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lm2597/lm2597hv simple switcher ? power converter 150 khz 0.5a step-down voltage regulator, with features general description the lm2597/lm2597hv series of regulators are monolithic integrated circuits that provide all the active functions for a step-down (buck) switching regulator, capable of driving a 0.5a load with excellent line and load regulation. these de- vices are available in fixed output voltages of 3.3v, 5v, 12v, and an adjustable output version, and are packaged in an 8-lead dip and an 8-lead surface mount package. this series of switching regulators is similar to the lm2594 series, with additional supervisory and performance features added. requiring a minimum number of external components, these regulators are simple to use and include internal frequency compensation 2 , improved line and load specifications, fixed-frequency oscillator, shutdown /soft-start, error flag delay and error flag output. the lm2597/lm2597hv series operates at a switching fre- quency of 150 khz thus allowing smaller sized filter compo- nents than what would be needed with lower frequency switching regulators. because of its high efficiency, the cop- per traces on the printed circuit board are normally the only heat sinking needed. a standard series of inductors (both through hole and sur- face mount types) are available from several different manu- facturers optimized for use with the lm2597/lm2597hv se- ries. this feature greatly simplifies the design of switch-mode power supplies. other features include a guaranteed 4 % tolerance on out- put voltage under all conditions of input voltage and output load conditions, and 15 % on the oscillator frequency. ex- ternal shutdown is included, featuring typically 85 a standby current. self protection features include a two stage current limit for the output switch and an over temperature shutdown for complete protection under fault conditions. the lm2597hv is for use in applications requiring and input voltage up to 60v. features n 3.3v, 5v, 12v, and adjustable output versions n adjustable version output voltage range, 1.2v to 37v (57v for hv version) 4 % max over line and load conditions n guaranteed 0.5a output current n available in 8-pin surface mount and dip-8 package n input voltage range up to 60v n 150 khz fixed frequency internal oscillator n shutdown /soft-start n out of regulation error flag n error output delay n bias supply pin (v bs ) for internal circuitry improves efficiency at high input voltages n low power standby mode, i q typically 85 a n high efficiency n uses readily available standard inductors n thermal shutdown and current limit protection applications n simple high-efficiency step-down (buck) regulator n efficient pre-regulator for linear regulators n on-card switching regulators n positive to negative converter typical application (fixed output voltage versions) simple switcher ? and switchers made simple ? are registered trademarks of national semiconductor corporation. ds012440-1 2 patent number 5,382,918. march 1998 lm2597/lm2597hv simple switcher power converter 150 khz 0.5a step-down voltage regulator, with features ? 1998 national semiconductor corporation ds012440 www.national.com
absolute maximum ratings (note 1) if military/aerospace specified devices are required, please contact the national semiconductor sales office/ distributors for availability and specifications. maximum supply voltage (v in ) lm2597 45v lm2597hv 60v sd /ss pin input voltage (note 2) 6v delay pin voltage (note 2) 1.5v flag pin voltage ?0.3 v 45v bias supply voltage (v bs ) ?0.3 v 30v feedback pin voltage ?0.3 v +25v output voltage to ground (steady state) ?1v power dissipation internally limited storage temperature range ?65c to +150c esd susceptibility human body model (note 3) 2 kv lead temperature m8 package vapor phase (60 sec.) +215c infrared (15 sec.) +220c n package (soldering, 10 sec.) +260c maximum junction temperature +150c operating conditions temperature range ?40c t j +125c supply voltage lm2597 4.5v to 40v lm2597hv 4.5v to 60v lm2597/lm2597hv-3.3 electrical characteristics specifications with standard type face are for t j = 25c, and those with boldface type apply over full operating tempera- ture range. v inmax = 40v for the lm2597 and 60v for the lm2597hv symbol parameter conditions lm2597/lm2597hv-3.3 units (limits) typ limit (note 4) (note 5) system parameters (note 6) test circuit figure 12 v out output voltage 4.75v v in v inmax , 0.1a i load 0.5a 3.3 v 3.168/ 3.135 v(min) 3.432/ 3.465 v(max) h efficiency v in = 12v, i load = 0.5a 80 % lm2597/lm2597hv-5.0 electrical characteristics specifications with standard type face are for t j = 25c, and those with boldface type apply over full operating tempera- ture range. v inmax = 40v for the lm2597 and 60v for the lm2597hv symbol parameter conditions lm2597/lm2597hv-5.0 units (limits) typ limit (note 4) (note 5) system parameters (note 6) test circuit figure 12 v out output voltage 7v v in v inmax , 0.1a i load 0.5a 5 v 4.800/ 4.750 v(min) 5.200/ 5.250 v(max) h efficiency v in = 12v, i load = 0.5a 82 % www.national.com 2
lm2597/lm2597hv-12 electrical characteristics specifications with standard type face are for t j = 25c, and those with boldface type apply over full operating tempera- ture range. v inmax = 40v for the lm2597 and 60v for the lm2597hv symbol parameter conditions lm2597/lm2597hv-12 units (limits) typ limit (note 4) (note 5) system parameters (note 6) test circuit figure 12 v out output voltage 15v v in v inmax , 0.1a i load 0.5a 12 v 11.52/ 11.40 v(min) 12.48/ 12.60 v(max) h efficiency v in = 25v, i load = 0.5a 88 % lm2597/lm2597hv-adj electrical characteristics specifications with standard type face are for t j = 25c, and those with boldface type apply over full operating tempera- ture range. v inmax = 40v for the lm2597 and 60v for the lm2597hv symbol parameter conditions lm2597/lm2597hv-adj units (limits) typ limit (note 4) (note 5) system parameters (note 6) test circuit figure 12 v fb feedback voltage 4.5v v in v inmax , 0.1a i load 0.5a 1.230 v v out programmed for 3v. circuit of figure 12 . 1.193/ 1.180 v(min) 1.267/ 1.280 v(max) h efficiency v in = 12v, v out = 3v, i load = 0.5a 80 % all output voltage versions electrical characteristics specifications with standard type face are for t j = 25c, and those with boldface type apply over full operating tempera- ture range. unless otherwise specified, v in = 12v for the 3.3v, 5v, and adjustable version and v in = 24v for the 12v ver- sion. i load = 100 ma. symbol parameter conditions lm2597/lm2597hv-xx units (limits) typ limit (note 4) (note 5) device parameters i b feedback bias current adjustable version only, v fb = 1.235v 10 50/ 100 na f o oscillator frequency (note 7) 150 khz 127/ 110 khz(min) 173/ 173 khz(max) v sat saturation voltage i out = 0.5a (notes 8 and 9) 0.9 v 1.1/ 1.2 v(max) dc max duty cycle (on) (note 9) 100 % min duty cycle (off) (note 10) 0 i cl current limit peak current, (notes 8 and 9) 0.8 a 0.65/ 0.58 a(min) 1.3/ 1.4 a(max) i l output leakage current (notes 8, 10 and 11) output = 0v 50 a(max) output = ?1v 2 ma 15 ma(max) i q operating quiescent sd /ss pin open, v bs pin open(note 10) 5ma current 10 ma(max) 3 www.national.com
all output voltage versions electrical characteristics (continued) specifications with standard type face are for t j = 25c, and those with boldface type apply over full operating tempera- ture range. unless otherwise specified, v in = 12v for the 3.3v, 5v, and adjustable version and v in = 24v for the 12v ver- sion. i load = 100 ma. symbol parameter conditions lm2597/lm2597hv-xx units (limits) typ limit (note 4) (note 5) device parameters i stby standby quiescent sd /ss pin = 0v (note 10)lm2597 85 a current 200/ 250 a(max) lm2597hv 140 250/ 300 a(max) q ja thermal resistance n package, junction to ambient (note 12) 95 c/w m package, junction to ambient (note 12) 150 shutdown/soft-start control test circuit of figure 12 v sd shutdown threshold 1.3 v voltage low, (shutdown mode) 0.6 v(max) high, (soft-start mode) 2 v(min) v ss soft-start voltage v out = 20 % of nominal output voltage 2 v v out = 100 % of nominal output voltage 3 i sd shutdown current v shutdown = 0.5v 5a 10 a(max) i ss soft-start current v soft-start = 2.5v 1.6 a 5 a(max) flag/delay control test circuit of figure 12 regulator dropout low (flag on) 96 % detector 92 % (min) threshold voltage 98 % (max) vf sat flag output saturation i sink = 3 ma 0.3 v voltage v delay = 0.5v 0.7/ 1.0 v(max) if l flag output leakage current v flag = 40v 0.3 a delay pin threshold 1.25 v voltage low (flag on) 1.21 v(min) high (flag off) and v out regulated 1.29 v(max) delay pin source v delay = 0.5v 3 a current 6 a(max) delay pin saturation low (flag on) 55 mv 350/ 400 mv(max) bias supply i bs bias supply pin current v bs = 2v (note 10) 120 a 400 a(max) v bs = 4.4v (note 10) 4 ma 10 ma(max) i q operating quiescent current v bs = 4.4v , v in pin current(note 10) 1 2 ma note 1: absolute maximum ratings indicate limits beyond which damage to the device may occur. operating ratings indicate conditions for which the device is in- tended to be functional, but do not guarantee specific performance limits. for guaranteed specifications and test conditions, see the electrical characteristics. note 2: voltage internally clamped. if clamp voltage is exceeded, limit current to a maximum of 1 ma. note 3: the human body model is a 100 pf capacitor discharged through a 1.5k resistor into each pin. note 4: typical numbers are at 25c and represent the most likely norm. note 5: all limits guaranteed at room temperature (standard type face) and at temperature extremes (bold type face). all room temperature limits are 100 % produc- tion tested. all limits at temperature extremes are guaranteed via correlation using standard statistical quality control (sqc) methods. all limits are used to calculate average outgoing quality level (aoql). www.national.com 4
