1 description LT3922 36v 2a synchronous 2mhz boost led driver demonstration circuit dc2247a is a 36v 2a synchronous 2mhz boost led driver featuring the lt ? 3922. it drives a single string of leds at 330ma up to 34v when v in is between 7v and 28v . it runs down to 4v in with reduced i led and can withstand v in as high as 36v , but has over - voltage lockout (ovlo) set for 28v for this application. dc2247 a runs at 2mhz switching frequency. spread spectrum frequency modulation (ssfm) can be turned on with a simple jumper, reducing emi. dc2247a comes with low emi features including optimized layout, ssfm, and input and output filters. it passes cispr 25 class 5 conducted and radiated emi. it is protected against both open and short led conditions and reports the faults. the LT3922 has an input voltage range from 2.8v to 36v. its internal synchronous 2a 40v switches allow up to 34v of leds on the output with room for overvoltage protection and overshoot during an open led event. it has adjustable switching frequency between 200khz and 2.5mhz. it can be synchronized to an external source or programmed with ssfm for low emi. the pwmtg high side pwm mosfet driver assists with short-circuit protection and versatility. LT3922 can be configured as boost, boost-buck, or buck mode led driver and maintain all of its low-emi, pwm dimming and fault diagnostic features. the LT3922 can be pwm dimmed with an external pwm signal or an internally-generated pwm signal. dc2247a has a jumper that can be set to switch between internally- generated pwm signal, externally-generated pwm signal, and no pwm signal (100% on). it can be analog dimmed with a control voltage on its ctrl pin. when run with both pwm dimming and ssfm, the spread spectrum aligns itself with the pwm signal for flicker-free operation. 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. small ceramic input and output capacitors are used to save space and cost. the board is designed with tiny, high frequency capacitors on both sides of the v out pins for a reduction in radiated emi. the open led overvoltage protection uses the ics constant voltage regulation loop to regulate the output to approximately 37.5v if the led string is opened although it may reach almost 40v peak during transient from running leds to open. there is a protection diode from led + to gnd to prevent negative ringing during a short circuit. the output current can be monitored through the ismon output pin. undervoltage and overvoltage lockout can be adjusted on the circuit with a few simple resistor choices. there is an emi filter on the input of dc2247 a. there is also a small ferrite bead output filter. these filters, combined with proper board layout and ssfm are effective to help the pcb pass cispr 25 class 5 conducted and radiated emi. please follow the recommended layout and four-layer thickness of dc2247a for low emi applications. the LT3922 data sheet gives a complete description of the part, operation and applications information. the data sheet must be read in conjunction with this demo manual for dc2247a. the LT3922eufd is assembled in a 28- lead plastic 4mm x 5mm qfn package with a thermally enhanced ground pad. proper board layout is essential for maximum thermal performance. see the data sheet section layout considerations. design files for this circuit board are available at http://www.linear.com/demo/dc2247a dc2247af demo manual dc2247a
