LINEAR Application Note AN32 High Efficiency Linear Regulators User Guide

Application Note 32 March 1989

High Effi ciency Linear Regulators Jim Williams

Introduction

Linear voltage regulators continue to enjoy widespread use switching supply output. Figure 1 shows such an arrange- despite the increasing popularity of switching approaches. ment. The main output (“A”) is stabilized by feedback to Linear regulators are easily implemented, and have much the switching regulator. Usually, this output supplies most better noise and drift characteristics than switchers. Ad- of the power taken from the circuit. Because of this, the ditionally, they do not radiate RF, function with standard amount of energy in the transformer is relatively unaf- magnetics, are easily frequency compensated, and have fected by power demands at the “B” and “C” outputs. fast response. Their largest disadvantage is ineffi ciency. This results in relatively constant “B” and “C” regulator Excess energy is dissipated as heat. This elegantly sim- input voltages. Judicious design allows the regulators to plistic regulation mechanism pays dearly in terms of lost run at or near their dropout voltage, regardless of loading power. Because of this, linear regulators are associated or switcher input voltage. Low dropout regulators thus with excessive dissipation, ineffi ciency, high operating save considerable power and dissipation.

temperatures and large heat sinks. While linears cannot L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear

Technology Corporation. All other trademarks are the property of their respective owners.

compete with switchers in these areas they can achieve signifi cantly better results than generally supposed. New

components and some design techniques permit reten- +VIN

tion of linear regulator’s advantages while improving VREG “C” effi ciency.

One way towards improved effi ciency is to minimize the input-to-output voltage across the regulator. The smaller this term is, the lower the power loss. The minimum input/ VREG “B” output voltage required to support regulation is referred to as the “dropout voltage.” Various design techniques and technologies offer different performance capabilities. Appendix A, “Achieving Low Dropout,” compares some

approaches. Conventional three terminal linear regulators have a 3V dropout, while newer devices feature 1.5V drop- out (see Appendix B, “A Low Dropout Regulator Family”)

SWITCHING at 7.5A, decreasing to 0.05V at 100μA. REGULATOR

Regulation from Stable Inputs

AN32 F01

Lower dropout voltage results in signifi cant power sav- ings where input voltage is relatively constant. This is

Figure 1. Typical Switching Supply Arrangement Showing Linear normally the case where a linear regulator post-regulates a Post-Regulators

an32f

AN32-1