Designing With Low-Dropout Voltage Regulators Wolbert Application
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Micrel's Guide
Designing With Low-Dropout Voltage Regulators
Wolbert Applications Engineering Manager
Revised Edition, December 1998
Table Contents
Micrel Semiconductor
1849 Fortune Drive Jose, 95131 Phone: (408) 944-0800 Fax: (408) 944-0970
Index
Micrel Semiconductor
Designing With Regulators
Micrel, High Performance Analog Power Company
Micrel Semiconductor designs, develops, manufactures, markets high performance analog power integrated circuits worldwide basis. These circuits used wide variety electronic products, including those cellular communications, portable desktop computers, industrial electronics. Micrel History Since founding 1978 independent test facility integrated circuits, Micrel maintained reputation excellence, quality customer responsiveness that second none. 1981 Micrel acquired first independent semiconductor processing facility. Initially focusing custom specialty fabrication other manufacturers, Micrel eventually expanded develop line semicustom standard product Intelligent Power integrated circuits. 1993, with continued success these ventures, Micrel acquired 57,000 facility 1995 expanded campus into 120,000 facility. Class facility allowed Micrel extend process foundry capabilities with full complement CMOS/DMOS/Bipolar/NMOS/PMOS processes. Incorporating metal gate, silicon gate, dual metal, dual poly feature sizes down micron, Micrel able offer customers unique design fabrication tools. Micrel Today Beyond Building strength innovator process test technology, Micrel expanded diversified business becoming recognized leader high performance analog power control management markets. company's initial public offering December 1994 recent ISO9001 compliance just more steps Micrel's long range strategy become preeminent supplier high performance analog power management control ICs. staying close customer markets they serve, Micrel will continue remain focused cost effective standard product solutions ever changing world. niche Micrel carved itself involves:
High Performance.precision voltages, high technology (Super PNPprocess, patented circuit techniques, etc.) combined with safety features overcurrent, overvoltage, overtemperature protection Analog.we control continuously varying outputs voltage current opposed digital ones zeros (although often throw "mixed signal" i.e. analog with digital controls bring best both worlds) Power ICs.our products involve high voltage, high current, both
this expertise address following growing market segments:
Power supplies Battery powered computer, cellular phone, handheld instruments Industrial display systems Desktop computers Aftermarket automotive Avionics Plus many others
Copyright 1998 Micrel, Inc. rights reserved. part this publication reproduced used form means without written permission Micrel, Incorporated. Some products this book protected more following patents: 4,914,546; 4,951,101; 4,979,001; 5,034,346; 5,045,966; 5,047,820; 5,254,486; 5,355,008. Additional patents pending.
Designing With Regulators
Micrel Semiconductor
Designing With Regulators
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Contents
Contributors:
Section Introduction: Low-Dropout Linear Regulators
What Linear Regulator? Regulators? Basic Design Issues What "Low-Dropout" Linear Regulator?. Linear Regulators Switching Regulators
Prefers Linear Dropout Regulators?
Section Low-Dropout Regulator Design Charts
Regulator Selection Charts Regulator Selection Table Maximum Power Dissipation Package Type Typical Thermal Characteristics Output Current Junction Temperature Voltage Differential Junction Temperature Rise Available Output Current Differential Voltage
Section Using Linear Regulators
General Layout Construction Considerations
Layout
Bypass Capacitors Output Capacitor Circuit Board Layout
Assembly
Lead Bending Heat Sink Attachment
Output Voltage Accuracy
Adjustable Regulator Accuracy Analysis Improving Regulator Accuracy Regulator Reference Circuit Performance
Design Issues General Applications
Noise Noise Reduction Stability Efficiency Building Adjustable Regulator Allowing Output
Reference Generates "Virtual VOUT" Op-Amp Drives Ground Reference
Systems With Negative Supplies
Designing With Regulators
Micrel Semiconductor
Designing With Regulators
High Input Voltages Controlling Voltage Regulator Turn-On Surges
Simplest Approach Improving Simple Approach. Eliminating Initial Start-Up Pedestal
Current Sources
Simple Current Source Super Current Source Accurate Current Source Using Amps
Low-Cost Power Supply
Computer Power Supplies
Dropout Requirements 3.xV Conversion Circuits.
Method Monolithic Method MIC5156 "Super LDO" Method MIC5158 "Super LDO" Method Current Boost MIC2951
Adjust Resistor Values 3.3V 2.xV Conversion Improving Transient Response Accuracy Requirements Multiple Output Voltages Multiple Supply Sequencing Thermal Design
Portable Devices
Design Considerations
Small Package Needed Self Contained Power Current (And Voltage) Output Noise Requirement Dropout Battery Life Ground Current Battery Life
Battery Stretching Techniques
Sleep Mode Switching Power Sequencing
Multiple Regulators Provide Isolation
Thermal Management
Thermal Primer
Thermal Parameters Thermal/Electrical Analogy
Calculating Thermal Parameters
Calculating Maximum Allowable Thermal Resistance
Maximum Junction Temperature? Heat Sink Charts High Current Regulators. Thermal Examples Heat Sink Selection Reading Heat Sink Graphs Power Sharing Resistor
Designing With Regulators
Micrel Semiconductor
Designing With Regulators
Multiple Packages Heat Sink.
Paralleled Devices Heat Sink Example
Heat Sinking Surface Mount Packages
Determining Heat Sink Dimensions SO-8 Calculations: Comments.
Linear Regulator Troubleshooting Guide
Section Linear Regulator Solutions
Super PNPRegulators.
Super beta Circuitry Dropout Voltage Ground Current Fully Protected
Current Limiting Overtemperature Shutdown Reversed Input Polarity Overvoltage Shutdown
Variety Packages Choose Five Terminal Regulators? Compatible Pinouts Stability Issues Paralleling Bipolar Regulators
Micrel's Unique "Super LDOTM".
Micrel's Super Family MIC5156 MIC5157 MIC5158 3.3V, Regulator Application Comparison With Monolithics.
Similarities Monolithics Differences from Monolithics
Unique Super Applications
Super High-Current Regulator Selecting Current Limit Threshold Sense Resistor Power Dissipation Kelvin Sensing
Alternative Current Sense Resistors Overcurrent Sense Resistors from Board Traces
Resistor Design Method Design Example Calculate Sheet Resistance Calculate Minimum Trace Width Calculate Required Trace Length Resistor Layout Thermal Considerations
Design Aids Highly Accurate Current Limiting Protecting Super from Long-Term Short Circuits
Designing With Regulators
Micrel Semiconductor
Designing With Regulators
Section Omitted Section Package Information.
Packaging Automatic Handling
Tape Reel Ammo Pack Pricing
Tape Reel Standards Packages Available Tape Reel
Package Orientation Linear Regulator Packages
8-Pin Plastic 14-Pin Plastic 8-Pin SOIC 14-Pin SOIC TO-92 SOT-223 SOT-143 (M4) SOT-23 (M3) SOT-23-5 (M5) MSOP-8 [MM8TM] (MM) 3-Lead TO-220 5-Lead TO-220 5-Lead TO-220 Vertical Lead Bend Option (-LB03) 5-Lead TO-220 Horizontal Lead Bend Option (-LB02) 3-Lead TO-263 5-Lead TO-263 Typical 3-Lead TO-263 Layout Typical 5-Lead TO-263 Layout 3-Lead TO-247 (WT) 5-Lead TO-247 (WT)
Section Appendices
Appendix Table Standard Resistor Values. Appendix Table Standard ±10% Resistor Values Appendix SINK Calculator.
Section Low-Dropout Voltage Regulator Glossary Section References Section Index Section Worldwide Representatives Distributors
Micrel Sales Offices U.S. Sales Representatives U.S. Distributors International Sales Representatives Distributors
Designing With Regulators
Micrel Semiconductor
Designing With Regulators
Contributors:
Jerry Kmetz Mike Mottola Cecil Brian Huffman Marvin Vander Kooi Claude Smithson
Micrel Semiconductor
1849 Fortune Drive Jose, 95131 Phone: (408) 944-0800 Fax: (408) 944-0970 http://www.micrel.com
Designing With Regulators
Micrel Semiconductor
Designing With Regulators
Section Introduction: Low-Dropout Linear Regulators
What Linear Regulator?
linear voltage regulators have been around decades. These simple-to-use devices appear nearly every type electronic equipment, where they produce clean, accurate output voltage used sensitive components. Historically, linear regulators with outputs have been expensive limited current applications. However, Micrel Semiconductor's unique "Super PNPTM" line dropout regulators provides amperes current with dropout voltages less than 0.6V, guaranteed. lower cost product line outputs same currents with only dropout. These dropout voltages guarantee microprocessor gets clean, well regulated supply that quickly reacts processor-induced load changes well input supply variations. dropout linear voltage regulator easy-to-use, cost, high performance means powering your systems. op-amp increases drive pass element, which increases output voltage. Conversely, output rises above desired point, reduces drive. These corrections performed continuously with reaction time limited only speed output transistor loop. Real linear regulators have number other features, including protection from short circuited loads overtemperature shutdown. Advanced regulators offer extra features such overvoltage shutdown, reversed-insertion reversed polarity protection, digital error indicators that signal when output correct.
Regulators?
Their most basic function, voltage regulation, provides clean, constant, accurate voltage circuit. Voltage regulators fundamental block power supplies most electronic equipment. regulator benefits applications include:
Input
Output
Accurate supply voltage Active noise filtering Protection from overcurrent faults Inter-stage isolation (decoupling) Generation multiple output voltages from single source
Useful constant current sources
Figure shows several typical applications linear voltage regulators. traditional power supply appears Figure 1-2(A). Here, linear regulator performs ripple rejection, eliminating hum, output voltage regulation. power supply output voltage will clean constant, independent line voltage variations. Figure 1-2(B) uses low-dropout linear regulator provide constant output voltage from battery, battery discharges. dropout regulators excellent this application since they allow more usable life from given battery. Figure 1-2(C) shows linear regulator configured "post regulator" switching power
Ground
Figure 1-1. basic linear regulator schematic.
typical linear regulator diagram shown Figure 1-1. pass transistor controlled operational amplifier which compares output voltage reference. output voltage drops,
Section Introduction
Designing With Regulators
Micrel Semiconductor
supply. Switching supplies known excellent efficiency, their output noisy; ripple degrades regulation performance, especially when powering analog circuits. linear regulator following switching regulator provides active filtering greatly improves output accuracy composite supply. Figure 1-2(D) demonstrates, some linear regulators serve double duty both regulator power ON/OFF control. some applications, especially radio systems, different system blocks often powered from different regulators-even they same supply voltage-because isolation (decoupling) high gain regulator provides.
Designing With Regulators
use, with their output voltages accurately trimmed factory-but only your application uses available voltage. Adjustables allow using voltage custom-tailored your circuit.
Maximum output current parameter generally used group regulators. Larger maximum output currents require larger, more expensive regulators.
Dropout voltage next major parameter. This
minimum additional voltage input that still produces regulated output. example, Micrel 5.0V Super regulator will provide regulated output with input voltage 5.3V above. 300mV term dropout voltage. linear regulator world, lower dropout voltage, better.
Basic Design Issues
Let's review most important parameters voltage regulators:
Ground current supply current used
regulator that does pass into load. ideal regulator will minimize ground current. This parameter sometimes called quiescent current, this usage incorrect PNP-pass element regulators.
Output voltage important parameter, this
reason most designers purchase regulator. Linear regulators available both fixed output voltage adjustable configurations. Fixed voltage regulators offer enhanced ease-of-
Input
Low-Dropout Linear Regulator Output
Battery
Low-Dropout Linear Regulator Output
Standard Power Supplies
Battery Powered Applications
Enable
Low-Dropout Linear Regulator Low-Dropout Linear Regulator Low-Dropout Linear Regulator Low-Dropout Linear Regulator
Output
Switching Regulator (High efficiency, noisy output) Input
Battery
Low-Dropout Linear Regulator Clean Output
Enable
Output
Enable
Output
Enable
Output
Post-Regulator Switching Supplies
"Sleep-mode" Inter Stage Isolation Decoupling
Figure 1-2. Typical Linear Regulator Applications
Designing With Regulators
Section Introduction
Micrel Semiconductor
(MIN) (Q1) (Q2) VSAT current source used) Input current source resistor Drive Current Output
Designing With Regulators
(MIN) VSAT (Q2) +VBE (Q1) Input Drive Current
VREF
(MIN) VSAT
Output
Input
Output
Drive Current
VREF
VREF
Standard NPN-pass transistor regulator
(MIN) (ON)(Q1) IOUT Input current source resistor
NPN-pass regulator with reduced dropout
Low-Dropout PNP-pass transistor regulator
(MIN) (ON)(Q1) IOUT Output Input charge pump voltage multiplier VREF
Output
VREF
P-Channel MOSFET-pass transistor regulator
N-Channel MOSFET-pass transistor regulator
Figure 1-3. Five Major Types Linear Regulators
Efficiency amount usable (output) power
achieved from given input power. With linear regulators, efficiency approximately output voltage divided input voltage.
lators require only 0.3V headroom, would provide regulated output with only 5.3V input. Figure shows five major types linear regulators: "Classic" NPN-based regulators that require excess input voltage function. "Low Dropout NPN" regulators, with output base drive circuit. These devices reduce dropout requirement 1.5V. True dropout PNP-based regulators that need 0.3V 0.6V extra operation. P-channel CMOS output regulators. These devices have very dropout voltages currents require large area (hence higher costly than bipolar versions) have high internal drive current requirements when working with noisy inputs widely varying output currents.
