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? semiconductor components industries, llc, 2002 january, 2002 rev. 3 1 publication order number: lm2576/d lm2576 3.0 a, 15 v, step-down switching regulator the lm2576 series of regulators are monolithic integrated circuits ideally suited for easy and convenient design of a stepdown switching regulator (buck converter). all circuits of this series are capable of driving a 3.0 a load with excellent line and load regulation. these devices are available in fixed output voltages of 3.3 v, 5.0 v, 12 v, 15 v, and an adjustable output version. these regulators were designed to minimize the number of external components to simplify the power supply design. standard series of inductors optimized for use with the lm2576 are offered by several different inductor manufacturers. since the lm2576 converter is a switchmode power supply, its efficiency is significantly higher in comparison with popular threeterminal linear regulators, especially with higher input voltages. in many cases, the power dissipated is so low that no heatsink is required or its size could be reduced dramatically. a standard series of inductors optimized for use with the lm2576 are available from several different manufacturers. this feature greatly simplifies the design of switchmode power supplies. the lm2576 features include a guaranteed 4% tolerance on output voltage within specified input voltages and output load conditions, and 10% on the oscillator frequency ( 2% over 0 c to 125 c). external shutdown is included, featuring 80 m a (typical) standby current. the output switch includes cyclebycycle current limiting, as well as thermal shutdown for full protection under fault conditions. features ? 3.3 v, 5.0 v, 12 v, 15 v, and adjustable output versions ? adjustable version output voltage range, 1.23 to 37 v 4% maximum over line and load conditions ? guaranteed 3.0 a output current ? wide input voltage range ? 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 ? moisture sensitivity level (msl) equals 1 applications ? simple highefficiency stepdown (buck) regulator ? efficient preregulator for linear regulators ? oncard switching regulators ? positive to negative converter (buckboost) ? negative stepup converters ? power supply for battery chargers http://onsemi.com see detailed ordering and shipping information in the package dimensions section on page 24 of this data sheet. ordering information 1 5 to220 tv suffix case 314b 1 5 heatsink surface connected to pin 3 to220 t suffix case 314d pin 1. v in 2. output 3. ground 4. feedback 5. on /off d 2 pak d2t suffix case 936a heatsink surface (shown as terminal 6 in case outline drawing) is connected to pin 3 see general marking information in the device marking section on page 24 of this data sheet. device marking information 1 5
lm2576 http://onsemi.com 2 figure 1. block diagram and typical application 7.0 v - 40 v unregulated dc input l1 100 m h gnd +v in 1 c in 100 m f 3on /off 5 output 2 feedback 4 d1 1n5822 c out 1000 m f typical application (fixed output voltage versions) representative block diagram and typical application unregulated dc input +v in 1 c out feedback 4 c in l1 d1 r2 r1 1.0 k output 2 gnd 3 on /off 5 reset latch thermal shutdown 52 khz oscillator 1.235 v band-gap reference freq shift 18 khz comparator fixed gain error amplifier current limit driver 1.0 amp switch on /off 3.1 v internal regulator regulated output v out load output voltage versions 3.3 v 5.0 v 12 v 15 v r2 ( w ) 1.7 k 3.1 k 8.84 k 11.3 k for adjustable version r1 = open, r2 = 0 w lm2576 5.0 v regulated output 3.0 a load this device contains 162 active transistors. absolute maximum ratings (absolute maximum ratings indicate limits beyond which damage to the device may occur.) rating symbol value unit maximum supply voltage v in 45 v on /off pin input voltage 0.3 v v +v in v output voltage to ground (steadystate) 1.0 v power dissipation case 314b and 314d (to220, 5lead) p d internally limited w thermal resistance, junctiontoambient r q ja 65 c/w thermal resistance, junctiontocase r q jc 5.0 c/w case 936a (d 2 pak) p d internally limited w thermal resistance, junctiontoambient r q ja 70 c/w thermal resistance, junctiontocase r q jc 5.0 c/w storage temperature range t stg 65 to +150 c minimum esd rating (human body model: c = 100 pf, r = 1.5 k w ) 2.0 kv lead temperature (soldering, 10 seconds) 260 c maximum junction temperature t j 150 c
lm2576 http://onsemi.com 3 operating ratings (operating ratings indicate conditions for which the device is intended to be functional, but do not guarantee specific performance limits. for guaranteed specifications and test conditions, see the electrical characteristics.) rating symbol value unit operating junction temperature range t j 40 to +125 c supply voltage v in 40 v system parameters ([note 1] test circuit figure 15) electrical characteristics (unless otherwise specified, v in = 12 v for the 3.3 v, 5.0 v, and adjustable version, v in = 25 v for the 12 v version, and v in = 30 v for the 15 v version. i load = 500 ma. for typical values t j = 25 c, for min/max values t j is the operating junction temperature range that applies [note 2], unless otherwise noted.) characteristics symbol min typ max unit lm25763.3 ([note 1] test circuit figure 15) output voltage (v in = 12 v, i load = 0.5 a, t j = 25 c) v out 3.234 3.3 3.366 v output voltage (6.0 v v in 40 v, 0.5 a i load 3.0 a) v out v t j = 25 c 3.168 3.3 3.432 t j = 40 to +125 c 3.135 3.465 efficiency (v in = 12 v, i load = 3.0 a) h 75 % lm25765 ([note 1] test circuit figure 15) output voltage (v in = 12 v, i load = 0.5 a, t j = 25 c) v out 4.9 5.0 5.1 v output voltage (8.0 v v in 40 v, 0.5 a i load 3.0 a) v out v t j = 25 c 4.8 5.0 5.2 t j = 40 to +125 c 4.75 5.25 efficiency (v in = 12 v, i load = 3.0 a) h 77 % lm257612 ([note 1] test circuit figure 15) output voltage (v in = 25 v, i load = 0.5 a, t j = 25 c) v out 11.76 12 12.24 v output voltage (15 v v in 40 v, 0.5 a i load 3.0 a) v out v t j = 25 c 11.52 12 12.48 t j = 40 to +125 c 11.4 12.6 efficiency (v in = 15 v, i load = 3.0 a) h 88 % lm257615 ([note 1] test circuit figure 15) output voltage (v in = 30 v, i load = 0.5 a, t j = 25 c) v out 14.7 15 15.3 v output voltage (18 v v in 40 v, 0.5 a i load 3.0 a) v out v t j = 25 c 14.4 15 15.6 t j = 40 to +125 c 14.25 15.75 efficiency (v in = 18 v, i load = 3.0 a) h 88 % lm2576 adjustable version ([note 1] test circuit figure 15) feedback voltage (v in = 12 v, i load = 0.5 a, v out = 5.0 v, t j = 25 c) v out 1.217 1.23 1.243 v feedback voltage (8.0 v v in 40 v, 0.5 a i load 3.0 a, v out = 5.0 v) v out v t j = 25 c 1.193 1.23 1.267 t j = 40 to +125 c 1.18 1.28 efficiency (v in = 12 v, i load = 3.0 a, v out = 5.0 v) h 77 % 1. external components such as the catch diode, inductor, input and output capacitors can affect switching regulator system perf ormance. when the lm2576 is used as shown in the figure 15 test circuit, system performance will be as shown in system parameters section . 2. tested junction temperature range for the lm2576: t low = 40 c t high = +125 c
