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Mitchell Specifying right reference applying correctly more difficult task than might first surmise, considering that references only 3-terminal devices. Although word "accuracy" most often spoken reference references, dangerous this word freely because mean different things different people. Even more perplexing fact that reference classified application panacea another. This application note will familiarize reader with various aspects reference "accuracy" present some tips extracting maximum performance from reference. with other specialized electronic fields, field monolithic references vocabulary. We've already learned first word reference vocabulary, "accuracy." This yardstick with which references graded compared. Unfortunately, there least five good units gauging accuracy. keep from reaching full understanding topic, industry pundits special technique called "unit-hopping" confuse confound everyone from newcomer seasoned veteran. mention accuracy figure pundit quickly hops unit that cannot follow line reasoning. Figure neutralizes pundits' callous intentions allows possessor unit-hop with equal ease full comprehension. Refer Figure read this application note. Today's reference technology divided along lines: bandgap references, which balance temperature coefficient forward-biased diode junction against that (see Appendix buried Zeners (see Appendix which subsurface breakdown achieve outstanding long-term stability noise. With exceptions, both reference types additional on-chip circuitry further minimize temperature drift trim output voltage exact value. Bandgap references generally used systems bits; buried Zeners take over from there higher accuracy systems.
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1024 2048 4096 8192 16,384 32,768 65,536 -100 131,072 262,144 -110 524,288 1,048,576 2,097,152 -130 4,194,304 8,388,608 16,777,216 -120
0.02 0.001 0.0005 -140
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10,000 5,000 3,000 2,000 1,000
0.05 0.03
0.01 0.005 0.003 0.002
0.0003 0.0002 0.0001
Figure Accuracy Translator
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DIGITS MIRRORED SCALE
BITS
PERCENT
COUNTS
Application Note
circuits systems, monolithic references face competition from discrete Zener diodes 3-terminal voltage regulators only where accuracy concern. Zeners voltage regulators commonplace; these represent 5-bit accuracy. other spectrum-laboratory standards-the performance best monolithic references exceeded only saturated Weston cells Josephson arrays, leaving monolithic references command every conceivable circuit system application. Reference accuracy comprises multiple electrical specifications. These summarized Table Most commonly specified circuit designers initial accuracy. This measure output voltage error expressed percent volts. Initial accuracy specified room temperature (25°C), with fixed input voltage zero load current, shunt references, fixed bias current.
Table Reference Accuracy Specifications
NORMALIZED OUTPUT VOLTAGE
2ppm/°C 40ppm/°C, with devices falling outside this range. properly applied LTZ1000 temperature stabilized reference demonstrate 0.05ppm/°C. Tempco specified average over operating temperature range units ppm/°C mV/°C. This average calculated what called "box" method. Figure shows method tempco figures defined calculated. reference question (LT®1019 bandgap) tested over specified operating temperature range. minimum maximum recorded output voltages applied equation shown, resulting average temperature coefficient expressed V/°C. This further manipulated find ppm/°C, used data sheet. tempco average over operating range, rather than incremental slope measured specific point. case LT1021 LT1236, incremental slope 25°C also guaranteed.
1.003 1.002 1.001 1.000 0.999 0.998 LT1019 CURVE 10ppm/°C FULL TEMP RANGE "BOX"
PARAMETER DESCRIPTION PREFERRED UNIT(S) Initial Accuracy Initial Output Voltage 25°C VMAX Temperature ppm/°C Coefficient Total Temperature Range
Long-Term Stability Noise Change Output Time Measured Over 1000 Hours More 0.1Hz 10Hz 10Hz 1kHz ppmkh µVP-P, ppmP-P µVRMS, ppmRMS
Tight initial accuracy concern systems where calibration either inconvenient impossible. More commonly, absolute accuracy only secondary concern, final trim performed finished product reconcile summation system inaccuracies. final trim affects considerable cost savings eliminating need tight initial accuracy every reference, DAC, ADC, amplifier transducer system. Monolithic reference initial accuracy ranges from 0.02% representing 1LSB error 6-bit 12-bit systems. Weston cells Josephson arrays clock 1ppm 10ppm 0.02ppm initial accuracy, respectively (0.02ppm less than 1LSB error 25-bit system). Temperature-induced changes reference output voltage quickly overshadow tight initial accuracy specification. Considerable effort therefore expended minimize temperature coefficient (tempco) reference. Most references guaranteed range
0.997
5ppm/°C 70°C "BOX" AVERAGE TEMPERATURE COEFFICIENT TEMPERATURE (°C)
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Figure Method Expresses Absolute Output Accuracy Over Temperature Drift Term
data sheet figure tempco used directly calculate output voltage tolerance over entire operating temperature range. device with tempco 10ppm/°C, specified 70°C, could drift 700ppm from initial value (about counts 12-bit system). 0.1% reference with 700ppm tempco error guaranteed 0.17% accurate over entire operating temperature range. exceptions this rule LT1004 LT1034, which simply guarantee absolute output voltage accuracy over entire operating temperature range. LT1009 LT1029 combination two, called "bow
