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lm2576/lm2576hv www.artschip.com 1 lm2576/lm2576hv series simple switcher ? 3a step-down voltage regulator general description the lm2576 series of regulators are monolithic integrated circuits that provide all the ac tive functions for a step-down (buck) switching regulator, capable of driving 3a load with excellent line and load regulation. these devices are available in fixed output voltages of 3. 3v, 5v,12v,15v and an adjustable output version. requiring a minimum number of external components, these regulators are simple to use and include internal frequency compensation and a fixed-frequency oscillator. the lm2576 series offers a high-efficiency replacement for popular three-terminal linear regulators. it substantially reduces the size of the heat sink, and in some cases no heat sink is required. a standard series of inductors optimized for use with the lm2576 are available from several different manufacturers. the feature greatly simplifies the design of switch- mode power supplies. other features in clude a guaranteed 4% tolerance on output voltage within specified input voltages and output load conditions, and 10% on the oscillator frequency. external shutdown is included, featuring 50 0 a (typical) standby current. the output switch includes cycle- by-cycle current limiting, as well as thermal shutdown for full protection under fault conditions. features 3.3v, 5v, 12v, 15v, and adjustable output versions adjustable version output voltage range,1.23v to 37v (57v for hv version) 4% max over line and load conditions guaranteed 3a output current wide input voltage range, 40v up to 60v for hv version requires only 4 external components 52 khz fixed frequency internal oscillator ttl shutdown capability, low power standby mode high efficiency uses readily available standard inductors thermal shutdown and current limit protection p+ product enhancement tested applications simple high-efficiency step-down (buck) regulator efficient pre-regulator for linear regulators on-card switching regulators positive to negative converter (buck-boost) typical application (fixed output voltage versions) figure 1.
lm2576/lm2576hv www.artschip.com 2 block diagram 3.3v r2=1.7k 5v, r2=3.1k 12v, r2=8.84k 15v, r2=11.3k for adj. version r1=open, r2=0 $ patent pending ordering information output voltage temperature range 3.3 5.0 12 15 adj ns package number package type lm2576hvs-3.3 lm2576hvs-5.0 lm2576h vs-12 lm2576hvs-15 lm2576hvs-adj lm2576s-3.3 lm2576s-5.0 lm2576s -12 lm2576s-15 lm2576s-adj ts5b lm2576hvsx-3.3 lm2576hvsx-5.0 lm2576h vsx-12 lm2576hvsx-15 lm2576hvsx-adj lm2576sx-3.3 lm2576sx-5.0 lm2576 sx-12 lm2576sx-15 lm2576sx-adj ts5b tape & reel to-263 lm2576hvt-3.3 lm2576hvt-5.0 lm2576hvt -12 lm2576hvt-15 lm2576hvt-adj lm2576t-3.3 lm2576t-5.0 lm2576t -12 lm2576t-15 lm2576t-adj t05a lm2576hvt-3.3 flow lb03 lm2576hvt-5.0 flow lb03 lm2576hvt-12 flow lb03 lm2576hvt-15 flow lb03 lm2576hvt-adj flow lb03 -40 t a 125 lm2576t-3.3 flow lb03 lm2576t-5.0 flow lb03 lm2576t-12 flow lb03 lm2576t-15 flow lb03 lm2576t-adj flow lb03 t05d to-220
lm2576/lm2576hv www.artschip.com 3 absolute maximum ratings (note 1) if military/aerospace specified devices are required, please contact the national semiconductor sales office/distributors for ava ilability and specifications. maximum supply voltage lm2576 45v lm2576hv 63v /off pin input voltage -0.3v ? v ? +v in output voltage to ground (steady state) -1v power dissipation internally limited storage temperature range -65 to +150 maximum junction temperature 150 minimum esd rating (c =100pf, r=1.5k $ ) 2kv lead temperature (soldering, 10 seconds) 260 operating ratings temperature range lm2576/lm2576hv -40 ? t j ? +125 supply voltage lm2576 40v lm2576hv 60v lm2576-3.3, lm2576hv-3.3 electrical characteristics specifications with standard type face are for t j =25 , and those with boldface type apply over full operating te mperature range. lm2576-3.3 lm2576hv-3.3 symbol parameter conditions typ limit (note 2) units(limits) system parameters (note 3) test circuit figure 2 v out output voltage v in =12v, i load =0.5a circuit of figure 2 3.3 3.234 3.366 v v(min) v(max) v out output voltage lm2576 6v ? v in ? 40v,0.5a ? i load ? 3a circuit of figure 2 3.3 3.168/3.135 3.432/3.465 v v(min) v(max) v out output voltage lm2576hv 6v ? v in ? 60v, 0.5a ? i load ? 3a circuit of figure 2 3.3 3.168/3.135 3.450/3.482 v v(min) v(max) efficiency v in =12v, i load =3a 75 % lm2576-5.0, lm2576hv-5.0 electrical characteristics specifications with standard type face are for t j =25 , and those with figure 2 boldface type apply over full operating temperature range. lm2576-5.0 lm2576hv-5.0 symbol parameter conditions typ limit (note 2) units(limits) system parameters (note 3) test circuit figure 2 v out output voltage v in =12v, i load =0.5a circuit of figure 2 5.0 4.900 5.100 v v(min) v(max) v out output voltage lm2576 0.5a ? i load ? 3a, 8v ? v in ? 40v circuit of figure 2 5.0 4.800/4.750 5.200/5.250 v v(min) v(max) v out output voltage lm2576hv 0.5a ? i load ? 3a, 8v ? v in ? 60v circuit of figure 2 5.0 4.800/4.750 5.225/5.275 v v(min) v(max) efficiency v in =12v, i load =3a 77 %
