LT3461 1 3461i 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. final electrical specifications n integrated schottky rectifier n 1.3mhz switching frequency n high output voltage: up to 38v n 300ma integrated switch n 12v at 70ma from 5v input n 5v at 115ma from 3.3v input n wide input range: 2.5v to 16v n uses small surface mount components n low shutdown current: <1 m a n soft-start n low profile (1mm) sot-23 (thinsot tm ) package applicatio s u features descriptio u typical applicatio u 1.3mhz step-up dc/dc converter in thinsot september 2003 the lt ? 3461 is a general purpose step-up dc/dc con- verter. the device switches at 1.3mhz, allowing the use of tiny, low cost and low height capacitors and inductors. an integrated schottky rectifier results in lower parts cost and smaller converter footprint. the constant frequency re- sults in low, predictable output noise that is easy to filter. the high voltage switch in the LT3461 is rated at 40v, making the device ideal for boost converters up to 38v. the LT3461 can generate 12v at up to 70ma from a 5v supply. the LT3461 is available in a low profile (1mm) sot-23 package. , ltc and lt are registered trademarks of linear technology corporation. n digital cameras n ccd bias supply n xdsl power supply n tft-lcd bias supply n local 5v or 12v supply n medical diagnostic equipment n battery backup figure 1. 5v to 12v, 70ma step-up dc/dc converter efficiency thinsot is a trademark of linear technology corporation v in 5v l1 15 h 261k 30.1k c2 2.2 f 15pf c1 1 f v out 12v 70ma 1 2 v in v out sw 3 4 5 6 fb shdn gnd LT3461 c1: taiyo yuden lmk212bj105md c2: taiyo yuden x7r emk316bj225ml l1: murata lqh32cn150k53 3461 f01 off on load current (ma) 0 efficiency (%) 85 80 75 70 65 60 3461 fo1a 20 30 10 40 50 60 70 v in = 5v v in = 3.3v
LT3461 2 3461i input voltage (v in ) .................................................. 16v v out , sw voltage .................................................... 40v fb voltage ................................................................. 5v shdn voltage .......................................................... 16v operating ambient temperature range (note 2) .................. C 40 c to 85 c maximum junction temperature .......................... 125 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c absolute axi u rati gs w ww u package/order i for atio uu w (note 1) electrical characteristics the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c, v in = 3v, v shdn = 3v, unless otherwise noted. parameter conditions min typ max units minimum operating voltage 2.5 v maximum operating voltage 16 v feedback voltage 1.235 1.255 1.275 v l 1.225 1.280 v feedback line regulation 0.005 %/v fb pin bias current l 40 100 na supply current fb = 1.3v, not switching 2.8 3.6 ma shdn = 0v 0.1 0.5 m a switching frequency l 1.0 1.3 1.7 mhz maximum duty cycle l 92 % switch current limit 300 420 600 ma switch v cesat i sw = 250ma 260 350 mv switch leakage current v sw = 5v 0.01 1 m a schottky forward voltage i schottky = 250ma 800 1100 mv schottky reverse leakage v out C sw = 40v 0.03 4 m a shdn voltage high 1.5 v shdn voltage low 0.4 v shdn pin bias current 35 50 m a consult ltc marketing for parts specified with wider operating temperature ranges. order part number s6 part marking ltaeb LT3461es6 t jmax = 125 c, q ja = 256 c/w in free air q jc = 120 c/w q ja = 120 c on board over ground plane note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: the LT3461e is guaranteed to meet specifications from 0 c to 70 c. specifications over the C40 c to 85 c operating temperature range are assured by design, characterization and correlation with statistical process controls. 6 v in 5 v out 4 shdn sw 1 top view s6 package 6-lead plastic tsot-23 gnd 2 fb 3