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Application Note
tie" "butterfly" method (see LT1009 data sheet detailed explanation). Neither bandgap buried Zener, their basic form, inherently drift. Special on-chip circuitry used improve tempco reference core. buried Zener first-order compensated against temperature changes adding junction diode. Zener itself measures 2mV/°C diode 2mV/°C. combination series cancel about 0.2mV/°C (30ppm/°C) total Interestingly, this very close tempco saturated Weston cell, which measures 40µV/°C, 39ppm/°C. Weston cells held temperature-controlled bath; monolithic buried Zener references further compensated against temperature changes carefully adding fractional and/or terms output. Post-manufacturing trims used both bandgap buried Zener products further minimize tempco finished reference. Another detractor from accuracy long-term stability. output reference changes, usually direction, ages. effect logarithmic; that output changes less less time progresses. units long-term stability, ppm/kh 1000 hours), reflect logarithmic decline output change time. Because long-term changes output small occur over course months years, impossible devise affordable manufacturing test guarantee true stability references. Instead, this parameter characterized aging dozens units temperature-controlled chamber 25°C 30°C 1000 hours more. Note that absolute temperature unimportant, must remain invariant during course test. Mathematically extrapolating long-term stability data from high temperature, accelerated life tests leads erroneously optimistic room temperature results. When long-term stability guaranteed, done means 4-week burn-in, during which multiple output voltage measurements made. Even with this elaborate, costly procedure, guaranteed limit about three four times typical drift. Unless product designed frequent calibration relatively performance, long-term stability important aspect reference performance. Products designed long calibration cycle must hold their accuracy extended periods time without intervention. These products demand references with good longterm stability. expect buried Zeners perform better than 20ppm/kh, bandgaps between 20ppm 50ppm/kh. Some this drift attributed trim compensation circuitry wrapped around reference core. LTZ1000 dispenses with trim compensation overhead favor on-chip heater. remaining Zener/diode core drifts 0.5ppm/kh first year operation, approaching stability Weston cell. Most long-term stability figures shown reference data sheets devices metal packages, where assembly package stresses minimized. expect somewhat less performance same reference plastic package. last factor that affects accuracy short-term variation output voltage, otherwise known noise. Reference noise typically characterized over frequency ranges: 0.1Hz 10Hz short-term, peak-to-peak drift, 10Hz 1kHz total "wideband" noise. Noise voltage usually proportional output voltage, output noise expressed constant voltage options given reference. Wideband noise ranges from 4ppm 16ppm bandgap references, 0.17ppm 0.5ppm buried Zeners. Noise improves with increased reference current, regardless reference type. since reference core operating current internally, noise characteristics cannot changed except external filtering (the LT1027 features noise filtering pin). LT1034 LTZ1000 buried Zeners externally accessible, allowing user increase bias current reduce noise. Adding output bypassing external compensation will affect character reference's noise. particular, compensation "peaky," spot noise will likely rise peak somewhere 100Hz 10kHz range. Critical damping will eliminate this noise peak. Reference noise affect dynamic range high resolution system, obscuring small signals. frequency noise also complicates measurement output voltage. Modern, high accuracy digital voltmeters average many readings help filter frequency noise effects provide stable reading reference's true output voltage.
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Application Note
ESSENTIAL FEATURES There styles references: shunt, functionally equivalent Zener diode; series, unlike 3-terminal regulator. Bandgaps buried Zeners available both configurations (see Figure Some series references designed also operate shunt mode simply biasing output leaving input open circuit. Series-mode references have advantage that they draw only load quiescent current from input supply, whereas shunt references must biased with current that exceeds maximum quiescent maximum expected load currents. Since they biased resistor, shunt references operate very wide range input voltages. About half LTC's reference offerings include external (customer) trimming. Some designed precision trimming reference output, whereas others have wide trim range, allowing output voltage adjusted several percent above below intended operating point. load current steps must handled, transient response important. Transient response varies widely from reference reference comprises three distinct qualities: turn-on characteristics, small-signal output impedance high frequency settling behavior when subjected fast, transient load. References exhibit these qualities because almost contain amplifier buffer and/or scale output. LT1009 optimized fast start-up characteristics, settles little over 1µs, shown Figure some references, optimum settling obtained with external compensation network. shown Figure 2µF/2 damper optimizes settling high frequency output impedance LT1019 reference. Fastest settling obtained with LT1027, which settles bits accuracy 2µs. This impressive feat illustrated oscillograph Figure which clearly shows output recovering from 10mA load step.
