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 HV pin is over 25 V to 500 V, the FL7701 maintains a 15.5 VDC at the VCC pin. The FL7701 also has a UVLO block for stable operation. When the VCC voltage reaches higher than VCCST+, the UVLO block starts operation.hen the VCC drops below the VCCST-, IC operation stops. The internal DAC_OUT reference signal is dependent on the VCC 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. 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 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 time. When a DC source connects to the IC, the internal reference immediately changes to DC waveform. Vsup D1 + VLED - Iline LED Load L FL7701 VCC VSUP_SEN DAC C ZCD_OUT HV Device HV DAC: Digital to Analog HV Device : High-Voltage Device DAC_OUT Driver S Reference R OUT Isw Q CS GND Figure 1. Basic Block of FL7701 © 2012 Fairchild Semiconductor Corporation Rev. 1.0.1 • 11/9/12 Figure 2. FL7701 Operation IL 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 reference is fixed at a certain DC level. In this time, the IC continually tries to find input phase information from the VCC 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. www.fairchildsemi.com AN-9744 APPLICATION NOTE 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. Figure 3. DC Input Condition Vp Vp / 2 Figure 6. Figure 4. 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 . 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. 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: 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 VCC pin for supply voltage stabilization. f OSC = Vbridge Internal Reference Bridge Diode Output Voltage 2.02 × 109 [Hz] RT (1) Output Open-Circuit Protection Input Voltage Peak 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. VCC ZCD BD DAC_OUT EMI filter LED L1 L D1 Figure 5. Internal PFC Function HV C1 The FL7701 detects the VCC 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. © 2012 Fairchild Semiconductor Corporation Rev. 1.0.1 • 11/9/12 D2 ADIM C2 OUT RT VCC FL7701 R3 C3 L2 Figure 7. CS C4 GND R1 R2 LED Open Condition www.fairchildsemi.com 2 AN-9744 APPLICATION NOTE For example, if VIN(max) = 220 V, η =85% and ten LEDs are in series connection, the minimum duty ratio is: 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 EdgeBlanking (LEB) time of 350 ns; the IC stops operation. VCC ZCD 10 × 3.5 = 0.132 0.85 × 2 × 220 Step 2: Maximum Duty Ratio Similar to Step 1, calculate maximum duty ratio as: HV JFET VCC Dmin = UVLO time D max = ZCD nV F (3) η × Vin (min) DAC Soft start TSD Digital Block RT S Oscillator Reference - [%] Duty 50 R + CS LEB GND 60 OUT Q 40 Leading Edge Blanking + 30 AOCP 2.5V Figure 8. 20 AOCP Function 10 Analog Dimming Function 0 0 The Analog Dimming (ADIM) function adjusts the output LED current by changing the voltage level of the ADIM pin. Figure 9. Application Information Vin (min) = Table 1 shows one example of a design target using the FL7701 device. Specification Frequency 45 kHz Output Voltage 35 VF=3.5 V, n=10 Output LED Current RMS 0.3 ILED(rms) Output LED Current Peak 0.5 ILED(peak) Input Voltage (Max.) 220 VAC(rms) [ms] nV F 35 = = 82.35[V ] η × Dmax 0.85 × 0.5 (4) 311V Note DCM Expected min. input voltage (CCM) : Vin(min)=82.35V DCM time Current Limit on the DAC reference Average LED Current(ILED(ave) i CCM 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. nVF η × Vin (max) 15 Duty Variation vs. Time Input voltage Target Design Specification Item Dmin = 10 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), recalculate the minimum input voltage for CCM operation: 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 VAC. Table 1. 