What "Low-Dropout" Linear Regulator?
dropout regulator class linear regulator that designed minimize saturation output pass transistor drive requirements. low-dropout linear regulator will operate with input voltages only slightly higher than desired output voltage. example, "classic" linear regulators, such 7805 LM317 need about higher input voltage given output voltage. output, these older devices need input. comparison, Micrel's Super beta dropout regu-
Section Introduction
Designing With Regulators
Micrel Semiconductor
Regulator controllers. These integrated circuits that provide reference control functions linear regulator, have pass element board. They provide advantage optimizing area cost higher current applications suffer disadvantage being multiple package solution. graph efficiency different classes linear regulators very significant differences input output voltages (see Figure 1-4). higher voltages, however, these differences diminish. 3.3V high current linear regulator controller such Micrel MIC5156 approach 100% efficiency input voltage approaches dropout. LM317 3.3V will have miserable efficiency only about dropout threshold.
Designing With Regulators
Furthermore, applications using input-tooutput voltage differentials, efficiency that bad! example, 3.3V microprocessor application, linear regulator efficiency approaches 66%. applications with current subcircuits care that regulator efficiency less than optimum power lost negligible overall.
Prefers Linear Dropout Regulators?
that price sensitive applications prefer linear regulators over their sampled-time counterparts. design decision especially clear makers communications equipment small devices battery operated systems current devices high performance microprocessors with sleep mode (fast transient recovery required) proceed through this book, will find numerous other applications where linear regulator best power supply solution.
Linear Regulators Switching Regulators
Linear regulators less energy efficient than switching regulators. continue using them? Depending upon application, linear regulators have several redeeming features: lower output noise important radios other communications equipment faster response input output transients easier because they require only filter capacitors operation generally smaller size magnetics required) less expensive (simpler internal circuitry magnetics required)
EFFICIENCY DROPOUT
MIC5200 MIC5203 MIC5201 MIC2920
MIC5156/7/8 MIC29150 MIC29300 MIC29500 MIC29750
LT1086 LT1085 LT1084 LT1083
78L05 LM340 LM317 LM350 LM396
OUTPUT CURRENT
Figure 1-4. Linear Regulator Efficiency Dropout
Designing With Regulators
Section Introduction
Micrel Semiconductor
Designing With Regulators
Section Low-Dropout Regulator Design Charts
Regulator Selection Charts
Output Current Accuracy Noise Single Dual
Dual Single ±1.0% Dual Single -180mA ±3.0% Dual Single
MIC5210 MSOP-8 MIC5205 SOT23-5 MIC5202 SO-8 LP2950 TO-92 MIC2950 TO-92 MIC5200 SO-8, SOT-223, MSOP-8 MIC5207 SOT23-5, TO-92 MIC5211 SOT23-6 MIC5208 MSOP-8 MIC5203 SOT-143, SOT23-5 MIC5219 SOT23-5, MSOP-8 MIC5209 SOT223, SO-8, TO263-5
Without Error Flag
Dual 150mA Noise Bypass 3.0, 3.3, 3.6, 4.0, 5.0V 150mA LDOw/ Noise Bypass 2.8, 3.0, 3.3, 3.6, 3.8, 4.0, 5.0V, Dual 100mA 3.0, 3.3, 4.5, 4.85, 5.0V 100mA Second Source '2950 5.0V 150mA Upgrade '2950 5.0V 100mA 3.0, 3.3, 4.85, 5.0V
With Error Flag
MIC5206 SOT23-5, MSOP-8
150mA LDOs Noise Bypass 2.5, 3.0, 3.3, 3.6, 4.0, 5.0V,
LP2951 SO-8, PDIP-8
100mA Second Source '2951 4.85, 5.0V,
MIC2951 150mA Upgrade '2951 SO-8, PDIP-8, MSOP-8 3.3, 4.85, 5.0V,
180mA 1.8, 2.5, 3.0, 3.3, 3.6, 3.8, 5.0V, Dual 50mA µCap 2.5, 3.0, 3.3, 3.6, 5.0, Mixed 3.3/5.0V Dual 50mA µCap 3.0, 3.3, 3.6, 4.0, 5.0V 80mA µCap 2.8, 3.0, 3.3, 3.6, 3.8, 4.0, 5.0V 500mA Peak 3.0, 3.3, 3.6, 5.0V, 500mA 1.8, 2.5, 3.0, 3.3, 5.0V, 200mA 3.0, 3.3, 4.85, 5.0V, 250mA 5.0V, 400mA 3.3, 4.85, 5.0V 400mA 500mA 2.5, 3.3, 5.0V 750mA 3.3, 5.0, 12.0V 750mA
Single
±1.0%
MIC5201 SOT223, SO-8
200mA 500mA
Single
MIC2954 TO220, SOT223, SO-8, TO92 MIC2920 TO220, SOT223 MIC29202 TO220, TO263
MIC5216 SOT23-5, MSOP-8 MIC29201 TO220, TO263, SO-8 MIC29204 SO-8
500mA Peak 3.0, 3.3, 3.6, 5.0V 400mA 3.3, 4.85, 5.0V 400mA
±3.0%
Single
MIC5237 TO220, TO263 MIC2937A TO220, TO263 MIC29372 TO220, TO263
750mA
±1.0%
Single
MIC29371 TO220, TO263
750mA 3.3, 5.0V
Figure 2-1a. 750mA Regulator Selection Guide
Shaded boxes denote automotive load dump protected devices
Section Design Charts
Designing With Regulators
Micrel Semiconductor
Designing With Regulators
Output Current
Accuracy
Error Flag
Low-Dropout Devices
MIC29151 TO220, TO263 MIC2940A TO220, TO263 1.5A 3.3, 5.0, 1.25A 3.3, 5.0, 1.25A 1.5A 3.3, 5.0, 12.0V 1.5A 3.0A 3.3, 5.0, 3.0A 3.0A 3.3, 5.0, 12.0V 3.0A 3.0A Cost 3.3, 5.0V 3.0A Cost 5.0A 3.3, 5.0V 5.0A 7.5A 3.3, 5.0V 5.0A 3.3, 5.0V 5.0A 5.0A Cost 3.3, 5.0V 5.0A Cost 7.5A 3.3, 5.0V 7.5A Cost 7.5A 3.3, 5.0V 7.5A Cost Controller 3.3, 5.0V, Controller (w/Charge Pump) 3.3, 5.0, Controller (w/Charge Pump) 5.0V,
Ultra-Low-Dropout Devices
MIC39151 TO263 1.5A 1.8, 2.5V
1.5A
±1.0%
MIC2941A TO220, TO263 MIC29150 TO220, TO263 MIC29152 TO220, TO263 MIC29301 TO220, TO263 MIC29303 TO220, TO263
MIC39100 SOT223 MIC39150 TO220, TO263
1.0A 1.8, 2.5, 3.3V 1.5A 1.8, 2.5V
MIC39301 TO263, TO220
3.0A 1.8, 2.5V
3.0A
±1.0%
MIC29300 TO220, TO263
MIC29302 TO220, TO263 MIC29310 TO220, TO263 MIC29312 TO220, TO263 MIC29501 TO220, TO263
MIC39300 TO220, TO263
3.0A 1.8, 2.5V
MIC29503 TO220, TO263 MIC29751 TO247
5.0A -7.5A
±1.0%
MIC29500 TO220, TO263 MIC29502 TO220, TO263
MIC29510 TO220 MIC29512 TO220 MIC29750 TO247 MIC29752 TO247 MIC29710 TO220 MIC29712 TO220 MIC5156 SO-8, PDIP-8
>7.5A
±1.0%
MIC5157 SO-14, PDIP-14 MIC5158 SO-14, PDIP-14
Figure 2-1b. >7.5A Regulator Selection Guide
Shaded boxes denote automotive load dump protected devices
Designing With Regulators
Section Design Charts
Section Design Charts Designing With Regulators
Micrel Semiconductor
Regulator Selection Table
(Sorted Output Current Rating)
Device MIC5208 MIC5211 MIC5203 MIC5200 MIC5202 LP2950 LP2951
Output Standard Output Voltage Current 4.75 4.85 50mA 50mA 80mA 100mA 100mA 100mA 100mA
Adj.
(max.)
Dropout Accuracy (IMAX, 25°C) 250mV 250mV 300mV 230mV
Current Error Enable/ Thermal Limit Flag Shutdown Shutdown
Rev. Input Load Protection Dump
Packages MSOP-8 SOT-23-6 SOT-143, SOT-23-5 SOP-8, SOT-223, MSOP-8 SOP-8 TO-92 DIP-8, SOP-8
225mV
MIC2950 150mA MIC2951 150mA MIC5205 150mA MIC5206 150mA MIC5210 150mA MIC5207 180mV MIC5201 200mA MIC2954 250mA MIC2920A 400mA MIC29201 400mA MIC29202 400mA MIC29204 400mA MIC5216 500mA(1) MIC5219 500mA(1) MIC5209 500mA MIC5237 500mA MIC2937A 750mA MIC29371 MIC29372 750mA 750mA
%,1% 380mV 1/2%,1% 380mV 1/2%,1% 300mV 1/2%,1% 300mV 165mV 165mV
165mV 165mA 270mV 375mV 450mV 450mV 450mV 450mV 300mV 300mV 300mV 300mV 370mV 370mV 370mV
TO-92 DIP-8, SOP-8, MSOP-8 SOT-23-5 SOT-23-5, MSOP-8 MSOP-8 SOT-23-5, TO-92SP SOP-8, SOT-223 TO-92,TO-220,SOT-223 TO-220, SOT-223 TO-220-5, TO-263-5 TO-220-5, TO-263-5 SOP-8, DIP-8 SOT-23-5, MSOP-8 SOT-23-5, MSOP-8 SOP-8, SOT-223, TO-263-5 TO-220, TO-263 TO-220, TO-263 TO-220-5, TO-263-5 TO-220-5, TO-263-5
Designing With Regulators
Output Standard Output Voltage Adj. Dropout Device Current 4.75 4.85 (max.) Accuracy (IMAX, 25°C) MIC2940A 1.25A 400mV MIC2941A 1.25A 400mV MIC29150 1.5A 350mV MIC29151 1.5A 350mV MIC29152 1.5A 350mV MIC29153 1.5A 26VSP 350mV MIC39150 1.5A 350mV MIC39151 1.5A 350mV MIC29300 370mV MIC29301 370mV MIC29302 370mV MIC29303 370mV MIC29310 600mV MIC29312 600mV MIC39300 400mV MIC39301 400mV MIC29500 370mV MIC29501 370mV MIC29502 370mV MIC29503 370mV MIC29510 700mV MIC29512 700mV MIC29710 7.5A 700mV MIC29712 7.5A 700mV MIC29750 7.5A 425mV MIC29751 7.5A 425mV MIC29752 7.5A 425mV MIC5156 MIC5157 MIC5158
Current Error Enable/ Thermal Limit Flag Shutdown Shutdown
Rev. Input Load Protection Dump
Designing With Regulators Section Design Charts
Micrel Semiconductor
Packages TO-220, TO-263 TO-220-5, TO-263-5 TO-220, TO-263 TO-220-5, TO-263-5 TO-220-5, TO-263-5 TO-220-5, TO-263-5 TO-220, TO-263 TO-220-5, TO-263-5 TO-220, TO-263 TO-220-5, TO-263-5 TO-220-5, TO-263-5 TO-220-5, TO-263-5 TO-220, TO-263 TO-220-5, TO-263-5 TO-220, TO-263 TO-220-5, TO-263-5 TO-220 TO-220-5, TO-263-5 TO-220-5, TO-263-5 TO-220-5, TO-263-5 TO-220, TO-263 TO-220-5 TO-220 TO-220-5 TO-247 TO-247-5 TO-247-5 SOP-8, DIP-8 SOP-14, DIP-14 SOP-14, DIP-14
Designing With Regulators
Special order. Contact factory.
