1 LT1268b/LT1268 7.5a, 150khz switching regulators s f ea t u re d u escriptio n wide input voltage range: 3.5v to 30v n low quiescent current: 7ma n internal 7.5a switch n very few external parts required n self-protected against overloads n available in standard and surface mount 5-pin packages n can be externally synchronized (see lt1072 data sheet) the LT1268b and LT1268 are monolithic high power switching regulators. identical to the popular lt1070, except for switching frequency (150khz) and higher switch current, they can be operated in all standard switching configurations including buck, boost, flyback, and inverting. a high current, high efficiency switch is included on the die along with all oscillator, control, and protection circuitry. integration of all functions allows the LT1268 to be built in standard 5-pin power packages. this makes it extremely easy to use and provides bust proof operations similar to that obtained with 3-pin linear regulators. the LT1268 operate with supply voltages from 3.5v to 30v and draw only 7ma quiescent current. by utilizing current mode switching techniques, it provides excellent ac and dc load and line regulation. the LT1268 use an adaptive anti-sat switch drive to allow very wide ranging load currents with no loss in efficiency. an externally activated shutdown mode reduces total supply current to 100 m a typical for standby operation. user note: this data sheet is only intended to provide specifications, graphs, and a general functional description of the LT1268b/LT1268. application circuits are included to show the capability of the LT1268b/LT1268. a complete design manual (an19) should be obtained to assist in developing new designs. this manual contains a comprehensive discussion of both the lt1070 and the external components used with it, as well as complete formulas for calculating the values of these components. the manual can also be used for the LT1268b/LT1268 factoring in the higher switch current rating and higher operating frequency. u a o pp l ic at i ty p i ca l boost regulator with 5.3v 1% output load current (a) 0 efficiency (%) 80 90 4 LT1268 ?ta02 70 60 1 2 3 100 v in = 5.3v v in = 4.7v v in = 5v efficiency of 5.3v boost converter v in v sw LT1268b l1 8 m h, 5a* d1 mbr1035 d2, d3 ? 1n4001 c2 0.068 c3 390 m f 16v r1 4.043k 0.1% r2 1.24k 0.1% r3 220 w fb gnd v c case l2 2 m h, 5a** c1 220 m f 16v + + c4 390 m f 16v v out 5.3v ?% 3.75a v in 4.7v to 5.3v LT1268 ?ta01 coiltronics ctx8-5-52 coiltronics ctx2-5-52 optional. d2 and d3 are used to increase undervoltage lockout from 2.7v to ? 4v. * ** ? + c5 0.02 u s a o pp l ic at i n high efficiency boost converter n pc power supply with multiple outputs n battery upconverter n negative-to-positive converter
2 LT1268b/LT1268 a u g w a w u w a r b s o lu t exi t i s supply voltage ....................................................... 30v switch output voltage ............................................ 60v feedback pin voltage (transient, 1ms) ................ 15v operating junction temperature range operating ............................................... 0 c to 125 c short-circuit ......................................... 0 c to 140 c storage temperature range ................. C 65 c to 150 c lead temperature (soldering, 10 sec).................. 300 c order part number wu u package / o rder i for atio order part number t jmax = 125 c, q jc = 2 c/w, q ja = 50 c/w LT1268ct LT1268bct t jmax = 125 c, q jc = 2 c/ w, q ja = see notes 5 and 6 q package 5-lead dd front view v in v sw gnd fb v c 5 4 3 2 1 LT1268cq LT1268bcq t package 5-lead to-220 v in v sw gnd fb v c front view 5 4 3 2 1 symbol parameter conditions min typ max units v ref reference voltage measured at LT1268b 1.235 1.244 1.253 v feedback pin (note 4) l 1.224 1.244 1.264 v reference voltage LT1268 1.224 1.244 1.264 v l 1.214 1.244 1.274 v i b feedback input current v fb = v ref 350 750 na l 1100 na g m error amplifier transconductance d i c = 25 m a 3000 4400 6000 m mho l 2400 7000 m mho error amplifier source or sink current v c = 1.5v 150 200 350 m a l 120 400 m a error amplifier clamp voltage hi clamp, v fb = 1v 1.80 2.30 v lo clamp, v fb = 1.5v 0.25 0.38 0.52 v reference voltage line regulation 3v v in v max , v c = 0.8v l 0.03 %/v a v error amplifier voltage gain 0.9v v c 1.4v 500 800 v/v minimum input voltage l 2.8 3.0 v i q supply current 3v v in v max , v c = 0.6v 7 10 ma control pin threshold duty cycle = 0 0.7 0.9 1.08 v l 0.5 1.25 v bv output switch breakdown voltage 3v v in v max , i sw = 1.5ma l 60 75 v v sat output switch-on resistance t j 100 c 0.12 0.18 w (note 1, 3) t j 125 c 0.22 w control voltage to switch 12 a/v current transconductance i lim switch current limit (note 3, 6) duty cycle = 50%, t j 100 c l 7.50 15 a duty cycle = 65%, t j 100 c l 6.50 14 a v in = 15v, v c = 0.5v, v fb = v ref , switch pin open, unless otherwise noted. e lectr ic al c c hara terist ics consult factory for industrial and military parts
