1 lt1944 dual micropower step-up dc/dc converter february 2001 the lt ? 1944 is a dual micropower step-up dc/dc con- verter in a 10-pin msop package. each converter is designed with a 350ma current limit and an input voltage range of 1.2v to 15v, making the lt1944 ideal for a wide variety of applications. both converters feature a quies- cent current of only 20 m a at no load, which further reduces to 0.5 m a in shutdown. a current limited, fixed off-time control scheme conserves operating current, resulting in high efficiency over a broad range of load current. the 36v switch allows high voltage outputs up to 34v to be easily generated in a simple boost topology without the use of costly transformers. the lt1944s low off-time of 400ns permits the use of tiny, low profile inductors and capaci- tors to minimize footprint and cost in space-conscious portable applications. n low quiescent current: 20 m a in active mode <1 m a in shutdown mode n operates with v in as low as 1.2v n low v cesat switch: 250mv at 300ma n tiny 10-pin msop package n uses small surface mount components n high output voltage: up to 34v n lcd bias n handheld computers n battery backup n digital cameras , ltc and lt are registered trademarks of linear technology corporation. applicatio s u features descriptio u typical applicatio u 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. v in sw1 fb2 lt1944 4 3 l1 4.7 h d1 shdn2 324k 1m c2 10 f 5v 80ma 30v 8ma v in 2.7v to 4.2v 1944 ta01 gnd 7 pgnd 9 pgnd 6 810 sw2 4.7pf c1 4.7 f fb1 2 5 1 shdn1 l2 10 h d2 2m 86.6k c3 1 f 4.7pf c1: taiyo yuden jmk212bj475 c2: taiyo yuden jmk316bj106 c3: taiyo yuden gmk316bj105 d1, d2: on semi mbr0540 l1: murata lqh3c4r7 l2: murata lqh3c100 load current (ma) 0.1 efficiency (%) 1 10 100 1944 ta01a 90 85 80 75 70 65 60 55 50 v in = 4.2v v in = 2.7v dual output (5v, 30v) boost converter 5v output efficiency final electrical specifications
2 lt1944 absolute axi u rati gs w ww u package/order i for atio uu w (note 1) v in , shdn1, shdn2 voltage ................................... 15v sw1, sw2 voltage .................................................. 36v fb1, fb2 voltage .......................................................v in current into fb1, fb2 pins ..................................... 1ma junction temperature ........................................... 125 c operating temperature range (note 2) .. C 40 c to 85 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c order part number lt1944ems ms10 part marking electrical characteristics the l denotes the specifications which apply over the full operating parameter conditions min typ max units minimum input voltage 1.2 v quiescent current, each switcher not switching 20 30 m a v shdn = 0v 1 m a fb comparator trip point l 1.205 1.23 1.255 v fb comparator hysteresis 8mv output voltage line regulation 1.2v < v in < 12v 0.05 0.1 %/v fb pin bias current (note 3) v fb = 1.23v l 30 80 na switch off time v fb > 1v 400 ns v fb < 0.6v 1.5 m s switch v cesat i sw = 300ma 250 350 mv switch current limit 300 350 400 ma shdn pin current v shdn = 1.2v 2 3 m a v shdn = 5v 8 12 m a shdn input voltage high 0.9 v shdn input voltage low 0.25 v switch leakage current switch off, v sw = 5v 0.01 5 m a note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: the lt1944 is guaranteed to meet performance specifications from 0 c to 70 c. specifications over the C 40 c to 85 c operating temperature range are assured by design, characterization and correlation with statistical process controls. note 3: bias current flows into the fb pin. t jmax = 125 c, q ja = 160 c/w temperature range, otherwise specifications are at t a = 25 c. v in = 1.2v, v shdn = 1.2v unless otherwise noted. lttr consult factory for parts specified with wider operating temperature ranges. 1 2 3 4 5 fb1 shdn1 gnd shdn2 fb2 10 9 8 7 6 sw1 pgnd v in pgnd sw2 top view ms10 package 10-lead plastic msop
3 lt1944 typical perfor a ce characteristics uw switch saturation voltage (v cesat ) quiescent current feedback pin voltage and bias current temperature ( c) ?0 ?5 0 25 50 75 100 switch voltage (v) 1944 g01 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 i switch = 500ma i switch = 300ma temperature ( c) ?0 feedback voltage (v) 1944 g02 1.25 1.24 1.23 1.22 1.21 1.20 bias current (na) 50 40 30 20 10 0 current voltage 25 0 25 50 75 100 temperature ( c) quiescent current ( a) 1944 g03 25 23 21 19 17 15 ?0 ?5 0 25 50 75 100 v in = 12v v in = 1.2v v fb = 1.23v not switching pi fu ctio s uuu fb1 (pin 1): feedback pin for switcher 1. set the output voltage by selecting values for r1 and r2. shdn1 (pin 2): shutdown pin for switcher 1. tie this pin to 0.9v or higher to enable device. tie below 0.25v to turn it off. gnd (pin 3): ground. tie this pin directly to the local ground plane. shdn2 (pin 4): shutdown pin for switcher 2. tie this pin to 0.9v or higher to