78K0/IB2 - Renesas Electronics
Contents
1. 10 A SSI NG UNS NENNT rt 11 2c NANI NOMD Or LEDS m un tees 12 2 Ghanoine LED Cte I TR T 12 4 3 Adjusting Buck Converter Ton Toft 12 5 Triac Controlled Dummmg 13 6 Benefits to Using 78K0 Ix2 for Triac Controlled Dmmmmg 15 7 Triac Dimmable LED Drive Reference Uesgn 16 NECI e m E 17 t DO 19 7 3 Over current Protection ccccccseccccesceeceeeceececeeeeeceeeeeceaeeeseeessaeeessaeeeseaeeeseueeseaeesseeesseeeesaneesseas 20 TAH Performance Evaluation RE WEE 22 Lo Ee LN 24 0e ROION mL 27 9 1 LED ODrHVEITSOIOFel I BS usi icc dn uode Ia 27 FREVISION ROCO MT S 29 General Precautions in the Handling of MPU MCU Products ee 30 RO1ANO520EU0100 Rev 1 00 Page 2 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide 1 Overview 1 1 LED Drives Ordinary LEDs used in signaling applications are simple low voltage low current device that do not have enough light output to compete with incandescent or fluorescent light sources A special line of high brightness LEDs can run at much higher currents and with the aid of special lenses and phosphor coatings can deliver quality light that is brighter and more uniform than previously mentioned sources Compared with 10 20mA forward curren2. 25 5 8717 10 2223uH Tripple 0 1 Minimum inductor value is chosen to be the maximum of the two 10 Lmin 2235 1 1 2458uH 3 5 Ton Toff Adjustment Will select a standard 2 2 mH inductor which will give us slightly higher frequency and will calculate Ton and Toff for the buck converter PWM drive signal based on Lmin L 10 L 2200uH Lmin L 0 9 21980uH _ Lmin ripple _ 1980 0 1 10 Ton 1138us VLon 174 Ton 1138 25 46 cycles e 6 Toff Lmin ripple 1980 0 110 L 776515 VLoff 25 5 Toff 7765 25 311 cycles Ton and Toff are expressed as multiples of in 25ns clock cycles and will be the values written to timer TMX1 compare registers Ton and Toff limit the output voltage The minimum output voltage of the buck converter is also limited by the delay in the current sense circuit and internal comparator delay filters The total time delay will be Toff delay in the current sense circuit The typical current sense circuit delay is 300ns With Toff value calculated in this design example the minimum time transistor T is on we can calculate the minimum voltage at the output of the buck converter Toff 311 cycles 25ns 311 7765ns Ton minimum 300ns due to delay in current sense circuit Duty 300ns 300ns 7765ns 3 7 Vout Vin DUTY 200V 3 7 7 4V CAUTION In this case this minimum voltage of 7 4 V is critical and limits the minimum number of LEDs we can drive If the added for
3. increase ripple current or increase PWM Frequency target L off 1 87E 03 Henries L pick 1 87E 03 Henries L min DEED Calculated minimum inductor value with 10 tolerance Rsense watts 0 294 Watts Check Inductor Value L selected 2206 03 Henries Selecta real inductor based on Limin in B26 Litolerance 10 Inductor Tolerance Limin 2 E 03 Henries PWM Tanimax 3 04E 06 Seconds dt ldiv 25ns Cycles Enter as BUCK TON MAK BUCK TOFF this value PWM Toff 5 50E 06 Seconds dt ldi v PWM Tonttyp 5 13E 07 Seconds dt ldi v PWM Freq 1535 8 50 Hz 0 35483871 Max Duty 139 16123 Max Vout 38 25ns Cycles Enter as BLICK TOFF value Figure8 Buck Calculator RO1AN0520EU0100 Rev 1 00 Page 11 of 28 May 13 2011 RENESAS 78KO IB2 Triac Dimmable LED Drive Design and User Guide 4 1 Changing Number of LEDs When changing the number of LEDs PWM Ton and PWM Toff need to be recalculated and this can be easily done with the excel calculator buck_calculator xls by simply plugging in the new number The sample code developed to support this application note has two defined values BUCK_TOFF and BUCK_TON_MAX used as load values for timer TX1 compare registers to generate the PWM signal driving the buck switch QI BUCK_TOFF corresponds to Toff and PWM TON MAX corresponds to PWM Ton PWM Toff The minimum number of LEDs for the resulting PWM Toff will be limited by the minimum forward voltage drop as shown in the desig
4. 16 Demonstration unit At the low end of the dimming range when the load is very light flicker may occur due to the fact that the load current may be lower than the triac holding current To eliminate the flicker turn on the load switch located at the back side of the enclosure This will connect a light load of 10 K across the dimmer output and line neutral RO1AN0520EU0100 Rev 1 00 Page 19 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide 7 3 Over current Protection The board is protected against over currents through a 0 5 ohm sensing resistor Rs placed in the return path of the common ground The negative voltage across the resistor is summed with an external reference voltage Vref and amplified by the internal OP Amp as seen in Figure 17 Over current protectionThe output of the OP Amp is connected to the input of the internal comparator CMPI DC BUS Rectified e AC Input o I LED L Y D ki Rs AS LED Current 0 5 Sensing Zero current Shunt detection Resistor TMX10 CMPO Vs Dimming R3 R4 AMP o gt CMP1 IC 78KO 1B2 Internal Vref Figure 17 Over current protection When the comparator input falls bellow an internal reference voltage the comparator output trips the high impedance line and disables both the PFC and the Buck timer output The equations used to calculate the over current trip value are the ones for an inverting summing
5. Billings Switchmode Pover Supply Handbook 2005 McGraw Hill e Robert W Erickson DC DC Power Converters University of Colorado Department of Electrical and Computer Engineering Article in Wiley Encyclopedia of Electrical and Electronics Engineering RO1AN0520EU0100 Rev 1 00 Page 27 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide Website and Support Renesas Electronics Website http www renesas com Inquiries http www renesas com inquiry All trademarks and registered trademarks are the property of their respective owners RO1AN0520EU0100 Rev 1 00 Page 28 of 28 May 13 2011 RENESAS Revision Record Description Rev Date Page Summary 1 00 May 13 11 First edition issued General Precautions in the Handling of MPU MCU Products The following usage notes are applicable to all MPU MCU products from Renesas For detailed usage notes on the products covered by this manual refer to the relevant sections of the manual If the descriptions under General Precautions in the Handling of MPU MCU Products and in the body of the manual differ from each other the description in the body of the manual takes precedence 1 Handling of Unused Pins Handle unused pins in accord with the directions given under Handling of Unused Pins in the manual The input pins of CMOS products are generally in the high impedance state In operation with an unused pin in the open circuit s
