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lt3470a 1 3470afb typical application features applications description micropower buck regulator with integrated boost and catch diodes the lt ? 3470a is a micropower step-down dc/dc con- verter that integrates a 440ma power switch, catch diode and boost diode into low profile 2mm 3mm dfn package. the lt3470a combines burst mode and continuous operation to allow the use of tiny inductor and capacitors while providing a low ripple output to loads of up to 250ma. with its wide input range of 4v to 40v, the lt3470a can regulate a wide variety of power sources, from 2-cell li-ion batteries to unregulated wall transformers and lead-acid batteries. quiescent current in regulation is just 35a in a typical application while a zero current shutdown mode disconnects the load from the input source, simplifying power management in battery-powered systems. fast cur- rent limiting and hysteretic control protects the lt3470a and external components against shorted outputs, even at 40v input. the lt3470a has higher output current and improved start-up and dropout performance compared to the lt3470. efficiency and power loss vs load current n low quiescent current: 35a at 12v in to 3.3v out n integrated boost and catch diodes n input range: 4v to 40v n 3.3v at 250ma from 4v to 40v input n 5v at 250ma from 5.7v to 40v input n low output ripple: <10mv n < 1a in shutdown mode n output voltage: 1.25v to 16v n hysteretic mode control C low ripple burst mode ? operation at light loads C continuous operation at higher loads n solution size as small as 50mm 2 n low profile (0.75mm) 2mm 3mm thermally enhanced 8-lead dfn package n automotive battery regulation n power for portable products n distributed supply regulation n industrial supplies n wall transformer regulation l , lt, ltc, ltm, linear technology, the linear logo and burst mode are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. v in boost lt3470a sw shdn 0.22f 22pf 22f 2.2f v in 5.7v to 40v v out 5v 250ma 604k 1% 200k 1% 33h bias fb gnd off on 3470a ta01a load current (ma) 30 efficiency (%) power loss (mw) 40 60 80 90 0.1 10 100 300 3470a ta01b 20 1 70 50 10 1 1000 100 10 0.1 v in = 12v
lt3470a 2 3470afb pin configuration absolute maximum ratings v in , shdn voltage ................................................... 40v boost pin voltage .................................................. 47v boost pin above sw pin ........................................ 25v fb voltage .................................................................. 5v bias voltage .............................................................15v sw voltage ................................................................v in maximum junction temperature lt3470ae, lt3470ai ........................................ 125c lt3470ah ......................................................... 150c operating temperature range (note 2) lt3470ae ............................................C 40c to 85c lt3470ai ........................................... C40c to 125c lt3470ah .......................................... C40c to 150c storage temperature range .................. C65c to 150c lead temperature (soldering, 10 sec) .................. 300c (note 1) top view 9 ddb8 package 8-lead (3mm 2mm) plastic dfn 5 6 7 8 4 3 2 1 fb bias boost sw shdn nc v in gnd ja = 80c/w exposed pad (pin 9) is gnd, must be soldered to pcb order information lead free finish tape and reel part marking * package description temperature range lt3470aeddb#pbf lt3470aeddb#trpbf ldpr 8-lead (3mm 2mm) plastic dfn C40c to 85c lt3470aiddb#pbf lt3470aiddb#trpbf ldpr 8-lead (3mm 2mm) plastic dfn C40c to 125c lt3470ahddb#pbf lt3470ahddb#trpbf ldpr 8-lead (3mm 2mm) plastic dfn C40c to 150c consult ltc marketing for parts specified with wider operating temperature ranges. *the temperature grade is identified by a l abel on the shipping container. consult ltc marketing for information on non-standard lead based finish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
