1 ltc1574 ltc1574-3.3/ltc1574-5 high efficiency step-down dc/dc converters with internal schottky diode the ltc ? 1574 is a family of easy-to-use current mode dc/dc converters ideally suited for 9v to 5v, 5v to 3.3v and inverting operation. with an internal 0.9 w switch (at a supply voltage of 12v) and a low forward drop schottky diode (0.450v typ at 200ma, t a = 25 c), the ltc1574 requires only three external components to construct a complete high efficiency dc/dc converter. under no load condition, the ltc1574 draws only 130 m a. in shutdown, it draws a mere 2 m a making this converter ideal for battery-powered applications. in dropout, the internal p-channel mosfet switch is turned on continu- ously allowing the user to maximize the life of the battery source. the maximum inductor current of the ltc1574 family is pin selectable to either 340ma or 600ma, optimizing efficiency for a wide range of applications. operation up to 200khz permits the use of small surface mount inductors and capacitors. for applications requiring higher output current or ultra- high efficiency, see the ltc1148 or ltc1265 data sheets. for detailed applications information, see the ltc1174 data sheet. high efficiency step-down converter ltc1574-5 efficiency n inverting converters n step-down converters n memory backup supply n portable instruments n battery-powered equipment n distributed power systems n high efficiency: up to 94% n usable in noise-sensitive products n peak inductor current independent of inductor value n short-circuit protection n internal low forward drop schottky diode n only three external components required n wide v in range: 4v to 18.5v (absolute maximum) n low dropout operation n low-battery detector n pin selectable current limit n internal 0.9 w power switch: v in < 11v n standby current: 130 m a n active low micropower shutdown features descriptio u applicatio s u typical applicatio u and ltc are registered trademarks and lt is a trademark of linear technology corporation. v in 100 m h ? 22 m f* 35v 100 m f* 10v 1574 ta01 5v 175ma 5 2, 4, 13, 15 7 10 3, 14 12 11 6 v in 5.5v to 16v gnd ltc1574-5 lb in lb out i pgm shdn v out sw * avx tpsd226k035 ** avx tpsd107k010 ? coiltronics ctx100-4 + + load current (ma) 1 efficiency (%) 100 95 90 85 80 75 70 10 100 1574 ta02 200 v in = 6v v in = 9v l = 100 m h v out = 5v i pgm = 0v
2 ltc1574 ltc1574-3.3/ltc1574-5 order part number ltc1574cs ltc1574cs-3.3 ltc1574cs-5 consult factory for industrial and military grade parts. (note 1) (voltage referred to gnd pin) input supply voltage (pin 5) ................. C 0.3v to 18.5v switch current (pin 3, 14) ........................................ 1a switch voltage (pin 3, 14) .......................... v in C 18.5v operating temperature range .................... 0 c to 70 c junction temperature (note 2) ............................ 125 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c symbol parameter conditions min typ max units i fb feedback current into pin 10 ltc1574 1 m a v fb feedback voltage ltc1574 l 1.20 1.25 1.30 v v out regulated output voltage ltc1574-3.3 l 3.14 3.30 3.46 v ltc1574-5 l 4.75 5.00 5.25 v d v out output voltage line v in = 6v to 12v, i load = 100ma, i pgm = v in (note 3) 10 70 mv regulation output voltage load ltc1574-3.3 (note 3) 20ma < i load < 175ma, i pgm = 0v C 5 C 70 mv regulation 20ma < i load < 400ma, i pgm = v in C45 C70 mv ltc1574-5 (note 3) 20ma < i load < 175ma, i pgm = 0v C 5 C 70 mv 20ma < i load < 400ma, i pgm = v in C50 C70 mv i q input dc supply current (note 4) active mode 4v < v in < 16v, i pgm = 0v 450 600 m a sleep mode 4v < v in < 16v 130 180 m a shutdown (note 5) shdn = 0v, 4v < v in < 16v 2 25 m a v lbtrip low-battery trip point 1.25 1.4 v i lbin current into pin 12 0.5 m a i lbout current sunk by pin 11 v lbout = 0.4v, v lbin = 0v 0.5 1.0 1.5 ma v lbout = 5v, v lbin = 10v 1.0 m a v hyst comparator hysteresis 7.5 15 30 mv i peak current limit i pgm = v in , v out = 0v l 0.54 0.60 0.78 a i pgm = 0v, v out = 0v l 0.27 0.34 0.50 a r on on resistance of switch l 0.9 1.55 w t off switch off time v out at regulated value 3 4 5 m s v ih shdn pin high minimum voltage at pin 7 for device to be active 1.2 v v il shdn pin low maximum voltage at pin 7 for device to be in shutdown 0.75 v absolute axi u rati gs w ww u package/order i for atio uu w top view s package 16-lead plastic so *adjustable output version t jmax = 125 c, q ja = 110 c/w 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 nc gnd sw gnd v in i pgm shdn nc nc gnd sw gnd lb in lb out v out (v fb *) nc electrical characteristics the l denotes specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v in = 9v, shdn = v in , i pgm = 0v, unless otherwise specified.
