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IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 1 IML8681 application notes table of content 1. ic description ................................. ................................................... ................................................... ..........2 2. features ....................................... ................................................... ................................................... ............2 3. package and pin diagrams ....................... ................................................... .................................................2 4. typical application circuit and waveforms ...... ................................................... ...........................................3 5. evaluation modules ............................. ................................................... ................................................... .....3 6. key parameters by order number ................. ................................................... ............................................4 7. operation theory ............................... ................................................... ................................................... ......5 8. application information ........................ ................................................... ................................................... ....9 8.1 selection of driving scheme ................... ................................................... .......................................9 8.2 total v f of the led string ................................ ................................................... ............................ 13 8.3 selection for option .......................... ................................................... .......................................... 16 8.4 v f variation ........................................ ................................................... ......................................... 17 8.5 thermal protection ............................ ................................................... .......................................... 18 8.6 layout suggestion ............................. ................................................... ......................................... 19 free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 2 1. ic description the IML8681 is a two terminal current controller (t tcc) for regulating the current flowing through an led string according to the voltage established acr oss the led string. when the IML8681 is configured with an led string i n the application, the IML8681 can work as voltage controlled current source, current regulator, or cu t-off, depending on the voltage across the device a nd its placement in the string. the typical application fo r the device is to power an led string from an ac v oltage source as in an led light bulb. no external components are needed saving pcb area a nd reducing total bom cost. the device allows the designer flexibility in pcb layout to meet vari ous shape requirements and constraints. it is espec ially suitable for replacing incandescent light bulb and linear type fluorescent lamp. 2. features system all solid state components no electrolytic capacitor needed compact size high power factor and low total harmonic distortio n performance high efficiency flexible pcb layout style chip 80v input sustaining voltage. various bypass-current options available for diffe rent applications. can be parallel connected to increase the current drive capability. chip-on-board process available. applications ac led lighting engine. led light bulb. led light tube. 3. package and pin diagrams dfn 2mm 2mm nc ms 1 ano de ( a ) nc catho de ( k ) ms 2 1 2 3 6 5 4 sot-89-5l ms1 cathode anode ms2 nc nc cathode anode to-252-2l free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 3 4. typical application circuit and waveforms i led rectified v ac t t 5. evaluation modules free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 4 6. key parameters by order number package type: dfn ? 2mm x 2mm ? 