Output current limited package layout. Maximum output current dropout voltage determined choice external MOSFET. 3.3V, selectable operation. Adjustable operation.
Micrel Semiconductor
Designing With Regulators
TO-247 (WT)
TO-220
TO-263
SOT-223
DIP-8 SO-8 MSOP-8 MM-8(MM) TO-92
SOT-23-5 (M5)
SOT-143 (M4)
minimum point each line Figure shows package power dissipation capability using "worst case" mounting techniques. maximum point shows power capability with very good (not infinite, though) heat sink. example, through-hole TO-220 packages dissipate less than without heat sink, over with good sink. chart approximate, assumes ambient temperature 25°C. Packages shown their approximate relative size.
Section Design Charts Designing With Regulators
Figure
Maximum Power Dissipation Package Type
Micrel Semiconductor
Designing With Regulators
Table 2-2. Typical Thermal Characteristics
Device
MIC5203BM4 MIC5200BM MIC5200BS MIC5202BM LP2950BZ LP2951BM MIC2950BZ MIC2951BM MIC2951BN MIC5205BM5 MIC5206BM5 MIC5206BMM MIC5207BM5 MIC5201BM MIC5201BS MIC2954BM MIC2954BS MIC2954BT MIC2954BZ MIC2920ABS MIC2920ABT MIC29202BU MIC29203BU MIC29204BM MIC2937ABT MIC2937ABU MIC29371BT MIC29371BU MIC29372BT MIC29372BU MIC29373BT MIC29373BU MIC2940ABT MIC2940ABU MIC2941BT MIC2941BU MIC29150BT MIC29150BU MIC29151BT MIC29151BU MIC29152BT MIC29152BU MIC29153BT MIC29153BU MIC29300BT MIC29300BU MIC29301BT MIC29301BU MIC29302BT MIC29302BU MIC29303BT MIC29303BU MIC29310BT MIC29312BT MIC29500BT MIC29500BU MIC29501BT MIC29501BU MIC29502BT MIC29502BU MIC29503BT MIC29503BU MIC29510BT MIC29512BT MIC29710BT MIC29712BT MIC29750BWT MIC29751BWT MIC29752BWT
"Typical" heat sink
Equivalent Thermal Graph (Figures 2-6, 2-7)
Designing With Regulators
Section Design Charts
Micrel Semiconductor
Designing With Regulators
MIC5200
JUNCTION TEMPERATURE (°C)
Output Current Junction Temperature Voltage Differential
(Figure 2-6)
These graphs show junction temperature with given output current input-output voltage differential. Ambient temperature 25°C. thermal resistance used calculations shown under each graph. This resistance assumes that heat sink suitable size particular regulator employed; higher current regulator circuits generally require larger heat sinks. Refer Thermal Management, Section definitions details. example, MIC5203-3.3BM4, supplying 50mA with 6.3V input (VIN VOUT 3V), will have junction temperature approximately (Figure (A)).
0.3V 0.01 0.02 0.03 0.04 0.05 0.06 0.07 OUTPUT CURRENT 0.08 0.09
Figure (B). SO-8 with 160°C/W
MIC5203BM4
JUNCTION TEMPERATURE (°C)
JUNCTION TEMPERATURE (°C)
MIC5205
0.3V
0.3V
0.02
0.04 0.06 OUTPUT CURRENT
0.08
0.05 OUTPUT CURRENT
0.15
Figure (A). SOT-143 with 250°C/W
Figure (C). SOT-23-5 with 220°C/W
Section Design Charts
Designing With Regulators
Micrel Semiconductor
MIC5201BM
JUNCTION TEMPERATURE (°C)
JUNCTION TEMPERATURE (°C)
Designing With Regulators
MIC2920
0.3V
0.3V
0.05
0.15 OUTPUT CURRENT
0.05
0.10
0.15 0.20 0.25 0.30 OUTPUT CURRENT
0.35
0.40
Figure (D). High Current SO-8 with 160°C/W
Figure (F). TO-263 with 40°C/W
MIC5201BS
JUNCTION TEMPERATURE (°C)
MIC2937ABU
JUNCTION TEMPERATURE (°C)
0.3V 0.05 0.15 OUTPUT CURRENT
0.3V OUTPUT CURRENT
Figure (E). SOT-223 with 50°C/W
Figure (G). TO-263 with 40°C/W
Designing With Regulators
Section Design Charts
Micrel Semiconductor
MIC29150
JUNCTION TEMPERATURE (°C) 0.3V
Designing With Regulators
MIC29710
JUNCTION TEMPERATURE (°C)
0.3V
OUTPUT CURRENT
OUTPUT CURRENT
Figure (H). TO-220 with 15°C/W
Figure (K). TO-220 with 6°C/W
MIC29500
JUNCTION TEMPERATURE (°C)
MIC29750
JUNCTION TEMPERATURE (°C)
0.3V
0.3V OUTPUT CURRENT
OUTPUT CURRENT
Figure (J). TO-220 with 6°C/W
Figure (L). TO-247 with 4°C/W
Section Design Charts
Designing With Regulators
Micrel Semiconductor
Designing With Regulators
MIC5205BM5
0.15 0.14 0.13 0.12 0.11 OUTPUT CURRENT 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 steps, units 100°
Junction Temperature Rise Available Output Current Differential Voltage
(Figure 2-7)
These graphs show available thermally-limited steady-state output current with given thermal resistance input-output voltage differential. assumed (thermal resistance from junction ambient) shown below each graph. Refer Thermal Management Section definitions details. example, Figure shows that MIC5205BM5, with across (VIN VOUT supplying 120mA, will have temperature rise 80°C (when mounted normally).
VOUT
Figure (C). SOT-23-5 with 220°C/W
MIC5203BM4
0.08 0.07 steps, units OUTPUT CURRENT 100° 0.05 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.02 0.01 0.04 0.02 VOUT
MIC5201BM
0.06 OUTPUT CURRENT
0.04
steps, units 100°
0.03
VOUT
Figure (A). SOT-143 with 250°C/W
Figure (D). SO-8 with 140°C/W
Designing With Regulators
Section Design Charts
Micrel Semiconductor
MIC5201BS
0.20 0.18 0.16 0.14 OUTPUT CURRENT 0.12 0.10 0.08 0.06 0.20 0.04 0.02 0.05 VOUT 0.15 0.10 steps, units 100° OUTPUT CURRENT 0.75 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25
Designing With Regulators
MIC2937A
steps, units 100°
VOUT
Figure (E). SOT-223 with 50°C/W
Figure (G). TO-263 with 40°C/W
MIC2920A
0.40 0.35 0.30 OUTPUT CURRENT steps, units OUTPUT CURRENT
MIC29150
0.25 100° 0.20
steps, units 100°
0.15
0.10
0.05
VOUT
VOUT
Figure (F). TO-263 with 40°C/W
Figure (H). TO-220 with 15°C/W
Section Design Charts
Designing With Regulators
Micrel Semiconductor
MIC29300
OUTPUT CURRENT OUTPUT CURRENT
Designing With Regulators
MIC29710
steps, units 100°
100°
steps, units
VOUT VOUT
Figure (I). TO-220 with 10°C/W
Figure (K). TO-220 with 6°C/W
MIC29500
OUTPUT CURRENT OUTPUT CURRENT VOUT steps, units 100°
MIC29750
steps, units 100°
VOUT
Figure (J). TO-220 with 6°C/W
Figure (L). TO-247 with 4°C/W
Designing With Regulators
Section Design Charts
Micrel Semiconductor
Designing With Regulators
Section Using Linear Regulators
General Layout Construction Considerations
Layout
Although often considered "just D.C. Circuit", low-dropout linear regulators actually built with moderately high frequency transistors because rapid response input voltage output current changes demand excellent high frequency performance. These characteristics place some requirements bypass capacitors board layout. Bypass Capacitors Low-dropout linear regulators need capacitors both their input output. input capacitor provides bypassing internal used voltage regulation loop. output capacitor improves regulator response sudden load changes, case Super PNPdevices, provides loop compensation that allows stable operation. input capacitor monolithic regulators should feature inductance generally good high frequency performance. Capacitance critical except systems where excessive input ripple voltage present. capacitor must, minimum, maintain input voltage minimum value above dropout point. Otherwise, regulator ceases regulation becomes merely saturated switch. AC-line powered system, where regulator mounted within centimeters from main filter capacitor, additional capacitors often unnecessary. 0.1µF ceramic directly adjacent regulator always good choice, however. regulator farther away from filter capacitor, local bypassing mandatory. With high current MIC5157 MIC5158 Super LDOregulator controllers, input capacitor should medium sized (10µF larger) (effective series resistance) type.
Output Capacitor Super regulators require certain minimum value output capacitance operation-below this minimum value, output exhibit oscillation. output capacitor inside voltage control loop necessary loop stabilization. Minimum recommended values listed each device data sheet. There maximum value- output capacitor increased without limit.1 Excellent response high frequency load changes (load current transient recovery) demands inductance, ESR, high frequency filter capacitors. Stringent requirements solved paralleling multiple medium sized capacitors. Capacitors should chosen comparing their lead inductance, ESR, dissipation factor. Multiple small medium sized capacitors provide better high frequency characteristics than single capacitor same total capacity since lead inductance multiple capacitors reduced paralleling. Although capacitance value filter increased without limit, paralleled capacitors drops below certain (device family dependent) threshold, zero transfer plot appears, lowering phase margin decreasing stability. With some devices, especially MIC5157 MIC5158 Super LDO, this problem solved using input decoupling capacitor. Worst-case situations require changes higher output capacitors-perhaps increasing both capacitance using different chemistry-or, last resort, adding small series resistance between regulator capacitor(s).
NOTE Truly huge output capacitors will extend start-up time, since regulator must charge them. This time determined capacitor value current limit value regulator.
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
Circuit Board Layout Stray capacitance inductance upset loop compensation promote instability. Excessive input lead resistance increases dropout voltage, excessive output lead resistance reduces output load regulation. Ground loops also cause both problems. Careful layout solution. Reduce stray capacitance inductance placing bypass filter capacitors close regulator. Swamp parasitic reactances using 0.1µF ceramic capacitor equivalent) parallel with regulator input filter capacitor. Designers batterypowered circuits often overlook finite high-frequency impedance their cells. ceramic capacitor solves many unexpected problems. Excessive lead resistance, causing unwanted voltage drops ruining load regulation, solved merely increasing conductor size. Regulators with remote sensing capability-like Micrel adjustables-may utilize Kelvin-sense connection directly load. Figure shows, additional pair wires feeds back load voltage regulator sense input.2 This lets regulator compensate line drop. Kelvin sense leads carry only small voltage-programming resistor current, they very narrow traces small diameter wire. judicious layout especially important remote-sensed designs, since these long, high impedance leads susceptible noise pickup.
VOUT Remote Sense
Designing With Regulators
ground lead filter capacitor (see Figure 3-2). ripple current, which several times larger than average current, create voltage drop ground line, raising voltage relative load. regulator attempts compensate, load regulation suffers. Solve problem ensuring rectifier current flows directly into filter capacitor.
Input
Low-Dropout Linear Regulator
Ripple Current
VOUT
Trace Resistance
VOUT VREG (IRIPPLE RTRACE) Where IRIPPLE
Figure 3-2. Ground Loop Ripple Currents Degrade Output Accuracy
Figure shows ideal layout remotesensed loads. single point ground practical, load regulation improved employing large ground plane.
VOUT Input MIC29302 0.1µF Ripple Current VREG
VOUT VREG
RTRACE)
Trace Resistance
Figure 3-3. Regulator Layout With Remote Voltage Sensing
VREG Trace Resistance
Assembly
power regulator circuits built like other analog system. Surface mounted systems assembled using normal reflow similar), techniques. Larger leaded packages require special lead bending before installation; specific lead bend options available from Micrel, assembler bend them. When power demands force heat sink, extra care must applied during assembly soldering. assembly discussion will focus popular TO-220 package generally applicable other package types.
Figure 3-1. Remote Voltage Sense (Kelvin) Connections
common ground loop problem occurs when rectifier ripple current flows through regulator's
NOTE internal reference most Micrel regulators positioned between adjust ground, unlike older "classic" regulator designs. This technique, while providing excellent performance with Micrel regulators, does work with older voltage regulators; fact, reduces their output voltage accuracy.