LT3461 3 3461i temperature ( c) fb voltage (v) 1.28 1.27 1.26 1.25 1.24 1.23 1.22 3461 g03 duty cycle (%) 10 current limit (ma) 90 80 60 40 20 3461 g02 30 50 70 480 360 240 120 0 3461 g05 3461 g06 temperature ( c) ?0 frequency (mhz) 1.6 1.5 1.4 1.3 1.2 1.1 1.0 20 60 3461 g01 ?0 0 40 80 100 shdn pin voltage (v) 1.3 current limit (ma) 1.9 2.3 3461 g04 1.5 1.7 2.1 480 420 360 300 240 180 120 60 0 t a = 25 c t a = 25 c shdn pin voltage (v) 0 shdn pin current ( a) 16 4812 320 280 240 200 160 120 80 40 0 t a = 25 c forward voltage (mv) schottky current (ma) 800 1000 600 400 200 500 400 300 200 100 0 t a = 25 c ?0 20 60 ?0 0 40 80 100 typical perfor a ce characteristics uw oscillator frequency current limit fb pin voltage current limit in soft start mode switching waveform circuit of figure 1 load transient response circuit of figure 1 schottky forward voltage shdn pin current v sw 5v/div v out 50mv/div 0.5 s/div 3461 g08 i load = 60ma 70ma v out 100mv/div 50 s/div 3461 g09 i load 35ma
LT3461 4 3461i operatio u figure 3. suggested layout the LT3461 uses a constant frequency, current mode control scheme to provide excellent line and load regula- tion. operation can be best understood by referring to the block diagram in figure 2. at the start of each oscillator cycle, the sr latch is set, which turns on the power switch q1. a voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the pwm comparator a2. when this voltage exceeds the level at the negative input of a2, the sr latch is reset turning off the power switch. the level at the negative input of a2 is set by the error amplifier a1, and is simply an amplified version of the difference between the feedback voltage and the reference voltage of 1.255v. in this manner, the error amplifier sets the correct peak current level to keep the output in regulation. if the error amplifiers output increases, more current is deliv- ered to the output; if it decreases, less current is delivered. layout hints the high speed operation of the LT3461 demands careful attention to board layout. you will not get advertised performance with careless layout. figure 3 shows the recommended component placement. block diagra w + + + 6 1 3 2 v out v out r1 (external) r2 (external) r s (external) c s (external) fb shutdown shdn ramp generator 1.255v reference 1.3mhz oscillator r s q 4 a1 a2 comparator driver r c c c sw q1 0.1 gnd v in fb 3461 f02 r s , c s optional soft-start components 5 figure 2. block diagram uu u pi fu ctio s sw (pin 1): switch pin. connect inductor here. minimize trace at this pin to reduce emi. gnd (pin 2): ground pin. tie directly to local ground plane. fb (pin 3): feedback pin. reference voltage is 1.255v. connect resistor divider tap here. minimize trace area at fb. set v out according to v out = 1.255v (1 + r1/r2). shdn (pin 4): shutdown pin. tie to 1.5v or higher to enable device; 0.4v or less to disable device. also func- tions as soft-start. use rc filter (47k, 47nf typ) as shown in figure 2. v out (pin 5): output pin. connect to resistor divider. put capacitor close to pin and close to gnd plane. v in (pin 6): input supply pin. must be locally bypassed. r2 r1 gnd c3 l1 c1 v out v in shutdown + c2 + 3461 f03