lt3470a 3 3470afb electrical characteristics the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. v in = 10v, v shdn = 10v, v boost = 15v, v bias = 3v unless otherwise specified. parameter conditions min typ max units minimum input voltage 4v quiescent current from v in v shdn = 0.2v v bias = 3v, not switching v bias = 0v, not switching 0.1 10 40 0.5 18 55 a a a quiescent current from bias v shdn = 0.2v v bias = 3v, not switching v bias = 0v, not switching 0.1 30 0.1 0.5 60 1.5 a a a fb comparator trip voltage v fb falling 1.228 1.250 1.265 v fb pin bias current (note 3) v fb = 1v h-grade 35 35 35 80 150 225 na na na fb voltage line regulation 4v < v in < 40v 0.0006 0.02 %/v minimum switch off-time (note 5) 500 ns maximum duty cycle 90 95 % switch leakage current 0.7 1.5 a switch v cesat i sw = 100ma 150 mv switch v cesat without boost v boost = v sw 0.9 1.2 v switch top current limit v fb = 0v 320 440 560 ma switch bottom current limit v fb = 0v 280 ma catch schottky drop i sw = 100ma 600 mv catch schottky reverse leakage v sw = 10v 0.2 2 a boost schottky drop i bias = 50ma 690 775 mv boost schottky reverse leakage v sw = 10v, v bias = 0v 0.2 2 a minimum boost voltage (note 4) 1.7 2.2 v boost pin current i sw = 100ma 2.3 5 ma bias pin preload v boost = 10v 50 ma shdn pin current v shdn = 2.5v 1 5 a shdn input voltage high 2v shdn input voltage low 0.2 v 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 lt3470ae is guaranteed to meet performance speci? cations from 0c to 85c. speci? cations over the C40c to 85c operating temperature range are assured by design, characterization and correlation with statistical process controls. the lt3470ai speci? cations are guaranteed over the C40c to 125c temperature range. lt3470ah speci? cations are guaranteed over the C40c to 150c temperature range. note 3: bias current ? ows out of the fb pin. note 4: this is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. note 5: this parameter is assured by design and correlation with statistical process controls.
lt3470a 4 3470afb typical performance characteristics efficiency, v out = 3.3v efficiency, v out = 5v v fb vs temperature top and bottom switch current limits (v fb = 0v) vs temperature v in quiescent current vs temperature bias quiescent current (bias > 3v) vs temperature shdn bias current vs temperature load current (ma) 50 efficiency (%) 70 90 40 60 80 0.1 10 100 3470a g01 30 1 l = toko d52lc 47h t a = 25c v in = 7v v in = 36v v in = 24v v in = 12v t a = 25c unless otherwise noted. temperature (c) C50 current limit (ma) 550 25 3470a g04 400 300 C25 0 50 250 200 600 500 450 350 75 100 150 125 temperature (c) C50 C25 0 v in current (a) 20 50 0 50 75 3470a g05 10 40 30 25 100 150 125 bias < 3v bias > 3v temperature (c) C50 bias current (a) 20 25 30 25 75 3470a g06 15 10 C25 0 50 100 150 125 5 0 temperature (c) C50 0 s hdn current (a) 1 3 4 5 50 9 3470a g07 2 0 C25 75 100 25 150 125 6 7 8 v shdn = 36v v shdn = 2.5v temperature (c) C50 1.240 v fb (v) 1.245 1.250 1.255 1.260 C25 0 25 50 3470a g03 75 100 150 125 load current (ma) 50 efficiency (%) 70 90 40 60 80 0.1 10 100 3470a g02 30 1 l = toko d52lc 47h t a = 25c v in = 12v v in = 36v v in = 24v
lt3470a 5 3470afb fb bias current (v fb = 1v) vs temperature typical performance characteristics fb bias current (v fb = 0v) vs temperature switch v cesat (i sw = 100ma) vs temperature boost diode v f (i f = 50ma) vs temperature catch diode v f (i f = 100ma) vs temperature diode leakage (v r = 36v) vs temperature t a = 25c unless otherwise noted. temperature (c) C50 C25 0 fb current (na) 20 60 50 0 50 75 3470a g08 10 40 30 25 100 150 125 temperature (c) C50 fb current (a) 80 100 120 25 75 3470a g09 60 40 C25 0 50 100 150 125 20 0 temperature (c) C50 switch v cesat (mv) 200 250 300 25 75 3470a g10 150 100 C25 0 50 100 150 125 50 0 temperature (c) C50 schottky v f (v) 0.7 25 3470a g11 0.4 0.2 C25 0 50 0.1 0 0.8 0.6 0.5 0.3 75 100 150 125 temperature (c) C50 0.4 0.5 0.7 25 75 3470a g12 0.3 0.2 C25 0 50 100 150 125 0.1 0 0.6 schottky v f (v) temperature (c) C50 C25 0 schottky diode leakage (ma) 30 60 0 50 75 3470a g13 20 15 10 50 40 25 55 5 45 35 25 100 150 catch boost 125
