quick start guide for demonstration circuit 502 1.25mhz w i de input range step dow n dc/dc converter lt1767 description demonstration circuit 502 is a 1.25mhz 1.5a monolithic step-down dc/dc switching converter using the lt1767 buck regulator. with its fast switching speed, 1.5a internal power switch, and wide input voltage range, the lt1767 is a very versatile and powerful ic for dc/dc converters that require compact space and a wide va- riety of input voltage ranges. the constant 1.25mhz switching frequency allows all of the components to be very small, surface mount devices. the current-mode control topology creates fast transient response and good loop stability with a minimum number of external compensation com- ponents and ceramic input and output capacitors. the low resistance inter- nal power switch (0.22 ? ) maintains high efficiencies >80% and up to 90%. the shdn pin and 6ua shutdown current extends battery life. the wide v in range of the lt1767 allows step-down configurations from 3vin up to 25vin. synchronization of switching frequen- cies up to 2mhz is possible. demonstration circuit 502 is a step- down converter using the lt1767ems8e. the circuit is designed for 6.6-25v input to 5.0v output @1a or for 4.5- 25v ? input to 3.3v output @1a for ca- ble modem, handheld, automotive, and desktop computer applications. the output voltage is jumper selectable (5v or 3.3v). this board was designed for wide input voltage range applica- tions where simplicity, small circuit size, and low component count are im- portant as well as up to 1a of load current. the use of ceramic capaci- tors not only demonstrates small size and low cost, but the advantage of current-mode control in step-down ap- plications with a simple compensation network and a place-holder for a feedforward capacitor for more rugged stability and optimal transient re- sponse if required. design files for this circuit board are available. call the ltc factory. ? higher input voltages may pulse-skip due to minimum on-time restrictions. compensation component changes may be necessary to opti- mize pulse-skipping during high-temperature, high-voltage conditions and maintain control of switch current. table 1. typical performance summary p a r a m e t e r c o n d i t i o n v a l u e input voltage range v out = 5v (jumper selection) 6.6v?25v input voltage range v out = 3.3v (jumper selection) 4.5v?18v v out v out = 5v (jumper selection) 5v 3% v out v out = 3.3v (jumper selection) 3.3v 3% maximum output current 1a typical switching frequency 1.25mhz typical efficiency v in = 12v, v out = 5v, i out = 1a 84% 1 1
quick start guide for demonstration circuit 502 1.25mhz w i de input range step dow n dc/dc converter quick start procedure demonstration circuit 502 is easy to set up to evaluate the performance of the lt1767. refer to figure 1 for proper measurement equipment setup and follow the procedure below: note: make sure that the input voltage does not exceed 25v. 1. connect the 7v?25v or 4.5v?18v in- put power supply to the vin and gnd terminals on the board. 2. connect an ammeter in series with the input supply to measure input current. 3. connect either power resistors or an electronic load to the vout and gnd terminals on the board. 4. connect an ammeter in series with output load to measure output cur- rent. 5. connect a voltmeter across the vin and gnd terminals to measure input voltage. 6. connect a voltmeter across the vout and gnd terminal to measure output voltage. 7. after all connections are made, turn on input power and verify that the output voltage is either 5.0v or 3.3v (the output voltage jumper setting). 8. the synchronization and shutdown functions are optional and their pins can be left floating (discon- nected) if their functions are not being used. load +? +? +? +? vout 3.3v 5v vin gnd sync v out gnd shdn linear t echnology (408) 432-1900 www .linear .com +? + ? + ? l tc1767ems8e 1.25mhz wide input range step down dc/dc converter demo cir cuit 502 figure 1. proper measurement equipment setup 2 2
quick start guide for demonstration circuit 502 1.25mhz w i de input range step dow n dc/dc converter 0 10 20 30 40 50 60 70 80 90 100 0. 01 0. 1 1 lo a d c u r r ent ( m a ) 3. 3v out e ffici e n c y ( % ) 5v in 8v in 12v in 18v in 0 10 20 30 40 50 60 70 80 90 100 0. 01 0. 