QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B 4.5A BOOST LED DRIVER WITH PWM DIMMING LT3478-1, LT3478 DESCRIPTION Demonstration circuit 949A-A/949A-B is a 4.5A peak details in the Soft-Start section in the LT3478- switch current boost LED driver with PWM dimming 1/LT3478 datasheet. featuring the LT3478-1/LT3478. DC949A-A uses the The switching frequency is set at 1MHz and can be LT3478-1, a fixed 1A LED driver with the LED current adjusted by changing a single resistor. Please read sense resistor inside the IC. DC949A-B has the the datasheet for details on setting the switching fre- LT3478 adjustable LED current converter requiring an quency and selecting components such as the induc- external sense resistor (R11). Both circuits are opti- tor and capacitors. The maximum rating on V is 36V. S mized to drive a 350mA LED string with a total LED However, in a boost converter, the maximum normal voltage above the input voltage and below 40V. The operating voltage of V is the maximum LED voltage S wide input voltage range, internal 4.5A npn power connected to the output. Using a higher voltage on V S switch, floating current sense amplifier, LED current results in loss of control of LED current. dimming control, inrush protection, programmable Modifications can be made to DC949A in order to switching frequency, soft-start, and overvoltage pro- convert the board from a boost LED driver to either a tection make the LT3478-1/LT3478 an extremely buck mode or buck-boost mode LED driver. The powerful and versatile LED driving IC. It can be used LT3003 three string ballaster is also available as an for boost, buck mode, and buck-boost mode LED option for any configuration of the DC949A. Please drivers. consult the factory for customization details. Some The LT3478-1/LT3478 has programmable overvoltage schematics are supplied in this quick-start guide. protection in case the LEDs are open or not properly The LT3478-1, LT3478 datasheet gives a complete attached to the terminals on the PCB. The shutdown description of the part, operation and applications function is activated by pulling the shutdown terminal information. The datasheet must be read in conjunc- to ground. In shutdown the boost configuration al- tion with this Quick Start Guide for demonstration lows the input voltage to be seen on the output and if circuit 949A-A/949A-B. The LT3478-1, LT3478 is as- the attached LED array leaks current at the input volt- sembled in a small 16-lead plastic TSSOP FE package age, the LEDs may not completely turn off although with a thermally enhanced ground pad. Proper board IC switching will cease. Tying the PWM terminal low layout is essential for maximum thermal perform- will disable the LEDs in shutdown or during normal ance. See the datasheet section Layout Considera- running operation. PWM dimming can be applied to tions. DC949A as built. CTRL dimming can be used for ana- log control of LED current if the resistor networks Design files for this circuit board are available. Call attached to the CTRL1 and/or CTRL2 pins are re- the LTC factory. moved. Inrush protection limits the V input current S LT is a trademark of Linear Technology Corporation to 6A during a hotplug or startup condition. Soft-start prevents high inrush current in most conditions. See 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B 4.5A BOOST LED DRIVER WITH PWM DIMMING Table 1. Typical Performance Summary for both DC949A-A and DC949A-B PARAMETER CONDITION VALUE (TYPICAL) Input Voltage V Range V > 25V, I = 350mA 2.8 25V S LED LED Input Voltage V Range Non-transient 2.8 16V IN I 350mA LED Maximum Output Voltage 36V Open LED Output Voltage 36V to 40V Switching Frequency R6 = 31.6k 1MHz Efficiency V =12V V =5V V =31.5V I =.35A 88% S IN LED LED Maximum PWM Dimming Ratio V =12V V =5V V =31.5V I =.35A 3000:1 S IN LED LED 3. With power off, connect the V power supply to S the V terminal and connect the VIN power sup- S ply to the VIN terminal within the ranges speci- QUICK START PROCEDURE fied on the PCB. 4. Connect the PWM and VIN terminals. Demonstration circuit 949A-A/949A-B is easy to set 5. Turn the V and VIN power supplies on. up to evaluate the performance of the LT3478-1, S LT3478. Follow the procedure below: 6. Release the shutdown/GND connection. NOTE: Make sure that the V DC input voltage does S 7. Observe the LED string running at full 350mA not exceed 25V and the VIN DC input voltage does LED current at 100% duty cycle. not exceed 16V. 8. For PWM dimming, connect a PWM 100Hz or 1. Connect a string of LEDs with forward voltage higher signal to the PWM terminal. less than 36V, but greater than the V input volt- S 9. Observe the reduction of brightness in the LED age, to the LED+ and LED- terminals on the PCB string. as shown. 2. Connect the shutdown terminal to GND. 2