Inrush Currents Tamed – Part 3

In part 1 and part 2 of my blog, we looked at the capabilities that the EDA tools provided in the area of supply network analysis as well as the different methods of power shut off control. In this third blog, we will look at inter-domain switch control and role it plays in further taming the inrush currents.

Inter-Domain Switching Control

We must also consider the inter-domain switching control methods during our supply network analysis. Here each method provides a way to control the simultaneous switching of multiple logic domains.

  • No control – logic domains may all transition at the same time. This can cause higher peak inrush currents to occur and require longer switch control delays, additional decoupling capacitance or larger physical power rails.
  • Sequential domain list – reduces the simultaneous transitions to one by following a fixed sequence for the domain transitions. This limits the inrush current to a single domain at a time, but increases total transition time.
  • First level grouping based on state followed by a sequential domain list – reduces the simultaneous transitions to one by processing the domains in a series of groups defined by the operating point state then, within each group following a fixed sequence for the domain transitions. This allows the transition order to be modified per operating point state. The inrush current is again limited to a single domain at a time, but increases total transition time.
  • Cost model per supply – reduces the simultaneous transitions to a smaller number based on a cost model per domain and a limit per supply. Domains with a small cost may transition if the supply remains under a limit. Domains with a large cost may have to wait until other transitions complete. This reduces the total time to transition a series of domains and still maintain an acceptable inrush current.

Dependent Factors

The proper control method to choose will be dependent on a number of factors.

  • Size of the logic domains – determine the number and size of power switches needed. Larger domains will produce larger inrush currents given the same switching control. Smaller domains may not require any switching control or inter-domain control.
  • Supply network structure and proximity to other logic domains – determine if the supply rails can handle the instantaneous currents and IR drops due to the location of the power switches and the neighboring domains within the supply network.
  • Simultaneous power switching with other logic domains – verify if inter-domain control is sufficient. Large domains may be required to be switched in isolation, while multiple smaller domains could be switched together.
  • Required speed of the power transition – verify if the transition time is acceptable. Modify the switch control sequence or inter-domain control as needed.
  • Process technology and operating voltage – affects the analysis tools and acceptable values for supply or ground sags.

Taming the Inrush

Taming the inrush currents on an SoC may require the application of many different techniques. Sonics’ solution to taming the inrush currents is to provide a full featured set of control methods in the EPU so that a designer can quickly choose which control techniques to apply in each situation to best solve the problem with the minimal amount of logic.

Go ahead and create more power domains and gate them more aggressively. There is a solution to taming the inrush currents waiting.

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