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LTC6655 6655fa typical a pplica t ion descrip t ion 0.25ppm noise, low drift precision buffered reference family the ltc ? 6655 is a complete family of precision bandgap voltage references, offering exceptional noise and drift performance. this low noise and drift is ideally suited for the high resolution measurements required by instrumenta- tion and test equipment. in addition, the LTC6655 is fully specifed over the temperature range of C40c to 125c, ensuring its suitability for demanding automotive and industrial applications. advanced curvature compensation allows this bandgap reference to achieve a drift of less than 2ppm/c with a predictable temperature characteristic and an output voltage accurate to 0.025%, reducing or eliminating the need for calibration. the LTC6655 can be powered from as little as 500mv above the output voltage to as much as 13.2v. superior load regulation with source and sink capability, coupled with exceptional line rejection, ensures consistent per- formance over a wide range of operating conditions. a shutdown mode is provided for low power applications. available in a small msop package, the LTC6655 family of references is an excellent choice for demanding preci- sion applications. l , lt, ltc, ltm, linear technology and the linear logo are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. basic connection fea t ures a pplica t ions n low noise: 0.25ppm p-p (0.1hz to 10hz) 625nv p-p for the LTC6655-2.5 n low drift: 2ppm/c max n high accuracy: 0.025% max n fully specifed from C40c to 125c n 100% tested at C40c, 25c and 125c n load regulation: <10ppm/ma n sinks and sources current: 5ma n low dropout: 500mv n maximum supply voltage: 13.2v n low power shutdown: <20a max n available output voltages: 1.25v, 2.048v, 2.5v, 3v, 3.3v, 4.096v, 5v n available in an 8-lead msop package n instrumentation and test equipment n high resolution data acquisition systems n weigh scales n precision battery monitors n high temperature applications n precision regulators n medical equipment n high output current precision reference LTC6655-2.5 v in shdn c out 10f v out 6655 ta01a v out_f v out_s 3v < v in 13.2v gnd c in 0.1f low frequency 0.1hz to 10hz noise (LTC6655-2.5) 500nv/div 6655 ta01b 1s/div
LTC6655 6655fa input voltage v in to gnd .......................................... C0.3v to 13.2v shdn to gnd ........................... C0.3v to (v in + 0.3v) output voltage: v out_f ...................................... C0.3v to (v in + 0.3v) v out_s ..................................................... C0.3v to 6v ou tput short-circuit duration ...................... indefnite operating temperature range (note 2) . C40c to 125c storage temperature range (note 2) ..... C65c to 150c lead temperature range (soldering, 10 sec) (note 3) ................................................................. 300c (note 1) 1 2 3 4 shdn v in gnd* gnd 8 7 6 5 gnd* v out_f v out_s gnd* top view ms8 package 8-lead plastic msop t jmax = 150c, ja = 300c/w *connect pins to device gnd (pin 4) p in c on f igura t ion a bsolu t e maxi m u m r a t ings o r d er i n f or m a t ion lead free finish tape and reel part marking package description specified temperature range LTC6655bhms8-1.25#pbf LTC6655bhms8-1.25#trpbf ltfdg 8-lead plastic msop C40c to 125c LTC6655chms8-1.25#pbf LTC6655chms8-1.25#trpbf ltfdg 8-lead plastic msop C40c to 125c LTC6655bhms8-2.048#pbf LTC6655bhms8-2.048#trpbf ltfdh 8-lead plastic msop C40c to 125c LTC6655chms8-2.048#pbf LTC6655chms8-2.048#trpbf ltfdh 8-lead plastic msop C40c to 125c LTC6655bhms8-2.5#pbf LTC6655bhms8-2.5#trpbf ltfcy 8-lead plastic msop C40c to 125c LTC6655chms8-2.5#pbf LTC6655chms8-2.5#trpbf ltfcy 8-lead plastic msop C40c to 125c LTC6655bhms8-3#pbf LTC6655bhms8-3#trpbf ltfdj 8-lead plastic msop C40c to 125c LTC6655chms8-3#pbf LTC6655chms8-3#trpbf ltfdj 8-lead plastic msop C40c to 125c LTC6655bhms8-3.3#pbf LTC6655bhms8-3.3#trpbf ltfdk 8-lead plastic msop C40c to 125c LTC6655chms8-3.3#pbf LTC6655chms8-3.3#trpbf ltfdk 8-lead plastic msop C40c to 125c LTC6655bhms8-4.096#pbf LTC6655bhms8-4.096#trpbf ltfdm 8-lead plastic msop C40c to 125c LTC6655chms8-4.096#pbf LTC6655chms8-4.096#trpbf ltfdm 8-lead plastic msop C40c to 125c LTC6655bhms8-5#pbf LTC6655bhms8-5#trpbf ltfdn 8-lead plastic msop C40c to 125c LTC6655chms8-5#pbf LTC6655chms8-5#trpbf ltfdn 8-lead plastic msop C40c to 125c consult ltc marketing for parts specifed with wider operating temperature ranges. consult ltc marketing for information on non-standard lead based fnish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel specifcations, go to: http://www.linear.com/tapeandreel/