SHUNT
SERIES
VREF
VREF
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Figure References Supplied Either 2-Terminal Zener Style 3-Terminal Voltage Regulator Style
LT1019
VOLTAGE SWING
TANTALUM
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OUTPUT
OUTPUT INPUT
Figure Optimum Settling Realized with Compensation Output
TIME (µs)
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INPUT
VOUT 400µV/DIV COUPLED 10mA LOAD STEP 2µs/DIV
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Figure LT1009 Optimized Rapid Settling Power-Up
Figure LT1027 Optimized Fast Settling Response Load Steps
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Application Note
REFERENCE PITFALLS References look deceptively simple use, like other precision product, maximum performance necessarily easy achieve. Here common pitfalls reference users face, ways beat them. Current-Hungry Loads Most references specified maximum load currents shunt currents) 10mA 20mA. Nevertheless, best performance obtained running reference maximum current. number effects, including thermal gradients across thermocouples formed between leads external circuit connections, limit short-term stability output voltage. Adding external pass transistor, shown Figure removes load current from reference. loads greater than 300µA, pass transistor carries almost current eliminates short-term thermal drift. This circuit also useful applications requiring more than 20mA, easily supports 100mA, limited only transistor beta dissipation.
(VOUT 1.4V) 1.8k 2N2905 LT1460-10 100mA
access die, extra pins 8-pin package used effect post-package trimming. some ICs, "NC" means "this floating, hook whatever want." case reference, means "don't connect anything this pin." That includes board leakage, well intentional connections. External connections will, best, cause output voltage shifts and, worst, permanently shift output voltage spec. similar caution applies TRIM references with adjustable outputs. TRIM akin amplifier's summing node; inject current into TRIM pin-unless want trim output, course. Here board leakage capacitive coupling noise sources pitfalls avoid. Board Leakage specter entered field references: board leakage caused residues water-soluble flux. effect unlike that produced sticky juice extravasated from ruptured electrolytic capacitor. Leakage from ground, supply rails other circuit potentials into trim other sensitive pins through conductive flux residues will cause output voltage shifts. Even leakage paths shift reference spec, external leakage manifest itself long-term output voltage drift, resistance flux residue changes with shifts relative humidity diffusion external contaminants. Water-soluble flux residues must removed from board package surfaces, completely avoided. case, author observed LT1009 shifted spec gross leakage path approximately between trim nearby power supply trace. leakage traced watersoluble flux. Figure shows good reference with only very small leakage. hypothetical industrial control board contains LT1027A producing various data acquisition circuits. nearby trace carries 24V. Just 147M leakage into noise filtering (NR) causes typical device shift +200ppm, spec. Clearly, circuit trace doesn't belong anywhere near 0.02% reference. This example oversimplified clearly demonstrates potential disaster.
SOLID TANT
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Figure External Transistor Useful Boosting Output Current Well Removing Load Current from Reference. This Trick Works 3-Terminal References
"NC" Pins references need only three external connections, they supplied 8-pin packages? There several reasons, we'll cover here postpackage trimming. guarantee tight output tolerances, some factory trimming necessary after device been packaged. packaged form longer have direct
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Application Note
tightly packed circuit board leave choice agglomerate incompatible traces. this case, guard ring eliminate reference shift (see Figure output reference divided down 4.4V, equal potential pin, used bias guard ring encircling trace connecting noise filter capacitor. This reduces effect board leakage paths more than orders magnitude, shunting errant leakage away from guarded traces.
147M MYLAR LT1027 VOUT
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LT1236-5 standard trim circuit modified, shown Figure prevent upsetting references' inherently temperature coefficients. Trimming LT1027 little effect output voltage tempco, needs special consideration. Always check reference data sheet specific recommendations.
