National Semiconductor Chris Richardson January 7, 2008 Introduction The LM3404/04HV is a buck regulator derived controlled current source designed to drive a series string of high power, high brightness LEDs (HBLEDs) such as the Luxeon™ K2 Emitter at forward currents of up to 1.0A. The board can accept an input voltage ranging from 6V to 42V when using the LM3404. When using the pin-for-pin compatible LM3404HV the upper bound of input voltage is 75V. The converter output voltage adjusts as needed to maintain a constant current through the LED array. The LM3404/04HV is a true step-down regulator with an output voltage range extending from a VO (MIN) of 200 mV (the reference voltage) to a VO(MAX) determined by the minimum off time (typically 300 ns). It can maintain regulated current through any number of LEDs as long as the combined forward voltage of the array does not exceed VO(MAX).. Circuit Performance LM3404 The LM3404 circuit and BOM have been designed to provide a constant forward current of 1.0A to a single LED with a forward voltage of approximately 3.7V (Typical of white, blue, and green LEDs using InGaN technology) from an input of 24V ±10%. Over the input voltage range of 6V to 42V the average LED current, IF, is 1.0A ±10%, the ripple current, ΔiF, will not exceed 400 mAP-P, and the switching frequency is 450 kHz ± 10%. Circuit Performance LM3404HV The LM3404HV circuit and BOM have been designed to provide a constant forward current of 1.0A to a single LED with a forward voltage of approximately 3.7V from an input of 48V ±10%. Over the input voltage range of 6V to 75V the average LED current, IF, is 1.0A ±10%, the ripple current, ΔiF, will not exceed 400 mAP-P, and the switching frequency is 200 kHz ± 10%. Connecting to LED Array The LM3404/04HV Evaluation Board includes a female 6-pin SIP, J1, connector as well as two standard 94mil turret connectors for the cathode and anode connections of the LED array. Figure 1 shows the pinout of J1. Solid 18 or 20 gauge wire with about 1 cm of insulation stripped away makes a convenient, solderless connection to J1. 20215901 FIGURE 1. Setting the LED Current The default forward current ILED delivered to the LED array is 1.0A, typical of many 3W LEDs. To adjust this value the current setting resistor RSNS can be changed according to the following equation: LM3404/LM3404HV Evaluation Board LM3404/LM3404HV Evaluation Board tSNS = 220 ns This resistor should be rated to handle the power dissipation of the LED current. For example, the closest 5% tolerance resistor to set an LED current of 1.0A is 0.22 Ω. In steady state this resistor will dissipate (1.0 x 0.22) = 220 mW, indicating that a resistor with a 1/4W power rating is appropriate. PWM Dimming The DIM1 terminal on the PCB provides an input for a pulse width modulation signal for dimming of the LED array. In order to fully enable and disable the LM3404/04HV the PWM signal should have a maximum logic low level of 0.8V and a minimum logic high level of 2.2V. The maximum PWM dimming frequency, minimum PWM duty cycle and maximum duty cycle are illustrated in Figure 2. PWM frequency should be at least one order of magnitude below the LM3404/04HV switching frequency. The interval tD represents the delay from a logic high at the DIM pin to the onset of the output current. The quantities tSU and tSD represent the time needed for the output current to slew up to steady state and slew down to zero, respectively. Typical response times for the standard LM3404 and LM3404HV demo boards circuits are shown in the Typical Performance Characteristics section. AN-1545 © 2008 National Semiconductor Corporation 202159 www.national.com AN-1545 20215902 FIGURE 2. PWM Dimming Limits The logic of DIM1 is direct, hence the LM3404/04HV will deliver regulated output current when the voltage at DIM1 is high, and the current output is disabled when the voltage at DIM1 is low. Connecting a constant logic low will disable the output, and the LM3404/04HV is enabled if the DIM pin is open-circuited. The DIM1 function disables only the power MOSFET, leaving all other circuit blocks functioning to minimize the converter response time. The DIM2 terminal provides a second method for PWM dimming by connecting to the gate of an optional MOSFET, Q1. Note that Q1 is not provided on the standard BOM, and must be added for the DIM2 function to operate. Q1 provides a parallel path for the LED current. This small MOSFET can be turned on and off much more quickly than the LM3404/04HV can shutdown the internal MOSFET, providing faster response time for higher frequency and/or greater resolution in the PWM dimming signal. The trade-off in this method is that the full current flows through Q1 while the LED is off, resulting in lower efficiency. The logic of DIM2 is inverted, hence the LM3404/04HV will deliver regulated output current when the voltage at DIM2 is low, and the current output is disabled when the voltage