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lt3502/lt3502a 1 3502fd features applications description 1.1mhz/2.2mhz, 500ma step-down regulators in 2mm 2mm dfn and ms10 the lt ? 3502/lt3502a are current mode pwm step-down dc/dc converters with an internal 500ma power switch, in tiny 8-lead 2mm 2mm dfn and 10-lead ms10 packages. the wide input voltage range of 3v to 40v makes the lt3502/lt3502a suitable for regulating power from a wide variety of sources, including 24v industrial supplies and automotive batteries. its high operating frequency allows the use of tiny, low cost inductors and capacitors, resulting in a very small solution. constant frequency above the am band avoids interfering with radio reception, making the lt3502a particularly suitable for automotive applications. cycle-by-cycle current limit and frequency foldback provide protection against shorted outputs. soft-start and frequency foldback eliminates input current surge during start-up. da current sense provides further protec- tion in fault conditions. an internal boost diode reduces component count. 3.3v step-down converter n 3v to 40v input voltage range n 500ma output current n switching frequency: 2.2mhz (lt3502a), 1.1mhz (lt3502) n 800mv feedback voltage n short-circuit robust n soft-start n low shutdown current: <2a n internally compensated n internal boost diode n thermally enhanced 2mm 2mm 8-lead dfn and 10-lead ms10 package n automotive systems n battery-powered equipment n wall transformer regulation n distributed supply regulation lt3502a 12v in ef? ciency v in 0.1 f 3502 ta01a 31.6k 10k 6.8 h shdn boost sw lt3502a bd gnd da fb off on 10 f 1 f v in 4.7v to 40v v out 3.3v 500ma load current (a) 0 0 efficiency (%) 10 30 40 50 0.2 0.4 0.5 90 3502 ta01b 20 0.1 0.3 60 70 80 3.3v out 5v out typical application l , lt, ltc, ltm, linear technology and the linear logo are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners.
lt3502/lt3502a 2 3502fd pin configuration absolute maximum ratings input voltage (v in ) ....................................................40v boost voltage .........................................................50v boost pin above sw pin ...........................................7v fb voltage ...................................................................6v shdn voltage ...........................................................40v (note 1) order information lead free finish tape and reel part marking* package description temperature range lt3502edc#pbf lt3502edc#trpbf lclv 8-lead 2mm 2mm plastic dfn C40c to 125c lt3502idc#pbf lt3502idc#trpbf lclv 8-lead 2mm 2mm plastic dfn C40c to 125c lt3502aedc#pbf lt3502aedc#trpbf lclt 8-lead 2mm 2mm plastic dfn C40c to 125c lt3502aidc#pbf lt3502aidc#trpbf lclt 8-lead 2mm 2mm plastic dfn C40c to 125c lt3502ems#pbf lt3502ems#trpbf ltdtr 10-lead plastic msop C40c to 125c lt3502ims#pbf lt3502ims#trpbf ltdtr 10-lead plastic msop C40c to 125c lt3502aems#pbf lt3502aems#trpbf ltdts 10-lead plastic msop C40c to 125c lt3502aims#pbf lt3502aims#trpbf ltdts 10-lead plastic msop C40c to 125c consult ltc marketing for parts speci? ed with wider operating temperature ranges. *the temperature grade is identi? ed by a label on the shipping container. consult ltc marketing for information on non-standard lead based ? nish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci? cations, go to: http://www.linear.com/tapeandreel/ top view v in bd fb shdn sw boost da gnd dc package 8-lead (2mm s 2mm) plastic dfn 9 4 1 2 3 6 5 7 8 ja = 102c/w exposed pad (pin 9) is gnd, must be soldered to pcb 1 2 3 4 5 sw boost nc da gnd 10 9 8 7 6 v in nc bd fb shdn top view ms package 10-lead plastic msop ja = 110c/w bd voltage ..................................................................7v operating junction temperature range (note 2) lt3502ae, lt3502e .......................... C40c to 125c lt3502ai, lt3502i ............................ C40c to 125c storage temperature range .................. C65c to 150c
lt3502/lt3502a 3 3502fd note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2. the lt3502edc and lt3502aedc are guaranteed to meet performance speci? cations from 0c to 125c junction temperature range. speci? cations over the C 40c to 125c operating junction temperature range are assured by design, characterization and correlation electrical characteristics the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 10v, v shdn = 5v, v boost = 15v. with statistical process controls. the lt3502idc and lt3502aidc are guaranteed over the C 40c to 125c operating junction temperature range. note 3: current limit guaranteed by design and/or correlation to static test. slope compensation reduces current limit at higher duty cycle. note 4: current ? ows into pin. note 5: current ? ows out of pin. parameter conditions min typ max units undervoltage lockout 2.6 2.8 3 v quiescent current at shutdown v shdn = 0v 0.5 2 a quiescent current not switching 1.5 2 ma feedback voltage 2mm 2mm dfn 2mm 2mm dfn ms10 ms10 l l 0.785 0.79 0.780 0.786 0.8 0.8 0.8 0.8 0.813 0.81 0.816 0.813 v v v v reference voltage line regulation 0.005 %/v fb pin bias current (note 5) l 15 50 na switching frequency i da < 500ma (lt3502a) i da < 500ma (lt3502a) i da < 500ma (lt3502) i da < 500ma (lt3502) l l 1.9 1.8 0.9 0.8 2.25 2.25 1.1 1.1 2.7 2.8 1.3 1.4 mhz mhz mhz mhz maximum duty cycle 100ma load (lt3502a) 100ma load (lt3502) 70 80 80 90 % % switch v cesat i sw = 500ma 450 mv switch current limit (note 3) 0.75 0.9 1.1 a switch active current sw = 10v (note 4) sw = 0v (note 5) 95 8 130 30 a a boost pin current i sw = 500ma 10 13 ma minimum boost voltage above switch i sw = 500ma 1.9 2.2 v boost schottky forward drop i out = 100ma 0.8 1 v da pin current to stop osc 500 650 ma shdn bias current v shdn = 5v v shdn = 0v 55 80 1 a a shdn input voltage high 2v shdn input voltage low 0.3 v
