AND8298 High Intensity LED Drivers Using NCP3065/NCV3065 Prepared by: Petr Konvicny ON Semiconductor http://onsemi.com Introduction High brightness LEDs are a prominent source of light and have better efficiency and reliability than conventional light sources. Improvements in high brightness LEDs present the potential for creative new lighting solutions that offer an improved lighting experience while reducing energy demand. LEDs require constant current driver solutions due to their wide forward voltage variation and steep V/I transfer function. For applications that are powered from low voltage AC sources typically used in landscape lighting or low voltage DC sources that may be used in automotive applications, high efficiency driver that can operate over wide range of input voltages to drive series strings of one to several LEDs. OSRAM OSTAR™, TopLED™ and Golden Dragon™. Configurations like this are found in 12 VDC track lighting applications, automotive applications, and low voltage AC landscaping applications as well as track lighting such as under−cabinet lights and desk lamps that might be powered from standard off−the−shelf 5 VDC and 12 VDC wall adapters. The NCP3065/NCV3065 can operate as a switcher or as a controller. These options are shown bellow. The brightness of the LEDs or light intensity is measured in Lumens and is proportional to the forward current flowing through the LED. The light efficiency can vary with the current flowing through the LED string. The NCP3065 is rated for commercial/industrial temperature ranges and the NCV3065 is automotive qualified. Demo Board Design Versions The demo boards are designed to display the full functionality and flexibility of NCP3065 as a driver to drive various LEDs at the low voltage AC and DC sources. The components are selected for the 15 W LED driver application. Based on this circuit, there are many possible configurations with different input voltages and output power levels that could be derived by making some minor components changes. Table 1 shows these different circuit solutions. Each application is described by the schematic and the bill of material and it has the option of LED dimming by using an external PWM signal. Component Selection Inductor When selecting an inductor there is a trade off between inductor size and peak current. In normal applications the ripple current can range from 15% to 100%. The trade off being that with small ripple current the inductance value increases. The advantage is that you can maximize the current out of the switching regulator. Figure 1. Buck Demo Board NCP/NCV3065 Demo Board This application note describes a DC−DC converter circuits that can easily be configured to drive LEDs at several different output currents and can be configured for either AC or DC input. The NCP3065/NCV3065 can be configured in a several driver topologies to a drive string of LEDs: be it traditional low power LEDs or high brightness high power LEDs such as the Lumileds Luxeon™ K2 and Rebel series, the CREE XLAMP™ 4550 or XR series, the © Semiconductor Components Industries, LLC, 2009 April, 2009 − Rev. 1 With Output Capacitor Operation A traditional buck topology includes an inductor followed by an output capacitor which filters the ripple. The capacitor is placed in parallel with the LED or array of LEDs to lower LED ripple current. With this approach the output inductance can be reduced which makes the inductance 1 Publication Order Number: AND8298/D AND8298 Output Capacitor