lt3470a 6 3470afb boost diode forward voltage minimum input voltage, v out = 3.3v minimum input voltage, v out = 5v typical performance characteristics boost diode current (ma) 0 schottky v f (v) 500 600 700 200 3470a g17 400 300 0 50 100 150 100 200 900 800 switch v cesat boost pin current catch diode forward voltage switch current (ma) 0 400 500 700 300 400 3470a g14 300 200 100 200 500 100 0 600 switch v cesat (mv) switch current (ma) 0 8 10 14 300 400 3470a g15 6 4 100 200 500 2 0 12 boost pin current (ma) catch diode current (ma) 0 schottky v f (v) 0.4 0.6 400 3470a g16 0.2 0 100 200 300 1.0 0.8 load current (ma) 0 3.0 input voltage (v) 3.5 4.0 4.5 5.0 5.5 6.0 50 100 150 200 250 3470a g18 t a = 25c v in to run/start load current (ma) 0 input voltage (v) 6 7 250 3470a g19 5 4 50 100 150 200 8 t a = 25c v in to run/start t a = 25c unless otherwise noted.
lt3470a 7 3470afb shdn (pin 8): the shdn pin is used to put the lt3470a in shutdown mode. tie to ground to shut down the lt3470a. apply 2v or more for normal operation. if the shutdown feature is not used, tie this pin to the v in pin. nc (pin 7): this pin can be left floating, connected to v in , or tied to gnd. v in (pin 6): the v in pin supplies current to the lt3470as internal regulator and to the internal power switch. this pin must be locally bypassed. gnd (pin 5): tie the gnd pin to a local ground plane below the lt3470a and the circuit components. return the feedback divider to this pin. sw (pin 4): the sw pin is the output of the internal power switch. connect this pin to the inductor, catch diode and boost capacitor. boost (pin 3): the boost pin is used to provide a drive voltage, which is higher than the input voltage, to the internal bipolar npn power switch. bias ( pin 2): the bias pin connects to the internal boost schottky diode and to the internal regulator. tie to v out when v out > 2.5v or to v in otherwise. when v bias > 3v the bias pin will supply current to the internal regulator. fb ( pin 1): the lt3470a regulates its feedback pin to 1.25v. connect the feedback resistor divider tap to this pin. set the output voltage according to v out = 1.25v (1 + r1/r2) or r1 = r2 (v out /1.25 C 1). exposed pad ( pin 9): ground. must be soldered to pcb. pin functions block diagram C + C + rq ? sq 500ns one shot v ref 1.25v burst mode detect sw gnd 3470a bd fb r2 r1 shdn enable v in v in nc bias boost l1 c2 c3 v out g m c1
lt3470a 8 3470afb figure 1. operating waveforms of the lt3470a converting 12v to 5v using a 33h inductor and 10f output capacitor operation the lt3470a uses a hysteretic control scheme in conjunc- tion with burst mode operation to provide low output ripple and low quiescent current while using a tiny inductor and capacitors. operation can best be understood by studying the block diagram. an error amplifier measures the output voltage through an external resistor divider tied to the fb pin. if the fb voltage is higher than v ref , the error amplifier will shut off all the high power circuitry, leaving the lt3470a in its micropower state. as the fb voltage falls, the error amplifier will enable the power section, causing the chip to begin switching, thus delivering charge to the output capacitor. if the load is light the part will alternate between micropower and switching states to keep the output in regulation (see figure 1a). at higher loads the part will switch continuously while the error amp servos the top and bottom current limits to regulate the fb pin voltage to 1.25v (see figure 1b). the switching action is controlled by an rs latch and two current comparators as follows: the switch turns on, and the current through it ramps up until the top current comparator trips and resets the latch causing the switch to turn off. while the switch is off, the inductor current ramps down through the catch diode. when both the bot- tom current comparator trips and the minimum off-time one-shot expires, the latch turns the switch back on thus completing a full cycle. the hysteretic action of this control scheme results in a switching frequency that depends on inductor value, input and output voltage. since the switch only turns on when the catch diode current falls below threshold, the part will automatically switch slower to keep inductor current