LT3461 5 3461i inrush current the LT3461 has a built-in schottky diode. when supply voltage is applied to the v in pin, the voltage difference between v in and v out generates inrush current flowing from input through the inductor and the schottky diode to charge the output capacitor to v in . the maximum nonrepetitive surge current the schottky diode in the LT3461 can sustain is 1.5a. the selection of inductor and capacitor value should ensure the peak of the inrush current to be below 1.5a. the peak inrush current can be calculated as follows: i v l r l lc r l p in = ? ? ? ? ? ? ? ? ? ? ? ? = + = + () . exp tan sin tan . . 06 15 2 1 15 4 11 2 2 w a w w a w a a w where l is the inductance, r is the resistance of the inductor and c is the output capacitance. for low dcr inductors, which is usually the case for this application, the peak inrush current can be simplified as follows: i v l p in = ? ? ? ? . exp 06 2 w a w p table 3 gives inrush peak currents for some component selections. table 3. inrush peak current v in (v) r ( w )l ( m h) c ( m f) i p (a) 5 0.5 22 1 0.70 5 0.5 33 1 0.60 applicatio s i for atio wu uu
LT3461 6 3461i typical applicatio s u 3.3v to 5v step-up converter efficiency v in 3.3v l1 4.7 h 45.3k 15k c2 1 f 15pf c1 1 f v out 5v 90ma 1 2 v in v out sw 3 4 5 6 fb shdn gnd LT3461 c1, c2: taiyo yuden x7r emk316bj105 l1: murata lqh32cn4r7m33 or equivalent 3461 ta03a off on load current (ma) 0 efficiency (%) 85 80 75 70 65 60 80 3461 ta03b 40 60 20 10 70 30 50 90 5v to 12v with soft-start circuit input current and output voltage v in 5v l1 15 h 261k 30.1k c2 2.2 f 15pf c1 1 f v out 12v 70ma 1 2 v in v out sw 3 4 5 6 fb shdn gnd LT3461 3461 ta02a control signal 47nf 47k c1: taiyo yuden lmk212bj105md c2: taiyo yuden x7r emk316bj225ml l1: murata lqh32cn150k53 control signal 5v/div v out 5v/div i n 50ma/div 1ms/div 3461 ta02b
LT3461 7 3461i package descriptio u s6 package 6-lead plastic tsot-23 (reference ltc dwg # 05-08-1636) 1.50 ?1.75 (note 4) 2.80 bsc 0.30 ?0.45 6 plcs (note 3) datum ? 0.09 ?0.20 (note 3) s6 tsot-23 0302 2.90 bsc (note 4) 0.95 bsc 1.90 bsc 0.80 ?0.90 1.00 max 0.01 ?0.10 0.20 bsc 0.30 ?0.50 ref pin one id note: 1. dimensions are in millimeters 2. drawing not to scale 3. dimensions are inclusive of plating 4. dimensions are exclusive of mold flash and metal burr 5. mold flash shall not exceed 0.254mm 6. jedec package reference is mo-193 3.85 max 0.62 max 0.95 ref recommended solder pad layout per ipc calculator 1.4 min 2.62 ref 1.22 ref
LT3461 8 3461i linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear.com ? linear technology corporation 2003 lt/tp 0903 1k ? printed in usa related parts part number description comments lt1615/lt1615-1 constant off-time, step-up dc/dc converter 350ma switch, v out to 36v, i q = 20 m a, thinsot package lt1944/lt1944-1 dual constant off-time, step-up dc/dc converter 350ma switch 2, v out to 36v, i q = 20 m a, ms10 package ltc3400/ltc3400b 1.2mhz, synchronous step-up dc/dc converter 600ma switch, v in : 0.85v to 5v, i q = 19 m a, thinsot package ltc3401 3mhz, synchronous step-up dc/dc converter 1a switch, v in : 0.85v to 5v, i q = 38 m a, ms10 package ltc3402 3mhz, synchronous step-up dc/dc converter 2a switch, v in : 0.85v to 5v, i q = 38 m a, ms10 package lt3460 1.3mhz step-up dc/dc converter 350ma switch, v out to 36v, thinsot, sc70 packages lt3464 constant off-time, micropower step-up dc/dc converter 85ma switch, internal schottky diode and output disconnect pnp, i q = 25 m a lt3465 1.2mhz white led step-up dc/dc converter drives up to 6 leds, inherently matches led current with built-in schottky typical applicatio s u 5v to 36v step-up converter v in 5v l1 47 h 280k 10k c2 0.47 f 50v 22pf c1 1 f 16v v out 36v 18ma 1 2 v in v out sw 3 4 5 6 fb shdn gnd LT3461 c1: taiyo yuden x7r emk316bj105 c2: murata grm42-6x7r474k50 l1: murata lqh32cn470 3461 ta05 off on efficiency load current (ma) 0 efficiency (%) 16 3461 ta05b 81214 6 24 10 80 75 70 65 60 55 50 18
|