lm2576/lm2576hv www.artschip.com 4 lm2576-12, lm2576hv-12 electrical characteristics specifications with standard type face are for t j =25 , and those with boldface type apply over full operating te mperature range. lm2576-12 lm2576hv-12 symbol parameter conditions typ limit(note2) units(limits) system parameters (note 3) test circuit figure 2 v out output voltage v in =25v, i load =0.5a circuit of figure 2 12 11.76 12.24 v v(min) v(max) v out output voltage lm2576 0.5a ? i load ? 3a 15v ? v in ? 40v circuit of figure 2 12 11.52/11.40 12.48/12.60 v v(min) v(max) v out output voltage lm2576hv 0.5a ? i load ? 3a, 15v ? v in ? 60v circuit of figure 2 12 11.54/11.40 12.54/12.66 v v(min) v(max) efficiency v in =15v, i load =3a 88 % lm2576-15, lm2576hv-15 electrical characteristics specifications with standard type face are for t j =25 , and those with boldface type apply over full operating te mperature range. lm2576-15 lm2576hv-15 symbol parameter conditions typ limit(note2) units(limits) system parameters (note 3) test circuit figure 2 v out output voltage v in =25v, i load =0.5a circuit of figure 2 15 14.70 15.30 v v(min) v(max) v out output voltage lm2576 0.5a ? i load ? 3a 18v ? v in ? 40v circuit of figure 2 15 14.40/14.25 15.60/15.75 v v(min) v(max) v out output voltage lm2576hv 0.5a ? i load ? 3a, 18v ? v in ? 60v circuit of figure 2 15 14.40/14.25 15.68/15.83 v v(min) v(max) efficiency v in =18v, i load =3a 88 %
lm2576/lm2576hv www.artschip.com 5 lm2576-adj, lm2576hv-adj electrical characteristics specifications with standard type face are for t j =25 , and those with boldface type apply over full operating temperature range. lm2576-adj lm2576hv-adj symbol parameter conditions typ limit(note2) units(limits) system parameters (note 3) test circuit figure 2 v out feedback voltage v in =12v, i load =0.5a v out =5v circuit of figure 2 1.230 1.217 1.243 v v(min) v(max) v out feedback voltage lm2576 0.5a ? i load ? 3a 8v ? v in ? 40v v out =5v,circuit of figure 2 1.230 1.193/1.180 1.267/1.280 v v(min) v(max) v out feedback voltage lm2576hv 0.5a ? i load ? 3a, 8v ? v in ? 60v v out =5v,circuit of figure 2 1.230 1.193/1.180 1.273/1.286 v v(min) v(max) efficiency v in =12v, i load =3a, v out =5v 77 % all output voltage versions electrical characteristics specifications with standard type face are for t j =25 , and those with boldface type apply over full o perating temperature range. unless otherwise specified, v in =12v for the 3.3v, 5v, and adjustable version, v in =25v for the 12v version, and v in =30v for the 15v version. i load =500ma lm2576-xx lm2576hv-xx symbol parameter conditions typ limit(note2) units(limits) device parameters i b feedback bias current v out =5v(adjustable version only) 50 100/500 na f o oscillator frequency (note 11) 52 47/42 58/63 khz khz (min) khz (max) v sat saturation voltage i out =3a (note 4) 1.4 1.8/2.0 v v(max) dc max duty cycle (on) (note 5) 98 93 % %(min) i cl current limit (note 4,11) 5.8 4.2/3.5 6.9/7.5 a a(min) a(max) i l output leakage current (note 6,7); output = 0v output=-1v output=-1v 7.5 2 30 ma(max) ma ma(max) i q quiescent current (note 6) 5 10 ma ma(max) i stby standby quiescent current /off pin =5v(off) 50 200 0 a 0 a(max) ja ja jc ja thermal resistance t package, junction to ambient (note 8) t package, junction to ambient (note 9) t package, junction to case s package, junction to ambient (note 10) 65 45 2 50 /w /off control test circuit figure2 v ih v out =30v 1.4 2.2/2.4 v(min) v il /off pin logic input level v out =nominal output volt age 1.2 1.0/0.8 v(max) i ih /off pin=5v (off) 12 30 0 a 0 a(max) i il /off pin input current /off pin=0v (on) 0 10 0 a 0 a(max)
lm2576/lm2576hv www.artschip.com 6 note 1: absolute maximum ratings indicate limits beyond which damage to the device may occur. operati ng ratings indicate conditions fo r which the device is intended to be functional, but do not guarantee specific performance limits. for guaranteed specifications and test c onditions, see the electrical characteristics. note 2: all limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). all room temperat ure limits are 100% production tested. all limits at tem perature extremes are guaranteed via correla tion using standard statistical quality co ntrol (sqc) methods. note 3: external components such as the catch diode, inductor, input and output capacitors can affect switching regulator system perfo rmance. when the lm2576/lm2576hv is used as shown in the figure 2 test circui t, system performance will be as shown in system parameters sec tion of electrical characteristics. note 4: output pin sourcing current. no diode , inductor or capacitor connected to output. note 5: feedback pin removed from output and connected to 0v. note 6: feedback pin removed from output and connected to +12v for the adjustable, 3.3v, and 5v versions, and +25v for the 12v and 15v versions, to force the output transistor off. note 7: v in =40v (60v for high voltage version). note 