at DIM2 is high. Connecting a constant logic high to the DIM2 will turn off the LED but will not shut down the LM3404/04HV. Output Open Circuit With either DIM terminal floating or connected to logic high, the LM3404/04HV will begin to operate as soon as it has an input of at least 6V. In the case that the input is powered but no LED array is connected the output voltage will rise to equal the input voltage. The output of the circuit is rated to 50V (LM3404) or 100V (LM3404HV) and will not suffer damage, however care should be taken not to connect an LED array if the output voltage is higher than the target forward voltage of the LED array in steady state. Alternatively, a zener diode and zener current limiting resistor can be placed in the positions Z1 and RZ. In the case of an accidental open circuit at the output Z1 will enter reverse bias and attempt to pull the CS pin voltage up to the output voltage. An internal comparator monitors the CS pin voltage and will disable the internal MOSFET in this case. The result is a low power hiccup mode, designed to prevent excessive voltage at the output and thermal stress on the inductor, internal MOSFET, and input voltage source. Low Power Shutdown The LM3404/04HV can be placed into a low power shutdown (typically 90 µA) by grounding the OFF* terminal. During normal operation this terminal should be left open-circuit. www.national.com 2 AN-1545 20215904 FIGURE 3. Standard Schematic TABLE 1. BOM 1 (LM3404) ID Part Number Type Size Parameters Qty Vendor U1 LM3404 LED Driver SO-8 42V, 1.0A 1 NSC L1 SLF10145T- 330M1R6 Inductor 10.1 x 10.1 x 4.5mm 33µH, 1.6A, 82mΩ 1 TDK D1 CMSH2-60M Schottky Diode SMA 60V, 2A 1 Central Semi Cf VJ0805Y104KXXAT Capacitor 0805 100nF 10% 1 Vishay Cb VJ0805Y103KXXAT Capacitor 0805 10nF 10% 1 Vishay Cin C3225X7R1H335M Capacitor 1210 3.3µF, 50V 1 TDK Rsns ERJ8BQFR22V Resistor 1206 0.22Ω 1% 1 Panasonic Ron CRCW08056812F Resistor 0805 68.1kΩ 1% 1 Vishay 0Ω Rz CRCW08050R00F Resistor 0805 1 Vishay DIM1, DIM2, OFF* 160-1512 Terminal 0.062” 3 Cambion GND1, GND2, GND3, VIN, Vo/ LED+, CS/LED- 160-1026 Terminal 0.094” 6 Cambion J1 535676-5 Connector Custom 1 Tyco/AMP 3 6 Pins www.national.com AN-1545 TABLE 2. BOM 2 (LM3404HV) ID Part Number Type U1 LM3404HV LED Driver L1 SLF12555T- 680M1R3 Inductor D1 CMSH2-100 Schottky Diode Cf VJ0805Y104KXXAT Capacitor Size Parameters Qty Vendor MSOP-8 75V, 0.5A 1 NSC 12.5 x 12.5 x 5.5mm 68µH, 1.3A, 83mΩ 1 TDK SMB 100V, 2A 1 Central Semi 0805 100nF 10% 1 Vishay Cb VJ0805Y103KXXAT Capacitor 0805 10nF 10% 1 Vishay Cin C4532X7R2A225M Capacitor 1812 2.2µF, 100V 1 TDK Co1 C3216X7R2A105M Capacitor 1206 1.0µF, 100V 1 TDK Rsns ERJ8BQFR22V Resistor 1206 0.22Ω 1% 1 Panasonic Ron CRCW08051743F Resistor 0805 174kΩ 1% 1 Vishay Rz CRCW08050R00F Resistor 0805 0Ω 1 Vishay DIM1, DIM2, OFF* 160-1512 Terminal 0.062” 3 Cambion GND1, GND2, GND3, VIN, Vo/ LED+, CS/LED- 160-1026 Terminal 0.094” 6 Cambion J1 535676-5 Connector Custom 1 Tyco/AMP 6 Pins 20215905 FIGURE 4. Complete Evaluation Board Schematic www.national.com 4 AN-1545 Typical Performance Characteristics Efficiency for Table 1 TA = 25°C, IF = 1.0A Efficiency for Table 2 TA = 25°C, IF = 1.0A 20215908 20215909 IF vs VIN for Table 1 TA = 25°C IF vs VIN for Table 2 TA = 25°C 20215910 20215911 DIM Pin Enable for Table 1 DIM Pin Disable for Table 1 20215912 20215913 5 www.national.com AN-1545 Switching Waveforms for Table 1 Output Ripple Current for Table 1 20215914 20215915 DIM Pin Enable for Table 2 DIM Pin Disable for Table 2 20215916 20215917 Switching Waveforms for Table 2 Output Ripple Current for Table 2 20215918 www.national.com 20215919 6 AN-1545 Layout 20215906 Top Layer and Top Overlay 7 www.national.com AN-1545 20215907 Bottom Layer and Bottom Overlay www.national.com 8 AN-1545 Notes 9 www.national.com LM3404/LM3404HV Evaluation Board Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: Products Design Support Amplifiers www.national.com/amplifiers WEBENCH www.national.com/webench Audio www.national.com/audio Analog University www.national.com/AU Clock Conditioners www.national.com/timing App Notes www.national.com/appnotes Data Converters www.national.com/adc Distributors www.national.com/contacts Displays www.national.com/displays Green Compliance www.national.com/quality/green Ethernet www.national.com/ethernet Packaging www.national.com/packaging Interface www.national.com/interface Quality and Reliability www.national.com/quality LVDS www.national.com/lvds Reference Designs www.national.com/refdesigns Power Management www.national.com/power Feedback www.national.com/feedback Switching Regulators www.national.com/switchers LDOs www.national.com/ldo LED Lighting www.national.com/led PowerWise www.national.com/powerwise Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors Wireless (PLL/VCO) www.national.com/wireless THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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