LTC6655 6655fa a v ailable op t ions output voltage initial accuracy temperature coefficient part number ? 1.250 0.025% 0.05% 2ppm/c 5ppm/c LTC6655bhms8-1.25 LTC6655chms8-1.25 2.048 0.025% 0.05% 2ppm/c 5ppm/c LTC6655bhms8-2.048 LTC6655chms8-2.048 2.500 0.025% 0.05% 2ppm/c 5ppm/c LTC6655bhms8-2.5 LTC6655chms8-2.5 3.000 0.025% 0.05% 2ppm/c 5ppm/c LTC6655bhms8-3.0 LTC6655chms8-3.0 3.300 0.025% 0.05% 2ppm/c 5ppm/c LTC6655bhms8-3.3 LTC6655chms8-3.3 4.096 0.025% 0.05% 2ppm/c 5ppm/c LTC6655bhms8-4.096 LTC6655chms8-4.096 5.000 0.025% 0.05% 2ppm/c 5ppm/c LTC6655bhms8-5 LTC6655chms8-5 ? see order information section for complete part number listing. e lec t rical c harac t eris t ics the l denotes the specifcations which apply over the full operating temperature range, otherwise specifcations are at t a = 25c. v in = v out + 0.5v, v out_s connected to v out_f , unless otherwise noted. parameter conditions min typ max units output voltage LTC6655b LTC6655c C0.025 C0.05 0.025 0.05 % % output voltage temperature coeffcient (note 4) LTC6655b LTC6655c l l 1 2.5 2 5 ppm/c ppm/c line regulation v out + 0.5v v in 13.2v, shdn = v in l 5 25 40 ppm/v ppm/v load regulation (note 5) i source = 5ma l 3 15 ppm/ma ppm/ma i sink = 5ma l 10 30 ppm/ma ppm/ma operating voltage (note 6) LTC6655-1.25, LTC6655-2.048, LTC6655-2.5 i source = 5ma, v out error 0.1% l 3 13.2 v LTC6655-3, LTC6655-3.3, LTC6655-4.096, LTC6655-5 i source = 5ma, v out error 0.1% i out = 0ma, v out error 0.1% l l v out + 0.5 v out + 0.2 13.2 13.2 v v output short-circuit current short v out to gnd short v out to v in 20 20 ma ma shutdown pin (shdn) logic high input voltage logic high input current, shdn = 2v l l 2.0 12 v a logic low input voltage logic low input current, shdn = 0.8v l l 0.8 15 v a supply current no load l 5 7 7.5 ma ma shutdown current shdn tied to gnd l 20 a
LTC6655 6655fa e lec t rical c harac t eris t ics the l denotes the specifcations which apply over the full operating temperature range, otherwise specifcations are at t a = 25c. v in = v out + 0.5v, v out_s connected to v out_f , unless otherwise noted. parameter conditions min typ max units output voltage noise (note 7) 0.1hz f 10hz 10hz f 1khz 0.25 0.67 ppm p-p ppm rms turn-on time 0.1% settling, c out = 2.7f 400 s long-term drift of output voltage (note 8) 60 ppm/khr hysteresis (note 9) ?t = C0c to 70c ?t = C40c to 85c ?t = C40c to 125c 30 35 60 ppm ppm ppm note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: precision may be affected if the parts are stored outside of the specifed temperature range. large temperature changes may cause changes in device performance due to thermal hysteresis. for best performance, extreme temperatures should be avoided whenever possible. note 3: the stated temperature is typical for soldering of the leads during manual rework. for detailed ir refow recommendations, refer to the applications information section. note 4: temperature coeffcient is measured by dividing the maximum change in output voltage by the specifed temperature range. note 5: load regulation is measured on a pulse basis from no load to the specifed load current. load current does not include the 2ma sense current. output changes due to die temperature change must be taken into account separately. note 6: excludes load regulation errors. minimum supply for the l tc6655 - 1.25, LTC6655-2.048 and LTC6655-2.5 is set by internal circuitry supply requirements, regardless of load condition. minimum supply for the LTC6655-3, LTC6655-3.3, LTC6655-4.096 and LTC6655-5 is specifed by load current. note 7: peak-to-peak noise is measured with a 2-pole highpass flter at 0.1hz and 3-pole lowpass flter at 10hz. the unit is enclosed in a still-air environment to eliminate thermocouple effects on the leads, and the test time is 10 seconds. due to the statistical nature of noise, repeating noise measurements will yield larger and smaller peak values in a given measurement interval. by repeating the measurement for 1000 intervals, each 10 seconds long, it is shown that there are time intervals during which the noise is higher than in a typical single interval, as predicted by statistical theory. in general, typical values are considered to be those for which at least 50% of the units may be expected to perform similarly or better. for the 1000 interval test, a typical unit will exhibit noise that is less than the typical value listed in the electrical characteristics table in more than 50% of its measurement intervals. see application note 124 for noise testing details. rms noise is measured with a spectrum analyzer in a shielded environment. note 8: long-term stability typically has a logarithmic characteristic and therefore, changes after 1000 hours tend to be much smaller than before that time. total drift in the second thousand hours is normally less than one-third that of the frst thousand hours with a continuing trend toward reduced drift with time. long-term stability is also affected by differential stresses between the ic and the board material created during board assembly. note 9: hysteresis in output voltage is created by mechanical stress that differs depending on whether the ic was previously at a higher or lower temperature. output voltage is always measured at 25c, but the ic is cycled to the hot or cold temperature limit before successive measurements. hysteresis is roughly proportional to the square of the temperature change. for instruments that are stored at well controlled temperatures (within 20 or 30 degrees of operational temperature), hysteresis is usually not a signifcant error source.
