces and vias. This ensures the application processor’s internal transistors will be operating within their specified voltage ranges for proper functionality. The amount of IR drop that will be encounter is based upon amount power drawn for a desired Use Case and PCB trace (widths, geometry and number of parallel traces) and via (size, type and number) characteristics.

Components that are distant from their power source are particularly susceptible to IR drop. Designs that rely on battery power must minimize voltage drops to avoid unacceptable power loss that can negatively impact system performance. Early assessments a PDN’s static (DC) performance helps to determine basic power distribution parameters such as best system input power point, optimal PCB layer stackup, and copper area needed for load currents.

Figure 7-3 Depiction of Sheet Resistivity and Resistance

Ohm’s Law (V = I × R) relates conduction current to voltage drop. At DC, the relation coefficient is a constant and represents the resistance of the conductor. Even current carrying conductors will dissipate power at high currents even though their resistance may be very small. Both voltage drop and power dissipation are proportional to the resistance of the conductor.

Figure 7-4 shows a PCB-level static IR drop budget defined between the power management device (PMIC) pins and the application processor’s balls when the PMIC is supplying power.

• It is highly recommended to physically place the PMIC as close as possible to the processor and on the same side. The orientation of the PMIC vs. the processor should be aligned to minimize distance for the highest current rail.

Figure 7-4 Static IR Drop Budget for PCB Only

The system-level IR drop budget is made up of three portions: on-chip, package, and PCB board. Static IR or dc analysis/design methodology consists of designing the PDN such that the voltage drop (under dc operating conditions) across power and ground pads of the transistors of the application processor device is within a specified value of the nominal voltage for proper functionality of the device.

A PCB system-level voltage drop budget for proper device functionality is typically 1.5% of nominal voltage. For a 1.35-V supply, this would be ≤20 mV.

To accurately analyze PCB static IR drop, the actual geometry of the PDN must be modeled properly and simulated to accurately characterize long distribution paths, copper weight impacts, electro-migration violations of current-carrying vias, and "Swiss-cheese” effects via placement has on power rails. It is recommended to perform the following analyses:

• Lumped resistance/IR drop analysis
• Distributed resistance/IR drop analysis

The NOTE above means the SMPS feedback signals and returns must be routed across PCB and connected to the Device input power ball for which a particular SMPS is supplying power. This feedback loop provides compensation for some of the voltage drop encountered across the PDN within limits. As such, the effective resistance of the PDN within this loop should be determined in order to optimize voltage compensation loop performance. The resistance of two PDN segments are of interest: one from the power inductor/bulk power filtering capacitor node to the Processor’s input power and second is the entire PDN route from SMPS output pin/ball to the Processor input power.

In the following sections each methodology is described in detail and an example has been provided of analysis flow that can be used by the PCB designer to validate compliance to the requirements on their PCB PDN design.