lm2576 http://onsemi.com 4 device parameters electrical characteristics (unless otherwise specified, v in = 12 v for the 3.3 v, 5.0 v, and adjustable version, v in = 25 v for the 12 v version, and v in = 30 v for the 15 v version. i load = 500 ma. for typical values t j = 25 c, for min/max values t j is the operating junction temperature range that applies [note 2], unless otherwise noted.) characteristics symbol min typ max unit all output voltage versions feedback bias current (v out = 5.0 v [adjustable version only]) i b na t j = 25 c 25 100 t j = 40 to +125 c 200 oscillator frequency [note 3] f osc khz t j = 25 c 52 t j = 0 to +125 c 47 58 t j = 40 to +125 c 42 63 saturation voltage (i out = 3.0 a [note 4]) v sat v t j = 25 c 1.5 1.8 t j = 40 to +125 c 2.0 max duty cycle (aono) [note 5] dc 94 98 % current limit (peak current [notes 3 and 4]) i cl a t j = 25 c 4.2 5.8 6.9 t j = 40 to +125 c 3.5 7.5 output leakage current [notes 6 and 7], t j = 25 c i l ma output = 0 v 0.8 2.0 output = 1.0 v 6.0 20 quiescent current [note 6] i q ma t j = 25 c 5.0 9.0 t j = 40 to +125 c 11 standby quiescent current (on /off pin = 5.0 v (aoffo)) i stby m a t j = 25 c 80 200 t j = 40 to +125 c 400 on /off pin logic input level (test circuit figure 15) v v out = 0 v v ih t j = 25 c 2.2 1.4 t j = 40 to +125 c 2.4 v out = nominal output voltage v il t j = 25 c 1.2 1.0 t j = 40 to +125 c 0.8 on /off pin input current (test circuit figure 15) m a on /off pin = 5.0 v (aoffo), t j = 25 c i ih 15 30 on /off pin = 0 v (aono), t j = 25 c i il 0 5.0 3. the oscillator frequency reduces to approximately 18 khz in the event of an output short or an overload which causes the regu lated output voltage to drop approximately 40% from the nominal output voltage. this self protection feature lowers the average dissipation of the ic by lowering the minimum duty cycle from 5% down to approximately 2%. 4. output (pin 2) sourcing current. no diode, inductor or capacitor connected to output pin. 5. feedback (pin 4) removed from output and connected to 0 v. 6. feedback (pin 4) removed from output and connected to +12 v for the adjustable, 3.3 v, and 5.0 v versions, and +25 v for the 12 v and 15 v versions, to force the output transistor aoffo. 7. v in = 40 v.
lm2576 http://onsemi.com 5 i q , quiescent current (ma) 40 typical performance characteristics (circuit of figure 15) v out , output voltage change (%) v out , output voltage change (%) , standby quiescent current ( t j , junction temperature ( c) i o , output current (a) t j , junction temperature ( c) v in , input voltage (v) v in , input voltage (v) input - output differential (v) t j , junction temperature ( c) figure 2. normalized output voltage t j , junction temperature ( c) figure 3. line regulation figure 4. dropout voltage figure 5. current limit figure 6. quiescent current figure 7. standby quiescent current i load = 200 ma i load = 3.0 a v in = 12 v v in = 40 v l1 = 150 m h r ind = 0.1 w i load = 500 ma i load = 3.0 a v out = 5.0 v measured at ground pin t j = 25 c v on/off = 5.0 v m a) 1.0 0.6 0.2 0 -0.2 -0.4 -1.0 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 2.0 1.5 1.0 0.5 0 6.5 6.0 5.5 5.0 4.5 4.0 20 18 16 14 12 10 8.0 6.0 4.0 200 180 160 140 120 100 80 60 20 0 125 100 75 50 25 0 -25 -50 40 35 30 25 20 15 10 5.0 0 125 100 75 50 25 0 -25 -50 125 100 75 50 25 0 -25 -50 40 35 30 25 20 15 10 5.0 0 125 100 75 50 25 0 -25 -50 -0.8 -0.6 0.4 0.8 v in = 20 v i load = 500 ma normalized at t j = 25 c i load = 500 ma t j = 25 c 3.3 v, 5.0 v and adj 12 v and 15 v v in = 25 v i stby
lm2576 http://onsemi.com 6 v sat , saturation voltage (v) 2.0 2.5 3.0 4.0 i b , feedback pin current (na) , standby quiescent current ( m a) i stby , input voltage (v) t j , junction temperature ( c) switch current (a) normalized frequency (%) t j , junction temperature ( c) figure 8. standby quiescent current v in , input voltage (v) figure 9. switch saturation voltage figure 10. oscillator frequency figure 11. minimum operating voltage figure 12. feedback pin current v in = 12 v normalized at 25 c t j = 25 c adjustable version only 200 180 140 120 100 80 60 40 20 0 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 8.0 6.0 4.0 2.0 0 -2.0 -4.0 -6.0 -8.0 -10 5.0 4.5 3.5 1.5 1.0 0.5 0 40 30 25 20 15 10 5 0 0 0.5 1.0 1.5 2.0 3.0 125 100 75 50 25 0 -25 -50 125 100 75 50 25 0 -25 -50 t j , junction temperature ( c) adjustable version only 100 80 60 40 20 0 -20 -40 -60 -80 -100 125 100 75 50 25 0 -25 -50 160 35 2.5 -40 c 25 c 125 c v out 1.23 v i load = 500 ma typical performance characteristics (circuit of figure 15) v in
lm2576 http://onsemi.com 7 2.0 a 0 0 a b c 100 m s/div 5 m s/div figure 13. switching waveforms figure 14. load transient response vout = 15 v a: output pin voltage, 10 v/div b: inductor current, 2.0 a/div c: inductor current, 2.0 a/div, accoupled d: output ripple voltage, 50 mv/ddiv, accoupled horizontal time base: 5.0 m s/div 50 v 0 4.0 a 2.0 a 100 mv output voltage change 0 3.0 a 2.0 a 1.0 a 0 4.0 a - 100 mv load current typical performance characteristics (circuit of figure 15) d
lm2576 http://onsemi.com 8 figure 15. typical test circuit d1 mbr360 l1 100 m h output 2 4 feedback c out 1000 m f c in 100 m f lm2576 fixed output 1 5 3on /off gnd v in load v out d1 mbr360 l1 100 m h output 2 4 feedback c out 1000 m f c in 100 m f lm2576 adjustable 1 5 3on /off gnd v in load v out 5,000 v fixed output voltage versions adjustable output voltage versions v out v ref 1.0 r2 r1 r2 r1 v out v ref 1.0 where v ref = 1.23 v, r1 between 1.0 k and 5.0 k r2 r1 c in 100 m f, 75 v, aluminium electrolytic c out 1000 m f, 25 v, aluminium electrolytic d1 schottky, mbr360 l1 100 m h, pulse eng. pe92108 r1 2.0 k, 0.1% r2 6.12 k, 0.1% 7.0 v - 40 v unregulated dc input 7.0 v - 40 v unregulated dc input pcb layout guidelines as in any switching regulator, the layout of the printed circuit board is very important. rapidly switching currents associated with wiring inductance, stray capacitance and parasitic inductance of the printed circuit board traces can generate voltage transients which can generate electromagnetic interferences (emi) and affect the desired operation. as indicated in the figure 15, to minimize inductance and ground loops, the length of the leads indicated by heavy lines should be kept as short as possible. for best results, singlepoint grounding (as indicated) or ground plane construction should be used. on the other hand, the pcb area connected to the pin 2 (emitter of the internal switch) of the lm2576 should be kept to a minimum in order to minimize coupling to sensitive circuitry. another sensitive part of the circuit is the feedback. it is important to keep the sensitive feedback wiring short. to assure this, physically locate the programming resistors near to the regulator, when using the adjustable version of the lm2576 regulator.
