www.fa irchildsemi.com ? 2012 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 11/9/12 AN-9744 smart led lamp driver ic with pfc function introduction the fl7701 is a pwm peak current controller for a buck converter topology operating in continuous conduction mode (ccm) with an intelligent pfc function using a digital control algorithm. the fl7701 has an internal self- biasing circuit that is a current source using a high-voltage switching device. when the input voltage is applied to the hv pin is over 25 v to 500 v, the fl7701 maintains a 15.5 v dc at the vcc pin. the fl7701 also has a uvlo block for stable operation. when the v cc voltage reaches higher than v ccst+ , the uvlo block starts operation.hen the v cc drops below the v ccst-, ic operation stops. hysteresis is provided for stable operation of the ic when input the voltage is in noisy circumstances or unstable conditions. the fl7701 has a ?smart? internal block for ac input condition. if an ac sour ce with 50 hz or 60 hz is applied, the ic automatically changes the internal reference to adjust to input conditions w ith an internal fixed transient time. when a dc source connect s to the ic, the internal reference immediately ch anges to dc waveform. reference r s q driver i sw l il led load d1 v sup dac: digital to analog hv device : high-voltage device + v led - fl7701 i line out hv device hv v cc gnd c vsup_sen dac_out zcd_out cs dac figure 1. basic block of fl7701 the internal dac_out reference signal is dependent on the v cc voltage. using the dac_out signal and internal clock, clk_gen; the fl7701 automatically makes a digital reference signal, dac_out. if the fl7701 cannot detect the zcd_out signal, the ic has an abnormal internal reference signal. in this situation, this phenomenon causes a lighting flicker. figure 2. fl7701 operation soft-start function the fl7701 has an internal soft-start to reduce inrush current at ic startup. when the ic starts operation, the internal reference of the ic slowly increases up to a fixed level for around seven cycles . after settling down this transient period, the internal re ference is fixed at a certain dc level. in this time, the ic continually tries to find input phase information from the v cc pin. if the ic succeeds in getting phase information from the vcc, the ic automatically follows a similar shape reference, which it made during the transient times, seven periods. if not, the ic has a dc reference level.
AN-9744 application note ? 2012 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 11/9/12 2 figure 3. dc input condition figure 4. ac input condition internal power factor (pf) function the fl7701 application circuit does not use the input electrolytic capacitor for voltage rectification after a bridge diode because this system de sign results in a high pulse shape input current. this pulse shape current contains many harmonic components, so the total system cannot have high pf. to get high pf performance, the fl7701 uses a different approach. the fl7701 has an intelligent in ternal pfc function that does not require additional detection pins or other components. the ic does not need a bulk capacitor on the vcc pin for supply voltage stabilization. dac_out bridge diode output voltage input voltage peak zcd vcc v bridge figure 5. internal pfc function the fl7701 detects the v cc changing point for making the zero crossing detection (zcd) signal, which is an internal timing signal for making dac_out. normally, a capacitor connected to the vcc pin is used for voltage stabilization and acts as low-pass filter or noise-canceling filter. this increases the ability to get a stable timing signal at the vcc pin, even is there may be noise on other pins. to precisely and reliably calculate the input voltage phase on the vcc pin, the fl7701 uses a digital technique (sigma/delta modulation/demodulation). after finishing this digital technique, the fl7701 has new reference that is the same phase as input voltage, as shown in figure 6. /2 vp vp figure 6. internal reference this signal enters the final comparator and current information from the sensing resistor. pin 1 is compared. as a result, the fl7701 has a high power factor and can operate as a normal peak current controller as shown in figure 6, in the dc input condition. the relationship between ac input mode and dc input mode is 2 . output frequency programming the fl7701 can program output frequency using an rt resistor or with the rt pin in open condition. the fl7701 can have a fixed output frequency around 45 khz when the rt pin is left open. for increasing system reliability, a small-value capacitor is recommended below 100 nf in rt- open condition. the relations hip between output frequency and the rt resistor is: rt f osc 9 10 02 . 2 = [hz] (1) output open-circuit protection the recommended connection method is shown in figure 7. the fl7701 has a high-voltage power supply circuit, which self biases using high-voltage process device. if the led does not connect to the chip, the ic cannot start. led l fl7701 out hv emi filter bd cs gnd vcc rt adim c1 c2 l1 l2 d1 r1 r2 r3 d2 c3 c4 figure 7. led open condition