6 pin (halogen free ) part number 1 st line marking target led v f in each segment mode 0 (ms2=l; ms1=l) mode 1 (ms2=l; ms1=h) mode 2 (ms2=h; ms1=l) mode 3 (ms2=h; ms1=h) i peak (ma) v on,th (v) v off,th (v) i peak (ma) v on,th (v) v off,th (v) i peak (ma) v on,th (v) v off,th (v) i peak (ma) v on,th (v) v off,th (v) IML8681nl-k11 8k11 65v led 38 n/a n/a 30.4 64 55 20.9 60.8 52.25 12.5 57.6 49.5 IML8681nl-k21 8k21 51 40.8 28.1 16.8 IML8681nl-k31 8k31 64 51.2 35.2 21.1 IML8681nl-k41 8k41 52v led 44 n/a n/a 35.2 50.5 45.7 24.2 48.0 43.4 14.5 45.5 41.1 IML8681nl-k51 8k51 46 36.8 25.3 15.2 IML8681nl-k61 8k61 58 46.4 31.9 19.1 IML8681nl-k71 8k71 61 48.8 33.6 20.1 IML8681nl-k81 8k81 73 58.4 40.2 24.1 IML8681nl-k91 8k91 76 60.8 41.8 25.1 IML8681nl-n11 8n11 36v led or lower 71 n/a n/a 56.8 35.5 30.5 39.1 33.7 29.0 23.4 32.0 27.5 IML8681nl-n21 8n21 78 62.4 42.9 25.7 IML8681nl-n31 8n31 86 68.8 47.3 28.4 IML8681nl-n41 8n41 95 76.0 52.3 31.4 IML8681nl-n51 8n51 104 83.2 57.2 34.3 IML8681nl-n61 8n61 114 91.2 62.7 37.6 IML8681nl-n71 8n71 119 95.2 65.5 39.3 IML8681nl-n81 8n81 130* 104.0 71.5 42.9 IML8681nl-n91 8n91 143* 114.4 78.7 47.2 IML8681nl-k12 8k12 65v led 38 n/a n/a 30.4 62 53 20.9 58.9 50.35 12.5 55.8 47.7 IML8681nl-k22 8k22 51 40.8 28.1 16.8 IML8681nl-k32 8k32 64 51.2 35.2 21.1 IML8681nl-k42 8k42 52v led 44 n/a n/a 35.2 49.1 44.2 24.2 46.6 42.0 14.5 44.2 39.8 IML8681nl-k52 8k52 46 36.8 25.3 15.2 IML8681nl-k62 8k62 58 46.4 31.9 19.1 IML8681nl-k72 8k72 61 48.8 33.6 20.1 IML8681nl-k82 8k82 73 58.4 40.2 24.1 IML8681nl-k92 8k92 76 60.8 41.8 25.1 IML8681nl-n12 8n12 36v led or lower 71 n/a n/a 56.8 34.5 29.5 39.1 32.8 28.0 23.4 31.1 26.6 IML8681nl-n22 8n22 78 62.4 42.9 25.7 IML8681nl-n32 8n32 86 68.8 47.3 28.4 IML8681nl-n42 8n42 95 76.0 52.3 31.4 IML8681nl-n52 8n52 104 83.2 57.2 34.3 IML8681nl-n62 8n62 114 91.2 62.7 37.6 IML8681nl-n72 8n72 119 95.2 65.5 39.3 IML8681nl-n82 8n82 130* 104.0 71.5 42.9 IML8681nl-n92 8n92 143* 114.4 78.7 47.2 note: when using the n81/n91 or n82/n92 options, u sing 2 pieces of k31/n11 or k32/n12, respectively, co nnected in parallel for u4 in place of a single n81/n91 or n82 /n92 is recommended in order to improve the thermal dissipation (please refer to the typical application circuit). because u4 is always turned on and condu cting high current during operation, the junction temperature may rise high enough to activate otp in a single n81 /n91 or n82/n92 chip used as u4. with proper thermal design , there is no thermal issue for u1, u2, or u3 using a single n81/n91 or n82/n92 chip. free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 5 7. operation theory the IML8681 is a two terminal current controller (t tcc) that regulates the current flowing through an led string according to the voltage established acr oss the string. the typical application for the dev ice is to power an led string from an ac voltage source as in an led light bulb or light tube. the IML8681 has 4 functional pins -- anode (a), cat hode (k), ms1, and ms2. in typical applications, the led string is divided into several segments. th e anode pin is connected to the most positive termi nal of the led segment and the cathode pin is connected to the most negative terminal. the ms1 and ms2 pins are used for device mode selection. one of four dev ice modes (mode 0 through mode 3) is selected by connecting ms1 and/or ms2 to the cathode (logic 0) or by leaving both pins open (internally pulled up to logic 1). the regulating current (i peak ), turn on voltage (v on,th ), and turn off voltage (v off,th ) are different for each mode. please refer to the diagram and the table in below. in mode 0, the chip is connected to the led string in series and acts as a constant current sink, regulating the current through the string. i n modes 1 through 3, the device, connected in parallel with a string segment, acts as a current shunt or bypass when the voltage across the segment, v ak , is below the mode threshold. the following explains the operation theory in deta il. the ms1 and ms2 pins will not be shown in the applications drawings to simplify the drawings. ms1 and ms2 pins connection for each mode cur rent and on/off voltage thresholds for each mode the i-v curve of the IML8681 operating in mode 0 is shown in below. there are two operating