Micrel’s Guide to
`
`Designing With
`Low-Dropout
`Voltage
`Regulators
`
`Bob Wolbert
`Applications Engineering Manager
`
`Revised Edition, December 1998
`
`Micrel Semiconductor
`1849 Fortune Drive
`San Jose, CA 95131
`Phone: + 1 (408) 944-0800
`Fax: + 1 (408) 944-0970
`
`MICROCHIP TECHNOLOGY INC. EXHIBIT 1028
`Page 1 of 109
`
`

`

`Micrel Semiconductor
`Designing With LDO Regulators
`Micrel, The High Performance Analog Power IC Company
`Micrel Today and Beyond
`Micrel Semiconductor designs, develops, manu-
`factures, and markets high performance analog power
`Building on its strength as an innovator in pro-
`integrated circuits on a worldwide basis. These cir-
`cess and test technology, Micrel has expanded and
`cuits are used in a wide variety of electronic prod-
`diversified its business by becoming a recognized
`ucts, including those in cellular communications, por-
`leader in the high performance analog power control
`table and desktop computers, and in industrial elec-
`and management markets.
`tronics.
`
`Micrel History
`Since its founding in 1978 as an independent
`test facility of integrated circuits, Micrel has maintained
`a reputation for excellence, quality and customer re-
`sponsiveness that is second to none.
`
`In 1981 Micrel acquired its first independent
`semiconductor processing facility. Initially focusing
`on custom and specialty fabrication for other IC manu-
`facturers, Micrel eventually expanded to develop its
`own line of semicustom and standard product Intelli-
`gent Power integrated circuits. In 1993, with the con-
`tinued success of these ventures, Micrel acquired a
`new 57,000 sq. ft. facility and in 1995 expanded the
`campus into a 120,000 sq. ft. facility. The new Class
`10 facility has allowed Micrel to extend its process
`and foundry capabilities with a full complement of
`CMOS/DMOS/Bipolar/NMOS/PMOS processes. In-
`corporating metal gate, silicon gate, dual metal, dual
`poly and feature sizes down to 1.5 micron, Micrel is
`able to offer its customers unique design and fabrica-
`tion tools.
`
`The company’s initial public offering in Decem-
`ber of 1994 and recent ISO9001 compliance are just
`two more steps in Micrel’s long range strategy to be-
`come the preeminent supplier of high performance
`analog 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 changing
`world.
`
`The niche Micrel has carved for itself involves:
`
`• High Performance.....precision voltages, high tech-
`nology (Super b eta PNP™ process, patented circuit
`techniques, etc.) combined with the new safety
`features of overcurrent, overvoltage, and overtem-
`perature protection
`• Analog.....we control continuously varying outputs of
`voltage or current as opposed to digital ones and
`zeros (although we often throw in “mixed signal” i.e.
`analog with digital controls to bring out the best of
`both worlds)
`• Power ICs.....our products involve high voltage, high
`current, or both
`We use this expertise to address the following
`growing market segments:
`
`1. Power supplies
`2. Battery powered computer, cellular phone,
`and handheld instruments
`3. Industrial & display systems
`4. Desktop computers
`5. Aftermarket automotive
`6. Avionics
`7. 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.