all output voltage versions electrical characteristics (continued) note 6: external components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. when the lm2597/ lm2597hv is used as shown in the figure 12 test circuit, system performance will be as shown in system parameters section of electrical characteristics. note 7: the switching frequency is reduced when the second stage current limit is activated. the amount of reduction is determined by the severity of current over- load. note 8: no diode, inductor or capacitor connected to output pin. note 9: feedback pin removed from output and connected to 0v to force the output transistor switch on. note 10: feedback pin removed from output and connected to 12v for the 3.3v, 5v, and the adj. version, and 15v for the 12v version, to force the output transistor switch off. note 11: v in = 40v for the lm2597 and 60v for the lm2597hv. note 12: junction to ambient thermal resistance with approximately 1 square inch of printed circuit board copper surrounding the leads. additional copper area will lower thermal resistance further. see application hints in this data sheet and the thermal model in switchers made simple ? software. typical performance characteristics normalized output voltage ds012440-2 line regulation ds012440-3 efficiency ds012440-4 switch saturation voltage ds012440-5 switch current limit ds012440-6 dropout voltage ds012440-7 quiescent current ds012440-8 standby quiescent current ds012440-9 minimum operating supply voltage ds012440-10 5 www.national.com
typical performance characteristics (continued) feedback pin bias current ds012440-11 flag saturation voltage ds012440-12 switching frequency ds012440-13 soft-start ds012440-14 shutdown /soft-start current ds012440-15 delay pin current ds012440-16 v in and v bs current vs v bs and temperature ds012440-17 soft-start response ds012440-18 shutdown /soft-start threshold voltage ds012440-25 www.national.com 6
typical performance characteristics (continued) connection diagrams and ordering information continuous mode switching waveforms v in = 20v, v out = 5v, i load = 400 ma l = 100 h, c out = 120 f, c out esr = 140 m w ds012440-19 a: output pin voltage, 10v/div. b: inductor current 0.2a/div. c: output ripple voltage, 20 mv/div. horizontal time base: 2 s/div. discontinuous mode switching waveforms v in = 20v, v out = 5v, i load = 200 ma l = 33 h, c out = 220 f, c out esr = 60 m w ds012440-20 a: output pin voltage, 10v/div. b: inductor current 0.2a/div. c: output ripple voltage, 20 mv/div. horizontal time base: 2 s/div. load transient response for continuous mode v in = 20v, v out = 5v, i load = 200 ma to 500 ma l = 100 h, c out = 120 f, c out esr = 140 m w ds012440-21 a: output voltage, 50 mv/div. (ac) b: 200 ma to 500 ma load pulse horizontal time base: 50 s/div. load transient response for discontinuous mode v in = 20v, v out = 5v, i load = 100 ma to 200 ma l = 33 h, c out = 220 f, c out esr = 60 m w ds012440-22 a: output voltage, 50 mv/div. (ac) b: 100 ma to 200 ma load pulse horizontal time base: 200 s/div. 8lead dip (n) ds012440-23 top view order number lm2597n-3.3, lm2597n-5.0, lm2597n-12 or lm2597n-adj lm2597hvn-3.3, lm2597hvn-5.0, lm2597hvn-12 or lm2597hvn-adj see ns package number n08e 8lead surface mount (m) ds012440-24 top view order number lm2597m-3.3, lm2597m-5.0, lm2597m-12 or lm2597m-adj lm2597hvm-3.3, lm2597hvm-5.0, lm2597hvm-12 or lm2597hvm-adj see ns package number m08a 7 www.national.com
lm2597/lm2597hv series buck regulator design procedure (fixed output) procedure (fixed output voltage version) example (fixed output voltage version) given: v out = regulated output voltage (3.3v, 5v or 12v) v in (max) = maximum dc input voltage i load (max) = maximum load current given: v out = 5v v in (max) = 12v i load (max) = 0.4a 1. inductor selection (l1) a. select the correct inductor value selection guide from fig- ure 3 , figure 4 ,or figure 5 . (output voltages of 3.3v, 5v, or 12v respectively.) for all other voltages, see the design pro- cedure for the adjustable version. b. from the inductor value selection guide, identify the induc- tance region intersected by the maximum input voltage line and the maximum load current line. each region is identified by an inductance value and an inductor code (lxx). c. select an appropriate inductor from the four manufacturer's part numbers listed in figure 7 . 1. inductor selection (l1) a. use the inductor selection guide for the 5v version shown in figure 4 . b. from the inductor value selection guide shown in figure 4 , the inductance region intersected by the 12v horizontal line and the 0.4a vertical line is 100 h, and the inductor code is l20. c. the inductance value required is 100 h. from the table in figure 7 , go to the l20 line and choose an inductor part num- ber from any of the four manufacturers shown. (in most in- stance, both through hole and surface mount inductors are available.) 2. output capacitor selection (c out ) a. in the majority of applications, low esr (equivalent series resistance) electrolytic capacitors between 82 f and 220 f and low esr solid tantalum capacitors between 15 f and 100 f provide the best results. this capacitor should be lo- cated close to the ic using short capacitor leads and short copper traces. do not use capacitors larger than 220 f. for additional information, see section on output capaci- tors in application information section. b. to simplify the capacitor selection procedure, refer to the quick design component selection table shown in figure 1 . this table contains different input voltages, output voltages, and load currents, and lists various inductors and output ca- pacitors that will provide the best design solutions. c. the capacitor voltage rating for electrolytic capacitors should be at least 1.5 times greater than the output voltage, and often much higher voltage ratings are needed to satisfy the low esr requirements for low output ripple voltage. d. for computer aided design software, see switchers made simple ? (version 4.1 or later). 2. output capacitor selection (c out ) a. see section on output capacitors in application infor- mation section. b. from the quick design component selection table shown in figure 1 , locate the 5v output voltage section. in the load cur- rent column, choose the load current line that is closest to the current needed in your application, for this example, use the 0.5a line. in the maximum input voltage column, select the line that covers the input voltage needed in your application, in this example, use the 15v line. continuing on this line are recommended inductors and capacitors that will provide the best overall performance. the capacitor list contains both through hole electrolytic and surface mount tantalum capacitors from four different capaci- tor manufacturers. it is recommended that both the manufac- turers and the manufacturer's series that are listed in the table be used. in this example aluminum electrolytic capacitors from several different manufacturers are available with the range of esr numbers needed. 120 f 25v panasonic hfq series 120 f 25v nichicon pl series c. for a 5v output, a capacitor voltage rating at least 7.5v or more is needed. but, in this example, even a low esr, switch- ing grade, 120 f 10v aluminum electrolytic capacitor would exhibit approximately 400 m w of esr (see the curve in figure 16 for the esr vs voltage rating). this amount of esr would result in relatively high output ripple voltage. to reduce the ripple to 1 % of the output voltage, or less, a capacitor with a higher voltage rating (lower esr) should be selected. a 16v or 25v capacitor will reduce the ripple voltage by approxi- mately half. www.national.com 8
lm2597/lm2597hv series buck regulator design procedure (fixed output) (continued) procedure (fixed output voltage version) example (fixed output voltage version) 3. catch diode selection (d1) a. the catch diode current rating must be at least 1.3 times greater than the maximum load current. also, if the power supply design must withstand a continuous output short, the diode should have a current rating equal to the maximum cur- rent limit of the lm2597. 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. c. this diode must be fast (short reverse recovery time) and must be located close to the lm2597 using short leads and short printed circuit traces. because of their fast switching speed and low forward voltage drop, schottky diodes provide the best performance and efficiency, and should be the first choice, especially in low output voltage applications. ultra-fast recovery, or high-efficiency rectifiers also provide good re- sults. ultra-fast recovery diodes typically have reverse recov- ery times of 50 ns or less. rectifiers such as the 1n4001 se- ries are much too slow and should not be used. 3. catch diode selection (d1) a. refer to the table shown in figure 10 . in this example, a 1a, 20v, 1n5817 schottky diode will provide the best perfor- mance, and will not be overstressed even for a shorted out- put. 4. input capacitor (c in ) a low esr aluminum or tantalum bypass capacitor is needed between the input pin and ground to prevent large voltage transients from appearing at the input. in addition, the rms current rating of the input capacitor should be selected to be at least 1 2 the dc load current. the capacitor manufacturers data sheet must be checked to assure that this current rating is not exceeded. the curve shown in figure 15 shows typical rms current ratings for several different aluminum electrolytic capacitor values. this capacitor should be located close to the ic using short leads and the voltage rating should be approximately 1.5 times the maximum input voltage. if solid tantalum input capacitors are used, it is recommended that they be surge current tested by the manufacturer. use caution when using ceramic capacitors for input bypass- ing, because it may cause severe ringing at the v in pin. for additional information, see section on input capaci- tors in application information section. 4. input capacitor (c in ) the important parameters for the input capacitor are the input voltage rating and the rms current rating. with a nominal in- put voltage of 12v, an aluminum electrolytic capacitor with a voltage rating greater than 18v (1.5 x v in ) would be needed. the next higher capacitor voltage rating is 25v. the rms current rating requirement for the input capacitor in a buck regulator is approximately 1 2 the dc load current. in this example, with a 400 ma load, a capacitor with a rms cur- rent rating of at least 200 ma is needed. the curves shown in figure 15 can be used to select an appropriate input capaci- tor. from the curves, locate the 25v line and note which ca- pacitor values have rms current ratings greater than 200 ma. either a 47 f or 68 f, 25v capacitor could be used. for a through hole design, a 68 f/25v electrolytic capacitor (panasonic hfq series or nichicon pl series or equivalent) would be adequate. other types or other manufacturers ca- pacitors can be used provided the rms ripple current ratings are adequate. for surface mount designs, solid tantalum capacitors are rec- ommended. the tps series available from avx, and the 593d series from sprague are both surge current tested. 9 www.national.com
