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LINEAR AN82-1 handbook

1. lol AN82 F14 Figure 14 Slotting the Area Around the Reference Can Help Isolate it from Board Stress if Properly Applied See Text Temperature Induced Noise Even though references operate on very meager supply currents dissipation in the reference is enough to cause small temperature gradients in the package leads Varia tions in thermal resistance caused by uneven air flow lead to differential lead temperatures thereby causing thermoelectric voltage noise at the output of the reference Figure 15 dramatically demonstrates this effect The first half of the plot was made with an LT1021H 7 buried Zener reference which was shielded from ambient air with a small foam cup Dart Container Corporation Stock No 8J8 or similar The cup was removed at six minutes elapsed time for the second half of the test Ambient in both cases was alab bench top with no excessive turbulence from air conditioners opening closing doors foot traffic or 547 exhaust Removing the foam cup increased the output noise by almost an order of magnitude in the 0 01Hz to 10Hz band The Kovar leads ofthe TO 5 working against copper circuit traces are the primary culprit Copper lead frames used on DIP and surface mount packages are not nearly as sensi tive to airturbulence because they are intrinsically matched Still external components create thermocouples of their own with potentials of 10uV C or more per junction Ina LT1021 7 ref
2. 1 where AVgg Offset voltage k Boltzmann s constant 1 381 10723 Joules K T absolute temperature 298K at room q charge of an electron 1 6 e 10 9 Cou lombs and Je emitter current density The actual units of area used to calculate Jg and Je cancel each other so that only the area ratio is important Similarly only the current ratio is important If we restrict ourselves to using two identical transistors Equation 1 reduces to AVpe Vpe1 Vee2 KT q In lc1 lc2 2 where lc collector current See Figure B 1 The tem perature coefficient is given by TC dAVgg dT k q In lc1 lc2 3 where k q 86 3uV C VBE1 Calculating the current ratio required to produce 2 18mV C corresponding to 650mV offset we find that it is unmanageably large about 9 44 e 10 9 1 In practice a much smaller offset is generated by a AVpe cell and then amplified to 650mV As an example see Figure B 2 Using a 10 1 current ratio we find a room temperature offset from Equation 2 of 59 2mV and a temperature coeffi cient of 199uV C Applying a gain of slightly less than eleven brings us to 650mV and 2 18mV C Adding a PNP emitter follower to the output of this circuit forms acrude bandgap reference with an output voltage equal to the sum of 650mV and the PNP s Vpe Assuming Vee 600mV the output would be 1 25V The reference could be further improved by trimming the gain of eleven so
3. REFERENCE PITFALLS References look deceptively simple to use but like any other precision product maximum performance is not necessarily easy to achieve Here are a few common pitfalls reference users face and ways to beat them Current Hungry Loads Most references are specified for maximum load currents or shunt currents of 10mA to 20mA Nevertheless best performance is not obtained by running the reference at maximum current A number of effects including thermal gradients across the die and thermocouples formed between the leads and external circuit connections may limit the short term stability of the output voltage Adding an external pass transistor as shown in Figure 7 removes the load current from the reference For loads greater than 300yA the pass transistor carries almost all of the current and eliminates short term thermal drift This circuitis also useful for applications requiring more than 20mA and easily supports up to 100mA limited only by transistor beta and dissipation Vt gt Vout 1 4V 2N2905 IN LT1460 10 OUT GND 10V AT 100mA 2uF SOLID TANT AN82 FO7 Figure 7 An External Transistor is Useful for Boosting Output Current as Well as for Removing Load Current from the Reference This Trick Works On All 3 Terminal References NC Pins If references need only two or three external connections why are they supplied in 8 pin packages There are several reasons but t