LTC6655 6655fa typical p er f or m ance c harac t eris t ics characteristic curves are similar for most voltage options of the LTC6655. curves from the LTC6655-1.25, LTC6655-2.5 and the LTC6655-5 represent the range of performance across the entire family of references. characteristic curves for other output voltages fall between these curves and can be estimated based on their voltage output. 1.25v load regulation (sinking) 1.25v output voltage noise spectrum 1.25v sinking current with a 3.3f output capacitor 1.25v sourcing current with a 3.3f output capacitor 1.25v shutdown supply current vs input voltage 1.25v v out distribution 1.25v low frequency 0.1hz to 10hz noise 1.25v output voltage temperature drift 1.25v load regulation (sourcing) 200nv/ div 6655 g01 1s/div temperature (c) ?50 ?25 1.2496 output voltage (v) 1.2498 1.2504 0 50 75 6655 g02 1.2502 1.2500 25 100 125 3 typical units output current (ma) ?20 output voltage change (ppm) 0 20 ?30 ?10 10 0.001 0.1 1 10 6655 g03 ?40 0.01 125c 25c ?40c output current (ma) 40 output voltage change (ppm) 80 120 160 200 0.001 0.1 1 10 6655 g04 0 0.01 125c 25c ?40c frequency (khz) 10 noise voltage (nv/ hz) 15 25 35 40 0.01 1 10 1000 6655 g05 5 0.1 100 30 20 0 2.7f 10f 100f i out 0ma 5ma v out 10mv/div c out = 3.3f 200s/div 6655 g06 i out ?5ma 0ma v out 10mv/div c out = 3.3f 200s/div 6655 g07 input voltage (v) 0 8 10 14 6 10 6655 g08 6 4 2 4 8 12 14 2 0 12 supply current (a) 125c 25c ?40c v out (v) 1.2495 0 number of parts 10 20 30 40 50 60 t a = 25c 1.2498 1.2500 1.2503 1.2505 6655 g09
LTC6655 6655fa 2.5v load regulation (sinking) 2.5v supply current vs input voltage 2.5v shutdown supply current vs input voltage 2.5v minimum v in C v out differential (sourcing) 2.5v minimum v in C v out differential (sinking) 2.5v output voltage noise spectrum 2.5v low frequency 0.1hz to 10hz noise 2.5v output voltage temperature drift 2.5v load regulation (sourcing) typical p er f or m ance c harac t eris t ics characteristic curves are similar for most voltage options of the LTC6655. curves from the LTC6655-1.25, LTC6655-2.5 and the LTC6655-5 represent the range of performance across the entire family of references. characteristic curves for other output voltages fall between these curves and can be estimated based on their voltage output. 500nv/ div 6655 g10 1s/div temperature (c) output voltage (v) 6655 g11 2.4990 2.4995 2.5000 2.5005 2.5010 ?50 0 50 100 150 3 typical units output current (ma) output voltage change (ppm) 6655 g12 ?50 ?40 ?30 ?20 ?10 0 10 0.001 0.01 0.1 1 10 125c 25c ?40c output current (ma) output voltage change (ppm) 6655 g13 ?20 0 40 80 120 20 60 100 140 160 0.001 0.01 0.1 1 10 125c 25c ?40c input voltage (v) supply current (ma) 6655 g14 0 1 2 3 4 5 6 7 8 0 2 4 6 8 10 12 14 125c 25c ?40c input voltage (v) supply current (a) 6655 g15 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 125c 25c ?40c input ? output voltage (v) 6655 g16 0.01 0.1 1 10 0.01 0.1 1 output current (ma) 125c 25c ?40c 6655 g17 0.01 0.1 1 10 ?0.15 ?0.05 0.05 0.15 output current (ma) input ? output voltage (v) 125c 25c ?40c c out = 100f frequency (khz) 60 noise voltage (nv hz) 100 0.01 10 100 1000 0 20 0.1 1 120 80 40 6655 f01 c out = 2.7f c out = 10f
LTC6655 6655fa 2.5v power supply rejection ratio vs frequency 2.5v output impedance vs frequency 2.5v line regulation 2.5v v out distribution 2.5v temperature drift distribution 2.5v shdn input voltage thresholds vs v in typical p er f or m ance c harac t eris t ics characteristic curves are similar for most voltage options of the LTC6655. curves from the LTC6655-1.25, LTC6655-2.5 and the LTC6655-5 represent the range of performance across the entire family of references. characteristic curves for other output voltages fall between these curves and can be estimated based on their voltage output. v out (v) 2.4992 0 number of parts 20 40 60 2.4996 2.5000 2.5004 2.5008 80 100 10 30 50 70 90 6655 g19 t a = 25c drift (ppm/c) 0 number of parts 8 10 12 2.8 6 4 0.8 1.6 0.4 1.2 2 2.4 2 0 14 ?40c to 125c 6655 g20 v in (v) v trip (v) 6655 g21 v th_up v th_dn 0.0 0.5 1.0 1.5 2.0 2.5 2 4 6 8 10 12 14 frequency (khz) power supply rejection ratio (db) 40 60 100 80 20 0 120 6655 g22 10.1 10 100 0.01 0.001 c out = 2.7f c out = 10f c out = 100f frequency (khz) output impedence () 101 100 0.1 0.001 0.01 1000 6655 g23 0.01 0.1 1 10 c out = 2.7f c out = 10f c out = 100f input voltage (v) 0 output voltage (v) 2.501 2.500 4 8 2 6 10 12 14 2.499 2.498 2.502 6655 g24 125c 25c ?40c