8: junction to ambient thermal resistance (no external heat si nk) for the 5 lead to-220 package mounted vertically, with 1 / 2 inch leads in a socket, or on a pc board with minimum copper area. note 9: junction to ambient thermal resistanc e (no external heat sink) for the 5 l ead to-220 package mounted vertically, with 1 / 4 inch leads soldered to a pc board containing approximately 4 square inches of copper area surrounding the leads. note 10: if the to-263 package is used, the thermal resistance can be r educed by increasing the pc board copper area thermally connected to the package. using 0.5 square inches of copper area, ja is 50 /w, with 1 square inch of copper area, ja is 37 /w, and with 1.6 or more square inches of copper area, ja is 32 /w. note 11: the oscillator frequency reduces to approximately 11khz in the ev ent of and output short or an overload which causes the regul ated output voltage to drop approximately 40% from the nominal output voltage. this self protection feature lowers the average power dissip ation of the ic by lowering the minimum duty cycle from 5% down to approximately 2%. typical performance characteristics (circuit of figure 2)
lm2576/lm2576hv www.artschip.com 7 typical performance characteristics (circuit of figure 2) (continued)
lm2576/lm2576hv www.artschip.com 8 typical performance characteristics (circuit of figure 2) (continued)
lm2576/lm2576hv www.artschip.com 9 typical performance characteristics (circuit of figure 2 ) (continued) feedback voltage vs duty cycle feedback pin current maximum power dissipation (to-263) (see note 10) switching waveforms v out =15v a: output pin voltage, 50v/div b: output pin current, 2a/div c: inductor current, 2a/div d: output ripple voltage, 50mv/div. ac-coupled. horizontal time base: 5 0 s/div load transient response
lm2576/lm2576hv www.artschip.com 10 test circuit and layout guidelines as in any switching regulator, layout is very important. rapidly switching currents associated with wiring inductance generate voltage transients which can cause problems. for minimal inductance and ground loops, the length of the leads indicated by heavy lines should be kept as short as possible. single-point grounding (as indicated) or ground plane construction should be used for best results. when using the adjustable version. physically locate the programming resistors near the regulator, to keep the sensitive feedback wiring short. c in -----100 0 f, 75v, aluminum electrolytic c out -----1000 0 f, 25v, aluminum electrolytic d 1 ----- schottky, mbr360 l 1 ----- 100 0 h, pulse eng. pe-92108 r 1 ----- 2k, 0.1% r 2 ----- 6.12k, 0.1% where v ref = 1.23v, r1 between 1k and 5k. figure2
lm2576/lm2576hv www.artschip.com 11 lm2576 series buck regulator design procedure procedure (fixed output voltage versions) example (fixed output voltage versions) given: v out = regulated output voltage (3.3v, 5v, 12v, or 15v) v in (max) = maximum input voltage l load (max)=maximum load current 1. inductor selection (l1) a. select the correct inductor value selection guide from figures 3,4,5 or figure 6 .(output voltages of 3.3v, 5v, 12v or 15v respectively). for other output voltages, see the design procedure for the adjustable version. b. from the inductor value selection guide, identify the inductance region intersected by v in (max) and l load (max), and note the inductor code for that region. c. identify the inductor value from the inductor code, and select an appropriate inductor from the table shown in figure3 . part numbers are listed for three inductor manufacturers. the inductor chosen must be rate d for operation at the lm2576 switching frequency (52 khz) and for a current rating of 1.15 x l load . for additional inductor information, see the inductor section in the application hints section of this data sheet. 2. output capacitor selection (c out ) a. the value of the output capacitor together with the inductor defines the dominate pole-pair of the switching regulator loop. for stable operation and an acceptable output ripple voltage, (approximately 1% of the out put voltage) a value between 100 0 f and 470 0 f is recommended. b. the capacitor?s voltage rating should be at least 1.5 times greater than the output voltage. for a 5v regulator, a rating of at least 8v is appropriate, and a 10v or 15v rating is recommended. higher voltage electrolytic capacitors generally have lower esr numbers, and for this reason it may be necessary to select a capacitor rated for a higher voltage than would normally be needed. 3. catch diode selection (d1) a. the catch-diode current rating must be at least 1.2 ti mes greater than the maximum load current. also, if the power supply design must withstand a continuous output short, t he diode should have a current rating equal to the maximum current limit of the lm2576. the most stressful condition for this diode is an overload or shorted output condition. b. the reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. 