AN Smart LED Lamp Driver IC with PFC Function. Introduction. Soft-Start Function. Internal Digital Reference by ZCD.

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1 AN-9744 Smar LED Lamp Driver IC wih PFC Funcion Inroducion The FL7701 is a PWM peak curren conroller for a buck converer opology operaing in Coninuous Conducion Mode (CCM) wih an inelligen PFC funcion using a digial conrol algorihm. The FL7701 has an inernal selfbiasing circui ha is a curren source using a high-volage swiching device. When he inpu volage is applied o he pin is over 25 V o 500 V, he FL7701 mainains a 15.5 V DC a he pin. The FL7701 also has a UVLO block for sable operaion. When he V CC volage reaches higher han V CT+, he UVLO block sars operaion. When he V CC drops below he V CT-, IC operaion sops. Please noe ha his applicaion noe could also be used for FLS0116, which is a Smar LED Lamp driver IC wih he same feaures and funcions as FL7701 bu includes buil-in 1 A, 500 V MOSFET. Inernal Digial Reference by ZCD Hyseresis is provided for sable operaion of he IC when he inpu volage is in noisy circumsances or unsable condiions. The FL7701 has a smar inernal block for AC inpu condiion. If an AC source wih 50 Hz or 60 Hz is applied, he IC auomaically changes he inernal reference o adjus o inpu condiions wih an inernal fixed ransien. When a DC source connecs o he IC, he inernal reference immediaely changes o DC waveform. The inernal DAC_ reference signal is dependen on he V CC volage. Using he DAC_ signal and inernal clock, CLK_GEN; he FL7701 auomaically makes a digial reference signal, DAC_. If oo high of a capacior value is conneced o, he volage may no drop enough during he AC inpu volage valley region. The ZCD_ signal does no rigger correcly and he IC will have an abnormal inernal reference signal, see Figure 2 for a normal rigger sequence. This abnormal inernal reference signal causes LED ligh flicker. Sof-Sar Funcion The FL7701 has an inernal sof-sar o reduce inrush curren a IC sarup. When he IC sars operaion, he inernal reference of he IC slowly increases up o a fixed level for around seven cycles. Afer his ransien period, he inernal reference is fixed a a cerain DC level, see Figure 3. In his ransien, he IC coninually ries o find inpu phase informaion from he pin. If he IC succeeds in deecing phase informaion from he inpu, he IC auomaically follows a similar shape o he AC inpu volage, see Figure 4. If no, he IC has a DC reference level. Vsup Iline D1 + VLED - L LED Load IL Drain Volage,Vdrain FL7701 VSUP_SEN Device VSUP_ SEN C DAC ZCD_ DAC_ DAC: Digial o Analog Device : High-Volage Device Reference S Q R Driver Isw ZCD_ CLK_ GEN Figure 1. Basic Block of FL7701 DAC_ Gae Volage, VG Figure 2. FL7701 Operaion Rev /30/14