LTC6655 6655fa 5v load regulation (sinking) 5v supply current vs input voltage 5v output voltage noise spectrum 5v minimum v in -v out differential (sourcing) 5v minimum v in -v out differential (sinking) 5v start-up response with a 3.3f output capacitor 5v low frequency 0.1hz to 10hz noise 5v output voltage temperature drift 5v load regulation (sourcing) typical p er f or m ance c harac t eris t ics characteristic curves are similar for most voltage options of the LTC6655. curves from the LTC6655-1.25, LTC6655-2.5 and the LTC6655-5 represent the range of performance across the entire family of references. characteristic curves for other output voltages fall between these curves and can be estimated based on their voltage output. temperature (c) ?50 4.9985 output voltage (v) 4.9990 4.9995 5.0000 5.0010 5.0005 ?25 0 25 50 6655 g26 75 100 125 3 typical units 500nv/ div 6655 g25 1s/div output current (ma) output voltage change (ppm) 0.01 ?50 ?10 0 10 0.1 1 10 6655 g27 ?20 ?30 ?40 125c 25c ?40c output current (ma) output voltage change (ppm) 0.01 ?20 60 80 100 0.1 1 10 6655 g28 40 20 0 125c 25c ?40c input voltage (v) 0 supply current (ma) 4 5 6 6 10 6655 g29 3 2 2 4 8 12 14 1 0 125c 25c ?40c frequency (khz) 0.01 80 noise voltage (nv/ hz) 100 120 140 160 0.1 1 10 100 1000 6655 g30 60 40 20 0 180 200 2.7f 10f 100f input-output voltage (v) 0.01 0.01 output current (ma) 1 10 0.1 1 6655 g31 0.1 125c 25c ?40c input-output voltage (v) ?0.3 0.01 output current (ma) 0.1 1 10 ?0.2 ?0.1 0 0.1 6655 g32 125c 25c ?40c v in 2v/div v out 2v/div c out = 3.3f 400s/div 6655 g33
LTC6655 6655fa p in func t ions shdn (pin 1): shutdown input. this active low input powers down the device to <20a. if left open, an inter- nal pull-up resistor puts the part in normal operation. it is recommended to tie this pin high externally for best performance during normal operation. v in (pin 2): power supply. bypass v in with a 0.1f, or larger, capacitor to gnd. gnd (pin 4): device ground. this pin is the main ground and must be connected to a noise-free ground plane. v out_s (pin 6): v out sense pin. connect this pin at the load and route with a wide metal trace to minimize load regulation errors. this pin sinks 2ma. output error is r trace ? 2ma, regardless of load current. for load currents <100a, tie directly to v out_f pin. v out_f (pin 7): v out force pin. this pin sources and sinks current to the load. an output capacitor of 2.7f to 100f is required. gnd (pins 3, 5, 8): internal function. ground these pins. bloc k d iagra m ? + v out_f 7 2 v out_s 6655 bd 6 bandgap v in 1 4 shdn gnd gnd 3,5,8
LTC6655 0 6655fa a pplica t ions i n f or m a t ion bypass and load capacitors the LTC6655 voltage references require a 0.1f or larger input capacitor located close to the part to improve power supply rejection. an output capacitor with a value between 2.7f and 100f is also required. the output capacitor has a direct effect on the stability, turn-on time and settling behavior. choose a capacitor with low esr to insure stability. resistance in series with the output capacitor (esr) introduces a zero in the output buffer transfer function and could cause instability. the 2.7f to 100f range includes several types of capacitors that are readily available as through-hole and surface mount components. it is recommended to keep esr less than or equal to 0.1?. capacitance and esr are both frequency dependent. at higher frequencies capacitance drops and esr increases. to insure stable operation the output ca- pacitor should have the required values at 100khz. in order to achieve the best performance, caution should be used when choosing a capacitor. x7r ceramic ca- pacitors are small, come in appropriate values and are relatively stable over a wide temperature range. however, for a low noise application x7r capacitors may not be suitable since they may exhibit a piezoelectric effect. the mechanical vibrations cause a charge displacement in the ceramic dielectric and the resulting perturbation can look like noise. if x7r capacitors are necessary, a thorough bench evaluation should be completed to verify proper performance. for very low noise applications where every nanovolt counts, flm capacitors should be considered for their low noise and lack of piezoelectric effects. film capaci- tors such as polyester, polystyrene, polycarbonate, and polypropylene have good temperature stability. additional care must be taken as polystyrene and polypropylene have an upper temperature limit of 85c to 105c. above these temperatures, the working voltages need to be derated according to manufacturers specifcations. another type of flm capacitor is polyphenylene sulfde (pps). these devices work over a wide temperature range, are stable, and have large capacitance values beyond 1f. in general, flm capacitors are found in surface mount and leaded packages. table 1 is a partial list of capacitor companies and some of their available products. in voltage reference applications, flm capacitor lifetime is affected by temperature and applied voltage. when polyester capacitors are operated beyond their rated temperatures (some capacitors are not rated for operation above 85c) they need to be derated. voltage derating is usually accomplished as a ratio of applied voltage to rated voltage limit. contact specifc flm capacitor manufacturers to determine exact lifetime and derating information. the lifetime of x7r capacitors is long, especially for reference applications. capacitor lifetime is degraded by operating near or exceeding the rated voltage, at high