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We have investigated the many factors that must be considered in designing an optimal power supply. Take a step-by-step approach and consider a few insider tips to make the task easier. This is part two of a two-part article. Part one focuses on how to minimize power consumption by adequately using multiple power operating modes in processor-based designs. Part two provides tips for optimizing resources and extending battery life by designing supplies more efficiently.
Power supplies can become a major source of unnecessary power consumption because their efficiency typically varies with voltage, current and temperature. Unfortunately, conversion efficiency can sometimes be quite low.
For example, although a linear regulator may achieve remarkable 89 percent efficiency when generating a 3.3-V memory rail from 3.7-V, its efficiency drops down to 29 percent when generating a 1.1-V core voltage from the same battery 3.7-V rail. In other words, 71 percent of the power going into the regulator is dissipated. However, there are techniques available to mitigate these losses. Sometimes the answer is to power different embedded processor's power supply rails with complementary power topologies.
Integrating the most efficient power supply into a design is often considered important enough that developers will design their own supplies rather than choosing a plug-in solution. Many digital designers, however, have limited power supply design experience, particularly when multiple power rails are involved-- which is typical when powering most DSPs.
While DC/DC power supply control IC suppliers typically provide detailed collateral to assist with circuit design, the most important work may actually take place before the actual designing starts. Only the project team can identify system-level and application requirements, determine power requirements such as voltage and current per rail as well as determine a power budget.
At the top level of this analysis, identifying and quantifying the application requirements are an important step. Cost is always a critical factor and in power supply design, BOM and design cost budgets are usually constrained because the power supply is often one of the last parts of the product to be designed. The physical size of the supply and the amount of heat that can dissipate are also important application-level boundary conditions. Another aspect, and something not often considered, is the design expertise within the team.
System requirements are a familiar topic and power supplies have a few unique considerations. In the short list below, the first three considerations are obvious but the last two may not be.
- Input power voltage and type: batteries, wall plug or both
- Voltage rails: how many, voltage, current, tolerance and accuracy
- Current levels, including dynamic or peak as well as standby
- Sequencing " for example, DSPs usually have very specific power up requirements
- Monotonic startup.
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