lm2576 http://onsemi.com 9 pin function description pin symbol description (refer to figure 1) 1 v in this pin is the positive input supply for the lm2576 stepdown switching regulator. in order to minimize voltage transients and to supply the switching currents needed by the regulator, a suitable input bypass capacitor must be present (c in in figure 1). 2 output this is the emitter of the internal switch. the saturation voltage v sat of this output switch is typically 1.5 v. it should be kept in mind that the pcb area connected to this pin should be kept to a minimum in order to minimize coupling to sensitive circuitry. 3 gnd circuit ground pin. see the information about the printed circuit board layout. 4 feedback this pin senses regulated output voltage to complete the feedback loop. the signal is divided by the internal resistor divider network r2, r1 and applied to the noninverting input of the internal error amplifier. in the adjustable version of the lm2576 switching regulator this pin is the direct input of the error amplifier and the resistor network r2, r1 is connected externally to allow programming of the output voltage. 5 on /off it allows the switching regulator circuit to be shut down using logic level signals, thus dropping the total input supply current to approximately 80 m a. the threshold voltage is typically 1.4 v. applying a voltage above this value (up to +v in ) shuts the regulator off. if the voltage applied to this pin is lower than 1.4 v or if this pin is left open, the regulator will be in the aono condition. design procedure buck converter basics the lm2576 is a abucko or stepdown converter which is the most elementary forwardmode converter. its basic schematic can be seen in figure 16. the operation of this regulator topology has two distinct time periods. the first one occurs when the series switch is on, the input voltage is connected to the input of the inductor. the output of the inductor is the output voltage, and the rectifier (or catch diode) is reverse biased. during this period, since there is a constant voltage source connected across the inductor, the inductor current begins to linearly ramp upwards, as described by the following equation: i l(on) v in v out t on l during this aono period, energy is stored within the core material in the form of magnetic flux. if the inductor is properly designed, there is sufficient energy stored to carry the requirements of the load during the aoffo period. figure 16. basic buck converter d v in r load l c out power switch the next period is the aoffo period of the power switch. when the power switch turns off, the voltage across the inductor reverses its polarity and is clamped at one diode voltage drop below ground by the catch diode. the current now flows through the catch diode thus maintaining the load current loop. this removes the stored energy from the inductor. the inductor current during this time is: i l(off) v out v d t off l this period ends when the power switch is once again turned on. regulation of the converter is accomplished by varying the duty cycle of the power switch. it is possible to describe the duty cycle as follows: d t on t , where t is the period of switching. for the buck converter with ideal components, the duty cycle can also be described as: d v out v in figure 17 shows the buck converter, idealized waveforms of the catch diode voltage and the inductor current. power switch figure 17. buck converter idealized waveforms power switch off power switch off power switch on power switch on v on(sw) v d (fwd) time time i load (av) i min i pk diode diode power switch diode voltage inductor current
lm2576 http://onsemi.com 10 procedure (fixed output voltage version) in order to simplify the switching regulator design, a stepbystep design procedure and some examples are provided. procedure example given parameters: v out = regulated output voltage (3.3 v, 5.0 v, 12 v or 15 v) v in(max) = maximum input voltage i load(max) = maximum load current given parameters: v out = 5.0 v v in(max) = 15 v i load(max) = 3.0 a 1. controller ic selection according to the required input voltage, output voltage and current, select the appropriate type of the controller ic output voltage version. 1. controller ic selection according to the required input voltage, output voltage, current polarity and current value, use the lm25765 controller ic 2. input capacitor selection (c in ) to prevent large voltage transients from appearing at the input and for stable operation of the converter, an aluminium or tantalum electrolytic bypass capacitor is needed between the input pin +v in and ground pin gnd. this capacitor should be located close to the ic using short leads. this capacitor should have a low esr (equivalent series resistance) value. 2. input capacitor selection (c in ) a 100 m f, 25 v aluminium electrolytic capacitor located near to the input and ground pins provides sufficient bypassing. 3. catch diode selection (d1) a. since the diode maximum peak current exceeds the regulator maximum load current the catch diode current rating must be at least 1.2 times greater than the maximum load current. for a robust design the diode should have a current rating equal to the maximum current limit of the lm2576 to be able to withstand a continuous output short b. the reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. 3. catch diode selection (d1) a. for this example the current rating of the diode is 3.0 a. b. use a 20 v 1n5820 schottky diode, or any of the suggested fast recovery diodes shown in table 1. 4. inductor selection (l1) a. according to the required working conditions, select the correct inductor value using the selection guide from figures 18 to 22 . b. from the appropriate inductor selection guide, identify the inductance 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. c. select an appropriate inductor from the several different manufacturers part numbers listed in table 2. the designer must realize that the inductor current rating must be higher than the maximum peak current flowing through the inductor. this maximum peak current can be calculated as follows : where t on is the aono time of the power switch and for additional information about the inductor, see the inductor section in the aapplication hintso section of this data sheet. i p(max) i load(max) v in v out t on 2l t on v out v in x 1.0 f osc 4. inductor selection (l1) a. use the inductor selection guide shown in figures 19. b. from the selection guide, the inductance area intersected by the 15 v line and 3.0 a line is l100. c. inductor value required is 100 m h. from table 2 , choose an inductor from any of the listed manufacturers.
lm2576 http://onsemi.com 11 procedure (fixed output voltage version) (continued) in order to simplify the switching regulator design, a stepbystep design procedure and some examples are provided. procedure example 5. output capacitor selection (c out ) a. since the lm2576 is a forwardmode switching regulator with voltage mode control, its open loop 2pole1zero frequency characteristic has the dominant polepair determined by the output capacitor and inductor values. for stable operation and an acceptable ripple voltage, (approximately 1% of the output voltage) a value between 680 m f and 2000 m f is recommended. b. due to the fact that the higher voltage electrolytic capacitors generally have lower esr (equivalent series resistance) numbers, the output capacitor's voltage rating should be at least 1.5 times greater than the output voltage. for a 5.0 v regulator, a rating at least 8.0 v is appropriate, and a 10 v or 16 v rating is recommended. 5. output capacitor selection (c out ) a. c out = 680 m f to 2000 m f standard aluminium electrolytic. b. capacitor voltage rating = 20 v. procedure (adjustable output version: lm2576adj) procedure example given parameters: v out = regulated output voltage v in(max) = maximum dc input voltage i load(max) = maximum load current given parameters: v out = 8.0 v v in(max) = 25 v i load(max) = 2.5 a 1. programming output voltage to select the right programming resistor r1 and r2 value (see figure 2) use the following formula: resistor r1 can be between 1.0 k and 5.0 k w . (for best temperature coefficient and stability with time, use 1% metal film resistors). v out v ref 1.0 r2 r1 r2 r1 v out v ref 1.0 where v ref = 1.23 v 1. programming output voltage (selecting r1 and r2) select r1 and r2: r2 = 9.91 k w , choose a 9.88 k metal film resistor. r2 r1 v out v ref 1.0 1.8 k 8.0 v 1.23 v 1.0 v out 1.23 1.0 r2 r1 select r1 = 1.8 k w 2. input capacitor selection (c in ) to prevent large voltage transients from appearing at the input and for stable operation of the converter, an aluminium or tantalum electrolytic bypass capacitor is needed between the input pin +v in and ground pin gnd this capacitor should be located close to the ic using short leads. this capacitor should have a low esr (equivalent series resistance) value . for additional information see input capacitor section in the aapplication hintso section of this data sheet. 2. input capacitor selection (c in ) a 100 m f, 150 v aluminium electrolytic capacitor located near the input and ground pin provides sufficient bypassing. 3. catch diode selection (d1) a. since the diode maximum peak current exceeds the regulator maximum load current the catch diode current rating must be at least 1.2 times greater than the maximum load current. for a robust design, the diode should have a current rating equal to the maximum current limit of the lm2576 to be able to withstand a continuous output short. b. the reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. 3. catch diode selection (d1) a. for this example, a 3.0 a current rating is adequate. b. use a 30 v 1n5821 schottky diode or any suggested fast recovery diode in the table 1.