temperature, with ac ripple or some combination of these. most reference applications have ac ripple only during transient events. table 1. film capacitor companies company dielectric available capacitance temperature range type cornell dublier polyester 0.5f to 10f C55c to 125c dme dearborn electronics polyester 0.1f to 12f C55c to 125c 218p, 430p, 431p, 442p, and 410p tecate polyester 0.01f to 18f C40c to 105c 901, 914, and 914d wima polyester 10f to 22f C55c to 100c mks 4, mks 2-xl vishay polyester 1000pf to 15f C55c to 125c mkt1820 vishay polycarbonate 0.01f to 10f C55c to 100c mkc1862, 632p dearborn electronics polyphenylene sulfde (pps) 0.01f to 15f C55c to 125c 820p, 832p, 842p, 860p, and 880p wima polyphenylene sulfde (pps) 0.01f to 6.8f C55c to 140c smd-pps
LTC6655 6655fa 3.5v 3v v in v out 50mv/div c out = 3.3f 6655 f04 400s/div a pplica t ions i n f or m a t ion the choice of output capacitor also affects the bandwidth of the reference circuitry and resultant noise peaking. as shown in figure 1, the bandwidth is inversely proportional to the value of the output capacitor. noise peaking is related to the phase margin of the output buffer. higher peaking generally indicates lower phase mar- gin. other factors affecting noise peaking are temperature, input voltage, and output load current. start-up and load transient response results for the transient response plots (figures 3 to 8) were produced with the test circuit shown in figure 2 unless otherwise indicated. the turn-on time is slew limited and determined by the short-circuit current, the output capacitor, and output voltage as shown in the equation: t v c i on out out sc = ? for example, the LTC6655-2.5v, with a 3.3f output capacitor and a typical short-circuit current of 20ma, the start-up time would be approximately: 2 5 3 3 10 0 02 412 6 . ? . ? . ? v f a s = the resulting turn-on time is shown in figure 3. here the output capacitor is 3.3f and the input capacitor is 0.1f. figure 4 shows the output response to a 500mv step on v in . the output response to a current step sourcing and sinking is shown in figures 5 and 6, respectively. figure 7 shows the output response as the current goes from sourcing to sinking. shutdown mode the LTC6655 family of references can be shut down by tying the shdn pin to ground. there is an internal pull-up resistor tied to this pin. if left unconnected this pin rises to v in and the part is enabled. due to the low internal pull-up current, it is recommended that the shdn pin be pulled high externally for normal operation to prevent accidental LTC6655-2.5 100 v out 1,2 7 6 3,4,5,8 c in 0.1f c out 3.3f v gen 6655 f02 0.5v v in 3v figure 2. transient load test circuit figure 3. start-up response figure 4. output response with a 500mv step on v in figure 1. output voltage noise spectrum v in 2v/div v out 1v/div c out = 3.3f 6655 f03 200s/div c out = 100f frequency (khz) 60 noise voltage (nv hz) 100 0.01 10 100 1000 0 20 0.1 1 120 80 40 6655 f01 c out = 2.7f c out = 10f
LTC6655 6655fa c out = 3.3f 6655 f08 1ms/div v out 1v/div shdn 2v/div c out = 3.3f 6655 f07 200s/div 2ma ?2ma i out v out 10mv/div a pplica t ions i n f or m a t ion figure 8. shutdown response with 5ma source load LTC6655-2.5 v in gnd shdn 2n7002 v out_f v out_s to c 3v v in 13.2v v out c1 1f c2 10f 6655 f09 figure 9. open-drain shutdown circuit figure 7. output response showing a sinking to sourcing transition shutdown due to system noise or leakage currents. the turn-on/turn-off response due to shutdown is shown in figure 8. to control shutdown from a low voltage source, a mosfet can be used as a pull-down device as shown in figure 9. note that an external resistor is unnecessary. a mosfet with a low drain-to-source leakage over the operating temperature range should be chosen to avoid inadvertently pulling down the shdn pin. a resistor may be added from shdn to v in to overcome excessive mosfet leakage. the shdn thresholds have some dependency on v in and temperature as shown in the typical performance characteristics section. avoid leaving shdn at a voltage between the thresholds as this will cause an increase in supply current due to shoot-through current. 0ma ?5ma i out v out 10mv/div c out = 3.3f 6655 f05 200s/div 5ma 0ma i out v out 10mv/div c out = 3.3f 6655 f06 200s/div figure 6. output response with 5ma load step sinking figure 5. output response with a 5ma load step sourcing long-term drift long-term drift cannot be extrapolated from accelerated high temperature testing. this erroneous technique gives drift numbers that are wildly optimistic. the only way long-term drift can be determined is to measure it over the time interval of interest. the LTC6655 long-term drift data was collected on 80 parts that were soldered into printed circuit boards similar to a real world application. the boards were then placed into a constant temperature oven with a t a = 35c, their outputs were scanned regularly and measured with an 8.5 digit dvm. typical long-term drift is illustrated in figure 10.