AN-9744 application note ? 2012 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 11/9/12 3 inductor short-circuit protection the fl7701 has an abnormal over-current protection (aocp) function. if the voltage of the led current-sensing resistor is higher than 2.5 v, even within leading edge- blanking (leb) time of 350 ns; the ic stops operation. gnd rt out hv vcc reference jfet aocp uvlo q r s tsd leading edge blanking zcd dac soft start digital block cs oscillator - + leb - + time vcc zcd 2.5v figure 8. aocp function analog dimming function the analog dimming (adim) function adjusts the output led current by changing the voltage level of the adim pin. application information the fl7701 is an innovative buck converter control ic designed for led applications. it can operate from dc and ac input voltages without limitation and its input voltage level can be up to 308 v ac . table 1 shows one example of a design target using the fl7701 device. table 1. target design specification item specification note frequency 45 khz output voltage 35 v f =3.5 v, n=10 output led current rms 0.3 i led(rms) output led current peak 0.5 i led(peak) input voltage (max.) 220 v ac(rms) step 1: minimum duty ratio the fl7701 has a fixed internal duty ratio range between 2% and 50%. this range depe nds on the input voltage and the number of leds in the string. (max) min in f v nv d = (2) where is efficiency of system; v in(max) is maximum input voltage; v f is forward-drop voltage of led; and n is led number in series connection. for example, if v in(max) = 220 v, =85% and ten leds are in series connection, the minimum duty ratio is: 132 . 0 220 2 85 . 0 5 . 3 10 min = = d step 2: maximum duty ratio similar to step 1, calculate maximum duty ratio as: (min) max in f v nv d = (3) 0 10 20 30 40 50 60 051015 duty [%] [ms] figure 9. duty variation vs. time the fl7701 has a 50% maximum duty cycle to prevent sub-harmonic instability. assume the minimum input voltage enters 50% duty ratio. using equation (2), re- calculate the minimum input voltage for ccm operation: ] [ 35 . 82 5 . 0 85 . 0 35 max (min) v d nv v f in = = = (4) input voltage average led current(i led(ave ) t on t off d min 1-d min ccm expected min. input voltage (ccm) : v in(min) =82.35v 311v dcm dcm time time current limit on the dac reference i figure 10. estimated waveforms
AN-9744 application note ? 2012 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 11/9/12 4 step 3: maximum on/off time the fl7701 has internally fixed maximum duty ratio around 0.5 to prevent sub-harm onic instability. assume the maximum on/off time. for example, the maximum on/off time at 45 khz operation condition is: 11 . 11 90000 1 2 1 = = = = s off on f t t [ s] step 4: calculate the led current ripple, ? i the figure 11 shows the typical led current waveforms of a fl7701 application. for more stable or linear led current, operate in ccm. figure 11. target waveforms of led current using the typical led current waveform in figure 11, derive the formula as: 2 ) . ( ) ( i i i peak ave led peak led + = or 2 ) . ( (min) i i i peak ave led led ? = (5) in table 1, the desired led current average is always located between led p eak current value, i led(peak) =500 ma, which is limited by the ic itself, and the led minimum current. using this characteristic, the inductor value for the desired output current ripple range ( ? i ) is: ) ( 2 ) . ( ) ( peak ave led peak led i i i ? = or ) ( 2 (min) ) . ( led peak ave led i i i ? = (6) where 2 ) . ( ) ( peak ave led rms led i i = from the table 1, the target led current rms is defined as 0.3 a and the led current peak is set to 0.5 a. ] [ 1516 . 0 ) 3 . 0 2 5 . 0 ( 2 ) 2 ( 2 ) ( ) ( a i i i rms led peak led = ? ? = ? ? = step 5: inductance derive one more formula for the minimum inductance value of the inductor using the step 4 results: [] mh i f d n v l s f 5 . 4 1516 . 0 45000 ) 132 . 0 1 ( 10 5 . 3 ) 1 )( ( min = ? = ? = (7) figure 12. current ripple ( ? i) vs. inductance figure 13. expected waveforms step 6: sensing resistor the fl7701 was calculated the sensing resistor value as: 1 5 . 0 5 . 0 ) ( = = = peak led cs i v r [ ] (8) the power rating is under 0.25 w even when considering power consumption at peak-current condition. step 7: frequency set resistor ] [ 919 . 44 10 0213 . 2 1 9 = ? ? = k f r sw t (9) if there is not connected r t resistance to the operation frequency is 45 khz.