lt3502/lt3502a 4 3502fd lt3502 maximum load current v out = 5v, l = 22h switch voltage drop lt3502 maximum load current v out = 3.3v, l = 15h switch current (a) 0 500 25c 125c 600 700 0.8 3502 g09 400 300 0.2 0.4 0.6 1.0 200 100 0 v ce (mv) C40c v in (v) 0 load current (a) 0.5 0.6 0.7 40 3502 g07 0.4 0.3 0 0.1 10 20 30 0.2 0.9 0.8 minimum typical v in (v) 0 load current (a) 0.5 0.6 0.7 40 3502 g08 0.4 0.3 0 0.1 10 20 30 0.2 0.9 0.8 minimum typical lt3502 5v out ef? ciency lt3502a maximum load current v out = 3.3v, l = 6.8h lt3502a maximum load current v out = 5v, l = 10h load current (a) 0 efficiency (%) 60 80 100 0.4 3502 g04 40 20 50 70 90 30 10 0 0.1 0.2 0.3 0.5 24v in 12v in v in (v) 0 0 load current (a) 0.1 0.3 0.4 0.5 1.0 0.7 10 20 typical 3502 g05 0.2 0.8 0.9 0.6 30 40 minimum v in (v) 0 0 load current (a) 0.1 0.3 0.4 0.5 1.0 0.7 10 20 3502 g06 0.2 0.8 0.9 0.6 30 40 minimum typical typical performance characteristics lt3502a 3.3v out ef? ciency lt3502a 5v out ef? ciency lt3502 3.3v out ef? ciency load current (a) 0 0 efficiency (%) 10 30 40 50 0.2 0.4 0. 5 90 3502 g01 20 0.1 0.3 60 70 80 24v in 12v in load current (a) 0 0 efficiency (%) 10 30 40 50 0.2 0.4 0. 5 90 3502 g02 20 0.1 0.3 60 70 80 24v in 12v in load current (a) 0 efficiency (%) 60 80 100 0.4 3502 g03 40 20 50 70 90 30 10 0 0.1 0.2 0.3 0.5 5v in 24v in 12v in (t a = 25c unless otherwise noted)
lt3502/lt3502a 5 3502fd switching frequency soft-start ( shdn ) uvlo temperature (c) C50 2.0 2.5 3.5 100 3502 g10 1.5 1.0 0 50 150 0.5 0 3.0 v in (v) temperature (c) C50 frequency (mhz) 1.0 1.5 150 3502 g11 0.5 0 0 50 100 2.5 2.0 lt3502a lt3502 shdn pin voltage (mv) 0 C0.1 switch current limit (a) 0 0.2 0.3 0.4 0.9 0.6 400 800 1000 3502 g12 0.1 0.7 0.8 0.5 200 600 1200 1400 1600 typical performance characteristics (t a = 25c unless otherwise noted) lt3502a maximum v in for full frequency (v out = 3.3v) shdn pin current shdn pin voltage (v) 0 0 shdn pin current (a) 50 150 200 250 10 20 25 45 3502 g13 100 515 30 35 40 300 temperature (c) C50 0 current limit (a) 0.1 0.3 0.4 0.5 1.0 0.7 0 50 3502 g14 0.2 0.8 0.9 0.6 100 150 da valley current limit sw peak current limit load current (a) 0 0 v in (v) 5 15 20 25 0.4 45 3502 g16 10 0.2 0.1 0.5 0.6 0.3 0.7 30 35 40 t a = 25c t a = 85c switch current limit lt3502a maximum v in for full frequency (v out = 5v) load current (a) 0 0 v in (v) 5 15 20 25 0.4 45 3502 g17 10 0.2 0.1 0.5 0.6 0.3 0.7 30 35 40 t a = 85c t a = 25c lt3502 maximum v in for full frequency (v out = 3.3v) load current (a) 0 0 v in (v) 5 15 20 25 0.4 45 3502 g18 10 0.2 0.1 0.5 0.6 0.3 0.7 30 35 40 t a = 85c t a = 25c switch current limit duty cycle (%) 0 0 current limit (a) 0.2 0.4 0.6 0.8 1.0 1.2 50 100 3502 g15 lt3502a lt3502
lt3502/lt3502a 6 3502fd lt3502a typical minimum input voltage (v out = 3.3v) load current (a) 0.001 7 6 5 4 3 2 1 0 3502 g19 0.01 0.1 1 v in (v) lt3502a typical minimum input voltage (v out = 5v) load current (a) 0.001 4 v in (v) 6 8 0.01 0.1 1 3502 g20 2 3 5 7 1 0 lt3502 typical minimum input voltage (v out = 3.3v) load current (a) 0.001 7 6 5 4 3 2 1 0 3502 g21 0.01 0.1 1 v in (v) typical performance characteristics (t a = 25c unless otherwise noted) continuous mode waveform discontinuous mode waveform v sw 5v/div i l 200ma/div v out 20mv/div 200ns/div 3502 g23 v in = 12v v out = 3.3v l = 6.8h c out = 10f i out = 250ma v sw 5v/div i l 200ma/div v out 20mv/div 200ns/div 3502 g24 v in = 12v v out = 3.3v l = 6.8 h c out = 10 f i out = 30ma lt3502 typical minimum input voltage (v out = 5v) load current (a) 0.001 4 v in (v) 6 8 0.01 0.1 1 3502 g22 2 3 5 7 1 0
lt3502/lt3502a 7 3502fd pin functions v in (pin 1/pin 10): the v in pin supplies current to the lt3502/lt3502as internal regulator and to the internal power switch. this pin must be locally bypassed. bd (pin 2/pin 8): the bd pin is used to provide current to the internal boost schottky diode. fb (pin 3/pin 7): the lt3502/lt3502a regulate their feedback pin to 0.8v. connect the feedback resistor di- vider tap to this pin. set the output voltage according to v out = 0.8 (1 + r1/r2). a good value for r2 is 10k. shdn (pin 4/pin 6): the shdn pin is used to put the lt3502 in shutdown mode. tie to ground to shut down the lt3502/lt3502a. tie to 2v or more for normal operation. if the shutdown feature is not used, tie this pin to the v in pin. the shdn pin also provides soft-start and frequency foldback. to use the soft-start feature, connect r3 and c4 to the shdn pin. shdn pin voltage should not be higher than v in . gnd (pin 5/pin 5): ground pin. da (pin 6/pin 4): connect the catch diode (d1) anode to this pin. this pin is used to provide frequency foldback in extreme situations. boost (pin 7/pin 2): the boost pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar npn power switch. connect a boost capacitor from this pin to sw pin. sw (pin 8/pin 1): the sw pin is the output of the internal power switch. connect this pin to the inductor, catch diode and boost capacitor. (dfn/ms)