regions ? the linear region and the constant current (saturat ion) region. when the voltage across the anode pin (a) and the cathode pin (k), v ak , is lower than the dropout voltage (v drop ), the conduction current, i ak , increases as v ak rises. when v ak rises beyond v drop , the chip enters the constant current region and i ak will be kept at predetermined level, i peak , as v ak increases. power dissipation is most important whe n operating in mode 0. use the exposed thermal pad to increase power dissi pation. soldering the thermal pad of the part to a copper foil on the pcb will improve thermal conduct ivity. IML8681 v ak i ak anode (a) cathode (k) 0 v ak i ak constant current region i ak =i peak v drop i peak linear region i ak inceases as v ak rises i-v curve of the IML8681 operating in mode 0 mode ms2 ms1 i peak v on,th / v off,th 3 1 1 33% 89%/76% 2 1 0 55% 94%/80% 1 0 1 80% 98%/85% 0 0 0 100% n/a free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 6 the generic i-v curve of the IML8681 operating in modes 1 through 3 is shown in below. there are three operation regions; the linear region, the constant current region, and the cut-off region. when the vo ltage across the anode pin (a) and the cathode pin (k), v ak , is lower than the dropout voltage, v drop , the conduction current, i ak , increases as v ak rises. when v ak rises beyond v drop , the chip enters constant current region and i ak will be kept at predetermined level, i peak . when v ak rises beyond v off,th , the chip enters the cut-off region and i ak drops to 0. when v ak drops below v on,th , the chip enters the constant current region again and i ak increases to i peak . IML8681 v ak i ak anode (a) cathode (k) v off,th 0 v ak i ak constant current region i ak =i peak v drop i peak linear region i ak inceases as v ak rises v on,th cut - off region i ak =0 i-v curve of the IML8681 operating in modes 1 throu gh 3 to control the current flowing through the led stri ng, the IML8681 can be connected to led string either in series or in parallel. please refer to th e 2-stage driving scheme as an example, shown below . the led string is divided into 2 segments, led1 and led 2, connected in series. the IML8681 chip u1 is connected in parallel with led1 and chip u2 connect ed in series with the led string. a rectified ac so urce is applied to the led string network, as shown in t he figure, below. u1 is operated in mode 1, 2 or 3, and u2 is operated in mode 0 with a higher current setting , i peak2 , than u1. v ac ~ v ak1 i led1 i ak 1 i led 2 u1 IML8681 v ak2 u2 IML8681 led1 led2 v led2 mode 0 mode 1/2/3 2-stage driving scheme the operation of the 2-stage driving scheme is expl ained with the circuit and waveforms shown below. for illustration purposes, one rectified v ac cycle is divided into 8 periods, t 0 to t 7 . free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 7 during the period t 0 , as v ac ramps up, the system is working at stage 1, and u1 is on. the current flows through u1, led2, and u2, and increases as the rect ified v ac rises. during t 1 , the system is still working at stage 1. when the rectified v ac rises to v drop , the current is clamped at i peak1 by u1, and v ak1 keeps rising. during t 2 , v ak1 rises above v off,th and the system enters stage 2. u1 turns off and th e current flows through led1, led2 and u2. the current increases again unti l it is clamped at i peak2 by u2 during t 3 . as v ac ramps down, the current is still clamped at i peak2 by u2 during t 4 . in the period of t 5 , the current starts to decrease when v ak2 drops below v drop of u2. during t 6 , v ak1 drops below v on,th , and u1 is turned on again. the system re-enters stage 1. the current is clamped at i peak1 by u1. during t 7 , v ak2 drops below v drop of u1 and the current decreases to 0. the system op eration repeats for each rectified v ac cycle. v ac ~ v ak1 i led 1 i ak 1 i led2 u1 IML8681 v ak2 u2 IML8681 led1 led2 v led2 v ac ~ v ak1 i led 1 i ak 1 i led2 u1 IML8681 v ak2 u2 IML8681 led1 led2 v led2 stage 1: u1 and u2 are on stag e 2: u1 is off and u2 is on free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 8 v ak2 i led1 v ak 1 v ac rectified ac