designing with low dropout voltage regulator.pdf

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Micrels Guide to

Designing WithLow-Dropout

VoltageRegulators

Bob Wolbert

Applications Engineering Manager

Micrel Semiconductor1849 Fortune DriveSan Jose, CA 95131

Phone: + 1 (408) 944-0800Fax: + 1 (408) 944-0970

Revised Edition, December 1998

Table of Contents

Index


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Micrel Semiconductor Designing With LDO Regulators

Designing With LDO Regulators 2

Micrel, The High Performance Analog Power IC CompanyMicrel Semiconductor designs, develops, manu-

factures, and markets high performance analog powerintegrated circuits on a worldwide basis. These cir-cuits are used in a wide variety of electronic prod-ucts, including those in cellular communications, por-

table and desktop computers, and in industrial elec-tronics.

Micrel HistorySince its founding in 1978 as an independent

test facility of integrated circuits, Micrel has maintaineda reputation for excellence, quality and customer re-sponsiveness that is second to none.

In 1981 Micrel acquired its first independentsemiconductor processing facility. Initially focusingon custom and specialty fabrication for other IC manu-

facturers, Micrel eventually expanded to develop itsown line of semicustom and standard product Intelli-gent Power integrated circuits. In 1993, with the con-tinued success of these ventures, Micrel acquired anew 57,000 sq. ft. facility and in 1995 expanded thecampus into a 120,000 sq. ft. facility. The new Class10 facility has allowed Micrel to extend its processand foundry capabilities with a full complement ofCMOS/DMOS/Bipolar/NMOS/PMOS processes. In-corporating metal gate, silicon gate, dual metal, dualpoly and feature sizes down to 1.5 micron, Micrel isable to offer its customers unique design and fabrica-tion tools.

Micrel Today and BeyondBuilding on its strength as an innovator in pro-

cess and test technology, Micrel has expanded anddiversified its business by becoming a recognizedleader in the high performance analog power control

and management markets.The companys initial public offering in Decem-

ber of 1994 and recent ISO9001 compliance are justtwo more steps in Micrels long range strategy to be-come the preeminent supplier of high performanceanalog power management and control ICs. By stay-ing close to the customer and the markets they serve,Micrel will continue to remain focused on cost effec-tive standard product solutions for an ever changingworld.

The niche Micrel has carved for itself involves:

High Performance .....precision voltages, high tech-nology (Super eta PNP process, patented circuittechniques, etc.) combined with the new safetyfeatures of overcurrent, overvoltage, and overtem-perature protection

Analog .....we control continuously varying outputs ofvoltage or current as opposed to digital ones andzeros (although we often throw in mixed signal i.e.analog with digital controls to bring out the best ofboth worlds)

Power ICs .....our products involve high voltage, highcurrent, or both

We use this expertise to address the followinggrowing market segments:

1. Power supplies2. Battery powered computer, cellular phone,

and handheld instruments3. Industrial & display systems4. Desktop computers5. Aftermarket automotive6. Avionics7. Plus many others

Copyright 1998 Micrel, Inc.All rights reserved. No part of this publication may be reproduced or used in any form or by any means

without written permission of Micrel, Incorporated.Some products in this book are protected by one or more of the following patents: 4,914,546; 4,951,101;4,979,001; 5,034,346; 5,045,966; 5,047,820; 5,254,486; and 5,355,008. Additional patents are pending.


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3 Designing With LDO Regulators


ContentsContributors: ......................................................................................................... 7

Section 1. Introduction:

Low-Dropout Linear Regulators .................................................. 8What is a Linear Regulator? ............................................................................... 8Why Use Regulators? ........................................................................................... 8Basic Design Issues .............................................................................................. 9What is a Low-Dropout Linear Regulator?................................................ 10Linear Regulators vs. Switching Regulators ................................................. 11

Who Prefers Linear Low Dropout Regulators? .................................................. 11

Section 2. Low-Dropout RegulatorDesign Charts ................................................................................ 12

Regulator Selection Charts ............................................................................... 12Regulator Selection Table ................................................................................. 14Maximum Power Dissipation by Package Type........................................... 16 Typical Thermal Characteristics ..................................................................... 17Output Current vs. Junction Temperature and Voltage Differential ....... 18

Junction Temperature Rise vs. Available Output Currentand Differential Voltage .............................................................................. 21

Section 3. Using LDO Linear Regulators ..................................... 24General Layout and Construction Considerations ...................................... 24

Layout ....................................................................................................................... 24Bypass Capacitors ...................... ...................... ....................... ...................... .......... 24Output Capacitor .................... ....................... ....................... ...................... ............. 24Circuit Board Layout .................... ...................... ....................... ....................... ....... 25

Assembly ................................................................................................................... 25Lead Bending ..................... ...................... ....................... ....................... .................. 26Heat Sink Attachment ..................... ...................... ....................... ...................... ..... 26

Output Voltage Accuracy .................................................................................. 27 Adjustable Regulator Accuracy Analysis............................................................ 27 Improving Regulator Accuracy ............................................................................. 28

Regulator & Reference Circuit Performance ....................................................... 29Design Issues and General Applications ...................................................... 31 Noise and Noise Reduction .................................................................................... 31Stability .................................................................................................................... 31LDO Efficiency ......................................................................................................... 31Building an Adjustable Regulator Allowing 0V Output................................... 31

Reference Generates a Virtual VOUT .................... ....................... .................... 31Op-Amp Drives Ground Reference ..................... ....................... ...................... .... 32

Systems With Negative Supplies .......................................................................... 32

Click Any Item toJump to Page


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High Input Voltages ................................................................................................ 33Controlling Voltage Regulator Turn-On Surges.................................................. 33

The Simplest Approach .................... ...................... ....................... ...................... ... 34Improving the Simple Approach....................... ...................... ....................... ....... 34Eliminating Initial Start-Up Pedestal .................... ....................... ...................... ... 35

Current Sources ........................................................................................................ 36Simple Current Source ...................... ...................... ....................... ...................... ... 36The Super LDO Current Source .................... ....................... ....................... .......... 36Accurate Current Source Using Op Amps.................... ....................... ................ 36

A Low-Cost 12V & 5V Power Supply ................................................................... 36Computer Power Supplies................................................................................ 38

Dropout Requirements ............................................................................................ 385V to 3.xV Conversion Circuits.............................................................................. 39

Method 1: Use a Monolithic LDO ....................... ....................... ...................... ..... 39Method 2: The MIC5156 Super LDO ....................... ...................... ................... 39Method 3: The MIC5158 Super LDO ....................... ...................... ................... 40Method 4: Current Boost a MIC2951 ........................................ ...................... ...... 40

Adjust Resistor Values............................................................................................ 403.3V to 2.xV Conversion.......................................................................................... 41 Improving Transient Response .............................................................................. 41 Accuracy Requirements .......................................................................................... 42 Multiple Output Voltages ...................................................................................... 43 Multiple Supply Sequencing .................................................................................. 44Thermal Design ........................................................................................................ 44

Portable Devices ................................................................................................. 45Design Considerations ............................................................................................ 45

Small Package Needed ..................... ...................... ....................... ...................... ... 45Self Contained Power ................... ....................... ....................... ...................... ...... 45

Low Current (And Low Voltage) ............................ ....................... ....................... 45Low Output Noise Requirement ...................... ....................... ...................... ........ 45Dropout and Battery Life ...................................... ....................... ....................... ... 46Ground Current and Battery Life ..................... ....................... ...................... ........ 46

Battery Stretching Techniques ............................................................................... 46Sleep Mode Switching ...................... ...................... ....................... ...................... ... 46Power Sequencing .................... ....................... ...................... ....................... ........... 46

Multiple Regulators Provide Isolation................................................................ 46Thermal Management ....................................................................................... 47

A Thermal Primer .................................................................................................... 47 Thermal Parameters .................... ....................... ...................... ....................... ........ 47

Thermal/Electrical Analogy .................... ....................... ....................... ................ 47Calculating Thermal Parameters .......................................................................... 48Calculating Maximum Allowable Thermal Resistance ............................. ........ 49

Why A Maximum Junction Temperature? ............................................................ 49 Heat Sink Charts for High Current Regulators................................................... 50Thermal Examples ................................................................................................... 51 Heat Sink Selection ................................................................................................. 52Reading Heat Sink Graphs..................................................................................... 52Power Sharing Resistor .......................................................................................... 53


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Multiple Packages on One Heat Sink................................................................... 54Paralleled Devices on a Heat Sink Example ..................... ...................... ............. 55