lt3502/lt3502a 8 3502fd block diagram r driver q1 s osc slope comp frequency foldback int reg and uvlo v c g m 0.8v 3502 bd q q boost bd sw da gnd fb r2 r1 v out l1 c3 c1 d1 v in c2 v in on off c4 r3 shdn 1 4 3 5 6 8 7 2
lt3502/lt3502a 9 3502fd the lt3502/lt3502a are constant frequency, current mode step-down regulators. an oscillator enables an rs ? ip-? op, turning on the internal 500ma power switch q1. an ampli? er and comparator monitor the current ? owing between the v in and sw pins, turning the switch off when this current reaches a level determined by the voltage at v c . an error ampli? er measures the output voltage through an external resistor divider tied to the fb pin and servos the v c node. if the error ampli? ers output increases, more current is delivered to the output; if it decreases, less current is delivered. an active clamp (not shown) on the v c node provides current limit. the v c node is also clamped to the voltage on the shdn pin; soft-start is implemented by generating a voltage ramp at the shdn pin using an external resistor and capacitor. the shdn pin voltage during soft-start also reduces the oscillator frequency to avoid hitting current limit during start-up. an internal regulator provides power to the control cir- cuitry. this regulator includes an undervoltage lockout to prevent switching when v in is less than ~3v. the shdn pin is used to place the lt3502/lt3502a in shutdown, disconnecting the output and reducing the input current to less than 2a. the switch driver operates from either v in or from the boost pin. an external capacitor and the internal diode are used to generate a voltage at the boost pin that is higher than the input supply. this allows the driver to fully saturate the internal bipolar npn power switch for ef? cient operation. a comparator monitors the current ? owing through the catch diode via the da pin and reduces the lt3502/ lt3502as operating frequency when the da pin current exceeds the 650ma valley current limit. this frequency foldback helps to control the output current in fault conditions such as shorted output with high input volt- age. the da comparator works in conjunction with the switch peak current limit comparator to determine the maximum deliverable current of the lt3502/lt3502a. the peak current limit comparator is used in normal current mode operations and is used to turn off the switch. the da valley current comparator monitors the catch diode current and will delay switching until the catch diode current is below the 650ma limit. maximum deliverable current to the output is therefore limited by both switch peak current limit and da valley current limit. operation
lt3502/lt3502a 10 3502fd applications information figure 1. continuous mode operation near minimum on-time of 60ns fb resistor network the output voltage is programmed with a resistor divider between the output and the fb pin. choose the 1% resis- tors according to: r1 = r2 v out 0.8v ?1 ? ? ? ? ? ? r2 should be 20k or less to avoid bias current errors. reference designators refer to the block diagram. input voltage range the input voltage range for the lt3502/lt3502a applica- tions depends on the output voltage and on the absolute maximum ratings of the v in and boost pins. the minimum input voltage is determined by either the lt3502/lt3502as minimum operating voltage of 3v, or by its maximum duty cycle. the duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: dc = v out + v d v in ?v sw + v d where v d is the forward voltage drop of the catch diode (~0.4v) and v sw is the voltage drop of the internal switch (~0.45v at maximum load). this leads to a minimum input voltage of: v in(min) = v out + v d dc max ?v d + v sw with dc max = 0.80 for the lt3502a and 0.90 for the lt3502. the maximum input voltage is determined by the absolute maximum ratings of the v in and boost pins. for ? xed frequency operation, the maximum input voltage is determined by the minimum duty cycle dc min : v in(max) = v out + v d dc min ?v d + v sw dc min = 0.15 for the lt3502a and 0.08 for the lt3502. v sw 20v/div v out 100mv/div 1 s/div v in = 33v, v out = 3.3v l = 6.8 h, c out = 10 f, i out = 250ma 3502 f01 i l 500ma/div note that this is a restriction on the operating input volt- age for ? xed frequency operation; the circuit will tolerate transient inputs up to the absolute maximum ratings of the v in and boost pins. the input voltage should be limited to the v in operating range (40v) during overload conditions. minimum on-time the lt3502/lt3502a will still regulate the output at input voltages that exceed v in(max) (up to 40v), however, the output voltage ripple increases as the input voltage is increased. as the input voltage is increased, the part is required to switch for shorter periods of time. delays associated with turning off the power switch dictate the minimum on-time of the part. the minimum on-time for the lt3502/lt3502a is 60ns (figure 1). when the required on-time decreases below the mini- mum on-time of 60ns, instead of the switch pulse width becoming narrower to accommodate the lower duty cycle requirement, the switch pulse width remains fixed at 60ns. the inductor current ramps up to a value exceed- ing the load current and the output ripple increases. the part then remains off until the output voltage dips below the programmed value before it begins switching again (figure 2). provided that the load can tolerate the increased output voltage ripple and that the components have been properly selected, operation above v in(max) is safe and will not damage the part.