4. input capacitor (c in ) an aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation. given: v out =5v v in (max) =15v l load (max)=3a 1. inductor selection (l1) a . use the selection guide shown in figure 4 , b. from the selecti on guide, the inductance area intersected by the 15v line and 3a line is l100. c. inductor value required is 100 0 h. from the table in figure 3. choose aie 415-0930. pulse engineering pe92108, or renco rl2444. 2. output capacitor selection (c out ) a. c out =680 0 f to 2000 0 f standard aluminum electrolytic. b. capacitor voltage rating=20v. 3. catch diode selection (d1) a. for this example, a 3a current rating is adequate. b. use a 20v 1n5823 or sr302 schottky diode, or any of the suggested fast-recoverv diodes shown in figure8. 4. input capacitor (c in ) a 100 0 f, 25v aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing.
lm2576/lm2576hv www.artschip.com 12 lm25756 series buck regulator design procedure (continued) inductor value selection guides (for continuous mode operation) figure 3. lm2576(hv) -3.3 figure 4. lm2576(hv)-5.0 figure 5. lm2576(hv)-12 figure 6. lm2576(hv)-15
lm2576/lm2576hv www.artschip.com 13 lm2576 series buck regulator design procedure (continued) figure 7. lm2576(hv)-adj procedure (adjustable output voltage versions) example (adjustable output voltage versions) given: v out =regulated output voltage v in (max) =maximum input voltage l load (max)=maximum load current f=switching frequency (fixed at 52khz) 1. programming output voltage (selecting r1 and r2, as shown in figure 2) use the following formula to select the appropriate resistor values. r 1 can be between 1k and 5k. (for best temperature coefficient and stability with time, use 1% metal film resistors) given: v out =10v v in (max) =25v i load (max)=3a f=52khz 1. programming output voltage (selecting r1 and r2) r 2 = 1k (8.13 -1) = 7.13k, closest 1% value is 7.15k
lm2576/lm2576hv www.artschip.com 14 lm2576 series buck regulator design procedure (continued) procedure (adjustable output voltage versions) example (adjustable output voltage versions) 2. inductor selection (l1) a. calculate the inductor volt microsecond constant, e t (v 0 s), from the following formula: b. use the e t value from the previous formula and match it with e t number on the vertical axis of the inductor value selection guide shown in figure 7 . c. on the horizontal axis, select the maximum load current. d. identify the inductance re gion intersected by the e t value and the maximum load current value, and note the inductor code for that region . e. identify the inductor value from the table shown in figure 9. part numbers are listed for three inductor manufacturers. the inductor chosen must be rated for operation at the lm2576 switching frequency (52khz) and for a current rating of 1.15 x l load . for additional inductor informati on, see the inductor section in the application hints section of this data sheet. 3. output capacitor selection (c out ) a . the value of the output capacitor together with the inductor defines the dominate pole-pair of the switching regula tor loop. for stable operation, the capacitor must satisfy the following requirement: the above formula yields capacitor values between 10 0 f and 2200 0 f that will satisfy the loop requirements for stable operation. but to achieve an acc eptable output ripple voltage, (approximately 1% of the output voltage) and transient response, the output capacitor may need to be several times larger than the above formula yields. b. the capacitor?s voltage rating should be at last 1.5 times greater than the output voltage. for a 10v regulator, a rating of at least 15v or more is recommended. higher voltage electrolytic capacitors generally have lower esr numbers, and for this reason it may be necessary to select a capacitor rate for a higher voltage than would normally be needed. 4.catch diode selection (d1) a. the catch-diode current rating must be at least 1.2 times greater than the maximum load current. also, if the power supply design must withstand a continuous output short, the dio de should have a current rating equal to the maximum current limit of the lm2576. the most stressful condition for this diode is and overload or shorted output. see diode selection guide in figure 8 . b. the reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. 5.input capacitor (c in ) an aluminum or tantalum electrolytic bypass capacitor located close to the regulator is needed for stable operation. 2.inductor selection (l1) a. calculate e t(v 0 s ) b. e t=115 v 0 s c. i load (max)=3a d. inductance region = h150 e. inductor value = 150 0 h choose from aie part #415-0936 pulse engineeri ng part #pe-531115, or renco part #rl2445. 3.output capacitor selection (c out ) however, for acceptable output ripple voltage select c out ? 680 0 f c out =680 0 f electrolytic capacitor 4.catch diode selection (d1) a. for this example, a 3.3a current rating is adequate. b. use a 30v 31dq03 schottky diode, or any of the suggested fast-recovery diodes in figure8 . 5.input capacitor ( c in ) a 100 0 f aluminum electrolytic capacitor located near the input and ground pins provides sufficient bypassing.