Heat Sinking Surface Mount Packages ................................................................ 56Determining Heat Sink Dimensions ....................... ...................... ....................... . 56SO-8 Calculations: ...................... ...................... ....................... ...................... .......... 57Comments ..................... ....................... ....................... ...................... ....................... . 58

Linear Regulator Troubleshooting Guide ..................................................... 59Section 4. Linear Regulator Solutions .......................................... 60

Super eta PNP Regulators........................................................................... 60Super beta PNP Circuitry ....................................................................................... 61Dropout Voltage....................................................................................................... 61Ground Current ........................................................................................................ 62 Fully Protected ......................................................................................................... 62

Current Limiting ...................... ...................... ....................... ...................... ............. 62Overtemperature Shutdown ...................... ...................... ....................... ............... 62Reversed Input Polarity ..................... ...................... ....................... ...................... .. 62Overvoltage Shutdown ..................... ...................... ....................... ...................... .. 63

Variety of Packages ................................................................................................. 63Why Choose Five Terminal Regulators? .............................................................. 63Compatible Pinouts ................................................................................................ 63Stability Issues ........................................................................................................ 64Paralleling Bipolar Regulators ............................................................................. 64

Micrels Unique Super LDO..................................................................... 66 Micrels Super LDO Family ................................................................................... 66The MIC5156............................................................................................................. 66The MIC5157 and MIC5158 .................................................................................... 663.3V, 10A Regulator Application ........................................................................... 66

Comparison With Monolithics.............................................................................. 67 Similarities to Monolithics ...................... ....................... ...................... .................. 67Differences from Monolithics ...................... ....................... ...................... ............. 67

Unique Super LDO Applications .......................................................................... 67 Super High-Current Regulator .................... ....................... ...................... ............. 67Selecting the Current Limit Threshold ..................... ....................... ..................... 69Sense Resistor Power Dissipation ..................... ....................... ...................... ....... 69Kelvin Sensing ...................... ...................... ....................... ...................... ................ 69

Alternative Current Sense Resistors..................................................................... 69Overcurrent Sense Resistors from PC Board Traces .......................................... 69

Resistor Design Method ................... ....................... ....................... ...................... .. 70Design Example ....................... ....................... ...................... ....................... ............ 70Calculate Sheet Resistance ...................... ....................... ...................... .................. 71Calculate Minimum Trace Width ...................... ....................... ...................... ....... 71Calculate Required Trace Length ...................................... ...................... .............. 71Resistor Layout .................... ....................... ...................... ....................... ................ 71Thermal Considerations ................... ....................... ....................... ...................... .. 71

Design Aids ............................................................................................................... 71 Highly Accurate Current Limiting ........................................................................ 71Protecting the Super LDO from Long-Term Short Circuits............................... 71


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Section 5. Omitted ............................................................................ 74Section 6. Package Information...................................................... 75

Packaging for Automatic Handling ................................................................ 76Tape & Reel ..................... ...................... ....................... ....................... ...................... 76Ammo Pack .................... ...................... ....................... ...................... ....................... 76Pricing ..................... ...................... ....................... ....................... ...................... ........ 76

Tape & Reel Standards............................................................................................ 76Packages Available in Tape & Reel ...................................................................... 76

Package Orientation ........................................................................................... 77Linear Regulator Packages ............................................................................... 78

8-Pin Plastic DIP (N) .......................................... ...................... ....................... ........ 7814-Pin Plastic DIP (N) .................... ...................... ....................... ....................... ..... 788-Pin SOIC (M) .................... ....................... ...................... ....................... ................. 7914-Pin SOIC (M) ...................... ...................... ....................... ...................... .............. 79TO-92 (Z) ...................... ....................... ....................... ...................... ....................... .. 80SOT-223 (S) ..................... ...................... ....................... ....................... ...................... 80

SOT-143 (M4) ..................... ...................... ....................... ....................... ................... 81SOT-23 (M3) ....................... ....................... ...................... ....................... ................... 81SOT-23-5 (M5) ...................... ...................... ....................... ....................... ................ 82MSOP-8 [MM8] (MM) ...................... ...................... ....................... ...................... 823-Lead TO-220 (T) ..................... ....................... ....................... ...................... ........... 835-Lead TO-220 (T) ..................... ....................... ....................... ...................... ........... 835-Lead TO-220 Vertical Lead Bend Option (-LB03) .............................. .............. 845-Lead TO-220 Horizontal Lead Bend Option (-LB02) .............. ....................... . 843-Lead TO-263 (U) .................... ....................... ...................... ....................... ........... 855-Lead TO-263 (U) .................... ....................... ...................... ....................... ........... 85Typical 3-Lead TO-263 PCB Layout .................... ...................... ....................... ..... 86Typical 5-Lead TO-263 PCB Layout .................... ...................... ....................... ..... 863-Lead TO-247 (WT) .................... ....................... ...................... ....................... ........ 875-Lead TO-247 (WT) .................... ....................... ...................... ....................... ........ 88

Section 7. Appendices ...................................................................... 89Appendix A. Table of Standard 1% Resistor Values.................................... 90Appendix B. Table of Standard 5% and 10% Resistor Values.............. 91Appendix C. LDO SINK for the HP 48 Calculator....................................... 92

Section 8. Low-Dropout Voltage Regulator Glossary ................ 95Section 9. References ........................................................................ 97

Section 10. Index ............................................................................... 98Section 11. WorldwideRepresentatives and Distributors ............................................ 100

Micrel Sales Offices ......................................................................................... 100U.S. Sales Representatives .............................................................................. 101U.S. Distributors ............................................................................................... 103International Sales Representatives and Distributors .............................. 107


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Contributors:

Jerry Kmetz

Mike Mottola

Jim CecilBrian Huffman

Marvin Vander Kooi

Claude Smithson

Micrel Semiconductor1849 Fortune DriveSan Jose, CA 95131

Phone: + 1 (408) 944-0800Fax: + 1 (408) 944-0970

http://www.micrel.com


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Section 1: Introduction 8 Designing With LDO Regulators

Section 1. Introduction:Low-Dropout Linear Regulators

op-amp increases drive to the pass element, whichincreases output voltage. Conversely, if the outputrises above the desired set point, the op amp reducesdrive. These corrections are performed continuouslywith the reaction time limited only by the speed of theop amp and output transistor loop.

Real linear regulators have a number of otherfeatures, including protection from short circuitedloads and overtemperature shutdown. Advancedregulators offer extra features such as overvoltageshutdown, reversed-insertion and reversed polarity

protection, and digital error indicators that signal whenthe output is not correct.

Why Use Regulators?Their most basic function, voltage regulation,

provides clean, constant, accurate voltage to a cir-cuit. Voltage regulators are a fundamental block inthe power supplies of most all electronic equipment.

Key regulator benefits and applications include:

Accurate supply voltage Active noise filtering Protection from overcurrent faults Inter-stage isolation (decoupling) Generation of multiple output voltages from a

single source

Useful in constant current sourcesFigure 1-2 shows several typical applications for

linear voltage regulators. A traditional AC to DC powersupply appears in Figure 1-2(A). Here, the linear regu-lator performs ripple rejection, eliminating AC hum,and output voltage regulation. The power supply out-put voltage will be clean and constant, independentof AC line voltage variations. Figure 1-2(B) uses alow-dropout linear regulator to provide a constantoutput voltage from a battery, as the battery dis-charges. Low dropout regulators are excellent for thisapplication since they allow more usable life from agiven battery. Figure 1-2(C) shows a linear regulatorconfigured as a post regulator for a switching power

What is a Linear Regulator?IC linear voltage regulators have been around

for decades. These simple-to-use devices appear innearly every type of electronic equipment, where theyproduce a clean, accurate output voltage used bysensitive components.

Historically, linear regulators with PNP outputshave been expensive and limited to low current ap-plications. However, Micrel Semiconductors uniqueSuper eta PNP line of low dropout regulators

provides up to 7.5 amperes of current with dropoutvoltages less than 0.6V, guaranteed. A lower costproduct line outputs the same currents with only 1Vof dropout. These low dropout voltages guarantee themicroprocessor gets a clean, well regulated supplythat quickly reacts to processor-induced load changesas well as input supply variations.

The low dropout linear voltage regulator is aeasy-to-use, low cost, yet high performance meansof powering your systems.

Input

Ground

Output

Figure 1-1. A basic linear regulator schematic.