smaller and less expensive. Alternatively, the circuit could be run at lower frequency with the same inductor value which improves the efficiency and expands the output voltage range. Equation 2 is used to calculate the capacitor size based on the amount of LED ripple. When you choose output capacitor we have to think about its value, ESR and ripple current. C OUT + No Output Capacitor Operation V IN * V OUT DI MAX T ON (eq. 2) Current Feedback Loop A constant current buck regulator such as the NCP3065 focuses on the control of the current through the load, not the voltage across it. The switching frequency of the NCP3065 is in the range of 100 kHz − 300 kHz which is much higher than the human eye can detect. This allows us to relax the ripple current specification to allow higher peak to peak values. This is achieved by configuring the NCP3065 in a continuous conduction buck configuration with low peak to peak ripple thus eliminating the need for an output filter capacitor. The important design parameter is to keep the peak current below the maximum current rating of the LED. Using 15% peak−to−peak ripple results in a good compromise between achieving max average output current without exceeding the maximum limit. This saves space and reduces part count for applications that require a compact footprint. For the common LED currents such as the 350 mA, 700 mA, 1000 mA we setup inductor ripple current to the $52.5 mA, $105 mA, $150 mA. With respect these requirements we are able to select inductor value (Equation 1). L+ V IN * (1 * D) * D DI + DV * 8 * f 8 * L * f 2 * DV OUT To drive LEDs in a constant current mode, the feedback for the regulator is taken by sensing the voltage drop across the sensing resistor R12, see Figures 2 or 8. The RC circuit (R10 & C5) between the sense resistor and the feedback pin improves converter transient response. The low feedback reference voltage of 235 mV allows the use of low power and lower cost sense resistor. Equation 3 calculates the sense resistor value. I OUT + LED current (mA) R sense + 0.235 V R sense [A] (eq. 3) Sensing resistor value (mW) 350 680 1/4W 700 330 1/4W 1000 220 1/4W (eq. 1) http://onsemi.com 2 V REF AND8298 Table 1. COMPONENTS CHANGES FOR DIFFERENT CONFIGURATIONS LED Driver VIN ILED VF L COUT R8 Application (V) (mA) (V) (mH) (mF) (W) 12 VDC 1 W LED 10 − 14 350 3.6 47 100 12k 150 0 3k3 47 100 16k 150 0 12k 12 VDC 3 W LED 10 − 14 12 VDC 5 W LED 10 − 14 24 VDC 5 W LED 700 or 350 700 or 1000 21 − 27 24 VDC 10 W LED 350 21 − 27 700 3.6 or 7.2 7.2 or 3.6 14 14 BUCK 12 VAC 1 W LED 14 − 20 12 VAC 3 W LED 14 − 20 12 VAC 5 W LED Vout NCP3065 14 − 20 350 700 or 350 700 or 1000 3.6 or 7.2 7.2 or 3.6 12 VAC 5 W 14 − 20 350 14 12 VAC 15 W 21 − 27 1000 14 100 12k 0 12k 68 100 160k 220 0 39k 68 100 150k 220 0 100k 47 100 7k5 220 0 7k5 47 100 22k 220 0 22k 47 100 36k 220 0 100k/16k 47 100 NU 220 0 NU 47 100 82k average current through the LEDs Figure 5. The component value of the RC filter are dependent on the PWM frequency. Due to this, the frequency has to be higher. Figure 17 illustrates the linearity of the digital dimming function with a 200 Hz digital PWM. The dimming frequency range for digital input mode is basically from 200 Hz to 1 kHz. For frequencies below 200 Hz the human eye will see the flicker. The low dimming frequencies are EMI convenient and an impact to it is small. The Figure 3 shows us an example of solution A, which uses the COMP pin to perform the dimming function and Figure 4 show us an example