LT1236-5 LT1021-5 TRIM
VOUT 1N4148
REFACC_08.eps
Figure LT1021 LT1236 Output Trim Made Temperature Insensitive Addition Diode Resistor
Figure Board Leakage Wreak Havoc with Precision Reference. Here, 147M Leakage Path Pushes Output Spec
147M GUARD RING LT1027 VOUT
Burn-In Most manufacturers high-accuracy systems their products through burn-in procedure. Burn-in solves problems once: relieves stresses built into reference circuit board during assembly ages reference beyond highest long-term drift region, which occurs when power first applied part. typical burn-in procedure calls operating board 125°C ambient hours. main concern stress relief, shorter, unpowered burn-in cycle used. Board Stress Burn-in help "relax" stuffed board, additional mechanical stress introduced when board mounted into product. Stress directly measurable effect reference output. stress changes over period time, manifest itself unacceptable long-term drift. Circuit boards perfectly elastic, bending forces cause permanent deformation permanent step-change reference output voltage. Devices metal (TO-5 TO-46) packages largely immune board stress, owing rigidity package flexibility leads. Plastic surface mount packages another matter. Board stress effects easily observed monitoring output reference while applying bending force
220k
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Figure Adding Guard Ring Protects Against Errant Leakage Paths
Trim-Induced Temperature Drift About half LTC's reference offerings include external (customer) trimming. Trimming necessary calibrate system, also adversely affect tempco reference. example, LT1019 bandgap reference, external trim resistors won't match tempco internal resistors. mismatch causes small (1ppm/°C) worst-case shift output voltage tempco, explained data sheet. LT1021-5
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Application Note
board. controlled experiment performed measure effect board stress LT1460CS8-2.5 surface mount reference. Devices were mounted center rectangular boards, shown Figure boards were then deflected out-of-plane mils inch, shown steps through Figure shows effect output representative sample measured over eight cycles flexure.
original board showed about 60ppm peak-to-peak shift. board then slotted vertical mill, forming 0.5" 0.5" with reference located center (also illustrated Figure 11). test continued with slotted configuration, output voltage variations were reduced count (10µV) meter, approximately 4ppm peak-to-peak. This represents tenfold improvement stress-induced output voltage shift. Several other techniques employed minimize this effect, without resorting milled board. Anything that done restrict board from bending helpful. small, thick board better than large, thin board. Stiffeners help immunize board against flexure. Mount circuit board with grommets, flexible standoffs card-cage style that minimal force applied mounting holes board.
SLOTTED AREA APPROXIMATELY INCH INCH
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Part placement orientation just important. board squeezed from opposite edges, bending force tends concentrate line down center. Locate reference away from middle board. Since longer side board more flexible than shorter, locate reference along shorter edge. These recommendations generalities; placement, mounting method orientation other components assemblies circuit board will influence mechanical strengths weaknesses circuit board. Bench tests indicate that strongest axis plastic packages along shorter dimension body plastic. Figure shows correct orientation surface mount parts. Note that part's longest axis placed perpendicular that circuit board. devices Figure shown center board illustrative purposes only; comments about placement still apply. spite precautions, extraneous effects adversely affect reference's resistance board stress. Watch adhesives solder flux debris under package. These will create pressure points induce unpredictable stresses package. board been subjected high bending force, some glass fibers layers break shear apart, permanently weakening board. Subsequent bending forces will concentrate their stress points thus weakened. Figure shows various schemes routing stress-relief slots circuit board, along with optimum package
Figure Reference Sensitivity Stress Evaluated Assembling Devices Circuit Board Flexing, Shown Steps Through
ORIGINAL CIRCUIT BOARD
OUTPUT DEVIATION (ppm)
SLOTTED CIRCUIT BOARD
4ppm 10µV METER RESOLUTION DEFECTION NUMBER
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Figure Isolating Stress Slotting Circuit Board Reduces Reference Variations More Than Order Magnitude (LTC1460S8-2.5)
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Application Note