1 1 lo a d c u r r ent ( m a ) 5v out e f fici e n cy ( % ) 8v in 12v in 18v in 24v in figure 2. efficiency for v out = 3.3v figure 3. efficiency for v out = 5v minimum input voltage the minimum lt1767 operating input voltage is 3.0v. however, a dc/dc buck converter needs to have an input voltage greater than the output volt- age by a certain margin in order to provide the desired regulated output voltage. maximum duty cycle, switch on-resistance, and inductor and diode dc losses all play a part in deter- mining the minimum input voltage for a selected output over the full cur- rent range of the application. for this demonstration board, with up to 1a output current, the minimum input voltages required to maintain regu- lated output voltages is listed be- low. customizable output voltages 2.5v and 1.8v are very common and are thus additionally listed in the table (see customizing the board below). if the input voltage falls below the minimum input voltage listed below, the output voltage can drop accord- ingly from the programmed output voltage. this mode is called maximum duty cycle drop-out. table 2. minimum input voltage for regulated output v out v in (minimum) 5 . 0 v 6 . 6 v 3 . 3 v 4 . 5 v 2 . 5 v 3 . 5 v 1 . 8 v 3 . 0 v customizing the board the components built on the board are optimized for a wide input voltage range. the bandwidth can be increased for more specific input voltages such as 12v 10% or 5v 10% if desired. some typical applications with typi- cal input voltage ranges that are smaller than the wide input voltage range of the part are included below. the adjustable feedback resistors al- low the output voltage to be custom- ized. for output voltages below 3.0v, the boost diode should be moved from d2 to d3 to provide the minimum boost 3 3
quick start guide for demonstration circuit 502 1.25mhz w i de input range step dow n dc/dc converter voltage required for the internal power switch. make sure that the boost capacitor (c8) has a voltage rating greater than or equal to the output voltage for applications where the boost diode is placed in d2. how- ever, the boost capacitor must have a voltage rating greater than the input voltage whenever the boost diode is placed in position d3. vin d1 ups120 c8 0.1uf 5v @1a ceramic r1 31.6k u1 lt1767ems8e /shdn fb vc gnd boost vsw sync vin 12v +/-10% vout l1 cdrh5d18-5r4 c4 100pf r3 4.7k ceramic r2 10.0k d2 cmdsh-3 c5 4.7uf 6.3v x5r 0805 c3 1.5nf c2 2.2uf 25v x7r 1206 vout vin l1 cdrh4d28-4r7 c8 0.1uf ceramic c4 100pf r2 10.0k d1 ups120 12v +/-10% c2 2.2uf 25v x7r 1206 c3 2.2nf r1 17.4k r3 3.3k ceramic d2 cmdsh-3 3.3v @1a u1 lt1767ems8e /shdn fb vc gnd boost vsw sync vin c5 4.7uf 6.3v x5r 0805 r1 10.7k ceramic ceramic 5v +/-10% u1 lt1767ems8e /shdn fb vc gnd boost vsw sync vin c5 4.7uf 6.3v x5r 0805 c3 1.5nf l1 1008ps-272m 2.5v @1a vout d1 ups120 r2 10.0k c8 0.1uf c4 100pf d3 cmdsh-3 c2 2.2uf 10v x5r 0805 r3 3.3k vin 4 4
quick start guide for demonstration circuit 502 1.25mhz w i de input range step dow n dc/dc converter l1 1008ps-272m c5 4.7uf 6.3v x5r 0805 c4 100pf 1.8v @1a ceramic vout r2 10.0k r1 4.99k c2 2.2uf 10v x5r 0805 ceramic d1 ups120 3-5.5v d3 cmdsh-3 c8 0.1uf c3 1.5nf u1 lt1767ems8e /shdn fb vc gnd boost vsw sync vin r3 2.2k vin 5 5
5 5 4 4 3 3 2 2 1 1 d d c c b b a a vin sy nc shdn vout 5. 0v 3. 3v gnd gnd * 3v-2 5v 5v/3 .3v@1a * see quick- start guide. expo sed pad solder to ground vi n minimum vou t 6.6 5 4.5 3.5 3 3.3 2.5 1.8 a dc 502a tuesday, august 06, 2002 1 1 lt1767ems8e, 1.25mhz wide input range step down dc /dc converter june wu 2/12/02 keith s. 2/12/02 size scal e: cage code dwg no rev sheet of filename: title contr act no. ap provals date drawn checked approve d engineer designer technology 16 30 mccarthy blvd. milpitas, ca 95035 phone: (408)432-1900 fax: (408)434-0507 vfb vout vfb vout u1 lt1767em s8e 1 2 3 6 7 8 5 4 boost vin vsw fb vc sync shdn gnd d3 cmdsh-3 opt. r2 31 .6k c7 080 5 opt. jp1 1 2 3 d1 ups14 0 c3 68 00pf e1 r3 2.2k c5 10 0pf opt. e5 d2 cmdsh-3 r5 100k opt. e6 c8 0. 1uf c2 10 uf 6. 3v r1 10.0k e3 c4 10 0pf l1 cdrh 5d18-5r4 r4 39 .2k c1 2.2 uf 25v e2 c6 taj opt. e4
linear technology corporation lt1767ems8e bill of material demo bd. #502a item qty reference part description manufacture / part # 1 1 c1 cap., x7r, 2.2uf, 25v, 10% 1206 tdk, c3216x7r1e225k 2 1 c2 cap., x5r, 10uf, 6.3v, 20% 0805 tdk, c2010x5r0j106m 3 1 c3 cap., x7r, 6800pf, 50v, 20% 0603 avx, 06035c682mat2a 4 1 c4 cap., npo, 100pf, 50v, 10% 0603 avx, 06035a101kat2a 5 0 c5 (opt.) cap., 100pf, 0603 6 0 c6 (opt.) 7 0 c7 (opt.) cap., 0805 8 1 c8 cap., x7r, 0.1uf, 16v, 20% 0603 taiyo yuden, emk107bj104ma 9 1 d1 schottky barrier rec., ups140 microsemi., ups140 10 1 d2 diode, cmdsh-3, sod-323 central semi., cmdsh-3 11 0 d3 (opt.) diode, cmdsh-3, sod-323 central semi., cmdsh-3 12 6 e1-e6 turret, mill-max 2501-2 13 2 jp1,jp2 0.079 single row header, 3 pin comm con, 2802s-03-g1 14 1 jp1 shunt, comm con, ccij2mm-138g 15 1 l1 inductor, 5.4uh sumida, cdrh5d18-5r4 16 1 r1 res.,chip, 10.0k, 1/16w, 5% 0603 aac, cr16-103jm 17 1 r2 res.,chip, 31.6k, 1/16w, 1% 0603 aac, cr16-3162fm 18 1 r3 res.,chip, 2.2k, 1/16w, 5% 0603 aac, cr16-222jm 19 1 r4 res.,chip, 39.2k, 1/16w, 5% 0603 aac, cr16-3922fm 20 0 r5 (opt.) res., chiip 0603 21 1 u1 i.c., lt1767ems8e ms8 exposed linear tech. lt1767ems8e 22 1 printed circuit board demo board DC502A page 1 - of - 1
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