A typical linear regulator diagram is shown inFigure 1-1. A pass transistor is controlled by an op-erational amplifier which compares the output volt-age to a reference. As the output voltage drops, the


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Designing With LDO Regulators 9 Section 1: Introduction


supply. Switching supplies are known for excellent ef-ficiency, but their output is noisy; ripple degradesregulation and performance, especially when power-ing analog circuits. The linear regulator following theswitching regulator provides active filtering and greatlyimproves the output accuracy of the composite sup-ply. As Figure 1-2(D) demonstrates, some linear regu-

lators serve a double duty as both regulator and powerON/OFF control. In some applications, especially ra-dio systems, different system blocks are often pow-ered from different regulatorseven if they use thesame supply voltagebecause of the isolation (de-coupling) the high gain regulator provides.

Basic Design IssuesLets review the most important parameters of

voltage regulators:

Output voltage is an important parameter, as thisis the reason most designers purchase a regula-tor. Linear regulators are available in both fixedoutput voltage and adjustable configurations.Fixed voltage regulators offer enhanced ease-of-

use, with their output voltages accurately trimmedat the factorybut only if your application usesan available voltage. Adjustables allow using avoltage custom-tailored for your circuit.

Maximum output current is the parameter gener-ally used to group regulators. Larger maximumoutput currents require larger, more expensiveregulators.

Dropout voltage is the next major parameter. Thisis the minimum additional voltage on the input thatstill produces a regulated output. For example, aMicrel 5.0V Super eta PNP regulator will pro-vide regulated output with an input voltage of 5.3Vor above. The 300mV term is the dropout volt-age. In the linear regulator world, the lower thedropout voltage, the better.

Ground current is the supply current used by theregulator that does not pass into the load. An idealregulator will minimize its ground current. Thisparameter is sometimes called quiescent current,but this usage is incorrect for PNP-pass elementregulators.

Figure 1-2. Typical Linear Regulator Applications

(D) Sleep-mode and Inter Stage Isolation or De- coupling

(C) Post-Regulator for Switching Supplies

AC or DCInput

CleanDC Output

Low-DropoutLinear Regulator

Switching Regulator(High efficiency,but noisy output)

Output 1Low-Dropout

Linear Regulator

Battery Low-DropoutLinear Regulator



Output 2

Output 3

Output 4

Enable 1

Enable 2

Enable 3

Enable 4

(A) Standard Power Supplies (B) Battery Powered Applications

AC Input

DC Output


DC Output


Battery


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Section 1: Introduction 10 Designing With LDO Regulators

Efficiency is the amount of usable (output) powerachieved from a given input power. With linearregulators, the efficiency is approximately theoutput voltage divided by the input voltage.

What is a Low-DropoutLinear Regulator?

A low dropout regulator is a class of linear regu-

lator that is designed to minimize the saturation ofthe output pass transistor and its drive requirements.A low-dropout linear regulator will operate with inputvoltages only slightly higher than the desired outputvoltage. For example, classic linear regulators, suchas the 7805 or LM317 need about 2.5 to 3V higherinput voltage for a given output voltage. For a 5V out-put, these older devices need a 8V input. By com-parison, Micrels Super beta PNP low dropout regu-

Input Output

VREF

VDO (MIN) = VBE (Q1) + V BE (Q2) + VSAT current source (if used)

+

DriveCurrent

Q1

Q2current sourceor resistor

Input Output

VREF

VDO (MIN) = VSAT (Q2) +V BE (Q1)

+

DriveCurrent

Q1

Q2

Input Output

VREF

VDO (MIN) = VSAT

+

DriveCurrent

Input Output

VREF

VDO (MIN) = R DS (ON) (Q1) IOUT

+

Q1chargepump

voltagemultiplier

Input Output

VREF

VDO (MIN) = R DS (ON) (Q1) IOUT

+

Q1current source

or resistor

(D) P-Channel MOSFET-pass transistor regulator (E) N-Channel MOSFET-pass transistor regulator

Figure 1-3. The Five Major Types of Linear Regulators

(A) Standard NPN-pass transistor regulator

(C) Low-Dropout PNP-pass transistor regulator

(B) NPN-pass regulator with reduced dropout

lators require only 0.3V of headroom, and would pro-vide regulated output with only 5.3V of input.

Figure 1-3 shows the five major types of linearregulators:

A. Classic NPN-based regulators that require 2.5to 3V of excess input voltage to function.

B. Low Dropout NPN regulators, with a NPN out-put but a PNP base drive circuit. These devicesreduce the dropout requirement to 1.2 to 1.5V.

C. True low dropout PNP-based regulators that need0.3V to 0.6V extra for operation.

D. P-channel CMOS output regulators. These de-vices have very low dropout voltages at low cur-rents but require large die area (hence highercostly than bipolar versions) and have high inter-nal drive current requirements when working withnoisy inputs or widely varying output currents.


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Designing With LDO Regulators 11 Section 1: Introduction


E. Regulator controllers. These are integrated cir-cuits that provide the reference and control func-tions of a linear regulator, but do not have thepass element on board. They provide the advan-tage of optimizing die area and cost for highercurrent applications but suffer the disadvantageof being a multiple package solution.

If we graph the efficiency of the different classesof linear regulators we see very significant differencesat low input and output voltages (see Figure 1-4). Athigher voltages, however, these differences dimin-ish. A 3.3V high current linear regulator controller suchas the Micrel MIC5156 can approach 100% efficiencyas the input voltage approaches dropout. But anLM317 set to 3.3V at 1A will have a miserable effi-ciency of only about 50% at its dropout threshold.

Linear Regulators vs.Switching Regulators

Linear regulators are less energy efficient thanswitching regulators. Why do we continue usingthem? Depending upon the application, linear regu-lators have several redeeming features:

lower output noise is important for radios and othercommunications equipment

faster response to input and output transients easier to use because they require only filter ca-

pacitors for operation generally smaller in size (no magnetics required) less expensive (simpler internal circuitry and no

magnetics required)

Figure 1-4. Linear Regulator Efficiency at Dropout

Furthermore, in applications using low input-to-output voltage differentials, the efficiency is not allthat bad! For example, in a 5V to 3.3V microproces-sor application, linear regulator efficiency approaches66%. And applications with low current subcircuitsmay not care that regulator efficiency is less thanoptimum as the power lost may be negligible overall.

Who Prefers Linear Low Dropout Regulators?

We see that price sensitive applications preferlinear regulators over their sampled-time counterparts.The design decision is especially clear cut for mak-ers of:

communications equipment small devices battery operated systems

low current devices high performance microprocessors with sleep

mode (fast transient recovery required)As you proceed through this book, you will find

numerous other applications where the linear regu-lator is the best power supply solution.

100

50

80

60

81OUTPUT CURRENT (A)

LM340LM317 LM350 LM396

LT1083LT1086 LT1085 LT1084

MIC5156/7/8

MIC5203MIC5200

MIC29750MIC29300 MIC29500MIC29150MIC2920

MIC5201

1000.40.20.1 4 6

78L05

2

E F F I C I E N C Y A T D R O P O U T ( % )


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Section 2: Design Charts 12 Designing With LDO Regulators

Section 2. Low-Dropout RegulatorDesign Charts

Regulator Selection Charts

OutputCurrent Accuracy

LowNoise

Singleor Dual

0 180mA

200mA 500mA

Without Error Flag With Error Flag

MIC5210 Dual 150mA LDO w/ Noise BypassMSOP-8 3.0, 3.3, 3.6, 4.0, 5.0V

MIC5205 150mA LDOw/ Noise BypassSOT23-5 2.8, 3. 0, 3.3, 3.6, 3.8, 4. 0, 5.0V, Adj

MIC5202 Dual 100mA LDOSO-8 3.0, 3.3, 4.5, 4.85, 5.0V

LP2950 100mA LDO Second Source to '2950

TO-92 5.0VMIC2950 150mA LDO Upgrade to '2950TO-92 5.0V

MIC5200 100mA LDOSO-8, SOT-223, MSOP-8 3. 0, 3.3, 4.85, 5.0V

MIC5207 180mA LDOSOT23-5, TO-92 1.8, 2.5, 3.0, 3.3, 3.6, 3.8, 5.0V, Adj

MIC5211 Dual 50mA Cap LDOSOT23-6 2.5, 3.0, 3.3, 3.6, 5.0, Mixed 3 .3/5.0V

MIC5208 Dual 50mA Cap LDOMSOP-8 3.0, 3.3, 3.6, 4.0, 5.0V

MIC5203 80mA Cap LDOSOT-143, SOT23-5 2.8, 3.0, 3.3, 3.6, 3.8, 4.0, 5.0V

MIC5206 150mA LDOs w Noise BypassSOT23-5, MSOP-8 2.5, 3.0, 3.3, 3.6, 4.0, 5.0V, Adj