enable device. tie below 0.25v to turn it off. fb2 (pin 5): feedback pin for switcher 2. set the output voltage by selecting values for r1b and r2b. sw2 (pin 6): switch pin for switcher 2. this is the collector of the internal npn power switch. minimize the metal trace area connected to the pin to minimize emi. pgnd (pins 7, 9): power ground. tie these pins directly to the local ground plane. both pins must be tied. v in (pin 8): input supply pin. bypass this pin with a capacitor as close to the device as possible. sw1 (pin 10): switch pin for switcher 1. this is the collector of the internal npn power switch. minimize the metal trace area connected to the pin to minimize emi. temperature ( c) switch off time (ns) 1944 g04 550 500 450 400 350 300 250 ?0 ?5 0 25 50 75 100 v in = 1.2v v in = 12v temperature ( c) peak current (ma) 1944 g05 400 350 300 250 200 150 100 50 0 ?0 ?5 0 25 50 75 100 v in = 12v v in = 1.2v shutdown pin voltage (v) shutdown pin current ( a) 1944 g03 25 20 15 10 5 0 0 5 10 15 25 c 100 c switch off time shutdown pin current switch current limit
4 lt1944 block diagra w figure 1. lt1944 block diagram operatio u the lt1944 uses a constant off-time control scheme to provide high efficiencies over a wide range of output current. operation can be best understood by referring to the block diagram in figure 1. q1 and q2 along with r3 and r4 form a bandgap reference used to regulate the output voltage. when the voltage at the fb1 pin is slightly above 1.23v, comparator a1 disables most of the internal cir- cuitry. output current is then provided by capacitor c2, which slowly discharges until the voltage at the fb1 pin drops below the lower hysteresis point of a1 (typical hysteresis at the fb pin is 8mv). a1 then enables the internal circuitry, turns on power switch q3, and the current in inductor l1 begins ramping up. once the switch current reaches 350ma, comparator a2 resets the one- shot, which turns off q3 for 400ns. l1 then delivers current to the output through diode d1 as the inductor current ramps down. q3 turns on again and the inductor current ramps back up to 350ma, then a2 resets the one- shot, again allowing l1 to deliver current to the output. this switching action continues until the output voltage is charged up (until the fb1 pin reaches 1.23v), then a1 turns off the internal circuitry and the cycle repeats. the lt1944 contains additional circuitry to provide protection during start-up and under short-circuit conditions. when the fb1 pin voltage is less than approximately 600mv, the switch off-time is increased to 1.5 m s and the current limit is reduced to around 250ma (70% of its normal value). this reduces the average inductor current and helps minimize the power dissipation in the power switch and in the external inductor and diode. the second switching regulator operates in the same manner. + + 8 400ns one-shot driver reset driver reset enable 42mv 0.12 a2 a1 q3 9 3 r4 140k r3 30k r6 40k r5 40k q2 x10 q1 1 v in fb1 2 shdn1 10 sw1 pgnd gnd 1944 bd l1 c2 v out1 v in d1 r2 (external) r1 (external) v out1 c1 + + 400ns one-shot enable 42mv 0.12 a2b a1b q3b 7 r4b 140k r3b 30k r6b 40k r5b 40k q2b x10 q1b 5 v in fb2 4 shdn2 6 sw2 pgnd l2 c3 v out2 v in d2 r2b (external) r1b (external) v out2
5 lt1944 choosing an inductor several recommended inductors that work well with the lt1944 are listed in table 1, although there are many other manufacturers and devices that can be used. consult each manufacturer for more detailed information and for their entire selection of related parts. many different sizes and shapes are available. use the equations and recommenda- tions in the next few sections to find the correct inductance value for your design. table 1. recommended inductors part value ( m h) max dcr ( w ) vendor lqh3c4r7 4.7 0.26 murata lqh3c100 10 0.30 (714) 852-2001 lqh3c220 22 0.92 www.murata.com cd43-4r7 4.7 0.11 sumida cd43-100 10 0.18 (847) 956-0666 cdrh4d18-4r7 4.7 0.16 www.sumida.com cdrh4d18-100 10 0.20 do1608-472 4.7 0.09 coilcraft do1608-103 10 0.16 (847) 639-6400 do1608-223 22 0.37 www.coilcraft.com inductor selectionboost regulator the formula below calculates the appropriate inductor value to be used for a boost regulator using the lt1944 (or at least provides a good starting point). this value pro- vides a good tradeoff in inductor size and system perfor- mance. pick a standard inductor close to this value. a larger value can be used to slightly increase the available output current, but limit it to around twice the value calculated below, as too large of an inductance will in- crease the output voltage ripple without providing much additional output current. a smaller value can be used (especially for systems with output voltages greater than 12v) to give a smaller physical size. inductance can be calculated as: l vv v i t out in min d lim off = -+ () where v d = 