lm2597/lm2597hv series buck regulator design procedure (fixed output) (continued) conditions inductor output capacitor through hole surface mount output load max input inductance inductor panasonic nichicon avx tps sprague voltage current voltage (h) ( # ) hfq series pl series series 595d series (v) (a) (v) (f/v) (f/v) (f/v) (f/v) 3.3 0.5 5 33 l14 220/16 220/16 100/16 100/6.3 7 47 l13 120/25 120/25 100/16 100/6.3 10 68 l21 120/25 120/25 100/16 100/6.3 40 100 l20 120/35 120/35 100/16 100/6.3 6 68 l4 120/25 120/25 100/16 100/6.3 0.2 10 150 l10 120/16 120/16 100/16 100/6.3 40 220 l9 120/16 120/16 100/16 100/6.3 5 0.5 8 47 l13 180/16 180/16 100/16 33/25 10 68 l21 180/16 180/16 100/16 33/25 15 100 l20 120/25 120/25 100/16 33/25 40 150 l19 120/25 120/25 100/16 33/25 9 150 l10 82/16 82/16 100/16 33/25 0.2 20 220 l9 120/16 120/16 100/16 33/25 40 330 l8 120/16 120/16 100/16 33/25 12 0.5 15 68 l21 82/25 82/25 100/16 15/25 18 150 l19 82/25 82/25 100/16 15/25 30 220 l27 82/25 82/25 100/16 15/25 40 330 l26 82/25 82/25 100/16 15/25 15 100 l11 82/25 82/25 100/16 15/25 0.2 20 220 l9 82/25 82/25 100/16 15/25 40 330 l17 82/25 82/25 100/16 15/25 figure 1. lm2597/lm2597hv fixed voltage quick design component selection table www.national.com 10
lm2597/lm2597hv series buck regulator design procedure (adjustable output) procedure (adjustable output voltage version) example (adjustable output voltage version) given: v out = regulated output voltage v in (max) = maximum input voltage i load (max) = maximum load current f = switching frequency (fixed at a nominal 150 khz). given: v out = 20v v in (max) = 28v i load (max) = 0.5a f = switching frequency (fixed at a nominal 150 khz). 1. programming output voltage (selecting r 1 and r 2 ,as shown in figure 12 ) use the following formula to select the appropriate resistor values. select a value for r 1 between 240 w and 1.5 k w . the lower re- sistor values minimize noise pickup in the sensitive feedback pin. (for the lowest temperature coefficient and the best sta- bility with time, use 1 % metal film resistors.) 1. programming output voltage (selecting r 1 and r 2 ,as shown in figure 12 ) select r 1 to be 1 k w ,1 % . solve for r 2 . r 2 = 1k (16.26 ? 1) = 15.26k, closest 1 % value is 15.4 k w . r 2 = 15.4 k w . 2. inductor selection (l1) a. calculate the inductor volt microsecond constant e t (v s), from the following formula: where v sat = internal switch saturation voltage = 0.9v and v d = diode forward voltage drop = 0.5v b. use the e t value from the previous formula and match it with the e t number on the vertical axis of the inductor value selection guide shown in figure 6 . c. on the horizontal axis, select the maximum load current. d. identify the inductance region intersected by the e t value and the maximum load current value. each region is identified by an inductance value and an inductor code (lxx). e. select an appropriate inductor from the four manufacturer's part numbers listed in figure 7 . 2. inductor selection (l1) a. calculate the inductor volt microsecond constant (e t), b. e t = 35.2 (v s) c. i load (max) = 0.5a d. from the inductor value selection guide shown in figure 6 , the inductance region intersected by the 35 (v s) horizontal line and the 0.5a vertical line is 150 h, and the inductor code is l19. e. from the table in figure 7 , locate line l19, and select an in- ductor part number from the list of manufacturers part num- bers. 11 www.national.com
lm2597/lm2597hv series buck regulator design procedure (adjustable output) (continued) procedure (adjustable output voltage version) example (adjustable output voltage version) 3. output capacitor selection (c out ) a. in the majority of applications, low esr electrolytic or solid tantalum capacitors between 82 f and 220 f provide the best results. this capacitor should be located close to the ic using short capacitor leads and short copper traces. do not use capacitors larger than 220 f. for additional informa- tion, see section on output capacitors in application in- formation section. b. to simplify the capacitor selection procedure, refer to the quick design table shown in figure 2 . this table contains dif- ferent output voltages, and lists various output capacitors that will provide the best design solutions. c. the capacitor voltage rating should be at least 1.5 times greater than the output voltage, and often much higher volt- age ratings are needed to satisfy the low esr requirements needed for low output ripple voltage. 3. output capacitor seiection (c out ) a. see section on c out in application information section. b. from the quick design table shown in figure 2 , locate the output voltage column. from that column, locate the output voltage closest to the output voltage in your application. in this example, select the 24v line. under the output capacitor sec- tion, select a capacitor from the list of through hole electrolytic or surface mount tantalum types from four different capacitor manufacturers. it is recommended that both the manufactur- ers and the manufacturers series that are listed in the table be used. in this example, through hole aluminum electrolytic capacitors from several different manufacturers are available. 82 f 50v panasonic hfq series 120 f 50v nichicon pl series c. for a 20v output, a capacitor rating of at least 30v or more is needed. in this example, either a 35v or 50v capacitor would work. a 50v rating was chosen because it has a lower esr which provides a lower output ripple voltage. other manufacturers or other types of capacitors may also be used, provided the capacitor specifications (especially the 100 khz esr) closely match the types listed in the table. re- fer to the capacitor manufacturers data sheet for this informa- tion. 4. feedforward capacitor (c ff ) (see figure 12 ) for output voltages greater than approximately 10v, an addi- tional capacitor is required. the compensation capacitor is typically between 50 pf and 10 nf, and is wired in parallel with the output voltage setting resistor, r 2 . it provides additional stability for high output voltages, low input-output voltages, and/or very low esr output capacitors, such as solid tantalum capacitors. this capacitor type can be ceramic, plastic, silver mica, etc. (because of the unstable characteristics of ceramic capacitors made with z5u material, they are not recommended.) 4. feedforward capacitor (c ff ) the table shown in figure 2 contains feed forward capacitor values for various output voltages. in this example ,a1nfca- pacitor is needed. www.national.com 12
lm2597/lm2597hv series buck regulator design procedure (adjustable output) (continued) procedure (adjustable output voltage version) example (adjustable output voltage version) 5. catch diode selection (d1) a. the catch diode current rating must be at least 1.3 times greater than the maximum load current. also, if the power supply design must withstand a continuous output short, the diode should have a current rating equal to the maximum cur- rent limit of the lm2597. 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. c. this diode must be fast (short reverse recovery time) and must be located close to the lm2597 using short leads and short printed circuit traces. because of their fast switching speed and low forward voltage drop, schottky diodes provide the best performance and efficiency, and should be the first choice, especially in low output voltage applications. ultra-fast recovery, or high-efficiency rectifiers are also a good choice, but some types with an abrupt turn-off characteristic may cause instability or eml problems. ultra-fast recovery diodes typically have reverse recovery times of 50 ns or less. recti- fiers such as the 1n4001 series are much too slow and should not be used. 5. catch diode selection (d1) a. refer to the table shown in figure 10 . schottky diodes pro- vide the best performance, and in this example a 1a, 40v, 1n5819 schottky diode would be a good choice. the 1a diode rating is more than adequate and will not be overstressed even for a shorted output. 6. input capacitor (c in ) a low esr aluminum or tantalum bypass capacitor is needed between the input pin and ground to prevent large voltage transients from appearing at the input. in addition, the rms current rating of the input capacitor should be selected to be at least 1 2 the dc load current. the capacitor manufacturers data sheet must be checked to assure that this current rating is not exceeded. the curve shown in figure 15 shows typical rms current ratings for several different aluminum electrolytic capacitor values. this capacitor should be located close to the ic using short leads and the voltage rating should be approximately 1.5 times the maximum input voltage. if solid tantalum input capacitors are used, it is recomended that they be surge current tested by the manufacturer. use caution when using ceramic capacitors for input bypass- ing, because it may cause severe ringing at the v in pin. for additional information, see section on input capacitor in application information section. 