LP2951 100mA LDO Second Source to '2951SO-8, PDIP-8 4.85, 5.0V, Adj

MIC2951 150mA LDO Upgrade to '2951SO-8, PDIP-8, MSOP-8 3.3, 4.85, 5.0V, Adj

MIC5219 500mA Peak LDOSOT23-5, MSOP-8 3.0, 3.3, 3.6, 5.0V, Adj

MIC5209 500mA LDOSOT223, SO-8, TO263-5 1.8, 2.5, 3.0, 3.3, 5.0V, Adj

MIC5201 200mA LDOSOT223, SO-8 3.0, 3.3, 4.85, 5.0V, Adj

MIC2954 250mA LDOTO220, SOT223, SO-8, TO92 5.0V, Adj

MIC2920 400mA LDOTO220, SOT223 3.3, 4.85, 5.0V

MIC29202 400mA LDOTO220, TO263 Adj

MIC5237 500mA LDOTO220, TO263 2.5, 3.3, 5.0V

MIC29371 750mA LDOTO220, TO263 3.3, 5.0V

MIC2937A 750mA LDOTO220, TO263 3.3, 5.0, 12.0V

MIC29372 750mA LDOTO220, TO263 Adj

750mA

MIC5216 500mA Peak LDOSOT23-5, MSOP-8 3.0, 3.3, 3.6, 5.0V

MIC29201 400mA LDOTO220, TO263, SO-8 3.3, 4.85, 5.0V

MIC29204 400mA LDOSO-8 Adj

Yes

No

Yes

No

1.0%

3.0%

Dual

Single

Dual

Single

Dual

Single

1.0%

3.0%

Yes

No

1.0%

Single

Single

Single

Single

No

No

Figure 2-1a. 0 to 750mA LDO Regulator Selection Guide

Shaded boxes denote automotive load dump protected devices


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Designing With LDO Regulators 13 Section 2: Design Charts


OutputCurrent Accuracy Error Flag

1A 1.5A

3.0A

Low-Dropout Devices Ultra-Low-Dropout Devices

MIC29151 1.5A LDOTO220, TO263 3.3, 5.0, 12V

MIC2940A 1.25A LDO

TO220, TO263 3.3, 5.0, 12VMIC2941A 1.25A LDOTO220, TO263 Adj

MIC29150 1.5A LDOTO220, TO263 3.3, 5.0, 12.0V

MIC29152 1.5A LDOTO220, TO263 Adj

MIC29301 3.0A LDOTO220, TO263 3.3, 5.0, 12V


MIC39151 1.5A LDOTO263 1.8, 2.5V

MIC39150 1.5A LDOTO220, TO263 1.8, 2.5V

MIC29300 3.0A LDOTO220, TO263 3.3, 5.0, 12.0V


MIC29310 3.0A Low Cost LDOTO220, TO263 3.3, 5.0V

MIC29312 3.0A Low Cost LDOTO220, TO263 Adj

MIC29501 5.0A LDOTO220, TO263 3.3, 5.0V


MIC29751 7.5A LDOTO247 3.3, 5.0V

MIC39300 3.0A LDO

TO220, TO263 1.8, 2.5V

MIC29500 5.0A LDOTO220, TO263 3.3, 5.0V


MIC29510 5.0A Low Cost LDOTO220 3.3, 5.0V

MIC29512 5.0A Low Cost LDOTO220 Adj

MIC29750 7.5A LDOTO247 3.3, 5.0V


MIC29710 7.5A LDOTO220 3.3, 5.0V


MIC39301 3.0A LDOTO263, TO220 1.8, 2.5V

MIC39100 1.0A LDOSOT223 1.8, 2.5, 3.3V

MIC5156 LDO ControllerSO-8, PDIP-8 3.3, 5.0V, Adj

MIC5157 LDO Controller (w/Charge Pump)SO-14, PDIP-14 3.3, 5.0, 12V

MIC5158 LDO Controller (w/Charge Pump)SO-14, PDIP-14 5.0V, Adj

1.0%

1.0%

5.0A -7.5A

1.0%

Yes

No

Yes

No

Yes

No

>7.5A 1.0% Yes

Figure 2-1b. 1A to >7.5A LDO Regulator Selection Guide

Shaded boxes denote automotive load dump protected devices


14/109

S ect i on2 : D esi gnC h ar t s

1 4

D esi gni ngWi t h L D OR egul at or s

Output Standard Output Voltage Adj. Dropout Current Error Enable/ ThermalDevice Current 1.8 2.5 2.8 3.0 3.3 3.6 3.8 4.0 4.75 4.85 5.0 12 (max.) Accuracy (IMAX, 25

C) Limit Flag Shutdown Shutdown

MIC5208 50mA 2 3% 250mV MIC5211 50mA 2 3% 250mV MIC5203 80mA 3% 300mV MIC5200 100mA 1% 230mV MIC5202 100mA 2 1% 225mV LP2950 100mA 1 2%,1% 380mV LP2951 100mA 29V 1 2%,1% 380mV MIC2950 150mA 1 2%,1% 300mV MIC2951 150mA 29V 1 2%,1% 300mV MIC5205 150mA 16V 1% 165mV MIC5206 150mA 16V 1% 165mV MIC5210 150mA 2 1% 165mV MIC5207 180mV 16V 3% 165mA MIC5201 200mA 16V 1% 270mV MIC2954 250mA 29V 1 2% 375mV MIC2920A 400mA 1% 450mV MIC29201 400mA SP 1% 450mV MIC29202 400mA 26V 1% 450mV MIC29204 400mA 26V 1% 450mV MIC5216 500mA (1) 12V 1% 300mV MIC5219 500mA (1) 12V 1% 300mV MIC5209 500mA 16V 1% 300mV MIC5237 500mA 16V 3% 300mV MIC2937A 750mA 1% 370mV MIC29371 750mA SP 1% 370mV MIC29372 750mA 26V 1% 370mV

Regulator Selection (Sorted by Output Current Ra


15/109

D esi gni ngWi t h L D OR egul at or s

1 5

S ect i on2 : D esi gnC h ar t s

Output Standard Output Voltage Adj. Dropout Current Error Enable/ ThermalDevice Current 1.8 2.5 2.8 3.0 3.3 3.6 3.8 4.0 4.75 4.85 5.0 12 (max.) Accuracy (IMAX, 25

C) Limit Flag Shutdown Shutdow

MIC2940A 1.25A 1% 400mV MIC2941A 1.25A 26V 1% 400mV MIC29150 1.5A 1% 350mV MIC29151 1.5A 1% 350mV MIC29152 1.5A 26V 1% 350mV

MIC29153 1.5A 26VSP 1% 350mV MIC39150 1.5A 1% 350mV MIC39151 1.5A 1% 350mV MIC29300 3A 1% 370mV MIC29301 3A 1% 370mV MIC29302 3A 26V 1% 370mV MIC29303 3A 26V 1% 370mV MIC29310 3A 2% 600mV MIC29312 3A 16V 2% 600mV MIC39300 3A 1% 400mV

MIC39301 3A 1% 400mV MIC29500 5A 1% 370mV MIC29501 5A 1% 370mV MIC29502 5A 26V 1% 370mV MIC29503 5A 26V 1% 370mV MIC29510 5A 2% 700mV MIC29512 5A 16V 2% 700mV MIC29710 7.5A 2% 700mV MIC29712 7.5A 16V 2% 700mV MIC29750 7.5A 1% 425mV

MIC29751 7.5A 1% 425mV MIC29752 7.5A 26V 1% 425mV MIC5156 (2) 36V 1% (2) MIC5157 (2) (3) (3) (3) 1% (2) MIC5158 (2) (4) (4) 1% (2)

SP Special order. Contact factory.1 Output current limited by package and layout.2 Maximum output current and dropout voltage are determined by the choice of external MOSFET.3 3.3V, 5V, or 12V selectable operation.4 5V or Adjustable operation.


16/109


17/109


18/109



25

35

45

55

65

75

85

95

105

115

125

0 0.02 0.04 0.06 0.08

J U N C T I O N T E M P E R A T U R E ( C )

OUTPUT CURRENT (A)

MIC5203BM4

4V

5V

6V7V8V9V10V

3V

2V

1V

0.3V

25

35

45

55

65

75

85

95

105

115

125

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

J U N C T I O N T E M P E R A T U R

E ( C )

OUTPUT CURRENT (A)

MIC5200

4V

5V

6V

7V8V9V10V

3V

2V

1V

0.3V

Output Current vs. JunctionTemperature and VoltageDifferential(Figure 2-6)

These graphs show the junction temperaturewith a given output current and input-output voltagedifferential. Ambient temperature is 25 C. The ther-mal resistance used for the calculations is shownunder each graph. This resistance assumes that aheat sink of suitable size for the particular regulatoris employed; higher current regulator circuits gener-ally require larger heat sinks. Refer to Thermal Man- agement , in Section 3 , for definitions and details.

For example, a MIC5203-3.3BM4, supplying50mA and with 6.3V on its input (V IN V OUT = 3V), willhave a junction temperature of approximately 63

(Figure 2-6 (A)).

Figure 2-6 (A). SOT-143 with JA = 250 C/W Figure 2-6 (C). SOT-23-5 with JA = 220 C/W

Figure 2-6 (B). SO-8 with JA = 160 C/W

25

35

45

55

65

75

85

95

105

115

125

0 0.05 0.1 0.15


OUTPUT CURRENT (A)

MIC5205

4V5V6V7V8V

9V10V

3V

2V

1V

0.3V


19/109



25

35

45

55

65

75

85

95

105

115

125

0 0.05 0.1 0.15 0.2


E ( C )

OUTPUT CURRENT (A)

MIC5201BM

4V5V6V

7V8V9V

10V3V

2V

1V

0.3V

25

35

45

55

65

75

85

95

105

115

125

0 0.05 0.1 0.15 0.2


OUTPUT CURRENT (A)

MIC5201BS

4V

5V

6V

7V

8V

9V

10V

3V

2V

1V

0.3V

25

35

45

55

65

75

85

95

105

115

125

0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40


OUTPUT CURRENT (A)

MIC2920

4V

5V

6V

7V8V9V10V

3V

2V

1V

0.3V

25

35

45

55

65

75

85

95

105

115

125

0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.650.70 0.75


OUTPUT CURRENT (A)

MIC2937ABU

4V

5V6V7V8V9V

10V

3V

2V

1V

0.3V

Figure 2-6 (D). High Current SO-8 with JA = 160 C/W

Figure 2-6 (F). TO-263 with JA = 40 C/W

Figure 2-6 (G). TO-263 with JA = 40 C/W Figure 2-6 (E). SOT-223 with JA = 50 C/W


20/109



25

35

45

55

65

75

85

95

105

115

125

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0


OUTPUT CURRENT (A)

MIC29500

1V

0.3V

2V

3V

4V

5V6V7V8V

9V10V

25

35

45

55

65

75

85

95

105

115

125

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5


E ( C )

OUTPUT CURRENT (A)

MIC29150

4V

5V6V

7V8V9V

10V

3V

2V

1V

0.3V

25

35

45

55

65

75

85

95

105

115

125

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5


E ( C )

OUTPUT CURRENT (A)

MIC29710

1V

0.3V

2V

3V

4V5V6V7V

8V9V

10V

25

35

45

55

65

75

85

95

105

115

125

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5


OUTPUT CURRENT (V)

MIC29750

4V5V6V

7V8V9V10V

3V

2V

1V

0.3V

Figure 2-6 (H). TO-220 with JA = 15 C/W Figure 2-6 (K). TO-220 with JA = 6 C/W

Figure 2-6 (L). TO-247 with JA = 4 C/W Figure 2-6 (J). TO-220 with JA = 6 C/W


21/109



0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

O U T P U T C U R R E N T ( A )

VIN VOUT

MIC5203BM4

10

100

10 steps,units in C.

50

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

0.12

0.13

0.14

0.15

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14


VIN VOUT

MIC5205BM5

50

10

100


0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0 2 4 6 8 10 12 14 16 18 20 22 24


VIN VOUT

MIC5201BM

100


50

10

Junction Temperature Rise vs.Available Output Currentand Differential Voltage(Figure 2-7)

These graphs show the available thermally-lim-ited steady-state output current with a given thermalresistance and inputoutput voltage differential. Theassumed JA (thermal resistance from junction toambient) is shown below each graph. Refer to Ther- mal Management in Section 3 for definitions anddetails.

For example, Figure 2-7 (C) shows that theMIC5205BM5, with 3V across it (V IN = VOUT + 3V) andsupplying 120mA, will have a temperature rise of 80 C(when mounted normally).

Figure 2-7 (C). SOT-23-5 with JA = 220 C/W

Figure 2-7 (A). SOT-143 with JA = 250 C/W Figure 2-7 (D). SO-8 with JA = 140 C/W


22/109


23/109



0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

0 2 4 6 8 10 12 14 16 18 20 22 24


VIN VOUT

MIC29710


50

100

10

0.00.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

0 2 4 6 8 10 12 14 16 18 20 22 24


VIN VOUT

MIC29750

10

100


50

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 2 4 6 8 10 12 14 16 18 20 22 24


VIN VOUT

MIC29500

50

100


10

0

0.5

1.0

1.5

2.0

2.5

3.0

0 2 4 6 8 10 12 14 16 18 20 22 24


VIN VOUT

MIC29300

100


50

10

Figure 2-7 (I). TO-220 with JA = 10 C/W Figure 2-7 (K). TO-220 with JA = 6 C/W

Figure 2-7 (L). TO-247 with JA = 4 C/W Figure 2-7 (J). TO-220 with JA = 6 C/W


24/109


25/109

Designing With LDO Regulators 25 Section 3: Using LDO Linear Regulators


Circuit Board LayoutStray capacitance and inductance may upset

loop compensation and promote instability. Exces-sive input lead resistance increases the dropout volt-age, and excessive output lead resistance reducesoutput load regulation. Ground loops also cause bothproblems. Careful layout is the solution.

Reduce stray capacitance and inductance byplacing bypass and filter capacitors close to the regu-lator. Swamp parasitic reactances by using a 0.1 Fceramic capacitor (or equivalent) in parallel with theregulator input filter capacitor. Designers of battery-powered circuits often overlook the finite high-fre-quency impedance of their cells. The ceramic capaci-tor solves many unexpected problems.

Excessive lead resistance, causing unwantedvoltage drops and ruining load regulation, is solved

by merely increasing conductor size. Regulators withremote sensing capabilitylike all Micreladjustablesmay utilize a Kelvin-sense connectiondirectly to the load. As Figure 3-1 shows, an addi-tional pair of wires feeds back the load voltage to theregulator sense input. 2 This lets the regulator com-pensate for line drop. As the Kelvin sense leads carryonly the small voltage-programming resistor current,they may be very narrow traces or small diameterwire. A judicious layout is especially important in re-mote-sensed designs, since these long, high imped-ance leads are susceptible to noise pickup.

VOUT@IDC OUT

TraceResistance

RL

R1

R2

ADJ

VREG

IN OUT

GND

R e m o

t e S e n s e

VIN

GND

Figure 3-1. Remote Voltage Sense (Kelvin)Connections

A common ground loop problem occurs whenrectifier ripple current flows through the regulators

ground lead on its way to the filter capacitor (see Fig-ure 3-2). The ripple current, which is several timeslarger than the average DC current, may create avoltage drop in the ground line, raising its voltage rela-tive to the load. As the regulator attempts to compen-sate, load regulation suffers. Solve the problem byensuring rectifier current flows directly into the filter

capacitor.

AC Input

VOUT@IDC OUT


Ripple CurrentTrace

Resistance

+

VOUT = VREG + (IRIPPLE R TRACE )Where I RIPPLE >> IDC OUT

V R E G

Figure 3-2. Ground Loop and Ripple Currents Degrade Output Accuracy

Figure 3-3 shows an ideal layout for remote-sensed loads. If a single point ground is not practical,load regulation is improved by employing a largeground plane.

AC Input

VOUT@IDC OUT

MIC29302

Ripple Current

TraceResistance

VOUT = VREG + (2 I DC OUT RTRACE )

RL

R1

R2

ADJ

0.1FVREG

Figure 3-3. Regulator Layout With Remote Voltage Sensing

AssemblyLow power regulator circuits are built like any

other analog system. Surface mounted systems areassembled using normal reflow (or similar), tech-niques. Larger leaded packages may require speciallead bending before installation; specific lead bend

options are available from Micrel, or the assemblermay bend them. When power demands force the useof a heat sink, extra care must be applied during as-sembly and soldering. Our assembly discussion willfocus on the popular TO-220 package but it is gener-ally applicable to other package types.NOTE 2: The internal reference in most Micrel regulators is

positioned between the adjust pin and ground, unlike theolder classic NPN regulator designs. This technique,while providing excellent performance with Micrel regu-lators, does not work with the older voltage regulators; infact, it reduces their output voltage accuracy.


26/109


27/109


28/109


Section 3: Using LDO Linear Regulators 28 Designing With Linear Regulators

age varies from minimum to 12V using 1%, 0.5%,0.25%, and 0.1% resistors. The more expensive,tighter accuracy resistors provide improved tolerance,but it is still limited by the adjustable regulators 2%internal reference.

0

0.20.40.60.81.01.21.41.61.82.0

1 2 3 4 5 6 7 8 9 10 11 12

E R R O R P E R C E N T A G E

OUTPUT VOLTAGE

1%

0.5%

0.25%

0.1%

Figure 3-7. Worst-Case Output Tolerance

A better method is possible: increase the overallaccuracy of the regulator by employing a precisionreference in the feedback loop.

Improving Regulator AccuracyAchieving a worst-case error of 2.5%, includ-

ing all D/C and A/C error terms, is possible by in-creasing the basic accuracy of the regulator itself, butthis is expensive since high current regulators havesignificant self-heating. Its internal reference mustmaintain accuracy across a wide temperature range.Testing for this level of performance is time consum-ing and raises the cost of the regulator, which is un-acceptable for extremely price-sensitive marketplaces.Some systems require better than 2% accuracy. Thishigh degree of accuracy is possible using Micrel'sLM4041 voltage reference instead of one of the pro-gramming resistors (refer to Figure 3-8). The regula-

tor output voltage is the sum of the internal referenceand the LM4041s programmed voltage (Equation3-3).

(3-3) V OUT = VREF Regulator + VLM4041= 1.240 + V LM4041

The benefit of this circuit is the increased accu-racy possible by eliminating the multiplicative effect

of the regulators internal reference. In normal con-figurations, the reference error is multiplied up by theresistor ratio, keeping the error percentage constant.With this circuit, the error voltage is within 25mV, ab-solute. Another benefit of this arrangement is that theLM4041 is not a dissipative device: there is only asmall internal temperature rise to degrade accuracy.

Additionally, both references are operating in their low-sensitivity range so we get less error contribution fromthe resistors. A drawback of this configuration is thatthe minimum output voltage is now the sum of bothreferences, or about 2.5V. The adjustable LM4041 isavailable in accuracies of 0.5% and 1%, which al-lows better overall system output voltage accuracy.

Equation 3-4 presents the formula for theLM4041-ADJ output voltage. Note the output voltagehas a slight effect on the reference. Refer to theLM4040 data sheet for full details regarding this sec-

ond-order coefficient.

(3-4) V VV

V1.233

R1b

R1a1LM4041 OUT

REF

OUT= + +

Actually, the voltage drop across R1b is slightlyhigher than that calculated from Equation 3-4. Ap-proximately 60nA of current flows out of the LM4041FB terminal. With large values of R1b, this currentcreates millivolts of higher output voltage; for bestaccuracy, compensate R1b by reducing its size ac-cordingly. This error is +1mV with R1b = 16.5k .Equation 3-5 shows the nominal output voltage forthe composite regulator of Figure 4.

(3-5)

V

1.233R1b

R1a1

1.00130.0013R1b

R1a

60nA R1b 1.240OUT =+

+ ( ) +

Note that the tolerance of R2 has no effect onoutput voltage accuracy. It sets the diode reverse (op-erating) current and also allows the divider currentfrom R1a and R1b to pass. With R2 = 1.2k , 1mA ofbias flows. If R2 is too small (less than about 105 ,the maximum reverse current of the LM4041-ADJ isexceeded. If it is too large with respect to R1a andR1b then the circuit will not regulate. The recom-mended range for R2 is from 121 to R1a 10 .


29/109



Figure 3-10 shows the resistor error contribu-tion to the LM4041C reference output voltage toler-ance. Figure 3-11 shows the worst-case output volt-age error of the composite regulator circuit using vari-ous resistor tolerances, when a 0.5% LM4041C ref-erence is employed. The top four traces reflect useof 1%, 0.5%, 0.25%, and 0.1% resistors. Table 3-1lists the production accuracy obtained with the low-cost LM4041C and standard 1% resistors as well asthe improvement possible with 0.1% resistors.

00.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

1 2 3 4 5 6 7 8 9 10

E R R O R P E R C E N T A G E

OUTPUT VOLTAGE

1%

0.25%

0.1%

0.5%

Figure 3-10. Resistor Tolerance Effects on LM4041Voltage Reference Accuracy

Figure 3-8. Improved Accuracy Composite Regulator Circuit

0

2

4

6

8

10

12

0 1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

8 0 0

9 0 0

O U T P U T V O L T A G E ( V )

RESISTOR R1b (k )

Figure 3-9. Output Voltage vs. R1b (See Figure 3-8)

Regulator & Reference Circuit Performance

With this circuit we achieve much improved ac-curacies. Our error terms are:

25mV (constant) from the MIC29512

0.5% from the LM4041C

+ 0 to 2% from R1a and R1b

0.5% + 25mV to Total Error Budget2.5% + 25mV

MIC29512BTMIC29712BT

R2330(tolerance not critical)

R1a120k

R1b

LM4041-ADJ

VIN VOUT

1.233V


30/109



Composite StandardVOUT Circuit Circuit

2.50V 1.6% 3.0%3.00V 1.9% 3.2%3.30V 2.1% 3.3%3.50V 2.1% 3.2%5.00V 2.4% 3.5%6.00V 2.4% 3.6%8.00V 2.5% 3.7%10.00V 2.5% 3.8%11.00V 2.5% 3.8%

Table 3-2. Comparing the Worst-Case Output Voltage Error for the Two Topologies With

Typical Output Voltages

00.70.91.11.31.5

1.71.92.12.32.5

2 . 5

3 . 5

4 . 5

5 . 5

6 . 5

7 . 5

8 . 5

9 . 5

1 0

. 5

1 1

. 5

E R R O R P E R

C E N T A G E

OUTPUT VOLTAGE

1%

0.1%

0.25%

0.5%

Figure 3-11. Composite Regulator Accuracy

What does the extra complexity of the compos-ite regulator circuit of Figure 3-8 buy us in terms ofextra accuracy? With precision components, we mayachieve tolerances better than 1% with the compos-ite regulator, as compared to a theoretical best caseof somewhat worse than 2% with the standard regu-lator and resistor configuration. Figure 3-12 and Table

VOUT 1% Resistors 0.1% Resistors

2.50V

1.54%

1.50%

2.90V 1.88% 1.41%3.00V 1.94% 1.39%3.30V 2.07% 1.34%3.45V 2.12% 1.31%3.525V 2.14% 1.30%3.60V 2.16% 1.29%5.00V 2.36% 1.13%6.00V 2.41% 1.07%8.00V 2.46% 0.98%10.00V 2.49% 0.92%11.00V 2.49% 0.90%

Table 3-1. Worst-Case Output Voltage Error for Typical Operating Voltages Using the LM4040C

( 0.5% Accuracy Version)

3-2 show the accuracy difference between the cir-cuits as the output voltage changes. The accuracydifference is the tolerance of the two-resistor circuitminus the tolerance of the composite circuit. Both tol-erances are the calculated worst-case value, using1% resistors. This figure shows the composite circuitis always at least 1% better than the standard con-

figuration. Both the figure and the table assume stan-dard 1% resistors and the LM4041C-ADJ (0.5%) ref-erence.

00.20.40.60.81.01.21.41.61.82.0

2 3 4 5 6 7 8 9 10 11 12

A c c u r a c y

D i f f e r e n c e

( % )

OUTPUT VOLTAGE (V)

Figure 3-12. Accuracy difference between the Standard Two-Resistor Circuit and the Composite

Circuit of Figure 3-8


31/109



Design Issues andGeneral Applications

Noise and Noise ReductionMost of the output noise caused by a LDO regu-

lator emanates from the voltage reference. While

some of this noise may be shunted to ground by theoutput filter capacitor, bypassing the reference at ahigh impedance node provides more attenuation fora given capacitor value. The MIC5205 and MIC5206use a lower noise bandgap reference and also pro-vide external access to this reference. A small value(470pF or so) external capacitor attenuates outputnoise by about 10dB for a 5 volt output.

All of Micrels adjustable regulators allow a simi-lar technique. By shunting one of the voltage program-ming resistors with a small-value capacitor, the high

frequency gain of the regulator is reduced whichserves to reduce high frequency noise. The capaci-tor should be placed across the resistor connectingbetween the feedback pin and the output (R1 on datasheet schematics).

StabilityLow dropout linear regulators with a PNP out-

put require an output capacitor for stable operation.See Stability Issues in Section 4, Linear RegulatorSolutions for a discussion on stability with Super etaPNP regulators.

The Super LDO is more stable than the mono-lithic devices and rarely needs much attention to guar-antee stability. Micrels Unique Super LDO , also inSection 4, discusses the few parameters requiringvigilance.

LDO EfficiencyThe electrical efficiency of all electronic devices

is defined as P OUT P IN. A close efficiency approxi-mation for linear regulators is

VOUTEff = VIN

This approximation neglects regulator operatingcurrent, but is very accurate (usually within 1%) forSuper eta PNP and Super LDO regulators with their

very low housekeeping power draw. The full formulais:

VIN (IGND ) + (VIN VOUT ) IOUTEff =

VOUT IOUTBuilding an Adjustable Regulator

Allowing 0V Output Some power supplies, especially laboratory

power supplies and power systems demanding well-controlled surge-free start-up characteristics, requirea zero-volt output capability. In other words, an ad-

justable laboratory power supply should provide arange than includes 0V. However, as shown in Fig-ure 3-13, a typical adjustable regulator does not fa-cilitate adjustment to voltages lower than V REF (theinternal bandgap voltage). Adjustable regulator ICsare designed for output voltages ranging from theirreference voltage to their maximum input voltage(minus dropout); the reference voltage is generallyabout 1.2V. The lowest output voltage available fromthis circuit is provided when R1 = 0 . For theMIC29152 LDO regulator, V REF = 1.240V, soVOUT (min) = V REF (1+R1/R2), or 1.240V.

Typical LDO Regulator

VREF

VOUT

R12M1%

R2102k 1%VADJ

OUTIN

GND

C IN22F

ADJ

MIC29152

VIN

COUT22F

(1.24 25V)(26V)

VOUT (max) = VREF 1R1R2

Figure 3-13. Typical Adjustable Regulator

Two designs work around the minimum outputvoltage limitation. The first uses a low-cost referencediode to create a virtual V OUT that cancels the ref-erence. The second uses op-amps to convince theregulator adjust pin that zero volts is a proper outputlevel. In both cases, the feedback-loop summing junc-tion must be biased at V REF to provide linear opera-tion.

Reference Generates a Virtual V OUTFigure 3-14 shows a simple method of achiev-

ing a variable output laboratory supply or a less-than-1.2V fixed-output supply. The circuit uses a secondbandgap reference to translate the regulators outputup to a virtual V OUT and then uses that virtual V OUTas the top of a feedback divider. The output voltageadjusts from 0V to about 20V.


32/109


33/109



Split SupplyLoad

MIC29xxx +V

GND

V V

VIN

Figure 3-16. Diode Clamp Allows Start-Up in Split-Supply System

High Input VoltagesIf the input voltage ranges above the maximum

allowed by the regulator, a simple preregulator circuitmay be employed, as shown in Figure 3-17. A pre-regulator is a crude regulator which drops extra volt-age from the source to a value somewhat lower thanthe maximum input allowed by the regulator. It alsohelps thermal design by distributing the power dissi-pation between elements. The preregulator need nothave good accuracy or transient response, sincethese parameters will be cleaned up by the finalregulator.

MIC29150-12VIN

Rz

Rd

10F

Dz26V

200mW

Q

0.1F

+12V1A

22F

Figure 3-17. Preregulator Allows High Input Supply

Figure 3-17 shows the generic circuit. Table 3-3provides component values for a typical application:+12V output at 1A. With up to 40V of input, no Rd isrequired. Above 40V, heat sinking is eased by powersharing with Rd. Note that a minimum input voltageis also listed; the composite regulator enters dropout

below this minimum value. Assumptions made includea Q1 beta of 1000 and zener diode dissipation of200mW. The MIC29150 dissipates a maximum of13W; Q1 generates less than 15W of heat.

VMAX VMIN Rz Rd

30V 15V 1.1k 040V 17.5V 3.6k 050V 23V 6.2k 10 60V 34V 8.87k

20

Table 3-3. Component Values for Figure 3-17

Controlling Voltage Regulator Turn-On Surges

When a power supply is initially activated, in-rush current flows into the filter capacitors. The sizeof this inrush surge is dependent upon the size of thecapacitors and the slew rate of the initial power-onramp. Since this ramp plays havoc with the upstreampower source, it should be minimized. Employing the

minimum amount of capacitance is one method, butthis technique does not solve the general problem.Slew rate limiting the power supply is a good solutionto the general problem.

The turn-on time interval of a voltage regulatoris essentially determined by the bandwidth of the regu-lator, its maximum output current (in current limit),and the load capacitance. To some extent, the risetime of the applied input voltage (which is normallyquite short, tens of milliseconds, or less) also affectsthe turn-on time. However, the regulator output volt-

age typically steps abruptly at turn-on. Increasing theturn-on interval via some form of slew-limiting de-creases the surge current seen by both the regulatorand the system. These applications describe circuitrythat changes the step-function to a smoother RCcharge waveform.

Various performance differences exist betweenthe three circuits that are presented. These are:

(1) whether stability is impacted

(2) whether start-up output voltage is 0V

(3) whether the circuit quickly recovers from a mo-mentarily interrupted input voltage or a shortedoutput.

Table 3-4 summarizes each circuits features.


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The Simplest ApproachFigure 3-18 illustrates a typical LDO voltage

regulator, the MIC29152, with an additional capaci-tor (C T) in parallel with the series leg (R1) of the feed-back voltage divider. Since the voltage (V ADJ) willbe maintained at V REF by the regulator loop, theoutput of this circuit will still rapidly step to V REF (andthen rise slowly). Since V REF is usually only about1.2V, this eliminates a large part of the surge current.


VREF

VOUT

CT0.33F

R1300k

R2100k

VADJ

OUTIN

GND

C IN22F

ADJ

MIC29152

VIN

COUT22F

Figure 3-18. Simplest Slow Turn-On Circuit

As C T charges, the regulator output (V OUT ) as-ymptotically approaches the desired value. If a turn-on time of 300 milliseconds is desired then about threetime constants should be allowed for charge time:3t = 0.3s, or t = 0.1s = R1 CT = 300k 0.33 F.

0

0

2

0.2

4

0.4 0.6 0.8

10

5

1.0

0

O U T P U T V O L T A G E

( V )

I N P U T V O L T A G E ( V )

TIME (s)

Figure 3-19. Turn-On Behavior for Circuit of Figure 3-18

Figure 3-19 shows the waveforms of the circuitof Figure 3-18. This circuit has three shortcomings:(1) the approximately 1.2V step at turn-on, (2) theaddition of capacitor C T places a zero in the closed-loop transfer function (which affects frequency andtransient responses and can potentially cause stabil-ity problems) and (3) the recovery time associatedwith a momentarily short-circuited output may be un-acceptably long 3.

Improving the Simple ApproachFigure 3-20 addresses the problems of poten-

tial instability and recovery time. Diode D1 is addedto the circuit to decouple the (charged) capacitor fromthe feedback network, thereby eliminating the effectof C T on the closed-loop transfer function. Becauseof the non-linear effect of D1 being in series with C T,there is a slightly longer tail associated with ap-proaching the final output voltage at turn-on. In theevent of a momentarily shorted output, diode D2 pro-vides a low-impedance discharge path for C

T and

thus assures the desired turn-on behavior.Typical LDO Regulator

VREF

VOUT

CT0.33F

R1300k

R2100k

VADJ

OUTIN

GND

C IN22F

ADJ

MIC29152

VIN

COUT22F

D1, D2 = 1N4148D2

D1

Figure 3-20. Improved Slow Turn-On Circuit

Figure 3-21 shows the waveforms of the circuitof Figure 3-20. Note that the initial step-function out-put is now 0.6V higher than with the circuit of Figure3-18. This (approximately) 1.8V turn-on pedestal may

Circuit Stability Start-Up V IN Interrupt V OUT ShortFigure Impacted? Pedestal? Recovery? Recovery?3-18 yes 1.2V no no

3-20 no 1.8V no yes

3-22 no 0V yes no

NOTE 3: This is because when the output is shorted, C T isdischarged only by R2; if the short is removed before C Tis fully discharged the regulator output will not exhibit thedesired turn-on behavior.

Table 3-4. Slow Turn-On Circuit Performance Features


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be objectionable, especially in applications where thedesired final output voltage is relatively low.