3 LT1268b/LT1268 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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. symbol parameter conditions min typ max units d i in supply current increase 25 45 ma/a d i sw during switch-on time f switching frequency 120 150 180 khz l 120 180 khz dc max maximum switch duty cycle 65 85 92 % shutdown mode supply current 3v v in v max , v c = 0.05v 100 500 m a shutdown mode threshold voltage 3v v in v max 100 150 250 mv l 50 300 mv v in = 15v, v c = 0.5v, v fb = v ref , switch pin open, unless otherwise noted. e lectr ic al c c hara terist ics the l denotes specifications which apply over the full operating temperature range. note 1: measured with v c in hi clamp, v fb = 0.8v. note 2: for duty cycles (dc) between 50% and 65%, minimum guaranteed switch current is given by i lim = 6.25 (1.7 C dc). note 3: minimum current limit is reduced by 0.5a at 125 c. 100 c test limits are guaranteed by correlation to 125 c tests. note 4: LT1268b reference voltage is specified at 9mv to guarantee 1% output voltage accuracy when 0.1% external resistors are used to set output voltage. to maintain output accuracy under load, load current should be taken from the case and the ground pin should be connected separately to output ground. see an19 for details. note 5: the q package is intended for surface mount without a separate heat sink. see graph for thermal resistance as a function of the mounting area. this curve assumes no other heat dissipators adjacent to package. note 6: maximum switch current may be limited by package power dissipation, especially for the surface mount (q) package. this package has a thermal resistance of 20 c/w to 50 c/w (see graph). the following formula will allow an estimate of maximum continuous switch current as a function of power loss and duty cycle. see an19 for more details. i max = p r sw dc p = power dissipation due to switch current r sw = switch-on resistance ? 0.15 w dc = switch duty cycle in a typical application where thermal resistance is 30 c/w, maximum power might be limited to 2w and power allocated to switch loss is 1.5w. for a duty cycle of 40%, this yields i max = = 5a 1.5 0.15 0.4 obviously, a combination of high thermal resistance and high duty cycle may restrict switch current to a value well below the 7.5a electrical limit. typical perfor a ce characteristics wu duty cycle (%) 0 switch current (a) 18 24 80 LT1268 ?tpc01 12 6 0 20 40 60 100 70 10 30 50 90 55? 125? 25? switch current limit vs duty cycle top surface copper area (cm 2 ) 0 0 q (?/w) 10 20 30 40 60 5 10 15 20 LT1268 ?603 25 30 50 single-sided board, 1oz copper double-sided board, 1oz copper bottom copper area >100cm 2 switch current (a) 0 switch saturation voltage (v) 1.2 1.6 8 LT1268 ?tpc02 0.8 0.4 1.0 1.4 0.6 0.2 0 2 4 6 10 7 1 3 5 9 t j = 25? switch saturation voltage q package thermal resistance
4 LT1268b/LT1268 linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7487 (408) 432-1900 l fax : (408) 434-0507 l telex : 499-3977 ? linear technology corporation 1994 1268fs, sn1268 lt/gp 0194 10k rev 0 ? printed in the usa u a o pp l ic at i ty p i ca l boost converter (5v to 12v) q package 5-lead dd t package 5-lead to-220 v in 20 m h 5v v sw LT1268ct LT1268 ?ta03 fb 1k 1.24k 10.7k 12v, 1.5a mbr1020 1 m f 1000 m f 25v v c gnd + + package descriptio u dimensions in inches (millimeters) unless otherwise noted. 0.022 ?0.005 (0.559 ?0.127) 0.105 ?0.008 (2.667 ?0.203) 0.004 +0.008 0.004 () 0.102 +0.203 0.102 0.050 ?0.012 (1.270 ?0.305) 0.059 (1.499) typ 0.050 ?0.008 (1.270 ?0.203) 0.175 ?0.008 (4.445 ?0.203) 0.060 (1.524) 0.401 ?0.015 (10.185 ?0.381) 15?typ 0.032 ?0.008 (0.813 ?0.203) 0.331 +0.012 0.020 () 8.407 +0.305 0.508 0.143 +0.012 0.020 () 3.632 +0.305 0.508 0.067 ?0.010 (1.702 ?0.254) 0.028 ?0.035 (0.711 ?0.889) 0.057 ?0.077 (1.448 ?1.956) 0.139 ?0.153 (3.531 ?3.886) dia 0.079 ?0.135 (2.007 ?3.429) 0.380 ?0.420 (9.652 ?10.668) 0.560 ?0.650 (14.224 ?16.510) 0.866 ?0.913 (21.996 ?23.190) 0.970 ?1.050 (24.64 ?26.67) 0.304 ?0.380 (7.722 ?9.652) 0.700 ?0.728 (17.780 ?18.491) 0.035 ?0.055 (0.889 ?1.397) 0.169 ?0.185 (4.293 ?4.699) 0.079 ?0.115 (2.007 ?2.921) 0.149 ?0.230 (3.785 ?5.842) 0.620 ?0.020 (15.75 ?0.508) 0.460 ?0.500 (11.68 ?12.70) 0.210 ?0.240 (5.334 ?6.096) 0.055 ?0.090 (1.397 ?2.286) 0.015 ?0.025 (0.381 ?0.635)
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