3 ltc1574 ltc1574-3.3/ltc1574-5 the l denotes specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v in = 9v, shdn = v in , i pgm = 0v, unless otherwise specified. symbol parameter conditions min typ max units i ih shdn pin input current shdn = 16v 2 m a i il shdn pin input current 0 shdn 0.8v 0.5 m a v f schottky diode forward voltage forward current = 200ma 0.450 0.570 v i r schottky reverse current reverse voltage = 5v 10 25 m a reverse voltage = 18.5v 100 250 m a note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: t j is calculated from the ambient temperature t a and power dissipation p d according to the following formulas: t j = t a + (p d ? 110 c/w) note 3: guaranteed by design. note 4: does not include schottky reverse current. dynamic supply current is higher due to the gate charge being delivered at the switching frequency. note 5: current into pin 5 only, measured without electrolytic input capacitor. typical perfor m a n ce characteristics u w efficiency vs input voltage efficiency vs load current efficiency vs load current switch resistance vs input voltage switch leakage current vs temperature efficiency using different types of inductor core material electrical characteristics load current (ma) 1 efficiency (%) 10 100 1574 ?tpc01 500 100 90 80 70 60 50 l = 50 m h v out = 3.3v i pgm = v in coil = ctx50-4 v in = 5v v in = 9v load current (ma) 1 efficiency (%) 10 100 1574 ?tpc02 400 100 95 90 85 80 75 70 l = 50 m h v out = 5v i pgm = v in coil = ctx50-4 v in = 6v v in = 9v input voltage (v) 5 efficiency (%) 7 9 10 14 1574 ?tpc03 68 11 12 13 95 94 93 92 91 90 89 v out = 5v l = 100 m h coil = ctx100-4 i load = 300ma i pgm = v in i load = 100ma i pgm = 0v load current (ma) 1 efficiency (%) 10 100 500 100 90 80 70 60 50 v in = 5v v out = 3.3v i pgm = v in ctx50-4 ctx50-4p 1574 ?tpc04 temperature ( c) 0 leakage current (na) 180 160 140 120 100 80 60 40 20 0 40 80 100 1574 ?tpc05 20 60 v in = 13.5v input voltage (v) 4 rds (on) ( w ) 12 1574 ?tpc06 6 8 10 14 16 18 20 t a = 25 c 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7
4 ltc1574 ltc1574-3.3/ltc1574-5 v out or v fb (pin 10): for the ltc1574, this pin connects t o the main voltage comparator input. on the ltc1574-5 and ltc1574-3.3, this pin goes to an internal resistive divider which sets the output voltage . lb out (pin 11): open drain of an n-channel pull-down. this pin will sink current when (pin 12) lb in goes below 1.25v. lb in (pin 12): the (C) input of the low-battery voltage comparator. the (+) input is connected to a reference voltage of 1.25v. nc (pins 1, 8, 9, 16): no connection. gnd (pins 2, 4, 13, 15): ground. sw (pins 3, 14): drain of p-channel mosfet switch and cathode of schottky diode. v in (pin 5): input supply voltage. it must be decoupled close to ground (pin 4). i pgm (pin 6): this pin selects the current limit of the p-channel switch. with i pgm = v in , the current trip point is 600ma and with i pgm = 0v, the current trip point is reduced to 340ma. shdn (pin 7): pulling this pin to ground keeps the internal switch off and puts the ltc1574 in micropower shutdown. operating frequency and inductor since the ltc1574 utilizes a constant off-time architecture, its operating frequency is dependent on the value of v in . the frequency of operation can be expressed as: f t vv vv hz off in out in d = - + ? ? ? ? () 1 where t off = 4 m s and v d is the voltage drop across the internal schottky diode. note that the operating frequency is a function of the input and output voltage. although the size of the inductor does not affect the fre- quency or inductor peak current, it does affect the ripple current. the peak-to-peak ripple current is