ST AN2129 Application note handbook
Contents
1. 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 instead of 350mA That means less than 3 difference what should be still acceptable especially considering 5 precision of the current sensing am plifier in L6902D ky 719 AN2129 APPLICATION NOTE The average value of the output current during dimming is depicted in Figure 14 and Figure 15 Almost ideal dimming curve can be observed during digital control Figure 15 On the an alog dimming curve Figure 14 it can be seen that current is already zero for 3 1V in place of 3 3V This behavior is caused by the use of industrial resistances E24 values instead of the exact values calculated from Equation 1 and Equation 2 and it allows to have LED safely off when maximum dimming voltage is applied Figure 14 Output Current during analog Figure 15 Output current during digital dimming dimming Iren mA Ilen mA T T T 1 5 2 2 5 Vdimm V T T T 1 0 20 40 60 80 100 120 Duty Cycle 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 Desc2. Lit vil it M 2 0ms 5 0MSis A Ch2 172mA IN T L EAEE DERNE Bw 200nsipt eared Perr Ped Te A S A EEE B mn Ch2 10 0mA Q By M 4 0ps 2 5GS s 400ps pt A Ch2 342mA iti E eres Ch2 100m Q 4 6 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 Figure 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 voltage 350mA output current 100 0 Efficiency foe ol O 70 0 T r 1 0 5 10 15 20 25 30 Input Voltage V e1LED 2 LEDs 3LEDs 4 LEDs 5 LEDs e 6 LEDs Figure 12 Efficiency vs number of LEDs Figure 13 Output current variation 25V 100 360 95 ya 350 ae a T A X 5 340 gt Fa o o g 80 5 330 o E 75 y 6 W 2 320 70 65 310 60 o 0 1 2 3 4 5 6 7 f 0 1 2 3 4 5 6 7
3. SHUTDOWN PEAK TO PEAK CURRENT LIMITING vB P 5 tL OSCILLATOR FREQUENCY SHIFTER 4 Application Board An application board using the dimming principles described above has been designed and its schematic is in Figure 5 There is only a single 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 application 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 OV the maximum output 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 10kQ 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 4 219 ky AN2129 APPLICATION NOTE Figure 4 Connecting the potentiometer for analog dimmi
4. AN2129 ky APPLICATION NOTE DIMMING OF SUPER HIGH BRIGHTNESS LEDS WITH L6902D 1 Introduction Thanks to the high efficiency and reliability super 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 like 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 converters 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 output 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 usually a simple potentiometer is enough Figure 1 Analog current control Figure 2 Average current contro
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6. l by PWM time 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 microcontroller 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 general purpose 3 3Volts precise 2 refer ence voltage or 2 5A typical value internal current limit for short circuit protection Rev 2 AN2129 0705 1 9 AN2129 APPLICATION NOTE 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 VOLTAGES MONITOR INHIBIT SUPPLY as Current_E A p Q THERMAL gt
7. mils 46 83 mm ial MF E gt 350mA L 1350 mils 34 29 mm LT Super High Brightness LED Dimming Table 1 Bill of materials Type Reference Part Supplier Order Code Ceramic Capacitor C1 10uF N A Tantal Capacitor 10pHF 35V Capacitor SMD 0805 22nF Capacitor SMD 0805 220pF Schotky Diode Connector 8 24V Connector Daan Input Connector GND N A Coil 100uH 1 2A 0 339 W rth Elektronik 744 562 0 Resistor SMD 0805 R4 1k Resistor SMD 0805 Resistor SMD 0805 Resistor SMD 0805 Resistor SMD 0805 Converter U1 L6902D STMicroelectronics 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 Rsense i e neglecting voltage drop on the resistors R4 R5 and R6 the value of Rsense 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 2857Q thus the nearest higher standard e g E24 series value for Rsense should be selected e g 0 330 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 a
8. nd 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 4 9 ky AN2129 APPLICATION NOTE Figure 7 Dimming network with zero dimming voltage maximum current 2 goi R4 cs 3 R6 R5 Rsense a Dimming Input Vdimm OV R5 R6 1 R 100mV LED sense Eq 1 R4 R5 R4 R6 R5 R6 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 CS4 R4 cS 3 R6 R5 Rsense Dimming Input V dimm 3 3V R4 R5 100mV Vdimmax R4 A5 R4 R6 A5 R6 ae 5 9 AN2129 APPLICATION NOTE Both equations Equation 1 and Equation 2 must be valid together i e two equations for three variables ILED Rsense and Vgimmax should be selected before One resistor must be chosen before and than the other resistors calculated from the equations mentioned This process should be iterative calculated for different chosen resistors to get resi
9. ng The second dimming method implemented on this board is a PWM control of average LED cur rent This control needs a digital 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 STPS340U 8 24V CS 22nF 25V CERAMIC 3 3V Vref Dimming Input 3 9 AN2129 APPLICATION NOTE Figure 6 PCB layout 2 1844
10. ription of Changes 02 Mar 2005 1 First Issue 05 Jul 2005 2 Corrected the Eq 2 to page 5 9 8 9 4 AN2129 APPLICATION NOTE The present note which is for guidance only aims at providing customers with information regarding their products in order for them to save time As a result STMicroelectronics shall not be held liable for any direct indirect or consequential damages with respect to any claims arising from the content of such a note and or the use made by customers of the information contained herein in connection with their products Information furnished is believed to be accurate and reliable However STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics Specifications mentioned in this publication are subject to change without notice This publication supersedes and replaces all information previously supplied STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics The ST logo is a registered trademark of STMicroelectronics All other names are the property of their respective owners 2005 STMicroelectronics All rights reserved STMicroelectronics group of companies Austral
11. stor 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 mA Vaimmax V Rsense MQ R4 O R5 O R6 2 350 3 3 330 8200 27000 1000 3 3 120 910 5600 24000 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 ak 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 have 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 1uF but on the other side it must be taken in account that it will rise the current ripple Figure 9 PWM dimming 50 Figure 10 Current ripple 1 LED 15V input 0 dimming Tek Run Sample 8715 Acgs 11 Aug 04 09 33 49 pot ae a pert pt Tete T Tek Stopped 24980 Acgs 12 Aug 04 11 46 50 RE merre Anna ES T HA H 4 ee ie HHH ant T HEHH HHH
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ST AN2129 Application note handbook