1 description lt 3909 2-string 50ma, 2mhz step-up led driver with 1.8% current matching demonstration circuit 2176 a features the lt ? 3909 , a 2-string 50ma, 2 mhz step-up led driver with 1.8% current matching. the LT3909 precisely regulates two led strings for equal current, supports led dimming that reduces brightness down to a 1/40,000 ratio, protects against faults and also reports faults. the LT3909 consists of a step-up converter with a fixed 2 mhz switching frequency and current sources for the led strings. the operating voltage range for dc2176a is from 7 v to 36v. the maximum current in each led string is 40ma and is programmed by 24.9 k i set resistor r1. the i set resistor can be changed to 20.0 k to program the string current to 50ma C see the data sheet for an application example with 50ma string current. the LT3909 utilizes voltage feedback during pwm dim - ming and when there are faults on both led strings. volt- age feedback resistors r2 and r3 on dc2176a optimize operation for leds that are 35 v maximum. efficiency is 86% at 12 v input, 35 v for the led string forward voltages and 40ma per string. led current can be reduced by an external voltage applied to the ctrl terminal. resistor r8 pulls up ctrl to the intv cc voltage of the LT3909 so the led current is at the 40ma maximum when there is no external ctrl voltage applied. there are placeholders to add voltage dividers either from v in or intv cc to the ctrl pin. a voltage divider from v in to the ctrl pin can be used to reduce led current at low input voltages. a resistor divider from intv cc to ctrl allows an ntc to reduce led current at high temperatures. there are placeholders for an rc filter between the ctrl terminal and the ctrl pin. the rc filter limits the ramp time of the ctrl pin and prevent false faults when ctrl ramps too quickly. the fault terminal is pulled up by 100 k r6 to v in , but there is a placeholder to pull up fault to intv cc instead. l , lt, ltc, ltm, linear technology and the linear logo are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. resistor divider r4 and r5 from the input to the en/uvlo pin programs the circuit to turn off when v in decreases below 5 v with 0.9 v of hysteresis. the circuit can also be turned off by connecting the en/uvlo terminal to ground. an external voltage must be applied to the pwm terminal in order for dc2176a to operate. the external voltage can be a dc level or an appropriate pwm dimming signal. a common frequency for pwm dimming is 100 hz. there is a placeholder for a resistor to pull up pwm to intv cc so an external voltage on pwm for operation is unnecessary. there is a placeholder that allows an external diode to be placed in parallel with the internal LT3909 rectifier for higher current capability. dc2176a uses an inductor that is optimized for high efficiency, and the input supply for power to the inductor can be separated from the LT3909 v in pin to allow operation from two input sources. ceramic capacitors are used at the input, output and for intv cc . a 50v tantalum polymer capacitor protects the input from ringing that may occur when the input is hot- plugged using long leads. there are placeholders occupied by 0805 size 0 jumpers on the back of the pcb for two led strings and also placeholders for sod323 diodes to clamp the led1 and led2 voltages from ringing below ground. 2-string operation, close current matching, an extremely wide dimming range and fault protection make the LT3909 ideal for functionally advanced back lighting and illumi - nation cir cuits. dc2176a features the LT3909edd in a thermally enhanced 12-lead 3 mm 3 mm dfn package. the LT3909 data sheet must be read in conjunction with this demo manual to properly use or modify dc2176a. design files for this circuit board are available at http://www . linear.com/demo/dc2176a dc2176af demo manual dc2176a