6. PWM duty cycle can be adjusted to maintain a certain value Figure 4 DC DC Boost Converter RO1AN0520EU0100 Rev 1 00 Page 5 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide If the load is nonlinear or inductive the current seen at the AC input side will lag behind the voltage and it may also be distorted with large peaks causing a power factor of less than 1 Close to unity power factor is desirable to reduce harmonic distortions that can creep back into the line as well as to eliminate the excessive currents that lead to overheating of the supply lines which result in power losses To achieve this the DC DC boost circuit switch can be controlled in such a way that the current stays in phase with the voltage and the distortions are eliminated There are three common methods of controlling the current through the load based on the time the switch is held off after it was previously turned on to allow the current build up through the inductor One method called the continuous conduction mode CCM will turn the transistor off for a very short time so that the current never drops to 0 The second method is the critical conduction mode CRM in which the transistor is off until the entire energy stored in the inductor is discharged into the capacitor and the current decreases to 0 at which time the transistor is turned on again and the cycle repeats The third method called the discontinuous conduction mode le
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8. a practical example on how to calculate Toff and maximum Ton The number of LEDs we chose to drive for this example will be 7 at a constant current of 350 mA and a typical forward voltage drop of 3 5V The buck input voltage is 200V and the inductor value is 1mH The acceptable ripple current will be 100mA 3 1 Design Data Vboost 200 V buck input DC voltage Rds on 1 2 ohm MOSFET drain source on resistance Iled 0 35 A LED constant current Iripple 0 1 A acceptable LED ripple current Vf LED 3 5 V LED forward voltage at rated current of LEDs 7 number of series connected LEDs Vf LED chain 24 5 forward voltage of LED chain Vd IV forward voltage of flyback diode Rsense 2 4 ohm current sensing resistor F 100000 Hz target frequency 3 2 Buck Converter Target PWM Duty Ton and Toff Duty o A fe oS El 212 834 Vboost Rds Iled Rsense Iled 200 1 2 0 35 2 4 0 35 Ton I NE 1283ns F Duty 100000 Ton cycles Ton 25 5l cycles Toff x Ton 10000ns 1283ns 8717us Toff _ cycles Toff 25 349 cycles 3 3 Inductor Voltage VLon and VLoff VLon Vboost Rds Iled Rsense Iled 200 1 2 0 35 2 4 0 35 174V VLoff Vf Vd 24 5 1 25 5V RO1AN0520EU0100 Rev 1 00 Page 9 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide 3 4 Inductor Calculation VLon Ton 174 1283 10 Lon 2235uH Tripple 0 1 9 io VLoff Toff
9. can be found at the end of this document The board is a compact 2 x 2 50 8 mm x 50 8 mm 4 layer PCB populated on both sides The code is programmed into the MCU and it only needs the input AC power the triac and the LEDs connected CAUTION Users of this board must be aware that high voltage up to 450V DC is present and there is a high risk of electric shock if the board or any components are touched The design topology is non isolated meaning that the LED output is directly connected to the high voltage DC bus The low voltage ground for the MCU and digital components is common to the high voltage ground and it is connected to the AC neutral i e Power Line referenced Do not touch any of the components on the board while the AC power is applied Debug port Buck inductor LED connector AC connector 78K0 IB2 Bulk capacitor Figure 13 Triac Dimmer Reference Design To download or to debug the code a custom 6 pin connector was used To connect the MINICUBE2 we made a small interface PCB shown in Figure 14 below Please note that the debug port is not isolated and connecting MINICUBE2 to the PC while the AC power to the board is applied may result in damage to the PC and it also poses the risk of electric shock Use a USB isolation hub such as the B amp B UISOHUBA when debugging or flash programming this board RO1AN0520EU0100 Rev 1 00 Page 16 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and Use
10. mA Efficiency 96 Table1 Test Results The total system efficiency is greater than 8496 even at 5096 dimming levels and the power factor exceeds 0 98 at 10096 For 90V AC input the power factor is greater than 0 9 at 50 dimming and greater than 0 87 at 50 dimming for 115VAC and 130VAC input voltages RO1AN0520EU0100 Rev 1 00 Page 23 of 28 May 13 2011 RENESAS Triac Dimmable LED Drive Design and User Guide 78KO IB2 7 5 Schematic 9jD2365 wes oo DH Share 95D2 9 Ping pexceug Ag 30 Ap posnasj3 mv MOHIKGS AU emp OL Zz v0 Yaliyo SOINOMNIO2313 23N BOS ch 1M 8 O0N 8 LE Has xong 4 i Ce e SEA GC Era CH CZ HALWE oud 4 gege n tta quo T T fad JT M T M3 dT Mg Vd C 3 ig LRA ELI sg Gti Jipa id iY LEJ l ERIE 12384 tei TSAOS Duk Ap pawsau5 GELDE i 204 uyr Waya KAYEO gS DE d pov 6606 XXd XX XXi XXM XXd XXN KA Za FHA Sg NO TL BIET O n u GI FS co 3B dn Fd iM xol Kd LAE gto Giro Ped ws ite LOOL EAMEL Ed us ene g LIDL ty Dx Ed eA Ld rM Eg d o LZ Ed Sen Du ERLL DOR EL Ll SECH 33651 12 Ea GEO IS EX EE Pd 13535 PAIR Ee BIS OW IS Ed IEN OL OVIS OA SSA Oe Ea HY I a cy 145 EE vHY ELT ep Lo Od Bhd 108 CE WHIA KOR WA amp iH r id AE TITHI CC L Page 24 of 28 rCENESAS RO1ANO520EU0100 Rev 1 00 May 13 2011 Triac Dimmable LE
11. D Drive Design and User Guide 78KO IB2 Wana AHI SJ3Jlidd S 30 15008 OVIML am ava Loop 94d v Old JN v SOINGDHLOA ISA JAN ANS ddi AG Adel AZL aep i AGE a Te Se E ee PH ews e rs e LE 5T i rav us L 5 5 ie Ke 1 CH 5 5 C En um ARI Vi nU E ID0EBEG1 5 ei im HDIIAS 244 gt ALIROJ ug A vw Gun in ro i La 2 i E l I ge DER v aig am S bd Ld AS AGE AZ Azi J GKG ui ar GA E Gig EH AGE B JF d T wv c s au II ARE aT Di MAC ep eg j5H3EA Jid EIS aee Bi A i DH ENIS LIA e 2 wep xL aT est CT T m E Ko ta ele HA oci 83 d zEUb BiU Y vv Je Stree rn um i c pe LEGURA Eg 3H A FEE n 80 WOM DL z E g aT san AI Aij eno 1S00B8 24d ri pel Ke WELLES LUA He e t ong chu n a ra E 4 siu a i E IGET 3 S Y ge d A Gay DURO FZEAM Y an 3n L KA BEZ DND Alor cou LI iz ra ja pron LECKHLA KL Uz a PER WEEZELOAR I HE RE E WEG Eu ap L CCS Wii EEN 2 gn 2 iy 7 ni Mire ach er X wa ApH HOEE I i po ia WW eis ui B LH2 wore eb 7777 a j KEE iH z e NW omms DE JA Lan F n a Di en D e pm cm c Anen na DN Y IN2 Page 25 of 28 44 NEC SAS RO1ANO520EU0100 Rev 1 00 May 13 2011 Triac Dimmable LED Drive Design and User Guide TE at JeAlgJ aam 22 03 vV2ld 3v SOINDYLIA l3 JAN Sa na Wine H
12. Ero TEE TXOCR1 nterlock li riot PFC off pulse l pos I RS MOMS TXOCRO CMP2 4 Internal PFC on pulse width Zero current RE E detection Triac angle TMOO decoder TI 00 Figure 5 PFC Implementation with 78K0 IB2 RO1AN0520EU0100 Rev 1 00 Page 6 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide The CRM algorithm uses a constant ON time but to maintain a constant boost voltage on the output the ON time needs to be adjusted in a control loop With the Triac varying the voltage on the input Ton will be larger at low levels and shorter at the higher levels The load presented by the LED Buck driver will also lead to variations in the ON time The initial ON time is selected for 120V nominal input voltage About every 2ms the PFC boosted voltage is measured and the on time is adjusted up or down The aux winding of the PFC inductor detects when current falls to zero using comparator 2 When zero current is detected by the voltage going below 0 3V for 100ns timer T XO is restarted turning on the PFC switch for the on time pulse As a safeguard a 0 05 ohm sense resistor is used in the ground return path to the bridge rectifier The sensed voltage is amplified by the on board Op Amp and the output of the Op Amp is used to trigger comparator 1 which is configured to set timer TKO and TX1 outputs to Hi Z External circuitry makes certain the PFC and Buck switches are off when the microcontrol