under control during start-up or short-circuit conditions. the switch driver operates from either the input or from the boost pin. an external capacitor and internal diode is 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 efficient operation. if the shdn pin is grounded, all internal circuits are turned off and v in current reduces to the device leakage current, typically 100na. (1a) burst mode operation (1b) continuous operation v out 20mv/div i l 100ma/div 1ms/div v out 20mv/div i l 100ma/div 5s/div 3470a f01a no load 10ma load v out 20mv/div i l 100ma/div 1s/div v out 20mv/div i l 100ma/div 1s/div 3470a f1b 200ma load 150ma load
lt3470a 9 3470afb input voltage range the minimum input voltage required to generate a par- ticular output voltage in an lt3470a application is limited by either its 4v undervoltage lockout 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 Cv sw + v d where v d is the forward voltage drop of the catch diode (~0.6v) and v sw is the voltage drop of the internal switch at maximum load (~0.4v). given dc max = 0.90, this leads to a minimum input voltage of: v in(min) = v out + v d dc max ? ? ? ? ? ? + v sw Cv d this analysis assumes the part has started up such that the capacitor tied between the boost and sw pins is charged to more than 2v. for proper start-up, the minimum input voltage is limited by the boost circuit as detailed in the section boost pin considerations. the maximum input voltage is limited by the absolute maximum v in rating of 40v, provided an inductor of suf- ficient value is used. inductor selection the switching action of the lt3470a during continuous operation produces a square wave at the sw pin that results in a triangle wave of current in the inductor. the hysteretic mode control regulates the top and bottom current limits (see electrical characteristics) such that the average induc- tor current equals the load current. for safe operation, it must be noted that the lt3470a cannot turn the switch on for less than ~150ns. if the inductor is small and the input voltage is high, the current through the switch may exceed safe operating limit before the lt3470a is able to turn off. to prevent this from happening, the following equation provides a minimum inductor value: l min = v in(max) ?t on-time(min) i max applications information where v in(max) is the maximum input voltage for the ap- plication, t on-time(min) is ~150ns and i max is the maximum allowable increase in switch current during a minimum switch on-time (150ma). while this equation provides a safe inductor value, the resulting application circuit may switch at too high a frequency to yield good efficiency. it is advised that switching frequency be below 1.2mhz during normal operation: f = 1C dc () v d + v out () l? i l where f is the switching frequency, i l is the ripple current in the inductor (~200ma), v d is the forward voltage drop of the catch diode, and v out is the desired output voltage. if the application circuit is intended to operate at high duty cycles (v in close to v out ), it is important to look at the calculated value of the switch off-time: t off-time = 1C dc f the calculated t off-time should be more than lt3470as minimum t off-time (see electrical characteristics), so the application circuit is capable of delivering full rated output current. if the full output current of 250ma is not required, the calculated t off-time can be made less than minimum t off-time possibly allowing the use of a smaller inductor. see table 1 for an inductor value selection guide. table 1. recommended inductors for loads up to 250ma v out v in up to 16v v in up to 40v 2.5v 10h 33h 3.3v 10h 33h 5v 15h 33h 12v 33h 47h choose an inductor that is intended for power applications. table 2 lists several manufacturers and inductor series. for robust output short-circuit protection at high v in (up to 40v) use at least a 33h inductor with a minimum 450ma saturation current. if short-circuit performance is not required, inductors with i sat of 300ma or more may be used. it is important to note that inductor saturation current is reduced at high temperaturessee inductor vendors for more information.