LTC6655 6655fa figure 10. long-term drift a pplica t ions i n f or m a t ion figure 11. hysteresis plot C40c to 125c v in (v) 0 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0 6655 f12 5 10 no load 15 power (w) 5ma load figure 12. LTC6655-2.5 power consumption v in (v) 0 105 115 125 12 6655 f13 95 85 3 6 9 15 75 65 55 maximum ambient operating temperature (c) no load 5ma load figure 13. LTC6655-2.5 maximum ambient operating temperature hysteresis thermal hysteresis is a measure of change of output voltage as a result of temperature cycling. figure 11 illustrates the typical hysteresis based on data taken from the LTC6655-2.5. a proprietary design technique minimizes thermal hysteresis. power dissipation power dissipation for the LTC6655 depends on v in and load current. figure 12 illustrates the power consump- tion versus v in under a no-load and 5ma load condition at room temperature for the LTC6655-2.5. other voltage options display similar behavior. the msop8 package has a thermal resistance ( ja ) equal to 300c/w. under the maximum loaded condition, the increase in die temperature is over 35c. if operated at these conditions with an ambient temperature of 125c, the absolute maximum junction temperature rating of the device would be exceeded. although the maximum junction temperature is 150c, for best performance it is recommended to not exceed a junction temperature of 125c. the plot in figure 13 shows the recommended maximum ambient temperature limits for differing v in and load conditions using a maximum junction temperature of 125c. distribution (ppm) ?90 number of units 20 25 50 15 10 ?50 ?10 ?70 90 ?30 10 70 30 110 5 0 30 6655 f11 hours 0 long-term drift (ppm) 40 80 120 2000 6655 f10 0 ?40 ?80 500 1000 1500 2500 4 typical units LTC6655-2.5
LTC6655 6655fa a pplica t ions i n f or m a t ion LTC6655-2.5 2 7 2ma load star minimize resistance of metal 6 4 6655 f15 + figure 15. kelvin connection for good load regulation output current <100a), v out_s should be tied to v out_f by the shortest possible path to reduce errors caused by resistance in the sense trace. careful attention to grounding is also important, espe- cially when sourcing current. the return load current can produce an i ? r drop causing poor load regulation. use a star ground connection and minimize the ground to load metal resistance. although there are several pins that are required to be connected to ground, pin 4 is the actual ground for return current. optimal noise performance the LTC6655 offers extraordinarily low noise for a bandgap referenceonly 0.25ppm in 0.1hz to 10hz. as a result, system noise performance may be dominated by system design and physical layout. some care is required to achieve the best possible noise performance. the use of dissimilar metals in component leads and pc board traces creates thermocouples. varia- tions in thermal resistance, caused by uneven air fow, create differential lead temperatures, thereby causing thermoelectric voltage noise at the output of the refer- ence. minimizing the number of thermocouples, as well as limiting airfow, can substantially reduce these errors. additional information can be found in linear technology application note 82. position the input and load capacitors close to the part. although the LTC6655 has a dc psrr of over 100db, the power supply should be as stable as possible to guarantee optimal performance. a plot of the 0.1hz to 10hz low frequency noise is shown in the typical performance characteristic section. noise performance can be further improved by wiring several LTC6655s in parallel as shown in the typical applications section. with this technique the noise is reduced by n , where n is the number of LTC6655s in parallel. pc board layout the LTC6655 reference is a precision device that is factory trimmed to an initial accuracy of 0.025%, as shown in the typical performance characteristic section. the mechanical stress caused by soldering parts to a printed circuit board may cause the output voltage to shift and the temperature coeffcient to change. to reduce the effects of stress-related shifts, mount the reference near the short edge of a printed circuit board or in a corner. in addition, slots can be cut into the board on two sides of the device to reduce mechanical stress. a thicker and smaller board is stiffer and less prone to bend. finally, use stress relief, such as fexible standoffs, when mounting the board. additional precautions include making sure the solder joints are clean and the board is fux free to avoid leakage paths. a sample pcb layout is shown in figure 14. load regulation to take advantage of the v out kelvin force/sense pins, the v out_s pin should be connected separately from the v out_f pin as shown in figure 15. the v out_s pin sinks 2ma, which is unusual for a kelvin connection. however, this is required to achieve the ex- ceptional low noise performance. the i ? r drop on the v out_s line directly affects load regulation. the v out_s trace should be as short and wide as practical to minimize series resistance the v out_s trace adds error as r trace ? 2ma, so a 0.1 trace adds 200v error. the v out_f pin is not as important as the v out_s pin in this regard. an i ? r drop on the v out_f pin increases the minimum supply voltage when sourcing current, but does not directly affect load regulation. for light loading of the output (maximum figure 14. sample pcb layout 6655 f14 gnd v out v in
LTC6655 6655fa a pplica t ions i n f or m a t ion noise specifcation noise in any frequency band is a random function based on physical properties such as thermal noise, shot noise, and ficker noise. the most precise way to specify a random error such as noise is in terms of its statistics, for example as an rms value. this allows for relatively simple maximum error estimation, generally involving assumptions about noise bandwidth and crest factor. unlike wideband noise, low frequency noise, typically specifed in a 0.1hz to 10hz band, has traditionally been specifed in terms of expected error, illustrated as peak-to-peak error. low frequency noise is generally measured with an oscilloscope over a 10 second time frame. this is a pragmatic approach, given that it can be diffcult to measure noise accurately at low frequencies, and that it can also be diffcult to agree on the statistical characteristics of the noise, since ficker noise dominates the spectral density. while practical, a random sampling of 10 second intervals is an inadequate method for representation of low frequency noise, especially for systems where this noise is a dominant limit of system performance. given the random nature of noise, the output voltage may be observed over many time intervals, each giving different results. noise specifcations that were determined using this method are prone to subjectivity, and will tend toward a mean statistical value, rather than the maximum noise that is likely to be produced by the device in question. because the majority of voltage reference data sheets express low frequency noise as a typical number, and as it tends to be illustrated with a repeatable plot near the mean of a distribution of peak-to-peak values, the LTC6655 data sheet provides a similarly defned typical specifcation in order to allow a reasonable direct comparison against similar products. data produced with this method gener- ally suggests that in a series of 10 second output voltage measurements, at least half the observations should have a peak-to-peak value that is below this number. for example, the LTC6655-2.5 measures less than 0.25ppm p-p in at least 50% of the 10 second observations. as mentioned above, the statistical distribution of noise is such that if observed for long periods of time, the peak error in output voltage due to noise may be much larger than that observed in a smaller interval. the likely maximum error due to noise is often estimated using the rms value, multiplied by an estimated crest factor, assumed to be in the range of 6 to 8.4. this maximum possible value will only be observed if the output voltage is measured for very long periods of time. therefore, in addition to the common method, a more thorough approach to measuring noise has been used for the LTC6655 (described in detail in linear technologys an124) that allows more information to be obtained from the result. in particular, this method characterizes the noise over a signifcantly greater length of time, resulting in a more complete description of low frequency noise. the peak-to-peak voltage is measured for 10 second intervals over hundreds of intervals. in ad- dition, an electronic peak-detect circuit stores an objective value for each interval. the results are then summarized in terms of the fraction of measurement intervals for which observed noise is below a specifed level. for example, the LTC6655-2.5 measures less than 0.27ppm p-p in 80% of the measurement intervals, and less than 0.295ppm p-p in 95% of observation intervals. this statistical variation in noise is illustrated in table 2 and figure 17. the test circuit is shown in figure 16. table 2 low frequency noise (ppm p-p ) 50% 0.246 60% 0.252 70% 0.260 80% 0.268 90% 0.292 this method of testing low frequency noise is superior to more common methods. the results yield a comprehensive statistical description, rather than a single observation. in addition, the direct measurement of output voltage over time gives an actual representation of peak noise, rather than an estimate based on statistical assumptions such as crest factor. additional information can be derived from a measurement of low frequency noise spectral density, as shown in figure 18. it should be noted from figure 18 that the LTC6655 has not only a low wideband noise, but an exceptionally low ficker noise corner of 1hz! this substantially reduces low frequency noise, as well as long-term variation in peak noise.