13. IH awd TU own kA SRC F DO oM tayo Pop Of IN N 4490 fil ZC ISHE wind K HS pk JSHISENYS n H p Dem i m 5 H2 57 We Page 26 of 28 434 NC SAS 78KO IB2 ALOU St E H zh im Eng Ba 231 L Am 7 bb T ASI 154 P Hi F gt d an d Ja 8 an 2 nega bk 0 v EN3 ls a 931 A T 1 5DCBA Yana 031 zs jd 558ur c waeGlei ISL An ld 0S9HREZ6 OZE b IS bi RO1ANO520EU0100 Rev 1 00 May 13 2011 78K0 IB2 Triac Dimmable LED Drive Design and User Guide 8 References 78K0 IX2 User s Manual Hardware RO1UHOO10EJO400 Applilet EZ for HCD Controller User Manual U19178EJ5 VOUMOO 8 Bit MCUs for Lighting U9560EU2VOPBOO Application Notes e Controlling Fluorescent Lamp Ballasts by using 78KO Ix2 U19665EJ2VOANOO e Controlling High Brightness LED by using 78K0 Ix2 U19666EJ1VOANOO e DMX512 78KO Ix2 Series Lighting ASSP U19885EEIVOANOO Demonstration Kits e 78KO Ix2 LED Control Demonstration Kit for NEC Electronics 78KO Ix2 Lighting ASSP Microcontroller U19935EE1 VOUMOO e Lighting Communications Demonstration Board for Lighting communication data protocols U19936EE1 VOUMOO DevCon 2010 A17C Powering driving and dimming high brightness LEDs with Renesas lighting microcontrollers available on DevCon website 8 1 LED Driver References e Steve Winder Power Supplies for LED Driving 2008 Elsevier Inc www elsevier com e Keith
14. Nn Ho ANE AE jj Fie a Se ee Se aa i me e dap ese M i is i Measure Pi TEA Pakia Para P4 P5 PBwidthiF 1 value 47 99693 kHz 1 6519 status At 1 Tt Timebase 40 0 paT wWSteam 20 0 us 100M5 50 GS s Edge Figure 9 Buck Ton too short Ton has to be adjusted so that the inductor can charge up in one pulse as seen in Figure 10 below The first pulse is wider because we allow a larger Ton The subsequent pulses are narrower as the comparator threshold is reached LeCroy j j ci d peel a pmo rei tnis Head D ue s Enron a my Les edet eet Ai amt pia e ei Delt 7 i Measure PI TEA FPZ pkpkici Dimmer P4 P5 Pa widthtF 1 value 33 085113 kHz 1 619 status m Tt 1r Timebase 30 0 us 10 0 us Stop 5 0 G amp s Edge 310 mv Megative Stream 500 kS Figure 10 Buck Ton adjusted Other practical consideration for choosing the right values for Ton and Toff is to allow the buck frequency to be as low as possible to improve efficiency and reduce the switch transistor and fly back diode heat dissipation 5 Triac Controlled Dimming The Triac dimming level set by the dimming knob needs to be decoded and used as the reference input for the LED dimming The 16 bit timer TMOO with a 78 125kHz clock measures the positive pulse width on the AC sense line at input pin TIOOO on the microcontroller To implement dimming based on the Triac s coduction angle a unique timer interl
15. Ton on cycle by cycle basis as the comparator detects the maximum allowed current through the LEDs This cycle by cycle current limiting during Ton will result in a constant average current through the LEDs One of the basic requirements for this scheme to work is to have Ton set large enough to allow the comparator to trip before Ton expires To help with the design we implemented a buck converter calculator tool in Excel called buck calculator xIs The calculator takes in the data shown in the design example and generates Ton and Toff in 25ns clock cycles for timer TX1 The excel calculator page shown in Figure 8 bellow can be provided along with the sample code Units Description LED Volts Boost Voltage y Ohms Buck mosfet on state resistance Yhonzi Amps Average current through LEDs Percent Ripple current peak to peak far inductor Lbu 0 10000025 Amps Lbuck ripple current calculated Volts Forward voltage of single LED in array Count Number of LEDS in array 35 Volts LED forward voltage calculated Vdbuck Volts Buck diode forward voltage at Iff LED Rsense Ohms Value of sense resistor PWM Frequer Hz Target PWM frequency Calculated Values do not enter anything below this line Duty 8 1195 Percent PWM Ton 5 21E 07 Seconds Rdc Iripple Lbuck Rds on Ton Check PWM Toff 5 19E 06 Seconds 24 VL on 3 59E 02 Volts VL off 36 Volts L on 1 87E 3 Henries TIP For lower inductance value
16. amplifier RO1AN0520EU0100 Rev 1 00 Page 20 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide Vout Ven La Yee uM a x 14 RA R1 R2 R1 R2 R3 Design example for Vin 115VAC Pout 20W LED current 350mA R1 R3 1K R2 RA 15K Efficiency n 85 At full load the current through the sensing resistor is P 2 Is out 0 A Vin an 115 0 85 Where Is 1s the current through the shunt resistor Vin is the input AC voltage The voltage on the sensing resistor in reference to GND is Vs Rs Is 0 5 0 204 0 102V Solving Vout for Vref 2 5V reveals Vout 0 102 M 25d ia 0 97 V 16 16 1 We can choose the over current trip value to be 1 5 times Zs for full load Recalculating Vout reveals Vout OC 0 153 ae 2 5 L gu Pm 0 205 V 16 16 l The Op Amp output is fed into comparator 1 input CMP1 We will set the internal reference voltage to 0 200 V which will be the trip value for the over current shutdown If we recalculate for 220V AC input the values found will be Vout 1 735 V Vout OC 1 35V The comparator threshold can be set to 1 35V If we increase the input voltage to 265 VAC the values will be Vout 1 86V Vout OC 1 54V The comparator threshold can be set to 1 55V which is still bellow the maximum allowable setting of 1 6V for the internal reference voltage With the resistor values chosen we can cover both 115VAC a
17. capabilities Although the LED is a very simple low voltage low current device to make it compete with its predecessors it needs a fair amount of other LEDs working together under the control of relatively complex electronics circuits The key to getting the maximum light output of and LED is to drive it with a constant current that is right at or bellow the rated spec Providing this constant current can be accomplished by simply using a series resistor but this is not what an energy efficient solution should look like Instead a full on off current chopping method with a resulting average current through the LED is used to get maximize efficiency while making sure that the LED works in a save operating region Target Devices 78K0 Ix2 Family RO1AN0520EU0100 Rev 1 00 Page 1 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide Contents DAEEC 3 P LED RN 3 SCH lun un EEE E E EE E EEE E EE 3 2 Microcontroller Based Triac Dimmable LED Drve 4 2 2 Power Factor Correction PFC DC DC Boost converter 5 2 3 DC DC Buck Converter Constant Current LED Drmve 7 SEEN erimus eowcEE 9 Me SUC A e Tc 9 3 2 Buck Converter Target PWM Duty Ton and Tote 9 3 3 Inductor Voltage d Beie ECH le Vi aaa aaa 9 34 Wie e de e TU te EE 10 29 TOn Ken AdUS ilie r