of solution B. The behavior of the NCP3065 with dimming you can see in Figures 15 and 16 and dimming linearity in the Figure 17. As you can see in these figures there aren’t any delays in the rise or fall edges, which give us the required dimming linearity. I = 350 mA 700 mA, 1000 mA Comp GND 3.6 47 150 Rsense Figure 2. NCP3065 Current Feedback Dimming Possibility The emitted LED light is proportional to average output (LED) current. The NCP3065 is capable of analog and digital PWM dimming. For the dimming we have three possibilities how to create it. We basically use a PWM signal with variable duty cycle for the managing output current value. The COMP or IPK pin of the NCP3065 is used to provide dimming capability. In digital input mode the PWM input signal inhibits switching of the regulator and reducing the average current through the LEDs. In analog input mode a PWM input signal is RC filtered and the resulting voltage is summed with the feedback voltage thus reduces the http://onsemi.com 3 AND8298 NCP3065 NCP3065 J2 +VIN J3 NC R1 + 0R10 C2 GND J5 IPK +VIN VCC J3 ON/OFF 1k2 Q2 BC817−LT1G R9 10k +VIN J3 + 0R10 10k GND J5 IPK VCC C2 COMP R11 ON/OFF 1k2 Q2 BC817−LT1G R10 1k 0805 1k C9 R10 J5 LED R12 Rsense $1% Figure 5. NCP3065 Dimming Solution C NC R9 R19 1k 0805 R12 Rsense $1% NCP3065 R1 IPK COMP R11 ON/OFF Figure 3. NCP3065 Dimming Solution A J2 NC VCC C2 J5 R10 1k 0805 0R10 + GND COMP R11 R1 J2 R12 Rsense $1% Figure 4. NCP3065 Dimming Solution B http://onsemi.com 4 AND8298 BOARD LAYOUT The layout of the evaluation board and schematic is shown below in Figure 6 and Figure 7. Figure 6. Demo board layout Top (Not in Scale) Figure 7. Demo Board Silk Screen Top http://onsemi.com 5 http://onsemi.com 6 Figure 8. 12 VDC and 24 VDC Input LED Driver Schematic R11 R11 ON/OFF 1k2 J6 ON/OFF 1k2 J6 +VAUX J4 GND J3 +VIN J2 R3 R4 R5 R6 R7 220 F/50V 0.1 F Q1 IPK NC SWC SWE 1k C5 15k R13 NU 1k 0805 100pF 1.8nF C3 10k CT R8 MMSD4148 D2 R10 R14 NU NCP3065 SOIC8 VCC TCAP COMP COMP GND VCC IPK NC U1 R15 R9 Q2 BC817−LT1G C2 + C4 1206 12061206 1206 1206 1206 R2 1%R BC807−LGT1G 0R10 R1 6x 1R0 Q5 MMBT3904LT1G Q4 NTF2955 C1 R12 Rsense 1% MBRS140LT3G 0.1 F 1206 D1 L1 GND J7 −LED J5 +LED C6 NU + J1 AND8298 CON3 J2 D3 D5 7 http://onsemi.com 1k2 0805 R3 C5 R2 1k 0805 100pF 1.8nF C2 10k 0805 NCP3065 SOIC8 CT R5 100nF C3 220mF/35V + SWC SWE COMP VCC TCAP COMPGND IPK NC U1 Q1 BC817−LT1G R4 VCC MBRS2040LT3 MBRS2040LT3 D4 MBRS2040LT3 MBRS2040LT3 C4 D2 VCC 0.15R/0.5W R1 R6 0.68W 1206 J4 LED C1 1mF 1206 R7 0.68W 1206 R8 0.68W 1206 Jumper1 Jumper2 J3 MBRS2040LT3 D1 L1 OUTPUT J1 AND8298 Figure 9. Schematic NCP3065 as Switcher in the AC Input LED Driver Application AND8298 Table 2. 12 VDC INPUT 1 W LED DRIVER WITHOUT OUTPUT CAPACITOR BILL OF MATERIALS Qty Reference Part Description 2 C1, C4 100 nF, Ceramic Capacitor Mfg P/N 1 C2 220 mF/50 V, Electrolytic Capacitor Package Mtg 1206 SMD G, 10x10.2 SMD 1 C3 1.8 nF, Ceramic Capacitor 0805 SMD 1 C5 100 pF, Ceramic Capacitor 0805 SMD 1 D1 1 A, 40 V Schottky Rectifier 1 D2 1 L1 1 Q4 1 EEEVFK1H221P Mfg Panasonic MBRS140LT3G ON Semiconductor SMB SMD Switching Diode MMSD4148T1G ON Semiconductor SOD123 SMD Surface Mount Power Inductor DO3340P−154MLD Coilcraft Power MOSFET, P−Channel NTF2955T1G ON Semiconductor SOT223 SMD Q5 General Purpose Transistor MMBT3904LT1G ON Semiconductor SOT23 SMD 1 R1 100 mW, 0.5 W 2010 SMD 1 R8 3k3, Resistor 0805 SMD 1 R9 10 kW, Resistor 0805 SMD 2 R10, R15 1 kW, Resistor 0805 SMD 1 R11 1.2 kW, Resistor 0805 SMD 1 R12 680 mW, $1% 1206 SMD 1 U1 DC−DC Controller SOIC8 SMD Package Mtg 1206 SMD G, 10x10.2 SMD 0805 SMD NCP3065 ON Semiconductor SMD Table 3. 