4. voltage regulators are commonplace these represent 4 or 5 bit accuracy Atthe other end ofthe spectrum laboratory standards the performance of the best monolithic references is exceeded only by saturated Weston cells and Josephson arrays leaving monolithic references in command of every conceivable circuit and system application Reference accuracy comprises multiple electrical specifi cations These are summarized in Table 1 Most com monly specified by circuit designers is initial accuracy This is a measure of the output voltage error expressed in percent or in volts Initial accuracy is specified at room temperature 25 C with a fixed input voltage and zero load current or for shunt references a fixed bias current Table 1 Reference Accuracy Specifications PARAMETER DESCRIPTION PREFERRED UNIT S Initial Accuracy Initial Output Voltage at 25 C V Temperature VMAX VIN ppm eC Coefficient Total Temperature Range Lona T Change in Output vs Time ong Term Measured Over 1000 Hours ppmvkh Stability or More Noise 0 1Hz to 10Hz uVp p ppmp p 10Hz to 1kHz UVRMS PPMRMS Tight initial accuracy is a concern in systems where calibration is either inconvenient or impossible More commonly absolute accuracy is only a secondary con cern as a final trim is performed on the finished product to reconcile the summation of all system inaccuracies A final trim affects considerable cost savings by elim
5. 1000 hours or more Note that the absolute temperature is unimportant but it must remain invariant during the course of the test Mathemati cally extrapolating long term stability data from high tem perature accelerated life tests leads to erroneously optimistic room temperature results When long term stability is guaranteed it is done by means of a 4 week burn in during which multiple output voltage measurements are made Even with this elaborate costly procedure the guaranteed limit is about three to four times the typical drift Unless the product is designed for frequent calibration or is relatively low performance long term stability may be an important aspect of reference performance Products designed for a long calibration cycle must hold their accuracy for extended periods of time without interven tion These products demand references with good long term stability You can expect buried Zeners to perform better than 20ppm Vkh and bandgaps between 20ppm and 50ppm vkh Some of this drift is attributed to the trim and compensation circuitry wrapped around the reference core The LTZ1000 dispenses with trim and compensation overhead in favor of an on chip heater The remaining Zener diode core drifts 0 5ppm Vkh in the first year of Operation approaching the stability of a Weston cell Most of the long term stability figures shown in LTC reference data sheets are for devices in metal can pack ages where assembly and pac
6. LT1495 op amp whose output saturates at 11mV from the negative rail Once powered there is no reason to turn the circuit off One AAA alkaline contains 1200mAH capacity enough to power the circuit throughout the 5 year shelf life of the battery Voltage output accuracy is about 0 17 and current output accuracy is about 1 2 Trim R1 to cali brate the voltage 1k per 0 1 and R3 to calibrate the output current 250Q per 0 1 Low noise synthesizers need quiet power supplies for their VCOs and other critical circuitry 3 terminal regulators exhibit far too much noise for this application calling J 2XAAA gt ALKALINE 1pyA R1 R2 R3 MARS SERIES IRC 512 992 7900 LT1634A 1 25 instead for a regulator constructed from a reference A practical example is shown in Figure 17 Current through the LT1021 5 reference is used to drive the base of a PNP pass device resulting in an available output current of at least 1A In this example the current is intentionally limited to 200mA by the addition of emitter degeneration and base clamping The low noise of the reference is preserved giving a 100 fold improvement over the noise of an equivalent 5V 3 terminal regulator not to mention improved initial accuracy and long term stability Typical output noise is 7uVp p over a 10kHz bandwidth 9V TO 12V INPUT KEA RED LED 7BD949 3 L10213 5V 200mA ouT OUTPUT D 2 02 GN L gue T TANT ZETEX INC 516 8
7. chip circuitry is used to improve the tempco of the reference core A buried Zener is first order compensated against temperature changes by adding a P N junction diode The Zener itself measures 2mV C and the diode 2mV C The combi nation of the two in series cancel to about 0 2mV C 30ppm C out ofa total of 7V Interestingly this is very close to the tempco of a saturated Weston cell which measures 40uV C or 39ppm C Weston cells are held in a temperature controlled bath monolithic buried Zener references are further compensated against tem perature changes by carefully adding fractional Vgg and or AVge terms to the output Post manufacturing trims are used on both bandgap and buried Zener products to further minimize tempco of the finished reference Another detractor from accuracy is ong term stability The output of a reference changes usually in one direc tion as itages The effect is logarithmic that is the output changes less and less as time progresses The units of long term stability ppm Vkh kh 1000 hours reflectthe logarithmic decline of the output change vs time Because long term changes in the output are small and occur over the course of months or years it is impossible to devise an affordable manufacturing test to guarantee the true stabil ity of all references Instead this parameter is character ized by aging dozens of units in a temperature controlled chamber at 25 C to 30 C for