lt3470a 10 3470afb applications information input capacitor step-down regulators draw current from the input sup- ply in pulses with very fast rise and fall times. the input capacitor is required to reduce the resulting voltage ripple at the v in pin of the lt3470a and to force this switching current into a tight local loop, minimizing emi. the input capacitor must have low impedance at the switching frequency to do this effectively. a 1f to 2.2f ceramic capacitor satisfies these requirements. if the input source impedance is high, a larger value ca- pacitor may be required to keep input ripple low. in this case, an electrolytic of 10f or more in parallel with a 1f ceramic is a good combination. be aware that the input capacitor is subject to large surge currents if the lt3470a circuit is connected to a low impedance supply, and that some electrolytic capacitors (in particular tantalum) must be specified for such use. output capacitor and output ripple the output capacitor filters the inductors ripple current and stores energy to satisfy the load current when the lt3470a is quiescent. in order to keep output voltage ripple low, the impedance of the capacitor must be low at the lt3470as switching frequency. the capacitors equiva- lent series resistance (esr) determines this impedance. choose one with low esr intended for use in switching regulators. the contribution to ripple voltage due to the esr is approximately i lim ? esr. esr should be less than ~150m. the value of the output capacitor must be large enough to accept the energy stored in the inductor without a large change in output voltage. setting this voltage step equal to 1% of the output voltage, the output capacitor must be: c out > 50 ? l ? i lim v out ? ? ? ? ? ? 2 where i lim is the top current limit with v fb = 0v (see electri- cal characteristics). for example, an lt3470a producing 3.3v with l = 33h requires 22f. the calculated value can be relaxed if small circuit size is more important than low output ripple. sanyos poscap series in b-case and provides very good performance in a small package for the lt3470a. similar performance in traditional tantalum capacitors requires a larger package (c-case). with a high quality capacitor filtering the ripple current from the inductor, the output voltage ripple is determined by the delay in the lt3470as feedback comparator. this ripple can be reduced further by adding a small (typically 22pf) phase lead capacitor between the output and the feedback pin. table 2. inductor vendors vendor url part series inductance range (h) size (mm) coilcraft www.coilcraft.com do1605 me3220 do3314 10 to 47 10 to 47 10 to 47 1.8 5.4 4.2 2.0 3.2 2.5 1.4 3.3 3.3 sumida www.sumida.com cr32 cdrh3d16/hp cdrh3d28 cdrh2d18/hp 10 to 47 10 to 33 10 to 47 10 to 15 3.0 3.8 4.1 1.8 4.0 4.0 3.0 4.0 4.0 2.0 3.2 3.2 toko www.tokoam.com db320c d52lc 10 to 27 10 to 47 2.0 3.8 3.8 2.0 5.0 5.0 wrth elektronik www.we-online.com we-pd2 typ s we-tpc typ s 10 to 47 10 to 22 3.2 4.0 4.5 1.6 3.8 3.8 coiltronics www.cooperet.com sd10 10 to 47 1.0 5.0 5.0 murata www.murata.com lqh43c lqh32c 10 to 47 10 to 15 2.6 3.2 4.5 1.6 2.5 3.2
lt3470a 11 3470afb applications information ceramic capacitors ceramic capacitors are small, robust and have very low esr. however, ceramic capacitors can cause problems when used with the lt3470a. not all ceramic capacitors are suitable. x5r and x7r types are stable over temperature and applied voltage and give dependable service. other types, including y5v and z5u have very large temperature and voltage coefficients of capacitance. in an application circuit they may have only a small fraction of their nominal capacitance resulting in much higher output voltage ripple than expected. ceramic capacitors are piezoelectric. the lt3470as switching frequency depends on the load current, and at light loads the lt3470a can excite the ceramic capacitor at audio frequencies, generating audible noise. since the lt3470a operates at a lower current limit during burst mode operation, the noise is typically very quiet to a ca- sual ear. if this audible noise is unacceptable, use a high performance electrolytic capacitor at the output. the input capacitor can be a parallel combination of a 2.2f ceramic capacitor and a low cost electrolytic capacitor. a final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the lt3470a. a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. if the lt3470a circuit is plugged into a live supply, the input volt- age can ring to twice its nominal value, possibly exceeding the lt3470as rating. this situation is easily avoided; see the hot-plugging safely section. boost and bias pin considerations capacitor c3 and the internal boost schottky diode (see block diagram) are used to generate a boost voltage that is higher than the input voltage. in most cases a 0.22f capacitor will work well. figure 2 shows two ways to ar- range the boost circuit. the boost pin must be more than 2.5v above the sw pin for best efficiency. for outputs of 3.3v and above, the standard circuit (figure 2a) is best. for outputs between 2.5v and 3v, use a 0.47f. for lower output voltages the boost diode can be tied to the input figure 2. two circuits for generating the boost voltage table 2. capacitor vendors 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) 749-9714 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 v in boost lt3470a (2a) (2b) sw c3 0.22f v in v out v boost C v sw ? v out max v boost ? v in + v out bias gnd v in boost lt3470a sw bias c3 0.22f v in v out 3470a f02 v boost C v sw ? v in max v boost ? 2?v in gnd