Designing With Regulators
Section Using Linear Regulators
Micrel Semiconductor
Lead Bending lead bending necessary, standard bend options offered Micrel whenever possible. These bending operations performed tooling developed specifically this purpose with safety package, die, internal wire bonds mind. Custom lead bending also available nominal charge. prototyping other quantity custom lead bending requirements, clamp leads junction case with long nosed pliers. Using your fingers another pair pliers, bend outer lead desired. Please observe following cautions: spread compress leads bend twist leads body junction: start bend least from body Maintain lead bend radius approximately re-bend leads multiple times
Designing With Regulators
mils inch with surface finish 1.5µm better minimum thermal resistance. Holes mounting screw should drilled deburred. Slightly oversized holes allow slippage during temperature cycling generally recommended. Heat sinks bare aluminum copper optimum heat radiators. Anodizing painting improves heat radiation capability. more details heat sinks, References. Thermal grease, thermal pads, other thermally conductive interface between package heat sink compensates surface flatness errors, mounting torque reduction over time, gaps, other sins, recommended. Heat sink manufacturers offer variety solutions with widely varying prices, installation ease, effectiveness. Many heat sinks available with mounting clips. These allow fast assembly and, when clip also presses against plastic body instead only metal tab, provide excellent heat contact area thermal resistance. Machine screws often used heat sink attachment (see Figure 3-4). Proper torque imperative; loose thermal interface resistance excessive; tight semiconductor will crack. 0.68N-m specification applies clean threads; ensure that thermal grease does interfere with threads.
6-32 Phillips Head Machine Screw Nylon Flat Washer TO-220 Package
Micrel TO-220 packages made from nickelplated tinned copper best electrical thermal performance. While rugged electrically, they susceptible mechanical stress fatigue. Please handle them with care! Heat Sink Attachment TO-220 package applications moderate (room) temperatures require heat sinking power dissipation less than watts. Otherwise, heat sinks necessary. minimum practical lead length heat travel more directly board, board itself heat sink. Attachment techniques vary depending upon heat sink type, which turn depends upon power dissipated. first consideration whether electrical isolation required. Micrel's Super regulators have grounded tab, which usually means insulation necessary. This helps reducing eliminating thermal resistances. Next, determine heat sink size. Thermal Management chapter details. standard commercial heat sink chosen, generally assume minimal surface roughness burrs. Otherwise, machining mounting necessary achieve flatness (peak-to-valley)
Apply Heat-Transfer Compound Between Surfaces
Flat Washer (Optional) Lock Washer 6-32 Maximum Torque: 0.68 in-lbs) (Caution: Excessive torque crack semiconductor)
Figure 3-4. Mounting TO-220 Packages Heat Sinks
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
Designing With Regulators
reference tolerance determine total regulator inaccuracy. sensitivity analysis this equation shows that error contribution adjust resistors
tol% Error Contribution tol%
Output Voltage Accuracy
Adjustable Regulator Accuracy Analysis
Micrel Regulators high accuracy devices with output voltages factory-trimmed much better than accuracy. Across operating temperature, input voltage, load current ranges, their worst-case accuracies still better than ±2%. adjustable regulators, output also depends upon accuracy programming resistors. Some systems require supply voltage accuracies better than ±2.5%-including noise transients. While noise generally major contributor output inaccuracy, load transients caused rapidly varying loads (such high-speed microprocessors), significant, even when using fast transient-response regulators high-quality filter capacitors.
Micrel Adjustable Regulator
(3-2)
VREF VOUT
VOUT
1.24V
VOUT 1.240
Since output voltage proportional product reference voltage ratio programming resistors, high output voltage, error contribution programming resistors each resistor's tolerance. standard resistors contribute much output voltage error. lower voltages, error less significant. Figure shows effects resistor tolerance regulator accuracy from minimum output voltage (VREF) 12V. minimum VOUT, theoretical resistor tolerance effect output accuracy. Resistor error increases proportionally with output voltage: output 2.5V, sensitivity factor 0.5; about 0.75; over 0.9. This means that with output, error contribution resistors 0.75 times tolerances, 0.75 1.5%. expected, more precise resistors offer more accurate performance.
Figure 3-5. adjustable linear regulator uses ratio resistors determine output voltage.
ERROR PERCENTAGE
Adjustable regulators ratio resistors multiply reference voltage produce desired output voltage (see Figure 3-5). formula output voltage from resistors presented Equation 3-1. (3-1)
VOUT VREF
0.1% OUTPUT VOLTAGE 0.25% 0.5%
basic MIC29512 production-trimmed reference (VREF) with better than accuracy fixed temperature 25°C. guaranteed better than over full operating temperature range, input voltage variations, load current changes. Since practical circuits experience large temperature swings should specification theoretical worst-case. This value assumes error contribution from programming resistors. Referring Figure Equation 3-1, that resistor tolerance (tol) must added
Figure 3-6. Resistor Tolerance Effects Adjustable Regulator Accuracy
output voltage error entire regulator system reference tolerance resistor error contribution. Figure shows this worstcase tolerance MIC29512 output volt-
Designing With Regulators
Section Using Linear Regulators
Micrel Semiconductor
varies from minimum using ±1%, ±0.5%, ±0.25%, ±0.1% resistors. more expensive, tighter accuracy resistors provide improved tolerance, still limited adjustable regulator's internal reference.
0.5% 0.25% 0.1% OUTPUT VOLTAGE
Designing With Regulators
regulator's internal reference. normal configurations, reference error multiplied resistor ratio, keeping error percentage constant. With this circuit, error voltage within 25mV, absolute. Another benefit this arrangement that LM4041 dissipative device: there only small internal temperature rise degrade accuracy. Additionally, both references operating their lowsensitivity range less error contribution from resistors. drawback this configuration that minimum output voltage both references, about 2.5V. adjustable LM4041 available accuracies ±0.5% ±1%, which allows better overall system output voltage accuracy. Equation presents formula LM4041-ADJ output voltage. Note output voltage slight effect reference. Refer LM4040 data sheet full details regarding this second-order coefficient. (3-4)
VREF VLM4041 VOUT 1.233 VOUT
better method possible: increase overall accuracy regulator employing precision reference feedback loop.
ERROR PERCENTAGE
Figure 3-7. Worst-Case Output Tolerance
Improving Regulator Accuracy
Achieving worst-case error ±2.5%, including error terms, possible increasing basic accuracy regulator itself, this expensive since high current regulators have significant self-heating. internal reference must maintain accuracy across wide temperature range. Testing this level performance time consuming raises cost regulator, which unacceptable extremely price-sensitive marketplaces. Some systems require better than accuracy. This high degree accuracy possible using Micrel's LM4041 voltage reference instead programming resistors (refer Figure 3-8). regulator output voltage internal reference LM4041's programmed voltage (Equation 3-3). (3-3) VOUT VREF Regulator VLM4041 1.240 VLM4041
Actually, voltage drop across slightly higher than that calculated from Equation 3-4. Approximately 60nA current flows LM4041 terminal. With large values R1b, this current creates millivolts higher output voltage; best accuracy, compensate reducing size accordingly. This error +1mV with 16.5k. Equation shows nominal output voltage composite regulator Figure (3-5)
1.233 (60nA R1b) 1.240 0.0013R1b 1.0013
VOUT
benefit this circuit increased accuracy possible eliminating multiplicative effect
Note that tolerance effect output voltage accuracy. sets diode reverse (operating) current also allows divider current from pass. With 1.2k, bias flows. small (less than about 105, maximum reverse current LM4041-ADJ exceeded. large with respect then circuit will regulate. recommended range from R1a/10.
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
Designing With Regulators
MIC29512BT MIC29712BT
1.233V 120k
VOUT
LM4041-ADJ
(tolerance critical)
Figure 3-8. Improved Accuracy Composite Regulator Circuit
OUTPUT VOLTAGE
Figure 3-10 shows resistor error contribution LM4041C reference output voltage tolerance. Figure 3-11 shows worst-case output voltage error composite regulator circuit using various resistor tolerances, when 0.5% LM4041C reference employed. four traces reflect 0.5%, 0.25%, 0.1% resistors. Table lists production accuracy obtained with lowcost LM4041C standard resistors well improvement possible with 0.1% resistors.
ERROR PERCENTAGE
RESISTOR
Figure 3-9. Output Voltage (See Figure 3-8)
0.5% 0.25% 0.1% OUTPUT VOLTAGE
Regulator Reference Circuit Performance
With this circuit achieve much improved accuracies. error terms are: 25mV 0.5%
(constant) from MIC29512
from LM4041C from
0.5% 25mV Total Error Budget 2.5% 25mV
Figure 3-10. Resistor Tolerance Effects LM4041 Voltage Reference Accuracy
Designing With Regulators
Section Using Linear Regulators
Micrel Semiconductor
ERROR PERCENTAGE
Designing With Regulators
show accuracy difference between circuits output voltage changes. accuracy difference tolerance two-resistor circuit minus tolerance composite circuit. Both tolerances calculated worst-case value, using resistors. This figure shows composite circuit always least better than standard configuration. Both figure table assume standard resistors LM4041C-ADJ (0.5%) reference.
0.5% 0.25% 0.1%
Accuracy Difference
10.5
11.5
OUTPUT VOLTAGE
OUTPUT VOLTAGE
Figure 3-11. Composite Regulator Accuracy
What does extra complexity composite regulator circuit Figure terms extra accuracy? With precision components, achieve tolerances better than with composite regulator, compared theoretical best case somewhat worse than with standard regulator resistor configuration. Figure 3-12 Table
VOUT
2.50V 2.90V 3.00V 3.30V 3.45V 3.525V 3.60V 5.00V 6.00V 8.00V 10.00V 11.00V
Resistors ±1.54% ±1.88% ±1.94% ±2.07% ±2.12% ±2.14% ±2.16% ±2.36% ±2.41% ±2.46% ±2.49% ±2.49%
0.1% Resistors ±1.50% ±1.41% ±1.39% ±1.34% ±1.31% ±1.30% ±1.29% ±1.13% ±1.07% ±0.98% ±0.92% ±0.90%
Figure 3-12. Accuracy difference between Standard Two-Resistor Circuit Composite Circuit Figure
VOUT
2.50V 3.00V 3.30V 3.50V 5.00V 6.00V 8.00V 10.00V 11.00V
Composite Circuit
±1.6% ±1.9% ±2.1% ±2.1% ±2.4% ±2.4% ±2.5% ±2.5% ±2.5%
Standard Circuit
±3.0% ±3.2% ±3.3% ±3.2% ±3.5% ±3.6% ±3.7% ±3.8% ±3.8%
Table 3-1. Worst-Case Output Voltage Error Typical Operating Voltages Using LM4040C 0.5% Accuracy Version)
Section Using Linear Regulators
Table 3-2. Comparing Worst-Case Output Voltage Error Topologies With Typical Output Voltages
Designing With Linear Regulators
Micrel Semiconductor
Designing With Regulators
very housekeeping power draw. full formula (IGND) (VIN VOUT) IOUT VOUT IOUT
Design Issues General Applications
Noise Noise Reduction
Most output noise caused regulator emanates from voltage reference. While some this noise shunted ground output filter capacitor, bypassing reference high impedance node provides more attenuation given capacitor value. MIC5205 MIC5206 lower noise bandgap reference also provide external access this reference. small value (470pF external capacitor attenuates output noise about 10dB volt output. Micrel's adjustable regulators allow similar technique. shunting voltage programming resistors with small-value capacitor, high frequency gain regulator reduced which serves reduce high frequency noise. capacitor should placed across resistor connecting between feedback output data sheet schematics).
Building Adjustable Regulator Allowing Output
Some power supplies, especially laboratory power supplies power systems demanding wellcontrolled surge-free start-up characteristics, require zero-volt output capability. other words, adjustable laboratory power supply should provide range than includes However, shown Figure 3-13, typical adjustable regulator does facilitate adjustment voltages lower than VREF (the internal bandgap voltage). Adjustable regulator designed output voltages ranging from their reference voltage their maximum input voltage (minus dropout); reference voltage generally about 1.2V. lowest output voltage available from this circuit provided when MIC29152 regulator, 1.240V, VOUT(min) VREF(1+R1/R2), 1.240V.
Typical Regulator (26V) 22µF MIC29152 VREF VADJ 102k COUT 22µF VOUT (1.24 25V)
Stability
dropout linear regulators with output require output capacitor stable operation. Stability Issues Section Linear Regulator Solutions discussion stability with Super regulators. Super more stable than monolithic devices rarely needs much attention guarantee stability. Micrel's Unique "Super LDO", also Section discusses parameters requiring vigilance.