12 VDC INPUT 1 W LED DRIVER WITH OUTPUT CAPACITOR BILL OF MATERIALS Qty Reference Part Description Mfg P/N 2 C1, C4 100 nF, Ceramic Capacitor 1 C2 220 mF/50 V, Electrolytic Capacitor 1 C3 1.8 nF, Ceramic Capacitor 1 C5 100 pF, Ceramic Capacitor 1 C6 100 mF/50 V, Electrolytic Capacitor 1 D1 1 D2 1 1 1 1 1 EEEVFK1H221P Mfg Panasonic 0805 SMD F, 8x10.2 SMD ON Semiconductor SMB SMD ON Semiconductor SOD123 SMD EEEVFK1H101P Panasonic 1 A, 40 V Schottky Rectifier MBRS140LT3G Switching Diode MMSD4148T1G L1 Surface Mount Power Inductor DO3316P−473MLD Coilcraft Q4 Power MOSFET, P−Channel NTF2955T1G ON Semiconductor SOT223 SMD Q5 General Purpose Transistor MMBT3904LT1G ON Semiconductor SOT23 SMD R1 100 mW, 0.5 W 2010 SMD R8 12k, Resistor 0805 SMD 1 R9 10 kW, Resistor 0805 SMD 2 R10, R15 1 kW, Resistor 0805 SMD 1 R11 1.2 kW Resistor 0805 SMD 1 R12 680 mW, $1% 1206 SMD 1 U1 DC−DC Controller SOIC8 SMD NCP3065 Table 4. 12 VDC Input 1 W LED Drivers Test Results Test Efficiency With Output Cap Without Output Cap Line regulation Output Current Ripple With Output Cap Without Output Cap Result 74% 72% $3% < 50 mA < 100 mA http://onsemi.com 8 ON Semiconductor SMD AND8298 Table 5. 12 VDC INPUT 3 W LED DRIVER WITHOUT OUTPUT CAPACITOR BILL OF MATERIALS Qty Reference Part Description 2 C1, C4 100 nF, Ceramic Capacitor Mfg P/N 1 C2 220 mF/50 V, Electrolytic Capacitor 1 C3 1.8 nF, Ceramic Capacitor 1 C5 100 pF, Ceramic Capacitor 1 D1 2 A, 40 V Schottky Rectifier 1 D2 1 L1 1 Q4 1 1 1 1 2 1 1 R12 330 mW, $1% 1 U1 DC−DC Controller EEEVFK1H221P Mfg Panasonic Package Mtg 1206 SMD G, 10x10.2 SMD 0805 SMD 0805 SMD MBRS240LT3G ON Semiconductor SMB SMD Switching Diode MMSD4148T1G ON Semiconductor SOD123 SMD Surface Mount Power Inductor DO3340P−154MLD Coilcraft Power MOSFET, P−Channel NTF2955T1G ON Semiconductor SOT223 SMD Q5 General Purpose Transistor MMBT3904LT1G ON Semiconductor SOT23 SMD R1 100 mW, 0.5 W 2010 SMD R8 12k, Resistor 0805 SMD R9 10 kW, Resistor 0805 SMD R10, R15 1 kW, Resistor 0805 SMD R11 1.2 kW, Resistor 0805 SMD 1206 SMD SOIC8 SMD NCP3065 ON Semiconductor SMD Table 6. 12 VDC INPUT 3 W LED DRIVER WITH OUTPUT CAPACITOR BILL OF MATERIALS Qty Reference Part Description 2 C1, C4 100 nF, Ceramic Capacitor Mfg P/N 1 C2 220 mF/50 V, Electrolytic Capacitor 1 C3 1.8 nF, Ceramic Capacitor, 1 C5 100 pF, Ceramic Capacitor, 1 C6 100 mF/50 V, Electrolytic Capacitor 1 D1 1 D2 1 EEEVFK1H221P Mfg Panasonic Package Mtg 1206 SMD G, 10x10.2 SMD 0805 SMD 0805 SMD F, 8x10.2 SMD ON Semiconductor SMB SMD ON Semiconductor SOD123 SMD EEEVFK1H101P Panasonic 2 A, 40 V Schottky Rectifier MBRS240LT3G Switching Diode MMSD4148T1G L1 Surface Mount Power Inductor DO3316P−473MLD Coilcraft 1 Q4 Power MOSFET, P−Channel NTF2955T1G ON Semiconductor SOT223 SMD 1 Q5 General Purpose Transistor MMBT3904LT1G ON Semiconductor SOT23 SMD 1 R1 100 mW, 0.5 W 2010 SMD 1 R8 16k, Resistor 0805 SMD 1 R9 10 kW, Resistor 0805 SMD 2 R10, R15 1 kW, Resistor 0805 SMD 1 R11 1.2 kW, Resistor 0805 SMD 1 R12 330 mW, $1% 1206 SMD 1 U1 DC−DC Controller SOIC8 SMD NCP3065 Table 7. 12 VDC Input 3 W LED Drivers Test Results Test Efficiency With Output Cap Without Output Cap Line regulation Output Current Ripple With Output Cap Without Output Cap Result 76% 76% $5% < 50 mA < 90 mA http://onsemi.com 9 ON Semiconductor SMD AND8298 Table 8. 