lm2576 http://onsemi.com 12 procedure (adjustable output version: lm2576adj) (continued) procedure example 4. inductor selection (l1) a. use the following formula to calculate the inductor volt x microsecond [v x m s] constant: b. match the calculated e x t value with the corresponding number on the vertical axis of the inductor value selection guide shown in figure 22. this e x t 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. c. next step is to identify the inductance region intersected by the e x t value and the maximum load current value on the horizontal axis shown in figure 25. d. from the inductor code, identify the inductor value. then select an appropriate inductor from table 2. the inductor chosen must be rated for a switching frequency of 52 khz and for a current rating of 1.15 x i load . the inductor current rating can also be determined by calculating the inductor peak current : where t on is the aono time of the power switch and for additional information about the inductor, see the inductor section in the aexternal componentso section of this data sheet. ext v in v out v out v in x 10 6 f[hz] [v x s] i p(max) i load(max) v in v out t on 2l t on v out v in x 1.0 f osc 4. inductor selection (l1) a. calculate e x t [v x m s] constant: b. e x t = 80 [v x m s] c. i load(max) = 2.5 a inductance region = h150 d. proper inductor value = 150 m h choose the inductor from table 2. ext ( 258.0 ) x 8.0 25 x 1000 52 80 [v x s] 5. output capacitor selection (c out ) a. since the lm2576 is a forwardmode switching regulator with voltage mode control, its open loop 2pole1zero frequency characteristic has the dominant polepair determined by the output capacitor and inductor values . for stable operation, the capacitor must satisfy the following requirement: b. capacitor values between 10 m f and 2000 m f will satisfy the loop requirements for stable operation. to achieve an acceptable output ripple voltage and transient response, the output capacitor may need to be several times larger than the above formula yields. c. due to the fact that the higher voltage electrolytic capacitors generally have lower esr (equivalent series resistance) numbers, the output capacitor's voltage rating should be at least 1.5 times greater than the output voltage. for a 5.0 v regulator, a rating of at least 8.0 v is appropriate, and a 10 v or 16 v rating is recommended. c out 13, 300 v in(max) v out xl[ m h] [ m f] 5. output capacitor selection (c out ) a. to achieve an acceptable ripple voltage, select c out = 680 m f electrolytic capacitor. c out 13, 300 x 25 8 x 150 332.5 m f
lm2576 http://onsemi.com 13 et, voltage time (v s) m 100 80 90 3.0 2.5 1.5 0.8 0.5 0.4 0.6 1.0 2.0 5.0 60 40 20 15 10 8.0 7.0 6.0 maximum input voltage (v) maximum input voltage (v) i l , maximum load current (a) i l , maximum load current (a) i l , maximum load current (a) maximum input voltage (v) i l , maximum load current (a) maximum input voltage (v) figure 18. lm25763.3 i l , maximum load current (a) figure 19. lm25765 l680 figure 20. lm257612 figure 21. lm257615 figure 22. lm2576adj lm2576 series buck regulator design procedures (continued) indicator value selection guide (for continuous mode operation) 60 40 20 15 12 10 9.0 8.0 7.0 60 40 35 25 20 18 30 16 15 14 60 40 30 25 22 20 19 18 17 300 70 60 50 45 40 35 30 25 20 3.0 2.5 1.5 0.8 0.5 0.3 0.3 3.0 0.8 0.6 0.5 0.4 0.3 3.0 2.0 1.5 0.5 0.3 l330 l470 l150 l220 0.4 0.6 1.0 2.0 l100 l68 l47 h470 h1000 h680 h220 h330 h150 l680 l330 l470 l150 l220 l100 l68 l47 1.2 h470 h1000 h680 h220 h330 h150 l680 l330 l470 l150 l220 l100 l68 h1500 1.0 1.5 2.0 2.5 h470 h1000 h680 h220 h330 h150 l680 l330 l150 l220 l100 l68 h1500 3.0 0.8 0.6 0.5 0.4 0.3 1.0 1.5 2.0 2.5 35 l470 h680 h220 h330 h150 l680 l330 l150 l220 l100 l68 h1500 l470 h2000 l47 h470 h1000 150 200 250 0.4 0.6 0.8 1.0 2.5
lm2576 http://onsemi.com 14 table 1. diode selection guide schottky fast recovery 3.0 a 4.0 6.0 a 3.0 a 4.0 6.0 a v r through hole surface mount through hole surface mount through hole surface mount through hole surface mount 20 v 1n5820 mbr320p sr302 sk32 1n5823 sr502 sb520 30 v 1n5821 mbr330 sr303 31dq03 sk33 30wq03 1n5824 sr503 sb530 50wq03 mur320 31df1 her302 murs320t3 murd320 30wf10 mur420 her602 murd620ct 50wf10 40 v 1n5822 mbr340 sr304 31dq04 sk34 30wq04 mbrs340t3 mbrd340 1n5825 sr504 sb540 mbrd640ct 50wq04 her302 (all diodes rated to at least 100 v) 30wf10 (all diodes rated to at least 100 v) (all diodes rated to at least 100 v) (all diodes rated to at least 100 v) 50 v mbr350 31dq05 sr305 sk35 30wq05 sb550 50wq05 100 v) 100 v) 100 v) 100 v) 60 v mbr360 dq06 sr306 mbrs360t3 mbrd360 50sq080 mbrd660ct note: diodes listed in bold are available from on semiconductor. table 2. inductor selection by manufacturer's part number inductor code inductor value tech 39 schott corp. pulse eng. renco l47 47 m h 77 212 671 26980 pe53112 rl2442 l68 68 m h 77 262 671 26990 pe92114 rl2443 l100 100 m h 77 312 671 27000 pe92108 rl2444 l150 150 m h 77 360 671 27010 pe53113 rl1954 l220 220 m h 77 408 671 27020 pe52626 rl1953 l330 330 m h 77 456 671 27030 pe52627 rl1952 l470 470 m h * 671 27040 pe53114 rl1951 l680 680 m h 77 506 671 27050 pe52629 rl1950 h150 150 m h 77 362 671 27060 pe53115 rl2445 h220 220 m h 77 412 671 27070 pe53116 rl2446 h330 330 m h 77 462 671 27080 pe53117 rl2447 h470 470 m h * 671 27090 pe53118 rl1961 h680 680 m h 77 508 671 27100 pe53119 rl1960 h1000 1000 m h 77 556 671 27110 pe53120 rl1959 h1500 1500 m h * 671 27120 pe53121 rl1958 h2200 2200 m h * 671 27130 pe53122 rl2448 note: *contact manufacturer
lm2576 http://onsemi.com 15 table 3. example of several inductor manufacturers phone/fax numbers pulse engineering, inc. phone fax + 16196748100 + 16196748262 pulse engineering, inc. europe phone fax + 3539324107 + 3539324459 renco electronics, inc. phone fax + 15166455828 + 15165865562 tech 39 phone fax + 33141151681 + 33147095051 schott corporation phone fax + 16124751173 + 16124751786 external components input capacitor (c in ) the input capacitor should have a low esr for stable operation of the switch mode converter a low esr (equivalent series resistance) aluminium or solid tantalum bypass capacitor is needed between the input pin and the ground pin, to prevent large voltage transients from appearing at the input. it must be located near the regulator and use short leads. with most electrolytic capacitors, the capacitance value decreases and the esr increases with lower temperatures. for reliable operation in temperatures below 25 c larger values of the input capacitor may be needed. also paralleling a ceramic or solid tantalum capacitor will increase the regulator stability at cold temperatures. rms current rating of c in the important parameter of the input capacitor is the rms current rating. capacitors that are physically large and have large surface area will typically have higher rms current ratings. for a given capacitor value, a higher voltage electrolytic capacitor will be physically larger than a lower voltage capacitor, and thus be able to dissipate more heat to the surrounding air, and therefore will have a higher rms current rating. the consequence of operating an electrolytic capacitor beyond the rms current rating is a shortened