VOUT (max) VREF
Figure 3-13. Typical Adjustable Regulator
designs work around minimum output voltage limitation. first uses low-cost reference diode create "virtual" VOUT that cancels reference. second uses op-amps convince regulator adjust that zero volts proper output level. both cases, feedback-loop summing junction must biased VREF provide linear operation. Reference Generates "Virtual VOUT" Figure 3-14 shows simple method achieving variable output laboratory supply less-than1.2V fixed-output supply. circuit uses second bandgap reference translate regulator's output "virtual VOUT" then uses that virtual VOUT feedback divider. output voltage adjusts from about 20V.
Efficiency
electrical efficiency electronic devices defined POUT PIN. close efficiency approximation linear regulators VOUT
This approximation neglects regulator operating current, very accurate (usually within Super Super regulators with their
Designing With Regulators
Section Using Linear Regulators
Micrel Semiconductor
When goes output about virtual VOUT bandgap voltage above ground, adjust input also bandgap voltage above ground. regulator's error amplifier loop satisfied that both inputs bandgap voltage keeps output voltage constant virtual VOUT tracks increases remaining bandgap voltage above actual VOUT, output rises from ground. maximum possible VOUT equals regulator's maximum input voltage minus approximately housekeeping voltage required current-source external bandgap reference. current source, composed 2N3687 JFET designed about 77µA. Seven microamperes resistor string (about times nominal 60nA input current regulator's adjust input) 70µA bandgap. optional, needed only load present. bleeds 70µA reference current satisfies minimum load current requirement regulator.
2N3697 VIRTUAL VOUT
Designing With Regulators
bottom feedback voltage divider operation identical standard adjustable regulator configuration, shown Figure 3-13 (when adjusted provide maximum output voltage). Conversely, when adjusted input voltage follower taken directly from output amplifier bottom voltage divider biased such that VADJ will equal VREF when VOUT Rotation results smooth variation output voltage from upper design value, which determined
Typical Regulator (26V) 22µF MIC29152 VREF VADJ 102k 100K
LM358
COUT 22µF 102k
VOUT (0V-25V)
VOUT (max) VREF
LM358
LM4041DIM3-1.2 BANDGAP REFERENCE
Figure 3-15. 0V-to-25V Adjustable Regulator
VOUT
MIC29152BT ADJUST 1.24V 180k
Figure 3-14. Adjust Zero Volt Circuit Using Reference Diode
drawback this simple design that voltage internal reference regulator must match external (LM4041) voltage output actually reach zero volts. practice, minimum output voltage from this simple circuit millivolts. Op-Amp Drives Ground Reference circuit Figure 3-15 provides adjustability down controlling ground reference feedback divider. uses regulator's internal bandgap reference provide both accuracy economy. Non-inverting amplifier senses VREF (via VADJ) provides gain just slightly more than unity. When adjusted supply ground voltage follower then ground also applied
gain amplifier 1.05, this example. Note that portion gain above unity reciprocal attenuation ratio afforded feedback divider i.e., provide optimal ratio matching, resistors have been chosen same values types their counterparts respectively.
Systems With Negative Supplies
common start-up difficulty occurs regulator output pulled below ground. This possible systems with negative power supplies. easy shown Figure 3-16: adding power diode, such 1N4001, from regulator output ground (with anode ground). This clamps worst-case regulator output voltage 0.6V 0.7V prevents start-up problems.
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
VMAX
MIC29xxx
Designing With Regulators
VMIN 17.5V 1.1k 3.6k 6.2k 8.87k
+VIN
Split Supply Load
Table 3-3. Component Values Figure 3-17
Figure 3-16. Diode Clamp Allows Start-Up Split-Supply System
Controlling Voltage Regulator TurnOn Surges
When power supply initially activated, inrush current flows into filter capacitors. size this inrush surge dependent upon size capacitors slew rate initial power-on ramp. Since this ramp plays havoc with upstream power source, should minimized. Employing minimum amount capacitance method, this technique does solve general problem. Slew rate limiting power supply good solution general problem. turn-on time interval voltage regulator essentially determined bandwidth regulator, maximum output current current limit), load capacitance. some extent, rise time applied input voltage (which normally quite short, tens milliseconds, less) also affects turn-on time. However, regulator output voltage typically steps abruptly turn-on. Increasing turn-on interval some form slew-limiting decreases surge current seen both regulator system. These applications describe circuitry that changes step-function smoother charge waveform. Various performance differences exist between three circuits that presented. These are: whether stability impacted whether start-up output voltage whether circuit quickly recovers from momentarily interrupted input voltage shorted output. Table summarizes each circuit's features.
High Input Voltages
input voltage ranges above maximum allowed regulator, simple preregulator circuit employed, shown Figure 3-17. preregulator crude regulator which drops extra voltage from source value somewhat lower than maximum input allowed regulator. also helps thermal design distributing power dissipation between elements. preregulator need have good accuracy transient response, since these parameters will "cleaned final regulator.
0.1µF 10µF 200mW 22µF MIC29150-12 +12V
Figure 3-17. Preregulator Allows High Input Supply
Figure 3-17 shows generic circuit. Table provides component values typical application: +12V output With input, required. Above 40V, heat sinking eased power sharing with Note that minimum input voltage also listed; composite regulator enters dropout below this minimum value. Assumptions made include beta 1000 zener diode dissipation 200mW. MIC29150 dissipates maximum 13W; generates less than heat.
Designing With Regulators
Section Using Linear Regulators
Micrel Semiconductor
Circuit Figure 3-18 3-20 3-22 Stability Impacted?
Designing With Regulators
Start-Up Interrupt VOUT Short Pedestal? Recovery? Recovery? 1.2V 1.8V
Table 3-4. Slow Turn-On Circuit Performance Features
Simplest Approach Figure 3-18 illustrates typical voltage regulator, MIC29152, with additional capacitor (CT) parallel with series (R1) feedback voltage divider. Since voltage (VADJ) will maintained VREF regulator loop, output this circuit will still rapidly step VREF (and then rise slowly). Since VREF usually only about 1.2V, this eliminates large part surge current.
Typical Regulator 22µF MIC29152 VREF 100k VADJ 300k 0.33µF COUT 22µF VOUT
Figure 3-19 shows waveforms circuit Figure 3-18. This circuit three shortcomings: approximately 1.2V step turn-on, addition capacitor places zero closedloop transfer function (which affects frequency transient responses potentially cause stability problems) recovery time associated with momentarily short-circuited output unacceptably long3. Improving Simple Approach Figure 3-20 addresses problems potential instability recovery time. Diode added circuit decouple (charged) capacitor from feedback network, thereby eliminating effect closed-loop transfer function. Because non-linear effect being series with there slightly longer "tail" associated with approaching final output voltage turn-on. event momentarily shorted output, diode provides low-impedance discharge path thus assures desired turn-on behavior.
Typical Regulator 22µF VREF 300k MIC29152 VADJ 1N4148 100k 0.33µF COUT 22µF VOUT
Figure 3-18. Simplest Slow Turn-On Circuit
charges, regulator output (VOUT) asymptotically approaches desired value. turnon time milliseconds desired then about three time constants should allowed charge time: 0.3s, 0.1s 300k 0.33µF.
INPUT VOLTAGE
Figure 3-20. Improved Slow Turn-On Circuit
OUTPUT VOLTAGE
Figure 3-21 shows waveforms circuit Figure 3-20. Note that initial step-function output 0.6V higher than with circuit Figure 3-18. This (approximately) 1.8V turn-on pedestal
TIME
Figure 3-19. Turn-On Behavior Circuit Figure 3-18
NOTE This because when output shorted, discharged only short removed before fully discharged regulator output will exhibit desired turn-on behavior.
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
objectionable, especially applications where desired final output voltage relatively low.
INPUT VOLTAGE
22µF
Designing With Regulators
Typical Regulator VREF MIC29152 VCONTROL 240k 1N4148 0.1µF 100k VADJ 300k COUT 22µF VOUT
240k
10µF
1N4001
Figure 3-22. Slow Turn-On Without Pedestal Voltage
OUTPUT VOLTAGE
TIME
Figure 3-21. Turn-On Behavior Figure 3-20
Eliminating Initial Start-Up Pedestal circuits Figures 3-18 3-19 depend upon existence output voltage create VADJ) and, therefore, produce initial step-function voltage pedestals about 1.2V 1.8V, seen Figures 3-19 3-21, respectively. approach Figure 3-22 facilitates placing output voltage origin zero volts because VCONTROL derived from input voltage. reactive component added feedback circuit. value should considerably smaller than assure that junction acts like voltage source driving primary timing control. sufficient current introduced into loop summing junction (via generate VADJ VREF, then VOUT will zero volts. charges VCONTROL decays, which would eventually result VADJ VREF. normal operation, VADJ VREF, VOUT becomes greater than zero volts. process continues until VCONTROL decays VREF 0.6V VOUT reaches desired value. This circuit requires regulator with enable function, (such MIC29152) because small spike generated coincident with application step-function input voltage. Capacitor resistor provide short hold-off timing function that eliminates this spike.
Figure 3-23 illustrates timing this operation. small initial delay (about milliseconds) time interval during which VADJ VREF. Since usually fairly consistent value chosen minimize this delay. Note that calculated based minimum foreseen described below), then higher values will produce additional delay before turn-on ramp begins. Conversely, VIN(max) used calculation then lower values will produce desired turn-on characteristic; instead, there will small initial step-function prior desired turn-on ramp. Recovery from momentarily shorted output addressed this circuit, interrupted input voltage handled properly. Notice that buildup regulator output voltage differs from waveforms Figures 3-19 3-21 that more ramp-like (less logarithmic). This because only initial portion charge waveform used; i.e., while VCONTROL VREF 0.6V. actual time constant used Figure 3-22 0.33 second, second. shown Figure 3-23, this provides about milliseconds ramp time, which corresponds first capacitor charge curve. calculated follows: turn-on time force VADJ 1.5V (just slightly higher than VREF) then
ICONTROL 1.5V
0.6V ICONTROL
Since MIC29152 low-dropout regulator, chosen VIN(min). This corresponds small (approximately 40msec) delay before out-
Designing With Regulators
Section Using Linear Regulators
Micrel Semiconductor
begins rise. With input initial delay considerably more noticeable.
INPUT VOLTAGE
Designing With Regulators
Super Current Source adjustable Super LDOs, MIC5156 MIC5158, feature linear current limiting. This referenced internal 35mV source. simple, high efficiency, high output current source built (Figure 3-25). Current source compliance excellent, ranging from zero volts dropout, which only IOUT (ON) 35mV (generally only hundred millivolts even 10A). Output current IOUT 35mV This circuit suffers from relatively poor accuracy, however, since 35mV threshold production trimmed. allow clamping output voltage maximum value, desired.
OUTPUT VOLTAGE
TIME
Figure 3-23. Turn-On Behavior Figure 3-22
MIC5158
Current Sources
Another major application voltage regulators current sources. Among other uses, most rechargeable batteries need some type constant current chargers. Simple Current Source Several techniques generating accurate output currents exist. simplest uses single resistor ground return lead (Figure 3-24). This technique works with Micrel adjustable regulators except MIC5205 MIC5206. output current VREF drawback this simple circuit that power supply ground load ground common. Also, compliance ranges from only VOUT (VDO VREF).
1.240V IOUT 1.240
Micrel Adjustable Regulator
IOUT
Figure 3-25. Simple Current Source Using Super
Accurate Current Source Using Amps High accuracy maintaining common ground both possible with alternative circuit using amps current MOSFET (Figure 3-26). This technique works with Micrel adjustable regulators except MIC52xx series. Compliance from VDO.
IOUT
Load
Low-Cost Power Supply
Taking advantage low-dropout voltage capability Micrel's regulators, build dual output linear power supply with excellent efficiency using cost 12.6V center-tapped "filament" transformer. Figure 3-27 shows schematic simple power supply. Using single center-tapped transformer bridge rectifier, both outputs available. Efficiency high because transformer's output voltage only slightly above desired outputs. 12.6V center tapped
Figure 3-24. Simple Current Source Uses Reference Resistor Return
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
Designing With Regulators
68µF
MIC29152
1N4148 1.24V +VIN
100m 100k 330µF
IOUT
MIC6211
Reduce
1000pF +VIN
1.240
0.01µF IOUT
IOUT
1.240
MIC6211
VN2222
1.24k
Figure 3-26. Current Source Using Pair Op-Amps
filament transformer decades-old design originally used powering vacuum tube heaters. perhaps most common transformer made. outside windings feed bridge rectifier filter capacitor output. MIC29150-12 produces regulated output. transformer center feeds filter capacitor MIC291505.0 directly-no rectifier diode needed.
This circuit scaled other output currents desired. Overall efficiency extremely high input voltage, heat sinking requirements minimal. final benefit: since power tabs TO-220 packages ground potential, single non-isolated, non-insulated heat sink used both regulators.
