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Keywords: LDO, linear regulator, linear voltage regulator, portable, batteries, alkaline, NiCd, NiMH,
Lithium, Li+, lithium-ion, switching regulator, efficiency, quiescent current, IQ, wireless
TUTORIAL 751
Linear Regulators in Portable Applications
Jul 22, 2002
Abstract: Linear regulators provide significant advantages over switching regulators in simplicity, cost,
and output noise, but not efficiency. When applied to battery-operated portable equipment, battery life is
more important than individual circuit efficiency, therefore, the choice between a low dropout (LDO) and
a switching regulator is not so obvious. In addition, the particular battery characteristics, whether alkaline,
NiCd, NiMH, or lithium (Li+), must be considered.
A similar version of this article appeared in the August, 1996 issue of Electronic Products.
Introduction
Advances in voltage-regulator design have helped to establish portable electronic products as the fastest
growing segment of the electronics industry. The demands for low cost, long battery life, and small size
in this market are changing and rearranging the design priorities for power supplies. The result is a
general rewrite of specifications for the latest generation of power-supply ICs.
Surprisingly, both switch-mode and linear voltage regulators have participated in this transformation.
Despite the predominant use of switch-mode regulators in portable products, the linear voltage regulator
remains a viable contender. Advances in operating current, dropout voltage, noise, and packaging make
the modern linear regulator very different from the LM309s and µA7805s used for most designs in the
past.
Three very important questions must be asked when designing power supplies for portable products: Can
a linear regulator work in my design? Will linear regulators (versus switching types) limit my battery life?
Which regulator specs are critical? These questions are explored in the following discussion with an
emphasis on portable and handheld applications. The issues discussed include a comparison of switch-
mode and linear strengths, the parameters important to battery life, when linear regulators should not be
used, how battery type can affect your design decisions, and how the characteristics of linear regulators
can help or hinder a portable design.
Linear versus Switching Regulators
Before dealing with the subtleties of linear regulators for portable designs, it is worthwhile to make a
comparison between the linear and switching types. In some cases, switching regulators can provide
major benefits in a portable design. For example, if a high-performance switch-mode converter exhibits
90% efficiency (in transforming battery power to system power), then a linear design is unlikely to extend
the battery life, unless the voltage differential between the battery and linear regulator is small.
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