5 Dmin 1-Dmin time ton toff Figure 10. Estimated Waveforms (2) where η is efficiency of system; VIN(max) is maximum input voltage; VF is forward-drop voltage of LED; and n is LED number in series connection. © 2012 Fairchild Semiconductor Corporation Rev. 1.0.1 • 11/9/12 www.fairchildsemi.com 3 AN-9744 APPLICATION NOTE 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 time. For example, the maximum on/off time at 45 kHz operation condition is: Step 5: Inductance Derive one more formula for the minimum inductance value of the inductor using the Step 4 results: L= t on = t off 1 1 = = = 11.11 [μs] 2 f s 90000 (VF × n)(1 − Dmin ) 3.5 ×10× (1 − 0.132) = = 4.5[mH] f s × Δi 45000× 0.1516 (7) 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 12. Figure 11. Current Ripple (∆I) vs. Inductance Target Waveforms of LED Current Using the typical LED current waveform in Figure 11, derive the formula as: Δi or 2 Δi − 2 I LED ( peak ) = I LED ( ave. peak ) + I LED (min) = I LED ( ave. peak ) (5) Figure 13. Step 6: Sensing Resistor The FL7701 was calculated the sensing resistor value as: In Table 1, the desired LED current average is always located between LED peak current value, ILED(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: Δi = 2( I LED ( peak ) − I LED ( ave. peak ) ) or Δi = 2( I LED ( ave. peak ) − I LED (min) ) Expected Waveforms R= VCS 0.5 = = 1 [Ω] I LED ( peak ) 0.5 (8) The power rating is under 0.25 W even when considering power consumption at peak-current condition. Step 7: Frequency Set Resistor 1 Rt = ⋅ 2.0213 ⋅ 109 = 44.919 [kΩ] f sw (6) I LED ( ave. peak ) Where I LED ( rms ) = (9) If there is not connected Rt resistance to the operation frequency is 45 kHz. 2 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. Δi = 2( I LED ( peak ) − 2 ⋅ I LED ( rms ) ) = 2(0.5 − 2 ⋅ 0.3) = 0.1516 [ A] © 2012 Fairchild Semiconductor Corporation Rev. 1.0.1 • 11/9/12 www.fairchildsemi.com 4 AN-9744 APPLICATION NOTE System Verification 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 14 shows the recommended circuit of a FL7701 system with just a few components. VDRAIN[100V/div] ILED[0.2A/div] Figure 14. Test Circuit Figure 17. Figure 15 and Figure 16 show the startup waveforms from a on FL7701 application in DC and AC input conditions at 220 V with ten LEDs. VCC[5V/div] VDRAIN[100V/div] ILED[0.2A/div] Input Source Changing: 45 Hz to 100 Hz VDRAIN[100V/div] ILED[0.2A/div] Figure 18. Figure 15. Input Source Changing: 100 Hz to 45 Hz The Figure 19 shows the analog dimming performance with changing VADIM. The output LED current changes according to the control voltage. Soft-Start Performance in DC Input Condition VCC[5V/div] VDRAIN[100V/div] ILED[0.2A/div] Figure 19. Figure 16. VADMIN vs. LED Current Soft-Start Performance in AC Input Condition © 2012 Fairchild Semiconductor Corporation Rev. 1.0.1 • 11/9/12 www.fairchildsemi.com 5 AN-9744 APPLICATION NOTE Design Tips 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-bypulse, 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. 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. VCC[10V/div] VDC[40V/div] VCS[1V/div] VGATE[7V/div] VDD[3V/div] ILED[0.2A/div]VD RVDRAIN[100V/div ] Figure 20. AOCP Function Figure 22. 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. 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. VDD[3V/div] ILED[0.1A/div] VDRAIN[100V/div] Figure 23. Figure 21. Circuit with Electrolytic Capacitor VDD[3V/div] Vgate[7V/div] ILED[0.2A/div] VDRAIN[100V/div] Typical Operating Waveforms Figure 24. © 2012 Fairchild Semiconductor Corporation Rev. 1.0.1 • 11/9/12 Typical with Bulk Capacitor www.fairchildsemi.com 6 AN-9744 APPLICATION NOTE 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: Figure 25. Example LED Layout Locate the IC on the external powering path. Separate power GND and signal GND. VCC capacitor should be located close to the VCC pin. 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. 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. © 2012 Fairchild Semiconductor Corporation Rev. 1.0.1 • 11/9/12 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. www.fairchildsemi.com 7