8. from newcomer to seasoned 0 02 200 veteran You mention an accuracy figure and the pundit 8192 w ali quickly hops to anew unit so that you cannot follow his line a of reasoning Figure 1 neutralizes the pundits callous 0 005 50 45 intentions and allows its possessor to unit hop with equal 32 768 0 003 30 ease and full comprehension Refer to Figure 1 as you read eae iii this application note iwi Ed Today s IC reference technology is divided along two gi pes r lines bandgap references which balance the tempera 262 144 n 3 ture coefficient of a forward biased diode junction against PP o gt that of a AVgg see Appendix B and buried Zeners see Appendix A which use subsurface breakdown to achieve 1 048 576 0 0990 e outstanding long term stability and low noise With few TM js 6 exceptions both reference types use additional on chip 7 circuitry to further minimize temperature drift and trim 4 194 304 a output voltage to an exact value Bandgap references are nae generally used in systems of up to 12 bits buried Zeners 7 40 take over from there in higher accuracy systems 16 777 216 af 47 LTC and LT are registered trademarks of Linear Technology Corporation Figure 1 Accuracy Translator LI nee AN82 1 Application Note 82 In circuits and systems monolithic references face com petition from discrete Zener diodes and 3 terminal voltage regulators only where accuracy is not a concern 5 Zeners and 3
9. in temperature As explained in the text Zener based references use a Zener and a forward biased diode connected in series to achieve near zero temperature coefficient and a bandgap relies on a AVgg in series with a forward biased diode An indispensable technique in integrated circuit design the AVgg is not widely known in other fields Before explaining the theory of AVgr let s skip ahead to the two most important results two identical diode or base emitter junctions running different currents produce dif ferent voltage drops The ratio of the currents controls the absolute value of the offset voltage Further this offset has a predictable positive temperature coefficient of approxi mately 3 4uV C for each room temperature millivolt of offset By combining the positive TC of a AVgg with the negative TC of a diode drop a zero TC bandgap reference is formed As we shall soon see it takes a AVgg offset of 650mV to cancel the 2 18mV C TC of a hypothetical diode Information furnished by Linear Technology Corporation is believed to be accurate and reliable However no responsibility is assumed for its use Linear Technology Corporation makes no represen tation that the interconnection of its circuits as described herein will not infringe on existing patent rights AN82 11 Application Note 82 Two transistors or diodes produce an offset given by the following equation AVpe Vpe1 Vee2 KT q In Jg1 JE2
10. that the AVpr exactly canceled the PNP s base emitter temperature coefficient IC bandgap references are con structed in a similar way The numbers have been massaged for those who want to reproduce the calculations or a combination of current and area scaling to achieve a 10 1 current density ratio in Equation 1 VBE2 AN82 F1B Figure B 1 The Current Ratio Required to Produce a Certain Vge Offset is Defined by Equations 1 and 2 VBE 600mV 2 18mV C 650mV 2 18mV C AN82 FB2 Figure B 2 A Bandgap Reference is Formed by Stacking a AVge Generator and a Vp Linear Technology Corporation 1630 McCarthy Blvd Milpitas CA 95035 7417 408 432 1900 FAX 408 434 0507 e www linear tech com AN82 12 an82f LT TP 1199 4K PRINTED IN USA TECHNOLOGY LINEAR TECHNOLOGY CORPORATION 1999
