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22 September 2009
SIGNAL CHAIN BASICS (Part 11): Introducing voltage- and power-conditioning circuits

The very basics of voltage-regulation circuits, from simple resistor through switching buck and boost converters

By Bill Klein, Senior Applications Engineer, Texas Instruments
Planet Analog
April 5, 2008 (12:00 PM EST)

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(Editor's Note: There are links to the previous parts of this series at the end, below the author's biography.)

For all signal-chain devices to operate properly, each requires a stable-voltage power source. Power-conditioning circuits fall into two major classes: a) linear and b) switching. While linear voltage controllers are usually the lowest efficiency, they also generate the least amount of noise, Figure 1.

Figure 1: Linear voltage regulator concept
(Click on image to enlarge)

If load-current requirements do not change with time, and the unregulated input supply is stable, a simple series resistance of the figure could adjust the supply voltage for a circuit. However, these conditions are not realistic in the real world. The circuit shown in Figure 2 provides an active control.

Figure 2: Active voltage regulator.
(Click on image to enlarge)

The Zener diode (Z1) provides a stable reference at the positive input of the op amp. The op amp adjusts its output until the sample voltage at the negative input formed by the R2-R3 voltage divider equals the Zener voltage. Output voltage control is then maintained over a wide range of load current and input voltage. Unfortunately, this circuit is not very efficient. Efficiency can be significantly improved with switching regulators.

Figure 3: Buck switching converter [and see video link, in the text below]
(Click on image to enlarge)

The circuit in Figure 3 is a buck converter (in the class of dc/dc switching converters), which produces an output voltage lower than the input voltage (see one-minute video here). The cycle starts with Q1 turned on. Thus, current flows from the supply through Q1 and L1 to the load. Current flowing through L1 develops a magnetic energy storage field within it. The controller opens Q1 at the proper switch point. The energy stored in L1 now acts as a source that continues to supply current to the load.

As the magnetic field within L1 collapses, the output voltage drops to a set point, and Q1 is turned back on to restore the output. During the phase with Q1 off, current returns through the free wheeling diode D1. In the linear pass regulator energy is lost as heat in the pass element. But, in the switching regulator the inductor acts as a storage element.

Figure 4: Boost switching regulator [and see video link, in the text below]
(Click on image to enlarge)

The boost switching regulator shown in Figure 4 produces an output voltage that is higher than the input (see one-minute video here). In the first phase, Q1 is turned on so the input current builds a magnetic field in L1. With diode D1 reverse biased, the output is isolated from the L1 circuit. Any current supplied to the load is from the output capacitor Cout. During the second phase of operation, Q1 is open and the energy from L1 is added to the input voltage to produce an output greater than the input.

Controllers for both switching regulators employ different algorithms to determine switching points between the phases, making it possible for switching regulators to achieve more than 90 percent efficiency.

About the author

William P. (Bill) Klein is a Senior Applications Engineer with the High Performance Analog group at Texas Instruments. Bill joined TI through its acquisition of Burr-Brown in August 2000. His experience as an analog circuit designer covers over 40 years in fields ranging from mineral exploration to medical nuclear imaging. One current role Bill has is hosting the Analog e-LAB Web Cast, presenting real world solutions to real world problems in analog circuit design. In addition to a BSEE from Arizona State University and registration as a Professional Engineer in the State of Arizona, he has authored numerous magazine articles, application notes and conference papers.

Previous installments of this series:

"SIGNAL CHAIN BASIC Series (Part 10): Exploring the Delta-Sigma Converter", www.planetanalog.com/features/showArticle.jhtml;?articleID=206903892, click here
"SIGNAL CHAIN BASIC Series (Part 9): SAR Converter Operation Explored", www.planetanalog.com/features/showArticle.jhtml;?articleID=206901015, click here
"SIGNAL CHAIN BASIC Series (Part 8): Flash- and Pipeline-Converter Operation Explored", www.planetanalog.com/features/showArticle.jhtml;?articleID=206504089, click here
"SIGNAL CHAIN BASIC Series (Part 7): Op Amp Performance Specification--Bias Current", www.planetanalog.com/features/showArticle.jhtml;?articleID=206101908, click here
"SIGNAL CHAIN BASIC Series (Part 6): Op Amp Input Voltage Offset", www.planetanalog.com/features/showArticle.jhtml;?articleID=205901111, click here
"SIGNAL CHAIN BASICS Series (Part 5): Introduction to the Instrumentation Amplifier", www.planetanalog.com/features/showArticle.jhtml;?articleID=205208593, click here
"SIGNAL CHAIN BASICS Series (Part 4): Introduction to analog/digital converter (ADC) types", www.planetanalog.com/features/showArticle.jhtml;?articleID=204803631, click here
"SIGNAL CHAIN BASICS Series (Part 3): Analog and the digital world", www.planetanalog.com/features/showArticle.jhtml;?articleID=204400376, click here
"SIGNAL CHAIN BASICS Series (Part 2): Op Amp--Basic operations", www.planetanalog.com/features/showArticle.jhtml;?articleID=203101699, click here
"SIGNAL CHAIN BASICS: Operational Amplifier--The Basic Building Block", www.planetanalog.com/features/showArticle.jhtml;?articleID=202801320, click here

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