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www.fairchildsemi.com 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 selfbiasing circuit that is a current source using a high-voltage switching device. When the input voltage is applied to the pin is over 25 V to 500 V, the FL7701 maintains a 15.5 V DC at the 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. When the V CC drops below the V CCST-, IC operation stops. Please note that this application note could also be used for FLS0116, which is a Smart LED Lamp driver IC with the same features and functions as FL7701 but includes built-in 1 A, 500 V MOSFET. Internal Digital Reference by ZCD Hysteresis is provided for stable operation of the IC when the input voltage is in noisy circumstances or unstable conditions. The FL7701 has a smart internal block for AC input condition. If an AC source with 50 Hz or 60 Hz is applied, the IC automatically changes the internal reference to adjust to input conditions with an internal fixed transient. When a DC source connects to the IC, the internal reference immediately changes to DC waveform. 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 too high of a capacitor value is connected to, the voltage may not drop enough during the AC input voltage valley region. The ZCD_OUT signal does not trigger correctly and the IC will have an abnormal internal reference signal, see Figure 2 for a normal trigger sequence. This abnormal internal reference signal causes LED light flicker. 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 this transient period, the internal reference is fixed at a certain DC level, see Figure 3. In this transient, the IC continually tries to find input phase information from the pin. If the IC succeeds in detecting phase information from the input, the IC automatically follows a similar shape to the AC input voltage, see Figure 4. If not, the IC has a DC reference level. Vsup Iline D1 + VLED - L LED Load IL Drain Voltage,Vdrain FL7701 VSUP_SEN Device VSUP_ SEN C DAC ZCD_OUT DAC_OUT DAC: Digital to Analog Device : High-Voltage Device Reference S Q R Driver OUT Isw ZCD_ OUT CS CLK_ GEN Figure 1. Basic Block of FL7701 DAC_ OUT Gate Voltage, VG Figure 2. FL7701 Operation Rev. 1.2 12/30/14
V RECTIFIED I LED To precisely and reliably calculate the input voltage phase on the 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. Sine wave generation (when PFC function is enabed.) Constant reference generation (when PFC function is disabled.) Figure 3. DC Input Condition ZCD_OUT Soft change period VRECTIFIED ILED 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 design 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 internal PFC function that does not require additional detection pins or other components. The IC does not need a bulk capacitor on the pin for supply voltage stabilization. The FL7701 detects the V CC changing point for generating the Zero Crossing Detection (ZCD) signal, which is an internal timing signal for generating DAC_OUT. Normally, a capacitor connected to the 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 pin, even is there may be noise on other pins. Reference Vp Figure 6. Vp / 2 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 RTopen condition. The relationship between output frequency and the RT resistor is: 2.0210 [Hz] (1) RT f OSC 9 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. Vbridge Bridge Diode Output Voltage BD EMI filter LED Input Voltage Peak JFET Output Voltage L1 D1 L ZCD Vdd Charging Voltage JFET Output Voltage t t C1 L2 C2 C3 R3 ADIM RT C4 OUT FL7701 CS D2 R1 R2 t DAC_OUT Figure 7. LED Open Condition t Figure 5. Internal PFC Function Rev. 1.2 12/30/14 2
ZCD AN-9744 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) of 350 ns; the IC stops operation. JFET For example, if V IN(max) = 220 V, η=85% and ten LEDs are in series connection, the minimum duty ratio is: D min 103.5 0.132 0.85 2 220 Step 2: Maximum Duty Ratio Similar to Step 1, calculate maximum duty ratio as: ZCD UVLO Dmax nv F (3) Vin(min) DAC Soft start TSD RT Oscillator Digital Block Reference - + S R Q OUT 60 [%] 50 Duty LEB Leading Edge Blanking CS 40 2.5V - + AOCP 30 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) 20 10 0 0 5 10 15 Figure 9. Duty Variation vs. Time [ms] The FL7701 has a 50% maximum duty cycle to prevent subharmonic instability. Assume the minimum input voltage enters 50% duty ratio. Using Equation (2), re-calculate the minimum input voltage for CCM operation: V in(min) DCM nvf 35 82.35[ V ] D 0.85 0.5 (4) Input voltage max Current Limit on the DAC reference 311V Expected min. input voltage (CCM) : Vin(min)=82.35V i Average LED Current(ILED(ave) DCM Step 1: Minimum Duty Ratio The FL7701 has a fixed internal duty ratio range between 2% and 50%. This range depends on the input voltage and the number of LEDs in the string. Dmin CCM 1-Dmin D min nv V F (2) in(max) Figure 10. ton toff Estimated Waveforms where η 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. Rev. 1.2 12/30/14 3