12 VDC INPUT 5 W LED DRIVER WITHOUT OUTPUT CAPACITOR BILL OF MATERIALS Qty Reference Part Description 2 C1, C4 100 nF, Ceramic Capacitor Mfg P/N 1 C2 220 mF/50 V, Electrolytic Capacitor Package Mtg 1206 SMD G, 10x10.2 SMD 1 C3 1.8 nF, Ceramic Capacitor 0805 SMD 1 C5 100 pF, Ceramic Capacitor 0805 SMD 1 D1 2 A, 40 V Schottky Rectifier 1 D2 1 L1 1 Q4 1 EEEVFK1H221P Mfg Panasonic MBRS240LT3G ON Semiconductor SMB SMD Switching Diode MMSD4148T1G ON Semiconductor SOD123 SMD Surface Mount Power Inductor DO3340P−154MLD Coilcraft Power MOSFET, P−Channel NTF2955T1G ON Semiconductor SOT223 SMD Q5 General Purpose Transistor MMBT3904LT1G ON Semiconductor SOT23 SMD 1 R1 100 mW, 0.5 W 2010 SMD 1 R8 12k, Resistor 0805 SMD 1 R9 10 kW, Resistor 0805 SMD 2 R10, R15 1 kW, Resistor 0805 SMD 1 R11 1.2 kW, Resistor 0805 SMD 1 R12 220 mW, $1% 1206 SMD 1 U1 DC−DC Controller SOIC8 SMD Package Mtg 1206 SMD G, 10x10.2 SMD NCP3065 ON Semiconductor SMD Table 9. 12 VDC INPUT 5 W LED DRIVER WITH OUTPUT CAPACITOR BILL OF MATERIALS Qty Reference Part Description Mfg P/N 2 C1, C4 100 nF, Ceramic Capacitor 1 C2 220 mF/50 V, Electrolytic Capacitor 1 C3 1.8n F, Ceramic Capacitor, 0805 SMD 1 C5 100 pF, Ceramic Capacitor, 0805 SMD 1 C6 100 mF/50 V, Electrolytic Capacitor EEEVFK1H101P Panasonic F, 8x10.2 SMD 1 D1 2 A, 40 V Schottky Rectifier MBRS240LT3G ON Semiconductor SMB SMD 1 D2 Switching Diode MMSD4148T1G ON Semiconductor SOD123 SMD 1 L1 Surface Mount Power Inductor DO3316P−473MLD Coilcraft 1 Q4 Power MOSFET, P Channel NTF2955T1G ON Semiconductor SOT223 SMD 1 Q5 General Purpose Transistor MMBT3904LT1G ON Semiconductor SOT23 SMD 1 R1 100 mW, 0.5 W 2010 SMD 1 R8 15k, resistor 0805 SMD 1 R9 10 kW, resistor 0805 SMD 2 R10, R15 1 kW, resistor 0805 SMD 1 R11 1.2 kW, resistor 0805 SMD 1 R12 220 mW, $1% 1206 SMD 1 U1 DC−DC controller SOIC8 SMD EEEVFK1H221P NCP3065 Table 10. 12 VDC Input 5 W LED Drivers Test Results Test Result Efficiency 75% Line regulation $4% Output Current Ripple With Output Cap Without Output Cap < 50mA < 110mA http://onsemi.com 10 Mfg Panasonic ON Semiconductor SMD 400 800 390 780 380 760 370 740 360 720 IOUT (mA) IOUT (mA) AND8298 350 340 680 330 660 320 640 310 620 300 600 9 10 11 12 13 14 15 14 16 17 18 19 20 VIN (V) Figure 10. Current Regulation, 12 VDC Input 1 W LED Driver Figure 11. Current Regulation, 12 VAC Input 3 W LED Driver 21 95 1100 90 EFFICIENCY (%) 1050 1000 950 85 80 75 900 850 10 15 VIN (V) 1150 IOUT (mA) 700 11 12 VIN (V) 13 14 70 14 Figure 12. Current Regulation, 12 VDC Input 5 W LED Driver 15 16 17 VIN (V) 18 19 Figure 13. 12 VAC Input 5 W LED Driver Efficiency http://onsemi.com 11 20 AND8298 Figure 14. 12 VDC, IOUT = 350 mA Input Inductor Ripple Without Output Capacitor, C1 Inductor Input, C4 Inductor Current Table 11. BUCK EFFICIENCY RESULTS FOR DIFFERENT RIPPLE WITH NO OUTPUT CAPACITOR Efficiency 1 LED, Vf = 3.6 V 2 LEDs, Vf = 3.6 V 4 LED, Vf = 14.4 V IOUT = 350 mA > 74% > 83% − IOUT = 700 mA > 76% > 83% − IOUT = 1000 mA > 75% − − IOUT = 350 mA > 70% > 80% > 87% IOUT = 700 mA > 72% > 82% − IOUT = 1000 mA > 70% − − IOUT = 350 mA − − > 82% IOUT = 700 mA − − > 86% IOUT = 1000 mA − − > 87% VIN = 12 VDC VIN = 12 VAC VIN = 24 VDC http://onsemi.com 12 AND8298 Figure 15. NCP3065 Behavior with Dimming, Frequency is 200 Hz, Duty Cycle 50% Figure 16. NCP3065 Dimming Behavior, Frequency 1 kHz, Duty Cycle 50% 800 24 VIN, VF 3.6 V 700 ILED (mA) 600 500 400 12 VIN, VF 3.6 V 300 200 24 VIN, VF 7.2 V 100 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) Figure 17. Output Current Dependency on the Dimming Duty Cycle Pulse feedback design The NCP3065 