lt3502/lt3502a 11 3502fd figure 2. pulse-skipping occurs when required on-time is below 60ns v sw 20v/div v out 100mv/div 1s/div v in = 40v, v out = 3.3v l = 6.8h, c out = 10f, i out = 250ma 3502 f02 i l 500ma/div applications information inductor selection and maximum output current a good ? rst choice for the inductor value is: l = 1.6(v out + v d ) for the lt3502a l = 4.6(v out + v d ) for the lt3502 where v d is the voltage drop of the catch diode (~0.4v) and l is in h. with this value there will be no subharmonic oscillation for applications with 50% or greater duty cycle. the inductors rms current rating must be greater than the maximum load current and its saturation current should be about 30% higher. for robust operation during fault conditions, the saturation current should be above 1.2a. to keep ef? ciency high, the series resistance (dcr) should be less than 0.1. table 1 lists several vendors and types that are suitable. there are several graphs in the typical performance characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages. low inductance may result in discontinuous mode opera- figure 3. pulse-skipping with large load current will be limited by the da valley current limit. notice the flat inductor valley current and reduced switching frequency as the input voltage increases, the inductor current ramps up quicker, the number of skipped pulses increases and the output voltage ripple increases. for operation above v in(max) the only component requirement is that the components be adequately rated for operation at the intended voltage levels. inductor current may reach current limit when operating in pulse-skipping mode with small valued inductors. in this case, the lt3502/lt3502a will periodically reduce its frequency to keep the inductor valley current to 650ma (figure 3). peak inductor current is therefore peak current plus minimum switch delay: 900ma + v in ?v out l ?60ns the part is robust enough to survive prolonged operation under these conditions as long as the peak inductor cur- rent does not exceed 1.2a. inductor current saturation and junction temperature may further limit performance during this operating regime. v sw 20v/div v out 100mv/div 1s/div v in = 40v, v out = 3.3v l = 6.8h, c out = 10f, i out = 500ma 3502 f03 i l 500ma/div table 1 vendor url part series inductance rate (h) size (mm) sumida www.sumida.com cdrh4d28 cdrh5d28 cdrh8d28 1.2 to 4.7 2.5 to 10 2.5 to 33 4.5 4.5 5.5 5.5 8.3 8.3 toko www.toko.com a916cy d585lc 2 to 12 1.1 to 39 6.3 6.2 8.1 8 wrth elektronik www.we-online.com we-tpc(m) we-pd2(m) we-pd(s) 1 to 10 2.2 to 22 1 to 27 4.8 4.8 5.2 5.8 7.3 7.3
lt3502/lt3502a 12 3502fd applications information tion, which is okay, but further reduces maximum load current. for details of the maximum output current and discontinuous mode operation, see linear technology application note 44. catch diode a low capacitance 500ma schottky diode is recommended for the catch diode, d1. the diode must have a reverse voltage rating equal to or greater than the maximum input voltage. the diodes inc. sbr1u40lp , on semi mbrm140, and diodes inc. dfls140 are good choices for the catch diode. input capacitor bypass the input of the lt3502/lt3502a circuit with a 1f or higher value ceramic capacitor of x7r or x5r type. y5v types have poor performance over temperature and applied voltage and should not be used. a 1f ceramic is adequate to bypass the lt3502/lt3502a and will easily handle the ripple current. however, if the input power source has high impedance, or there is signi? cant inductance due to long wires or cables, additional bulk capacitance may be necessary. this can be provided with a low performance electrolytic capacitor. step-down regulators draw current from the input supply in pulses with very fast rise and fall times. the input ca- pacitor is required to reduce the resulting voltage ripple at the lt3502/lt3502a and to force this very high frequency switching current into a tight local loop, minimizing emi. a 1f capacitor is capable of this task, but only if it is placed close to the lt3502/lt3502a and the catch diode (see the pcb layout section). a second precaution regarding the ceramic input capacitor concerns the maximum input volt- age rating of the lt3502/lt3502a. a ceramic input capaci- tor combined with trace or cable inductance forms a high quality (underdamped) tank circuit. if the lt3502/lt3502a circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the lt3502/lt3502as voltage rating. this situation is easily avoided; see the hot plugging safely section. output capacitor the output capacitor has two essential functions. along with the inductor, it ? lters the square wave generated by the lt3502/lt3502a to produce the dc output. in this role it determines the output ripple so low impedance at the switching frequency is important. the second function is to store energy in order to satisfy transient loads and stabilize the lt3502/lt3502as control loop. ceramic capacitors have very low equivalent series resistance (esr) and provide the best ripple performance. a good value is: c out = 33 v out for the lt3502a c out = 66 v out for the lt3502 where c out is in f. use an x5r or x7r type and keep in mind that a ceramic capacitor biased with v out will have less than its nominal capacitance. this choice will provide low output ripple and good transient response. transient performance can be improved with a high value capacitor, but a phase lead capacitor across the feedback resistor, r1, may be required to get the full bene? t (see the compensation section). for small size, the output capacitor can be chosen according to: c out = 25 v out where c out is in f. however, using an output capacitor this small results in an increased loop crossover frequency and increased sensitivity to noise. high performance electrolytic capacitors can be used for the output capacitor. low esr is important, so choose one that is intended for use in switching regulators. the esr should be speci? ed by the supplier and should be 0.1 or less. such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the capacitor must be large to achieve low esr. table 2 lists several capacitor vendors.