6. input capacitor (c in ) the important parameters for the input capacitor are the input voltage rating and the rms current rating. with a nominal in- put voltage of 28v, an aluminum electrolytic aluminum electro- lytic capacitor with a voltage rating greater than 42v (1.5 x v in ) would be needed. since the the next higher capacitor voltage rating is 50v, a 50v capacitor should be used. the ca- pacitor voltage rating of (1.5 x v in ) is a conservative guideline, and can be modified somewhat if desired. the rms current rating requirement for the input capacitor of a buck regulator is approximately 1 2 the dc load current. in this example, with a 400 ma load, a capacitor with a rms cur- rent rating of at least 200 ma is needed. the curves shown in figure 15 can be used to select an ap- propriate input capacitor. from the curves, locate the 50v line and note which capacitor values have rms current ratings greater than 200 ma. a 47 f/50v low esr electrolytic ca- pacitor capacitor is needed. for a through hole design, a 47 f/50v electrolytic capacitor (panasonic hfq series or nichicon pl series or equivalent) would be adequate. other types or other manufacturers ca- pacitors can be used provided the rms ripple current ratings are adequate. for surface mount designs, solid tantalum capacitors are rec- ommended. the tps series available from avx, and the 593d series from sprague are both surge current tested. to further simplify the buck regulator design procedure, na- tional semiconductor is making available computer design software to be used with the simple switcher line ot switching regulators. switchers made simple ? (version 4.1 or later) is available at national's web site, www.national.com. 13 www.national.com
lm2597/lm2597hv series buck regulator design procedure (adjustable output) (continued) output voltage (v) through hole output capacitor surface mount output capacitor panasonic nichicon pl feedforward avx tps sprague feedforward hfq series series capacitor series 595d series capacitor (f/v) (f/v) (f/v) (f/v) 1.2 220/25 220/25 0 220/10 220/10 0 4 180/25 180/25 4.7 nf 100/10 120/10 4.7 nf 6 82/25 82/25 4.7 nf 100/10 120/10 4.7 nf 9 82/25 82/25 3.3 nf 100/16 100/16 3.3 nf 12 82/25 82/25 2.2 nf 100/16 100/16 2.2 nf 15 82/25 82/25 1.5 nf 68/20 100/20 1.5 nf 24 82/50 120/50 1 nf 10/35 15/35 220 pf 28 82/50 120/50 820 pf 10/35 15/35 220 pf figure 2. output capacitor and feedforward capacitor selection table www.national.com 14
lm2597/lm2597hv series buck regulator design procedure inductor value selection guides (for continuous mode operation) ds012440-57 figure 3. lm2597/lm2597hv-3.3 ds012440-30 figure 4. lm2597/lm2597hv-5.0 ds012440-58 figure 5. lm2597/lm2597hv-12 ds012440-32 figure 6. lm2597/lm2597hv-adj 15 www.national.com
lm2597/lm2597hv series buck regulator design procedure (continued) induc- tance (h) cur- rent (a) schott renco pulse engineering coilcraft through surface through surface through surface surface hole mount hole mount hole mount mount l1 220 0.18 67143910 67144280 rl-5470-3 rl1500-220 pe-53801 pe-53801-s do1608-224 l2 150 0.21 67143920 67144290 rl-5470-4 rl1500-150 pe-53802 pe-53802-s do1608-154 l3 100 0.26 67143930 67144300 rl-5470-5 rl1500-100 pe-53803 pe-53803-s do1608-104 l4 68 0.32 67143940 67144310 rl-1284-68 rl1500-68 pe-53804 pe-53804-s do1608-68 l5 47 0.37 67148310 67148420 rl-1284-47 rl1500-47 pe-53805 pe-53805-s do1608-473 l6 33 0.44 67148320 67148430 rl-1284-33 rl1500-33 pe-53806 pe-53806-s do1608-333 l7 22 0.60 67148330 67148440 rl-1284-22 rl1500-22 pe-53807 pe-53807-s do1608-223 l8 330 0.26 67143950 67144320 rl-5470-2 rl1500-330 pe-53808 pe-53808-s do3308-334 l9 220 0.32 67143960 67144330 rl-5470-3 rl1500-220 pe-53809 pe-53809-s do3308-224 l10 150 0.39 67143970 67144340 rl-5470-4 rl1500-150 pe-53810 pe-53810-s do3308-154 l11 100 0.48 67143980 67144350 rl-5470-5 rl1500-100 pe-53811 pe-53811-s do3308-104 l12 68 0.58 67143990 67144360 rl-5470-6 rl1500-68 pe-53812 pe-53812-s do1608-683 l13 47 0.70 67144000 67144380 rl-5470-7 rl1500-47 pe-53813 pe-53813-s do3308-473 l14 33 0.83 67148340 67148450 rl-1284-33 rl1500-33 pe-53814 pe-53814-s do1608-333 l15 22 0.99 67148350 67148460 rl-1284-22 rl1500-22 pe-53815 pe-53815-s do1608-223 l16 15 1.24 67148360 67148470 rl-1284-15 rl1500-15 pe-53816 pe-53816-s do1608-153 l17 330 0.42 67144030 67144410 rl-5471-1 rl1500-330 pe-53817 pe-53817-s do3316-334 l18 220 0.55 67144040 67144420 rl-5471-2 rl1500-220 pe-53818 pe-53818-s do3316-224 l19 150 0.66 67144050 67144430 rl-5471-3 rl1500-150 pe-53819 pe-53819-s do3316-154 l20 100 0.82 67144060 67144440 rl-5471-4 rl1500-100 pe-53820 pe-53820-s do3316-104 l21 68 0.99 67144070 67144450 rl-5471-5 rl1500-68 pe-53821 pe-53821-s ddo3316-683 l26 330 0.80 67144100 67144480 rl-5471-1 e pe-53826 pe-53826-s e l27 220 1.00 67144110 67144490 rl-5471-2 e pe-53827 pe-53827-s e figure 7. inductor manufacturers part numbers coilcraft inc. phone (800) 322-2645 fax (708) 639-1469 coilcraft inc., europe phone +44 1236 730 595 fax +44 1236 730 627 pulse engineering inc. phone (619) 674-8100 fax (619) 674-8262 pulse engineering inc., phone +353 93 24 107 europe fax +353 93 24 459 renco electronics inc. phone (800) 645-5828 fax (516) 586-5562 schott corp. phone (612) 475-1173 fax (612) 475-1786 figure 8. inductor manufacturers phone numbers nichicon corp. phone (708) 843-7500 fax (708) 843-2798 panasonic phone (714) 373-7857 fax (714) 373-7102 avx corp. phone (803) 448-9411 fax (803) 448-1943 sprague/vishay phone (207) 324-7223 fax (207) 324-4140 figure 9. capacitor manufacturers phone numbers www.national.com 16
lm2597/lm2597hv series buck regulator design procedure (continued) block diagram vr 1a diodes surface mount through hole schottky ultra fast schottky ultra fast recovery recovery 20v all of these diodes are rated to 1n5817 all of these diodes are rated to at least 60v. sr102 at least 60v. mbrs130 1n5818 30v sr103 11dq03 mbrs140 murs120 1n5819 mur120 40v 10bq040 10bf10 sr104 her101 10mq040 11dq04 11df1 50v or more mbrs160 sr105 10bq050 mbr150 10mq060 11dq05 mbrs1100 mbr160 10mq090 sb160 sgl41-60 11dq10 ss16 figure 10. diode selection table ds012440-26 figure 11. 17 www.national.com
typical circuit and layout guidelines fixed output voltage versions ds012440-27 component values shown are for v in = 15v, v out = 5v, i load = 500 ma. c in e 47 f, 50v, aluminum electrolytic nichicon apl serieso c out e 120 f, 25v aluminum electrolytic, nichicon apl serieso d1 e 1a, 30v schottky rectifier, 1n5818 l1 e 100 h, l20 typical values c ss e 0.1 f c delay e 0.1 f r pull up e 4.7k *use bias supply pin for 5v and 12v versions adjustable output voltage versions ds012440-56 select r 1 to be approximately 1 k w ,usea1 % resistor for best stability. component values shown are for v in = 20v, v out = 10v, i load = 500 ma. c in e 68 f, 35v, aluminum electrolytic nichicon apl serieso c out e 120 f, 25v aluminum electrolytic, nichicon apl serieso d1 e 1a, 30v schottky rectifier, 1n5818 l1 e 150 h, l19 r 1 e1k w ,1 % r 2 e 7.15k, 1 % c ff e 3.3 nf, see application information section typical values c ss e 0.1 f c delay e 0.1 f r pull up e 4.7k *for output voltages between 4v and 20v figure 12. typical circuits and layout guides www.national.com 18
typical circuit and layout guidelines (continued) as in any switching regulator, layout is very important. rap- idly switching currents associated with wiring inductance can generate voltage transients which can cause problems. for minimal inductance and ground loops, the wires indicated by heavy lines should be wide printed circuit traces and should be kept as short as possible. for best results, ex- ternal components should be located as close to the switcher lc as possible using ground plane construction or single point grounding. if open core inductors are used, special care must be taken as to the location and positioning of this type of induc- tor. allowing the inductor flux to intersect sensitive feedback, lc groundpath and c out wiring can cause problems. when using the adjustable version, special care must be taken as to the location of the feedback resistors and the as- sociated wiring. physically locate both resistors near the ic, and route the wiring away from the inductor, especially an open core type of inductor. (see application section for more information.) application information pin functions +v in (pin 7)ethis is the positive input supply for the ic switching regulator. a suitable input bypass capacitor must be present at this pin to minimize voltage transients and to supply the switching currents needed by the regulator. ground (pin 6)ecircuit ground. output (pin 8)einternal switch. the voltage at this pin switches between (+v in ?v sat ) and approximately ?0.5v, with a duty cycle of v out /v in . to minimize coupling to sensi- tive circuitry, the pc board copper area connected to this pin should be kept to a minimum. feedback (pin 4)esenses the regulated output voltage to complete the feedback loop. shutdown /soft-start (pin 5)ethis dual function pin pro- vides the following features: (a) allows the switching regula- tor circuit to be shut down using logic level signals thus drop- ping the total input supply current to approximately 80 a. (b) adding a capacitor to this pin provides a soft-start feature which minimizes startup current and provides a controlled ramp up of the output voltage. error flag (pin 1)eopen collector output that provides a low signal (flag transistor on) when the regulated output voltage drops more than 5 % from the nominal output volt- age. on start up, error flag is low until v out reaches 95 % of the nominal output voltage and a delay time determined by the delay pin capacitor. this signal can be used as a reset to a microprocessor on power-up. delay (pin 2)eat power-up, this pin can be used to provide a time delay between the time the regulated output voltage reaches 95 % of the nominal output voltage, and the time the error flag output goes high. bias supply (pin 3)ethis feature allows the regulators in- ternal circuitry to be powered from the regulated output volt- age or an external supply, instead of the input voltage. this results in increased efficiency under some operating condi- tions, such as low output current and/or high input voltage. special note if any of the above four features (shutdown /soft-start, error flag, delay, or bias supply) are not used, the respective pins should be left open. external components soft-start capacitor c ss ea capacitor on this pin provides the regulator with a soft-start feature (slow start-up). when the dc input voltage is first applied to the regulator, or when the shutdown /soft-start pin is allowed to go high, a constant current (ap- proximately 5 a begins charging this capacitor). as the ca- pacitor voltage rises, the regulator goes through four operat- ing regions (see the bottom curve in figure 13 ). 