11. this effect without resorting to a milled board Anything that can be done to restrict the board from bending is helpful A small thick board is better than a large thin board Stiffeners help immunize the board against flexure Mount the circuit board with grommets flexible standoffs or card cage style so that minimal force is applied to the mounting holes and board Part placement and orientation are just as important If a board is squeezed from opposite edges the bending force tends to concentrate in a line down the center Locate the reference away from the middle of the board Since the longer side of a board is more flexible than the shorter locate the reference along the shorter edge These recom mendations are generalities the placement mounting method and orientation of other components and assem blies on the circuit board will influence the mechanical strengths and weaknesses of the circuit board Bench tests indicate that the strongest axis for plastic packages is along the shorter dimension of the body of the plastic Figure 13 shows the correct orientation for surface mount parts Note that the part s longest axis is placed perpendicular to that of the circuit board The devices in Figure 13 are shown in the center of the board for illustra tive purposes only comments about placement still apply In spite of all precautions extraneous effects may adversely affectthe reference s resistance to board stress Watch o
12. 64 7630 GLOWS IN CURRENT LIMIT DO NOT OMIT Figure 17 Ultralow Noise 5V 200mA Supply Output Noise is 7uVprms Over a 10Hz to 10kHz Bandwidth Reference Noise is Guaranteed to be Less Than 11uVpms Standard 3 Terminal Regulators Have One Hundred Times the Noise and No Guarantees AN82 F16 O O COM 1 5V Figure 16 This Pocket Reference Operates for Five Years on One Set of AAA Cells AN82 9 Application Note 82 CONCLUSION When specifying a reference keep in mind that initial accuracy temperature coefficient and long term stability all play a role in overall accuracy of the finished product By taking some care in applying the reference and by avoiding some key pitfalls the reference s inherent accu racy can be preserved FOR FURTHER READING Spreadbury Peter J The Ultra Zener A Portable Replacement for the Weston Cell by EEE Transactions on Instrumentation and Measurement Vol 40 No 2 April 1991 pp 343 346 Huffman Brian Application Note 42 Voltage Reference Circuit Collection Linear Technology Corporation June 1991 Lee Albert 4 5uA Li lon Battery Protection Circuit Linear Technology Volume 9 Number 2 June 1999 p 36 APPENDIX A BURIED ZENER LOW LONGTERM DRIFT AND NOISE The Zener diode has long been used in reference service in many noncritical applications Integrated circuit design ers sometimes use an NPN emitter base junction operat ing in reverse breakdow
13. Anode Highest Dopant Concentration Occurs Directly Under the Mask Opening AN82 10 Application Note 82 The last steps include a p base diffusion and an emitter diffusion located at the center of the Zener see Figure A 2 The emitter becomes the cathode whereas the combined isolation and base diffusion serve as the anode Breakdown occurs at the bottom of the of the cathode where the emitter and isolation base dopant concentra tions are richest Lighter doping concentrations result ina ACTIVE ISOLATIO higher breakdown voltage atthe iso buried layer base epi and iso epi junctions and at the outer fringes of the emitter diffusion ensuring that these areas are not active when the buried junction is biased into breakdown The resultis an extremely stable subsurface breakdown mecha nism that has near theoretical noise and is unaffected by surface contamination or oxide effects CATHODE n EMITTER p SO p ISO n BURIED LAYER D SUBSTRATE AN82 Fa2 Figure A 2 An Emitter Diffusion Forms the Cathode Breakdown Occurs Under the Center of the Emitter Where Both Emitter and Iso Base Dopant Concentrations are Highest APPENDIX B AVpe INTEGRATED CIRCUIT WORKHORSE Itis perhaps a cruel fate for IC designers that no single IC device or structure is invariant with changes in tempera ture Various combinations of devices have been devised to stabilize circuits against changes
14. Ni Application Note 82 TECHNOLOGY November 1999 Understanding and Applying Voltage References By Mitchell Lee Specifying the right reference and applying it correctly is a more difficult task than one might first surmise consid 7 ering that references are only 2 or 3 terminal devices Z Although the word accuracy is most often spoken in F reference to references it is dangerous to use this word PERCENT POWERS OF TEN PPM DVM DIGITS oa oO z too freely because it can mean different things to different a 0 people Even more perplexing is the fact that a reference a classified as a dog in one application is a panacea in 19 another This application note will familiarize the reader 18 5 with the various aspects of reference accuracy and A present some tips on extracting maximum performance 2 from any reference i a lone ERE As with other specialized electronic fields the field of 128 4 rans monolithic references has its own vocabulary We ve ee ms ae already learned the first word in our reference vocabulary 0 3 3 000 accuracy This is the yardstick with which references are 512 0 2 2 000 graded and compared Unfortunately there are at least five m A adao or six good units for gauging accuracy To keep you from reaching afull understanding of the topic industry pundits 2048 0 05 500 3z use a special technique called unit hopping to confuse ne 0 03 300 and confound everyone