2 quick s tart p roce d ure demonstration circuit dc2247 a is easy to set up to evaluate the performance of the LT3922 follow the procedure below: 1. with power off, connect a string of leds that will run with forward voltage less than or equal to 34v (at 330ma) to the led + and led C terminals on the pcb as shown in figure 1. 2. connect the en/uvlo terminal to gnd. 3. set jp1 to on and jp2 to ext/on for 100% always-on led operation. set jp3 to no ssfm to run without ssfm or external synchronization. 4. with power off, connect the input power supply to the low emi vin and gnd terminals. make sure that the input voltage will not exceed 36v. 5. turn the input power supply on and make sure the voltage is between 5v and 36v to start operation. 6. release the en/uvlo-to-gnd connection. 7. observe the led string running at the programmed led current. 8. to change the brightness with analog dimming, simply attach a voltage source to the ctrl terminal and set the voltage between 0 v and 1.5 v. see data sheet for details. 9. to change brightness with external pwm dimming, set jp1 to ext and jp2 to ext/on. attach a 3 v rectangular waveform with varying duty cycle to the pwm terminal. 10. to change brightness with internally-generated pwm dimming, set jp1 to int and jp2 to int. adjust the setting of the vr1 variable resistor with a small fat- head screwdriver to toggle between 0% and 100% pwm dimming duty cycle in 1/128 steps. 11. to enable spread spectrum frequency modulation, set jp3 to ssfm on. p erformance s ummary specifications are at t a = 25c parameter c onditions min typ max units input voltage low emi v in range operating i led > 0ma 4 28 v input voltage low emi v in range operating v led = 34v, i led = 330ma 7 28 v safe input voltage low emi v in range 0 36 v switching frequency r2 = 45.3k, ssfm = off 2 mhz switching frequency r2 = 45.3k, ssfm = on 2 to 2.5 mhz i led r1 = 0.3, 7.0v < low emi v in < 28v, v led = 34v 330 ma v led range r4 = 1m, r5 = 33.2k v in 34 v open led voltage v out r4 = 1m, r5 = 33.2k, open load 37.5 v efficiency (100% pwm dc) pvin = 12v, v led = 34v, i led = 330ma 90 % efficiency with emi filters removed pvin = 12v, v led = 34v, i led = 330ma 91 % internally-generated pwm dimming range operating jp1 = int, jp2 = int 1/128 100% internally-generated pwm dimming frequency operating jp1 = int, jp2 = int r10 = 332k 122 hz dc2247af demo manual dc2247a
3 figure 1. test procedure setup drawing for dc2247a quick s tart p roce d ure dc2247 f01 monitor fault flag monitor led current ? + ? + v in + ? v out i in input supply + ? i led ? + up to 34v leds set jp3 for sync and ssfm. connect external sync signal to sync terminal when ?ext sync? option is selected. set jp1 and jp2 to match ext: external pwm dimming int: internal pwm dimming on: 100% on, no pwm dimming adjust vr1 with screwdriver for internal pwm dimming duty cycle control 1v 2v or external pwm dimming optional sync internal pwm dimming 0v 2v optional analog dimming for led current reduction optional connect en/uvlo to gnd for shutdown dc2247af demo manual dc2247a
4 quick s tart p roce d ure figure 2. dc2247a efficiency and led current versus input voltage for 34v 330ma led load. efficiency remains high through the range due to synchronous switching. at low v in , i led can be reduced due to peak switch current limit figure 3. infinite-persist scope traces show pwm dimming and ssfm working together for flicker-free brightness control with both a) externally and b) internally generated pwm dimming a) b) v in (v) efficiency (%) dc2247 f02a 60 65 70 75 80 85 90 95 100 0 30 252015105 without emi filters with emi filter v in (v) led current dc2247 f02b 0.00 0.05 0.10 0.15 0.20 0.30 0.35 0.40 0 30 252015105 1s/div i led 200ma/div i l 1a/div v in = 12v, v led = 34v, i led = 330ma f sw = 2mhz, ssfm on 2000:1, 120hz external pwm dimming dc2247 f03a 10s/div i led 200ma/div i l 1a/div v in = 12v, v led = 34v, i led = 330ma f sw = 2mhz, ssfm on 128:1, 122hz internal pwm dimming v pwm = 1.01v dc2247 f03b dc2247af demo manual dc2247a