1. regulator in shutdown. when the sd /ss pin voltage is between 0v and 1.3v, the regulator is in shutdown, the out- put voltage is zero, and the ic quiescent current is approxi- mately 85 a. 2. regulator on, but the output voltage is zero. with the sd /ss pin voltage between approximately 1.3v and 1.8v, the internal regulatory circuitry is operating, the quiescent current rises to approximately 5 ma, but the output voltage is still zero. also, as the 1.3v threshold is exceeded, the soft-start capacitor charging current decreases from 5 a down to approximately 1.6 a. this decreases the slope of capacitor voltage ramp. 3. soft-start region. when the sd /ss pin voltage is be- tween 1.8v and 2.8v ( @ 25c), the regulator is in a soft-start condition. the switch (pin 8) duty cycle initially starts out very low, with narrow pulses and gradually get wider as the capacitor sd /ss pin ramps up towards 2.8v. as the duty cycle increases, the output voltage also increases at a con- trolled ramp up. see the center curve in figure 13 . the input supply current requirement also starts out at a low level for the narrow pulses and ramp up in a controlled manner. this is a very useful feature in some switcher topologies that re- quire large startup currents (such as the inverting configura- tion) which can load down the input power supply. note: the lower curve shown in figure 13 shows the soft-start region from 0 % to 100 % . this is not the duty cycle percentage, but the output volt- age percentage. also, the soft-start voltage range has a negative tem- perature coefficient associated with it. see the soft-start curve in the electrical characteristics section. 4. normal operation. above 2.8v, the circuit operates as a standard pulse width modulated switching regulator. the capacitor will continue to charge up until it reaches the inter- nal clamp voltage of approximately 7v. if this pin is driven from a voltage source, the current must be limited to about 1 ma. 19 www.national.com
application information (continued) ds012440-33 figure 13. soft-start, delay, error, output www.national.com 20
application information (continued) delay capacitor c delay eprovides delay for the error flag output. see the upper curve in figure 13 , and also refer to timing diagrams in figure 14 . a capacitor on this pin provides a time delay be- tween the time the regulated output voltage (when it is in- creasing in value) reaches 95 % of the nominal output volt- age, and the time the error flag output goes high. a 3 a constant current from the delay pin charges the delay ca- pacitor resulting in a voltage ramp. when this voltage reaches a threshold of approximately 1.3v, the open collec- tor error flag output (or power ok) goes high. this signal can be used to indicate that the regulated output has reached the correct voltage and has stabilized. if, for any reason, the regulated output voltage drops by 5 % or more, the error output flag (pin 1) immediately goes low (internal transistor turns on). the delay capacitor provides very little delay if the regulated output is dropping out of regulation. the delay time for an output that is decreasing is approximately a 1000 times less than the delay for the rising output. for a 0.1 f delay capacitor, the delay time would be approximately 50 ms when the output is rising and passes through the 95 % threshold, but the delay for the output drop- ping would only be approximately 50 s. r pull up ethe error flag output, (or power ok) is the collec- tor of a npn transistor, with the emitter internally grounded. to use the error flag, a pullup resistor to a positive voltage is needed. the error flag transistor is rated up to a maximum of 45v and can sink approximately 3 ma. if the error flag is not used, it can be left open. input capacitor c in ea low esr aluminum or tantalum bypass capacitor is needed between the input pin and ground pin. it must be lo- cated near the regulator using short leads. this capacitor prevents large voltage transients from appearing at the in- put, and provides the instantaneous current needed each time the switch turns on. the important parameters for the input capacitor are the voltage rating and the rms current rating. because of the relatively high rms currents flowing in a buck regulator's in- put capacitor, this capacitor should be chosen for its rms current rating rather than its capacitance or voltage ratings, although the capacitance value and voltage rating are di- rectly related to the rms current rating. the rms current rating of a capacitor could be viewed as a capacitor's power rating. the rms current flowing through the capacitors internal esr produces power which causes the internal temperature of the capacitor to rise. the rms current rating of a capacitor is determined by the amount of current required to raise the internal temperature approxi- mately 10c above an ambient temperature of 105c. the ability of the capacitor to dissipate this heat to the surround- ing air will determine the amount of current the capacitor can safely sustain. capacitors that are physically large and have a large surface area will typically have higher rms current ratings. for a given capacitor value, a higher voltage electro- lytic capacitor will be physically larger than a lower voltage capacitor, and thus be able to dissipate more heat to the sur- rounding air, and therefore will have a higher rms current rating. ds012440-34 figure 14. timing diagram for 5v output 21 www.national.com
application information (continued) the consequences of operating an electrolytic capacitor above the rms current rating is a shortened operating life. the higher temperature speeds up the evaporation of the ca- pacitor's electrolyte, resulting in eventual failure. selecting an input capacitor requires consulting the manu- facturers data sheet for maximum allowable rms ripple cur- rent. for a maximum ambient temperature of 40c, a gen- eral guideline would be to select a capacitor with a ripple current rating of approximately 50 % of the dc load current. for ambient temperatures up to 70c, a current rating of 75 % of the dc load current would be a good choice for a conservative design. the capacitor voltage rating must be at least 1.25 times greater than the maximum input voltage, and often a much higher voltage capacitor is needed to sat- isfy the rms current requirements. a graph shown in figure 15 shows the relationship between an electrolytic capacitor value, its voltage rating, and the rms current it is rated for. these curves were obtained from the nichicon aplo series of low esr, high reliability electro- lytic capacitors designed for switching regulator applications. other capacitor manufacturers offer similar types of capaci- tors, but always check the capacitor data sheet. astandardo electrolytic capacitors typically have much higher esr numbers, lower rms current ratings and typically have a shorter operating lifetime. because of their small size and excellent performance, sur- face mount solid tantalum capacitors are often used for input bypassing, but several precautions must be observed. a small percentage of solid tantalum capacitors can short if the inrush current rating is exceeded. this can happen at turn on when the input voltage is suddenly applied, and of course, higher input voltages produce higher inrush currents. sev- eral capacitor manufacturers do a 100 % surge current test- ing on their products to minimize this potential problem. if high turn on currents are expected, it may be necessary to limit this current by adding either some resistance or induc- tance before the tantalum capacitor, or select a higher volt- age capacitor. as with aluminum electrolytic capacitors, the rms ripple current rating must be sized to the load current. output capacitor c out ean output capacitor is required to filter the output and provide regulator loop stability. low impedance or low esr electrolytic or solid tantalum capacitors designed for switching regulator applications must be used. when select- ing an output capacitor, the important capacitor parameters are; the 100 khz equivalent series resistance (esr), the rms ripple current rating, voltage rating, and capacitance value. for the output capacitor, the esr value is the most important parameter. the output capacitor requires an esr value that has an up- per and lower limit. for low output ripple voltage, a low esr value is needed. this value is determined by the maximum allowable output ripple voltage, typically 1 % to 2 % of the out- put voltage. but if the selected capacitor's esr is extremely low, there is a possibility of an unstable feedback loop, re- sulting in an oscillation at the output. using the capacitors listed in the tables, or similar types, will provide design solu- tions under all conditions. if very low output ripple voltage (less than 15 mv) is re- quired, refer to the section on output voltage ripple and transients for a post ripple filter. an aluminum electrolytic capacitor's esr value is related to the capacitance value and its voltage rating. in most cases, higher voltage electrolytic capacitors have lower esr values (see figure 16 ). often, capacitors with much higher voltage ratings may be needed to provide the low esr values re- quired for low output ripple voltage. the output capacitor for many different switcher designs of- ten can be satisfied with only three or four different capacitor values and several different voltage ratings. see the quick design component selection tables in figure 1 and figure 2 for typical capacitor values, voltage ratings, and manufactur- ers capacitor types. electrolytic capacitors are not recommended for tempera- tures below ?25c. the esr rises dramatically at cold tem- peratures and typically rises 3x @ ?25c and as much as 10x at ?40c. see curve shown in figure 17 . solid tantalum capacitors have a much better esr spec for cold temperatures and are recommended for temperatures below ?25c. catch diode buck regulators require a diode to provide a return path for the inductor current when the switch turns off. this must be a fast diode and must be located close to the lm2594 using short leads and short printed circuit traces. because of their very fast switching speed and low forward voltage drop, schottky diodes provide the best performance, especially in low output voltage applications (5v and lower). ultra-fast recovery, or high-efficiency rectifiers are also a good choice, but some types with an abrupt turnoff charac- ds012440-28 figure 15. rms current ratings for low esr electrolytic capacitors (typical) ds012440-29 figure 16. capacitor esr vs capacitor voltage rating (typical low esr electrolytic capacitor) www.national.com 22