Step 3: Maximum On/Off Time The FL7701 has internally fixed maximum duty ratio around 0.5 to prevent sub-harmonic instability. Assume the maximum on/off. For example, the maximum on/off at 45 khz operation condition is: 1 1 t on toff 11.11 [μs] 2 f 90000 s Step 4: Calculate the LED Current Ripple, i Figure 11 shows the typical LED current waveforms of a FL7701 application. For more stable or linear LED current, operate in CCM. Step 5: Inductance Derive one more formula for the minimum inductance value of the inductor using the Step 4 results: ( V n)(1 Dmin ) 3.510 (1 0.132) L 4. f i 45000 0.1516 F 5 s mh (7) Current peak at LED average current maximum point DCM (ILED(ave.peak)) i Current peak at LED current maximum point (ILED(peak)) Average LED Current (ILED(ave)) 0.5 i 0.5 i CCM DCM Figure 12. Current Ripple ( I) vs. Inductance Current min at LED current maximum point (ILED(min)) Dmin ton toff 1-Dmin Figure 11. Target Waveforms of LED Current Using the typical LED current waveform in Figure 11, derive the formula as: I I LED ( peak) LED (min) i I LED ( ave. peak) or 2 i I LED ( ave. peak) 2 In Table 1, the desired LED current average is always located between LED peak 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: i ( I LED I ) or i 2 ( peak) LED ( ave. peak) 2 ( I LED ( ave. peak) I LED (min) ) where I I LED ( rms) LED ( ave. peak) 2 From the Table 1, the target LED current rms is defined as 0.3A and the LED current peak is set to 0.5 A. (5) (6) Figure 13. Expected Waveforms Step 6: Sensing Resistor The current sensing resistor value can be determined by: VCS 0.5 [] R 1 (8) I LED ( peak) 0.5 The power rating is under 0.25 W even when considering power consumption at peak-current condition. Step 7: Frequency Set Resistor 1 9 Rt 2.021310 44.919[ k] (9) f sw If the frequency setting resistor R t is not connected, the system operates at the IC s default switching frequency which is is 45 khz. i 2 ( I LED ( peak) 2 I LED ( rms) ) 2(0.5 2 0.3) 0.1516 [ A] Rev. 1.2 12/30/14 4
System Verification Figure 14 shows the recommended circuit of a FL7701 system with just a few components. 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. BD EMI filter L1 D1 LED L ADIM D2 C1 C2 R3 RT OUT FL7701 CS R1 ILED[0.2A/div] L2 C3 C4 R2 Figure 14. Test Circuit Figure 15 and Figure 16 show the startup waveforms from a FL7701 application in DC and AC input conditions at 220 V with ten LEDs. Figure 17. Input Source Changing: 45 Hz to 100 Hz [5V/div] ILED[0.2A/div] ILED[0.2A/div] Figure 15. [5V/div] ILED[0.2A/div] Soft-Start Performance in DC Input Condition Figure 18. Input Source Changing: 100 Hz to 45 Hz 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 Figure 16. Soft-Start Performance in AC Input Condition Rev. 1.2 12/30/14 5
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. [10V/div] VDC[40V/div] VCS[1V/div] VGATE[7V/div] 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. VDD[3V/div] ILED[0.2A/div]VD RVDRAIN[100V/div ] 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. VDD[3V/div] ILED[0.1A/div] 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. BD EMI filter Electrolytic Capacitor FRD LED L AC INPUT ADIM RT FL7701 OUT CS Figure 23. Circuit with Electrolytic Capacitor Figure 21. Typical Operating Waveforms VDD[3V/div] Vgate[7V/div] ILED[0.2A/div] Figure 24. Typical with Bulk Capacitor Rev. 1.2 12/30/14 6
Preventing LED Flicker Due to ZCD Error During Analog Dimming pin of FL7701 is usually connected behind the EMI filter components. Waveform of pin has distortion due to L-C resonance of the EMI filter and discharging of EMI capacitor. This could lead to waveform distortion as well as ZCD detection error as shown in Figure 25. The abnormal waveform could cause LED light flicker to happen. Minimum Dimming Range Operating range of analog dimming pin is 0.5 V ~ 3.5 V. If a dc signal is not injected into the ADIM pin, this pin is internally pulled high to 3.5 V and the system operates at full brightness. When voltage of ADIM pin is below 0.5 V, FL7701operates with minimum turn on which decides the minimum dimming current as shown in Figure 27. Figure 25. LED Flicker at Analog Dimming The reasons for the LED flickering are as follow; 1. EMI filter is optimized for the rated output power. When analog diming is used, the output power is decreased. As the output power decreases, the EMI filter capacitor is not fully discharged. 2. When the EMI filter capacitor is fully discharged, voltage does not have a drop point and this causes ZCD error 3. When ZCD error happens, FL7701 changes to DC mode operation for the next 6 rectified line cycles LED string has a flicker due to current deviation. In order to prevent ZCD error when FL7701 is used for analog dimming, connection point must change from the drain side to the input side via series connected diodes as shown in Figure 26. Figure 27. Example of Analog Dimming Curve 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 and signal. V CC capacitor should be located close to the pin. Powering Path LED Capacitor for pin Fuse 500mA/250V BD MB6S FLS0116 CS DRAIN One point P IC RT ADIM Analog Dimming Signal with MCU Figure 28. S Example LED Layout Figure 26. FLS0126 Application with Analog Dimming Rev. 1.2 12/30/14 7
Related Datasheets FL7701 - Smart LED Lamp Driver IC with PFC Function FLS0116 - MOSFET Integrated 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. FAIRCHILD 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 CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems 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. Rev. 1.2 12/30/14 8
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