Learning How To Power Circuits
: Description : Notes on different approaches and techniques for powering electronic circuits (with a focus on battery power)--drawing on the knowledge of those who actually understand this arcane art.
: Status : Research—don't take my word for it.
Types of Power Supply IC
This outline of available Power Supply IC options is from "Selecting the correct IC for power-supply applications" from Texas Instruments' Application Notes for Single Channel LDO page:
Inductive switchers are usually the best choice for highest efficiency. The inductive switcher circuits require a switching element, rectifier, inductor, and input and output capacitors. For many applications, the solution size can be reduced by choosing a device where the IC switching element and rectifier are internal. These circuits have typical efficiencies ranging from 80 to 96%, depending on the load. Switching converters usually require more space due to the size of the inductor and are generally more expensive. The switching converter also causes electromagnetic interference (EMI) radiation from the inductor and noise on the output due to the switching." Low-dropout linear regulators (LDOs) step down dc voltages by dropping the input voltage across a pass element. The benefit of this topology is that it requires only three parts (pass element and input/output capacitors). LDOs are usually a cheaper solution and are much less noisy than inductive switchers. The device input current is equal to the load current, so the efficiency of the solution is equal to the output-to-input voltage ratio. The drawback of this solution is the low efficiency for high input-to-output voltage ratios. All of the power is dissipated by the pass element, which means that an LDO is not an ideal solution for high-current applications where the input-to-output difference is large. These high-power applications require heat sinking, which increases the solution size." Charge pumps step up/down or invert dc voltages using “flying” capacitors as storage elements and use internal switches to connect the capacitors in such a way as to perform the desired dc/dc conversion. Charge pumps are generally cheaper than inductive switchers and do not emit EMI, but the output ripple is usually greater than that of inductive switchers. Charge pumps are limited in their output power, and the transient response is limited to the rate at which the flying capacitors can charge. Additionally, efficiency is usually very low in applications where the input voltage is near the output voltage."
This document also has a helpful case study working through how to choose one or more power supply ICs for a particular project.
Another issue to consider is the amount of heat the regulator needs to dissipate—this is related to the difference between the input and output voltages:
Where thermal considerations are challenging, another approach is to look at a switching regulator. Often, the additional efficiency that can be possible with a switching design can alleviate thermal issues faced when using a linear regulator approach. There are many considerations in migrating from a linear to a switching design that are beyond the scope of this application report.
Low-Dropout Regulators (LDOs)
A really good explanation of some of the terms used in discussing LDOs can be found in Understanding the Terms and Definitions of LDO Voltage Regulators--it includes helpful diagrams of how changes in one parameter affects the other parameters. (An almost identical document is Technical Review Of Low Dropout Voltage Regulator Operation And Performance but it does seem a little more technical—although one improvment is it includes larger diagrams.)
While not totally clear, this image (from another application note) visually shows how the voltage dropout affects the regulator's operation:
(Source: Digital Designer's Guide to Linear Voltage Regulators & Thermal Mgmt (LDO) (ti.com))
- How to Replace Tantalum Capacitors with Ceramic Capacitors in LDOs (ti.com)
- Extending the Input Voltage Range of an LDO Regulator with a FET (ti.com)
- ESR, Stability, and the LDO Regulator (ti.com) -- Includes section on how to identify regulator instability and "ringing" with the use of an oscilloscope.
- Fundamental Theory of PMOS Low-Dropout Voltage Regulators -- Fairly technical but a few helpful introductory explanations.
How to use specific solutions
Here are links to further pages detailing my research into implementing specific power supply solutions: