1/9 AN2129 application note 1 introduction thanks to the high efficiency and reliability, su per high brightness leds are becoming more and more important when compared to conventional light sources. although leds can be sup- plied directly from a simple voltage source (lik e battery with resistor), for most applications it is better to use a switching current source to get not even higher efficiency but also to get a better light output. this paper will focus on a l6902d based dc/dc converter with dimming interface. for more details about other conver ters and applications for leds available from stmicroelectronics please refer to other application notes ([1] and [2]). 2 dimming concepts there are two basic principles how the light o utput of the led can be controlled. since the light brightness is proportional to the current, both methods are dealing with current regulation. the first and the easiest way is to control the led current itself, with the principal sketch in figure 1, where current is changed proportionally with the dimming signal. disadvantage of this ana- log control is that there can be a significant change of color (wavelength difference could be several nanometers) in deep dimming (less that 10%). this potential disadvantage is compen- sated by a very simple control circuit (u sually a simple potentiometer is enough). figure 1. analog current control figure 2. average current control by pwm i time i time i max i average the second method is based on an average current control (digital control) as can be seen in figure 2. the current is switched between zero and the nominal current with a frequency high- er than 100hz (to avoid flickering). the change of duty cycle and hence the average current change will be seen as a brightness change, because human eye reaction is slow enough to "integrate" the light output and it will not be noticed as a blinking. this method avoids the color change problem, but on the other hand it needs more sophisti- cated control circuits (usually a microcon troller or another simple pwm generator). 3 l6902d dc/dc converter the l6902d is a complete and simple step down switching regulator with adjustable current and voltage feedback. thanks to its current control loop with external sense resistor it is able to work in a constant current mode, providing up to 1a output current with an accuracy of 5%. among other features there can be also found gen eral purpose 3.3volts precise (2%) refer- ence voltage or 2.5a (typical value) interna l current limit for short circuit protection. dimming of super high brightness leds with l6902d AN2129/0705 rev. 2
AN2129 application note 2/9 in figure 3 is the internal structure of the l6902d converter, the datasheet [3] should be re- ferred for more details. figure 3. l6902d block diagram (see [3] for details) 4 application board an application board using the dimming principl es described above has been designed and its schematic is in figure 5. there is only a si ngle dimming input connector on the board; usable for both dimming methods (either analog or pwm control can be used, as preferred). there were made some changes compared to the appl ication circuit presented in datasheet [3] al- lowing this dimming. first of all, the sense resistor has been moved from higher voltage path (coil output) to the lower one (output ground). then three resistors were added (r4, r5 and r6) for modifying the current sense feedback. a signal between 0 and 3.3v should be used for analog (peak current) dimming. when the dimming pin is grounded (0v) the maximum outp ut current is provided (350ma) and vice versa when 3.3v is applied to the pin, the current provided is zero and so the led is off. there are two more pins on the board: 3.3v reference voltage pin and ground pin (a jumper can be used to connect the dimming pin to the ground pin for the maximum output). for the easiest way of dimming just connect the 10k ? potentiometer between 3.3v and ground pins. the potentiom- eter slider should be connected to the dimming pin (as it can be seen in figure 4). vref good 3.8v comp vfb driver inhibit + - 1.235v e/a 1.235v thermal shutdown - + pwm oscillator out v cc gnd v ref supply trimming frequency shifter voltages monitor d q ck current limiting peak to peak + - current_e/a voltage_e/a cs- cs+
3/9 AN2129 application note figure 4. connecting the potentiometer for analog dimming the second dimming method implemented on this board is a pwm control of average led cur- rent. this control needs a digi tal pwm signal (amplitude can be either 3.3v or 5v) between dimming pin and ground pin. then varying the duty cycle will change the led brightness (100% means led off and 0% means led fully on). with the closer look on the application (figure 5) it is noticeable that cathode of the led must not be connected to the ground of the circuit, because there is a sense resistor between cath- ode and the ground. if by any accident, led cathode is grounded, the current feedback loop will be inactivated and the l6902d will set the maximum output voltage (as set by the voltage divider r1 and r3) regardless the current which can eventually destroy the led. also care must be taken on input voltage polarity together with output led polarity. if the input polarity is twisted, the whole ic could be damaged. while with the output polarity reversed, the board itself cannot be damaged, but the led will see the maximum voltage (as limited by the voltage divider r1 and r3) in reverse direction. figure 5. board schematic (order code stleddcdim-eval1) vcc 8 comp 4 gnd 7 vref 6 out 1 cs+ 2 cs- 3 fb 5 u1 l6902d r5 8k2 r6 27k 1 j4 dimming input + c1 10uf 25v ceramic c4 220pf c3 22nf r2 5k1 stps34ou d1 l1 100uh + c2 10uf 35v r1 9k1 r3 510 1 j6 gnd 1 j3 3.3v vref rsense 0.33 1 2 j1 8 - 24v 1 2 j2 output r4 1k
AN2129 application note 4/9 figure 6. pcb layout table 1. bill of materials the calculation of the resistor current feedback network can look relatively complicated, but with few simplifications it becomes easy to take in. first assumption is that all the current flows only through the r sense (i.e. neglecting voltage drop on the resistors r4,r5 and r6); the value of r sense is defined by the output current and the threshold voltage on cs+ pin (100mv). un- fortunately this calculation will give uncommon values (e.g. for 350ma it gives 0.2857 ? ) thus the nearest higher standard (e.g. e24 series) value for rsense should be selected (e.g. 0.33 ? ) and then the difference between ideal and standard value is compensated by r4, r5 and r6 to receive precise output current. the application is shifting between two limit states with dimming; maximum current (zero dim- ming voltage) and zero current (full dimming voltage). in figure 7, the dimming network with grounded dimming input (equation 1 describes the circuit) is shown, it means when the current flowing through the led is on its maximum (i.e. 350ma on this board). type reference part supplier order code ceramic capacitor c1 10uf n/a tantal capacitor c2 10f; 35v n/a capacitor smd 0805 c3 22nf n/a capacitor smd 0805 c4 220pf n/a schotky diode d1 stps340u stmicroelectronics connector j1 8 -24v n/a connector j2 output n/a connector j3 3.3v vref n/a connector j4 dimming input n/a connector j6 gnd n/a coil l1 100h; 1.2a; 0.33 ? wrth elektronik 744 562 0 resistor smd 2010 rsense 0.33 n/a resistor smd 0805 r1 9k1 n/a resistor smd 0805 r2 5k1 n/a resistor smd 0805 r3 510 n/a resistor smd 0805 r4 1k n/a resistor smd 0805 r5 8k2 n/a resistor smd 0805 r6 27k n/a converter u1 l6902d stmicroelectronics
5/9 AN2129 application note figure 7. dimming network with ze ro dimming voltage (maximum current) eq 1 the second limit state is depicted in figure 8. in this case the current through the rsense is zero (led is off) and thus on point a there is a zero voltage (i.e. ground). the equation 2 shows the calculation for this state. figure 8. dimming network with maximum dimming voltage (zero output current) eq 2 v dimm = 0v 100mv r5 r6 i led r sense ?? ? r4 r5 ? r4 r6 ? r5 r6 ? ++ -------------------------------------------------------------------------- - = v dimm = 3.3v 100mv v dimmax r4 r5 ? r4 r5 ? r4 r6 ? r5 r6 ? ++ -------------------------------------------------------------------------- - =
AN2129 application note 6/9 both equations (equation 1 and equation 2) must be valid together, i.e. two equations for three variables (i led , r sense and v dimmax should be selected before). one resistor must be chosen before and than the other resistors calcula ted from the equations mentioned. this process should be iterative (calculated for different chosen resistors) to get resistor values as close to the industrial standard values as possible. the table 2 can help for work simplification, be- cause it contains resistor values for the most common super high brightness leds. table 2. pre-calculated standard values for feedback loop * iled is a nominal led current obtained with minimum dimming voltage (vdimm=0v) ** vdimmax means dimming voltage for maximum dimming i.e. zero output current (iled=0a) 5 measurement a couple of measurements have been performed on the board; the results are on the graphs below. one up to six leds in serial string ha ve been used as load (golden dragon lw w5sg from osram) in figure 9 there is a led current waveform during dimming with pwm signal at 100hz fre- quency. it could be noticed a waveform rounding during turning-on and off, which is caused by charging the output capacitor c2. if the sharper on and off edges are needed a smaller capac- itor should be used (e.g. 1 f), but on the other side it must be taken in account that it will rise the current ripple. i led [ma]* v dimmax [v]** r sense [m ? ] r4 [ ? ]r5 [ ? ]r6 [ ? ] 350 3.3 330 1000 8200 27000 700 3.3 200 910 2400 20000 1000 3.3 120 910 5600 24000 figure 9. pwm dimming (50%) figure 10. current ripple (1 led, 15v input, 0% dimming)
7/9 AN2129 application note in figure 10 the detail of output current is depicted, where the ripple during all the measure- ment stayed below 5ma, (i.e. less than 2%). and as mentioned above, if a less wavy output is needed, bigger output capacitor should be used, but then a slower on and off edges will ap- pear. efficiency of the converter is processed in figu re 11 and figure 12, where it is showed that more difference between input and output voltage or lower load current, causes lower efficien- cy. for six leds in one serial string (voltage drop around 20volts) and input voltage 25v the efficiency was measured above 93%. figure 11. efficiency vs. input vo ltage (@ 350ma output current) 70.0 75.0 80.0 85.0 90.0 95.0 100.0 0 5 10 15 20 25 30 input voltage [v] efficiency [% ] 1 led 2 leds 3 leds 4 leds 5 leds 6 leds figure 12. efficiency vs. number of leds @ 25v figure 13. output current variation 60 65 70 75 80 85 90 95 100 01234567 leds efficiency [%] 300 310 320 330 340 350 360 01234567 leds output current [ma] increasing the number of leds in series in one string (on figure 13) a lower output current can be observed (for six leds it is 341ma inste ad of 350ma). that means less than 3% difference, what should be still acceptable especially considering 5% prec ision of the current sensing am- plifier in l6902d.
AN2129 application note 8/9 the average value of the output current during dimming is depicted in figure 14 and figure 15. almost ideal dimming curve can be observed du ring digital control (figure 15). on the an- alog dimming curve (figure 14) it can be seen th at current is already zero for 3.1v in place of 3.3v. this behavior is caused by the use of in dustrial resistances (e24 values) instead of the exact values calculated from equation 1 and eq uation 2 and it allows to have led safely off when maximum dimming voltage is applied. figure 14. output current during analog dimming figure 15. output current during digital dimming 6 references and related materials [1] an1891 - application ideas: driving leds using l497x, l597x, l692x dc-dc converters families [2] an1941 - low voltage led driver using l6920d, l4971 and l6902d [3] l6902d datasheet 7 revision history table 3. revision history date revision description of changes 02-mar-2005 1 first issue 05-jul-2005 2 corrected the eq. 2 to page 5/9
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