operating life. in order to assure maximum capacitor operating lifetime, the capacitor's rms ripple current rating should be: i rms > 1.2 x d x i load where d is the duty cycle, for a buck regulator d t on t v out v in and d t on t |v out | |v out | v in for a buck boost regulator. output capacitor (c out ) for low output ripple voltage and good stability, low esr output capacitors are recommended. an output capacitor has two main functions: it filters the output and provides regulator loop stability. the esr of the output capacitor and the peaktopeak value of the inductor ripple current are the main factors contributing to the output ripple voltage value. standard aluminium electrolytics could be adequate for some applications but for quality design, low esr types are recommended. an aluminium electrolytic capacitor's esr value is related to many factors such as the capacitance value, the voltage rating, the physical size and the type of construction. in most cases, the higher voltage electrolytic capacitors have lower esr value. often capacitors with much higher voltage ratings may be needed to provide low esr values that, are required for low output ripple voltage. the output capacitor requires an esr value that has an upper and lower limit as mentioned above, a low esr value is needed for low output ripple voltage, typically 1% to 2% of the output voltage. but if the selected capacitor's esr is extremely low (below 0.05 w ), there is a possibility of an unstable feedback loop, resulting in oscillation at the output. this situation can occur when a tantalum capacitor, that can have a very low esr, is used as the only output capacitor. at low temperatures, put in parallel aluminium electrolytic capacitors with tantalum capacitors electrolytic capacitors are not recommended for temperatures below 25 c. the esr rises dramatically at cold temperatures and typically rises 3 times at 25 c and as much as 10 times at 40 c. solid tantalum capacitors have much better esr spec at cold temperatures and are recommended for temperatures below 25 c. they can be also used in parallel with aluminium electrolytics. the value of the tantalum capacitor should be about 10% or 20% of the total capacitance. the output capacitor should have at least 50% higher rms ripple current rating at 52 khz than the peaktopeak inductor ripple current.
lm2576 http://onsemi.com 16 catch diode locate the catch diode close to the lm2576 the lm2576 is a stepdown buck converter; it requires a fast diode to provide a return path for the inductor current when the switch turns off. this diode must be located close to the lm2576 using short leads and short printed circuit traces to avoid emi problems. use a schottky or a soft switching ultrafast recovery diode since the rectifier diodes are very significant sources of losses within switching power supplies, choosing the rectifier that best fits into the converter design is an important process. schottky diodes provide the best performance because of their fast switching speed and low forward voltage drop. they provide the best efficiency especially in low output voltage applications (5.0 v and lower). another choice could be fastrecovery, or ultrafast recovery diodes. it has to be noted, that some types of these diodes with an abrupt turnoff characteristic may cause instability or emi troubles. a fastrecovery diode with soft recovery characteristics can better fulfill some quality, low noise design requirements. table 1 provides a list of suitable diodes for the lm2576 regulator. standard 50/60 hz rectifier diodes, such as the 1n4001 series or 1n5400 series are not suitable. inductor the magnetic components are the cornerstone of all switching power supply designs. the style of the core and the winding technique used in the magnetic component's design has a great influence on the reliability of the overall power supply. using an improper or poorly designed inductor can cause high voltage spikes generated by the rate of transitions in current within the switching power supply, and the possibility of core saturation can arise during an abnormal operational mode. voltage spikes can cause the semiconductors to enter avalanche breakdown and the part can instantly fail if enough energy is applied. it can also cause significant rfi (radio frequency interference) and emi (electromagnetic interference) problems. continuous and discontinuous mode of operation the lm2576 stepdown converter can operate in both the continuous and the discontinuous modes of operation. the regulator works in the continuous mode when loads are relatively heavy, the current flows through the inductor continuously and never falls to zero. under light load conditions, the circuit will be forced to the discontinuous mode when inductor current falls to zero for certain period of time (see figure 23 and figure 24). each mode has distinctively different operating characteristics, which can affect the regulator performance and requirements. in many cases the preferred mode of operation is the continuous mode. it offers greater output power, lower peak currents in the switch, inductor and diode, and can have a lower output ripple voltage. on the other hand 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 selection guide for the lm2576 regulator was added to this data sheet (figures 18 through 22). this guide assumes that the regulator is operating in the continuous mode, and selects an inductor that will allow a peaktopeak inductor ripple current to be a certain percentage of the maximum design load current. this percentage is allowed to change as different design load currents are selected. for light loads (less than approximately 300 ma) it may be desirable to operate the regulator in the discontinuous mode, because the inductor value and size can be kept relatively low. consequently, the percentage of inductor peaktopeak current increases. this discontinuous mode of operation is perfectly acceptable for this type of switching converter. any buck regulator will be forced to enter discontinuous mode if the load current is light enough. horizontal time base: 5.0 m s/div figure 23. continuous mode switching current waveforms vertrical resolution 1.0 a/div 2.0 a 0 a 2.0 a 0 a inductor current waveform power switch current waveform selecting the right inductor style some important considerations when selecting a core type are core material, cost, the output power of the power supply, the physical volume the inductor must fit within, and the amount of emi (electromagnetic interference) shielding that the core must provide. the inductor selection guide covers dif ferent styles of inductors, such as pot core, ecore, toroid and bobbin core, as well as different core materials such as ferrites and powdered iron from different manufacturers. for high quality design regulators the toroid core seems to be the best choice. since the magnetic flux is contained within the core, it generates less emi, reducing noise problems in sensitive circuits. the least expensive is the bobbin core type, which consists of wire wound on a ferrite rod core. this type of inductor generates more emi due to the fact that its core is open, and the magnetic flux is not contained within the core.