2 V RECTIFIED I LED To precisely and reliably calculae he inpu volage phase on he pin, he FL7701 uses a digial echnique (sigma/dela modulaion/demodulaion). Afer finishing his digial echnique, he FL7701 has new reference ha is he same phase as inpu volage, as shown in Figure 6. Sine wave generaion (when PFC funcion is enabed.) Consan reference generaion (when PFC funcion is disabled.) Figure 3. DC Inpu Condiion ZCD_ Sof change period VRECTIFIED ILED Figure 4. AC Inpu Condiion Inernal Power Facor (PF) Funcion The FL7701 applicaion circui does no use he inpu elecrolyic capacior for volage recificaion afer a bridge diode because his sysem design resuls in a high pulse shape inpu curren. This pulse shape curren conains many harmonic componens, so he oal sysem canno have high PF. To ge high PF performance, he FL7701 uses a differen approach. The FL7701 has an inelligen inernal PFC funcion ha does no require addiional deecion pins or oher componens. The IC does no need a bulk capacior on he pin for supply volage sabilizaion. The FL7701 deecs he V CC changing poin for generaing he Zero Crossing Deecion (ZCD) signal, which is an inernal iming signal for generaing DAC_. Normally, a capacior conneced o he pin is used for volage sabilizaion and acs as low-pass filer or noise-canceling filer. This increases he abiliy o ge a sable iming signal a he pin, even is here may be noise on oher pins. Reference Vp Figure 6. Vp / 2 Inernal Reference This signal eners he final comparaor and curren informaion from he sensing resisor. Pin 1 is compared. As a resul, he FL7701 has a high power facor and can operae as a normal peak curren conroller as shown in Figure 6, in he DC inpu condiion. The relaionship beween AC Inpu Mode and DC Inpu Mode is 2. Oupu Frequency Programming The FL7701 can program oupu frequency using an resisor or wih he pin in open condiion. The FL7701 can have a fixed oupu frequency around 45 khz when he pin is lef open. For increasing sysem reliabiliy, a small-value capacior is recommended below 100 nf in open condiion. The relaionship beween oupu frequency and he resisor is: [Hz] (1) f OSC 9 Oupu Open-Circui Proecion The recommended connecion mehod is shown in Figure 7. The FL7701 has a high-volage power supply circui, which self biases using high-volage process device. If he LED does no connec o he chip, he IC canno sar. Vbridge Bridge Diode Oupu Volage BD EMI filer LED Inpu Volage Peak JFET Oupu Volage L1 D1 L ZCD Vdd Charging Volage JFET Oupu Volage C1 L2 C2 C3 R3 ADIM C4 FL7701 D2 R1 R2 DAC_ Figure 7. LED Open Condiion Figure 5. Inernal PFC Funcion Rev /30/14 2

3 ZCD AN-9744 Inducor Shor-Circui Proecion The FL7701 has an Abnormal Over-Curren Proecion (AOCP) funcion. If he volage of he LED curren-sensing resisor is higher han 2.5 V, even wihin Leading Edge- Blanking (LEB) of 350 ns; he IC sops operaion. JFET For example, if V IN(max) = 220 V, η=85% and en LEDs are in series connecion, he minimum duy raio is: D min Sep 2: Maximum Duy Raio Similar o Sep 1, calculae maximum duy raio as: ZCD UVLO Dmax nv F (3) Vin(min) DAC Sof sar TSD Oscillaor Digial Block Reference - + S R Q 60 [%] 50 Duy LEB Leading Edge Blanking V - + AOCP 30 Figure 8. AOCP Funcion Analog Dimming Funcion The Analog Dimming (ADIM) funcion adjuss he oupu LED curren by changing he volage level of he ADIM pin. Applicaion Informaion The FL7701 is an innovaive buck converer conrol IC designed for LED applicaions. I can operae from DC and AC inpu volages wihou limiaion and is inpu volage level can be up o 308 V AC. Table 1 shows one example of a design arge using he FL7701 device. Table 1. Targe Design Specificaion Iem Specificaion Noe Frequency 45 khz Oupu Volage 35 V F=3.5 V, n=10 Oupu LED Curren RMS 0.3 I LED(rms) Oupu LED Curren Peak 0.5 I LED( Inpu Volage (Max.) 220 V AC(rms) Figure 9. Duy Variaion vs. Time [ms] The FL7701 has a 50% maximum duy cycle o preven subharmonic insabiliy. Assume he minimum inpu volage eners 50% duy raio. Using Equaion (2), re-calculae he minimum inpu volage for CCM operaion: V in(min) DCM nvf [ V ] D (4) Inpu volage max Curren Limi on he DAC reference 311V Expeced min. inpu volage (CCM) : Vin(min)=82.35V i Average LED Curren(ILED(ave) DCM Sep 1: Minimum Duy Raio The FL7701 has a fixed inernal duy raio range beween 2% and 50%. This range depends on he inpu volage and he number of LEDs in he sring. Dmin CCM 1-Dmin D min nv V F (2) in(max) Figure 10. on off Esimaed Waveforms where η is efficiency of sysem; V IN(max) is maximum inpu volage; V F is forward-drop volage of LED; and n is LED number in series connecion. Rev /30/14 3