LTC6655 6655fa a pplica t ions i n f or m a t ion + ? 100k 100k shield shielded can 1n4697 10v ac line ground 1300f 9v 100k* 10* + ? 1k* 200* 2k 450* 900* 15v 15v ?15v ?15v 1f 1f a1 lt1012 a2 lt1097 6655 f16 ? input q1 5 * = 1% metal film ** = 1% wirewound, ultronix105a = 1n4148 = 2n4393 = 1/4 ltc202 see appendix c for power, shielding and grounding scheme = tantalum,wet slug i leak < 5na see text/appendix b = polypropelene a4 330f output capacitors = <200na leakage at 1v dc at 25c q1, q2 = thermally mated 2sk369 (match v gs 10%) or lsk389 dual thermally lag see text a = 10 4 low noise pre-amp reference under test 0.15f 750* 10k ?15v q3 2n2907 q2 0.022f 1f **1.2k sd LTC6655 2.5v in s f + 1f 0.1f 124k* 124k* ? + a3 lt1012 1m* 10k* 100* 330* in out root-sum-square correction see text 330f 16v 330f 16v + + 330f 16v 330f 16v + ? a4 lt1012 0.1f 0.1f 10k a = 100 and 0.1hz to 10hz filter 1f rst + ? a5 1/4 lt1058 ? + a7 1/4 lt1058 1k peak to peak noise detector o to 1v = o to 1v + peak 4.7k p p 1f rst 15 0.1f + ? a6 1/4 lt1058 ? + a8 1/4 lt1058 1k ? peak 4.7k 10k 100k 100k p t t ? + dvm to oscilloscope input via isolated probe, 1v/div = 1v/div, referred to input, sweep = 1s/div from oscilloscope sweep gate output via isolation pulse transformer reset pulse generator 0.22f c2 rc2 +15 +15 clr2 +15 74c221 rst = q2 +v +15 a2 b2 10k bat-85 bat-85 10k + + 0.005f ?15 10k 0.005f figure 16. detailed noise test circuitry. see application note 124.
LTC6655 6655fa ir refow shift the mechanical stress of soldering a part to a board can cause the output voltage to shift. moreover, the heat of an ir refow or convection soldering oven can also cause the output voltage to shift. the materials that make up a semiconductor device and its package have different rates of expansion and contraction. after a part undergoes the extreme heat of a lead-free ir refow profle, like the one a pplica t ions i n f or m a t ion figure 19. lead-free refow profle figure 20. output voltage shift due to ir refow shown in figure 19, the output voltage shifts. after the device expands, due to the heat, and then contracts, the stresses on the die have changed position. this shift is similar, but more extreme than thermal hysteresis. experimental results of ir refow shift are shown below in figure 20. these results show only shift due to refow and not mechanical stress. minutes 0 temperature (c) 150 225 8 6655 f19 75 0 2 4 6 10 300 t = 150c t s = 190c t l = 217c t p = 260c 380s t p 30s t l 130s 40s 120s ramp down t s(max) = 200c ramp to 150c output voltage shift due to ir reflow (%) ?0.029 0 number of units 2 4 6 ?0.023 ?0.017 ?0.005 ?0.011 8 1 3 5 7 6655 f20 figure 17. low frequency noise histogram of the LTC6655-2.5 figure 18. LTC6655-2.5 low frequency noise spectrum peak-to-peak noise (nv) 450 0 number of observations 5 15 20 25 35 6655 f17 10 30 650 950 550 750 850 frequency (hz) 0.1 0 noise voltage (nv/ hz) 120 160 200 1 10 100 6655 f18 80 40
LTC6655 6655fa typical a pplica t ions LTC6655-2.5 gnd shdn v in v out_s v out_f v out c1 0.1f r1 bzx84c12 4v to 30v c2 10f 6655 ta02 extended supply range reference extended supply range reference LTC6655-2.5 gnd v in shdn v out_s v out_f v out 6v to 80v on semi mmbt5551 c1 0.1f r1 100k r2 4.7k bzx84c12 c2 10f 0.1f 6655 ta03 boosted output current LTC6655-2.5 gnd v in shdn v out_s v out + 1.8v to 13.2v v out_f v out c1 1f c3 0.1f r1 220 c2 10f 6655 ta04 2n2905 35ma max r2 1k c4 1f q1 2n2222 i max set by npn v out 6655 ta05 LTC6655-2.5 gnd shdn v in v out_s v out_f c2 4.7f c1 0.1f 4v to 13.2v boosted output current output voltage boost LTC6655-2.5 gnd shdn v in v out_s r r = 0k to 1k v out_f v out 2.5v to 4.5v c1 1f v in v out + 0.5v to 13.2v c2 10f 6655 ta07 v out = voltage option + 0.002 ? r this example uses 2.5v as the voltage option for r use a potentiometer that can handle 2ma, is low noise and has a low temperature coefficient low noise precision voltage boost circuit LTC6655-2.5 lt1677 gnd shdn v in v out_s v out_f v out 5v c1 1f v in v out + 0.5v to 13.2v c2 10f r1 10k r3 5k v in 6655 ta08 v out = voltage option ? (1 + r1/r2) this example uses 2.5v as the voltage option for r1 and r2 use vishay trimmed resistor array (vsr144 or mpm). with a precision array the matching and low tc will help preserve low drift. r3 = r1||r2 + ? r load r2 10k + ?