18. ectively vary the Triac conduction or phase angle and the resulting average AC voltage across the light bulb RO1AN0520EU0100 Rev 1 00 Page 3 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide Figure 2 Triac Conduction Triac dimmers also called wall dimmers are widely used in residential and commercial buildings with incandescent light bulbs but they present serious limitations and challenges when used with alternative lighting solutions such as CFLs or LEDs The biggest problem LED drive designers face is the fact that the load has to draw minimum current to maintain the Triac in conduction LED light engines draw a fraction of the power incandescent light bulbs draw causing the Triac to shut off during the conduction cycle resulting in visible light flicker To maintain a minimum current through the Triac often additional resistive loads are used but of course this increases power consumption and lower s the overall efficiency the LED lights are meant for Another problem Triacs have is the susceptibility to the switching noise the drive introduces causing shut off and flicker on the output Effective ways to deal with this is to provide adequate filtering As it will be seen in the following chapter LED drives are electronic circuits containing power switches and inductors seen from the AC mains side as nonlinear loads with poor power factor One of the basic requirements utility companies mandate fo
19. ent of Renesas Electronics 12 Please contact a Renesas Electronics sales office if you have any questions regarding the information contained in this document or Renesas Electronics products or if you have any other inquiries Note 1 Renesas Electronics as used in this document means Renesas Electronics Corporation and also includes its majority owned subsidiaries Note 2 Renesas Electronics product s means any product developed or manufactured by or for Renesas Electronics 24 NE SAS SALES OFFICES Renesas Electronics Corporation http www renesas com Refer to http www renesas com for the latest and detailed information Renesas Electronics America Inc 2880 Scott Boulevard Santa Clara CA 95050 2554 U S A Tel 1 408 588 6000 Fax 1 408 588 6130 Renesas Electronics Canada Limited 1101 Nicholson Road Newmarket Ontario L3Y 9C3 Canada Tel 1 905 898 5441 Fax 1 905 898 3220 Renesas Electronics Europe Limited Dukes Meadow Millboard Road Bourne End Buckinghamshire SL8 5FH U K Tel 44 1628 585 100 Fax 44 1628 585 900 Renesas Electronics Europe GmbH Arcadiastrasse 10 40472 Dusseldorf Germany Tel 49 211 65030 Fax 49 211 6503 1327 Renesas Electronics China Co Ltd 7th Floor Quantum Plaza No 27 ZhiChunLu Haidian District Beijing 100083 P R China Tel 86 10 8235 1155 Fax 86 10 8235 7679 Renesas Electronics Shanghai Co Ltd Unit 204 205 AZIA Center No 1233 Lujiazui Ring
20. forward voltage of 3 5 V per LED the maximum number of LEDs we can drive at 600 mA is 20 0 6 3 5 which is 9 To drive LEDs with higher currents that exceed the inductor current rating not only the inductor needs to be changed but it may also require a higher current recovery diode as well as a higher current boost inductor 4 3 Adjusting Buck Converter Ton Toff From the design example above Toff is 311 cycles and Ton is 46 cycles resulting in a switching frequency of 112 3KHz If we consider 90 efficiency these numbers will have to be increased by at least 10 to give us Toff 342 and Ton 51 The PWM dimming method described in the following chapter 5 uses timer TMH1 PWM output to gate buck timer TX output As it can be seen in the waveforms in Figure 9 and Figure 10 below Ton needs to be further increased to ensure proper inductor charge up after each dimming cycle Figure 9 shows the waveform on the buck sensing resistor After each dimming cycle the current through the inductor has to rise from 0 to 350 mA Iripple 2 50 mA If Ton is too short it takes multiple pulses to charge up the inductor This will most likely produce flicker on the output when the dimming is at the lower range RO1AN0520EU0100 Rev 1 00 Page 12 of 28 May 13 2011 RENESAS 78KO IB2 Triac Dimmable LED Drive Design and User Guide 3 Ki LL E E SONS ee ee Dou i i i H 3 4 a i mha H
21. ingement of patents copyrights or other intellectual property rights of third parties by or arising from the use of Renesas Electronics products or technical information described in this document No license express implied or otherwise is granted hereby under any patents copyrights or other intellectual property rights of Renesas Electronics or others You should not alter modify copy or otherwise misappropriate any Renesas Electronics product whether in whole or in part Descriptions of circuits software and other related information in this document are provided only to illustrate the operation of semiconductor products and application examples You are fully responsible for the incorporation of these circuits software and information in the design of your equipment Renesas Electronics assumes no responsibility for any losses incurred by you or third parties arising from the use of these circuits software or information When exporting the products or technology described in this document you should comply with the applicable export control laws and regulations and follow the procedures required by such laws and regulations You should not use Renesas Electronics products or the technology described in this document for any purpose relating to military applications or use by the military including but not limited to the development of weapons of mass destruction Renesas Electronics products and technology may not be used for or incorporated int
22. it until the target clock signal has stabilized When the clock signal is generated with an external resonator or from an external oscillator during a reset ensure that the reset line is only released after full stabilization of the clock signal Moreover when switching to a clock signal produced with an external resonator or by an external oscillator while program execution is in progress wait until the target clock signal is stable Differences between Products Before changing from one product to another i e to one with a different type number confirm that the change will not lead to problems The characteristics of MPU MCU in the same group but having different type numbers may differ because of the differences in internal memory capacity and layout pattern When changing to products of different type numbers implement a system evaluation test for each of the products Notice All information included in this document is current as of the date this document is issued Such information however is subject to change without any prior notice Before purchasing or using any Renesas Electronics products listed herein please confirm the latest product information with a Renesas Electronics sales office Also please pay regular and careful attention to additional and different information to be disclosed by Renesas Electronics such as that disclosed through our website Renesas Electronics does not assume any liability for infr