Input
MIC29150-12
12.0V
MIC29150-5.0 12.6V Filament Transformer
5.0V
Figure 3-27. Dual-Output Power Supply From Single Transformer Bridge Rectifier
Designing With Regulators
Section Using Linear Regulators
Micrel Semiconductor
Designing With Regulators
jumper-selected resistors. They fast starting, optionally provide ON/OFF control error flag that indicates power system trouble.
Computer Power Supplies
decreasing silicon geometries microprocessors memory have forced reduction operating voltage from longtime standard This rise sub-5V microprocessors, logic, memory components personal computer systems created demand lower voltage power supplies. Several options exist desktop computer system designer. these options provide both 3.3V 5.0V from main system power supply. Another existing high current supply employ dropout (LDO) linear regulator provide 3.3V. low-cost, production proven desktop computer power supplies output ±12V-but Redesigning system power supply would increase cost break long standing power supply motherboard connector standard which provision Further complicating matters that "3V" really defined. Microprocessor manufacturers produce devices requiring 2.9V, 3.3V, 3.38V, 3.45V, 3.525V, 3.6V, several other similar voltages. single standard been adopted. Designing stocking dedicated power supplies these different voltages would extremely difficult expensive. Also, motherboard makers want maximize their available market allowing many different microprocessors possible each board; this means they must design on-board supply that produces most popular voltages remain competitive. This even more important motherboard vendors sell boards sans-microprocessor. They must only provide expected voltages, they must simplify selection process that system integrators-and even some users- configure voltage properly. With operating voltage, microprocessor will generate errors; high voltage fatal. Instead, system integrators motherboards with on-board power supply, which converts readily available source into required voltage output. simplest, lowest cost solution this problem modern, very dropout version venerable linear regulator. This cost option, requiring only quick design work little motherboard space. Linear regulators provide clean, accurate output radiate RFI, government certification jeopardized. Adjustable linear regulators allow voltage selection means
Dropout Requirements
While linear regulators extremely easy use, design factor must considered: dropout voltage. example, regulator with volts dropout producing 3.3V output requires over volts input. Furthermore, reliable circuit operation requires operating linear regulator above dropout region-in other words, with higher than minimum input voltage. dropout, regulator regulating responds sluggishly load changes. What required dropout voltage performance? Let's assume have supply need provide 3.525V microprocessor. worst case occurs when input voltage from supply minimum output maximum. example will illustrate. 4.75V VOUT 3.525V 3.60V
Maximum Allowable Dropout Voltage:
1.15V
This simplified example does include effects power supply connector, microprocessor socket, board trace resistances, which would further subtract from required dropout voltage. Fast response load current changes (from processor recovering from "sleep" mode, example) occurs only when regulator away from dropout point. real systems, maximum dropout voltage between 0.6V ideal. Achieving this performance means output device must either bipolar transistor MOSFET. Historically, linear regulators with outputs have been expensive limited current applications. However, Super dropout regulators provide amperes current with dropout voltages less than 0.6V, guaranteed. lower cost product line outputs same currents with only dropout. These dropout voltages guarantee microprocessor gets clean, well regulated supply that quickly reacts processor-induced load changes well input supply variations. dropout linear voltage regulator easy-to-use, cost, high performance means
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
powering high performance voltage microprocessors. selecting modern dropout regulator, assure reliable operation under working conditions.
Designing With Regulators
Method MIC5156 "Super LDO" Micrel MIC5156 linear regulator controller that works with cost N-Channel power MOSFET produce very dropout regulator system. MIC5156 available small 8-pin SOIC standard 8-pin DIP, offers fixed 3.3V, 5.0V, user selectable (adjustable) voltage outputs. Figure shows entire schematic-two filter capacitors, MOSFET, printed circuit board trace about centimeter long (used current sense resistor) need fixed voltage version. adjustable part, resistors. MIC5156 requires additional power supply provide gate drive MOSFET: your PC's supply-the current drawn from supply very small; approximately milliampere. supply available, MIC5158 generates bias does need additional supply. Figure 3-30 shows typical 3.3V computer power supply application. MIC5156 provides regulated 3.3V using pass element also controls MOSFET switch supply.
+12V 0.1µF
Enable Shutdown
3.xV Conversion Circuits
Recommended circuits on-board desktop computer power supplies follow. high speed load changes common microprocessors, fast load transient response crucial. This means circuit layout bypass filter capacitor selection also critical. current levels, thermal considerations difficult; however, currents above amperes, resulting heat troublesome. Method Monolithic simplest method providing second computer motherboard using monolithic regulator. required voltage standard value, fixed-voltage regulator available. this ideal situation, your electrical design consists merely specifying suitable output filter capacitor. output voltage available from fixed regulator, adjustables used. They resistors program output voltage otherwise similar fixed versions. Figure 3-28 3-29 show fixed adjustable regulator applications.
MIC29710 VOUT
FLAG
MIC5156-3.3
Figure 3-28. Fixed Regulator Circuit Suitable Computer Power Supply Applications
MIC29712
47µF 0.035V ILIMIT
47µF
VOUT 3.3V,
SMP60N03-10L
Improves transient response load changes
VOUT
Figure 3-30. MIC5156 5V-to-3.3V Converter
When 3.3V output reached regulation, FLAG output goes high, enhancing which switches Load This circuit complies with requirements some dual-voltage microprocessors that require supply input remain below 3.0V until 3.3V supply input greater than 3.0V. optional current limiting sense resistor (RS) limits load current maximum. less costly designs, sense resistor's value function duplicated using techniques: solid piece copper wire with appropriate length di39 Section Using Linear Regulators
VOUT 1.240
Figure 3-29. Adjustable Regulator Circuit Suitable Computer Power Supply Applications
Designing With Regulators
Micrel Semiconductor
ameter (gauge) makes reasonably accurate lowvalue resistor. Another method uses printed circuit trace create sense resistor. resistance value function trace thickness, width, length. Alternative Resistors, Section current sense resistor details. NOTE: power MOSFET connected +5V. insulator between MOSFET heat sink, necessary. Method MIC5158 "Super LDO" Like MIC5156, MIC5158 linear regulator controller that works with cost N-Channel power MOSFET produce very dropout regulator system. MIC5158, however, generates bias voltage required drive N-channel MOSFET does require supply. on-board charge pump uses three capacitors takes care level shifting. Figure 3-31 shows MIC5158 circuit. idea motherboard manufacturer: build MIC5158 circuit plug-in daughterboard with three five pins that allow mount system board like monolithic regulator.
0.1µF
Designing With Regulators
down protection, requires numerous external components. recommended.
+4.75 5.25V Pass Element (TIP127 D45H8) +VOUT (3.3V 3.83V 0.1µF 0.1µF 0.1µF
+VIN LP2951 Feedback
680µF
VOUT
158k 95.3k
VOUT 1.235V
Figure 3-32. Transistor Boosts Current Output From MIC2951 Regulator
Adjust Resistor Values
Table shows recommended resistor values various voltages. values shown represent calculated closest-match desired voltage using standard tolerance resistors. Since Micrel's adjustable regulators high impedance feedback stage, large value adjust resistors generally recommended. Valid resistor values range from ohms about 500k. While MIC29152/29302/29502 have 1.240V reference, Super current boosted MIC2951 circuits 1.235V reference. Figs. 3-28 1.240V) Voltage 80.6k 16.9k 237k 107k 2.85 287k 221k 162k 121k 102k 71.5k 158k 105k 3.15 191k 124k 196k 118k 3.45 221k 124k 102k 53.6k 221k 107k 255k 115k 316k 137k 137k 52.3k Figs. 3-30, 1.235V) 53.6k 11.5k 301k 137k 187k 143k 137k 102k 150k 105k 154k 102k 158k 102k 178k 107k 191k 107k 383k 200k 221k 107k 115k 51.1k 232k 100k 107k 40.2k
3.3µF
FLAG
MIC5158
0.1µF (5V) 47µF
IRLZ44
COUT 47µF
VOUT 3.3V, 17.8k, 10.7k,
IRFZ44.
Figure 3-31. MIC5158 5V-to-3.3V Converter
Method Current Boost MIC2951 150mA MIC2951 gets capacity boost several amperes using external transistor. Figure 3-32 shows MIC2951 driving DH45H8 equivalent transistor achieve output. This circuit number problems, including poor stability large output capacitor required squelch oscillations), poor current limiting characteristics, poor load transient response, overtemperature shut-
Table 3-5. Suggested Adjust Resistor Values
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
Designing With Regulators
MIC29712 VOUT 49.9k 0.1µF 93.1k VOUT 3.525V nominal 330µF TPSE337M006R0100 tantalum
3.3V 2.xV Conversion
Like 3.3V conversion discussed above, dropping voltages below 3.3V from 3.3V rail useful application regulators. Here, regulator dropout voltage much more critical. Applications using 2.9V only have 400mV headroom when powered from perfect 3.3V supply. standard 3.3V supply tolerance ±300mV, headroom drops only 100mV. this situation, most reasonable solution Super circuits shown Figures 3-30 3-31. These circuits feature excellent efficiency-approximately 88%. Monolithic solutions powered from standard 3.3V 300mV supply become tenable with output voltages 2.5V below.
VOUT load (not shown): Intel® Power Validator
Figure 3-33. Load Transient Response Test Circuit. Super System Driving Intel Pentium "Validator" Test System
MIC29512 Load Transient Response (See Test Circuit Schematic)
Improving Transient Response
Modern low-voltage microprocessors have multiple operating modes maximize both performance minimize power consumption. They switch between these modes quickly, however, which places strain their power supply. Supply current variations several orders magnitude tens nanoseconds standard some processors-and they still require that their supply voltage remain within specification throughout these transitions. Micrel low-dropout regulators have excellent response variations input voltage load current. virtue their dropout voltage, these devices saturate into dropout readily similar NPN-based designs. 3.3V output Super will maintain full speed performance with input supply 4.2V, will still provide some regulation with supplies down 3.8V, unlike devices that require 5.1V more good performance become nothing more than resistor under 4.6V input. Micrel's regulators provide superior performance 3.3V" conversion applications, especially when tolerances considered. Figure 3-33 test schematic using Intel® PentiumValidator. Validator dynamic load which simulates Pentium processor changing states high speed. Using Figure 3-33, MIC29512 (Figure 3-34) tested with fast 200mA load transitions. MIC29712 tested with fast transitions between 200mA 7.5A (Figure 3-35).
OUTPUT VOLTAGE
+20mV 3.525V -20mV 1ms/division
LOAD CURRENT
200mA
Figure 3-34. MIC29512 Load Transient Response
MIC29712 Load Transient Response (See Test Circuit Schematic)
OUTPUT VOLTAGE
+50mV 3.525V -50mV 1ms/division 200mA
Figure 3-35. MIC29712 Load Transient Response. Load Varies from 200mA 7.5A
Designing With Regulators
LOAD CURRENT
Section Using Linear Regulators
Micrel Semiconductor
following photographs show transient response MIC5156 Super with IRL3103 power MOSFET (RDS (ON) 14m, Ciss 1600pF) driving Intel PentiumValidator. Figure 3-36 shows performance with four 330µF surface mount capacitors. peak transient response voltage -55mV attack +60mV turn-off. Figure 3-37 shows tremendous improvement another four 330µF capacitors make: with eight 330µF capacitors, transient peaks drop only approximately ±25mV. These measurements made with 5.0V, 12.0V, single 330µF bypass capacitor input MIC5156. both 5156 MIC5158 same error amplifier circuit, their transient response should similar. Furthermore, transient response MIC5156 does change input voltage (VDD) decreases from 5.0V down nearly dropout levels less than 3.6V input with 3.525V output).
Designing With Regulators
Accuracy Requirements
Microprocessors have various voltage tolerance requirements. Some happy with supplies that swing full ±10%, while others need better than ±2.5% accuracy proper operation. Fixed 3.3V devices operate well with these microprocessors, since Micrel guarantees better than across operating load current temperature ranges. Locating regulator close processor minimize lead resistance inductance only design consideration that necessary. Microprocessors that nonstandard varying voltages have problem: while basic adjustable regulator offers accuracy worst case over temperature extremes, error external programming resistors (either tolerance compromise resistance ratio that unavoidable when using standardized resistor values) directly appears output voltage error. error budget quickly disappears. Adjustable Regulator Accuracy Analysis, this section, discussion voltage tolerance sensitivity. When trace resistance effects considered, painfully apparent that this solution will provide needed ±2.5% accuracy. Resistors 0.1% tolerance step. Other ideas presented Improving Regulator Accuracy, also this section.