lm2576/lm2576hv www.artschip.com 15 schottky fast recovery v r 3a 4a-6a 3a 4a-6a 20v 1n5820 mbr320p sr302 1n5823 30v 1n5821 mbr330 31dq03 sr303 50wq03 1n5824 40v 1n5822 mbr340 31dqo4 sr304 mbr340 50wq04 1n5825 50v mbr350 31dq05 sr305 50wq05 60v mbr360 dq06 sr306 50wr06 50sq060 the following diodes are all rated to 100v 31df1 her302 the following diodes are all rated to 100v 50wf10 mur410 her602 figure 8. diode selection guide inductor code inductor value schott (note 12) pulse eng. (note 13) renco (note 14) l47 47 0 h 671 26980 pe-53112 rl2442 l68 68 0 h 671 26990 pe-92114 rl2443 l100 100 0 h 671 27000 pe-92108 rl2444 l150 150 0 h 671 27010 pe-53113 rl1954 l220 220 0 h 671 27020 pe-52626 rl1953 l330 330 0 h 671 27030 pe-52627 rl1952 l470 470 0 h 671 27040 pe-53114 rl1951 l680 680 0 h 671 27050 pe-52629 rl1950 h150 150 0 h 671 27060 pe-53115 rl2445 h220 220 0 h 671 27070 pe-53116 rl2446 h330 330 0 h 671 27080 pe-53117 rl2447 h470 470 0 h 671 27090 pe-53118 rl1961 h680 680 0 h 671 27100 pe-53119 rl1960 h1000 1000 0 h 671 27110 pe-53120 rl1959 h1500 1500 0 h 671 27120 pe-53121 rl1958 h2200 2200 0 h 671 27130 pe-53122 rl2448 note 12: schott corporation, (612) 475 -1173, 1000 parkers lake road, wayzata, mn 55391. note 13: pulse engineering, (619) 674-8100, p .o. box 12235, san diego, ca 92112. note 14: renco electronics incorporated, (516) 586-5566, 60 jeffryn blvd. east, deer park, ny 11729. figure 9. inductor selection by manufacturer?s part number application hints input capacitor (c in ) to maintain stability, the regulator input pin must be by-passed with at least a 100 0 f electrolytic capacitor. the capacitor?s leads must be kept short, and located near the regulator. if the operating temperature range includes temperatures below -25 , the input capacitor value may need to be larger. with most electrolytic capacitors, the capacitance value decreases and the esr increases with lower temperatures and age. paralleling a ceramic or solid tantalum capacitor will increase the regulator stability at cold temperatures. for maximum capacitor operating lifetime, the capacitor?s rms ripple current rating should be greater than
lm2576/lm2576hv www.artschip.com 16 application hints(continued) inductor selection all switching regulators have two basic modes of operation: continuous and discontinuous. the difference between the two types relates to the inductor current, whether it is flowing continuously, or if it drops to zero for a period of time in the normal switching cycle. each mode has distinctively different operating characteristics, whic h can affect the regulator performance and requirements. the lm2576 (or any of the simple switcher family) can be used for both continuous and discontinuous modes of operation. the inductor value selection guides in figure 3 through figure 7 were designed for buck regulator designs of the continuous inductor current type. when using inductor values shown in the inductor selection guide, the peak-to-peak inductor ripple current will be approximately 20% to 30% of the maximum dc current. with relatively heavy load currents, the circuit operates in the continuous mode (inducto r current always flowing), but under light load conditions, the circuit will be forced to the discontinuous mode (inductor current falls to zero for a period of time). this discontinuous m ode of operation is perfectly acceptable. for light loads (le ss than approximately 300ma) it may be desirable to operate the regulator in the discontinuous mode, primarily because of the lo wer inductor values required for the discontinuous mode. the selection guide chooses inductor values suitable for continuous mode operation, but if the inductor value chosen is prohibitively high, the designer sh ould investigate the possibility of discontinuous operation. the computer design software switchers made simple will provide all component values for discontinuous (as well as c ontinuous) mode of operation. inductors are available in differ ent styles such as pot core, toriod, e-frame, bobbin core, etc., as well as different core materials, such as ferrites and powdered iron. the least expensive, the bobbin core type, consists of wire wrapped on a ferrite rod core. this type of construction makes for an inexpensive inductor, but since the magnetic flux is not completely contained within the core, it generates more electromagnetic interference (emi). this emi can cause problems in sensitive circuits, or can give incorrect scope readings because of induced vo ltages in the scope probe. the inductors listed in the selection chart include ferrite pot core construction for aie, powdered iron toroid for pulse engineering, and ferrite bobbin core for renco. an inductor should not be operated beyond its maximum rated current because it may saturate. when an inductor begins to saturate, the inductance decreases rapidly and the inductor begins to look mainly resistive ( the dc resistance of winding). this will cause the switch current to rise very rapidly. different inductor types have different saturation characteristics, and this should be kept in mind when selecting an inductor. the inductor manufacturer?s data sheets include current and energy limits to avoid inductor saturation. inductor ripple current when the switcher is operating in the continuous mode, the inductor current waveform ranges from a triangular to a sawtooth type of waveform (depending on the input voltage). for a given input voltage and out put voltage, the peak-to-peak amplitude of this inductor current waveform remains constant. as the load current rises or falls, the entire sawtooth current waveform also rises or falls. the average