23. ler outputs are Hi Z Another key element for success is the size of the capacitor between the bridge rectifier and the PFC circuit When the Triac switches on at 50 dimming level a voltage spike of 170V at 120V AC is presented to the bridge rectifier circuit from the Triac output To maintain power factor across dimming it is important to keep this capacitor size as small as possible balancing noise levels with inrush current One indication that the capacitor is too large will be excessive heat in the bridge rectifier and inrush limiting resistor in the power supply 2 3 DC DC Buck Converter Constant Current LED Drive A Vpcin PWM T R By Comparator 0 Current sensing Figure 6 DC DC Buck Converter The buck constant current LED driver uses comparator 0 and a sensing resistor R to set a typical current through a chain of LEDs on the output The resistor value is chosen so that the voltage drop generated by the average current is within the 0 05V to 1 6V threshold range of the comparator For 1 ohm resistor value for example and a typical 350mA LED current this would be 0 35V To support higher or lower LED current with a given R we just need to change the comparator threshold voltage Resistor R should be as small as possible to reduce the losses The implementation of the DC buck converter with the KO IB2 is shown in Figure 7 below RO1AN0520EU0100 Rev 1 00 Page 7 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Dri
24. m CERN ESAS APPLICATION NOTE 78K0 IB2 RO1AN0520EU0100 Rev 1 00 Triac Dimmable LED Drive Design and User Guide May 13 2011 Introduction This application note provides a guide to designing Triac dimmable High Brightness LED drives with Renesas Electronics 8 bit 78KO IB2 microcontrollers MCUs This document includes e Overview of LED drives e Overview of Triac dimmers e Triac dimmable LED drive solution example e Reference design example with Renesas 78K0 IB2 microcontroller LED lighting is one of the most promising solutions to deal with the increasing energy demands as natural resources around the world are becoming more expensive and less viable to sustain the needs of ever increasing populations around the world The awareness of the impact energy production has on the environment as well as the continuous strive of human kind to make things better and more efficient caused an unprecedented concentration of R amp D efforts and capital investments in the development of the LED lighting industry Compared with the traditional incandescent light bulbs LED light engines are up to 80 more efficient in terms of light output and 10 times longer lasting Compared with other energy efficient lighting solutions such the compact fluorescent lights CFL or high intensity discharge HID lamps which contain harmful substances like mercury LED lights are less of a threat to the environment and offer far better color management and dimming
25. mming range can be set for all different Triacs we use In addition the Ix2 provides connectivity options through standard microcontroller interfaces such UART SPI or IIC to allow the control of the LEDs from a remote location or as part of a network such as ZigBee or power line communication The Ix2 supports the DALI protocol through its on chip dedicated DALI communication interface and DMX12 can be easily implemented through the UART port Dimming for example can be controlled from any of the listed network options through the same PWM method described in the application note but also from standard 0 10V DC devices through the ADC interface The same ADC can be used to connect temperature or occupancy sensors that can report back to a network controller In conclusion a Renesas 78KO Ix2 based LED drive solution expands the possibilities far beyond just replacing the light bulb with a more efficient one It can sense the environment think for itself and interact with other lights and systems to allow better and even more efficient management of energy resources RO1AN0520EU0100 Rev 1 00 Page 15 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide 7 Triac Dimmable LED Drive Reference Design The following chapters describe the reference design that was built based on the design guide we developed in the previous chapters The board picture is shown in Figure 13 Triac Dimmer Reference Design and the schematic
26. n example above If we use the value of 7 76 us from the design example above the minimum forward voltage will be 7 4V resulting in a minimum number of LEDs of 3 The maximum number of LEDs will be limited by the maximum power we can provide on the buck converter output The number of LEDs times the average LED current times the LED forward voltage drop cannot exceed this maximum output power If for example the rated output power is 20W the maximum number of LEDs will be limited to 16 4 2 Changing LED Current LED forward current differs from manufacturer to manufacturer or part number to part number The constant current regulation needs to match the LED forward current for the nominal output and should not exceed the value specified in the datasheet To adjust the LED current we simply need to adjust Comparator 0 threshold that is used to detect the current on a sensing resistor This threshold can be adjusted between 0 05V to 1 6V in 32 steps by changing the value of Comparator O configuration register CORVM To measure the LED current a common digital multi meter can be used Adjust the LED current to not exceed the maximum value specified in the datasheet at the maximum AC input voltage Maximum LED current ripple current cannot exceed the buck inductor rated current and the rated output power If the inductor rated current is 700 mA for a ripple current of 100mA than the maximum average LED current is 600mA With 600mA LEDs considering a typical
27. nd 220VAC operation RO1AN0520EU0100 Rev 1 00 Page 21 of 28 May 13 2011 RENESAS 7 4 Performance Evaluation Data The screen captures in Figure 18 bellow show test results for three input voltages 90V AC 115VAC and 130VAC with dimming levels of 100 50 and lowest level achieved Int ETTE TEE Ez YOKA JA CE Inorr Tra TZ 14 Trg Wt 9013 e 03059 o ME 2 01653 0236 129 e Mn IR tes e 9 26 20 49 9859 EL 0 9658 T ERE t LE e L 84449 0 3390 wz 0 1766 19 34 5 A e 10 01 0 9881 EL vu O 9ie 8554 E 8451 ikar Spal Ten T7 YUKIGA ia SN un ELI LET E LET E 0 3724 yasni west 11430 0 1798 n E Four qaad d E fuz gU 21 56 ec D 770701928 n0 L 31 E AC 0 9873 Bo a 1225 0732009 08728 oo UA Intey Reset Inten Reset 83 16 TN TF A P AS 84 20 TN iii XL EL AA LI EL Imie Esri IT RH sp z 3mo ai Tq mM E mM EJ EEREAFE E iting TW Trend 192 x SOTA FEE TERT Figure 18 Test Results RO1AN0520EU0100 Rev 1 00 May 13 2011 RENESAS TO ker 15 08 0 0371 0 38 44 31 tim SIS trend St 0 0004 0 02 ANC 0 6348 lr DTE 4 09 U3 wu 13 Ed Integ Reset 7 18 NE C SAL E Integ Eesti Spl 2p um EE SOM Thee FE EE Page 22 of 28 78K0 IB2 Triac Dimmable LED Drive Design and User Guide The meaningful data for tests conducted is shown in Table 1 Input Voltage VAC Dimming Level 76 LED Current