15. cremental slope measured at any specific point In the case of the LT1021 and LT1236 the incre mental slope at 25 C is also guaranteed 1 003 1 002 10ppm C FULL TEMP RANGE BOX 1 001 1 000 L e A a ge 0 999 Bppm ec 0 C TO 70 C BOX AVERAGE 3 TE PERATURE YAA Yu 2 COEFFICIENT TMAX Tmin C 0 997 1 L i L 50 25 0 25 50 75 TEMPERATURE C 0 998 NORMALIZED OUTPUT VOLTAGE 100 125 AN82 F02 Figure 2 The Box Method Expresses Absolute Output Accuracy Over Temperature as a Drift Term A data sheet figure for tempco can be used to directly calculate the output voltage tolerance over the entire operating temperature range A device with a tempco of 10ppm C specified for 0 C to 70 C could drift up to 700ppm from the initial value about 3 counts in a 12 bit system A 0 1 reference with 700ppm tempco error is guaranteed 0 17 accurate over its entire operating tem perature range Two exceptions to this rule are the LT1004 and LT1034 which simply guarantee absolute output voltage accuracy over the entire operating temperature range The LT1009 and LT1029 use a combination of the two called the bow AN82 2 Application Note 82 tie or butterfly method see the LT1009 data sheet for a detailed explanation Neither the bandgap nor the buried Zener in their basic form are inherently low drift Special on
16. erence this represents more than 1ppm C shift from each thermoelectric generator Temperature gradients across the circuit board and dissipation within external components can lead to the same kind of noise as shown in Figure 15 ___ LT1021 7 TO 5 PACKAGE f 0 01Hz TO 10Hz 20uV FOAM CUP REMOVED OUTPUT VOLTAGE NOISE 20uV DIV gt lt 0 2 4 6 8 10 12 TIME MINUTES AN82 F15 Figure 15 Air Turbulence Induces Low Frequency Noise and Compromises Reference Accuracy AN82 8 Application Note 82 Temperature gradients may arise from heat generators on the board Position the reference and its associated exter nal components far from heat sources and if necessary use routing techniques to create an isothermal island around the reference circuitry Minimize air movement either by adding a small enclosure around the reference circuitry or by encapsulating the reference circuitry in self expanding polyurethane foam REFERENCE APPLICATIONS The unique pocket reference shown in Figure 16 is a good match for a pair of AAA alkaline cells because the circuit draws less than 16uA supply current Two outputs are provided a buffered 1 5V voltage output and a regulated 1pA current source The current source compliance ranges from approximately 1V to 43V The reference is self biased completely eliminating line regulation as a concern Start up is guaranteed by the
17. f magnitude shunting the errant leakage away from the guarded traces OSV 10V I S 2147M S VIN I NR LT1027 Vout 5V Figure 8 Board Leakage Can Wreak Havoc with a Precision Reference Here a 147MQ Leakage Path to 24V Pushes the 5V Output Out of Spec N ESS lt as A Pati lt NA Vin e GUARD NR LT1027 Vout 5V Figure 9 Adding a Guard Ring Protects Against Errant Leakage Paths Trim Induced Temperature Drift About half of LTC s reference offerings include a pin for external customer trimming Trimming may be neces sary to calibrate the system but it can also adversely affect the tempco of the reference For example in the LT1019 bandgap reference external trim resistors won t match the tempco ofthe internal resistors The mismatch causes a small 1ppm C worst case shift in the output voltage tempco as explained onthe data sheet The LT1021 5 and LT1236 5 standard trim circuit can be modified as shown in Figure 10 to prevent upsetting the references inher ently low temperature coefficients Trimming the LT1027 has little effect on the output voltage tempco and it needs no special consideration Always check the reference data sheet for specific recommendations OUT Vout LT1236 5 N OR LT1021 5 R2 TRIM gt 50k 1N4148 GND m AN82F10 Figure 10 The LT1021 or LT1236 Output Trim is Made Temperature Insensitive by the Addition of a Diode and a Resi