Figure 3-36. Transient response MIC5156 Super driving Intel Pentium "Validator" microprocessor simulator. Output capacitance 330µF.
Figure 3-37. Transient response MIC5156 Super driving Intel Pentium "Validator" microprocessor simulator. Output capacitance 330µF.
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
Designing With Regulators
Input Output
Multiple Output Voltages
Another design parameter computer motherboard designers cope with need support different types microprocessors with layout. Since processors single family require different voltages, surprise that different processor types also need various supply voltages. Since expensive provide multiple variable outputs from system power supply, economical solution this problem generate switch between supplies directly motherboard. Occasionally, designer will lucky some motherboard options standard voltage from power supply. this case, switch higher voltage around generating lower voltage, shown Figure 3-38. This circuit designed allow Intel DX4ProcessorsTM, running 3.3V, operate same socket standard 486. DX4Processor hard wired ground, which provides switching needed automatically selecting supply voltage. Standard processors have connection this pin.
N-channel MOSFET
Source
4.7µF
MIC29302
ENABLE
(3A) 300k 47µF
ON/OFF (Optional)
220k
Voltage Selection Input High Open 3.3V
330k 2N2222 equiv.
180k
Figure 3-39. Adjustable analog switch provides selectable output voltages
Another method providing more output voltages socket with higher provided using Super LDO. Program adjustable MIC5156 MIC5158 shown Figure 340. When higher voltages chosen, regulator simply acts low-loss switch. transistor switch select lower voltage. This technique expanded number discrete voltages, desired. MIC5158 will operate from single input supply 3.0V greater. MIC5156 needs current supply provide gate bias pass MOSFET, this available, smaller than MIC5158 requires charge pump capacitors.
(+5V) ENABLE 47µF 12.1k MIC5158 "Super LDO" VOUT 0.1µF 0.1µF 10µF
100k
Input
MIC5014
Gate
Voltage Selection Input
High Open 3.3V
Input
Output
MIC29300-3.3
47µF
n.c. n.c.
16.9k
Figure 3-38. Switching 3.3V Microprocessor
This circuit capitalizes reversed-battery protection feature built into Micrel's Super regulators. regulators survive voltage forced their output that higher than their programmed output. this situation, regulator places pass transistor high impedance state. Only microamperes current leaks back into regulator under these conditions, which should negligible. Note that adjustable regulator could used place fixed voltage version shown. adjustable regulator analog switch will perform this task, shown Figure 3-39. Only supply maximum desired output voltage, higher) necessary.
330k 2N2222 equivalent
open) 3.3V High
Figure 3-40. MIC5158 with Selectable Output Voltages
Figure 3-41 switched voltage regulator that relies jumpers output voltage programming. While perhaps "elegant" previous techniques, provides full functionality flexibility. This circuit designed jumpers accidentally removed, output voltage drops lowest value. configuring jumpers shown, system relatively safe-if someone inadvertently removes
Designing With Regulators
Section Using Linear Regulators
Micrel Semiconductor
Designing With Regulators
Multiple Supply Sequencing
MIC29302BT
4.75V 5.25V 2.2µF
10µF
VOUT Microprocessor 2.90V 3.53V
3.38 3.45
Some microprocessors multiple supply voltages; voltage core, another cache memory, different I/O, example. Sequencing these supplies critical prevent latch-up. Figure 3-42 shows easy guaranteeing this sequencing using Micrel's regulators with enable control. output voltage Supply rises above regulator Supply starts Supply will never high until Supply active. Supply need higher output voltage; must only 2.4V above (necessary assure second regulator fully enabled). Note that Supply need enable pin. This technique works with MIC29151 through MIC29752 monolithic regulators well with Super (MIC5156/57/58). also applicable systems requiring number sequenced supplies, although simplicity only show supplies here.
3.30
3.53
2.90V
3.38V 3.38 3.30V 3.30 3.38
3.45V 3.30 3.45 3.38 3.53V 3.30 3.38
Voltage Jumper Positions
Thermal Design
Once electrical design your power system complete, must deal with thermal issues. While they terribly difficult, thermal design lightly covered most electrical engineering curriculum. Properly addressing thermal issues imperative system reliability, covered detail Thermal Management, later this section.
Figure 3-41. Jumper Selectable Output Voltages
jumpers, output voltage drops value. While system error-prone nonfunctional with this voltage, least microprocessor will survive.
47µF
MIC29712 VOUT 220µF
Supply 3.3V 7.5A
205k 124k
MIC29512 VOUT
Supply 2.5V 220µF (Sequenced After Supply
127k 124k VOUT 1.240 R1/R2)
Figure 3-42. Multiple Supply Sequencing
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
Designing With Regulators
Output Noise Requirement Cellular telephones, pagers, other radios have frequency synthesizers, preamplifiers, mixers that susceptible power supply noise. frequency synthesizer voltage controlled oscillator (VCO), block that determines operating frequency, produce noisy sine wave output wider bandwidth signal) noise present VCC. Making matters worse portable equipment designers, lower powered/lower cost VCOs generally more susceptible noise. Ideal VCOs produce single spectral line operating frequency. Real oscillators have sideband skirts; poor devices have broad skirts. Figure 3-43 shows measured phase noise from free running Murata MQE001-953 powered MIC5205 low-noise regulator. Note significant improvement when using noise bypass capacitor. Regulators optimized noise performance produce skirts similar worse than MIC5205 without bypass capacitors. Broad oscillator skirts decrease noise figure strong signal rejection capability receivers (reducing performance) broaden transmitted signal transmitters (possibly violation spectral purity regulations).
Capacitor 47pF Bypass
Portable Devices
Voltage regulators necessary almost electronic equipment, portable devices exception. Portable equipment includes cellular "wireless" telephones, radio receivers handheld transceivers, calculators, pagers, notebook computers, test equipment, medical appliances most other battery operated gear.
Design Considerations
Portable electronics characterized major distinguishing features: Small size Self-contained power source (batteries) Beyond these similarities, portable equipment power requirements vary much their intended application. Small Package Needed Portable devices are, definition, relatively small lightweight. Most circuitry surface mounted power dissipation normally minimized. Self Contained Power Most portable equipment battery powered. Batteries often largest heaviest component system, account more total volume mass portable device. Power conservation important design consideration. power components used power management techniques, such "sleep mode", help maximize battery life. Just never rich, one's batteries never last long enough! another battery-imposed limitation that batteries available discrete voltages, determined their chemical composition. Converting these voltages into constant supply suitable electronics regulator's most important task. Current (And Voltage) regulators used portable equipment usually output current devices, generally under 250mA, since their loads also (usually) current. portable devices have high voltage loads4 those that need little current.
0.25
2.25
4.25
6.25
8.25
10.25
12.25
14.25
16.25
18.25
20.25
22.25
-23.75
-21.75
-19.75
-17.75
-15.75
-13.75
-11.75
Frequency Offset from Carrier (kHz)
Figure 3-43. Low-Noise (MIC5205) Reduces Phase Noise
Although susceptible noise VCOs, preamplifiers mixers operating from noisy supplies also reduce receiver transmitter performance similar ways.
NOTE notable exceptions this statement fluorescent backlights notebook computers electroluminescent lamps telephones, watches, etc. These lamps must driven with switching regulator that boosts battery voltage-something linear regulator cannot
Designing With Regulators
Section Using Linear Regulators
24.25
-9.75
-7.75
-5.75
-3.75
-1.75
Micrel Semiconductor
Dropout Battery Life dropout regulators allow more operating lifetime from batteries generating usable output load well after standard regulators would saturated. This allows discharging batteries lower levels or-in many cases-eliminating cell from series string. Compared older style regulators with dropout, Micrel's 0.3V 0.6V LDOs allow eliminating alkaline, NiCd, NiMH cells. Ground Current Battery Life quiescent, ground, current regulators employed inside portable equipment also important. This current another load battery, should minimized.
Designing With Regulators
zero current.5 Designers updating older systems that used MOSFETs switching power regulators eliminate MOSFET. regulator serves switch, voltage regulator, current limiter, overtemperature protector. important features type portable equipment. Power Sequencing technique related Sleep Mode Switching Power Sequencing. This power control technique that enables power blocks short while then disables them. example, cellular telephone awaiting call, receiver power pulsed low-to-medium duty cycle. listens milliseconds each hundred milliseconds.
Multiple Regulators Provide Isolation
close proximity between different circuit blocks naturally required portable equipment increases possibility interstage coupling interference. Digital noise from microprocessor interfere with sensitive receiver preamplifier, example. common path this noise common supply bus. Linear regulators help this situation providing active isolation between load input supply. Noise from load that appears regulator's output greatly attenuated regulator's input. Figure 3-44 shows simplified block diagram cellular telephone power distribution system. Between five seven regulators used typical telephone, providing regulation, ON/OFF (sleep mode) switching, active isolation between stages.
NOTE real world, there such thing zero, Micrel's regulators pass only nanoamperes device leakage current when disabled-"virtually zero" current.
Battery Stretching Techniques
Sleep Mode Switching Sleep mode switching important technique battery powered devices. Basically, sleep mode switching powers down system blocks immediately required. example, while cellular phone must monitor incoming call, transmitter needed should draw power; shut off. Likewise, audio circuits powered down. Portable computers sleep mode switching spinning down hard disk drive powering down video display backlight, example. Simpler devices like calculators automatically turn after certain period inactivity. Micrel's regulators make sleep mode implementation easy because each family version with logic-compatible shutdown control. Many families feature "zero power" shutdown-when disabled, regulator fully powers down draws virtually
Power Switch
MIC5203
Microcontroller
MIC5207
Power
MIC5203
Audio, etc.
MIC5203
RF/IF Stages
MIC5205
Figure 3-44. Cellular Telephone Block Diagram
Section Using Linear Regulators Designing With Linear Regulators
Micrel Semiconductor
Designing With Regulators
Thermal resistance, heat sink ambient (free air) Ambient temperature Junction (die) temperature
Thermal Management
Thermal Primer
Micrel dropout (LDO) regulators very easy use. Only external filter capacitor necessary operation, electrical design effort minimal. many cases, thermal design also quite simple, aided dropout characteristic Micrel's LDOs. Unlike other linear regulators, Micrel's LDOs operate with dropout voltages 300mV-often less. resulting Voltage Current power loss quite small even with moderate output current. higher currents and/or higher input-to-output voltage differentials, however, selecting correct heat sink essential "chore".
Heat Sink
Package (case)
(junction)
TJ(MAX) Maximum allowable junction temperature Figure 3-46 shows thermal terms they apply linear regulators. "junction" "die" active semiconductor regulator; this heat source. package shown standard TO-220; "case" metal forming back package which acts heat spreader. heat sink interface between package ambient environment. Between each element-junction, package, heat sink, ambient-there exists interface thermal resistance. Between package junction case thermal resistance, Between package heat sink case-to-sink thermal resistance, between heat sink external surroundings heat sink ambient thermal resistance, total path from ambient
Heat Sink Ambient
Package (case)
(junction)
Figure 3-45. Regulator Mounted Heat Sink
Thermal Parameters Before working with thermal parameters, will define applicable symbols terms. Temperature difference, Heat flow (Watts) Thermal resistance (°C/W) Power Dissipation (Watts) Thermal resistance, junction (die) ambient (free air) Thermal resistance, junction (die) package (case) Thermal resistance, case (package) heat sink
Section Using Linear Regulators
rise
(temperature
Figure 3-46. Thermal Parameters
Thermal/Electrical Analogy those more comfortable with laws Kirchhoff than those Boyle Celsius, electrical metaphor simplifies thermal analysis. Heat flow current flow have similar characteristics. Table shows general analogy.
Designing With Regulators
Micrel Semiconductor
Thermal Parameter Power Thermal Resistance Temperature Difference Electrical Parameter Current Resistance Voltage
Designing With Regulators
This serves limit maximum heat sink size possible. Parameter TJ(MAX) Extenuating Circumstances heat sink size, design flow regulator size package type mounting technique package type regulator manufacturer lifetime considerations VIN, VOUT, IOUT
Table 3-6. Thermal/Electrical Analogy
formula constant heat flow equivalent electrical (Ohm's Law) form Electrically, voltage difference across resistor produces current flow. Thermally, temperature gradient across thermal resistance creates heat flow. From this, deduce that dissipate power heat need minimize temperature rise, must minimize thermal resistance. Taken another way, have given thermal resistance, dissipating more power will increase temperature rise. Thermal resistances like electrical resistances: series, they add; parallel, their reciprocals resulting inverted. general problem heat sinking power semiconductors simplified following electrical schematic (Figure 3-47).
