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august 2012 doc id 023412 rev 1 1/17 AN4140 application note stld41 output power capability introduction this application note is dedicated to the output power capability of the stld41, which is a white led driver for display backlight. the schematic, functional description and external components selection are also discussed in this application note. detailed behavior, if different led counts on the output and different currents through leds are used, is also shown. www.st.com
contents AN4140 2/17 doc id 023412 rev 1 contents 1 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3 application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4 component calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1 c in selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.2 c out selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.3 r sense value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.4 inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5 output power capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.1 with the lqh6ppn470m3 inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.2 with the mss1038473mlb inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.3 with the lps6235473mlb inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6 efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.1 with the lqh6ppn470m3 inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.2 with the mss1038473mlb inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 6.3 with the lps6235473mlb inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
AN4140 description doc id 023412 rev 1 3/17 1 description the stld41 is a boost converter that operates from 3.0 v to 21 v and can provide an output voltage as high as 38 v. it can drive up to 40 white leds connected in 4 strings of 10 leds in series. the total output current capability is 120 ma at an output voltage of 38 v. regulation is performed by the internal error amplifier which works with the feedback voltage from the sensing resistor. the device can be turned on/off by way of the logic signal connected to the en pin. the enable pin allows the device to be turned off, so reducing the current consumption to less than 1 a. the leds can be dimmed by applying a pwm signal to the pwm pin. soft-start with controlled inrush current limit, thermal shutdown and overvoltage protection are integrated functions of the device.
applications AN4140 4/17 doc id 023412 rev 1 2 applications mini pcs pmp & pnd printers game consoles. figure 1. package dfn8 (3 x 3 mm)
AN4140 application circuit doc id 023412 rev 1 5/17 3 application circuit figure 2. application schematic table 1. list of external components comp. manufacturer part number value size ratings l coilcraft murata coilcraft lps6235473mlb lqh6ppn470m43 mss1038473mlb 47 h 6 x 6 x 3.5 mm 6 x 6 x 4.3 mm 10 x 10 x 3.8 mm i = 1.2 a, dcr = 0.245 i = 1.25 a, dcr = 0.23 i = 2.22 a, dcr = 0.13 c in murata grm32er71h106k 10 f 1210 10%, x7r, 50 v c out murata grm31cr61h225ka88l 2.2 f 1206 15%, x5r, 50 v d stmicroelectronics stps1l40m 1 a 3.75 x 1 x 1.9 mm 40 v r sense 0.4 ? 1.6 0603
component calculation AN4140 6/17 doc id 023412 rev 1 4 component calculation 4.1 c in selection it is recommended to use 10 f as the input capacitor to achieve good stability of the device and low noise on the v in track. 4.2 c out selection it is recommended to use 2.2 f as the optimal value of the output capacitor to get the best compromise between output voltage ripple and load transient response. 4.3 r sense value r sense = v ref / i led r sense sensing resistor v ref reference voltage = 160 mv typical i led total led output current p rsense = r sense * i led2 p rsense power dissipation of the sensing resistor 4.4 inductor selection a thin shielded inductor with a low dc series resistance of winding is recommended for this application. to achieve a good efficiency in step-up mode, it is recommended to use an inductor with a dc series resistance r dcl = r d / 10 [ ? , ? , 1], where r d is the dynamic resistance of the led. equation 1 table 2. r sense calculated values i led 100 150 200 250 300 350 400 r sense 1.6 ? 1.1 ? 0.8 ? 0.65 ? 0.55 ? 0.45 ? 0.4 ?