AN-9744 application note ? 2012 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 11/9/12 5 system verification figure 14 shows the recommended circuit of a fl7701 system with just a few components. figure 14. test circuit figure 15 and figure 16 show the startup waveforms from a on fl7701 application in dc a nd ac input conditions at 220 v with ten leds. figure 15. soft-start performance in dc input condition figure 16. soft-start performance in ac input condition figure 17 and figure 18 show performance of fl7701 following the input source changes from high-line frequency, to lower frequency, then to higher frequency. figure 17. input source changing: 45 hz to 100 hz figure 18. input source changing: 100 hz to 45 hz the figure 19 shows the analog dimming performance with changing v adim . the output led current changes according to the control voltage. figure 19. v admin vs. led current i led [0.2a/div] v drain [100v/div] i led [0.2a/div] v drain [100v/div] v cc [5v/div] v drain [100v/div] i led [0.2a/div] v cc [5v/div] v drain [100v/div] i led [0.2a/div]
AN-9744 application note ? 2012 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 11/9/12 6 figure 20 shows the typical function of aocp performance. the fl7701 limits output led current pulse-by-pulse with leading-edge blanking (leb), ignoring current noise. even though the ic limits the output led current pulse-by- pulse, it cannot prevent inrush current during an inductor short. to prevent this kind of abnormal situation, the ic has an aocp function to protect the system. figure 20. aocp function figure 21 shows the typical waveforms of fl7701 system. the led current has the same phase as the input voltage source and rectified sinusoidal waveform. figure 21. typical operating waveforms design tips led current changing figure 22 shows the recommended circuit for achieving high pf. in this condition, the led current goes to 0 every half cycle period. figure 22. typical waveform to design around this, add an electrolytic capacitor in parallel to the led load, as shown in figure 23. this added capacitor provides a truer dc led current. figure 23. circuit with electrolytic capacitor figure 24. typical with bulk capacitor v cc [10v/div] v dc [40v/div] v cs [1v/div] v gate [7v/div] v dd [3v/div] i led [0.1a/div] v drain [100v/div] v dd [3v/div] i led [0.2a/div] v d r v drain [100v/div ] v dd [3v/div] v gate [7v/div] i led [0.2a/div] v drain [100v/div]
AN-9744 application note ? 2012 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 11/9/12 7 increasing system reliability to increase system reliability in noisy conditions, add a small capacitor with below 100 pf to the rt and adim pins. in normal conditions, these components are unnecessary. pcb layout guidelines the pcb layout is important because a common application would be to retrofit a lamp application, which requires a small product size. the ic could be affected by noise, so carefully follow the pcb layout guide lines: ? locate the ic on the external powering path. ? separate power gnd and signal gnd. ? v cc capacitor should be located close to the vcc pin. figure 25. example led layout related datasheets fl7701 ? smart led lamp driver ic with pfc function disclaimer fairchild semiconductor reserves the right to make changes without further notice to any products herein to improve reliability, function, or design. fa irchild does not assume any liability arising out 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 support policy fairchild?s products are not authorized for use as criti cal components in life support devices or systems without the express written approval of the preside nt of fairchild semiconductor corporation. as used herein: 1. life support devices or system s are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. a critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.
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