lt3502/lt3502a 13 3502fd table 2 vendor phone url part series comments panasonic (714) 373-7366 www.panasonic.com ceramic polymer, tantalum eef series kemet (864) 963-6300 www.kemet.com ceramic, tantalum t494,t495 sanyo (408)794-9714 www.sanyovideo.com ceramic polymer, tantalum poscap murata (404) 436-1300 www.murata.com ceramic avx www.avxcorp.com ceramic, tantalum tps series taiyo yuden (864) 963-6300 www.taiyo-yuden.com ceramic applications information figure 4 shows the transient response of the lt3502a with several output capacitor choices. the output is 3.3v. the load current is stepped from 150ma to 400ma and back to 150ma, and the oscilloscope traces show the output voltage. the upper photo shows the recommended value. the sec- ond photo shows the improved response (less voltage drop) resulting from a larger output capacitor and a phase lead capacitor. the last photo shows the response to a high performance electrolytic capacitor. transient performance is improved due to the large output capacitance. boost pin considerations capacitor c3 and the internal boost diode are used to generate a boost voltage that is higher than the input voltage. in most cases a 0.1f capacitor will work well. figure 5 shows two ways to arrange the boost circuit. the boost pin must be at least 2.2v above the sw pin for best ef? ciency. for outputs of 3v and above, the standard circuit (figure 5a) is best. for outputs less than 3v and above 2.5v, place a discrete schottky diode (such as the bat54) in parallel with the internal diode to reduce v d . the following equations can be used to calculate and minimize boost capacitance in f: 0.012/(v bd + v catch C v d C 2.2) for the lt3502a 0.030/(v bd + v catch C v d C 2.2) for the lt3502 v d is the forward drop of the boost diode, and v catch is the forward drop of the catch diode (d1). for lower output voltages the bd pin can be tied to an external voltage source with adequate local bypassing (figure 5b). the above equations still apply for calculating the optimal boost capacitor for the chosen bd voltage. the absence of bd voltage during start-up will increase minimum voltage to start and reduce ef? ciency. you must also be sure that the maximum voltage rating of boost pin is not exceeded. the minimum operating voltage of an lt3502/lt3502a application is limited by the undervoltage lockout (3v) and by the maximum duty cycle as outlined above. for proper start-up, the minimum input voltage is also limited by the boost circuit. if the input voltage is ramped slowly, or the lt3502/lt3502a is turned on with its shdn pin when the output is already in regulation, then the boost capacitor may not be fully charged. because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. this minimum load will depend on the input and output voltages, and on the arrangement of the boost circuit. the minimum load generally goes to zero once the circuit has started. figure 6 shows plots of minimum load to start and to run as a function of input voltage. in many cases the discharged output capacitor will present a load to the switcher which will allow it to start. the plots show the worst-case situation where v in is ramping very slowly. at light loads, the inductor current becomes discontinuous and the effective duty cycle can be very high. this reduces the minimum input voltage to approximately 400mv above v out . at higher load currents, the inductor current is continuous and the duty cycle is limited by the maximum duty cycle of the lt3502/lt3502a, requiring a higher input voltage to maintain regulation.
lt3502/lt3502a 14 3502fd applications information figure 5 v in bd gnd sw da boost v in lt3502 (5a) v out v boost C v sw v out max v boost v in + v out 3502 f05a v in bd gnd sw da boost v in v dd lt3502 (5b) v out v boost C v sw v dd max v boost v in + v dd 3502 f05b figure 4. transient load response of the lt3502a with different output capacitors as the load current is stepped from 150ma to 400ma. v in = 12v, v out = 3.3v, l = 6.8h 10f fb 32.4k i l 0.2a/div v out 0.1v/div ac coupled i l 0.2a/div v out 0.1v/div ac coupled 10s/div 10s/div i l 0.2a/div v out 0.1v/div ac coupled 10s/div 10k v out 3502 f04a 3502 f04b 3502 f04c fb v out 32.4k 10k 10f 2 50pf sanyo 4tpb100m fb v out + 32.4k 10k 100f
lt3502/lt3502a 15 3502fd applications information figure 6 (6a) lt3502a typical minimum input voltage, v out = 3.3v (6b) lt3502a typical minimum input voltage, v out = 5v (6c) lt3502 typical minimum input voltage, v out = 3.3v (6d) lt3502 typical minimum input voltage, v out = 5v soft-start the shdn pin can be used to soft start the lt3502/lt3502a, reducing the maximum input current during start-up. the shdn pin is driven through an external rc ? lter to create a voltage ramp at this pin. figure 7 shows the start-up waveforms with and without the soft-start circuit. by choosing a large rc time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot. choose the value of the resistor so that it can supply 80a when the shdn pin reaches 2v. short and reverse protection if the inductor is chosen so that it wont saturate excessively, the lt3502/lt3502a will tolerate a shorted output. when operating in short-circuit condition, the lt3502/lt3502a will reduce their frequency until the valley current is 650ma (figure 8a). there is another situation to consider in systems where the output will be held high when the input to the lt3502/lt3502a is absent. this may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode or-ed with the lt3502/lt3502as output. if the v in pin is allowed to ? oat and the shdn pin is held high (either by a logic signal load current (a) 0.001 7 6 5 4 3 2 1 0 3502 g19 0.01 0.1 1 v in (v) run start load current (a) 0.001 4 v in (v) 6 8 0.01 0.1 1 3502 g20 2 3 5 7 1 0 start run load current (a) 0.001 7 6 5 4 3 2 1 0 3502 g21 0.01 0.1 1 v in (v) run start load current (a) 0.001 4 v in (v) 6 8 0.01 0.1 1 3502 g22 2 3 5 7 1 0 start run