lt3470a 12 3470afb applications information (figure 2b). the circuit in figure 2a is more efficient because the boost pin current and bias pin quiescent current comes from a lower voltage source. you must also be sure that the maximum voltage ratings of the boost and bias pins are not exceeded. the lt3470a monitors the boost capacitor for sufficient voltage such that the switch is allowed to fully saturate. when boost voltage falls below adequate levels (1.8v typical) the switch will operate with about 1v of drop, and an internal current source will begin to pull 50ma (typi- cal) from the bias pin which is typically connected to the output. this current forces the lt3470a to switch more often and with more inductor current, which recharges the boost capacitor. when the boost capacitor voltage is above 1.8v (typical) the current source turns off, and the part may enter burstmode. this cycle will repeat anytime there is an undervoltage condition on the boost capaci- tor. see figure 3 for minimum input voltage for outputs of 3.3v and 5v. shorted input protection if the inductor is chosen so that it wont saturate exces- sively at the top switch current limit maximum of 525ma, an lt3470a buck regulator will tolerate a shorted output even if v in = 40v. there is another situation to consider in systems where the output will be held high when the input to the lt3470a 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 lt3470as output. if the v in pin is allowed to float and the shdn pin is held high (either by a logic signal or because it is tied to v in ), then the lt3470as internal circuitry will pull its quiescent current through its sw pin. this is fine if your system can tolerate a few ma in this state. if you ground the shdn pin, the sw pin current will drop to es- sentially zero. however, if the v in pin is grounded while the output is held high, then parasitic diodes inside the lt3470a can pull large currents from the output through the sw pin and the v in pin. figure 4 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. figure 3. the minimum input voltage depends on output voltage, load current and boost circuit minimum input voltage, v out = 3.3v minimum input voltage, v out = 5v figure 4. diode d1 prevents a shorted input from discharging a backup battery tied to the output; it also protects the circuit from a reversed input. the lt3470a runs only when the input is present hot-plugging safely v in boost lt3470a sw shdn 3470a f04 v in 100k d1 1m v out backup bias fb gnd load current (ma) 0 3.0 input voltage (v) 3.5 4.0 4.5 5.0 5.5 6.0 50 100 150 200 250 3470a f03a t a = 25c v in to run/start load current (ma) 0 input voltage (v) 6 7 250 3470a f03b 5 4 50 100 150 200 8 t a = 25c v in to run/start
lt3470a 13 3470afb applications information pcb layout for proper operation and minimum emi, care must be taken during printed circuit board layout. note that large, switched currents flow in the power switch, the internal catch diode and the input capacitor. the loop formed by these components should be as small as possible. further- more, the system ground should be tied to the regulator ground in only one place; this prevents the switched cur- rent from injecting noise into the system ground. 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 c2. additionally, the sw and boost nodes should be kept as small as possible. unshielded inductors can induce noise in the feedback path resulting in instability and increased output ripple. to avoid this problem, use vias to route the v out trace under the ground plane to the feedback divider (as shown in figure 5). finally, keep the fb node as small as possible so that the ground pin and ground traces will shield it from the sw and boost nodes. figure 5 shows component placement with trace, ground plane and via locations. include vias near the gnd pin, or pad, of the lt3470a to help remove heat from the lt3470a to the ground plane. figure 5. a good pcb layout ensures proper, low emi operation v out 3470a f05 shdn v in gnd
lt3470a 14 3470afb applications information hot-plugging safely the small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of lt3470a. however, these capacitors can cause problems if the lt3470a is 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 under damped tank circuit, and the volt- age at the v in pin of the lt3470a can ring to twice the nominal input voltage, possibly exceeding the lt3470as rating and damaging the part. if the input supply is poorly controlled or the user will be plugging the lt3470a into an energized supply, the input network should be designed to prevent this overshoot. figure 6 shows the waveforms that result when an lt3470a circuit is connected to a 24v supply through six feet of 24-gauge twisted pair. the first 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 the circuit. in figure 6b 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 filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit. an alternative solution is shown in figure 6c. 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 filtering. this solution is smaller and less expensive than the electrolytic capacitor. for high input voltages its impact on efficiency is minor, reducing efficiency less than one half percent for a 5v output at full load operating from 24v. high temperature considerations the die junction temperature of the lt3470a must be lower than the maximum rating of 125c (150c for h-grade). this is generally not a concern unless the ambi- ent temperature is above 85c. for higher temperatures, care should be taken in the layout of the circuit to ensure good heat sinking of the lt3470a. the maximum load current should be derated as the ambient temperature approaches the maximum junction rating. the die tem- perature is calculated by multiplying the lt3470a power dissipation by the thermal resistance from junction to ambient. power dissipation within the lt3470a can be estimated by calculating the total power loss from an efficiency measurement. thermal resistance depends on the layout of the circuit board and choice of package. the dfn package with the exposed pad has a thermal resistance of approximately 80c/w. finally, be aware that at high ambient temperatures the internal schottky diode will have significant leakage current (see typical performance characteristics) increasing the quiescent current of the lt3470a converter.
