National Semiconductor Application Note 2041 Clinton Jensen May 3, 2010 Introduction PWM Dimming The LM3434 is an adaptive constant on-time DC/DC buck constant current controller designed to drive a high brightness LEDs (HB LED) at high forward currents. It is a true current source that provides a constant current with constant ripple current regardless of the LED forward voltage drop. The board can accept an input voltage ranging from -9V to -30V w.r.t. GND. The output configuration allows the anodes of multiple LEDs to be tied directly to the ground referenced chassis for maximum heat sink efficacy when a negative input voltage is used. The LM3434 is capable if high speed PWM dimming in excess of 40kHz. Dimming is accomplished by shorting across the LED with a FET(s). Dimming FETs are included on the evaluation board for testing LEDs placed close to the board. The FETs on the evaluation board should be removed if using dimming FETs remotely placed close to the LED (recommended). If the FETs cannot be placed directly next to the LED then a snubber across the FETs may be required to protect the FETs and the LM3434 from v=Ldi/dt voltage transients induced by the fast current changes in the line inductance leading to the LED. This will slow the edges and limit PWM dimming capabilities at high frequencies. To use the dimming function apply square wave to the PWM test point on the board that has a positive voltage w.r.t. GND. When this pin is pulled high the dimming FET is enabled and the LED turns off. When it is pulled low the dimming FET is turned off and the LED turns on. A scope plot of PWM dimming is included in the Typical Performance Characteristics section showing 30kHz dimming at 50% duty cycle. LM3434 Board Description The evaluation board is designed to provide a constant current in the range of 4A to 20A. The LM3434 requires two input voltages for operation. A positive voltage with respect to GND is required for the bias and control circuitry and a negative voltage with respect to GND is required for the main power input. This allows for the capability of using common anode LEDs so that the anodes can be tied to the ground referenced chassis. The evaluation board only requires one input voltage of -9V to -30V with respect to GND. The positive voltage is supplied by the LM5002 circuit. The LM5002 circuit also provides a UVLO function to remove the possibility of the LM3434 from drawing high currents at low input voltages during startup. Initially the output current is set at the minimum of approximately 4A with the POT P1 fully counter-clockwise. To set the desired current level a short may be connected between LED+ and LED-, then use a current probe and turn the POT clockwise until the desired current is reached. The current may be adjusted with P1 up to 18A. 20A output may be acheived either by bypassing P1 and applying an analog voltage directly to ADJ or by adjusting the values of R1 and/or R2 to get higher than 1.5V with P1 fully clockwise. PWM dimming FETs are included on-board for testing when the LED can be connected directly next to the board. A shutdown test post on J2, ENA, is included so that startup and shutdown functions can be tested using an external voltage. LM3434 20A Evaluation Board LM3434 20A Evaluation Board High Current Operation and Component Lifetime When driving high current LEDs, particularly when PWM dimming, component lifetime may become a factor. In these cases the input ripple current that the input capacitors are required to withstand can become large. At lower currents long life ceramic capacitors may be able to handle this ripple current without a problem. At higher currents more input capacitance may be required. To remain cost effective this may require putting one or more aluminum electrolytic capacitors in parallel with the ceramic input capacitors. Since the operational lifetime of LEDs is very long (up to 50,000 hours) the longevity of an aluminum electrolytic capacitor can become the main factor in the overall system lifetime. The first consideration for selecting the input capacitors is the RMS ripple current they will be required to handle. This current is given by the following equation: Setting the LED Current The LM3434 evaluation board is designed so that the LED current can be set in multiple ways. There is a shunt on J2 initially connecting the ADJ pin to the POT allowing the current to be adjusted using the POT P1. This POT will apply a voltage to the ADJ pin between 0.3V and 1.5V w.r.t. GND to adjust the voltage across the sense resistor (RSENSE) R15. The shunt may also be removed and an external voltage positive w.r.t. GND can then be applied to the ADJ test point on the board. A 5mΩ resistor (two 10mΩ resistors in parallel) comes mounted on the board so using the VSENSE vs. VADJ graph in the Typical Performance Characteristics section the current can be set using the following equation: Alternatively the shunt can be removed and the ADJ test point can be connected to the VINX test point to fix VSENSE at 60mV. © 2010 National Semiconductor Corporation 301193 www.national.com AN-2041 ILED = VSENSE/RSENSE The parallel combination of the ceramic and aluminum electrolytic input capacitors must be able to handle this ripple current. The