0

0

2

0.2

4

0.4 0.6 0.8

10

5

1.0

0



TIME (s)

Figure 3-21. Turn-On Behavior of Figure 3-20

Eliminating Initial Start-Up PedestalThe circuits of Figures 3-18 and 3-19 depend

upon the existence of an output voltage (to createVADJ ) and, therefore, produce the initial step-func-tion voltage pedestals of about 1.2V and 1.8V, as canbe seen in Figures 3-19 and 3-21, respectively. Theapproach of Figure 3-22 facilitates placing the outputvoltage origin at zero volts because V CONTROL isderived from the input voltage. No reactive compo-

nent is added to the feedback circuit. The value ofRT should be considerably smaller than R3 to as-sure that the junction of R T and C T acts like a volt-age source driving R3 and so R T is the primary tim-ing control. If sufficient current is introduced into theloop summing junction (via R3) to generate V ADJ VREF , then V OUT will be zero volts. As R T chargesCT, VCONTROL decays, which would eventually re-sult in V ADJ < VREF . In normal operation, V ADJ =VREF , so V OUT becomes greater than zero volts.The process continues until V CONTROL decays toV

REF+ 0.6V and V

OUT reaches the desired value.

This circuit requires a regulator with an enable func-tion, (such as the MIC29152) because a small (< 2V)spike is generated coincident with application of astep-function input voltage. Capacitor C1 and resis-tor R4 provide a short hold-off timing function thateliminates this spike.


VREF

VOUT

R1300k

R2100k

VADJ

OUTIN

GND

C IN22F

ADJ

MIC29152

VIN

COUT22F

D1C1

0.1F RT

33k

CT10F

R4240k

EN

D21N4001

R3

240k

VCONTROL

1N4148

Figure 3-22. Slow Turn-On Without Pedestal Voltage

Figure 3-23 illustrates the timing of this opera-tion. The small initial delay (about 40 milliseconds) isthe time interval during which V ADJ > V REF . SinceVIN is usually fairly consistent in value R3 may bechosen to minimize this delay. Note that if R3 is cal-culated based on the minimum foreseen V IN (as de-scribed below), then higher values of V IN will pro-

duce additional delay before the turn-on ramp begins.Conversely, if V IN(max) is used for the calculation ofR3, then lower values of V IN will not produce the de-sired turn-on characteristic; instead, there will be asmall initial step-function prior to the desired turn-onramp. Recovery from a momentarily shorted outputis not addressed by this circuit, but interrupted inputvoltage is handled properly. Notice that the buildupof regulator output voltage differs from the waveformsof Figures 3-19 and 3-21 in that it is more ramp-like(less logarithmic). This is because only an initial por-tion of the RC charge waveform is used; i.e., while

VCONTROL > VREF + 0.6V. The actual time con-stant used for Figure 3-22 is 0.33 second, so 3t isone second. As shown by Figure 3-23, this providesabout 600 milliseconds of ramp time, which corre-sponds to the first 60% of the capacitor RC chargecurve. R3 is calculated as follows:

at turn-on time force V ADJ = 1.5V

( just slightly higher than VREF )

then I =V

R1 R2

R1 R2

CONTROL1 5.

+

and R3 =V 0.6V

IIN min

CONTROL

Since the MIC29152 is a low-dropout regulator,6V was chosen for V IN(min). This corresponds to thesmall (approximately 40msec) delay before the out-


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put begins to rise. With 7V input the initial delay isconsiderably more noticeable.

0

0

2

0.2

4

0.4 0.6 0.8

10

5

1.0

0



TIME (s)

Figure 3-23. Turn-On Behavior of Figure 3-22

Current SourcesAnother major application for voltage regulators

is current sources. Among other uses, most recharge-able batteries need some type of constant currentchargers.

Simple Current SourceSeveral techniques for generating accurate out-

put currents exist. The simplest uses a single resis-

tor in the ground return lead (Figure 3-24). This tech-nique works with all Micrel adjustable regulators ex-cept for the MIC5205 or the MIC5206. The outputcurrent is V REF R. A drawback of this simple circuitis that power supply ground and load ground are notcommon. Also, compliance ranges from 0V to onlyVOUT (VDO + VREF ).

Micrel AdjustableRegulator

1.240V

IOUT

VIN

ADJGND

R

IOUT = 1.240 / R

Load

+

Figure 3-24. Simple Current Source Uses Reference Resistor in V Return

The Super LDO Current SourceThe adjustable Super LDOs, MIC5156 and

MIC5158, feature linear current limiting. This is refer-enced to an internal 35mV source. A simple, high ef-ficiency, high output current source may be built (Fig-ure 3-25). Current source compliance is excellent,ranging from zero volts to V IN dropout, which isonly I OUT RDS (ON) + 35mV (generally only a fewhundred millivolts even at 10A). Output current is

IOUT = 35mV RsThis circuit suffers from relatively poor accuracy,

however, since the 35mV threshold is not productiontrimmed. R1 and R2 allow clamping the output volt-age to a maximum value, if desired.

GND

S

EA

MIC5158

GEN

VIN

R1

R2

D

RsVDD

IOUT

Figure 3-25. Simple Current Source Using the Super LDO

Accurate Current Source Using Op AmpsHigh accuracy and maintaining a common

ground are both possible with an alternative circuitusing two op amps and a low current MOSFET (Fig-ure 3-26). This technique works with all Micrel ad-

justable regulators except for the MIC52xx series.Compliance is from 0V to V IN VDO.

A Low-Cost 12V & 5V Power SupplyTaking advantage of the low-dropout voltage

capability of Micrels regulators, we may build a dualoutput 12V & 5V linear power supply with excellentefficiency using a low cost 12.6V center-tapped fila-ment transformer.

Figure 3-27 shows the schematic for the simplepower supply. Using a single center-tapped trans-former and one bridge rectifier, both 12V and 5V out-puts are available. Efficiency is high because thetransformers RMS output voltage is only slightlyabove our desired outputs. The 12.6V center tapped


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IN OUT

EN ADJ

GND

1k 3k

R3

100k

1000pF

0.01F

MIC6211

MIC6211

+VIN

4

3

1

1

5

2

3

4

+VIN5

2

1.24V

V2

V2

I1

R11.24k

VN2222

1N4148

10k

R2 100m

330F

IOUT1A

68F

4V to 6VMIC29152

IR1

R3

R2OUT =

1 240.

IR1

V V

R312=

1 240.

I V

R2OUT =

Reduce to 2kif VIN < 5V

Figure 3-26. Current Source Using a Pair of Op-Amps

filament transformer is a decades-old design origi-nally used for powering vacuum tube heaters. It isperhaps the most common transformer made. Theoutside windings feed the bridge rectifier and filtercapacitor for the 12V output. A MIC29150-12 pro-duces the regulated 12V output. The transformer cen-ter tap feeds the 5V filter capacitor and the MIC29150-5.0 directly no rectifier diode is needed.

This circuit may be scaled to other output cur-rents as desired. Overall efficiency is extremely highdue to the low input voltage, so heat sinking require-ments are minimal. A final benefit: since the powertabs of the TO-220 packages are at ground potential,a single non-isolated, non-insulated heat sink maybe used for both regulators.

AC Input

12.6V CTFilament

Transformer

MIC29150-12

MIC29150-5.0

12.0V

5.0V

Figure 3-27. A Dual-Output Power Supply From a Single Transformer and Bridge Rectifier


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of powering high performance low voltage micropro-cessors. By selecting a modern low dropout regula-tor, you assure reliable operation under all workingconditions.

5V to 3.xV Conversion CircuitsRecommended circuits for on-board desktop

computer power supplies follow. Due to the highspeed load changes common to microprocessors, fastload transient response is crucial. This means circuitlayout and bypass and filter capacitor selection is alsocritical. At low current levels, thermal considerationsare not difficult; however, at currents of above 3 am-peres, the resulting heat may be troublesome.

Method 1: Use a Monolithic LDOThe simplest method of providing a second V CC

on a computer motherboard is by using a monolithic

regulator. If the required voltage is a standard value,a fixed-voltage regulator is available. In this ideal situ-ation, your electrical design consists of merely speci-fying a suitable output filter capacitor. If the outputvoltage is not available from a fixed regulator,adjustables are used. They use two resistors to pro-gram the output voltage but are otherwise similar tothe fixed versions. Figure 3-28 and 3-29 show fixedand adjustable regulator applications.

MIC29710

OUT VOUTIN

GND

VIN

Figure 3-28. Fixed Regulator Circuit Suitable for Computer Power Supply Applications

MIC29712

OUT

ADJR1

VOUT

R2

EN

GND

VIN IN

OnOff

VOUT = 1.240R1R2

1

Figure 3-29. Adjustable Regulator Circuit Suitable for Computer Power Supply Applications

Method 2: The MIC5156 Super LDOThe Micrel MIC5156 is a linear regulator con-

designing with low dropout voltage regulator.pdf

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