28. o operate at either S5VAC to 135VAC or 175VAC to 265VAC ranges Make sure you have the correct version of the firmware before connecting the AC power The triac dimmer to be used can be any retail wall dimmer but please note that the dimming range and holding currents will differ and the results may differ If the holding current is higher when the dimmer is at lower position flicker may occur In our reference design we use a 10K 10W bleeder resistor connected across the AC input This resistor 1s optional and the value varies depending on the triac used Output 20 65VDC 350mA LED 7 W Input 85 265VAC Figure 15 Board connections To operate the board connect the LEDs and the dimmer as shown in Figure 15 and apply the AC power The LEDs should turn on and the intensity should follow the dimming level set by the triac control knob RO1AN0520EU0100 Rev 1 00 Page 18 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide 7 2 Demo For demonstration purposes a few fully enclosed units were built The demo shown in Figure 16 shows uses two 10W LED light modules to make up the full load of 20W Each module consists of 9 LEDs connected in series The LED maximum current was 350mA To operate the demo plug the power cord into the AC outlet and turn on the dimmer switch The LEDs should come on and the light intensity should follow the dimmer slider setting Triac Dimmer LED Module Figure
29. o any products or systems whose manufacture use or sale is prohibited under any applicable domestic or foreign laws or regulations Renesas Electronics has used reasonable care in preparing the information included in this document but Renesas Electronics does not warrant that such information is error free Renesas Electronics assumes no liability whatsoever for any damages incurred by you resulting from errors in or omissions from the information included herein Renesas Electronics products are classified according to the following three quality grades Standard High Quality and Specific The recommended applications for each Renesas Electronics product depends on the product s quality grade as indicated below You must check the quality grade of each Renesas Electronics product before using it in a particular application You may not use any Renesas Electronics product for any application categorized as Specific without the prior written consent of Renesas Electronics Further you may not use any Renesas Electronics product for any application for which it is not intended without the prior written consent of Renesas Electronics Renesas Electronics shall not be in any way liable for any damages or losses incurred by you or third parties arising from the use of any Renesas Electronics product for an application categorized as Specific or for which the product is not intended where you have failed to obtain the prior written consent of Renesas Elect
30. ocking feature of the KO IB2 microcontroller is employed The 8 Bit TMHI timer is configured as a PWM timer and its output is used to gate the output of the Buck converter PWM timer TX1 The Triac conduction angle as RO1AN0520EU0100 Rev 1 00 Page 13 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide measured by timer TMOO is translated into an 8 Bit number and loaded to timer TMH compare register CRCOO In effect the conduction angle controls the LED dimming in 256 steps through timer TMH duty cycle To compensate for the Triac asymmetric conduction quadrants an average of more than two measurements is used Pulses that are smaller than a minimum set value are rejected and are not used to adjust the running average The algorithm compares the measured average with the current timer TMH duty cycle setting and adjusts the duty cycle to compensate If the difference is significant the duty cycle is adjusted more quickly towards the new setting When the TMH PWM setting is within 30 counts of the average the PWM setting is adjusted by a smaller increment 1 This reduces flicker and provides a smooth ramp to the new level At power up the TMH PWM duty is set to the minimum level When the light is turned on at full brightness timer TMH PWM duty ramps up to full brightness providing a soft start effect At lower dimming levels this effect 1s less noticeable Timer TMH is used as a 1 2kHz PWM gating for
31. prove the quality and reliability of its products semiconductor products have specific characteristics such as the occurrence of failure at a certain rate and malfunctions under certain use conditions Further Renesas Electronics products are not subject to radiation resistance design Please be sure to implement safety measures to guard them against the possibility of physical injury and injury or damage caused by fire in the event of the failure of a Renesas Electronics product such as safety design for hardware and software including but not limited to redundancy fire control and malfunction prevention appropriate treatment for aging degradation or any other appropriate measures Because the evaluation of microcomputer software alone is very difficult please evaluate the safety of the final products or system manufactured by you Please contact a Renesas Electronics sales office for details as to environmental matters such as the environmental compatibility of each Renesas Electronics product Please use Renesas Electronics products in compliance with all applicable laws and regulations that regulate the inclusion or use of controlled substances including without limitation the EU RoHS Directive Renesas Electronics assumes no liability for damages or losses occurring as a result of your noncompliance with applicable laws and regulations 11 This document may not be reproduced or duplicated in any form in whole or in part without prior written cons
32. r Guide 7 1 Figure 14 MINICUBE2 interfaceing Operation CAUTION Before connecting the board to the AC power line please be aware that this is a non isolated design The user should not touch any part of this board to avoid the risk of electric shock The board specifications are as follows Input voltage 85 to 265VAC Output power 20W max LED voltage range 20 to 65VDC Constant LED current regulation Buck converter constant off PWM with hardware comparator LED current 350mA LED ripple current SOmA Active Power Factor Control Critical conduction mode using hardware comparator Power factor 20 95 maximum power 20W Dimming range 1 5 to 100 Power efficiency gt 80 full brightness 20W PFC boost voltage output stability 10 RO1AN0520EU0100 Rev 1 00 Page 17 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide B Over current and over voltage protections B 2 x 2 inch 4 layer PCB The LEDs used in this design were two modules of 9 LEDs each connected in series With 350mA and around 3 2V forward voltage per LED the two modules make up for 20W load The board has the code programmed and it is ready to operate once the As seen in Figure 15 the board has two sets of screw terminals one for the incoming AC voltage and one for the LEDs The board works with input voltages between 85 VAC and 265V AC but the firmaware has to be changed in order t