2 quick s tart p roce d ure to evaluate the performance of the LT3909 using dc2176 a, refer to figure 1 for the proper measurement equipment setup and then follow the procedure below: note: an external voltage must be applied to the pwm terminal in order for dc2176a to operate. see step 4 for details. 1. connect two strings of leds with a forward voltage of 35v or less and corresponding meters. 2. connect the en/uvlo terminal to gnd with a clip-on lead. 3. with the power off, connect the input power supply and corresponding meters. 4. apply an external voltage to the pwm terminal. the external voltage can be a 2 v to 5 v dc source or a 100hz or higher pwm dimming signal. install resistor r10 to pull up pwm to intv cc if an external voltage on pwm is not desired. 5. turn the v in power supply on and verify that the input voltage is between 7 v and 36 v. the v in voltage is al- lowed to exceed the led voltage by a few volts, but a v in that is much higher than the led voltages may activate overtemperature protection or dissipate too much power in the LT3909. 6. release the en/uvlo to gnd connection. 7. once the led strings are running at the programmed led current; adjust the input voltage within the operat - ing range and observe the led current regulation or reduction in brightness with led dimming. p erformance s ummary specifications are at t a = 25c parameters conditions min typ max units minimum input voltage, v in v led1 = v led2 = 35v, i led1 = i led2 40ma 7 v maximum input voltage, v in v led1 = v led2 = 35v, i led1 = i led2 40ma 36 v maximum led voltage, v led1 , v led2 r2 = 976k 1%, r3 = 31.6k 1% 35 v led current, i led1 , i led2 r1 = 24.9k, v ctrl = intv cc 40 ma input en/uvlo turn-on voltage, v in r4 = 402k, r5 = 130k, v in rising 5.9 v input en/uvlo turn-off voltage, v in r4 = 402k, r5 = 130k, v in falling 5 v efficiency v in = 12v, v led1 = v led2 = 35v, i led1 = i led2 = 40ma 86 % switching frequency 2 mhz dc2176af demo manual dc2176a
3 quick s tart p roce d ure figure 1. proper measurement equipment setup for dc2176a figure 2. efficiency vs input voltage v in with v led1 = v led2 = 35v and i led1 = i led2 = 40ma v in (v) 5 efficiency (%) 100 95 85 75 90 80 70 65 60 55 50 25 15 35 dc2176a f02 40 20 10 30 v led1 = v led2 = 35v i led1 = i led2 = 40ma dc2176af demo manual dc2176a
4 quick s tart p roce d ure figure 3. total output current i led1 + i led2 vs input voltage v in with v led1 = v led2 = 35v. uvlo occurs at v in = 5v input falling with 0.9v of hysteresis figure 4. led current when brightness is reduced to a 1/20,000 ratio by pwm dimming at 100hz v in (v) 0 5 i led1 + i led2 (ma) 90 70 50 80 60 40 30 20 10 0 25 15 35 dc2176a f03 40 20 10 30 v led1 = v led2 = 35v turn-on turn-off dc2176a f04 500ns/div v out 20v/div i led1 20ma/div i led2 20ma/div v pwm 2v/div v in = 12v v led1 = v led2 = 35v i led1 = i led2 = 40ma dc2176af demo manual dc2176a
5 p arts list item qty reference part description manufacturer/ part number required circuit components 1 1 c1 cap, 1f x7r 50v 10% 0805 murata grm21br71h105ka12l 2 1 c2 cap, 2.2f x7r 50v 20% 1206 tdk c3216x7r1h225m 3 1 c3 cap, 1f x5r 6.3v 20% 0603 tdk c1608x5r0j105m 4 1 l1 inductor, 6.8h 4020 wurth electronik 74437324068 5 1 r1 res, chip 24.9k 0.06w 1% 0603 vishay crcw060324k9fkea 6 1 r2 res, chip 976k 0.01w 1% 0603 vishay crcw0603976kfkea 7 1 r3 res, chip 31.6k 0.10w 1% 0603 vishay crcw060331k6fkea 8 1 r4 res, chip 402k 0.06w 1% 0603 vishay crcw0603402kfkea 9 1 r5 res, chip 130k 0.06w 1% 0603 vishay crcw0603130kfkea 10 1 u1 ic, led driver dfn(12) (dd) 3mm 3mm linear technology corporation LT3909edd#pbf additional demo board circuit components 1 0 c4 (opt) cap, 10nf x7r 100v 10% 0603 avx 06031c103 kat 2a 2 1 c5 cap, tant polymer 10f 50v 20% 7343 avx tcjd106m050r0120 3 0 d1 (opt) schottky diode, 1a sod-323 diodes inc sbr1a40s3 4 1 r6 res, chip 100k 0.06w 5% 0603 vishay crcw0603100kjea 5 0 r7, r9, r10, r11 (opt) res, 0603 6 1 r8 res, chip 100k 0.06w 1% 0603 vishay crcw0603100kfkea 7 1 r12 res, chip 1k 0.06w 5% 0603 vishay crcw06031k00jea 8 28 r13, r14, r15, r16, r17, r18, r19, r20, r21, r22, r23, r24, r25, r26, r 27, r28, r29, r30, r31, r32, r33, r34, r35, r36, r37, r38, r39, r40 res/jumper, chip 0 0.125w 0805 vishay crcw08050000zoea hardware: for demo board only 1 12 e1, e2, e3, e4, e5, e6 ,e7, e8, e9, e10, e11, e12 turret, testpoint mill max 2501-2-00-80-00-00-07-0 2 4 mh1 to mh4 stand-off, nylon 1/4" tall (snap on) keystone, 8831(snap on) dc2176af demo manual dc2176a