is a burst−mode architecture product which is similar but not exactly the same as a hysteretic architecture. The output switching frequency is dependent on the input and output conditions. The NCP3065 oscillator generates a constant frequency that is set by an external capacitor. This output signal is then gated by the peak current comparator and the oscillator. When the output current is above the threshold voltage the switch turns off. When the output current is below the threshold voltage the switch is turned on and gated with the oscillator. A simplified schematic is shown in Figure 18. This may cause possible overshoots on the output. Using the pulse feedback circuit will reduce this overshoot. This will result in a stabilized switching frequency and reduce the overshoot and output ripple. The pulse feedback circuit is implemented by adding an external resistor R8 between the CT pin and inductor input as shown in the buck schematic Figure 8. The resistor value is dependent on the input/output conditions and switching frequency. The typical range is 3k to 200k. Table 1 contains a list of typical applications and the recommended value for the pulse feedback resistor. Using an adjustable resistor in place of R8 when evaluating an application will allow the designer to optimize the value and make a final selection. http://onsemi.com 13 AND8298 Oscillator Output from Peak Current Comparator LED Vref + − VSENSE Figure 18. Burst−Mode Architecture Figures 19 and 20 show the effect of the pulse feedback resistor on the switching waveforms and load current ripple. This results in a fixed frequency switching with constant duty cycle, which is only dependent upon the input and output voltage ratio. When the ratio (VOUT/VIN) is near 1 (high duty cycle) over the entire input voltage range, the pulse feedback is not needed. Boost Converter Demo Board Figure 19. Switching Waveform Without Pulse Feedback Figure 21. Boost Demo Board Boost Converter Topology The Boost converter schematic is illustrated in Figure 22. When the low side power switch is turned on, current drawn from the input begins to flow through the inductor and the current Iton rises up. When the low side switch is turned off, the current Itoff circulates through diode D1 to the output capacitor and load. At the same time the inductor voltage is added with the input power supply voltage and as long as this is higher than the output voltage, the current continues to flow through the diode. Provided that the current through the inductor is always positive, the converter is operating in continuous conduction mode (CCM). On the next switching cycle, the process is repeated. Figure 20. Switching Waveform With Pulse Feedback http://onsemi.com 14 15 http://onsemi.com Figure 22. 12 VDC Input LED Driver Schematic R10 R10 ON/OFF 1k2 J6 ON/OFF 1k2 J6 +VAUX J4 GND J3 +VIN J2 Q1 R6 NU R11 Q2 BC817−LT1G 22mF/50V 0.1mF R5 C3 + R3 R4 C5 R2 BC807−LGT1G 0R15 R1 6x 1R0 $1% R7 SWC SWE 1k0 R8 NCP3065 VCC TCAP COMPGND IPK NC U1 L1 R9 Rsense MM3Z36VT1G D2 2.2nF C2 D1 MBRS140LT3G 0.1mF C2 C1 100mF/ 50V + −LED J5 GND J3 +LED J1 AND8298 AND8298 transistor is off. You could connect an external PWM signal to pin ON/OFF and a power source to pin +VAUX to realize the PWM dimming function. When the dimming signal exceeds the turn on threshold of the external PNP or NPN transistor, the comp pin will be pulled up. A TTL level input can also be used for dimming control. The range of the dimming frequency is from 100 Hz to 1 kHz, but