AN4140 component calculation doc id 023412 rev 1 7/17 where: i peak is peak inductor current i out is current sourced at the v out pin is the efficiency of the stld41 v out is output voltage at the v out pin v in is input voltage at the v bat pin l is inductance value of the inductor f is the switching frequency.
output power capability AN4140 8/17 doc id 023412 rev 1 5 output power capability all measurements were made at ambient temperature (24 c) and 3 different inductors. 5.1 with the lqh6ppn470m3 inductor figure 3. current capability with the lqh6ppn470m3 3 3.4 3.8 4.2 820 3.4 3.8 4.2 4.6 5 6 3 4.2 4.6 6 7 8 10 3.8 4.6 5 7 12 4.2 6 7 8 10 14 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 2.5 5.0 10.0 20.0 out-max [v] in [v] iout = 100 ma iout = 150 ma iout = 200 ma iout = 250 ma iout = 300 ma iout = 350 ma iout = 400 ma v v am12752v1
AN4140 output power capability doc id 023412 rev 1 9/17 5.2 with the mss1038473mlb inductor figure 4. current capability with the mss1038473mlb am12753v1 3 3.4 3.8 4.6 8 20 3 3.4 3.8 4.2 4.6 5 3 3.4 3.8 4.2 4.6 5 6 7 3.8 4.6 6 7 8 10 12 4.2 5 7 10 12 14 6 8 10 12 14 16 1 8 20 22 24 26 2 8 3 0 3 2 3 4 3 6 38 40 2.5 5.0 10.0 20.0 v out- max [v] v in [v] io u t = 100 ma io u t = 150 ma io u t = 200 ma io u t = 250 ma io u t = 3 00 ma io u t = 3 50 ma io u t = 400 ma
output power capability AN4140 10/17 doc id 023412 rev 1 5.3 with the lps6235473mlb inductor figure 5. current capability with the lps6235473mlb am12754v1 3 3.4 3.8 4.2 4.6 8 20 3 3.4 3.8 4.2 4.6 5 3 3.4 4.2 4.6 5 6 7 6 8 10 12 14 16 1 8 20 22 24 26 2 8 3 0 3 2 3 4 3 6 38 40 2.5 5.0 10.0 20.0 v out - max [v] v in [v] iout = 100 ma iout = 150 ma iout = 200 ma
AN4140 efficiency doc id 023412 rev 1 11/17 6 efficiency 6.1 with the lqh6ppn470m3 inductor figure 6. i out = 100 ma with the lqh6ppn470m3 figure 7. i out = 200 ma with the lqh6ppn470m3 am12755v1 55.00 57.00 59.00 61.00 6 3 .00 65.00 67.00 69.00 71.00 7 3 .00 75.00 77.00 79.00 8 1.00 83 .00 8 5.00 8 7.00 8 9.00 91.00 9 3 .00 95.00 2 3 4567 8 91011121 3 14 15 16 17 1 8 19 20 21 eff [ % ] v in [v ] vo u t = 12 v vo u t = 14 v vo u t = 17 v vo u t = 20 v vo u t = 2 3 v vo u t = 25 v vo u t = 2 8 v vo u t = 3 1 v vo u t = 33 v vo u t = 3 6 v am12756v1 55.00 57.00 59.00 61.00 6 3 .00 65.00 67.00 69.00 71.00 7 3 .00 75.00 77.00 79.00 8 1.00 83 .00 8 5.00 8 7.00 8 9.00 91.00 9 3 .00 95.00 97.00 2 3 4567 8 91011121 3 14 15 16 17 1 8 19 20 21 eff [ % ] v in [v] vo u t = 12 v vo u t = 14 v vo u t = 17 v vo u t = 20 v vo u t = 2 3 v vo u t = 26 v vo u t = 2 8 v vo u t = 3 1 v vo u t = 3 4 v vo u t = 3 6 v
efficiency AN4140 12/17 doc id 023412 rev 1 figure 8. i out = 300 ma with the lqh6ppn470m3 figure 9. i out = 400 ma with the lqh6ppn470m3 am12757v1 55.00 57.00 59.00 61.00 6 3 .00 65.00 67.00 69.00 71.00 7 3 .00 75.00 77.00 79.00 8 1.00 83 .00 8 5.00 8 7.00 8 9.00 91.00 9 3 .00 95.00 97.00 99.00 2 3 4567 8 91011121 3 14 15 16 17 1 8 19 20 21 eff [ % ] v in [v] vo u t = 12 v vo u t = 14 v vo u t = 17 v vo u t = 20 v vo u t = 2 3 v vo u t = 25 v vo u t = 2 8 v vo u t = 3 1 v vo u t = 33 v vo u t = 3 6 v am1275 8 v1 55.00 57.00 59.00 61.00 63.00 65.00 67.00 69.00 71.00 73.00 75.00 77.00 79.00 81.00 83.00 85.00 87.00 89.00 91.00 93.00 95.00 97.00 99.00 2 3 4 5 6 7 8 9 101112131415161718192021 e? [%] v in [v] vout = 9 v vout = 12 v vout = 15 v vout = 17 v vout = 20 v vout = 23 v vout = 26 v vout = 28 v vout = 31 v vout = 34 v