0.4v (schottky diode voltage), i lim = 350ma and t off = 400ns; for designs with varying v in such as battery powered applications, use the minimum v in value in the above equation. for most systems with output applicatio s i for atio wu u u voltages below 7v, a 4.7 m h inductor is the best choice, even though the equation above might specify a smaller value. this is due to the inductor current overshoot that occurs when very small inductor values are used (see current limit overshoot section). for higher output voltages, the formula above will give large inductance values. for a 2v to 20v converter (typical lcd bias application), a 21 m h inductor is called for with the above equation, but a 10 m h inductor could be used without excessive reduction in maximum output current. inductor selectionsepic regulator the formula below calculates the approximate inductor value to be used for a sepic regulator using the lt1944. as for the boost inductor selection, a larger or smaller value can be used. l vv i t out d lim off = + ? ? ? ? 2 current limit overshoot for the constant off-time control scheme of the lt1944, the power switch is turned off only after the 350ma current limit is reached. there is a 100ns delay between the time when the current limit is reached and when the switch actually turns off. during this delay, the inductor current exceeds the current limit by a small amount. the peak inductor current can be calculated by: ii vv l ns peak lim in max sat =+ - ? ? ? ? () 100 where v sat = 0.25v (switch saturation voltage). the current overshoot will be most evident for systems with high input voltages and for systems where smaller induc- tor values are used. this overshoot can be beneficial as it helps increase the amount of available output current for smaller inductor values. this will be the peak current seen by the inductor (and the diode) during normal operation. for designs using small inductance values (especially at input voltages greater than 5v), the current limit over- shoot can be quite high. although it is internally current
6 lt1944 applicatio s i for atio wu u u limited to 350ma, the power switch of the lt1944 can handle larger currents without problem, but the overall efficiency will suffer. best results will be obtained when i peak is kept below 700ma for the lt1944. capacitor selection low esr (equivalent series resistance) capacitors should be used at the output to minimize the output ripple voltage. multilayer ceramic capacitors are the best choice, as they have a very low esr and are available in very small packages. their small size makes them a good companion to the lt1944s ms10 package. solid tantalum capacitors (like the avx tps, sprague 593d families) or os-con capacitors can be used, but they will occupy more board area than a ceramic and will have a higher esr. always use a capacitor with a sufficient voltage rating. ceramic capacitors also make a good choice for the input decoupling capacitor, which should be placed as close as possible to the lt1944. a 4.7 m f input capacitor is suffi- cient for most applications. table 2 shows a list of several capacitor manufacturers. consult the manufacturers for more detailed information and for their entire selection of related parts. table 2. recommended capacitors capacitor type vendor ceramic taiyo yuden (408) 573-4150 www.t-yuden.com ceramic avx (803) 448-9411 www.avxcorp.com ceramic murata (714) 852-2001 www.murata.com setting the output voltage set the output voltage for each switching regulator by choosing the appropriate values for feedback resistors r1 and r2 (see figure 1). rr v v out 12 123 1 =- ? ? ? ? . diode selection for most lt1944 applications, the motorola mbr0520 surface mount schottky diode (0.5a, 20v) is an ideal choice. schottky diodes, with their low forward voltage drop and fast switching speed, are the best match for the lt1944. for higher output voltage applications the 30v mbr0530 or 40v mbr0540 can be used. many different manufacturers make equivalent parts, but make sure that the component is rated to handle at least 0.35a. lowering output voltage ripple using low esr capacitors will help minimize the output ripple voltage, but proper selection of the inductor and the output capacitor also plays a big role. the lt1944 pro- vides energy to the load in bursts by ramping up the inductor current, then delivering that current to the load. if too large of an inductor value or too small of a capacitor value is used, the output ripple voltage will increase because the capacitor will be slightly overcharged each burst cycle. to reduce the output ripple, increase the output capacitor value or add a 4.7pf feed-forward capaci- tor in the feedback network of the lt1944 (see the circuits in the typical applications section). adding this small, inexpensive 4.7pf capacitor will greatly reduce the output voltage ripple.