lm2576 http://onsemi.com 17 when multiple switching regulators are located on the same printed circuit board, open core magnetics can cause interference between two or more of the regulator circuits, especially at high currents due to mutual coupling. a toroid, pot core or ecore (closed magnetic structure) should be used in such applications. do not operate an inductor beyond its maximum rated current exceeding an inductor's maximum current rating may cause the inductor to overheat because of the copper wire losses, or the core may saturate. core saturation occurs when the flux density is too high and consequently the cross sectional area of the core can no longer support additional lines of magnetic flux. this causes the permeability of the core to drop, the inductance value decreases rapidly and the inductor begins to look mainly resistive. it has only the dc resistance of the winding. this can cause the switch current to rise very rapidly and force the lm2576 internal switch into cyclebycycle current limit, thus reducing the dc output load current. this can also result in overheating of the inductor and/or the lm2576. different inductor types have different saturation characteristics, and this should be kept in mind when selecting an inductor. 0.4 a 0 a 0.4 a 0 a inductor current waveform power switch current waveform figure 24. discontinuous mode switching current waveforms vertical resolution 200 ma/div horizontal time base: 5.0 m s/div general recommendations output voltage ripple and transients source of the output ripple since the lm2576 is a switch mode power supply regulator, its output voltage, if left unfiltered, will contain a sawtooth ripple voltage at the switching frequency. the output ripple voltage value ranges from 0.5% to 3% of the output voltage. it is caused mainly by the inductor sawtooth ripple current multiplied by the esr of the output capacitor. short voltage spikes and how to reduce them the regulator output voltage may also contain short voltage spikes at the peaks of the sawtooth waveform (see figure 25). these voltage spikes are present because of the fast switching action of the output switch, and the parasitic inductance of the output filter capacitor. there are some other important factors such as wiring inductance, stray capacitance, as well as the scope probe used to evaluate these transients, all these contribute to the amplitude of these spikes. to minimize these voltage spikes, low inductance capacitors should be used, and their lead lengths must be kept short. the importance of quality printed circuit board layout design should also be highlighted. unfiltered output voltage filtered output voltage horizontal time base: 5.0 m s/div figure 25. output ripple voltage waveforms vertrical voltage spikes caused by switching action of the output switch and the parasitic inductance of the output capacitor resolution 20 mv/div minimizing the output ripple in order to minimize the output ripple voltage it is possible to enlarge the inductance value of the inductor l1 and/or to use a larger value output capacitor. there is also another way to smooth the output by means of an additional lc filter (20 m h, 100 m f), that can be added to the output (see figure 34) to further reduce the amount of output ripple and transients. with such a filter it is possible to reduce the output ripple voltage transients 10 times or more. figure 25 shows the difference between filtered and unfiltered output waveforms of the regulator shown in figure 34. the lower waveform is from the normal unfiltered output of the converter, while the upper waveform shows the output ripple voltage filtered by an additional lc filter. heatsinking and thermal considerations the throughhole package to220 the lm2576 is available in two packages, a 5pin to220(t, tv) and a 5pin surface mount d 2 pak(d2t). although the to220(t) package needs a heatsink under most conditions, there are some applications that require no heatsink to keep the lm2576 junction temperature within the allowed operating range. higher ambient temperatures require some heat sinking, either to the printed circuit (pc) board or an external heatsink. the surface mount package d 2 pak and its heatsinking the other type of package, the surface mount d 2 pak, is designed to be soldered to the copper on the pc board. the copper and the board are the heatsink for this package and the other heat producing components, such as the catch diode and inductor. the pc board copper area that the package is soldered to should be at least 0.4 in 2 (or 260 mm 2 ) and ideally should have 2 or more square inches (1300 mm 2 ) of 0.0028 inch copper. additional increases of copper area
lm2576 http://onsemi.com 18 beyond approximately 6.0 in 2 (4000 mm 2 ) will not improve heat dissipation significantly. if further thermal improvements are needed, double sided or multilayer pc boards with large copper areas should be considered. in order to achieve the best thermal performance, it is highly recommended to use wide copper traces as well as large areas of copper in the printed circuit board layout. the only exception to this is the output (switch) pin, which should not have large areas of copper (see page 8 `pcb layout guideline'). thermal analysis and design the following procedure must be performed to determine whether or not a heatsink will be required. first determine: 1. p d(max) maximum regulator power dissipation in the application. 2. t a(max ) maximum ambient temperature in the application. 3. t j(max) maximum allowed junction temperature (125 c for the lm2576). for a conservative design, the maximum junction temperature should not exceed 110 c to assure safe operation. for every additional +10 c temperature rise that the junction must withstand, the estimated operating lifetime of the component is halved. 4. r q jc package thermal resistance junctioncase. 5. r q ja package thermal resistance junctionambient. (refer to absolute maximum ratings on page 2 of this data sheet or r q jc and r q ja values). the following formula is to calculate the approximate total power dissipated by the lm2576: p d = (v in x i q ) + d x i load x v sat where d is the duty cycle and for buck converter d t on t v o v in , i q (quiescent current) and v sat can be found in the lm2576 data sheet, v in is minimum input voltage applied, v o is the regulator output voltage, i load is the load current. the dynamic switching losses during turnon and turnoff can be neglected if proper type catch diode is used. packages not on a heatsink (freestanding) for a freestanding application when no heatsink is used, the junction temperature can be determined by the following expression: t j = (r q ja ) (p d ) + t a where (r q ja )(p d ) represents the junction temperature rise caused by the dissipated power and t a is the maximum ambient temperature. packages on a heatsink if the actual operating junction temperature is greater than the selected safe operating junction temperature determined in step 3, than a heatsink is required. the junction temperature will be calculated as follows: t j = p d (r q ja + r q cs + r q sa ) + t a where r q jc is the thermal resistance junctioncase, r q cs is the thermal resistance caseheatsink, r q sa is the thermal resistance heatsinkambient. if the actual operating temperature is greater than the selected safe operating junction temperature, then a larger heatsink is required. some aspects that can influence thermal design it should be noted that the package thermal resistance and the junction temperature rise numbers are all approximate, and there are many factors that will affect these numbers, such as pc board size, shape, thickness, physical position, location, board temperature, as well as whether the surrounding air is moving or still. other factors are trace width, total printed circuit copper area, copper thickness, single or doublesided, multilayer board, the amount of solder on the board or even color of the traces. the size, quantity and spacing of other components on the board can also influence its effectiveness to dissipate the heat. figure 26. inverting buckboost develops 12 v d1 1n5822 l1 68 m h output 2 4 feedback 12 to 40 v unregulated dc input c in 100 m f 1 5 3on /off gnd +v in -12 v @ 0.7 a regulated output c out 2200 m f lm257612 additional applications inverting regulator an inverting buckboost regulator using the lm257612 is shown in figure 26. this circuit converts a positive input voltage to a negative output voltage with a common ground by bootstrapping the regulators ground to the negative output voltage. by grounding the feedback pin, the regulator senses the inverted output voltage and regulates it. in this example the lm257612 is used to generate a 12 v output. the maximum input voltage in this case cannot exceed +28 v because the maximum voltage appearing across the regulator is the absolute sum of the input and output voltages and this must be limited to a maximum of 40 v.