application information (continued) teristic may cause instability or emi problems. ultra-fast re- covery diodes typically have reverse recovery times of 50 ns or less. rectifiers such as the 1n4001 series are much too slow and should not be used. 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, which can affect the regulators performance and requirements. most switcher designs will operate in the discontinuous mode when the load current is low. the lm2597 (or any of the simple switcher family) can be used for both continuous or discontinuous modes of opera- tion. in many cases the preferred mode of operation is the con- tinuous mode. it offers greater output power, lower peak switch, inductor and diode currents, and can have lower out- put ripple voltage. but it does require larger inductor values to keep the inductor current flowing continuously, especially at low output load currents and/or high input voltages. to simplify the inductor selection process, an inductor selec- tion guide (nomograph) was designed (see figure 3 through figure 6 ). this guide assumes that the regulator is operating in the continuous mode, and selects an inductor that will al- low a peak-to-peak inductor ripple current to be a certain percentage of the maximum design load current. this peak-to-peak inductor ripple current percentage is not fixed, but is allowed to change as different design load currents are selected. (see figure 18 .) by allowing the percentage of inductor ripple current to in- crease for low load currents, the inductor value and size can be kept relatively low. when operating in the continuous mode, the inductor current waveform ranges from a triangular to a sawtooth type of waveform (depending on the input voltage), with the average value of this current waveform equal to the dc output load current. inductors are available in different styles such as pot core, toroid, e-core, bobbin core, etc., as well as different core ma- terials, such as ferrites and powdered iron. the least expen- sive, the bobbin, rod or stick core, consists of wire wrapped on a ferrite bobbin. this type of construction makes for a in- expensive inductor, but since the magnetic flux is not com- pletely contained within the core, it generates more electro-magnetic interference (eml). this magnetic flux can induce voltages into nearby printed circuit traces, thus caus- ing problems with both the switching regulator operation and nearby sensitive circuitry, and can give incorrect scope read- ings because of induced voltages in the scope probe. also see section on open core inductors. the inductors listed in the selection chart include ferrite e-core construction for schott, ferrite bobbin core for renco and coilcraft, and powdered iron toroid for pulse engineer- ing. exceeding an inductor's maximum current rating may cause the inductor to overheat because of the copper wire losses, or the core may saturate. if the inductor begins to saturate, the inductance decreases rapidly and the inductor begins to look mainly resistive (the dc resistance of the winding). this can cause the switch current to rise very rapidly and force the switch into a cycle-by-cycle current limit, thus reducing the dc output load current. this can also result in overheat- ing of the inductor and/or the lm2597. different inductor types have different saturation characteristics, and this should be kept in mind when selecting an inductor. the inductor manufacturers data sheets include current and energy limits to avoid inductor saturation. discontinuous mode operation the selection guide chooses inductor values suitable for continuous mode operation, but for low current applications and/or high input voltages, a discontinuous mode design may be a better choice. it would use an inductor that would ds012440-37 figure 17. capacitor esr change vs temperature ds012440-31 figure 18. ( d i ind ) peak-to-peak inductor ripple current (as a percentage of the load current) vs load current 23 www.national.com
application information (continued) be physically smaller, and would need only one half to one third the inductance value needed for a continuous mode de- sign. the peak switch and inductor currents will be higher in a discontinuous design, but at these low load currents (200 ma and below), the maximum switch current will still be less than the switch current limit. discontinuous operation can have voltage waveforms that are considerable different than a continuous design. the out- put pin (switch) waveform can have some damped sinusoi- dal ringing present. (see photo titled; discontinuous mode switching waveforms) this ringing is normal for discontinu- ous operation, and is not caused by feedback loop instabili- ties. in discontinuous operation, there is a period of time where neither the switch or the diode are conducting, and the inductor current has dropped to zero. during this time, a small amount of energy can circulate between the inductor and the switch/diode parasitic capacitance causing this char- acteristic ringing. normally this ringing is not a problem, un- less the amplitude becomes great enough to exceed the in- put voltage, and even then, there is very little energy present to cause damage. different inductor types and/or core materials produce differ- ent amounts of this characteristic ringing. ferrite core induc- tors have very little core loss and therefore produce the most ringing. the higher core loss of powdered iron inductors pro- duce less ringing. if desired, a series rc could be placed in parallel with the inductor to dampen the ringing. the com- puter aided design software switchers made simple (ver- sion 4.1) will provide all component values for continuous and discontinuous modes of operation. output voltage ripple and transients the output voltage of a switching power supply operating in the continuous mode will contain a sawtooth ripple voltage at the switcher frequency, and may also contain short voltage spikes at the peaks of the sawtooth waveform. the output ripple voltage is a function of the inductor saw- tooth ripple current and the esr of the output capacitor. a typical output ripple voltage can range from approximately 0.5 % to 3 % of the output voltage. to obtain low ripple volt- age, the esr of the output capacitor must be low, however, caution must be exercised when using extremely low esr capacitors because they can affect the loop stability, result- ing in oscillation problems. if very low output ripple voltage is needed (less than 15 mv), a post ripple filter is recom- mended. (see figure 12 .) the inductance required is typi- cally between 1 h and 5 h, with low dc resistance, to maintain good load regulation. a low esr output filter ca- pacitor is also required to assure good dynamic load re- sponse and ripple reduction. the esr of this capacitor may be as low as desired, because it is out of the regulator feed- back loop. the photo shown in figure 19 shows a typical output ripple voltage, with and without a post ripple filter. when observing output ripple with a scope, it is essential that a short, low inductance scope probe ground connection be used. most scope probe manufacturers provide a special probe terminator which is soldered onto the regulator board, preferable at the output capacitor. this provides a very short scope ground thus eliminating the problems associated with the 3 inch ground lead normally provided with the probe, and provides a much cleaner and more accurate picture of the ripple voltage waveform. the voltage spikes are caused by the fast switching action of the output switch, the diode, and the parasitic inductance of the output filter capacitor, and its associated wiring. to mini- mize these voltage spikes, the output capacitor should be designed for switching regulator applications, and the lead lengths must be kept very short. wiring inductance, stray ca- pacitance, as well as the scope probe used to evaluate these transients, all contribute to the amplitude of these spikes. when a switching regulator is operating in the continuous mode, the inductor current waveform ranges from a triangu- lar to a sawtooth type of waveform (depending on the input voltage). for a given input and output voltage, the peak-to-peak amplitude of this inductor current waveform re- mains constant. as the load current increases or decreases, the entire sawtooth current waveform also rises and falls. the average value (or the center) of this current waveform is equal to the dc load current. if the load current drops to a low enough level, the bottom of the sawtooth current waveform will reach zero, and the switcher will smoothly change from a continuous to a discon- tinuous mode of operation. most switcher designs (irregard- less how large the inductor value is) will be forced to run dis- continuous if the output is lightly loaded. this is a perfectly acceptable mode of operation. in a switching regulator design, knowing the value of the peak-to-peak inductor ripple current ( d i ind ) can be useful for determining a number of other circuit parameters. param- eters such as, peak inductor or peak switch current, mini- mum load current before the circuit becomes discontinuous, output ripple voltage and output capacitor esr can all be calculated from the peak-to-peak d i ind . when the inductor nomographs shown in figure 3 through figure 6 are used to ds012440-39 figure 19. post ripple filter waveform ds012440-40 figure 20. peak-to-peak inductor ripple current vs load current www.national.com 24
