Don’t Assume Top LDO, Switcher Attributes Always Apply (.PDF Download)
Every design begins with some assumptions, because you have to start somewhere. But as the design moves along, it’s often important and necessary to double-check if those assumptions are actually valid for the specific case. When a design needs a small dc-dc converter, the choices are between the low-dropout (LDO) and switching topologies. Each has differing characteristics and attributes, but each also has a “tag” with which it’s closely associated.
For LDOs, the word is that they’re inefficient; for switchers, it is that they’re noisy. While these are true of each in general, certain situations arise in which they may not be true or relevant in a particular configuration.
Assumption #1: “LDOs are less efficient than switchers”
In general, yes. A typical LDO’s efficiency is in the 60% to 80% range, while a switcher is likely in the 75% to 95% range. But these basic numbers are only part of the story. What efficiency do you need, and what are you willing to pay for it? Also, depending on the dropout differential between VIN and VOUT, the LDO’s efficiency may be at the high end. This is the often the case in battery-powered applications, where the dropout voltage may be small until the battery reaches its discharge knee and drops below a viable operating value.