aluminum electrolytic in particular should be able to handle the ripple current without a significant rise in core temperature. A good rule of thumb is that if the case temperature of the capacitor is 5°C above the ambient board temperature then the capacitor is not capable of sustaining the ripple current for its full rated lifetime and a more robust or lower ESR capacitor should be selected. The other main considerations for aluminum electrolytic capacitor lifetime are the rated lifetime and the ambient operating temperature. An aluminum electrolytic capacitor comes with a lifetime rating at a given core temperature, such as 5000 hours at 105°C. As dictated by physics the capacitor AN-2041 Where LifeRATED is the rated lifetime at the rated core temperature TCORE. For example: If the ambient temperature is 85°C the core temperature is 85°C + 5°C = 90°C. (105°C 90°C)/7°C = 2.143. 2^2.413 = 4.417. So the expected lifetime is 5,000*4.417 = 22,085 hours. Long life capacitors are recommended for LED applications and are available with ratings of up to 20,000 hours or more at 105°C. lifetime should double for each 7°C below this temperature the capacitor operates at and should halve for each 7°C above this temperature the capacitor operates at. A good quality aluminum electrolytic capacitor will also have a core temperature of approximately 3°C to 5°C above the ambient temperature at rated RMS operating current. So as an example, a capacitor rated for 5,000 hours at 105°C that is operating in an ambient environment of 85°C will have a core temperature of approximately 90°C at full rated RMS operating current. In this case the expected operating lifetime of the capacitor will be approximately just over 20,000 hours. The actual lifetime (LifeACTUAL) can be found using the equation: 30119301 FIGURE 1. LM3434 Evaluation Board Schematic TABLE 1. BOM ID Part Number Type Size Qty Vendor U1 LM3434 LED Driver LLP-24 1 NSC U2 LM5002MA Boost Regulator SO-8 1 NSC C1 C0805C331J5GACTU Capacitor 0805 330pF, 50V 1 Kemet C2 GRM31CR60J476KE19L Capacitor 1206 47µF, 6.3V 1 Murata C3 EKY-350ELL151MHB5D Capacitor MULTICAP 150µF, 35V 1 United Chemicon C4, C5, C6 GRM32ER6YA106KA12 Capacitor 1210 10µF, 35V 2 Murata C7 C0805C104J5RACTU Capacitor 0805 0.1µF, 50V 1 Kemet C8, C13 HMK212BJ103KG-T Capacitor 0805 10nF, 100V 2 Taiyo Yuden www.national.com 2 Parameters Part Number Type Size Parameters Qty Vendor C9 OPEN C10, C11 GRM21BC81E475MA12 Capacitor 0805 0805 4.7µF, 25V 2 Murata C12 0805YD105KAT2A Capacitor 0805 1µF, 16V 1 AVX C14 B37941K9474K60 Capacitor 0805 0.47µF, 16V 1 EPCOS Inc . C15 GRM21BF51E225ZA01L Capacitor 0805 2.2µF, 25V 1 Murata C17 OPEN 0805 C18 08055C104JAT2A Capacitor 0805 0.1µF, 50V 1 AVX D1, D2 MBR0540 Diode SOD-123 40V, 500mA 2 Fairchild D3 MBRS240LT3 Diode SMB 40V, 2A 1 ON Semiconductor D4 OPEN J2 B8B-EH-A(LF)(SN) Connector 1 JST Sales America, Inc. Weidmuller SMB J1 1761582001 Connector 1 Jled 87438-0843 Connector 1 Molex L1 LPS3008-104ML Inductor 3008 100µH, 150mA 1 Coilcraft L2 SER2915H-103KL Inductor SER2900 10µH, 21.5A 1 Coilcraft L3, L4, L5, L6 MPZ2012S300A Ferrite Bead 0805 30Ω @ 100MHz 4 TDK L7 MPZ2012S101A Ferrite Bead 0805 100Ω @ 100MHz 1 TDK P1 3352T-1-103LF Potentiometer BOURNS2 10kΩ 1 Bourns Q1, Q2, Q3, Q4, Q5, Q6 Si7790DP FET PowerPAK 40V, 6mΩ 2 VishaySiliconix Q7 MMDT3906-7-F Dual PNP SOT363_N 1 Diodes Inc. Q8 ZXTN25040DFHTA NPN SOT-23B 1 Zetex Q9 ZXTP25040DFHTA PNP SOT-23B 1 Zetex R1 ERJ-6ENF2942V Resistor 0805 29.4kΩ 1 Panasonic R2 ERJ-6ENF2491V Resistor 0805 2.49kΩ 1 Panasonic R3, R30, R31 ERJ-6ENF1002V Resistor 0805 10kΩ 3 Panasonic R4 ERJ-6GEYJ393V Resistor 0805 39kΩ 1 Panasonic R5 ERJ-6GEYJ101V Resistor 0805 100Ω 1 Panasonic R7 OPEN R14 ERJ-6ENF49R9V Resistor 0805 49.9Ω 1 Panasonic R8 ERJ-6ENF2002V Resistor 0805 20kΩ 1 Panasonic R10 ERJ-6ENF4991V Resistor 0805 4.99kΩ 1 Panasonic R11, R12 ERJ-6ENF6192V Resistor 0805 61.9kΩ 2 Panasonic R13 ERJ-6GEYJ103V Resistor 0805 10kΩ 1 Panasonic R15a, R15b WSL25125R0100FEA Resistor CR6332-2512 0.01Ω 2 Vishay R16, R17, R18, R19, R20, R21 ERJ-6GEYJ2R7V Resistor 0805 2.7Ω 6 Panasonic R22 ERJ-6GEYJ100V Resistor 0805 10Ω 1 Panasonic R25 ERJ-6ENF7502V Resistor 0805 75kΩ 1 Panasonic R26 OPEN LED+, LED- 1502-2 Test Post TP 1502 0805 0.109" 2 Keystone ADJ, PWM, VINX 1593-2 Test Post TP 1593 0.084" 3 Keystone 3 www.national.com AN-2041 ID AN-2041 Typical Performance Characteristics Efficiency vs. LED Forward Voltage (VCGND - VEE = 9V) Efficiency vs. LED Forward Voltage (VCGND - VEE = 12V) 30119305 30119306 Efficiency vs. LED Forward Voltage (VCGND - VEE = 14V) VSENSE vs. VADJ 30119308 30119307 30kHz PWM Dimming Waveform Showing Inductor Ripple Current 30119309 ILED = 6A nominal, VIN = 3.3V, VEE = -12V Top trace: DIM input, 2V/div, DC Bottom trace: ILED, 2A/div, DC T = 10µs/div www.national.com 4 AN-2041 Layout 30119320 Top Layer and Top Overlay 30119321 Upper Middle Layer 5 www.national.com AN-2041 30119322 Lower Middle Layer 30119323 Bottom Layer and Bottom Overlay www.national.com 6 AN-2041 Notes 7 www.national.com LM3434 20A Evaluation Board Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise® Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise® Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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