Heat Flow Ambient
Power dissipation
Each regulator data sheet specifies junction case thermal resistance, Heat sink manufacturers specify (often graphically) each product. generally small compared maximum temperature Micrel regulators generally 125°C, unless specified otherwise data sheet. last remaining variable regulator power dissipation. Power dissipation linear regulator [(VIN VOUT) IOUT] (VIN IGND) Where: Power dissipation Input voltage applied regulator VOUT Regulator output voltage IOUT Regulator output current IGND Regulator biasing currents Proper design dictates worst case values parameters. Worst case high supply. Worst case VOUT thermal considerations lowest possible output voltage, subtracting tolerances from nominal output. IOUT taken highest steady-state value. ground current value comes from device's data sheet, from graph IGND IOUT.
Figure 3-47. Heat flow through interface resistances.
Summing these resistances, total thermal path heat generated regulator
Calculating Thermal Parameters
types thermal parameters exist; those control those fixed application physics). application itself determines which category parameters fit-some systems have specific form factor dictated other factors, example.
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
Calculating Maximum Allowable Thermal Resistance Given power dissipation, ambient operating temperature, maximum junction temperature regulator, maximum allowable thermal resistance readily calculated. (TJ(MAX) Maximum heat sink thermal resistance calculate thermal resistance (SA) required heat sink using following formula: --------
Designing With Regulators
Where: failure rate temperature (Kelvin) failure rate temperature MTTF1 mean time failure MTTF2 mean time failure activation energy electron volts (eV) Boltzmann's constant (8.617386 10-5 eV/K) activation energy determined long-term burn-in testing. average value 0.62eV determined, after considering temperature-related failure mechanisms, including silicon-related failure modes packaging issues, such attach, lead bonding, package material composition. Using reference temperature 125°C (498K) normalizing FITs, formula becomes:
0.62
Maximum Junction Temperature?
semiconductors, including regulators, have maximum junction temperature (TJ)? Heat natural enemy most electronic components, regulators exception. Semiconductor lifetimes, statistically specified mean time failure (MTTF) reduced significantly when they operated high temperatures. junction temperature, temperature silicon itself, most important temperature this calculation. Device manufacturers have this lifetime-versus-operating temperature trade-off mind when rating their devices. Power semiconductor manufacturers must also deal with inevitable temperature variations across surface, which more extreme wider temperature-range devices. Also, mechanical stress induced semiconductor, package, bond wires increased temperature cycling, such that caused turning equipment off. regulator running lower maximum junction temperature smaller temperature change, which creates less mechanical stress. expected failure rate under operating conditions very small, expressed FITs (failures time), which defined failures billion device hours. Deriving failure rate from operating life test temperature actual operating temperature performed using Arrhenius equation:
MTTF2 MTTF1
standard semiconductor reliability versus junction temperature characteristic shown Figure 3-48. that device operating 125°C relative lifetime 100. each 15°C rise junction temperature, MTTF halves. 150°C, drops about other hand, 100°C, life more than tripled, 70°C, 1800. designer equipment using LDOs, most important rule remember "cold cool; not". Minimizing regulator temperatures will maximize your product's reliability.
Arrhenius Plot
1x109 1x108
RELATIVE LIFETIME
1x107 1x106 1x105 1x104 1x103 1x102 1x101
JUNCTION TEMPERATURE (°C)
Figure 3-48. Typical MTTF Temperature Curve
Designing With Regulators
Section Using Linear Regulators
Micrel Semiconductor
Designing With Regulators
Heat Sink Charts High Current Regulators
heat sink plays important role high current regulator systems, directly affects safe operating area (SOA) semiconductor. following graphs, Figure 3-49 through 3-53, show maximum output current allowable with given heat
MIC29150
Infinite Sink
sink different input-output voltages ambient temperature 25°C. Three curves shown: heat sink, nominal heat sink, infinite heat sink Additional thermal design graphs appear Section
MIC29500/29510
Infinite Sink
OUTPUT CURRENT OUTPUT CURRENT
Heat Sink
Heat Sink VOUT VOUT
Figure 3-49. Maximum Output Current With Different Heat Sinks, MIC29150 Series
MIC29310
Infinite Sink
Figure 3-51. Maximum Output Current With Different Heat Sinks, MIC29500 Series
MIC29710
Infinite Sink
OUTPUT CURRENT OUTPUT CURRENT
8°C/W
5°C/W
Heat Sink VOUT
Heat Sink VOUT
Figure 3-50. Maximum Output Current With Different Heat Sinks, MIC29300 Series
Figure 3-52. Maximum Output Current With Different Heat Sinks, MIC29710/MIC29712
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
MIC29750
Infinite Sink
Designing With Regulators
Performing similar calculations 1.25A, 1.5A, 2.0A, 2.5A, 3.0A, 4.0A, 5.0A gives results shown Table 3-7. choose smallest regulator required current level minimize cost. Regulator IOUT 1.25A 1.5A 2.0A 2.5A 3.0A 4.0A 5.0A 10.5 SA(°C/W)
OUTPUT CURRENT
MIC29150 MIC29150
MIC29300 MIC29300 MIC29300 MIC29500
Heat Sink
VOUT
MIC29500
Figure 3-53. Maximum Output Current With Different Heat Sinks, MIC29750/MIC29752
Table 3-7. Micrel power dissipation heat sink requirements various 3.3V current levels.
Table shows effect maximum ambient temperature heat sink thermal properties. Lower thermal resistances require physically larger heat sinks. table clearly shows cooler running systems need smaller heat sinks, common sense suggests. Output Ambient Temperature 40°C 1.5A 24°C/W 5.1°C/W 50°C 21°C/W 4.1°C/W 60°C 17°C/W 3.2°C/W
Thermal Examples
Let's example. need design power supply voltage microprocessor which requires 3.3V will input from supply. choose MIC29300-3.3BT regulator. worst case high supply; this case, 5.25V. maximum temperature 125°C TO-220 package with 2°C/W mounting resistance (CS) 1°C/W2, will operate ambient temperature 50°C. Worst case VOUT thermal considerations minimum, 3.3V 3.234V.5 IOUT taken highest steady-state value. ground current value comes from device's data sheet, from graph IGND IOUT. Armed with this information, calculate thermal resistance (SA) required heat sink using previous formula: 50°C ---------- 1°C/W) °C/W 10.5W
Table 3-8. Ambient Temperature Affects Heat Sink Requirements
Although routine, these calculations become tedious. program written calculator available from Micrel that will calculate above parameters ease your design optimization process. will also graph resulting heat sink characteristics versus input voltage. Appendix program listing send e-mail Micrel apps@micrel.com request program "LDO SINK HP48".
NOTE Most Micrel regulators production trimmed better than accuracy under standard conditions. Across full temperature range, with load current input voltage variations, device output voltage varies less than ±2%.
Designing With Regulators
Section Using Linear Regulators
Micrel Semiconductor
Designing With Regulators
convection, sinks sizable, 1.5A (3.2W worst case package dissipation) feet/minute airflow, modest heat sinks adequate. Output Current
Airflow ft./min. (2m/sec) ft./min. (1.5m/sec) 1.5A Thermalloy 6049PB Thermalloy 6232 Thermalloy 6034 Thermalloy 6391B AAVID 504222B AAVID 563202B AAVID 593202B AAVID 534302B Thermalloy 7021B Thermalloy 6032 Thermalloy 6234B AAVID 577002 Thermalloy 6043PB Thermalloy 6045B AAVID 508122 AAVID 552022 AAVID 533302 Thermalloy 7025B Thermalloy 7024B Thermalloy 7022B Thermalloy 6101B
ft./min. (1m/sec)
Figure 3-54. "LDO SINK" Calculator Program Eases Tedious Thermal Calculations (See Appendix
Heat Sink Selection
With this information specify heat sink. worst case still (natural convection). heat sink should mounted that least 0.25 inches (about 6mm) separation exists between sides sink other components system case. Thermal properties maximized when heat sink mounted that natural vertical motion warm directed along long axis sink fins. fortunate enough have some forced airflow, reductions heat sink cost space possible characterizing speed-even slow stream significantly assists cooling. with natural convection, small allowing stream pass necessary. Fins should located maximize airflow along them. Orientation with respect vertical very important, airflow cooling dominates natural convection. example, will select heat sinks 1.5A outputs. consider four airflow cases: natural convection, feet/minute (1m/sec), feet/ minute (1.5m/sec), feet/minute (2m/sec). Table shows heat sinks these velocities; note rapid reduction size weight (fin thickness) when forced available. Consulting manufacturer's charts, variety sinks made that suitable application. (10.5W worst case package dissipation) natural
Natural Convection forced airflow)
AAVID 576000 AAVID 533602B AAVID 574802 AAVID 519922B 592502 AAVID 532802B 579302 Thermalloy 6299B Thermalloy 6238B Thermalloy 7023 Thermalloy 6038 Thermalloy 7038
Table 3-9. Commercial Heat Sinks 1.5A 5.0A Applications [Vertical Mounting Denoted (V); Means Horizontal Mounting]
Reading Heat Sink Graphs
Major heat sink manufacturers provide graphs showing their heat sink characteristics. standard graph (Figure 3-55) depicts different data: curve heat sink thermal performance still (natural convection); other shows performance possible with forced cooling. graphs should considered separately since they share common axes. Both measured using single device heat source: multiple regulators attached, thermal performance improves much one-third (see Multiple Packages Heat Sink, below).
Section Using Linear Regulators
Designing With Linear Regulators
Micrel Semiconductor
Velocity (ft/min) 1000
Designing With Regulators
Velocity (ft/min) 1000
Temperature Rise (°C)
Power Dissipation
Figure 3-57. Forced Convection Performance
Figure 3-55. Typical Heat Sink Performance Graph
Figure 3-56 shows natural convection portion curve. x-axis shows power dissipation y-axis represents temperature rise over ambient. While this curve nearly linear, does exhibit some droop larger temperature rises, representing increased thermodynamic efficiency with larger point curve, determined dividing temperature rise power dissipation. Figure 3-57 shows thermal resistance heat sink under forced convection. x-axis top, convention) velocity lineal units minute. y-axis right side)
Temperature Rise (°C)
Power Sharing Resistor
heat sink required applications still massive expensive. There better manage heat problems: take advantage very dropout voltage characteristic Micrel's Super PNPregulators dissipate some power externally series resistance. distributing voltage drop between this cost resistor regulator, distribute heating reduce size regulator heat sink. Knowing worst case voltages system peak current requirements, select resistor that drops portion excess voltage without sacrificing performance. maximum value resistor calculated from: (MIN) (VOUT (MAX) VDO) RMAX ---------------------- IOUT (PEAK) IGND Where:VIN (MIN) supply 4.75V) VOUT (MAX) maximum output voltage across full temperature range (3.3V 3.366V) worst case dropout voltage across full temperature range (600mV)
Power Dissipation
IOUT (PEAK) maximum 3.3V load current IGND regulator ground current. output example: 4.75 (3.366 0.6) 0.784V RMAX ------------------ ------ 0.154 0.08 5.08A
Figure 3-56. Natural Convection Performance
Designing With Regulators
Section Using Linear Regulators
Micrel Semiconductor
power drop across this resistor (RES) (IOUT (PEAK) IGND)2 4.0W. This subtracts directly from 10.5W regulator power dissipation that occurs without resistor, reducing regulator heat generation 6.5W. PD(Regulator) PD(R (RES) Considering resistor tolerances standard values leads 0.15 resistor. This produces nominal power savings 3.9W. With worstcase tolerances, regulator power dissipation drops 6.8W maximum. This heat drop reduces heat sinking requirements MIC29500 significantly. smaller heat sink with larger thermal resistance. Now, heat sink with 8.3°C/W thermal characteristics suitable-nearly factor better than without resistor. Table 3-10 lists representative heat sinks meeting these conditions.
0.15,
Designing With Regulators
Airflow
ft./min. (2m/sec)
Heat Sink Model
AAVID 530700 AAVID 574802 Thermalloy 6110 Thermalloy 7137, 7140 Thermalloy 7128 AAVID 57302 AAVID 530600 AAVID 577202 AAVID 576802 Thermalloy 6025 Thermalloy 6109 Thermalloy 6022 AAVID 575102 AAVID 574902 AAVID 523002 AAVID 504102 Thermalloy 6225 Thermalloy 6070 Thermalloy 6030 Thermalloy 6230 Thermalloy 6021, 6221 Thermalloy 7136, 7138 AAVID 563202 AAVID 593202 AAVID 534302 Thermalloy 6232 Thermalloy 6032 Thermalloy 6034 Thermalloy 6234
ft./min. (1.5m/sec)
ft./min. (1m/sec)
MIC29501-3.3
Control
3.3V Flag 47µF
Natura
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