voltage v led 2 i ak1 i peak1 v off,th i peak2 v drop v on,th t 6 t 3 t 5 t 1 t 2 t 0 t 7 i led2 v drop i peak1 i peak2 t 4 driving waveforms of 2-stage driving scheme free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 9 8. application information 8.1 selection of driving scheme there are 4 typical driving schemes for the IML8681 . stages 1 - 4 use 1 - 4 pieces of IML8681. the power factor (pf) and efficiency increase and t he total harmonic distortion (thd) decreases with each additional stage used. the driving scheme is v ery flexible, so the user can optimize cost and performance. the following describes the driving sc heme stages. a. single-stage driving scheme v ac ~ u 4 IML8681 led1 led2 led3 led4 i led i peak4 v ac i led single-stage driving scheme and corresponding wavefo rm in the single-stage driving scheme, only one iml868 1 chip (u4) is utilized as a constant current regulator to drive the entire led string. the curre nt flowing through the led string is clamped at i peak4 , the current setting of u4, when v ac is higher than the cut-in voltage of the led strin g. b. 2-stage driving scheme v ac ~ u1 IML8681 u 4 IML8681 led1 led2 led3 led4 i led v ac i led i peak4 i peak1 2-stage driving scheme and corresponding waveform free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 10 in the 2-stage driving scheme, two IML8681 chips, u 1 and u4, are used. u1 is connected in parallel with led1 and u4 is connected in series wi th the entire string. when u1 is turned on, it wil l clamp the led current at i peak1 . when u1 is turned off, u4 will clamp the led curr ent at i peak4 . beware that i peak4 must be higher than i peak1 . otherwise the circuit will operate as a single-stage driving scheme with u1 kept in the lin ear region. usually, u4 must be in mode 0, and it is recommended that u1 be in mode 1. c. 3-stage driving scheme v ac ~ u1 IML8681 u 4 IML8681 led1 led2 led3 u2 IML8681 led4 i led v ac i led i peak4 i peak1 i peak2 3-stage driving scheme and corresponding waveform in the 3-stage driving scheme, three IML8681 chips, u1, u2, and u4 are used. u1 is connected in parallel with led1; u2 is connected in parallel with led; and u4 is connected in series with the entire string. assuming i peak4 > i peak1 > i peak2 , u1, u2, and u4 are all turned on at the beginning of the cycle. because current of u2, i peak2 , is lowest, u2 will clamp the led current at i peak2 . when u2 turns off, u1 will then clamp the led current at i peak1 . when u1 and u2 turn off, u4 will clamp the led current at i peak4 . beware that i peak4 must be higher than i peak1 and i peak2 . the relationship between i peak1 and i peak2 determines the sequence that segments led1 and led 2 light up. if it is desired that led2 lights up earlier and longer than led1, i peak2 should be lower than i peak1 . usually, u4 must work in mode 0, and it is recommended that u1 and u2 work i n mode 1 and mode 2, respectively. free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 11 d. 4-stage driving scheme v ac ~ u1 IML8681 u 4 IML8681 led1 led2 led3 u2 IML8681 u3 IML8681 led4 i led i peak4 v ac i led i peak1 i peak2 i peak3 4-stage driving scheme and corresponding waveform in the 4-stage driving scheme, four IML8681 chips, u1, u2, u3, and u4 are used. u1, u2, and u3 are connected in parallel with led1, led2, and l ed3, respectively. u4 is connected in series with the entire string. assuming i peak4 > i peak1 > i peak2 > i peak3 . u1, u2, u3, and u4 are turned on at the beginning of the cycle. because the setting cur rent of u3, i peak3 , is lowest, u3 will clamp the led current at i peak3 . when u3 is turned off, u2 will then clamp the led current at i peak2 . when u2 is turned off, u1 will then clamp the led current at i peak1 . when u1, u2, and u3 are turned off, u4 will clamp the led current at i peak4 . beware that i peak4 must be higher than i peak1 , i peak2 , and i peak3 . the relationship between i peak1 , i peak2 , and i peak3 determines the sequence that segments led1, led2 a nd led3 light up. if led3 is planned to light up earlier and longer than led2 an d led1, i peak3 should be lower than i peak2 and i peak1 . usually, u4 works in mode 0, and it is