application information (continued) select an inductor value, the peak-to-peak inductor ripple current can immediately be determined. the curve shown in figure 20 shows the range of ( d i ind ) that can be expected for different load currents. the curve also shows how the peak-to-peak inductor ripple current ( d i ind ) changes as you go from the lower border to the upper border (for a given load current) within an inductance region. the upper border rep- resents a higher input voltage, while the lower border repre- sents a lower input voltage (see inductor selection guides). these curves are only correct for continuous mode opera- tion, and only if the inductor selection guides are used to se- lect the inductor value consider the following example: v out = 5v, maximum load current of 300 ma v in = 15v, nominal, varying between 11v and 20v. the selection guide in figure 4 shows that the vertical line for a 0.3a load current, and the horizontal line for the 15v in- put voltage intersect approximately midway between the up- per and lower borders of the 150 h inductance region. a 150 h inductor will allow a peak-to-peak inductor current ( d i ind ) to flow that will be a percentage of the maximum load current. referring to figure 20 , follow the 0.3a line approxi- mately midway into the inductance region, and read the peak-to-peak inductor ripple current ( d i ind ) on the left hand axis (approximately 150 ma p-p). as the input voltage increases to 20v, it approaches the up- per border of the inductance region, and the inductor ripple current increases. referring to the curve in figure 20 ,itcan be seen that for a load current of 0.3a, the peak-to-peak in- ductor ripple current ( d i ind ) is 150 ma with 15v in, and can range from 175 ma at the upper border (20v in) to 120 ma at the lower border (11v in). once the d i ind value is known, the following formulas can be used to calculate additional information about the switching regulator circuit. 1. peak inductor or peak switch current 2. minimum load current before the circuit becomes dis- continuous 3. output ripple voltage = ( d i ind )x(esr of c out ) = 0.150ax0.240 w = 36 mv p-p 4. open core inductors another possible source of increased output ripple voltage or unstable operation is from an open core inductor. ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to the other end. these magnetic lines of flux will induce a voltage into any wire or pc board copper trace that comes within the induc- tor's magnetic field. the strength of the magnetic field, the orientation and location of the pc copper trace to the mag- netic field, and the distance between the copper trace and the inductor, determine the amount of voltage generated in the copper trace. another way of looking at this inductive coupling is to consider the pc board copper trace as one turn of a transformer (secondary) with the inductor winding as the primary. many millivolts can be generated in a copper trace located near an open core inductor which can cause stability problems or high output ripple voltage problems. if unstable operation is seen, and an open core inductor is used, it's possible that the location of the inductor with re- spect to other pc traces may be the problem. to determine if this is the problem, temporarily raise the inductor away from the board by several inches and then check circuit op- eration. if the circuit now operates correctly, then the mag- netic flux from the open core inductor is causing the problem. substituting a closed core inductor such as a torroid or e-core will correct the problem, or re-arranging the pc layout may be necessary. magnetic flux cutting the ic device ground trace, feedback trace, or the positive or negative traces of the output capacitor should be minimized. sometimes, locating a trace directly beneath a bobbin in- ductor will provide good results, provided it is exactly in the center of the inductor (because the induced voltages cancel themselves out), but if it is off center one direction or the other, then problems could arise. if flux problems are present, even the direction of the inductor winding can make a difference in some circuits. this discussion on open core inductors is not to frighten the user, but to alert the user on what kind of problems to watch out for when using them. open core bobbin or asticko induc- tors are an inexpensive, simple way of making a compact ef- ficient inductor, and they are used by the millions in many dif- ferent applications. ds012440-41 circuit data for temperature rise curve (dip-8) capacitors through hole electrolytic inductor through hole, schott, 100 h diode through hole, 1a 40v, schottky pc board 4 square inches single sided 2 oz. copper (0.0028") figure 21. junction temperature rise, dip-8 25 www.national.com
application information (continued) thermal considerations the lm2597/lm2597hv is available in two packages, an 8-pin through hole dip (n) and an 8-pin surface mount so-8 (m). both packages are molded plastic with a copper lead frame. when the package is soldered to the pc board, the copper and the board are the heat sink for the lm2597 and the other heat producing components. for best thermal performance, wide copper traces should be used. pins should be soldered to generous amounts of printed circuit board copper, (one exception to this is the out- put (switch) pin, which should not have large areas of cop- per). large areas of copper provide the best transfer of heat (lower thermal resistance) to the surrounding air, and even double-sided or multilayer boards provide a better heat path to the surrounding air. unless power levels are small, sock- ets are not recommended because of the added thermal re- sistance it adds and the resultant higher junction tempera- tures. package thermal resistance and junction temperature rise numbers are all approximate, and there are many factors that will affect the junction temperature. some of these fac- tors include board size, shape, thickness, position, location, and even board temperature. other factors are, trace width, printed circuit copper area, copper thickness, single- or double-sided, multilayer board, and the amount of solder on the board. the effectiveness of the pc board to dissipate heat also depends on the size, quantity and spacing of other components on the board. furthermore, some of these com- ponents such as the catch diode will add heat to the pc board and the heat can vary as the input voltage changes. for the inductor, depending on the physical size, type of core material and the dc resistance, it could either act as a heat sink taking heat away from the board, or it could add heat to the board. the curves shown in figure 21 and figure 22 show the lm2597 junction temperature rise above ambient tempera- ture with a 500 ma load for various input and output volt- ages. the bias supply pin was not used (left open) for these curves. connecting the bias supply pin to the output voltage would reduce the junction temperature by approximately 5c to 15c, depending on the input and output voltages, and the load current. this data was taken with the circuit operating as a buck switcher with all components mounted on a pc board to simulate the junction temperature under actual op- erating conditions. this curve is typical, and can be used for a quick check on the maximum junction temperature for vari- ous conditions, but keep in mind that there are many factors that can affect the junction temperature. bias supply feature the bias supply (v bs ) pin allows the lm2597's internal cir- cuitry to be powered from a power source, other than v in , typically the output voltage. this feature can increase effi- ciency and lower junction temperatures under some operat- ing conditions. the greatest increase in efficiency occur with light load currents, high input voltage and low output voltage (4v to 12v). see efficiency curves shown in figure 23 and figure 24 . the curves with solid lines are with the v bs pin connected to the regulated output voltage, while the curves with dashed lines are with the v bs pin open. the bias supply pin requires a minimum of approximately 3.5v at room temperature (4v @ ?40c), and can be as high as 30v, but there is little advantage of using the bias supply feature with voltages greater than 15v or 20v. the current required for the v in pin is typically 4 ma. to use the bias supply feature with output voltages between 4v and 15v, wire the bias pin to the regulated output. since the v bs pin requires a minimum of 4v to operate, the 3.3v part cannot be used this way. when the v bs pin is left open, the intemal regulator circuitry is powered from the input voltage. ds012440-42 circuit data for temperature rise curve (surface mount) capacitors surface mount tantalum, molded ado size inductor surface mount, coilcraft do33, 100 h diode surface mount, 1a 40v, schottky pc board 4 square inches single sided 2 oz. copper (0.0028") figure 22. junction temperature rise, so-8 ds012440-43 figure 23. effects of bias supply feature on 5v regulator efficiency www.national.com 26