33. r LED drives is to have power factor correction PFC circuits This application note shows how the Renesas KO Ix2 dedicated lighting microcontroller can be effectively used to implement both the Triac dimmable LED drive and an active power factor correction function 2 Microcontroller Based Triac Dimmable LED Drive The basic requirement for a dimmable drive 1s to be able to track the Triac slider position which corresponds to the effective conduction angle and use this information to dim the light output on a chain of LEDs The useful range for the phase angle of a commercial Triac dimmer is between 45 to 135 degrees A block diagram of the drive is shown in Figure 3 RO1AN0520EU0100 Rev 1 00 Page 4 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide i Tac Output Bridge Power Factor Correction Rectifier PFC Triac Conduction Angle Detect 78K0 1B2 E Buck Constant Current To LEDs LED Driver Figure 3 LED Drive Block Diagram The bridge rectifier provides the 120 Hz unfiltered output to the PFC circuit and to a DC power supply circuit not shown in this block diagram The PFC block provides power factor correction and boosts the voltage to 200V DC to 400V DC to be used by the Buck converter stage The Buck converter converts the DC voltage to a lower value depending on the number of LEDs it is driving A conduction angle detect block is used for dimming reference The controller is
34. ronics The quality grade of each Renesas Electronics product is Standard unless otherwise expressly specified in a Renesas Electronics data sheets or data books etc Standard Computers office equipment communications equipment test and measurement equipment audio and visual equipment home electronic appliances machine tools personal electronic equipment and industrial robots High Quality Transportation equipment automobiles trains ships etc traffic control systems anti disaster systems anti crime systems safety equipment and medical equipment not specifically designed for life support Specific Aircraft aerospace equipment submersible repeaters nuclear reactor control systems medical equipment or systems for life support e g artificial life support devices or systems surgical implantations or healthcare intervention e g excision etc and any other applications or purposes that pose a direct threat to human life You should use the Renesas Electronics products described in this document within the range specified by Renesas Electronics especially with respect to the maximum rating operating supply voltage range movement power voltage range heat radiation characteristics installation and other product characteristics Renesas Electronics shall have no liability for malfunctions or damages arising out of the use of Renesas Electronics products beyond such specified ranges Although Renesas Electronics endeavors to im
35. tate extra electromagnetic noise is induced in the vicinity of LSI an associated shoot through current flows internally and malfunctions occur due to the false recognition of the pin state as an input signal become possible Unused pins should be handled as described under Handling of Unused Pins in the manual Processing at Power on The state of the product is undefined at the moment when power is supplied The states of internal circuits in the LSI are indeterminate and the states of register settings and pins are undefined at the moment when power is supplied In a finished product where the reset signal is applied to the external reset pin the states of pins are not guaranteed from the moment when power is supplied until the reset process is completed In a similar way the states of pins in a product that is reset by an on chip power on reset function are not guaranteed from the moment when power is supplied until the power reaches the level at which resetting has been specified Prohibition of Access to Reserved Addresses Access to reserved addresses is prohibited The reserved addresses are provided for the possible future expansion of functions Do not access these addresses the correct operation of LSI is not guaranteed if they are accessed Clock Signals After applying a reset only release the reset line after the operating clock signal has become stable When switching the clock signal during program execution wa
36. the Renesas 78K0 Ix2 8 bit dedicated lighting microcontrollers The right mix of peripherals and CPU performance make the 78K0 Ix2 an ideal device for lighting applications Three on board hardware comparators supporting two 16 bit PWM timers can drive external MOSFET switches for both the PFC circuit and the Buck converter with minimum CPU support The timers have Hi impedance states controlled by the comparators so that at a fault condition the external power circuits are protected 2 2 Power Factor Correction PFC DC DC Boost converter The PFC circuit is based on a typical DC DC boost power converter shown in Figure 4 DC DC Boost Converter bellow The role of the DC DC boost converter stage 1s to boost a rectified input voltage to 200V 400V DC needed for the DC DC buck converter stage In order to do that a switching device such as a MOSFET is driven by typically a PWM signal When the switching transistor is on the current flows and builds up through the inductor to the ground storing energy in the inductor When the transistor 1s off the energy stored in the inductor generates a voltage with positive polarity at the anode the diode The voltage seen on the anode of diode D is the sum of inductor voltage and VDCIn and it charges up capacitor C to a higher voltage than the input VDCIn making it a boost DC DC converter The voltage on the output can be precisely controlled by the duty cycle of the PWM signal By monitoring the output voltage the
37. timer TX1 output The value derived from T1000 input of timer TMM is scaled to a value between 3 and 254 and 1 2 and 100 Figures 11 and 12 bellow show the effect of dimming through timer TMH PWM method at 40 and near 100 LeCroy I l l NY I i NINDI C1 ae S BEER Teta ipu pus 1 ddl i i H d Measure PI TEA FPZ pkpkici Dimmer P4 PA Pa widthtF 1 value 43 702830 kHz 21B513V status A Tt Tt imebase DUU pst CTCE WStream 200 uel Stop Tb my 500M8 25655 Edge GE ative Figure 11 PWM Dimming at 40 LeCroy a NAM Dur act NR tL Rol ue GE 3 EN P del i o PB x TS ce m NEA I ee CRAT TEE HH tt at th tt tt tt HH Kees 1 i j m I INN TT ada TTT A i Bia Li Eii aiik MM d AMIN m T nt Wil ei Eeer EE ul MERE F ER ds idiei ia MALIA t citas ER iii idi liil bapti Measure D iTreocC 2 PzBkpkCT Dimmer P4 P5 Pa widthtF 1 value 417271018 kHz 1 606 status A Tt Tt Timebase 600 us Trigger De WY Stre aim 200 us stop FELT 500M5 245 6SfsfEdge Negative Figure 12 PWM Dimming near 100 RO1AN0520EU0100 Rev 1 00 Page 14 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide Using timer TMM measurement value of the Triac pulse width directly to set timer TMH duty cycle could result in flickering due to asymmetry in the Triac pulse width P