given by: i vv l ripple out d = + ? ? ? ? () - 410 6 a p-p when choosing a small inductor, core loss will increase due to higher ripple current. therefore, a low esr output capacitor has to be used. short-circuit protection the ltc1574 is protected from output short circuits by its internal current limit. depending on the condition of the low-battery detector the low-battery indicator senses the input voltage through an external resistive divider. this divided voltage connects to the C input of a voltage comparator (pin 12) which is compared with a 1.25v reference voltage. with the current i pgm pin, the limit is either set to 340ma or 600ma. in addition, the off-time of the switch is increased to allow the inductor current to decay far enough to prevent any current build-up (see figure 1). i pgm = v in i pgm = 0 gnd l = 100 m h v in = 13.5v 100ma/div 20 m s/div 1574 ? f01 figure 1. inductor current with output shorted uu u pi fu ctio s applicatio s i for atio wu uu
5 ltc1574 ltc1574-3.3/ltc1574-5 going into pin 12 being negligible, the following expres- sion is used for setting the trip limit: v r r lbtrip =+ ? ? ? ? 125 1 4 3 . difference between the absolute maximum voltage rating and the output voltage. a maximum of 12v is specified in figure 4, giving the circuit 1.5v of headroom for v in . note that the circuit can operate from a minimum of 4v, making it ideal for a four nicd cell application. for a higher output current circuit, please refer to the typical applications section. figure 2. low-battery comparator ltc1574 adjustable applications the ltc1574 develops a 1.25v reference voltage between the feedback terminal (pin 10) and ground (see figure 3). by selecting resistor r1, a constant current is caused to flow through r1 and r2 to set the overall output voltage. the regulated output voltage is determined by: v r r out =+ ? ? ? ? 125 1 2 1 . for most applications, a 30k resistor is suggested for r1. to prevent stray pickup, a 100pf capacitor is suggested across r1 located close to the ltc1574. figure 3. ltc1574 adjustable configuration inverting applications the ltc1574 can easily be set up for a negative output voltage. if C 5v is desired, the ltc1574-5 is ideal for this application as it requires the least components. figure 4 shows the schematic for this application. note that the output voltage is now taken off the gnd pins. therefore, the maximum input voltage is now determined by the figure 4. positive-to-negative 5v converter figure 5. low noise 5v to 3.3v regulator low noise regulators in some applications it is important not to introduce any switching noise within the audio frequency range. due to the nature of the ltc1574 during burst mode tm operation, there is a possibility that the regulator will introduce audio noise at some load currents. to circumvent this problem, a feed-forward capacitor can be used to shift the noise spectrum up and out of the audio band. figure 5 shows the low noise connection with c2 being the feed-forward capacitor. the peak-to-peak output ripple is reduced to 30mv over the entire load range. a toroidal surface mount burst mode is a trademark of linear technology corporation applicatio s i for atio wu uu ltc1574 + 1.25v reference r4 r3 12 v in 1574 ?f02 v fb r2 r1 10 v out 1574 ?f03 100pf ltc1574 12 ltc1574-5 lb in lb out i pgm gnd v in shdn v out sw 11 6 5 7 10 3, 14 2, 4, 13, 15 50 m h** v out ?v 45ma 47 m f* 16v 2 1574 ?f04 * ** avx tpsd476k016 coiltronics ctx50-4 input voltage 4v to 12v 0.1 m f + 47 m f* 16v 2 + v in l1** 100 m h 100 m f* 10v 1574 ?f05 5 2, 4, 13, 15 56k 33k 7 3, 14 10 12 11 6 v in 5v v out 3.3v 425ma gnd ltc1574 lb in lb out i pgm shdn sw v fb * avx tpsd107k010 ** coiltronics ctx100-4 100 m f* 10v c2 6.8nf + +