`
`Designing With LDO Regulators
`
`2
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`Micrel Semiconductor
`
`Designing With LDO Regulators
`
`Contents
`
`Contributors: ......................................................................................................... 7
`Section 1. Introduction:
`Low-Dropout␣ Linear␣ Regulators .................................................. 8
`What is a Linear Regulator? ............................................................................... 8
`Why Use Regulators? ........................................................................................... 8
`Basic Design Issues .............................................................................................. 9
`What is a “Low-Dropout” Linear␣ Regulator?................................................ 10
`Linear Regulators vs. Switching␣ Regulators ................................................. 11
`Who Prefers Linear Low Dropout Regulators? .................................................. 11
`Section 2. Low-Dropout Regulator
`Design Charts ................................................................................ 12
`Regulator Selection Charts ............................................................................... 12
`Regulator Selection Table ................................................................................. 14
`Maximum Power Dissipation by Package Type........................................... 16
` Typical Thermal Characteristics ..................................................................... 17
`Output Current vs. Junction Temperature and Voltage Differential ....... 18
`Junction Temperature Rise vs. Available Output Current
`and Differential Voltage .............................................................................. 21
`Section 3. Using LDO Linear Regulators ..................................... 24
`General Layout and Construction␣ Considerations ...................................... 24
`Layout ....................................................................................................................... 24
`Bypass Capacitors ................................................................................................... 24
`Output Capacitor ..................................................................................................... 24
`Circuit Board Layout............................................................................................... 25
`Assembly ................................................................................................................... 25
`Lead Bending ........................................................................................................... 26
`Heat Sink Attachment ............................................................................................. 26
`Output Voltage Accuracy .................................................................................. 27
`Adjustable Regulator Accuracy Analysis ............................................................ 27
`Improving Regulator Accuracy ............................................................................. 28
`Regulator & Reference Circuit Performance ....................................................... 29
`Design Issues and General␣ Applications ...................................................... 31
`Noise and Noise Reduction .................................................................................... 31
`Stability .................................................................................................................... 31
`LDO Efficiency ......................................................................................................... 31
`Building an Adjustable Regulator Allowing 0V Output ................................... 31
`Reference Generates a “Virtual VOUT” ............................................................... 31
`Op-Amp Drives Ground Reference ...................................................................... 32
`Systems With Negative Supplies .......................................................................... 32
`
`3
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`Designing With LDO Regulators
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`Micrel Semiconductor
`
`Designing With LDO Regulators
`
`High Input Voltages ................................................................................................ 33
`Controlling Voltage Regulator Turn-On␣ Surges.................................................. 33
`The Simplest Approach .......................................................................................... 34
`Improving the Simple Approach........................................................................... 34
`Eliminating Initial Start-Up Pedestal.................................................................... 35
`Current Sources ........................................................................................................ 36
`Simple Current Source ............................................................................................ 36
`The Super LDO Current Source ............................................................................ 36
`Accurate Current Source Using Op Amps........................................................... 36
`A Low-Cost 12V & 5V Power Supply ................................................................... 36
`Computer Power Supplies ................................................................................ 38
`Dropout Requirements ............................................................................................ 38
`5V to 3.xV Conversion Circuits.............................................................................. 39
`Method 1: Use a Monolithic LDO ......................................................................... 39
`Method 2: The MIC5156 “Super LDO” ................................................................ 39
`Method 3: The MIC5158 “Super LDO” ................................................................ 40
`Method 4: Current Boost a MIC2951 .................................................................... 40
`Adjust Resistor Values ............................................................................................ 40
`3.3V to 2.xV Conversion .......................................................................................... 41
`Improving Transient Response .............................................................................. 41
`Accuracy Requirements .......................................................................................... 42
`Multiple Output Voltages ...................................................................................... 43
`Multiple Supply Sequencing .................................................................................. 44
`Thermal Design ........................................................................................................ 44
`Portable Devices ................................................................................................. 45
`Design Considerations ............................................................................................ 45
`Small Package Needed ........................................................................................... 45
`Self Contained Power ............................................................................................. 45
`Low Current (And Low Voltage) .......................................................................... 45
`Low Output Noise Requirement ........................................................................... 45
`Dropout and Battery Life ....................................................................................... 46
`Ground Current and Battery Life .......................................................................... 46
`Battery Stretching Techniques ............................................................................... 46
`Sleep Mode Switching ............................................................................................ 46
`Power Sequencing ................................................................................................... 46
`Multiple Regulators Provide Isolation ................................................................ 46
`Thermal Management ....................................................................................... 47
`A Thermal Primer .................................................................................................... 47
`Thermal Parameters ................................................................................................ 47
`Thermal/Electrical Analogy .................................................................................. 47
`Calculating Thermal Parameters .......................................................................... 48
`Calculating Maximum Allowable Thermal␣ Resistance ..................................... 49
`Why A Maximum Junction Temperature? ............................................................ 49
`Heat Sink Charts for High Current Regulators................................................... 50
`Thermal Examples ................................................................................................... 51
`Heat Sink Selection ................................................................................................. 52
`Reading Heat Sink Graphs ..................................................................................... 52
`Power Sharing Resistor .......................................................................................... 53
`
`Designing With LDO Regulators
`
`4
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`Micrel Semiconductor
`
`Designing With LDO Regulators
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`Multiple Packages on One Heat Sink................................................................... 54
`Paralleled Devices on a Heat Sink Example ........................................................ 55
`Heat Sinking Surface Mount Packages ................................................................ 56
`Determining Heat Sink Dimensions ..................................................................... 56
`SO-8 Calculations: ................................................................................................... 57
`Comments................................................................................................................. 58
`Linear Regulator Troubleshooting Guide ..................................................... 59
`Section 4. Linear Regulator Solutions .......................................... 60
`Super bb bb beta PNP™ Regulators........................................................................... 60
`Super beta PNP Circuitry ....................................................................................... 61
`Dropout Voltage ....................................................................................................... 61
`Ground Current ........................................................................................................ 62
`Fully Protected ......................................................................................................... 62
`Current Limiting ...................................................................................................... 62
`Overtemperature Shutdown .................................................................................. 62
`Reversed Input Polarity .......................................................................................... 62
`Overvoltage Shutdown .......................................................................................... 63
`Variety of Packages ................................................................................................. 63
`Why Choose Five Terminal Regulators? .............................................................. 63
`Compatible Pinouts ................................................................................................ 63
`Stability Issues ........................................................................................................ 64
`Paralleling Bipolar Regulators ............................................................................. 64
`Micrel’s Unique “Super LDO™”..................................................................... 66
`Micrel’s Super LDO Family ................................................................................... 66
`The MIC5156 ............................................................................................................. 66
`The MIC5157 and MIC5158 .................................................................................... 66
`3.3V, 10A Regulator Application ........................................................................... 66
`Comparison With Monolithics.............................................................................. 67
`Similarities to Monolithics ..................................................................................... 67
`Differences from Monolithics ................................................................................ 67
`Unique Super LDO Applications .......................................................................... 67
`Super High-Current Regulator .............................................................................. 67
`Selecting the Current Limit Threshold ................................................................. 69
`Sense Resistor Power Dissipation ......................................................................... 69
`Kelvin Sensing ......................................................................................................... 69
`Alternative Current Sense Resistors ..................................................................... 69
`Overcurrent Sense Resistors from PC Board Traces .......................................... 69
`Resistor Design Method ......................................................................................... 70
`Design Example ....................................................................................................... 70
`Calculate Sheet Resistance ..................................................................................... 71
`Calculate Minimum Trace Width .......................................................................... 71
`Calculate Required Trace Length .......................................................................... 71
`Resistor Layout ........................................................................................................ 71
`Thermal Considerations ......................................................................................... 71
`Design Aids ............................................................................................................... 71
`Highly Accurate Current Limiting ........................................................................ 71
`Protecting the Super LDO from Long-Term Short Circuits ............................... 71
`
`5
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`Designing With LDO Regulators
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`Micrel Semiconductor
`Designing With LDO Regulators
`Section 5. Omitted ............................................................................ 74
`Section 6. Package Information...................................................... 75
`Packaging for Automatic Handling ................................................................ 76
`Tape & Reel ............................................................................................................... 76
`Ammo Pack .............................................................................................................. 76
`Pricing ....................................................................................................................... 76
`Tape & Reel Standards............................................................................................ 76
`Packages Available in Tape & Reel ...................................................................... 76
`Package Orientation ........................................................................................... 77
`Linear Regulator Packages ............................................................................... 78
`8-Pin Plastic DIP (N) ............................................................................................... 78
`14-Pin Plastic DIP (N) ............................................................................................. 78
`8-Pin SOIC (M) ......................................................................................................... 79
`14-Pin SOIC (M) ....................................................................................................... 79
`TO-92 (Z) ................................................................................................................... 80
`SOT-223 (S) ............................................................................................................... 80
`SOT-143 (M4) ............................................................................................................ 81
`SOT-23 (M3) .............................................................................................................. 81
`SOT-23-5 (M5) .......................................................................................................... 82
`MSOP-8 [MM8™] (MM) ......................................................................................... 82
`3-Lead TO-220 (T) .................................................................................................... 83
`5-Lead TO-220 (T) .................................................................................................... 83
`5-Lead TO-220 Vertical Lead Bend Option (-LB03) ............................................ 84
`5-Lead TO-220 Horizontal Lead Bend Option (-LB02) ...................................... 84
`3-Lead TO-263 (U) ................................................................................................... 85
`5-Lead TO-263 (U) ................................................................................................... 85
`Typical 3-Lead TO-263 PCB Layout ...................................................................... 86
`Typical 5-Lead TO-263 PCB Layout ...................................................................... 86
`3-Lead TO-247 (WT) ................................................................................................ 87
`5-Lead TO-247 (WT) ................................................................................................ 88
`Section 7. Appendices ...................................................................... 89
`Appendix A. Table of Standard 1% Resistor Values.................................... 90
`Appendix B. Table of Standard – 5% and – 10% Resistor Values.............. 91
`Appendix C. LDO SINK for the HP 48 Calculator....................................... 92
`Section 8. Low-Dropout Voltage Regulator Glossary ................ 95
`Section 9. References........................................................................ 97
`Section 10. Index ............................................................................... 98
`Section 11. Worldwide
`Representatives and Distributors ............................................ 100
`Micrel Sales Offices ......................................................................................... 100
`U.S. Sales Representatives .............................................................................. 101
`U.S. Distributors ............................................................................................... 103
`International Sales Representatives and Distributors .............................. 107
`
`Designing With LDO Regulators
`
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`Designing With LDO Regulators
`
`Micrel Semiconductor
`Contributors:
`
`Jerry Kmetz
`
`Mike Mottola
`
`Jim Cecil
`
`Brian Huffman
`
`Marvin Vander Kooi
`
`Claude Smithson
`
`Micrel Semiconductor
`1849 Fortune Drive
`San Jose, CA 95131
`Phone: + 1 (408) 944-0800
`Fax: + 1 (408) 944-0970
`http://www.micrel.com
`
`7
`
`Designing With LDO Regulators
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`Micrel Semiconductor
`
`Designing With LDO Regulators
`
`Section 1. Introduction:
`Low-Dropout␣ Linear␣ Regulators
`
`What is a Linear Regulator?
`IC linear voltage regulators have been around
`for decades. These simple-to-use devices appear in
`nearly every type of electronic equipment, where they
`produce a clean, accurate output voltage used by
`sensitive components.
`
`Historically, linear regulators with PNP outputs
`have been expensive and limited to low current ap-
`plications. However, Micrel Semiconductor’s unique
`“Super b eta PNP™” line of low dropout regulators
`provides up to 7.5 amperes of current with dropout
`voltages less than 0.6V, guaranteed. A lower cost
`product line outputs the same currents with only 1V
`of dropout. These low dropout voltages guarantee the
`microprocessor gets a clean, well regulated supply
`that quickly reacts to processor-induced load changes
`as well as input supply variations.
`
`The low dropout linear voltage regulator is a
`easy-to-use, low cost, yet high performance means
`of powering your systems.
`
`Input
`
`Output
`
`Ground
`
`Figure 1-1. A basic linear regulator schematic.
`
`A typical linear regulator diagram is shown in
`Figure 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
`
`op-amp increases drive to the pass element, which
`increases output voltage. Conversely, if the output
`rises above the desired set point, the op amp reduces
`drive. These corrections are performed continuously
`with the reaction time limited only by the speed of the
`op amp and output transistor loop.
`
`Real linear regulators have a number of other
`features, including protection from short circuited
`loads and overtemperature shutdown. Advanced
`regulators offer extra features such as overvoltage
`shutdown, reversed-insertion and reversed polarity
`protection, and digital error indicators that signal when
`the 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 in
`the 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 sources
`Figure 1-2 shows several typical applications for
`linear voltage regulators. A traditional AC to DC power
`supply 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, independent
`of AC line voltage variations. Figure 1-2(B) uses a
`low-dropout linear regulator to provide a constant
`output voltage from a battery, as the battery dis-
`charges. Low dropout regulators are excellent for this
`application since they allow more usable life from a
`given battery. Figure 1-2(C) shows a linear regulator
`configured as a “post regulator” for a switching power
`
`Section 1: Introduction
`
`8
`
`Designing With LDO Regulators
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`Micrel Semiconductor
`
`Designing With LDO Regulators
`
`supply. Switching supplies are known for excellent ef-
`ficiency, but their output is noisy; ripple degrades
`regulation and performance, especially when power-
`ing analog circuits. The linear regulator following the
`switching regulator provides active filtering and greatly
`improves 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 power
`ON/OFF control. In some applications, especially ra-
`dio systems, different system blocks are often pow-
`ered from different regulators—even if they use the
`same supply voltage—because of the isolation (de-
`coupling) the high gain regulator provides.
`
`Basic Design Issues
`Let’s review the most important parameters of
`voltage regulators:
`
`• Output voltage is an important parameter, as this
`is the reason most designers purchase a regula-
`tor. Linear regulators are available in both fixed
`output voltage and adjustable configurations.
`Fixed voltage regulators offer enhanced ease-of-
`
`use, with their output voltages accurately trimmed
`at the factory—but only if your application uses
`an available voltage. Adjustables allow using a
`voltage custom-tailored for your circuit.
`• Maximum output current is the parameter gener-
`ally used to group regulators. Larger maximum
`output currents require larger, more expensive
`regulators.