6 s chematic diagram ? ? ? ? ?? r9 (opt) e2 gnd e1 vin 7v \ 36v r5 130k u1 LT3909edd fault 8 pwm 7 intvcc 1 led1 10 vin 2 fb 6 en/uvlo 3 led2 9 iset 4 ctrl 5 vout 11 sw 12 gnd 13 c1 1f 50v 0805 r6 100k 5% e4 ctrl e7 gnd r3 31.6k l1 6.8h 74437324068 + c5 10f 50v 7343 r4 402k r1 24.9k e9 led1 35v ? led * e10 led2 35v ? led * c2 2.2f 50v 1206 e6 fault e5 pwm e8 gnd r2 976k 2. ? all ? resistors ? are ? 1%. e3 en/uvlo r12 1k 5% tp1 tp2 gnd d1 sbr1a40s3 (opt) c3 1f r11 (opt) e12 gnd c4 10nf (opt) e11 gnd r7 (opt) r8 100k r10 (opt) * see the performance summary for led current rating note: ? unless ? otherwise ? specified 1. ? all ? resistors ? and ? capacitors ? 0603. dc2176af demo manual dc2176a
7 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 representa- tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. s chematic diagram r19 0 0805 r18 0 0805 r17 0 0805 r16 0 0805 r15 0 0805 r14 0 0805 r13 0 0805 r40 0 0805 r20 0 0805 r21 0 0805 r22 0 0805 r23 0 0805 r24 0 0805 r25 0 0805 r32 0 0805 r31 0 0805 r30 0 0805 r29 0 0805 r28 0 0805 r27 0 0805 r26 0 0805 r39 0805 r33 0 0805 r34 0 0805 r35 0 0805 r36 0 0805 r37 0 0805 r38 0 0805 dc2176af demo manual dc2176a
8 linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com ? linear technology corporation 2015 lt 0915 ? printed in usa demonstration board important notice linear technology corporation (ltc) provides the enclosed product(s) under the following as is conditions: this demonstration board ( demo board) kit being sold or provided by linear technology is intended for use for engineering development or evaluation purposes only and is not provided by ltc for commercial use. as such, the demo board herein may not be complete in terms of required design-, marketing-, and/or manufacturing-related protective considerations, including but not limited to product safety measures typically found in finished commercial goods. as a prototype, this product does not fall within the scope of the european union directive on electromagnetic compatibility and therefore may or may not meet the technical requirements of the directive, or other regulations. if this evaluation kit does not meet the specifications recited in the demo board manual the kit may be returned within 30 days from the date of delivery for a full refund. the foregoing warranty is the exclusive warranty made by the seller to buyer and is in lieu of all other warranties, expressed, implied, or statutory, including any warranty of merchantability or fitness for any particular purpose. except to the extent of this indemnity, neither party shall be liable to the other for any indirect, special, incidental, or consequential damages. the user assumes all responsibility and liability for proper and safe handling of the goods. further, the user releases ltc from all claims arising from the handling or use of the goods. due to the open construction of the product, it is the users responsibility to take any and all appropriate precautions with regard to electrostatic discharge. also be aware that the products herein may not be regulatory compliant or agency certified (fcc, ul, ce, etc.). no license is granted under any patent right or other intellectual property whatsoever. ltc assumes no liability for applications assistance, customer product design, software performance, or infringement of patents or any other intellectual property rights of any kind. ltc currently services a variety of customers for products around the world, and therefore this transaction is not exclusive. please read the demo board manual prior to handling the product . persons handling this product must have electronics training and observe good laboratory practice standards. common sense is encouraged. this notice contains important safety information about temperatures and voltages. for further safety concerns, please contact a ltc application engineer. mailing address: linear technology 1630 mccarthy blvd. milpitas, ca 95035 copyright ? 2004, linear technology corporation dc2176af demo manual dc2176a
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