lt3502/lt3502a 16 3502fd applications information figure 7. to soft-start the lt3502a , add a resistor and capacitor to the shdn pin figure 8b. diode d4 prevents a shorted input from discharging a backup battery tied to the output; it also protects the circuit from a reversed input. the lt3502/lt3502a runs only when the input is present v in 3502 f08b shdn boost sw lt3502a bd d4 gnd da fb v in v out + figure 8a. the lt3502a reduces its frequency to below 500khz to protect against shorted output with 40v input v sw 10v/div 2 s/div v in = 40v v out = 0v l = 6.8 h c out = 10 f 3502 f08a i l 500ma/div run v sw 10v/div v in = 12v v out = 3.3v l = 6.8h c out = 10f v out 2v/div 5s/div i l 500ma/div v sw 10v/div v in = 12v v out = 3.3v l = 6.8h c out = 10f v out 2v/div 50s/div 3502 f07 i l 500ma/div shdn gnd 3502 f07a run 50k 0.1f shdn gnd 3502 f07b
lt3502/lt3502a 17 3502fd applications information or because it is tied to v in ), then the lt3502/lt3502as internal circuitry will pull its quiescent current through its sw pin. this is ? ne if your system can tolerate a few ma in this state. if you ground the shdn pin, the sw pin current will drop to essentially zero. however, if the v in pin is grounded while the output is held high, then parasitic diodes inside the lt3502/lt3502a can pull large currents from the output through the sw pin and the v in pin. figure 8b shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. hot plugging safely the small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of lt3502/lt3502a circuits. however, these capacitors can cause problems if the lt3502/lt3502a are plugged into a live supply (see linear technology application note 88 for a complete discussion). the low loss ceramic capacitor combined with stray inductance in series with the power source forms an underdamped tank circuit, and the voltage at the v in pin of the lt3502/lt3502a can ring to twice the nominal input voltage, possibly ex- ceeding the lt3502/lt3502as rating and damaging the part. if the input supply is poorly controlled or the user will be plugging the lt3502/lt3502a into an energized supply, the input network should be designed to prevent this overshoot. figure 9 shows the waveforms that result when an lt3502/lt3502a circuit is connected to a 24v supply through six feet of 24-gauge twisted pair. the ? rst plot is the response with a 2.2f ceramic capacitor at the input. the input voltage rings as high as 35v and the input current peaks at 20a. one method of damping the tank circuit is to add another capacitor with a series resistor to + + lt3502 2.2f v in 20v/div i in 5a/div 20s/div v in closing switch simulates hot plug i in (9a) (9b) (9c) low impedance energized 24v supply stray inductance due to 6 feet (2 meters) of twisted pair + + lt3502 2.2f 10f 35v ai.ei. lt3502 2.2f 0.1f 1 3502 f09 v in 20v/div i in 5a/div 20s/div v in 20v/div i in 5a/div 20s/div danger! ringing v in may exceed absolute maximum rating of the lt3502 figure 9. a well chosen input network prevents input voltage overshoot and ensures reliable operation when the lt3502 is connected to a live supply
lt3502/lt3502a 18 3502fd applications information figure 10. model for loop response C + C + 800mv sw v c lt3502 gnd 3502 f10 r1 out esr error amplifier current mode power stage fb r2 1m r c 150k c c 70pf c1 c1 g m = 100 a/v g m = 1a/v + c pl 0.5v the circuit. in figure 9b an aluminum electrolytic capacitor has been added. this capacitors high equivalent series resistance damps the circuit and eliminates the voltage overshoot. the extra capacitor improves low frequency ripple ? ltering and can slightly improve the ef? ciency of the circuit, though it is likely to be the largest component in the circuit. an alternative solution is shown in figure 9c. a 1 resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). a 0.1f capacitor improves high frequency ? ltering. this solution is smaller and less expensive than the electrolytic capacitor. for high input voltages its impact on ef? ciency is minor, reducing ef? ciency less than one half percent for a 5v output at full load operating from 24v. frequency compensation the lt3502/lt3502a use current mode control to regulate the output. this simpli? es loop compensation. in particular, the lt3502/lt3502a does not require the esr of the output capacitor for stability allowing the use of ceramic capacitors to achieve low output ripple and small circuit size. figure 10 shows an equivalent circuit for the lt3502/ lt3502a control loop. the error amp is a transconductance ampli? er with ? nite output impedance. the power section, consisting of the modulator, power switch and inductor, is modeled as a transconductance ampli? er generating an output current proportional to the voltage at the v c node. note that the output capacitor integrates this current, figure 11 bst da gnd d1 fb v in v out l1 c1 c2 c3 r1 r2 = via 3502 f11 bd shdn and that the capacitor on the v c node (c c ) integrates the error ampli? er output current, resulting in two poles in the loop. r c provides a zero. with the recommended output capacitor, the loop crossover occurs above the r c c c zero. this simple model works well as long as the value of the inductor is not too high and the loop crossover frequency is much lower than the switching frequency. with a larger ceramic capacitor (very low esr), crossover may be lower and a phase lead capacitor (c pl ) across the feedback divider may improve the phase margin and transient response. large electrolytic capacitors may have an esr large enough to create an additional zero, and the phase lead may not be necessary. if the output capacitor is different than the recommended capacitor, stability should be checked across all operat- ing conditions, including load current, input voltage and temperature. the lt1375 data sheet contains a more thorough discussion of loop compensation and describes how to test the stability using a transient load. pcb layout for proper operation and minimum emi, care must be taken during printed circuit board layout. figure 11 shows the recommended component placement with trace, ground plane and via locations. note that large, switched currents ? ow in the lt3502/lt3502as v in and sw pins, the catch diode (d1) and the input capacitor (c2).
