LM3434 20A Evaluation Board
LM3434 20A Evaluation Board
National Semiconductor Application Note 2041 Clinton Jensen May 3, 2010
Introduction
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.
PWM Dimming
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.
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: ILED = VSENSE/RSENSE Alternatively the shunt can be removed and the ADJ test point can be connected to the VINX test point to fix VSENSE at 60mV.
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
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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:
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.
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FIGURE 1. LM3434 Evaluation Board Schematic
TABLE 1. BOM ID U1 U2 C1 C2 C3 C4, C5, C6 C7 C8, C13 Part Number LM3434 LM5002MA C0805C331J5GACTU GRM31CR60J476KE19L EKY-350ELL151MHB5D GRM32ER6YA106KA12 C0805C104J5RACTU HMK212BJ103KG-T Type LED Driver Boost Regulator Capacitor Capacitor Capacitor Capacitor Capacitor Capacitor Size LLP-24 SO-8 0805 1206 MULTICAP 1210 0805 0805 330pF, 50V 47µF, 6.3V 150µF, 35V 10µF, 35V 0.1µF, 50V 10nF, 100V Parameters Qty 1 1 1 1 1 2 1 2 Vendor NSC NSC Kemet Murata United Chemicon Murata Kemet Taiyo Yuden
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ID C9 C10, C11 C12 C14 C15 C17 C18 D1, D2 D3 D4 J2 J1 Jled L1 L2 L3, L4, L5, L6 L7 P1 Q1, Q2, Q3, Q4, Q5, Q6 Q7 Q8 Q9 R1 R2 R3, R30, R31 R4 R5 R7 R14 R8 R10 R11, R12 R13 R15a, R15b R16, R17, R18, R19, R20, R21 R22 R25 R26 LED+, LEDADJ, PWM, VINX
Part Number OPEN GRM21BC81E475MA12 0805YD105KAT2A B37941K9474K60 GRM21BF51E225ZA01L OPEN 08055C104JAT2A MBR0540 MBRS240LT3 OPEN B8B-EH-A(LF)(SN) 1761582001 87438-0843 LPS3008-104ML SER2915H-103KL MPZ2012S300A MPZ2012S101A 3352T-1-103LF Si7790DP MMDT3906-7-F ZXTN25040DFHTA ZXTP25040DFHTA ERJ-6ENF2942V ERJ-6ENF2491V ERJ-6ENF1002V ERJ-6GEYJ393V ERJ-6GEYJ101V OPEN ERJ-6ENF49R9V ERJ-6ENF2002V ERJ-6ENF4991V ERJ-6ENF6192V ERJ-6GEYJ103V WSL25125R0100FEA ERJ-6GEYJ2R7V ERJ-6GEYJ100V ERJ-6ENF7502V OPEN 1502-2 1593-2
Type Capacitor Capacitor Capacitor Capacitor Capacitor Diode Diode
Size 0805 0805 0805 0805 0805 0805 0805 SOD-123 SMB SMB
Parameters 4.7µF, 25V 1µF, 16V 0.47µF, 16V 2.2µF, 25V 0.1µF, 50V 40V, 500mA 40V, 2A
Qty 2 1 1 1 1 2 1
Vendor Murata AVX EPCOS Inc . Murata AVX Fairchild ON Semiconductor JST Sales America, Inc. Weidmuller Molex Coilcraft Coilcraft TDK TDK Bourns VishaySiliconix Diodes Inc. Zetex Zetex Panasonic Panasonic Panasonic Panasonic Panasonic Panasonic Panasonic Panasonic Panasonic Panasonic Vishay Panasonic Panasonic Panasonic Keystone Keystone
Connector Connector Connector Inductor Inductor Ferrite Bead Ferrite Bead Potentiometer FET Dual PNP NPN PNP Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Resistor Test Post Test Post 3008 SER2900 0805 0805 BOURNS2 PowerPAK SOT363_N SOT-23B SOT-23B 0805 0805 0805 0805 0805 0805 0805 0805 0805 0805 CR6332-2512 0805 0805 0805 0805 TP 1502 TP 1593 0.109" 0.084" 29.4kΩ 2.49kΩ 10kΩ 39kΩ 100Ω 49.9Ω 20kΩ 4.99kΩ 61.9kΩ 10kΩ 0.01Ω 2.7Ω 10Ω 75kΩ 100µH, 150mA 10µH, 21.5A 30Ω @ 100MHz 100Ω @ 100MHz 10kΩ 40V, 6mΩ
1 1 1 1 1 4 1 1 2 1 1 1 1 1 3 1 1 1 1 1 2 1 2 6 1 1 2 3
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Typical Performance Characteristics
Efficiency vs. LED Forward Voltage (VCGND - VEE = 9V) Efficiency vs. LED Forward Voltage (VCGND - VEE = 12V)
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Efficiency vs. LED Forward Voltage (VCGND - VEE = 14V)
VSENSE vs. VADJ
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30kHz PWM Dimming Waveform Showing Inductor Ripple Current
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ILED = 6A nominal, VIN = 3.3V, VEE = -12V Top trace: DIM input, 2V/div, DC Bottom trace: ILED, 2A/div, DC T = 10µs/div
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Layout
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Top Layer and Top Overlay
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Upper Middle Layer
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Lower Middle Layer
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Bottom Layer and Bottom Overlay
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Notes
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LM3434 20A Evaluation Board
Notes
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