AN4140 efficiency doc id 023412 rev 1 13/17 6.2 with the mss1038473mlb inductor figure 10. i out = 100 ma with the mss1038473mlb figure 11. i out = 200 ma with the mss1038473mlb am12759v1 55.00 57.00 59.00 61.00 63.00 65.00 67.00 69.00 71.00 73.00 75.00 77.00 79.00 81.00 83.00 85.00 87.00 89.00 91.00 93.00 95.00 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 e? [%] v in [v] vout = 9 v vout = 12 v vout = 16 v vout = 19 v vout = 22 v vout = 25 v vout = 27 v vout = 30 v vout = 33 v vout = 36 v am12760v1 55.00 57.00 59.00 61.00 63.00 65.00 67.00 69.00 71.00 73.00 75.00 77.00 79.00 81.00 83.00 85.00 87.00 89.00 91.00 93.00 95.00 97.00 99.00 2 3 4 5 6 7 8 9 101112131415161718192021 e? [%] v in [v] vout = 10 v vout = 13 v vout = 16 v vout = 19 v vout = 22 v vout = 25 v vout = 27 v vout = 30 v vout = 33 v vout = 36 v
efficiency AN4140 14/17 doc id 023412 rev 1 figure 12. i out = 300 ma with the mss1038473mlb figure 13. i out = 400 ma with the mss1038473mlb am12761v1 55.00 57.00 59.00 61.00 63.00 65.00 67.00 69.00 71.00 73.00 75.00 77.00 79.00 81.00 83.00 85.00 87.00 89.00 91.00 93.00 95.00 97.00 99.00 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 e? [%] v in [v] vout = 10 v vout = 13 v vout = 17 v vout = 20 v vout = 23 v vout = 26 v vout = 28 v vout = 31 v vout = 33 v vout = 36 v am12762v1 55.00 57.00 59.00 61.00 63.00 65.00 67.00 69.00 71.00 73.00 75.00 77.00 79.00 81.00 83.00 85.00 87.00 89.00 91.00 93.00 95.00 97.00 99.00 2 3 4 5 6 7 8 9 101112131415161718192021 e? [%] v in [v] vout = 10 v vout = 13 v vout = 17 v vout = 20 v vout = 23 v vout = 26 v vout = 28 v vout = 30 v vout = 33 v vout = 36 v
AN4140 efficiency doc id 023412 rev 1 15/17 6.3 with the lps6235473mlb inductor figure 14. i out = 100 ma with the lps6235473mlb figure 15. i out = 200 ma with the lps6235473mlb am1276 3 v1 60.00 62.00 64.00 66.00 68.00 70.00 72.00 74.00 76.00 78.00 80.00 82.00 84.00 86.00 88.00 90.00 92.00 94.00 96.00 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 e? [%] v in [v] vout = 10 v vout = 13 v vout = 16 v vout = 19 v vout = 22 v vout = 25 v vout = 28 v vout = 30 v vout = 33 v vout = 36 v am12764v1 60.00 62.00 64.00 66.00 68.00 70.00 72.00 74.00 76.00 78.00 80.00 82.00 84.00 86.00 88.00 90.00 92.00 94.00 96.00 98.00 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 e? [%] v in [v] vout = 10 v vout = 13 v vout = 16 v vout = 19 v vout = 22 v vout = 25 v vout = 28 v vout = 30 v vout = 33 v vout = 36 v
revision history AN4140 16/17 doc id 023412 rev 1 7 revision history table 3. document revision history date revision changes 28-aug-2012 1 initial release.
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