recommende d that u1, u2, and u3 work in mode 1, mode 2, and mode 3, respectively. for the 4-stage driving scheme, it is not required that u1 works in mode 1, u2 works in mode 2, or u3 works in mode 3. only u4 is restricted to ope rate in mode 0 and with the highest current level. the system designer can arrange the sequence arbitr arily depending on system needs. flexibility is an important advantage of IML8681 driving system. the led segment lighting sequence for a 4-stage dri ving scheme is demonstrated in the figure below. in the example, the current path over the ri sing part of the v ac period is shown step by step. free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 12 v ac i ak1 i led i ak2 i ak3 i led,ak1 i led,ak2 i led,ak3 i ak4 u4 k a ms1 ms2 u1 u2 u3 k a ms2 ms1 k a ms2 ms1 k a ms2 ms1 v ac i ak1 i led i ak2 i ak3 i led,ak1 i led,ak2 i led,ak3 i ak4 u 4 k a ms1 ms2 u1 u2 u3 k a ms2 ms1 k a ms2 ms1 k a ms2 ms1 v ac i ak1 i led i ak2 i ak3 i led,ak1 i led,ak2 i led,ak3 i ak4 u4 k a ms1 ms2 u1 u2 u3 k a ms2 ms1 k a ms2 ms1 k a ms2 ms1 v ac i ak1 i led i ak2 i ak3 i led,ak1 i led,ak2 i led,ak3 u1 u2 u3 k a ms2 ms1 k a ms2 ms1 k a ms2 ms1 i ak4 u 4 k a ms1 ms2 i led i max 3 i max2 i max 1 i max4 rectified v ac t t v off,th 3 0 v ak i ak v drop i max3 v on,th3 i max2 i max1 i max4 v off,th2 v on,th2 v off,th1 v on,th1 u3 u2 u1 u4 4-stage driving scheme operation theory e. performance comparison of 4 driving schemes the typical cases of these 4 driving scheme were te sted. the test results and waveforms are shown, below. with more stages, the power factor (p f) and total harmonic distortion (thd) are significantly improved. power factor thd single-stage 0.86 58% 2-stage 0.93 33% 3-stage 0.96 20% 4-stage 0.97 15% 0.70 0.75 0.80 0.85 0.90 0.95 1.00 198 209 220 231 242 pf v ac (v) pf vs. vac 4 steps 3 steps 2 steps 1 step 0 15 30 45 60 75 90 198 209 220 231 242 thd (%) v ac (v) thd vs. vac 1 step 2 steps 3 steps 4 steps comparison of pf and thd for 1 through 4 stage drivi ng schemes free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 13 v ac ~ u4 IML8681 led1 led2 led3 led4 i led v ac ~ u1 IML8681 u4 IML8681 led1 led2 led3 led4 i led v ac ~ u1 IML8681 u4 IML8681 led1 led2 led3 u2 IML8681 led4 i led v ac ~ u1 IML8681 u4 IML8681 led1 led2 led3 u2 IML8681 u3 IML8681 led4 i led single stage power=7.2w pf=0.86 thd=58% 2-stage power=9.1w pf=0.93 thd=33% 3-stage power=9.9w pf=0.96 thd=20% 4-stage power=10.1w pf=0.97 thd=15% led current waveforms of 1 through 4 stage driving sc heme 8.2 total v f of the led string leds are current driven components which are very s ensitive to forward voltage (v f ). choosing a suitable v f for the led string can improve the performance of the led lighting system. to have a suitable total v f for the led string, we should consider the ac inpu t voltage and the variation in led v f . however, there are fluctuations on the ac input and variations around the led v f . both should be taken into consideration. generally, rectified v ac,max ? v ak,max < v f < rectified v ac,min ? v drop where, v ac,max is the maximum possible v ac voltage, usually 110% of the nominal v ac ; v ac,min is the minimum possible v ac voltage, usually 90% of the nominal v ac ; and v ak,max is the voltage rating of the anode pin to the cath ode pin of the IML8681, 90v. for example, for a v ac =220v power system, the led total v f is recommended to be in the range of 252v to 277v. in that case, 4 pieces of 67.5v hv le ds is very suitable for the application, as shown i n the example in below. free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 14 v ac bd u4 led4 u1 led1 k ms2 ms1 a u2 led2 k ms2 ms1 a u3 led3 k ms2 ms1 a k ms2 ms1 a led v f consideration vs. v ac fluctuation range however, in the range allowed, the led total v f is recommended to be as high as possible in order to achieve higher efficiency. therefore, the simple guideline is: rectified v ac,nominal x 83% < led total v f < rectified v ac,nominal x 89% please refer to the table in below for a summary of the recommended led total v f in 100v ac /110v ac /120v ac /220v ac power systems. for example, in a 110v ac power system, the total v f of the led string is