it is recommended to use frequency around 200 Hz as this is safely above the frequency where the human eye can detect the pulsed behavior, in addition this value is convenient to minimize EMI. There are two options to determine the dimming polarity. The first one uses the NPN switching transistor and the second uses a PNP switching transistor. The switch on/off level is dependent upon the chosen dimming topology. The external voltage source (VAUX) should have a voltage ranging from +5 VDC to +VIN. Figure 17 illustrates average LEDs current dependency on the dimming input signal duty cycle. For cycle by cycle switch current limiting a second comparator is used which has a nominal 200 mV threshold. The value of resistor R1 determines the current limit value and is configured according to the following equation. When operating in CCM the output voltage is equal to V OUT + V IN @ 1 1*D The duty cycle is defined as D+ t ON t ON ) t OFF + t ON T The input ripple current is defined as DI + V IN D f*L The load voltage must always be higher than the input voltage. This voltage is defined as V load + V sense ) n * V f where Vf = LED forward voltage, Vsense is the converter reference voltage, and n = number of LED’s in cluster. Since the converter needs to regulate current independent of load voltage variation, a sense resistor is placed across the feedback voltage. This drop is calculated as V sense + I load ) n * R sense The Vsense corresponds to the internal voltage reference or feedback comparator threshold. I pk(SW) + The maximum output voltage is clamped with an external zener diode, D2 with a value of 36 V which protects the NCP3065 output from an open LED fault. The demo board has a few options to configure it to your needs. You can use one 150 mW (R1) or a combination of parallel resistors such as six 1 W resistors (R2 − R7) for current sense. To evaluate the functionality of the board, high power LEDs with a typical Vf = 3.42 V @ 350 mA were connected in several serial combinations (4, 6, 8 LED’s string) and 4 chip and 6 chip LEDs with Vf =14V respectively Vf = 20.8 V @ 700 mA. Simple Boost 350 mA LED driver The NCP3065 boost converter is configured as a LED driver is shown in Figure 22. It is well suited to automotive or industrial applications where limited board space and a high voltage and high ambient temperature range might be found. The NCP3065 also incorporates safety features such as peak switch current and thermal shutdown protection. The schematic has an external high side current sense resistor that is used to detect if the peak current is exceeded. In the constant current configuration, protection is also required in the event of an open LED fault since current will continue to charge the output capacitor causing the output voltage to rise. An external zener diode is used to clamp the output voltage in this fault mode. Although the NCP3065 is designed to operate up to 40 V additional input transient protections might be required in certain automotive applications due to inductive load dump. The main operational frequency is determined by the external capacitor C4. The ton time is controlled by the internal feedback comparator, peak current comparator and main oscillator. The output current is configured by an internal feedback comparator with negative feedback input. The positive input is connected to an internal voltage reference of 0.235 V with 10% precision over temperature. The nominal LED current is setup by a feedback resistor. This current