LTC6655 6655fa typical a pplica t ions low noise statistical averaging reference e n = e n /n ; where n is the number of LTC6655s in parallel 6655 ta06a LTC6655-2.5 r1 32.4 gnd shdn v in v out v out_s v out_f c2 2.7f c1 0.1f LTC6655-2.5 r2 32.4 gnd shdn v in v out_s v out_f c4 2.7f c3 0.1f LTC6655-2.5 r3 32.4 gnd shdn v in v out_s v out_f c6 2.7f c5 0.1f LTC6655-2.5 r4 32.4 gnd shdn v in v out_s v out_f c8 2.7f c7 0.1f c9 4.7f 3v to 13.2v 200nv/ div 6655 ta06b 1s/div 320nv p-p 0.1hz to 10hz low frequency noise (0.1hz to 10hz) with four LTC6655-2.5 in parallel
LTC6655 0 6655fa p ac k age descrip t ion ms8 package 8-lead plastic msop (reference ltc dwg # 05-08-1660 rev f) msop (ms8) 0307 rev f 0.53 p 0.152 (.021 p .006) seating plane note: 1. dimensions in millimeter/(inch) 2. drawing not to scale 3. dimension does not include mold flash, protrusions or gate burrs. mold flash, protrusions or gate burrs shall not exceed 0.152mm (.006") per side 4. dimension does not include interlead flash or protrusions. interlead flash or protrusions shall not exceed 0.152mm (.006") per side 5. lead coplanarity (bottom of leads after forming) shall be 0.102mm (.004") max 0.18 (.007) 0.254 (.010) 1.10 (.043) max 0.22 ? 0.38 (.009 ? .015) typ 0.1016 p 0.0508 (.004 p .002) 0.86 (.034) ref 0.65 (.0256) bsc 0o ? 6o typ detail ?a? detail ?a? gauge plane 1 2 3 4 4.90 p 0.152 (.193 p .006) 8 7 6 5 3.00 p 0.102 (.118 p .004) (note 3) 3.00 p 0.102 (.118 p .004) (note 4) 0.52 (.0205) ref 5.23 (.206) min 3.20 ? 3.45 (.126 ? .136) 0.889 p 0.127 (.035 p .005) recommended solder pad layout 0.42 p 0.038 (.0165 p .0015) typ 0.65 (.0256) bsc
LTC6655 6655fa 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 representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. r e v ision h is t ory rev date description page number a 02/10 voltage options added (1.250, 2.048, 3.000, 3.300, 4.096, 5.000), refected throughout the data sheet 1 to 22
LTC6655 6655fa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com ? linear technology corporation 2009 lt 0210 rev a ? printed in usa r ela t e d p ar t s typical a pplica t ion low noise precision 24-bit analog-to-digital converter application ?2.5v 7.5v spi interface thermocouple 10f 0.1f LTC6655 v in shdn v out_f v out_s gnd gnd 3,5,8 4 1 2 6 7 v cc 5v 6655 ta09 1nf 1nf 0.01f 0.01f 50 2.5k 50 2.5k + ? + ? 1/2 ltc6241 1/2 ltc6241 ch0 ch1 ch2 ch3 ch4 ch5 ch6 ch7 ch8 ch9 ch10 ch11 ch12 ch13 ch14 ch15 com ref + ref ? gnd gnd gnd gnd gnd gnd gnd muxoutn adcinn muxoutp adcinp sdi sck sdo cs busy ext f o ltc2449 5k r ref 400 v ref r td v ref part number description comments lt ? 1236 precision low drift low noise reference 0.05% max, 5ppm/c max, 1ppm (peak-to-peak) noise lt1460 micropower series references 0.075% max, 10ppm/c max, 20ma output current lt1461 micropower series low dropout 0.04% max, 3ppm/c max, 50ma output current lt1790 micropower precision series references 0.05% max, 10ppm/c max, 60ma supply, sot23 package lt6650 micropower reference with buffer amplifer 0.5% max, 5.6a supply, sot23 package ltc6652 precision low drift low noise reference 0.05% max, 5ppm/c max, C40c to 125c, msop8 lt6660 tiny micropower series reference 0.2% max, 20ppm/c max, 20ma output current, 2mm 2mm dfn

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