4 Sep 3: Maximum On/Off Time The FL7701 has inernally fixed maximum duy raio around 0.5 o preven sub-harmonic insabiliy. Assume he maximum on/off. For example, he maximum on/off a 45 khz operaion condiion is: 1 1 on off [μs] 2 f s Sep 4: Calculae he LED Curren Ripple, i Figure 11 shows he ypical LED curren waveforms of a FL7701 applicaion. For more sable or linear LED curren, operae in CCM. Sep 5: Inducance Derive one more formula for he minimum inducance value of he inducor using he Sep 4 resuls: ( V n)(1 Dmin ) ( ) L 4. f i F 5 s mh (7) Curren peak a LED average curren maximum poin DCM (ILED(ave.) i Curren peak a LED curren maximum poin (ILED() Average LED Curren (ILED(ave)) 0.5 i 0.5 i CCM DCM Figure 12. Curren Ripple ( I) vs. Inducance Curren min a LED curren maximum poin (ILED(min)) Dmin on off 1-Dmin Figure 11. Targe Waveforms of LED Curren Using he ypical LED curren waveform in Figure 11, derive he formula as: I I LED ( LED (min) i I LED ( ave. or 2 i I LED ( ave. 2 In Table 1, he desired LED curren average is always locaed beween LED peak curren value, I LED( =500 ma, which is limied by he IC iself, and he LED minimum curren. Using his characerisic, he inducor value for he desired oupu curren ripple range ( i) is: i ( I LED I ) or i 2 ( LED ( ave. 2 ( I LED ( ave. I LED (min) ) where I I LED ( rms) LED ( ave. 2 From he Table 1, he arge LED curren rms is defined as 0.3A and he LED curren peak is se o 0.5 A. (5) (6) Figure 13. Expeced Waveforms Sep 6: Sensing Resisor The curren sensing resisor value can be deermined by: V 0.5 [] R 1 (8) I LED ( 0.5 The power raing is under 0.25 W even when considering power consumpion a peak-curren condiion. Sep 7: Frequency Se Resisor 1 9 R [ k] (9) f sw If he frequency seing resisor R is no conneced, he sysem operaes a he IC s defaul swiching frequency which is is 45 khz. i 2 ( I LED ( 2 I LED ( rms) ) 2( ) [ A] Rev /30/14 4

5 Sysem Verificaion Figure 14 shows he recommended circui of a FL7701 sysem wih jus a few componens. Figure 17 and Figure 18 show performance of FL7701 following he inpu source changes from high-line frequency, o lower frequency, hen o higher frequency. BD EMI filer L1 D1 LED L ADIM D2 C1 C2 R3 FL7701 R1 L2 C3 C4 R2 Figure 14. Tes Circui Figure 15 and Figure 16 show he sarup waveforms from a FL7701 applicaion in DC and AC inpu condiions a 220 V wih en LEDs. Figure 17. Inpu Source Changing: 45 Hz o 100 Hz [5V/div] Figure 15. [5V/div] Sof-Sar Performance in DC Inpu Condiion Figure 18. Inpu Source Changing: 100 Hz o 45 Hz Figure 19 shows he analog dimming performance wih changing V ADIM. The oupu LED curren changes according o he conrol volage. Figure 19. V ADMIN vs. LED Curren Figure 16. Sof-Sar Performance in AC Inpu Condiion Rev /30/14 5