recommended to be in the range of 129v~138v. for a 220v ac power system, the total v f of the led string is recommended to be in the rang e of 258v~277v. in these cases, the voltage gap between v ac peak and the total v f becomes the maximum voltage drop across the anode pin and the cathode pin of the IML8681 u4 chip series connected at the bottom of the led string. if the total v f is higher than v ac peak, i.e. v f =160v when v ac =110v ac , u4 will remain in the linear region and the led current will not reach the predetermined regulated level, as shown in the figure, below. on the other hand, if the total v f is much lower than v ac peak, i.e. v f =50v when v ac =110va c , the voltage across the anode pin and the cathode pin of u4 will be as high as 106v, which is higher than the maximum rating (90v) of the chip and may damage the chip. free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 15 ac input voltage (rms) dc peak voltage recommended total v f maximum voltage drop on u4 100v ac power system 110v (max.) 156v 117~125v 39v 100v (typ.) 141v 24v 90v (min.) 127v 10v 110v ac power system 121v (max.) 171v 129~138v 42v 110v (typ.) 156v 27v 99v (min.) 140v 11v 120v ac power system 132v (max.) 187v 140~151v 47v 120v (typ.) 170v 30v 108v (min.) 153v 13v 220v ac power system 242v (max.) 342v 258~277v 84v 220v (typ.) 311v 53v 198v (min.) 280v 22v led current waveforms under different total v f conditions with 110v ac the measured waveforms of v ac , led current, and voltage drop across u4 are shown , below. v ac vs. led current @ v ac =198v v ac vs. led current @ v ac =220v v ac vs. led current @ v ac =242v u4 v ak vs. led current @ v ac =198v u4 v ak vs. led current @ v ac =220v u4 v ak vs. led current @ v ac =242v free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 16 8.3 selection for option for better performance, consider not only the avera ge current, but also, the actual led v f at the expected peak current. the steps for selecting the most suitable option are: 1. determine the product type considering v ac range, power of the lamp, number of stages, and the led v f in each stage. 2. choose the most suitable options for the actual led v f at the expected peak current. step 1. determine the product type the IML8681 provides a number of options with diffe rent regulating currents and on/off voltages for different combinations of ac source, lamp power , and led v f in each segment. please refer to the table, below. when the led v f and product type are determined, the system design er can determine suitable options based on the table as a quick star t. led v f 67v 52v 36v led number 4 5 4 IML8681 chip number 4 4 4 v ac (v) 230 220 230 220 120 110 100 power 6w k11/k12 k11/k12 k41/k42 k51/k52 n11/n12 n21/n22 n31/n32 8w k21/k22 k21/k22 k61/k62 k71/k72 n41/n42 n51/n52 n61/n62 10w k31/k32 k31/k32 k81/k82 k91/k92 n71/n72 n81/n82 n91/n92 step 2. voltage matching for u1 for better voltage matching, the v on,th of stage 1 (u1) and the actual led v f at the expected peak current should be considered. please refer to the following figure. the driving scheme and the i-v curves of the led1 segment for a 2-stage ne twork are shown. the peak regulating current of u1 and u4, i peak1 and i peak4 , are also shown. v off,th1 of u1 should be lower than the v f of led1 at i led1 =i peak4 -i peak1 . otherwise, i led will be clamped at i peak4 and v ak1 will be clamped at the v f of led1 at i led1 =i peak1 -i peak4 . consequently, u1 will remain in the on-state during the v ac rising period, reducing efficiency. v on,th1 of u1 should be lower than the v f of led1 at i peak4 . otherwise, u1 will remain in the off-state until v ak of u1 ramps down to be lower than v off,th1 as v ac falls. as a result, the current notch will become more serious. note that, the maxi mum value of v on,th1 should be taken into consideration and the v f drops due to high temperature should also be consi dered. as a rule of thumb: v on,th1,max < (led1 v f @ i max4 @ 85 c) ? 