18. he one we ll cover here is post package trimming To guarantee tight output tolerances some factory trimming is necessary after the device has been packaged In packaged form we no longer have direct access to the die so the extra pins on an 8 pin package are used to effect post package trimming For some ICs NC means this pin is floating you can hook it up to whatever you want In the case of a reference it means don t connect anything to this pin That includes ESD and board leakage as well as inten tional connections External connections will at best cause output voltage shifts and at worst permanently shift the output voltage out of spec A similar caution applies to the TRIM pin on references with adjustable outputs The TRIM pin is akin to an amplifiers summing node do not inject current into a TRIM pin unless you want to trim the output of course Here board leakage or capacitive coupling to noise sources are pitfalls to avoid Board Leakage A new specter has entered the field of references board leakage caused by the residues of water soluble flux The effect is not unlike that produced by the sticky juice extravasated from a ruptured electrolytic capacitor Leak age from ground supply rails and other circuit potentials into NC trim and other sensitive pins through conductive flux residues will cause output voltage shifts Even if the leakage paths do not shift the reference out of spec exte
19. icates the measurement of output voltage Modern high accuracy digital voltmeters can average many readings to help filter low frequency noise effects and provide a stable reading of a reference s true output voltage AN82 3 Application Note 82 ESSENTIAL FEATURES There are two styles of references shunt functionally equivalent to a Zener diode and series not unlike a 3 terminal regulator Bandgaps and buried Zeners are available in both configurations see Figure 3 Some series references are designed to also operate in shunt mode by simply biasing the output pin and leaving the input pin open circuit Series mode references have the advantage that they draw only load and quiescent current from the input supply whereas shunt references must be biased with a current that exceeds the sum of the maxi mum quiescent and maximum expected load currents Since they are biased by a resistor shunt references can Operate on a very wide range of input voltages About half of LTC s reference offerings include a pin for external customer trimming Some are designed for b SERIES GND a VREF AN82 F03 Figure 3 References Are Supplied in Either 2 Terminal Zener Style a or 3 Terminal Voltage Regulator Style b VOLTAGE SWING V AN82 F04 Figure 4 The LT1009 is Optimized for Rapid Settling at Power Up precisio
20. inating the need for tight initial accuracy in every reference DAC ADC amplifier and transducer in the system Monolithic reference initial accuracy ranges from 0 02 to 1 representing 1LSB error in 6 bit to 12 bit systems Weston cells and Josephson arrays clock in at 1ppm to 10ppm and 0 02ppm initial accuracy respectively 0 02ppm is less than 1LSB error in a 25 bit system Temperature induced changes in reference output volt age can quickly overshadow a tight initial accuracy speci fication Considerable effort is therefore expended to minimize the temperature coefficient tempco of a refer ence Most references are guaranteed in the range of 2ppm C to 40ppm C with a few devices falling outside this range A properly applied LTZ1000 temperature stabi lized reference can demonstrate 0 05ppm C Tempco is specified as an average over the operating temperature range in units of ppm C or mV C This average is calculated in what is called the box method Figure 2 shows how box method tempco figures are defined and calculated The reference in question LT 1019 bandgap is tested over the specified operating tempera ture range The minimum and maximum recorded output voltages are applied to the equation shown resulting in an average temperature coefficient expressed in V C This is further manipulated to find ppm C as used in the data sheet The tempco is an average over the operating range rather than an in
