The linear voltage regulator was introduced in Part 11 of this series (link below). Successful application of this set of devices depends on an understanding of the specifications and knowledge of special circuit considerations that optimize the performance of this family of devices.

The first consideration is to determine the expected output voltage. The simplest of the linear regulators have a fixed output voltage set by internal connections similar to the R1/R2 network in Figure 1. Adjustable voltage regulators may have four pins: Vin, Vout, GND and Adjust. The adjust pin is to accommodate the voltage-setting resistors.

Figure 1: Basic linear voltage regulator

The output voltage of the three-terminal adjustable regulator shown in Figure 2:

Figure 2: Three-terminal adjustable voltage regulator

and is set by the relationship:

The voltage 1.25 V is the magnitude of the voltage reference. Dividing the numerator and denominator by R1 yields:

This is the transfer function for the non-inverting operational amplifier (op amp) developed in the first article in this series (Part 1, link below) published here on November 1, 2007.

Dynamic performance of a voltage regulator is measured by how well it holds the specified output voltage as input-supply voltage (line) or output-current demand (load) change. This is the response to both static changes and transient changes. Line regulation is usually specified at maximum load current. Load regulation may be specified at one or more line voltages.

As the input is unregulated, there may be some power-line frequency AC superimposed on the DC. This is input ripple and should be eliminated from the output. Another dynamic performance variable is output noise. Both of these usually can be reduced by increasing the filter capacitor on the regulator output, but at the cost of extended transient-recovery time.

Operation of the linear regulator depends on an internal gain stage. To prevent this gain stage from breaking into oscillation, a minimum output-filter capacitor (C2 in Figure 1) is usually required. An input-filter capacitor (C1 in Figure 1) will help the regulator service transient loads. This is especially needed if the regulator is some distance from the power source.

Many regulators cannot tolerate reverse voltage. If the output capacitor is charged to the output voltage and the input is shorted to ground, the regulator will see a reverse voltage and may latch in some unknown state, until all circuits are discharged. This fault condition may cause the regulator to self-destruct. To prevent this failure mode, a reverse-diode clamp (see Figure 3) may be required.

Figure 3: Reverse voltage clamp

Since there is no mechanism to store the unused energy input to the regulator, it must be dissipated as heat. This power is calculated as:

By using the input current in this calculation, the regulator quiescent or ground current is also considered.

In the case where the required load current is greater than the capability of the regulator, an external transistor can be added as shown in Figure 4. This is only possible with adjustable output regulators.

Figure 4: Increase I(load) with external transistor.

This circuit may help move heat from the regulator chip to a transistor on a heat sink. Reducing the power dissipated in the regulator can reduce output voltage drift due to temperature change.

If you have any questions about this or any of the articles in my Signal Chain Basics column, send an email to me at scb@list.ti.com.

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 BASICS (Part 18): The op amp as integrator", www.planetanalog.com/features/showArticle.jhtml;?articleID=209101070, click here
• "SIGNAL CHAIN BASICS (Part 17): Hysteresis--Understanding more about the analog voltage comparator", www.planetanalog.com/features/showArticle.jhtml;?articleID=208802817, click here
• "SIGNAL CHAIN BASICS (Part 16): Understanding the analog voltage comparator", www.planetanalog.com/features/showArticle.jhtml;?articleID=208403856, click here
• "SIGNAL CHAIN BASICS (Part 15): Analog/digital converter—dynamic parameters", www.planetanalog.com/features/showArticle.jhtml;?articleID=208401183, click here
• "SIGNAL CHAIN BASICS (Part 14): Analog/digital converter—static parameters", www.planetanalog.com/features/showArticle.jhtml;?articleID=207800114, click here
• "SIGNAL CHAIN BASICS (Part 13): Putting the Bode plot to use", www.planetanalog.com/features/showArticle.jhtml;?articleID=207403561, click here
• "SIGNAL CHAIN BASICS (Part 12): The Bode plot, an essential ac-parameter display tool", www.planetanalog.com/features/showArticle.jhtml;?articleID=207403561, click here
• "SIGNAL CHAIN BASICS (Part 11): Introducing voltage- and power-conditioning circuits", www.planetanalog.com/features/showArticle.jhtml;?articleID=207001505, click here
• "SIGNAL CHAIN BASICS (Part 10): Exploring the Delta-Sigma Converter", www.planetanalog.com/features/showArticle.jhtml;?articleID=206903892, click here
• "SIGNAL CHAIN BASICS (Part 9): SAR Converter Operation Explored", www.planetanalog.com/features/showArticle.jhtml;?articleID=206901015, click here
• "SIGNAL CHAIN BASICS (Part 8): Flash- and Pipeline-Converter Operation Explored", www.planetanalog.com/features/showArticle.jhtml;?articleID=206504089, click here
• "SIGNAL CHAIN BASICS (Part 7): Op Amp Performance Specification--Bias Current", www.planetanalog.com/features/showArticle.jhtml;?articleID=206101908, click here
• "SIGNAL CHAIN BASICS (Part 6): Op Amp Input Voltage Offset", www.planetanalog.com/features/showArticle.jhtml;?articleID=205901111, click here
• "SIGNAL CHAIN BASICS (Part 5): Introduction to the Instrumentation Amplifier", www.planetanalog.com/features/showArticle.jhtml;?articleID=205208593, click here
• "SIGNAL CHAIN BASICS (Part 4): Introduction to analog/digital converter (ADC) types", www.planetanalog.com/features/showArticle.jhtml;?articleID=204803631, click here
• "SIGNAL CHAIN BASICS (Part 3): Analog and the digital world", www.planetanalog.com/features/showArticle.jhtml;?articleID=204400376, click here
• "SIGNAL CHAIN BASICS (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