2v where, 2v is a guard band. after the option for u1 (mode 1) is determined, u4 (mode 0) is determined since u4 can be the same option. free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 17 v ac ~ u1 IML8681 u 4 IML8681 led1 led2 led3 led4 i led 0 v ak i ak v drop (u1) i peak 1 (u4) i peak 4 (u1) v off,th 1 (u1) v on,th1 v f @ i max4 i peak4 -i peak 1 i led1 driving scheme and i-v curves of the led string with 2-stage IML8681 if the 3-stage or 4-stage driving schemes are used, also follow the selection rules, above, to choose the best option for u1 (mode 1). the same op tion can be applied for u2 (mode 2) and u3 (mode 3). the settings of ms1 and ms2 by pcb layout will distinguish them. 8.4 v f variation when all of the leds are turned on, it is recommend ed that (led total v f ) + (v drop of u4) be lower than (rectified v ac peak) when v ac is the 90% of the nominal value (minimum fluctuati on level). in order not to damage u4, (rectified v ac peak) ? (led total v f ) should be lower than the voltage rating of u4 when v ac is 110% of the nominal value (maximum fluctuation level). however, the real led v f is distributed over a wide range in volume. for ex ample, for a 67v hv led, the real v f may be between 63.5v and 71.5v. the led v f is usually sorted into many bins during production. this makes the design of lighting netwo rk using the IML8681 a little complicated. take a 67v led as an example. please refer to the f igure, below. if v f is at the typical value (i.e. v f center bin = 67.5v) just match the v ac operating range. if v f is at the minimum value (i.e. v f bottom bin = 63.5v) u4 needs a higher voltage rating. the IML8681 just matches. if v f is at the maximum value (i.e. v f top bin = 71.5v) the top segment group of leds may not turn on when the v ac power fluctuates to the minimum level. free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 18 rectified v ac peak value @ v ac maximum (+10%) rectified v ac peak value @ v ac minimum (-10%) ? (rectified v ac,peak ) ? (led v f,total ) < u4 voltage rating ? in order not to damage the u4. ? (led v f,total ) + (v drop of u4) < (rectified v ac,peak ) ? in order to make all led turn on. led v f variation vs. v ac range so, if 67v leds are sorted in to 4 bins: bin 1 = 63 .5v ~ 65.5v; bin 2 = 65.5v ~ 67.5v; bin 3 = 67.5v ~ 69.5v; and bin 4 = 69.5v ~ 71.5v, it is rec ommended to select the IML8681 option according to the bin 2 v f value. then, both bin 2 and bin 3 leds can be us ed. for the leds in the bin 1 and bin 4, if a ~4% decrease in pf and an ~8% increase in thd are acceptable, then the leds in the bin 1 and bin 4 can be used in the same option (bom) as b in 2 and bin 3. if better performance is desired, different options for bin 1 and bin 4 are needed. f urther, if the 67v led is composed of discrete low voltage leds, such as 3v, 6v, or 9v, mixing of the bin 1 leds and bin 4 leds in lamp production is possible. in summary, if the IML8681 option is selected for t ypical v f , the decrease of pf and increase of thd is acceptable with v f 5% range. 8.5 thermal protection the thermal protection function is embedded to prev ent permanent damage caused by high junction temperature (t j ). please refer to the figure in below for the curv es and the measured waveforms. when the junction temperature, t j , is higher than 125c, the thermal protection is a ctivated and the peak current starts to ramp down linearly a t a rate of around -1.1%/c. this feature helps the system to achieve thermal steady state. free datasheet http://
IML8681 application note ? 2013 iml inc. all rights reserved. rev. 0.1 - jun. 2013 19 0 t j i ak i peak (170c) t tsd (125c) i peak /2 i tp linear ramp down type thermal protection function 8.6 layout suggestion the ms1 pin and ms2 pin are on the same side of the package as the anode (a) pin, not the cathode (k) pin. the ms1 and ms2 pins can be connec ted to k pin through the exposed pad. the nc pins (4 and 6) can also be connected to the exposed pad and the k pin to enhance the power dissipation ability. mode 1 nc ms1 anode (a) nc cathode (k) ms2 1 2 3 6 5 4 nc ms 1 anode (a ) nc cathode (k) ms2 1 2 3 6 5 4 nc ms1 anode (a) nc cathode (k) ms2 1 2 3 6 5 4 nc ms1 anode (a) nc cathode (k) ms2 1 2 3 6 5 4 mode 2 mode 0 mode 3 layout suggestion for improving the power dissipation ability free datasheet http://

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