application information (continued) shutdown /soft-start the circuit shown in figure 27 is a standard buck regulator with 24v in, 12v out, 100 ma load, and using a 0.068 f soft-start capacitor. the photo in figure 25 and figure 26 show the effects of soft-start on the output voltage, the input current, with, and without a soft-start capacitor. figure 25 also shows the error flag output going high when the output voltage reaches 95 % of the nominal output voltage. the re- duced input current required at startup is very evident when comparing the two photos. the soft-start feature reduces the startup current from 700 ma down to 160 ma, and delays and slows down the output voltage rise time. this reduction in start up current is useful in situations where the input power source is limited in the amount of current it can deliver. in some applications soft-start can be used to replace undervoltage lockout or delayed startup functions. if a very slow output voltage ramp is desired, the soft-start capacitor can be made much larger. many seconds or even minutes are possible. if only the shutdown feature is needed, the soft-start capaci- tor can be eliminated. ds012440-45 figure 24. effects of bias supply feature on 12v regulator efficiency ds012440-44 figure 25. output voltage, input current, error flag signal, at start-up, with soft-start ds012440-46 figure 26. output voltage, input current, at start-up, without soft-start ds012440-47 figure 27. typical circuit using shutdown /soft-start and error flag features 27 www.national.com
application information (continued) lnverting regulator the circuit in figure 28 converts a positive input voltage to a negative output voltage with a common ground. the circuit operates by bootstrapping the regulators ground pin to the negative output voltage, then grounding the feedback pin, the regulator senses the inverted output voltage and regu- lates it. this example uses the lm2597-5 to generate a ?5v output, but other output voltages are possible by selecting other out- put voltage versions, including the adjustable version. since this regulator topology can produce an output voltage that is either greater than or less than the input voltage, the maxi- mum output current greatly depends on both the input and output voltage. the curve shown in figure 29 provides a guide as to the amount of output load current possible for the different input and output voltage conditions. the maximum voltage appearing across the regulator is the absolute sum of the input and output voltage, and this must be limited to a maximum of 40v. in this example, when con- verting +20v to ?5v, the regulator would see 25v between the input pin and ground pin. the lm2597 has a maximum input voltage rating of 40v (60v for the lm2597hv). an additional diode is required in this regulator configuration. diode d1 is used to isolate input voltage ripple or noise from coupling through the c in capacitor to the output, under light or no load conditions. also, this diode isolation changes the topology to closely resemble a buck configuration thus pro- viding good closed loop stability. a schottky diode is recom- mended for low input voltages, (because of its lower voltage drop) but for higher input voltages, a 1n4001 diode could be used. because of differences in the operation of the inverting regu- lator, the standard design procedure is not used to select the inductor value. in the majority of designs, a 100 h, 1 amp inductor is the best choice. capacitor selection can also be narrowed down to just a few values. using the values shown in figure 28 will provide good results in the majority of invert- ing designs. this type of inverting regulator can require relatively large amounts of input current when starting up, even with light loads. input currents as high as the lm2597 current limit (ap- proximately 0.8a) are needed for 1 ms or more, until the out- put reaches its nominal output voltage. the actual time de- ds012440-48 figure 28. inverting ?5v regulator with shutdown and soft-start ds012440-49 figure 29. maximum load current for inverting regulator circuit www.national.com 28
application information (continued) pends on the output voltage and the size of the output capacitor. input power sources that are current limited or sources that can not deliver these currents without getting loaded down, may not work correctly. because of the rela- tively high startup currents required by the inverting topology, the soft-start feature shown in figure 28 is recommended. also shown in figure 28 are several shutdown methods for the inverting configuration. with the inverting configuration, some level shifting is required, because the ground pin of the regulator is no longer at ground, but is now at the negative output voltage. the shutdown methods shown accept ground referenced shutdown signals. undervoltage lockout some applications require the regulator to remain off until the input voltage reaches a predetermined voltage. figure 30 contains a undervoltage lockout circuit for a buck configu- ration, while figure 31 and figure 32 are for the inverting types (only the circuitry pertaining to the undervoltage lock- out is shown). figure 30 uses a zener diode to establish the threshold voltage when the switcher begins operating. when the input voltage is less than the zener voltage, resistors r1 and r2 hold the shutdown /soft-start pin low, keeping the regulator in the shutdown mode. as the input voltage ex- ceeds the zener voltage, the zener conducts, pulling the shutdown /soft-start pin high, allowing the regulator to begin switching. the threshold voltage for the undervoltage lockout feature is approximately 1.5v greater than the zener voltage. figure 31 and figure 32 apply the same feature to an invert- ing circuit. figure 31 features a constant threshold voltage for turn on and turn off (zener voltage plus approximately one volt). if hysteresis is needed, the circuit in figure 32 has a turn on voltage which is different than the turn off volt- age. the amount of hysteresis is approximately equal to the value of the output voltage. since the sd /ss pin has an in- ternal 7v zener clamp, r2 is needed to limit the current into this pin to approximately 1 ma when q1 is on. negative voltage charge pump occasionally a low current negative voltage is needed for bi- asing parts of a circuit. a simple method of generating a negative voltage using a charge pump technique and the switching waveform present at the out pin, is shown in fig- ure 33 . this unregulated negative voltage is approximately equal to the positive input voltage (minus a few volts), and can supply up to a 100 ma of output current. there is a re- quirement however, that there be a minimum load of several hundred ma on the regulated positive output for the charge pump to work correctly. also, resistor r1 is required to limit the charging current of c1 to some value less than the lm2597 current limit (typically 800 ma). this method of generating a negative output voltage without an additional inductor can be used with other members of the simple switcher family, using either the buck or boost topology. ds012440-50 figure 30. undervoltage lockout for a buck regulator ds012440-52 figure 31. undervoltage lockout without hysteresis for an inverting regulator ds012440-53 figure 32. undervoltage lockout with hysteresis for an inverting regulator 29 www.national.com
application information (continued) ds012440-51 figure 33. charge pump for generating a low current, negative output voltage www.national.com 30
application information (continued) typical surface mount pc board layout, fixed output (2x size) ds012440-54 c in e 10 f, 35v, solid tantalum, avx, atps serieso (surface mount, ado size) c out e 100 f, 10v solid tantalum, avx, atps serieso (surface mount, ado size) d1 e 1a, 40v surface mount schottky rectifier l1 e surface mount inductor, coilcraft do33 c ss e soft-start capacitor (surface mount ceramic chip capacitor) c d e delay capacitor (surface mount ceramic chip capacitor) r3 e error flag pullup resistor (surface mount chip resistor) typical surface mount pc board layout, adjustable output (2x size) ds012440-55 c in e 10 f, 35v, solid tantalum, avx, atps serieso (surface mount, ado size) c out e 68 f, 20v solid tantalum, avx, atps serieso (surface mount, ado size) d1 e 1a, 40v surface mount schottky rectifier l1 e surface mount inductor, coilcraft do33 c ss e soft-start capacitor (surface mount ceramic chip capacitor) c d e delay capacitor (surface mount ceramic chip capacitor) cff e feedforward capacitor (surface mount ceramic chip capacitor) r1 e output voltage program resistor (surface mount chip resistor) r2 e output voltage program resistor (surface mount chip resistor) r3 e error flag pullup resistor (surface mount chip resistor) figure 34. 2x printed circuit board layout 31 www.national.com
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physical dimensions inches (millimeters) unless otherwise noted 8-lead (0.150" wide) molded small outline package, order number lm2597m-3.3, lm2597m-5.0, lm2597m-12 or lm2597m-adj lm2597hvm-3.3, lm2597hvm-5.0, lm2597hvm-12 or lm2597hvm-adj ns package number m08a 33 www.national.com
physical dimensions inches (millimeters) unless otherwise noted (continued) life support policy national's products are not authorized for use as critical components in life support de- vices or systems without the express written approval of the president of national semi- conductor corporation. as used herein: 1. life support devices or systems are devices or sys- tems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose fail- ure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. a critical component in any component of a life support device or system whose failure to perform can be rea- sonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. national semiconductor corporation americas tel: 1-800-272-9959 fax: 1-800-737-7018 email: support@nsc.com www.national.com national semiconductor europe fax: +49 (0) 1 80-530 85 86 email: europe.support@nsc.com deutsch tel: +49 (0) 1 80-530 85 85 english tel: +49 (0) 1 80-532 78 32 fran?ais tel: +49 (0) 1 80-532 93 58 italiano tel: +49 (0) 1 80-534 16 80 national semiconductor asia pacific customer response group tel: 65-2544466 fax: 65-2504466 email: sea.support@nsc.com national semiconductor japan ltd. tel: 81-3-5620-6175 fax: 81-3-5620-6179 8-lead (0.300" wide) molded dual-in-line package, order number lm2597n-3.3, lm2597n-5.0, lm2597n-12 or lm2597n-adj lm2597hvn-3.3, lm2597hvn-5.0, lm2597hvn-12 or lm2597hvn-adj ns package number n08e lm2597/lm2597hv simple switcher power converter 150 khz 0.5a step-down voltage regulator, with features national does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and national reserves the right at any time without notice to change said circuitry and specifications.

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