dc value of this waveform is equal to the dc load current (in the buck regulator configuration). if the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and the swithcher will change to a discontinuous mode of operation. this is a perfectly acceptable mode of operation. any buck switching regulator (no matter how large the inductor value is) will be forced to run discontinuous if the load current is light enough. output capacitor an output capacitor is required to filter the output voltage and is needed for loop stability. the capacitor should be located near the lm2576 using short pc board traces. standard aluminum electrolytics are usually adequate, but low esr types are recommended for low output ripple voltage and good stability. the esr of a capacitor depends on many factors, some which are: the value, the voltage rating, physical size and the type of construction. in general, low value or low voltage (less than 12v) electrolytic capacitors usually have higher esr numbers. the amount of output ripple voltage is primarily a f unction of the esr (equivalent series resistance) of the output capacitor and the amplitude of the i nductor ripple current ( ? i ind ). see the section on inductor ripple current in application hints. the lower capacitor values (220 0 f - 1000 0 f) will allow typically 50mv to 150mv of output ripple voltage, while larger-value capacitors will reduce the ripple to approximately 20 mv to 50mv. output ripple voltage =( ? i ind ) (esr of c out ) to further reduce the output ripple voltage, several standard electrolytic capacitors may be paralleled, or a higher-grade capacitor may be used. such capacitors are often called ?high-frequency,? ?low-inductance,? or ?low-esr.? these will reduce the output ripple to 10mv or 20mv. however, when operating in the continuous m ode, reducing the esr below 0.03 $ can cause instability in the regulator. tantalum capacitors can have a very low esr, and should be carefully evaluated if it is the only output capacitor. because of their good low temperature charac teristics, a tantalum can be used in parallel with aluminum electrolytics, with the tantalum making up 10% or 20% of the total capacitance. the capacitor?s ripple current rating at 52khz should be at least 50% higher than the peak-to-peak inductor ripple current.
lm2576/lm2576hv www.artschip.com 17 application hints (continued) catch diode buck regulators require a diode to provide a return path for the inductor current when the switch is off. this diode should be located close to the lm2576 using short leads and short printed circuit traces. because of their fast switching speed and low forward voltage drop. schottky diodes provide t he best efficiency, especially in low output voltage switching regulators (less than 5v). fast-recovery, high-efficiency, or ultra-fast recovery diodes are also suitable, but some types with an abrupt turn-off characteristic may cause instability and emi problems, a fast-recovery diode with soft reco very characteristics is a better choice. standard 60hz diodes (e.g., 1n4001 or 1n5400, etc.) are also not suitable. see figure 8 for schottky and ?soft? fast-recovery diode selection guide. output voltage ripple and transients the output voltage of a switchin g power supply will contain a sawtooth ripple voltage at the switcher frequency, typically about 1% of the output voltage, and may also contain short voltage spikes at the peaks of the sawtooth waveform. the output ripple voltage is due mainly to the inductor sawtooth ripple current multiplied by t he esr of the output capacitor. (see the inductor selection in the application hints.) the voltage spikes are present bec ause of the fast switching action of the output switch, and the parasitic inductance of the output filter capacitor. to mini mize these voltage spikes, special low inductance capacitors can be used, and their lead lengths must be kept short. wiring inductance, stray capacitance, as well as the scope probe used to evaluate these transients, all contribute to the amp litude of these spikes. an additional small lc filter (20 0 h&100 0 f) can be added to the output (as shown in figure 15 ) to further reduce the amount of output ripple and transients. a 10x reduction in output ripple voltage and transients is possible with this filter. feedback connection the lm2576 (fixed voltage versions) feedback pin must be wired to the output voltage point of the switching power supply. when using the adjustable version, physically locate both output voltage programming resist ors near the lm2576 to avoid picking up unwanted noise. avoid using resistors greater than 100k $ because of the increased chance of noise pickup. /off input for normal operation, the /off pin should be grounded or driven with a low-level ttl voltage (typically below 1.6v). to put the regulator into standby mode, drive this pin with a high-level ttl or cmos signal. the /off pin can be safely pulled up to +v in without a resistor in series with it. the /off pin should not be left open. grounding to maintain output voltage stability, the power ground connections must be low-impedance (see figure 2). for the 5-lead to-220 and to-263 style package, both the tab and pin 3 are ground and either connection may be used, as they are both part of the same copper lead frame. heat sink/thermal considerations in many cases, only a small heat sink is required to keep the lm2576 junction temperature within the allowed operating range. for each application, to determine whether or not a heat sink will be required, the fo llowing must be identified: 1. maximum ambient temper ature (in the application). 