6 Figure 20 shows he ypical funcion of AOCP performance. The FL7701 limis oupu LED curren pulse-by-pulse wih Leading-Edge Blanking (LEB), ignoring curren noise. Even hough he IC limis he oupu LED curren pulse-by-pulse, i canno preven inrush curren during an inducor shor. To preven his kind of abnormal siuaion, he IC has an AOCP funcion o proec he sysem. [10V/div] VDC[40V/div] V[1V/div] VGATE[7V/div] Design Tips LED Curren Changing Figure 22 shows he recommended circui for achieving high PF. In his condiion, he LED curren goes o 0 every half cycle period. VDD[3V/div] VD RVDRAIN[100V/div ] Figure 20. AOCP Funcion Figure 21 shows he ypical waveforms of FL7701 sysem. The LED curren has he same phase as he inpu volage source and recified sinusoidal waveform. VDD[3V/div] ILED[0.1A/div] Figure 22. Typical Waveform To design around his, add an elecrolyic capacior in parallel o he LED load, as shown in Figure 23. This added capacior provides a ruer DC LED curren. BD EMI filer Elecrolyic Capacior FRD LED L AC INPUT ADIM FL7701 Figure 23. Circui wih Elecrolyic Capacior Figure 21. Typical Operaing Waveforms VDD[3V/div] Vgae[7V/div] Figure 24. Typical wih Bulk Capacior Rev /30/14 6

7 Prevening LED Flicker Due o ZCD Error During Analog Dimming pin of FL7701 is usually conneced behind he EMI filer componens. Waveform of pin has disorion due o L-C resonance of he EMI filer and discharging of EMI capacior. This could lead o waveform disorion as well as ZCD deecion error as shown in Figure 25. The abnormal waveform could cause LED ligh flicker o happen. Minimum Dimming Range Operaing range of analog dimming pin is 0.5 V ~ 3.5 V. If a dc signal is no injeced ino he ADIM pin, his pin is inernally pulled high o 3.5 V and he sysem operaes a full brighness. When volage of ADIM pin is below 0.5 V, FL7701operaes wih minimum urn on which decides he minimum dimming curren as shown in Figure 27. Figure 25. LED Flicker a Analog Dimming The reasons for he LED flickering are as follow; 1. EMI filer is opimized for he raed oupu power. When analog diming is used, he oupu power is decreased. As he oupu power decreases, he EMI filer capacior is no fully discharged. 2. When he EMI filer capacior is fully discharged, volage does no have a drop poin and his causes ZCD error 3. When ZCD error happens, FL7701 changes o DC mode operaion for he nex 6 recified line cycles LED sring has a flicker due o curren deviaion. In order o preven ZCD error when FL7701 is used for analog dimming, connecion poin mus change from he drain side o he inpu side via series conneced diodes as shown in Figure 26. Figure 27. Example of Analog Dimming Curve Increasing Sysem Reliabiliy To increase sysem reliabiliy in noisy condiions, add a small capacior wih below 100 pf o he and ADIM pins. In normal condiions, hese componens are unnecessary. PCB Layou Guidelines The PCB layou is imporan because a common applicaion would be o rerofi a lamp applicaion, which requires a small produc size. The IC could be affeced by noise, so carefully follow he PCB layou guide lines: Locae he IC on he exernal powering pah. Separae power and signal. V CC capacior should be locaed close o he pin. Powering Pah LED Capacior for pin Fuse 500mA/250V BD MB6S FLS0116 DRAIN One poin P IC ADIM Analog Dimming Signal wih MCU Figure 28. S Example LED Layou Figure 26. FLS0126 Applicaion wih Analog Dimming Rev /30/14 7

8 Relaed Daashees FL Smar LED Lamp Driver IC wih PFC Funcion FLS MOSFET Inegraed Smar LED Lamp Driver IC wih PFC Funcion DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITH FUHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPO POLICY FAIRCHILD S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPO DEVICES OR SYSTEMS WITH THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life suppor devices or sysems are devices or sysems which, (a) are inended for surgical implan ino he body, or (b) suppor or susain life, or (c) whose failure o perform when properly used in accordance wih insrucions for use provided in he labeling, can be reasonably expeced o resul in significan injury o he user. 2. A criical componen is any componen of a life suppor device or sysem whose failure o perform can be reasonably expeced o cause he failure of he life suppor device or sysem, or o affec is safey or effeciveness. Rev /30/14 8