21. kage stresses are mini mized You can expect somewhat less performance for the same reference in a plastic package One last factor that affects accuracy is short term variation of output voltage otherwise known as noise Reference noise is typically characterized over two frequency ranges 0 1Hzto 10Hzfor short term peak to peak drift and 10Hz to 1kHz for total wideband RMS noise Noise voltage is usually proportional to output voltage so the output noise expressed in ppm is constant for all voltage options of any given reference Wideband noise ranges from 4ppm to 16ppm RMS for bandgap references to 0 17ppm to 0 5ppm RMS for buried Zeners Noise improves with increased reference current regardless of reference type But since the reference core operating current is set internally the noise characteristics cannot be changed except by external filtering the LT1027 features a noise filtering pin The LT1034 and LTZ1000 buried Zeners are externally accessible allowing the user to increase the bias current and reduce noise Adding output bypassing or external compensation will affect the character of a reference s noise In particular if the compensation is peaky the spot noise will likely rise to a peak somewhere in the 100Hz to 10kHz range Critical damping will eliminate this noise peak Reference noise can affect the dynamic range of a high resolution system obscuring small signals Low fre quency noise also compl
22. n as a Zener reference Breakdown occurs at the surface of the die where the effects of contamination and oxide charge are most pronounced These junctions are noisy and suffer from unpredictable short and long term drift The buried Zener developed as a precision IC reference places the junction below the surface of the silicon well away from contamination and oxide effects The result is a Zener with excellent long term stability low noise and relatively accurate initial tolerance p SUBSTRATE Figure A 1 shows the first steps in fabricating a buried Zener A region of n buried layer is located beneath the Zener structure so as to shield subsequent diffusions from contact with the substrate After growth of the n epitaxial layer p isolation is diffused through a small opening at the center of the Zener At the same time isolation is diffused around the periphery to form a separate tub containing the entire Zener structure Isolation diffuses both downward and laterally The cen tral diffusion is shielded from contact with the substrate by the buried layer while the isolation walls are allowed to reach substrate and form an isolated tub It is important to note that the highest concentration of p occurs directly under the mask opening and thatthe dopant concentration is weakest at the fringes of a diffusion ISO DIFFUSION i CUT IN OXIDE rw n BURIED LAYER AN82 Fat Figure A 1 Iso is Diffused to Form the
23. n trimming of the reference output whereas oth ers have a wide trim range allowing the output voltage to be adjusted several percent above or below the intended operating point If load current steps must be handled transient response is important Transient response varies widely from refer ence to reference and comprises three distinct qualities turn on characteristics small signal output impedance at high frequency and settling behavior when subjected to a fast transient load References exhibit these qualities because almost all contain an amplifier to buffer and or scale the output The LT1009 is optimized for fast start up characteristics and it settles in a little over 1us as shown in Figure 4 For some references optimum settling is obtained with an external compensation network As shown in Figure 5 a 2uF 2Q damper optimizes the settling and high frequency output impedance ofan LT1019 reference Fastest settling is obtained with an LT1027 which settles to 13 bits accuracy in 2us This impressive feat is illustrated by the oscillograph of Figure 6 which clearly shows the output recovering from a 10mA load step VIN LT1019 2Q T0 52 2uF TANTALUM AN82 F05 Figure 5 Optimum Settling Realized with RC Compensation at Output Vout 400uV DIV AC COUPLED 10mA LOAD STEP AN82 F06 2us DIV Figure 6 The LT1027 is Optimized for Fast Settling in Response to Load Steps AN82 4 Application Note 82
24. rnal leakage can manifest itself as long term output voltage drift as the resistance of the flux residue changes with shifts in relative humidity and the diffusion of external contaminants Water soluble flux residues must be re moved from the board and package surfaces or com pletely avoided In one case the author observed an LT1009 shifted out of spec by a gross leakage path of approximately 80kQ between the trim pin and a nearby power supply trace The leakage was traced to water soluble flux Figure 8 shows how a good reference can go bad with only a very small leakage A hypothetical industrial control board contains an LT1027A producing 5V for various data acquisition circuits A nearby trace carries 24V Just 147MQ leakage into the noise filtering pin NR causes a typical device to shift 200ppm and out of spec Clearly a 24V circuit trace doesn t belong anywhere near a 0 02 reference This example is oversimplified but clearly dem onstrates the potential for disaster AN82 5 Application Note 82 A tightly packed circuit board may leave no choice but to agglomerate incompatible traces In this case use a guard ring to eliminate reference shift see Figure 9 The output of the reference is divided down to 4 4V equal to the potential on the NR pin and used to bias a guard ring encircling the trace connecting NR to the noise filter capacitor This reduces the effect of board leakage paths by more than two orders o