2. maximum regulator power dissipation (in application). 3. maximum allowed junction temperature (125 for the lm2576). for a safe, conservative design, a temperature approximately 15 cooler than the maximum temperatures should be selected. 4. lm2576 package thermal resistances ja and jc . total power dissipated by the lm2576 can be estimated as follows: p d = (v in )(i q )+(v o /v in )(i load )(v sat ) where i q (quiescent current) and v sat can be found in the characteristic curves shown previously, v in is the applied minimum input voltage, v o is the regulated output voltage, and i load is the load current. the dynamic losses during turn-on and turn-off are negligible if a schottky type catch diode is used. when no heat sink is used, the junction temperature rise can be determined by the following: ? t j =(p d ) ( ja ) to arrive at the actual operat ing junction temperature, add the junction temperature rise to the maximum ambient temperature. t j = ? t j + t a if the actual operating junction temperature is greater than the selected safe operating junction temperature determined in step 3, then a heat sink is required. when using a heat sink, the junc tion temperature rise can be determined by the following: ? t j =(p d ) ( jc + interface + heat sink) the operating junction temperature will be: t j =t a + ? t j as above, if the actual operati ng junction temperature is greater than the selected safe operating junction temperature, then a larger heat sink is required (one that has a lower thermal resistance). included on the switcher made simple design software is a more precise (non-linear) thermal model that can be used to determine junction temperature with different input-output parameters or different component values. it can also calculate the heat sink thermal resistance required to maintain the regulators junction temperature below the maximum operating temperature. additional applications inverting regulator figure 10 shows a lm2576-12 in a buck-boost configuration to generate a negative 12v output fr om a positive input voltage. this circuit bootstraps the regu lator?s ground pin to the negative output voltage, then by grounding the feedback pin, the regulator senses the inverted output voltage and regulates it to-12v. for an input voltage of 12v or more, the maximum available output current in this configurat ion is approximately 700ma. at lighter loads, the minimum input voltage required drops to approximately 4.7v.
lm2576/lm2576hv www.artschip.com 18 additional applications (continued) the switch currents in this bu ck-boost configuration are higher than in the standard buck-mode design, thus lowering the available output current. also, the start-up input current of the buck-boost converter is higher than the standard buck-mode regulator, and this may overload an input power source with a current limit less than 5a. using a delayed turn-on or an undervoltage lockout circuit (de scribed in the next section) would allow the input voltage to rise to a high enough level before the switcher would be allowed to turn on. because of the structural differences between the buck and the buck-boost regulator topologie s, the buck regulator design procedure section can not be used to select the inductor or the output capacitor. the recommended range of inductor values for the buck-boost design is between 68 0 h and 220 0 h, and the output capacitor values must be larger than what is normally required for buck designs. low input voltages or high output currents require a large value output capacitor (in the thousands of micro farads). the peak inductor current, which is the same as the peak switch current, can be calculated from the following formula: where f osc =52 khz. under normal continuous inductor current operating conditions, the minimum v in represents the worst case. select an inductor that is rated for the peak current anticipated. figure 10. inverting buck-boost develops-12v also, the maximum voltage appearing across the regulator is the absolute sum of the input and output voltage. for a -12v output, the maximum input voltage for the lm2576 is +28v, or +48v for the lm2576hv. the switchers made simple (version 3.0) design software can be used to determine the feasibility of regulator designs using different topologies, different i nput-output parameters, different components, etc. negative boost regulator another variation on the buck- boost topology is the negative boost configuration. the circuit in figure 11 accepts an input voltage ranging from -5v to -12v and provides a regulated -12v output. input voltages greater than -12v will cause the output to rise above -12v, but will not damage the regulator. typical load current 400ma for v in = -5.2v 750ma for v in = -7v note: heat sink may be required. figure 11. negative boost because of the boosting function of this type of regulator, the switch current is relatively high, especially at low input voltages. output load current limitations are a result of the maximum current rating of the switch. also, boost regulators can not provide current limiting load protec tion in the event of a shorted load, so some other means (such as a fuse) may be necessary. undervoltage lockout in some applications it is desir able to keep the regulator off until the input voltage reaches a certain threshold. an under voltage lockout circuit which accomplishes this task is shown in figure 12 , while figure 13 shows the same circuit applied to a buck-boost configuration. these circuits keep the regulator off until the input voltage reac hes a predetermined level. 01147616 note: complete circuit not shown. figure 12. undervoltage lockout for buck circuit.