lt3502/lt3502a 19 3502fd applications information the loop formed by these components should be as small as possible and tied to system ground in only one place. these components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. place a local, unbroken ground plane below these components, and tie this ground plane to system ground at one location, ideally at the ground terminal of the output capacitor c1. the sw and boost nodes should be as small as possible. finally, keep the fb node small so that the ground pin and ground traces will shield it from the sw and boost nodes. include vias near the exposed gnd pad of the lt3502/lt3502a to help remove heat from the lt3502/lt3502a to the ground plane. high temperature considerations the die temperature of the lt3502/lt3502a must be lower than the maximum rating of 125c. this is generally not a concern unless the ambient temperature is above 85c. for higher temperatures, care should be taken in the layout of the circuit to ensure good heat sinking of the lt3502/lt3502a. the maximum load current should be derated as the ambient temperature approaches 125c. the die temperature is calculated by multiplying the lt3502/lt3502a power dissipation by the thermal resistance from junction to ambient. power dissipation within the lt3502/lt3502a can be estimated by calculat- ing the total power loss from an ef? ciency measurement and subtracting the catch diode loss. thermal resistance depends on the layout of the circuit board, but 102c/w and 110oc/w are typical for the (2mm 2mm) dfn and ms10 packages respectively. figure 12. 15v step-down converter v in c3 0.1 f 1n4148 or other similar diodes 22pf 3502 f12 l1 33 h 10v shdn boost sw lt3502a bd gnd da fb off on c1 10 f r2 10k r1 180k c2 1 f v in 20v to 40v v out 15v 500ma c4 0.1f outputs greater than 7v note that for outputs above 7v, the input voltage range will be limited by the maximum rating of the boost pin. the sum of input and output voltages cannot exceed the boost pins 50v rating. the 15v circuit (figure 12) shows how to overcome this limitation using an additional zener diode. other linear technology publications application notes an19, an35 and an44 contain more detailed descriptions and design information for buck regulators and other switching regulators. the lt1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. design note 100 shows how to generate a bipolar output supply using a buck regulator.
lt3502/lt3502a 20 3502fd typical applications 0.8v step-down converter v in c3 0.1 f 0.1 f 3502 ta02a l1 3.3 h d1 shdn boost sw lt3502a bd gnd da fb off on c1 47 f c2 1 f v in 3v to 40v v bd 3v to 7v v out 0.8v 500ma c1: jmk212bj476mg c3: hmk212bj104mg l1: lqh43cn3r3m03 v in c3 0.1f c1: jmk316bj107ml l1: lqh43cn100k03 3502 ta02b l1 10h d1 shdn boost sw lt3502 bd gnd da fb off on c1 100f c2 1f v in 3v to 40v v bd 3v to 7v v out 0.8v 500ma 0.1f 1.8v step-down converter v in c3 0.1f 3502 ta03a l1 4.7h d1 shdn boost sw lt3502a bd gnd da fb off on c1 22f r2 10k r1 12.5k c2 1f v in 3v to 40v v bd 3v to 7v v out 1.8v 500ma c1: jmk212bj226mg l1: lqh43cn4r7m03 0.1f v in c3 0.1f 3502 ta03b l1 15h d1 shdn boost sw lt3502 bd gnd da fb off on c1 47f r2 10k r1 12.5k c2 1f v in 3v to 40v v bd 3v to 7v v out 1.8v 500ma c1: jmk212bj476mg l1: lqh55dn150m03 0.1f
lt3502/lt3502a 21 3502fd 2.5v step-down converter v in c3 0.1f 3502 ta04a l1 6.8h d1 shdn boost sw lt3502a bd gnd da fb off on c1 22f r2 10k r1 21.3k c2 1f v in 3.5v to 40v v bd 3v to 7v v out 2.5v 500ma c1: jmk212bj226mg l1: lqh43dn6r8m03 0.1f v in c3 0.1f 3502 ta04b l1 15h d1 shdn boost sw lt3502 bd gnd da fb off on c1 22f r2 10k r1 21.3k c2 1f v in 3.5v to 40v v bd 3v to 7v v out 2.5v 500ma c1: jmk212bj226mg l1: lqh55dn150m03 0.1f typical applications 3.3v step-down converter v in c3 0.1f 3502 ta05a l1 6.8h d1 shdn boost sw lt3502a bd gnd da fb off on c1 10f r2 10k r1 31.6k c2 1f v in 4.7v to 40v v out 3.3v 500ma c1: lmk316bj106ml-br l1: lqh43cn6r8m03 v in c3 0.1f 3502 ta05b l1 15h d1 shdn boost sw lt3502 bd gnd da fb off on c1 22f r2 10k r1 31.6k c2 1f v in 4.5v to 40v v out 3.3v 500ma c1: jmk212bj226mg l1: lqh55dn150m03
lt3502/lt3502a 22 3502fd package description dc8 package 8-lead plastic dfn (2mm 2mm) (reference ltc dwg # 05-08-1719 rev a) 2.00 p 0.10 (4 sides) note: 1. drawing is not a jedec package outline 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.15mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package 0.40 p 0.10 bottom viewexposed pad 0.64 p 0.10 (2 sides) 0.75 p 0.05 r = 0.115 typ r = 0.05 typ 1.37 p 0.10 (2 sides) 1 4 8 5 pin 1 bar top mark (see note 6) 0.200 ref 0.00 C 0.05 (dc8) dfn 0106 rev? 0.23 p 0.05 0.45 bsc 0.25 p 0.05 1.37 p 0.05 (2 sides) recommended solder pad pitch and dimensions apply solder mask to areas that are not soldered 0.64 p 0.05 (2 sides) 1.15 p 0.05 0.70 p 0.05 2.55 p 0.05 package outline 0.45 bsc pin 1 notch r = 0.20 or 0.25 s 45 o chamfer