6 ltc1574 ltc1574-3.3/ltc1574-5 for c out , the rms current rating should be at least: i i a rms peak rms ? () = 2 300ma absolute maximum ratings and latchup prevention the absolute maximum ratings specify that sw (pins 3, 14) can never exceed v in (pin 5) by more than 0.3v. normally this situation should never occur. it could, however, if the output is held up while the supply is pulled down. a condition where this could potentially occur is when a battery is supplying power to an ltc1574 regula- tor and also to one or more loads in parallel with the the regulators v in . if the battery is disconnected while the ltc1574 regulator is supplying a light load and one of the parallel circuits is a heavy load, the input capacitor of the ltc1574 regulator could be pulled down faster than the output capacitor, causing the absolute maximum ratings to be exceeded. the result is often a latchup which can be destructive if v in is reapplied. battery disconnect is pos- sible as a result of mechanical stress, bad battery contacts or use of a lithium-ion battery with a built-in internal disconnect. the user needs to assess his/her application to determine whether this situation could occur. if so, additional protection is necessary. prevention against latchup can be accomplished by simply connecting a schottky diode across the sw and v in pins as shown in figure 7. the diode will normally be reverse biased unless v in is pulled below v out at which time the diode will clamp the (v out C v in ) potential to less than the 0.6v required for latchup. note that a low leakage schottky should be used to minimize the effect inductor l1 is chosen for its excellent self-shielding prop- erties. open magnetic structures such as drum and rod cores are to be avoided since they inject high flux levels into their surroundings. this can become a major source of noise in any converter circuit. design example as a design example, assume v in = 9v (nominal), v out = 5v and i out = 350ma maximum. the ltc1574-5 is used for this application with i pgm (pin 6) connected to v in . the minimum value of l is determined by assuming the ltc1574-5 is operating in continuous mode. figure 6. continuous inductor current with i out = 350ma and i peak = 0.6a (i pgm = v in ), i v = 0.1a. the peak-to-peak ripple inductor current, i ripple , is 0.5a and is also equal to: i vv l ripple out d = + ? ? ? ? () - 410 6 a p-p solving for l in the above equation and with v d = 0.5v, l = 44 m h. the next higher standard value of l is 50 m h (example: coiltronics ctx50-4). the operating frequency, ignoring voltage across diode v d is: f v v khz out in ?- ? ? ? ? = 2 5 10 1 111 5 . with the value of l determined, the requirements for c in and c out are calculated. for c in , its rms current rating should be at least: i ivvv v a ma rms out out in out in rms = - () [] () = 12 174 / inductor current time i peak i v avg current = i out = = 350ma i peak + i v 2 1574 ?f06 applicatio s i for atio wu uu 1574 f07 v in v out latchup protection schottky sw ltc1574 + figure 7. preventing absolute maximum ratings from being exceeded