lt3470a 15 3470afb applications information figure 6. a well chosen input network prevents input voltage overshoot and ensures reliable operation when the lt3470a is connected to a live supply + lt3470a 2.2f v in 10v/div i in 10a/div 10s/div v in 10v/div i in 10a/div 10s/div v in 10v/div i in 10a/div 10s/div v in closing switch simulates hot plug i in (6a) (6b) (6c) low impedance energized 24v supply stray inductance due to 6 feet (2 meters) of twisted pair + lt3470a 2.2f 10f 35v ai.ei. lt3470a 2.2f 0.1f 1 3470a f06
lt3470a 16 3470afb applications information 3.3v step-down converter 5v step-down converter 2.5v step-down converter v in boost lt3470a sw shdn c3 0.22f, 6.3v 22pf c2 22f 3470a ta02 c1 1f v in 4v to 40v v out 3.3v 250ma r1 324k r2 200k c1: tdk c3216jb1h105m c2: ce jmk316 bj226ml-t l1: toko a993as-270m=p3 l1 33h bias fb gnd off on v in boost lt3470a sw shdn c3 0.22f, 6.3v 22pf c2 22f 3470a ta03 c1 1f v in 5.7v to 40v v out 5v 250ma r1 604k r2 200k l1 33h bias fb gnd off on c1: tdk c3216jb1h105m c2: ce jmk316 bj226ml-t l1: toko a914byw-330m=p3 v in boost lt3470a sw shdn c3 0.47f, 6.3v 22pf c2 22f 3470a ta04 c1 1f v in 4v to 40v v out 2.5v 250ma r1 200k r2 200k c1: tdk c3216jb1h105m c2: tdk c2012jb0j226m l1: sumida cdrh3d28 l1 33h bias fb gnd off on
lt3470a 17 3470afb typical applications 1.8v step-down converter 12v step-down converter v in boost lt3470a sw shdn bias c3 0.22f, 25v 22pf c2 22f 3470a ta05 c1 1f v in 4v to 23v v out 1.8v 250ma r1 147k r2 332k l1 22h fb gnd off on c1: tdk c3216jb1h105m c2: tdk c2012jb0j226m l1: murata lqh32cn150k53 v in boost lt3470a sw shdn c3 0.22f, 16v 22pf c2 10f 3470a ta06 c1 1f v in 15v to 34v v out 12v 250ma r1 866k r2 100k c1: tdk c3216jb1h105m c2: tdk c3216jb1c106m l1: murata lqh32cn150k53 l1 33h bias fb gnd off on
lt3470a 18 3470afb package description ddb package 8-lead plastic dfn (3mm 2mm) (reference ltc dwg # 05-08-1702 rev b) 2.00 0.10 (2 sides) note: 1. drawing conforms to version (wecd-1) in jedec package outline m0-229 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 0.10 bottom view?xposed pad 0.56 0.05 (2 sides) 0.75 0.05 r = 0.115 typ r = 0.05 typ 2.15 0.05 (2 sides) 3.00 0.10 (2 sides) 1 4 8 5 pin 1 bar top mark (see note 6) 0.200 ref 0 ?0.05 (ddb8) dfn 0905 rev b 0.25 0.05 2.20 0.05 (2 sides) recommended solder pad pitch and dimensions 0.61 0.05 (2 sides) 1.15 0.05 0.70 0.05 2.55 0.05 package outline 0.25 0.05 0.50 bsc pin 1 r = 0.20 or 0.25 45 chamfer 0.50 bsc
lt3470a 19 3470afb 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. revision history rev date description page number b 01/11 added h-grade part information. 2 to 5, 15 (revision history begins at rev b)