lt3502/lt3502a 23 3502fd information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. package description ms package 10-lead plastic msop (reference ltc dwg # 05-08-1661 rev e) msop (ms) 0307 rev e 0.53 0.152 (.021 .006) seating plane 0.18 (.007) 1.10 (.043) max 0.17 C?0.27 (.007 C .011) typ 0.86 (.034) ref 0.50 (.0197) bsc 12 3 45 4.90 0.152 (.193 .006) 0.497 0.076 (.0196 .003) ref 8 9 10 7 6 3.00 0.102 (.118 .004) (note 3) 3.00 0.102 (.118 .004) (note 4) note: 1. dimensions in millimeter/(inch) 2. drawing not to scale 3. dimension does not include mold flash, protrusions or gate burrs. mold flash, protrusions or gate burrs shall not exceed 0.152mm (.006") per side 4. dimension does not include interlead flash or protrusions. interlead flash or protrusions shall not exceed 0.152mm (.006") per side 5. lead coplanarity (bottom of leads after forming) shall be 0.102mm (.004") max 0.254 (.010) 0 C 6 typ detail a detail a gauge plane 5.23 (.206) min 3.20 C 3.45 (.126 C .136) 0.889 0.127 (.035 .005) recommended solder pad layout 0.305 0.038 (.0120 .0015) typ 0.50 (.0197) bsc 0.1016 0.0508 (.004 .002)
lt3502/lt3502a 24 3502fd linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2007 lt 0809 rev d ? printed in usa part number description comments lt1766 60v, 1.2a (i out ), 200khz, high ef? ciency step-down dc/dc converter v in : 5.5v to 60v, v out(min) = 1.2v, i q = 2.5ma, i sd = 25a, tssop16/tssop16e packages lt1933 500ma (i out ), 500khz, step-down switching regulator in sot-23 v in : 3.6v to 36v, v out(min) = 1.2v, i q = 1.6ma, i sd < 1a, thinsot? package lt1936 36v, 1.4a (i out ), 500khz, high ef? ciency step-down dc/dc converter v in : 3.6v to 36v, v out(min) = 1.2v, i q = 1.9ma, i sd < 1a, ms8e package lt1940 dual 25v, 1.4a (i out ), 1.1mhz, high ef? ciency step-down dc/dc converter v in : 3.6v to 25v, v out(min) = 1.20v, i q = 3.8ma, i sd < 30a, tssop16e package lt1976/ lt1977 60v, 1.2a (i out ), 200khz/500khz high ef? ciency step-down dc/dc converters with burst mode ? operation v in : 3.3v to 60v, v out(min) = 1.20v, i q = 100a, i sd < 1a, tssop16e package lt c 3407/ ltc3407-2 dual 600ma/800ma, 1.5mhz/2.25mhz, synchronous step-downdc/dc converters v in : 2.5v to 5.5v, v out(min) = 0.6v, i q = 40a, i sd <1a, 3mm 3mm dfn, ms10e package lt3434/ lt3435 60v, 1.2a (i out ), 200khz/500khz high ef? ciency step-down dc/dc converters with burst mode operation v in : 3.3v to 60v, v out(min) = 1.20v, i q = 100a, i sd < 1a, tssop16e package lt3437 60v, 400ma (i out ), micropower step-down dc/dc converter with burst mode operation v in : 3.3v to 60v, v out(min) = 1.25v, i q = 100a, i sd < 1a, dfn package lt3493 36v, 1.4a (i out ), 750khz, high ef? ciency step-down dc/dc converter v in : 3.6v to 36v, v out(min) = 0.8v, i q = 1.9ma, i sd < 1a, dfn package lt3501 dual 25v, 3a (i out ), 1.5mhz, high ef? ciency step-down dc/dc converter v in : 3.3v to 25v, v out(min) = 0.8v, i q = 3.7ma, i sd < 10a, tssop20e package lt3503 20v, 1a (i out ), 2.2mhz, high ef? ciency step-down dc/dc converter v in : 3.6v to 20v, v out(min) = 0.78v, i q = 1.9ma, i sd < 1a, 2mm 3mm dfn package lt3505 36v, 1.2a (i out ), 3mhz, high ef? ciency step-down dc/dc converter v in : 3.6v to 36v, v out(min) = 0.78v, i q = 2ma, i sd < 2a, 3mm 3mm dfn, ms8e packages lt3506/ lt3506a dual 25v, 1.6a (i out ), 575khz/1.1mhz, high ef? ciency step- down dc/dc converters v in : 3.6v to 25v, v out(min) = 0.8v, i q = 3.8ma, i sd < 30a, 4mm 5mm dfn package lt3508 dual 36v, 1.4a (i out ), 2.5mhz, high ef? ciency step-down dc/dc converter v in : 3.6v to 36v, v out(min) = 0.8v, i q = 4.3ma, i sd < 1a, 4mm 4mm qfn, tssop16e packages lt3510 dual 25v, 2a (i out ), 1.5mhz, high ef? ciency step-down dc/dc converter v in : 3.3v to 25v, v out(min) = 0.8v, i q = 3.7ma, i sd < 10a, tssop20e package ltc3548 dual 400ma + 800ma, 2.25mhz synchronous step-down dc/dc converter v in : 2.5v to 5.5v, v out(min) = 0.6v, i q = 40a, i sd < 1a, 3mm 3mm dfn, ms10e packages burst mode is a registered trademark of linear technology corporation. thinsot is a trademark of linear technology corporation. typical application 5v step-down converter v in c3 0.1f 3502 ta06a l1 10h d1 shdn boost sw lt3502a bd gnd da fb off on c1 10f r2 10k r1 52.3k c2 1f v in 6.7v to 40v v out 5v 500ma c1: lmk316bj106ml-br l1: lqh43cn100k03 v in c3 0.1f 3502 ta06b l1 22h d1 shdn boost sw lt3502 bd gnd da fb off on c1 22f r2 10k r1 52.3k c2 1f v in 6.4v to 40v v out 5v 500ma c1: lmk316bj106ml-br l1: lqh43cn100k03 related parts

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