lm2576 http://onsemi.com 19 this circuit configuration is able to deliver approximately 0.7 a to the output when the input voltage is 12 v or higher. at lighter loads the minimum input voltage required drops to approximately 4.7 v, because the buckboost regulator topology can produce an output voltage that, in its absolute value, is either greater or less than the input voltage. since the switch currents in this buckboost configuration are higher than in the standard buck converter topology, the available output current is lower. this type of buckboost inverting regulator can also require a larger amount of startup input current, even for light loads. this may overload an input power source with a current limit less than 5.0 a. such an amount of input startup current is needed for at least 2.0 ms or more. the actual time depends on the output voltage and size of the output capacitor. because of the relatively high startup currents required by this inverting regulator topology, the use of a delayed startup or an undervoltage lockout circuit is recommended. using a delayed startup arrangement, the input capacitor can charge up to a higher voltage before the switchmode regulator begins to operate. the high input current needed for startup is now partially supplied by the input capacitor c in . it has been already mentioned above, that in some situations, the delayed startup or the undervoltage lockout features could be very useful. a delayed startup circuit applied to a buckboost converter is shown in figure 27, figure 33 in the aundervoltage lockouto section describes an undervoltage lockout feature for the same converter topology. design recommendations: the inverting regulator operates in a different manner than the buck converter and so a different design procedure has to be used to select the inductor l1 or the output capacitor c out . the output capacitor values must be larger than what is normally required for buck converter designs. low input voltages or high output currents require a large value output capacitor (in the range of thousands of m f). the recommended range of inductor values for the inverting converter design is between 68 m h and 220 m h. to select an inductor with an appropriate current rating, the inductor peak current has to be calculated. the following formula is used to obtain the peak inductor current: where t on |v o | v in |v o | x 1.0 f osc , and f osc 52 khz. i peak i load (v in |v o |) v in v in xt on 2l 1 under normal continuous inductor current operating conditions, the worst case occurs when v in is minimal. figure 27. inverting buckboost regulator with delayed startup d1 1n5822 l1 68 m h output 2 4 feedback 12 v to 25 v unregulated dc input c in 100 m f /50 v 1 3 5on /off gnd +v in -12 v @ 700 m a regulated output c out 2200 m f /16 v lm257612 c1 0.1 m f r1 47 k r2 47 k figure 28. inverting buckboost regulator shutdown circuit using an optocoupler lm2576xx 1 3 5 gnd on /off +v in r2 47 k c in 100 m f note: this picture does not show the complete circuit. r1 47 k r3 470 shutdown input moc8101 -v out off on 5.0 v 0 +v in with the inverting configuration, the use of the on /off pin requires some level shifting techniques. this is caused by the fact, that the ground pin of the converter ic is no longer at ground. now, the on /off pin threshold voltage (1.3 v approximately) has to be related to the negative output voltage level. there are many different possible shut down methods, two of them are shown in figures 28 and 29.
lm2576 http://onsemi.com 20 figure 29. inverting buckboost regulator shutdown circuit using a pnp transistor note: this picture does not show the complete circuit. r2 5.6 k q1 2n3906 lm2576xx 1 3 5 gnd on /off r1 12 k -v out +v in shutdown input off on +v 0 +v in c in 100 m f negative boost regulator this example is a variation of the buckboost topology and it is called negative boost regulator. this regulator experiences relatively high switch current, especially at low input voltages. the internal switch current limiting results in lower output load current capability. the circuit in figure 30 shows the negative boost configuration. the input voltage in this application ranges from 5.0 v to 12 v and provides a regulated 12 v output. if the input voltage is greater than 12 v, the output will rise above 12 v accordingly, but will not damage the regulator. figure 30. negative boost regulator 1n5820 100 m h output 2 4 feedback v out = -12 v typical load current 400 ma for v in = -5.2 v 750 ma for v in = -7.0 v -5.0 v to -12 v c out 2200 m f low esr c in 100 m f lm257612 1 5 3 on /off gnd v in v in design recommendations: the same design rules as for the previous inverting buckboost converter can be applied. the output capacitor c out must be chosen larger than would be required for a what standard buck converter. low input voltages or high output currents require a large value output capacitor (in the range of thousands of m f). the recommended range of inductor values for the negative boost regulator is the same as for inverting converter design. another important point is that these negative boost converters cannot provide current limiting load protection in the event of a short in the output so some other means, such as a fuse, may be necessary to provide the load protection. delayed startup there are some applications, like the inverting regulator already mentioned above, which require a higher amount of startup current. in such cases, if the input power source is limited, this delayed startup feature becomes very useful. to provide a time delay between the time when the input voltage is applied and the time when the output voltage comes up, the circuit in figure 31 can be used. as the input voltage is applied, the capacitor c1 charges up, and the voltage across the resistor r2 falls down. when the voltage on the on /off pin falls below the threshold value 1.3 v, the regulator starts up. resistor r1 is included to limit the maximum voltage applied to the on /off pin. it reduces the power supply noise sensitivity, and also limits the capacitor c1 discharge current, but its use is not mandatory. when a high 50 hz or 60 hz (100 hz or 120 hz respectively) ripple voltage exists, a long delay time can cause some problems by coupling the ripple into the on /off pin, the regulator could be switched periodically on and off with the line (or double) frequency. figure 31. delayed startup circuitry r1 47 k lm2576xx 1 3 5 gnd on /off r2 47 k +v in +v in c1 0.1 m f c in 100 m f note: this picture does not show the complete circuit. undervoltage lockout some applications require the regulator to remain off until the input voltage reaches a certain threshold level. figure 32 shows an undervoltage lockout circuit applied to a buck regulator. a version of this circuit for buckboost converter is shown in figure 33. resistor r3 pulls the on /off pin high and keeps the regulator off until the input voltage reaches a predetermined threshold level with respect to the ground pin 3, which is determined by the following expression: v th v z1 1.0 r2 r1 v be ( q1 )
lm2576 http://onsemi.com 21 figure 32. undervoltage lockout circuit for buck converter r2 10 k z1 1n5242b r1 10 k q1 2n3904 r3 47 k v th 13 v c in 100 m f lm2576xx 1 3 5 gnd on /off +v in +v in note: this picture does not show the complete circuit. the following formula is used to obtain the peak inductor current: where t on |v o | v in |v o | x 1.0 f osc , and f osc 52 khz. i peak i load (v in |v o |) v in v in xt on 2l 1 under normal continuous inductor current operating conditions, the worst case occurs when v in is minimal. figure 33. undervoltage lockout circuit for buckboost converter r2 15 k z1 1n5242b r1 15 k q1 2n3904 r3 47 k v th 13 v c in 100 m f lm2576xx 1 3 5 gnd on /off +v in +v in v out note: this picture does not show the complete circuit. adjustable output, lowripple power supply a 3.0 a output current capability power supply that features an adjustable output voltage is shown in figure 34. this regulator delivers 3.0 a into 1.2 v to 35 v output. the input voltage ranges from roughly 3.0 v to 40 v. in order to achieve a 10 or more times reduction of output ripple, an additional lc filter is included in this circuit. figure 34. 1.2 to 35 v adjustable 3.0 a power supply with low output ripple d1 1n5822 l1 150 m h output 2 4 feedback r2 50 k r1 1.21 k l2 20 m h output voltage 1.2 to 35 v @ 3.0 a optional output ripple filter 40 v max unregulated dc input c out 2200 m f c1 100 m f c in 100 m f lm2574adj 1 5 3on /off gnd +v in
lm2576 http://onsemi.com 22 the lm25765 stepdown voltage regulator with 5.0 v @ 3.0 a output power capability. typical application with throughhole pc board layout figure 35. schematic diagram of the lm25765 stepdown converter d1 1n5822 l1 150 m h output 2 4 feedback unregulated dc input +v in = 7.0 to 40 v c out 1000 m f /16 v c1 100 m f /50 v lm25765 1 5 3on /off gnd +v in on /off regulated output v out1 = 5.0 v @ 3.0 a gnd in gnd out c1 100 m f, 50 v, aluminium electrolytic c2 1000 m f, 16 v, aluminium electrolytic d1 3.0 a, 40 v, schottky rectifier, 1n5822 l1 150 m h, rl2444, renco electronics figure 36. printed circuit board layout component side figure 37. printed circuit board layout copper side lm2576 u1 d1 c1 c2 l1 + + v ou t gnd out gnd in +v in on /off 00060_00 note: not to scale. note: not to scale.