38. ts of signaling LEDs high brightness LEDs can be driven with hundreds of milliamps To drive these LEDs efficiently special circuits needed to be developed The key to getting the maximum light output of and LED and maintain the same color temperature is to drive it with a very high constant current and manage to dissipate the excessive heat developed during the process A typical constant current drive uses a on off current chopping circuit to ensure an average current through the LED that is in the safe operating range 1 2 Triac Dimmers In the following paragraphs we will explain the basics of the Triac dimming and the challenges they pose to LED dimming drives Figure 1 shows a typical commercially available Triac dimmer circuit used to dim incandescent light bulbs Triac 120VAC Figure 1 Triac Dimmer By adjusting variable resistor VR capacitor C charges up with a delay set by the total RC constant Regardless of polarity when the voltage on the capacitor reaches a certain threshold the Diac starts to conduct and it triggers the conduction of the Triac An illustration of the conduction angle is shown in Figure 2 The Triac starts conducting after a delay set by the RC circuit and stays on until the first half cycle completes and AC voltage reaches zero On the following cycle the capacitor will charge up with the opposite polarity voltage and the Triac will turn on again after the delay set by the RC constant By adjusting VR we can eff
39. ts the current through the inductor decrease until it reaches 0 but the transistor is not turned on right away only after some delay The PFC circuit in this application note uses critical conduction mode active power factor correction and achieves a power factor of better than 95 Critical conduction mode CRM uses a constant ON time pulse to charge the inductor then allows the current to fully discharge before initiating the next constant ON time pulse One of the advantages of this method is that it eliminates reverse recovery losses in the boost diode and allows the use of a smaller lower cost diode Typical applications for this method are low power converters where losses are critical to the efficiency bottom line The implementation with the 78K0 IB2 microcontroller is shown in Figure 5 bellow The OFF time for the switching PWM signal can be precisely controlled by one of the built in comparators The algorithm uses only one 16 Bit timer and one comparator to implement the power factor correction in this case timer TXO and comparator 2 The only CPU intervention required is to adjust the ON time of TMXO to maintain the constant DC boost voltage on the output The ON time is adjusted in software based on the feedback provided by the 10 bit ADC as described bellow DC BUS VAC KN Rectified unfiltered Average AC Critical onduction Mode d CRM Zero current detection TMX0O ANIO mag ee HOH CH I output E
40. ulse rejection and cycle averaging are necessary for a stable flicker free light output 6 Benefits to Using 78K0 Ix2 for Tac Controlled Dimming As demonstrated throughout this application note the 78KO Ix2 dedicated lighting microcontroller can successfully address the basic requirements of Triac dimmable LED drives such as constant LED current control and power factor correction but is it justified to use a microcontroller for this type of application Most existing analog solutions achieve only 0 75 power factor which may be enough for residential lighting but not enough for industrial or commercial applications where the requirement is 0 9 The solution presented in this document uses active power factor correction to achieve better than 0 97 The constant current control implemented with the internal comparator has an adjustable threshold allowing great flexibility when changing the type or the number of LEDs and input voltage The designer must also be aware that not all Triacs are the same Conduction angle varies across different models or manufacturers Newer models advertise energy efficiency by simply limiting the conduction angle Some dimmers will dim down and others will cut off the light all together at some point The result is different voltage output and different dimming range By using a microcontroller such as the Ix2 with a simple push of a button both the low level and high level dimming can be calibrated so that the same di
41. ve Design and User Guide DC BUS Aq LED Ag j I nductor current LED Current Sensing Average LED current i A a I Internal L 222222 SUCRE MESE PN IE PTS Vref Current Sense I nput TMX1 CMPO 77 8 I mwa Internal CMP TMX1 output i i PWM Ton Toff O output CMPO TMX10 l Internal Vref Dimmino TMH1 Figure 7 Buck Converter Implementation with 78K0 IB2 Timer TX1 plus comparator 0 regulate the buck current using a constant off time algorithm The combination of the constant off time Toff and a maximum on time Ton limit the maximum voltage and set the maximum buck switching frequency To calculate the minimum off time an acceptable ripple current 100ma for a 350ma LED circuit for example has to be considered The constant off time buck algorithm works as follows When T is on timer TMXI counts up and the inductor current increases until the sensing comparator detects maximum value and resets timer TMXI The timer output and T will be off for a time interval set by the constant Toff During this time the inductor discharges by ripple current value and when Toff expires T is turned on again until the inductor current reaches again the comparator threshold or a the maximum time Ton RO1AN0520EU0100 Rev 1 00 Page 8 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide 3 Design Example In this chapter we will show
42. ward voltages of the series connected LEDs is less than 7 4V without changing the Toff parameter will cause damage to the device The buck circuit will not be able to limit the current and will try to dump RO1AN0520EU0100 Rev 1 00 Page 10 of 28 May 13 2011 RENESAS 78K0 IB2 Triac Dimmable LED Drive Design and User Guide 7 4V into a chain of diodes with a lower maximum voltage Since diodes have a very steep V I curve the I will increase exponentially leading to failure in the buck circuit Calculate Toff parameter using the buck calculator xls spreadsheet for your diodes before plugging in the board to make sure the parameters are compatible with your LED array The timer is programmed with the two intervals to happen in sequence minimum off time first maximum on time second The maximum on time will ordinarily be interrupted when the comparator resets the timer back to 0 restarting the minimum off time The counter will not start counting until the comparator has dropped below the comparator threshold for 300ns 4 Design Guidelines As previously shown the output voltage across the LED string is roughly the boost voltage times the buck converter frequency duty cycle For a given boost voltage number of LEDs inductor value and ripple current we can calculate Toff and Ton which will correspond to the buck converter frequency and duty cycle to produce the desired output voltage In this design we use a constant Toff method and limit
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