25. stor Burn In Most manufacturers of high accuracy systems run their products through a burn in procedure Burn in solves two problems at once it relieves stresses built into the refer ence and circuit board during assembly and it ages the reference beyond the highest long term drift region which occurs when power is first applied to the part A typical burn in procedure calls for operating the board at 125 C ambient for 168 hours If the main concern is stress relief a shorter unpowered burn in cycle can be used Board Stress Burn in can help relax a stuffed board but additional mechanical stress may be introduced when the board is mounted into the product Stress has a directly measur able effect on reference output If the stress changes over a period of time it may manifest itself as unacceptable long term drift Circuit boards are not perfectly elastic so bending forces may cause permanent deformation and a permanent step change in reference output voltage Devices in metal TO 5 and TO 46 packages are largely immune to board stress owing to the rigidity of the package and the flexibility of the leads Plastic and surface mount packages are another matter Board stress effects are easily observed by monitoring the output of a reference while applying a bending force to the AN82 6 Application Note 82 board A controlled experiment was performed to mea sure the effect of board stress on an LT1460CS8 2 5 S
26. urface mount reference Devices were mounted in the center of 7 x 9 rectangular boards as shown in Figure 11 The boards were then deflected out of plane 18 mils per inch as shown in steps 1 through 4 Figure 12 shows the net effect on the output of one representative sample measured over eight cycles of flexure x 9 a SLOTTED AREA APPROXIMATELY 1 2 INCH BY 1 2 INCH Cee a mezen Figure 11 Reference Sensitivity to Stress Was Evaluated by Assembling Devices On a 7 x 9 Circuit Board and Flexing as Shown in Steps 1 Through 4 160 ORIGINAL CIRCUIT BOARD m N 80 HI SLOTTED CIRCUIT BOARD OUTPUT DEVIATION ppm 40 4ppm 10uV METER RESOLUTION 0 10 20 30 40 DEFECTION NUMBER AN82 F12 Figure 12 Isolating Stress by Slotting the Circuit Board Reduces Reference Variations by More Than an Order of Magnitude LTC1460S8 2 5 The original board showed about 60ppm peak to peak shift The board was then slotted on a vertical mill forming a 0 5 x 0 5 tab with the reference located in its center also illustrated in Figure 11 The test continued with the slotted configuration and the output voltage variations were reduced to 1 count 10uV on the meter or approxi mately 4ppm peak to peak This represents a tenfold improvement in stress induced output voltage shift Several other techniques can be employed to minimize
27. ut for adhesives and solder and flux debris under the package These will create pressure points and induce unpredictable stresses in the package Ifa board has been subjected to a high bending force some of the glass fibers and layers may break or shear apart permanently weakening the board Subsequent bending forces will concentrate their stress at points thus weakened Figure 14 shows various schemes for routing stress relief slots on a circuit board along with optimum package LI WIR AN82 7 Application Note 82 Orientation Note that the longest axis of the reference is aligned with the tab not the shortest axis of the circuit board This is in anticipation of flexing forces transmitted into the tab The best orientation for the tab is in line with the longest axis of the board as in b c and d Bending forces along the weaker longer axis of the board could be coupled into a and e Note that the ICs are aligned to resist this force Use configuration c when the part is located along the longer edge of the board and d when itis located along the shorter edge Use b when the part is not located along any edge lt LONGEST DIMENSION gt k LONGEST AXIS 4 AN82 F13 Figure 13 Arranging the Longest Axes of the Board and Package in Perpendicularity Minimizes Stress Induced Output Changes

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