lm2576 http://onsemi.com 23 the lm2576adj stepdown voltage regulator with 8.0 v @ 1.0 a output power capability. typical application with throughhole pc board layout c1 100 m f, 50 v, aluminium electrolytic c2 1000 m f, 16 v, aluminium electrolytic d1 3.0 a, 40 v, schottky rectifier, 1n5822 l1 150 m h, rl2444, renco electronics r1 1.8 k w , 0.25 w r2 10 k w , 0.25 w figure 38. schematic diagram of the 8.0 v @ 3.0 a stepdown converter using the lm2576adj v ref = 1.23 v r1 is between 1.0 k and 5.0 k d1 1n5822 l1 150 m h output 2 r2 10 k r1 1.8 k regulated output filtered v out2 = 8.0 v @ 3.0 a unregulated dc input c2 1000 m f /16 v c1 100 m f /50 v lm2576adj 1 5 3on /off gnd +v in +v in = 10 v to 40 v 4 feedback v out v ref 1.0 r2 r1 on /off figure 39. printed circuit board layout component side figure 40. printed circuit board layout copper side lm2576 u1 d1 r1 r2 c1 + c2 + l1 on /off v out gnd out gnd in +v in 00059_00 note: not to scale. note: not to scale. references ? national semiconductor lm2576 data sheet and application note ? national semiconductor lm2595 data sheet and application note ? marty brown apractical switching power supply designo, academic press, inc., san diego 1990 ? ray ridley ahigh frequency magnetics designo, ridley engineering, inc. 1995
lm2576 http://onsemi.com 24 ordering information device nominal output voltage operating temperature range package shipping lm2576tvadj to220 (vertical mount) lm2576tadj 123vto37v to220 (straight lead) 50 units/rail lm2576d2tadj 1.23 v to 37 v d 2 pak (surface mount) lm2576d2tadjr4 d 2 pak (surface mount) 2500 tape & reel lm2576tv3.3 to220 (vertical mount) lm2576t3.3 33v to220 (straight lead) 50 units/rail lm2576d2t3.3 3.3 v d 2 pak (surface mount) lm2576d2tr43.3 d 2 pak (surface mount) 2500 tape & reel lm2576tv5 t 40 to +125 c to220 (vertical mount) lm2576t5 50v t j = 40 to +125 c to220 (straight lead) 50 units/rail lm2576d2t5 5.0 v d 2 pak (surface mount) lm2576d2tr45 d 2 pak (surface mount) 2500 tape & reel lm2576tv12 to220 (vertical mount) lm2576t12 12 v to220 (straight lead) 50 units/rail lm2576d2t12 d 2 pak (surface mount) lm2576tv15 to220 (vertical mount) lm2576t15 15 v to220 (straight lead) 50 units/rail lm2576d2t15 d 2 pak (surface mount) xxx = 3.3, 5.0, 12, 15, or adj a = assembly location wl = wafer lot y = year ww = work week to220 tv suffix case 314b 1 marking diagrams 5 to220 t suffix case 314d to220 t suffix case 314d d 2 pak d2t suffix case 936a lm 2576txxx awlyww lm 2576txxx awlyww 15 lm 2576d2txxx awlyww 15 lm 2576xxx awlyww 15
lm2576 http://onsemi.com 25 package dimensions to220 tv suffix case 314b05 issue j v q k f u a b g p m 0.10 (0.254) p m t 5x j m 0.24 (0.610) t optional chamfer s l w e c h n t seating plane notes: 1. dimensioning and tolerancing per ansi y14.5m, 1982. 2. controlling dimension: inch. 3. dimension d does not include interconnect bar (dambar) protrusion. dimension d including protrusion shall not exceed 0.043 (1.092) maximum. dim min max min max millimeters inches a 0.572 0.613 14.529 15.570 b 0.390 0.415 9.906 10.541 c 0.170 0.180 4.318 4.572 d 0.025 0.038 0.635 0.965 e 0.048 0.055 1.219 1.397 f 0.850 0.935 21.590 23.749 g 0.067 bsc 1.702 bsc h 0.166 bsc 4.216 bsc j 0.015 0.025 0.381 0.635 k 0.900 1.100 22.860 27.940 l 0.320 0.365 8.128 9.271 n 0.320 bsc 8.128 bsc q 0.140 0.153 3.556 3.886 s --- 0.620 --- 15.748 u 0.468 0.505 11.888 12.827 v --- 0.735 --- 18.669 w 0.090 0.110 2.286 2.794 5x d to220 t suffix case 314d04 issue e q 12345 u k d g a b 5 pl j h l e c m q m 0.356 (0.014) t seating plane t dim min max min max millimeters inches a 0.572 0.613 14.529 15.570 b 0.390 0.415 9.906 10.541 c 0.170 0.180 4.318 4.572 d 0.025 0.038 0.635 0.965 e 0.048 0.055 1.219 1.397 g 0.067 bsc 1.702 bsc h 0.087 0.112 2.210 2.845 j 0.015 0.025 0.381 0.635 k 0.990 1.045 25.146 26.543 l 0.320 0.365 8.128 9.271 q 0.140 0.153 3.556 3.886 u 0.105 0.117 2.667 2.972 notes: 1. dimensioning and tolerancing per ansi y14.5m, 1982. 2. controlling dimension: inch. 3. dimension d does not include interconnect bar (dambar) protrusion. dimension d including protrusion shall not exceed 10.92 (0.043) maximum.
lm2576 http://onsemi.com 26 package dimensions d 2 pak d2t suffix case 936a02 issue b 5 ref a 123 k b s h d g c e m l p n r v u terminal 6 notes: 1. dimensioning and tolerancing per ansi y14.5m, 1982. 2. controlling dimension: inch. 3. tab contour optional within dimensions a and k. 4. dimensions u and v establish a minimum mounting surface for terminal 6. 5. dimensions a and b do not include mold flash or gate protrusions. mold flash and gate protrusions not to exceed 0.025 (0.635) maximum. dim a min max min max millimeters 0.386 0.403 9.804 10.236 inches b 0.356 0.368 9.042 9.347 c 0.170 0.180 4.318 4.572 d 0.026 0.036 0.660 0.914 e 0.045 0.055 1.143 1.397 g 0.067 bsc 1.702 bsc h 0.539 0.579 13.691 14.707 k 0.050 ref 1.270 ref l 0.000 0.010 0.000 0.254 m 0.088 0.102 2.235 2.591 n 0.018 0.026 0.457 0.660 p 0.058 0.078 1.473 1.981 r 5 ref s 0.116 ref 2.946 ref u 0.200 min 5.080 min v 0.250 min 6.350 min 45 m 0.010 (0.254) t t optional chamfer
lm2576 http://onsemi.com 27 notes
lm2576 http://onsemi.com 28 on semiconductor and are trademarks of semiconductor components industries, llc (scillc). scillc reserves the right to make changes without further notice to any products herein. scillc makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does scillc assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. atypicalo parameters which may be provided in scill c data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. all operating parameters, including atypicalso must be validated for each customer application by customer's technical experts. scillc does not convey any license under its patent rights nor the rights of others. scillc products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body , or other applications intended to support or sustain life, or for any other application in which the failure of the scillc product could create a sit uation where personal injury or death may occur. should buyer purchase or use scillc products for any such unintended or unauthorized application, buyer shall indemnify and hold scillc and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthori zed use, even if such claim alleges that scillc was negligent regarding the design or manufacture of the part. scillc is an equal opportunity/affirmative action employer. publication ordering information japan : on semiconductor, japan customer focus center 4321 nishigotanda, shinagawaku, tokyo, japan 1410031 phone : 81357402700 email : r14525@onsemi.com on semiconductor website : http://onsemi.com for additional information, please contact your local sales representative. lm2576/d literature fulfillment : literature distribution center for on semiconductor p.o. box 5163, denver, colorado 80217 usa phone : 3036752175 or 8003443860 toll free usa/canada fax : 3036752176 or 8003443867 toll free usa/canada email : onlit@hibbertco.com n. american technical support : 8002829855 toll free usa/canada

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