7 ltc1574 ltc1574-3.3/ltc1574-5 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 represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. applicatio s i for atio wu uu on no-load supply current. schottky diodes such as mbr0530, bas85 and bat84 work well. another more serious effect of the protection diode leakage is that at no load with nothing to provide a sink for this leakage current, the output voltage can potentially float above the maximum allowable tolerance. to prevent this from occuring, a resistor must be connected between v out and ground with a value low enough to sink the maximum possible leakage current. low noise, high efficiency 3.3v regulator typical applicatio s u v in 5 100pf 2, 4, 13, 15 6 12 11 7 10 3, 14 50 m h ? 0.1 m f 6.8nf 22 m f* 25v 2 1574 ta03 v out 3.3v 450ma 56k 33k v in 4v to 12.5v gnd ltc1574 i pgm lb in lb out shdn v fb sw * avx tpsd226k025 ** avx tpsd107k010 ? coiltronics ctx50-4 100 m f** 10v 2 + + dimension in inches (millimeters) unless otherwise noted. u package descriptio s package 16-lead plastic small outline (narrow 0.150) (ltc dwg # 05-08-1610) 0.016 ?0.050 (0.406 ?1.270) 0.010 ?0.020 (0.254 ?0.508) 45 0 ?8 typ 0.008 ?0.010 (0.203 ?0.254) s16 1098 1 2 3 4 5 6 7 8 0.150 ?0.157** (3.810 ?3.988) 16 15 14 13 0.386 ?0.394* (9.804 ?10.008) 0.228 ?0.244 (5.791 ?6.197) 12 11 10 9 dimension does not include mold flash. mold flash shall not exceed 0.006" (0.152mm) per side dimension does not include interlead flash. interlead flash shall not exceed 0.010" (0.254mm) per side * ** 0.053 ?0.069 (1.346 ?1.752) 0.014 ?0.019 (0.355 ?0.483) typ 0.004 ?0.010 (0.101 ?0.254) 0.050 (1.270) bsc
8 ltc1574 ltc1574-3.3/ltc1574-5 1574fa lt/tp 1000 2k rev a ? printed in usa ? linear technology corporation 1995 related parts low dropout 5v step-down regulator with low-battery detection positive to C 5v converter high efficiency 3.3v regulator typical applicatio s u linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear-tech.com part number description comments lt ? 1074/lt1076 step-down switching regulator 100khz, 5a (lt1074) or 2a (lt1076) monolithic ltc1147 high efficiency step-down dc/dc controller 8-pin controller ltc1174 high efficiency step-down and inverting dc/dc converter 0.5a, burst mode operation, so-8 package, v in to 18v ltc1265 1.2a high efficiency step-down dc/dc regulator burst mode operation, monolithic lt1375/lt1376 1.5a 500khz step-down switching regulator high frequency small inductor lt1611 inverting 1.4mhz switching regulator in sot-23 C 5v at 150ma from 5v input, 1mv p-p output ripple, sot-23 package ltc1701 1mhz step-down dc/dc converter in sot-23 v in = 2.5v to 5.5v, i q = 135 m a, v out = 5v to 1.25v ltc1707 high efficiency synchronous step-down regulator v in = 2.85v to 8.5v, selectable burst mode operation, 600ma output current, so-8 package ltc1877/ltc1878 high efficiency synchronous step-down regulator 600ma at v in = 5v, 2.65v to 10v = v in , i q = 10 m a v in 5 2, 4, 13, 15 6 11 12 7 10 3, 14 l1 100 m h ? 0.1 m f 47 m f** 16v 2 1574 ta04 v out 5v 365ma *low- battery indicator 4.7k 162k 47.5k v in 5.5v to 12.5v gnd ltc1574-5 i pgm lb out lb in shdn v out sw 47 m f** 16v 2 * low-battery indicator is set up to trip at v in = 5.5v ** avx tpsd476k016 ? selection manufacturer part no. type coiltronics ctx100-4 surface mount sumida cd75-101 surface mount gowanda ga10-103k through hole + + v in 5 2, 4, 13, 15 6 12 11 7 10 3, 14 50 m h ? 0.1 m f 22 m f* 25v 2 1574 ta05 v out 3.3v 425ma v in 4v to 12.5v gnd ltc1574-3.3 i pgm lb in lb out shdn v out sw * avx tpsd226k025 ** avx tpsd476k016 ? coiltronics ctx50-4 47 m f* 16v 2 + + v in 5 2, 4, 13, 15 6 11 12 7 10 3, 14 l1 ? 50 m h 0.1 m f 1574 ta06 v out ?v *low- battery indicator 4.7k 280k 43k v in 4v to 12.5v gnd ltc1574-5 i pgm lb out lb in shdn v out sw 100 m f*** 10v * low-battery indicator is set to trip at v in = 4.4v ** avx tpsd106k035 *** avx tpsd107k010 ? selection manufacturer part no. type coiltronics ctx50-3 surface mount coilcraft dt3316-473 surface mount sumida cd54-470 surface mount gowanda ga10-472k through hole 10 m f** 35v 2 v in (v) i out (ma) 4 110 6 140 8 170 10 200 12.5 235 + +
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