is defined as: I OUT + 0.2 + 1.33 A 0.15 Number of LEDs String Forward Voltage at 255C Min Typ Max 4 11.16 13.68 15.96 6 16.74 20.52 23.94 8 22.32 27.36 31.92 The efficiency was calculated by measuring the input voltage and input current and LED current and LED voltage drop. The output current is dependent on the peak current, inductor value, input voltage and voltage drop value and of course on the switching frequency. I OUT + (D * D 2) * 0.235 R sense There are two approaches to implement LED dimming. Both use the negative comparator input as a shutdown input. When the pin voltage is higher than 0.235 V the switch D+ http://onsemi.com 16 ǒ I pk(SW) D V OUT ) V F * V IN V OUT ) V F * V SWCE * Ǔ V IN * V SWCE [*] 2*L*f [A] AND8298 is flat and represents normal operation, Region 3 occurs when VIN is greater than VOUT and there is no longer constant current regulation. Region 3 and 1 are included here for illustrative purposes as this is not a normal mode of operation. Figure 9 illustrates the additional circuitry required to support 12 VAC input signal which includes the addition of a bridge rectifier and input filter capacitor. The rectified dc voltage is VOUT Output Voltage VIN Input Voltage VF Schottky Diode Forward Voltage VSWCE Switch Voltage Drop Ipk(SW) Peak Switch Current D Duty Cycle L Inductor Value f Switching Frequency Line regulation curve in Figure 24 illustrates three distinct regions; in the first region, the peak current to the switch is exceeded tripping the overcurrent protection and causing the regulated current to drop, Region 2 is where the current V INDC + Ǹ2 * V AC [ 17 V DC 95 400 390 90 Boost 4LED 350 mA 380 EFFICIENCY (%) Boost 6LED 350 mA 370 ILOAD (mA) 85 80 Boost 6LED 350 mA Boost 4LED 350 mA 360 350 340 330 320 75 310 70 6 8 10 12 14 16 18 20 300 22 8 6 10 12 14 16 18 20 22 VIN (V) VIN (V) Figure 23. Boost Converter Efficiency for 4 or 6 LEDs and Output Current 350 mA Figure 24. Line Regulation for 4 or 6 LEDs and Output Current 350 mA Table 12. NCP3065 BOOST BILL OF MATERIALS Qty Reference Part Description Mfg P/N Mfg Package Mtg 1 C1 100 mF/50 V, Electrolytic Capacitor EEEVFK1H101P Panasonic F, 8x10.2 SMD 2 C2,C5 100 nF, Ceramic Capacitor 1206 SMD 1 C3 220 mF/50 V, Electrolytic Capacitor G, 10x10.2 SMD 1 C4 2.2 nF, Ceramic Capacitor 0805 SMD 1 D1 1 A, 40 V Schottky Rectifier MBRS140LT3G ON Semiconductor SMB SMD 1 D2 Zener Diode, 36 V MM3Z36VT1G ON Semiconductor SOD123 SMD 1 L1 Surface mount power inductor DO3340P−104MLD Coilcraft 1 Q2 General purpose transistor BC817−LT1G ON Semiconductor 1 R1 1 R8 1 EEEVFK1H221P Panasonic SMD SOT23 SMD 150 mW, 0.5 W 2010 SMD 1k, Resistor 0805 SMD R9 680 mW, $1% 1206 SMD 2 R10 1.2 kW, Resistor 0805 SMD 1 U1 DC−DC Controller SOIC8 SMD NCP3065 http://onsemi.com 17 ON Semiconductor AND8298 Conclusion for a variety of constant current buck and boost LED driver applications. In addition there is an EXCEL tool at the ON Semiconductor website for calculating inductor and other passive components if the design requirements differ from the specific application voltages and currents illustrated in these example. LEDs are replacing traditional incandescent and halogen lighting sources in architectural, industrial, residential and the transportation lighting. The key challenge in powering LED’s is providing a constant current source. The demo board for the NCP3065/NCV3065 can be easily configured ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. 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