orientation. Note that longest axis reference aligned with tab, shortest axis circuit board. This anticipation flexing forces transmitted into tab. best orientation line with longest axis board (b), (d). Bending forces along weaker (longer) axis board could coupled into (e). Note that aligned resist this force. configuration when part located along longer edge board, when located along shorter edge. when part located along edge. Temperature-Induced Noise Even though references operate very meager supply currents, dissipation reference enough cause small temperature gradients package leads. Variations thermal resistance, caused uneven flow, lead differential lead temperatures, thereby causing thermoelectric voltage noise output reference. Figure dramatically demonstrates this effect. first half plot made with LT1021H-7 buried Zener reference, which shielded from ambient with small foam (Dart Container Corporation Stock similar). removed minutes elapsed time second half test. Ambient both cases bench-top with excessive turbulence from conditioners, opening/closing doors, foot traffic exhaust. Removing foam increased output noise almost order magnitude 0.01Hz 10Hz band. Kovar leads TO-5 working against copper circuit traces primary culprit. Copper lead frames used surface mount packages nearly sensitive turbulence because they intrinsically matched. Still, external components create thermocouples their with potentials 10µV/°C more junction. LT1021-7 reference, this represents more than 1ppm/°C shift from each thermoelectric generator. Temperature gradients across circuit board dissipation within external components lead same kind noise shown Figure
LONGEST DIMENSION
LONGEST AXIS
MSOP
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Figure Arranging Longest Axes Board Package Perpendicularity Minimizes Stress-Induced Output Changes
OUTPUT VOLTAGE NOISE (20µV/DIV)
LT1021-7 (TO-5 PACKAGE) 0.01Hz 10Hz 20µV FOAM REMOVED
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TIME (MINUTES)
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Figure Slotting Area Around Reference Help Isolate from Board Stress Properly Applied (See Text)
Figure Turbulence Induces Frequency Noise Compromises Reference Accuracy
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Application Note
Temperature gradients arise from heat generators board. Position reference associated external components from heat sources and, necessary, routing techniques create isothermal island around reference circuitry. Minimize movement either adding small enclosure around reference circuitry, encapsulating reference circuitry self-expanding polyurethane foam. REFERENCE APPLICATIONS unique pocket reference shown Figure good match pair alkaline cells, because circuit draws less than 16µA supply current. outputs provided: buffered, 1.5V voltage output, regulated current source. current source compliance ranges from approximately 43V. reference self-biased, completely eliminating line regulation concern. Start-up guaranteed LT1495 amp, whose output saturates 11mV from negative rail. Once powered, there reason turn circuit off. alkaline contains 1200mAH capacity, enough power circuit throughout 5-year shelf life battery. Voltage output accuracy about 0.17% current output accuracy about 1.2%. Trim calibrate voltage 0.1%), calibrate output current (250 0.1%). noise synthesizers need quiet power supplies their VCOs other critical circuitry. 3-terminal regulators exhibit much noise this application, calling instead regulator constructed from reference. practical example shown Figure Current through LT1021-5 reference used drive base pass device, resulting available output current least this example, current intentionally limited 200mA addition emitter degeneration base clamping. noise reference preserved, giving 100-fold improvement over noise equivalent 3-terminal regulator, mention improved initial accuracy long-term stability. Typical output noise 7µVP-P over 10kHz bandwidth.
INPUT
LED** ZBD949* 1/2W 5V/200mA OUTPUT 47µF
LT1021-5
10µF TANT
ZETEX (516) 864-7630 GLOWS CURRENT LIMIT. OMIT.
REFACC_17.eps
Figure Ultralow Noise 200mA Supply Output Noise 7µVRMS Over 10Hz 10kHz Bandwidth. Reference Noise Guaranteed Less Than 11µVRMS. Standard 3-Terminal Regulators Have Hundred Times Noise Guarantees
ZTX214C 2XAAA ALKALINE CELLS
LT1495
249k 0.1%
MAR5 SERIES (512) 992-7900
Figure This Pocket Reference Operates Five Years Cells
LT1634A-1.25 LT1495 1.00M 0.1%
200k 0.1%
1.5V
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Application Note
CONCLUSION When specifying reference, keep mind that initial accuracy, temperature coefficient long-term stability play role overall accuracy finished product. taking some care applying reference, avoiding some pitfalls, reference's inherent accuracy preserved. FURTHER READING Spreadbury, Peter "The Ultra-Zener-A Portable Replacement Weston Cell?" IEEE Transactions Instrumentation Measurement, Vol. April 1991, 343-346 Huffman, Brian. Application Note Voltage Reference Circuit Collection. Linear Technology Corporation, June 1991. Lee, Albert. "4.5µA Li-Ion Battery Protection Circuit" Linear Technology, Volume Number June 1999, p.36.
