| Linear and Low-Dropout Regulators |
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Objective:
Understand how a linear or low-dropout regulator delivers a regulated output voltage; understand the difference between linear and low-dropout regulators.
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Online Seminar:
 The Evolution of Linear Regulators, with Focus on LDOs, 10/30/02, Muna Acosta
In this presentation we will review the details of linear voltage regulators and the evolution of the Low Dropout Voltage (LDO) regulator topology. The concepts of dropout voltage, power dissipation and efficiency are explained. We will discuss the reasons why choosing the output capacitor is very critical for stability, and how thermal shutdown protects a regulator circuit. After this seminar you should know the basic operation of an LDO regulator and how to successfully use them in your circuit design.
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Reading Assignment:
To understand how the three types of regulators provide control for the regulated output voltage, and to see how the differences in control method affect the frequency response of the regulators, read AN-1148, "Linear Regulators: Theory of Operation and Compensation."
For additional information, consult the Knowledge Base using the following queries:
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Research Assignment:
Using the datasheets for LP3985 (150mA), LM3480 (100mA), and LM340L (100mA), compare their performance and limitations (for the 5V output versions) -
- Identify which is the standard (NPN) regulator, which is the quasi-LDO regulator, and which is the LDO regulator.
- Which packages are the devices available in? check answer
- What type of pass device is used in each? check answer
Compare specifications:
- Which allows highest supply voltage? check answer
- Which allows lowest supply voltage (while maintaining regulated output)? check answer
- Which is lowest cost (check at www.national.com)? check answer
- Which has the lowest max. quiescent (no load) supply current? check answer
- How does the dropout voltage of the LM3480 and the LP3985 vary with temperature?
Check the LP3985 Application Hints:
- What is the function of the LP3985 Bypass pin? check answer
- What type of output capacitor should be used for most applications? check answer
Check thermal performance of LP3985 (with Vin=3.0V, Vout=2.8V, Iout=150mA, Ignd=140uA):
- What is the power dissipation of the LP3985? check answer
- What is the regulator efficiency? check answer
- If the thermal resistance qJA of the SOT23-5 package is 220oC/W, then what is the LP3985 junction temperature with an ambient temperature of 40oC? check answer
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Program Coordinator/Professor
Wanda Garrett
 Wanda Garrett is the Technical Communications Manager in the Web Business Group at National Semiconductor Corp. in Santa Clara, California. With 20 years experience in the design and application of DC/DC converters and related power management products, as well as being a key trainer on this subject, she is now a primary developer of National's Knowledge Base, the Online Seminar series, and other online technical tools. Before joining National in 1983, she received a BSEE from Washington State University.
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Quiz:
Take a quiz to check your understanding of this subject. Click here.
Answer all questions correctly to receive a certificate from Dean Bob Pease.
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