7 lt1944 typical applicatio s u v in sw1 fb2 lt1944 4 3 l1 4.7 h d1 shdn2 324k 1m c2 10 f 5v 40ma 3.3v 80ma v in 1.8v to 3v c1: taiyo yuden jmk212bj475 c2, c3: taiyo yuden jmk316bj106 d1, d2: on semi mbr0520 l1, l2: murata lqh3c4r7 (408) 573-4150 (408) 573-4150 (800) 282-9855 (814) 237-1431 1944 ta02 gnd 7 pgnd 9 pgnd 6 810 sw2 4.7pf c1 4.7 f fb1 2 5 1 shdn1 l2 4.7 h d2 1m 604k 4.7pf c3 10 f 2-cell dual output (3.3v, 5v) boost converter load current (ma) 0.1 efficiency (%) 1 10 100 1944 ta02a 90 85 80 75 70 65 60 55 50 v in = 3v v in = 1.8v load current (ma) 0.1 efficiency (%) 1 10 100 1944 ta02b 90 85 80 75 70 65 60 55 50 v in = 3v v in = 1.8v 2-cell to 5v efficiency 2-cell to 3.3v efficiency package descriptio u dimensions in millimeters (inches) unless otherwise noted. msop (ms10) 1100 * dimension does not include mold flash, protrusions or gate burrs. mold flash, protrusions or gate burrs shall not exceed 0.006" (0.152mm) per side ** dimension does not include interlead flash or protrusions. interlead flash or protrusions shall not exceed 0.006" (0.152mm) per side 0.021 0.006 (0.53 0.015) 0 ?6 typ seating plane 0.007 (0.18) 0.043 (1.10) max 0.007 ?0.011 (0.17 0.27) 0.005 0.002 (0.13 0.05) 0.034 (0.86) ref 0.0197 (0.50) bsc 12 3 45 0.193 0.006 (4.90 0.15) 8 9 10 7 6 0.118 0.004* (3.00 0.102) 0.118 0.004** (3.00 0.102) ms10 package 10-lead plastic msop (ltc dwg # 05-08-1661)
8 lt1944 linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear-tech.com ? linear technology corporation 2001 1944i lt/tp 0201 2k ? printed in usa related parts part number description comments lt1307 single-cell micropower 600khz pwm dc/dc converter 3.3v at 75ma from one cell, msop package lt1316 burst mode tm operation dc/dc with programmable current limit 1.5v minimum, precise control of peak current limit lt1317 2-cell micropower dc/dc with low-battery detector 3.3v at 200ma from two cells, 600khz fixed frequency lt1610 single-cell micropower dc/dc converter 3v at 30ma from 1v, 1.7mhz fixed frequency lt1611 1.4mhz inverting switching regulator in 5-lead sot-23 C 5v at 150ma from 5v input, tiny sot-23 package lt1613 1.4mhz switching regulator in 5-lead sot-23 5v at 200ma from 3.3v input, tiny sot-23 package lt1615 micropower dc/dc converter in 5-lead sot-23 20v at 12ma from 2.5v input, tiny sot-23 package lt1617 micropower inverting dc/dc converter in 5-lead sot-23 C15v at 12ma from 2.5v input, tiny sot-23 package lt1930a 2.2mhz boost dc/dc converter in sot-23 5v at 450ma from 3.3v, tiny sot-23 package burst mode is a trademark of linear technology corporation typical applicatio s u v in sw1 fb2 lt1944 4 3 l1 10 h d1 d2b d2a shdn2 140k 1m c2 2.2 f 10v 5ma 20v 500 a 6.5v 500 a 15ma v in 2.7v to 4.2v 1944 ta03 gnd 7 pgnd 9 pgnd 6 810 sw2 c8 1 f c1 4.7 f fb1 2 5 1 shdn1 l2 10 h d4 5 white leds 82.5 c4 0.1 f c3 0.1 f c6 2.2 f c7 0.1 f c1: taiyo yuden jmk212bj475 c2, c6: taiyo yuden lmk212bj225 c3, c4, c7: taiyo yuden emk107bj104 c5, c8: taiyo yuden tmk316bj105 d1, d4: on semi mbr0530 d2, d3: zetex bat54s l1, l2: sumida clq4d10-100 q1, q2: on semi mmbt3906 (408) 573-4150 (408) 573-4150 (408) 573-4150 (408) 573-4150 (800) 282-9855 (631) 543-7100 (847) 956-0666 (800) 282-9855 d3b d3a c5 1 f 140k q2 q1 four output power supply for color lcd displays
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