APPENDIX BURIED ZENER: LONGTERM DRIFT NOISE Zener diode long been used reference service many noncritical applications. Integrated circuit designers sometimes emitter-base junction operating reverse breakdown Zener reference. Breakdown occurs surface die, where effects contamination oxide charge most pronounced. These junctions noisy suffer from unpredictable short- long-term drift. buried Zener, developed precision reference, places junction below surface silicon, well away from contamination oxide effects. result Zener with excellent long-term stability, noise, relatively accurate initial tolerance. Figure shows first steps fabricating buried Zener. region buried layer located beneath Zener structure shield subsequent diffusions from contact with substrate. After growth epitaxial layer, isolation diffused through small opening center Zener. same time, isolation diffused around periphery form separate containing entire Zener structure. Isolation diffuses both downward laterally. central diffusion shielded from contact with substrate buried layer, while isolation walls allowed reach substrate form isolated tub. important note that highest concentration occurs directly under mask opening that dopant concentration weakest fringes diffusion.
DIFFUSION OXIDE
BURIED LAYER SUBSTRATE
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Figure A-1. Diffused Form Anode. Highest Dopant Concentration Occurs Directly Under Mask Opening
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Application Note
last steps include base diffusion emitter diffusion, located center Zener (see Figure A-2). emitter becomes cathode, whereas combined isolation base diffusion serve anode. Breakdown occurs bottom cathode, where emitter isolation base dopant concentrations richest. Lighter doping concentrations result
ACTIVE ZENER AREA ISOLATION ANODE
higher breakdown voltage iso-buried layer, base-epi iso-epi junctions, outer fringes emitter diffusion, ensuring that these areas active when buried junction biased into breakdown. result extremely stable subsurface breakdown mechanism that near-theoretical noise unaffected surface contamination oxide effects
CATHODE
BASE EMITTER BURIED LAYER SUBSTRATE
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Figure A-2. Emitter Diffusion Forms Cathode. Breakdown Occurs Under Center Emitter, Where Both Emitter Base Dopant Concentrations Highest
APPENDIX VBE: INTEGRATED CIRCUIT WORKHORSE perhaps, cruel fate designers that single device structure invariant with changes temperature. Various combinations devices have been devised stabilize circuits against changes temperature. explained text, Zener-based references Zener forward-biased diode connected series achieve near-zero temperature coefficient bandgap relies series with forward-biased diode. indispensable technique integrated circuit design, widely known other fields. Before explaining theory VBE, let's skip ahead most important results: identical diode base emitter) junctions running different currents produce different voltage drops. ratio currents controls absolute value offset voltage. Further, this offset predictable, positive temperature coefficient approximately 3.4µV/°C each room-temperature millivolt offset. combining positive with negative diode drop, zero bandgap reference formed. shall soon see, takes offset 650mV cancel 2.18mV/°C hypothetical diode.*
Information furnished Linear Technology Corporation believed accurate reliable. However, responsibility assumed use. Linear Technology Corporation makes representation that interconnection circuits described herein will infringe existing patent rights.
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Application Note
transistors diodes) produce offset given following equation: VBE1 VBE2 (kT/q) ln(JE1/JE2) where offset voltage, Boltzmann's constant (1.381 Joules/K), absolute temperature (298K room), charge electron (1.6 Coulombs), emitter current density. actual units area used calculate cancel each other, that only area ratio important. Similarly, only current ratio important. restrict ourselves using identical transistors, Equation reduces VBE1 VBE2 (kT/q) ln(IC1/IC2) where collector current (see Figure B-1). temperature coefficient given dVBE/dT (k/q) ln(IC1/IC2) where 86.3µV/°C. Calculating current ratio required produce +2.18mV/°C (corresponding 650mV offset) find that unmanageably large, about 9.44 1010:1. practice, much smaller offset generated cell then amplified 650mV. example, Figure B-2. Using 10:1 current ratio,** find room temperature offset from Equation 59.2mV, temperature coefficient 199µV/°C. Applying gain slightly less than eleven brings 650mV +2.18mV/°C. Adding emitter follower output this circuit forms crude "bandgap" reference, with output voltage equal 650mV PNP's VBE. Assuming 600mV, output would 1.25V. reference could further improved trimming gain eleven that exactly canceled PNP's base-emitter temperature coefficient. bandgap references constructed similar way.
*The numbers have been massaged those want reproduce calculations. **or combination current area scaling achieve 10:1 current density ratio Equation (1).
VBE1
VBE2
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Figure B-1. Current Ratio Required Produce Certain Offset Defined Equations
1.25V 0mV/°C
59.2mV
600mV -2.18mV/°C
650mV +2.18mV/°C
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Figure B-2. Bandgap Reference Formed Stacking Generator
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7417
(408)432-1900 FAX: (408) 434-0507 www.linear-tech.com
an82f LT/TP 1199 PRINTED
LINEAR TECHNOLOGY CORPORATION 1999

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