`• Dropout voltage is the next major parameter. This
`is the minimum additional voltage on the input that
`still produces a regulated output. For example, a
`Micrel 5.0V Super b eta PNP regulator will pro-
`vide regulated output with an input voltage of 5.3V
`or above. The 300mV term is the dropout volt-
`age. In the linear regulator world, the lower the
`dropout voltage, the better.
`• Ground current is the supply current used by the
`regulator that does not pass into the load. An ideal
`regulator will minimize its ground current. This
`parameter is sometimes called quiescent current,
`but this usage is incorrect for PNP-pass element
`regulators.
`
`AC Input
`
`Low-Dropout
`Linear Regulator
`
`Low-Dropout
`Linear Regulator
`
`DC Output
`
`Battery
`
`DC Output
`
`(A) Standard Power Supplies
`
`(B) Battery Powered Applications
`
`Switching Regulator
`(High efficiency,
`but noisy output)
`
`Low-Dropout
`Linear Regulator
`
`AC or DC
`Input
`
`Battery
`
`Clean
`DC Output
`
`Enable 1
`
`Enable 2
`
`Enable 3
`
`Enable 4
`
`Low-Dropout
`Linear Regulator
`
`Low-Dropout
`Linear Regulator
`
`Low-Dropout
`Linear Regulator
`
`Low-Dropout
`Linear Regulator
`
`Output 1
`
`Output 2
`
`Output 3
`
`Output 4
`
`(C) Post-Regulator for Switching Supplies
`
`(D) “Sleep-mode” and Inter Stage Isolation or De-
`coupling
`
`Figure 1-2. Typical Linear Regulator Applications
`
`Designing With LDO Regulators
`
`9
`
`Section 1: Introduction
`
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`Designing With LDO Regulators
`
`VDO (MIN) = VSAT (Q2) +VBE (Q1)
`
`VDO (MIN) = VSAT
`
`Output
`
`Input
`
`Output
`
`Drive
`Current
`
`+–
`
`VREF
`
`Q1
`
`Q2
`
`Drive
`Current
`
`Output
`
`Input
`
`+ –
`
`VREF
`
`Micrel Semiconductor
`
`VDO (MIN) = VBE (Q1) + VBE (Q2)
` + VSAT current source (if used)
`
`Q1
`
`Drive
`Current
`
`Input
`
`current source
`or resistor
`
`Q2
`
`+ –
`
`VREF
`
`(A) Standard NPN-pass transistor
`regulator
`
`(B) NPN-pass regulator with
`reduced dropout
`
`(C) Low-Dropout PNP-pass tran-
`sistor regulator
`
`VDO (MIN) = RDS (ON)(Q1) · IOUT
`
`VDO (MIN) = RDS (ON)(Q1) · IOUT
`
`Output
`
`Q1
`
`Output
`
`Input
`
`charge
`pump
`voltage
`multiplier
`
`+ –
`
`VREF
`
`Input
`
`current source
`or resistor
`
`Q1
`
`+–
`
`VREF
`
`(D) P-Channel MOSFET-pass transistor regulator
`
`(E) N-Channel MOSFET-pass transistor regulator
`
`Figure 1-3. The Five Major Types of Linear Regulators
`
`• Efficiency is the amount of usable (output) power
`achieved from a given input power. With linear
`regulators, the efficiency is approximately the
`output voltage divided by the input voltage.
`What is a “Low-Dropout”
`Linear␣ Regulator?
`A low dropout regulator is a class of linear regu-
`lator that is designed to minimize the saturation of
`the output pass transistor and its drive requirements.
`A low-dropout linear regulator will operate with input
`voltages only slightly higher than the desired output
`voltage. For example, “classic” linear regulators, such
`as the 7805 or LM317 need about 2.5 to 3V higher
`input voltage for a given output voltage. For a 5V out-
`put, these older devices need a 8V input. By com-
`parison, Micrel’s Super beta PNP low dropout regu-
`
`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 linear
`regulators:
`
`B.
`
`A.
`
`“Classic” NPN-based regulators that require 2.5
`to 3V of excess input voltage to function.
`“Low Dropout NPN” regulators, with a NPN out-
`put but a PNP base drive circuit. These devices
`reduce the dropout requirement to 1.2 to 1.5V.
`C. True low dropout PNP-based regulators that need
`0.3V 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 higher
`costly than bipolar versions) and have high inter-
`nal drive current requirements when working with
`noisy inputs or widely varying output currents.
`
`Section 1: Introduction
`
`10
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`Micrel Semiconductor
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`Furthermore, in applications using low input-to-
`output voltage differentials, the efficiency is not all
`that bad! For example,