lt3470a 20 3470afb linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2008 lt 0111 rev b ? printed in usa related parts part number description comments lt1616 25v, 500ma (i out ), 1.4mhz, high efficiency step-down dc/dc converter v in = 3.6v to 25v, v out = 1.25v, i q = 1.9ma, i sd = <1a, thinsot package lt1676 60v, 440ma (i out ), 100khz, high efficiency step-down dc/dc converter v in = 7.4v to 60v, v out = 1.24v, i q = 3.2ma, i sd = 2.5a, s8 package lt1765 25v, 2.75a (i out ), 1.25mhz, high efficiency step-down dc/dc converter v in = 3v to 25v, v out = 1.2v, i q = 1ma, i sd = 15a, s8, tssop16e packages lt1766 60v, 1.2a (i out ), 200khz, high efficiency step-down dc/dc converter v in = 5.5v to 60v, v out = 1.2v, i q = 2.5ma, i sd = 25a, tssop16/e package lt1767 25v, 1.2a (i out ), 1.25mhz, high efficiency step-down dc/dc converter v in = 3v to 25v; v out = 1.2v, i q = 1ma, i sd = 6a, ms8/e packages lt1776 40v, 550ma (i out ), 200khz, high efficiency step-down dc/dc converter v in = 7.4v to 40v; v out = 1.24v, i q = 3.2ma, i sd = 30a, n8, s8 packages lt c ? 1877 600ma (i out ), 550khz, synchronous step-down dc/dc converter v in = 2.7v to 10v; v out = 0.8v, i q = 10a, i sd = <1a, ms8 package ltc1879 1.2a (i out ), 550khz, synchronous step-down dc/dc converter v in = 2.7v to 10v; v out = 0.8v, i q = 15a, i sd = <1a, tssop16 package lt1933 36v, 600ma, 500khz, high efficiency step-down dc/dc converter v in = 3.6v to 36v; v out = 1.25v, i q = 2.5a, i sd = <1a, thinsot and 2mm 3mm dfn-6 package lt1934 34v, 250ma (i out ), micropower, step-down dc/dc converter v in = 3.2v to 34v; v out = 1.25v, i q = 12a, i sd = <1a, thinsot and 2mm 3mm dfn-6 package lt1956 60v, 1.2a (i out ), 500khz, high efficiency step-down dc/dc converter v in = 5.5v to 60v, v out = 1.2v, i q = 2.5ma, i sd = 25a, tssop16/e package ltc3405/ltc3405a 300ma (i out ), 1.5mhz, synchronous step-down dc/dc converter v in = 2.7v to 6v, v out = 0.8v, i q = 20a, i sd = <1a, thinsot package ltc3406/ltc3406b 600ma (i out ), 1.5mhz, synchronous step-down dc/dc converter v in = 2.5v to 5.5v, v out = 0.6v, i q = 20a, i sd = <1a, thinsot package ltc3411 1.25a (i out ), 4mhz, synchronous step-down dc/dc converter v in = 2.5v to 5.5v, v out = 0.8v, i q = 60a, i sd = <1a, ms package ltc3412 2.5a (i out ), 4mhz, synchronous step-down dc/dc converter v in = 2.5v to 5.5v, v out = 0.8v, i q = 60a, i sd = <1a, tssop16e package lt3430 60v, 2.75a (i out ), 200khz, high efficiency step-down dc/dc converter v in = 5.5v to 60v, v out = 1.2v, i q = 2.5ma, i sd = 30a, tssop16e package lt3470 40v, 200ma, micropower step-down dc/dc converter v in = 4v to 40v, v out = 1.25v, i q = 35a, i sd = <1a, dfn-8, thinsot packages

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