5 quick s tart p roce d ure figure 4. dc2247a conducted peak and average emipasses cispr 25 class 5 limits. figure 5. dc2247a radiated peak and average emi30mhz to 1ghzpasses cispr 25 class 5 limits mhz peak conducted emi (dbv) dc2247 f04a ?10 0 10 20 30 50 40 60 80 70 0.1 10 100 1 cispr 25 class 5 limits cispr 25 class 5 limits noise floor mhz peak radiated emi (dbv/m) dc2247 f05a ?10 0 10 20 30 40 50 0 1000 800 600 400 200 cispr 25 class 5 limits noise floor mhz average conducted emi (dbv) dc2247 f04b ?10 0 10 20 30 50 40 60 80 70 0.1 10 100 1 cispr 25 class 5 limits cispr 25 class 5 limits noise floor mhz average radiated emi (dbv/m) dc2247 f05b ?20 0 10 20 30 ?10 40 0 1000 800 600 400 200 cispr 25 class 5 limits noise floor figure 6. dc2247a radiated peak and average emi150khz to 30mhzpasses cispr 25 class 5 limits mhz peak radiated emi (dbv/m) dc2247 f06a 10 15 20 25 30 40 35 45 50 0.1 10 1 cispr 25 class 5 limits noise floor mhz average radiated emi (dbv/m) dc2247 f06b 0 5 10 20 15 25 30 35 40 0.1 10 1 cispr 25 class 5 limits noise floor dc2247af demo manual dc2247a
6 p arts list item qty reference part description manufacturer/part number required circuit components 1 1 c1 cap., x5r, 4.7f, 50v, 10%, 1206 murata, grm31cr71h475ka12l 2 1 c2 cap., x5r, 1f, 50v, 10%, 0603 tdk, c1608x5r1h105k080ab 3 1 c3 cap., x7r, 2.2f, 50v, 10%, 1206 murata, grm31cr71h225ka88k 4 2 c4, c5 cap., x5r, 0.47f, 50v, 10%, 0402 taiyo yuden, umk105abj474kv-f 5 1 c6 cap., x7r, 220pf, 50v, 10%, 0603 murata, grm188r71h221ka01d 6 1 c7 cap., x7r, 2.2f, 6.3v 10%, 0603 avx, 06036c225kat2a 7 1 c8 cap., x5r, 1f, 6.3v, 10%, 0402 murata, grm155r60j105ke19d 8 2 c9, c10 cap., x7r, 100nf, 6.3v, 10%, 0402 avx, 04026c104kat2a 21 1 l1 inductor, 4.7h, 4.45mm x 4.06mm wurth elektronik, 74437324047 22 1 m1 p-mosfet, si2319cds, sot23 vishay, si2319cds-t1-ge3 23 1 r1 res, chip, 0.3, 1/3w, 1%, 0805 susumu, rl1220s-r30-f 24 1 r2 res, chip, 45.3k, 1/16w, 1%, 0402 vishay, crcw040245k3fked 25 1 r3 res, chip, 24k, 1/10w, 1%, 0603 vishay, crcw060324k0fkea 26 1 r4 res, chip, 1m, 1/10w, 1%, 0603 vishay, crcw06031m00fkea 27 1 r5 res, chip, 33.2k, 1/10w, 1%, 0603 vishay, crcw060333k2fkea 28 1 r6 res, chip, 499k, 1/16w, 1%, 0402 vishay, crcw0402499kfked 29 1 r7 res, chip, 221k, 1/16w, 1%, 0402 vishay, crcw0402221kfked 30 1 r8 res, chip, 31.6k, 1/16w, 1%, 0402 vishay, crcw040231k6fked 31 1 r9 res, chip, 100k, 1/16w, 1%, 0402 vishay, crcw0402100kfked 32 1 r10 res, chip, 332k, 1/16w, 1%, 0402 vishay, crcw0402332kfked 36 1 u1 i.c., led driver, 28qfn 4x5 linear tech., LT3922eufd#pbf optional demo board circuit components 9 1 c11 cap., alum., 33f, 50v, 20%, 6.3mm x 7.7mm sun electronic industries corporation, 50ce33bs 10 2 c12, c13 cap., x7r, 0.1f, 50v, 10%, 0402 murata, grm155r71h104ke14d 11 1 c14 cap., x5r, 2.2f, 6.3v, 10%, 0402 murata, grm155r60j225me15d 12 0 c15, c17, c18 (opt) cap., option, 0603 13 0 c16, c19 (opt) cap., option, 0402 14 1 d1 diode, schottky, 40v, 1a, sod323f nxp, pmeg4010cej, 115 17 1 fb1 chip, bead, 600, 0805 wurth elektronik, 742792040 18 1 fb2 chip, bead, 1.5k, 0805 wurth elektronik, 742792097 33 1 r11 res, chip, 91k, 1/10w, 5%, 0603 vishay, crcw060391k0jnea 34 1 r12 res, chip, 51k, 1/10w, 1%, 0603 vishay, crcw060351k0fkea 35 0 r13, r14 (opt) res, option, 0402 37 1 vr1 trimmer., 100k, 0.25w, smd bourns, 3314j-1-104e hardware: for demo board only 15 6 e1, e3, e8, e11-e13 test point, turret, 0.094" mtg. hole mill-max, 2501-2-00-80-00-00-07-0 16 7 e2, e4-e7, e9, e10 test point, turret, 0.061" mtg. hole mill-max, 2308-2-00-80-00-00-07-0 19 2 jp1, jp3 header 3 pin 0.079 double row wurth elektronik, 62000621121 20 1 jp2 header 3 pin 0.079 single row wurth elektronik, 62000311121 38 3 xjp1, xjp2, xjp3 shunt, 0.079" center wurth elektronik, 60800213421 dc2247af demo manual dc2247a
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 dc2247af demo manual dc2247a
8 linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com ? linear technology corporation 2016 lt 0816 ? 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 whatsoeve r. 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 dc2247af demo manual dc2247a
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