lm2576/lm2576hv www.artschip.com 19 additional applications (continued) note: complete circuit not shown (see figure 10 ) figure 13. undervoltage lockout for buck-boost circuit delayed startup the /off pin can be used to provide a delayed start up feature as shown in figure 14 . with an input voltage of 20v and for the part values shown, the circuit provides approximately 10ms of delay time before the circuit beigns switching. increasing the rc time constant can provide longer delay times. but excessively large rc time constants can cause problems with input voltages that are high in 60hz or 120hz ripple, by coupling the ripple into the /off pin. adjustable output, low-ripple power supply a 3a power supply that features an adjustable output voltage is shown in figure 15 . an additional l-c filt er that reduces the output ripple by a factor of 10 or more is included in this circuit. 01147618 note: complete circuit not shown. figure 14. delayed startup 01147619 figure 15. 1.2v to 55v adjustable 3a power supply with low output ripple definition of terms buck regulator a switching regulator topology in which a higher voltage is converted to a lower voltage. also known as a step-down switching regulator. buck-boost regulator a switching regulator topology in which a positive voltage is converted to a negative volt age without a transformer. duty cycle(d) ratio of the output switch?s on- time to the oscillator period. catch diode or current steering diode the diode which provides a return path for the load current when the lm2576 switch is off. efficiency( ? ) the proportion of input power ac tually delivered to the load.
lm2576/lm2576hv www.artschip.com 20 definition of terms (continued) capacitor equivalent series resistance (esr) the purely resistive component of a real capacitor?s impedance ( see figure 16 ). it causes power loss resulting in capacitor heating ,which directly affects t he capacitor?s operating lifetime. when used as a switching regulator output filter, higher esr values result in higher output ripple voltages. figure 16. simple model of a real capacitor most standard aluminum electrolytic capacitors in the 100 0 f -1000 0 f range have 0.5 $ to 0.1 $ esr. higher-grade capacitors (?low-esr?, ?high-frequency?, or ?low-inductance?) in the 100 0 f -1000 0 f range generally have esr of less than 0.15 $ . equivalent series inductance (esl) the pure inductance component of a capacitor ( see figure 16 ). the amount of inductance is dete rmined to a large extent on the capacitor?s construction. in a buck regulator, this unwanted inductance causes voltage spikes to appear on the output. output ripple voltage the ac component of the switch ing regulator?s output voltage. it is usually dominated by the output capacitor?s esr multiplied by the inductor?s ripple current ( ? i ind ). the peak-to-peak value of this sawtooth ripple current can be determined by reading the inductor ripple current section of the application hints. capacitor ripple current rms value of the maximum allowable alternating current at which a capacitor can be operated continuously at a specified temperature. standby quiescent current (i stby ) supply current required by the lm2576 when in the standby mode ( /off pin is driven to ttl-high voltage, thus turning the output switch off). inductor ripple current ( ? i ind ) the peak-to-peak value of t he inductor current waveform, typically a sawtooth waveform when the regulator is operating in the continuous mode ( vs. discontinuous mode). continuous/discontinuo us mode operation relates to the inductor current. in the continuous mode, the inductor current is always flowing and never drops to zero, vs. the discontinuous mode, where the inductor current drops to zero for a period of time in the normal switching cycle. inductor saturation the condition which exists when an inductor cannot hold any more magnetic flux. when an inductor saturates, the inductor appears less inductive and the resistive component dominates. inductor current is then limited only by the dc resistance of the wire and the available source current. operating volt microsecond constant (e top) the product (in volt 0 s) of the voltage applied to the inductor and the time the voltage is applied. this e top constant is a measure of the energy handling capability of an inductor and is dependent upon the type of core , the core area, the number of turns, and the duty cycle. connection diagrams (note 15) straight leads 5-lead to-220(t) top view 01147621 lm2576t-xx or lm2576hvt-xx ns package number t05a to-263(s) 5-lead surface-mount package top view lm2576s-xx or lm2576hvs-xx ns package number ts5b lm2576sx-xx or lm2576hvsx-xx ns package number ts5b, tape and reel bent, staggered leads 5-lead to-220 (t) top view side view lm2576t-xx flow lb03 or lm2576hvt-xx flow lb03 ns package number t05d note15 :(xx indicates output voltage option. see ordering information table for complete part number.)
lm2576/lm2576hv www.artschip.com 21 physical dimensions inches (millimeters) unless otherwise noted
lm2576/lm2576hv www.artschip.com 22 physical dimensions inches (m illimeters) unless otherwise noted (continued)
lm2576/lm2576hv www.artschip.com 23 physical dimensions inches (millimeters) unless otherwise noted (continued)

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