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 switch current boost LED driver with PWM dimming featuring the LT3478-1/LT3478. DC949A-A uses the LT3478-1, a fixed 1A LED driver with the LED current sense resistor inside the IC. DC949A-B has the LT3478 adjustable LED current converter requiring an external sense resistor (R11). Both circuits are optimized to drive a 350mA LED string with a total LED voltage above the input voltage and below 40V. The wide input voltage range, internal 4.5A npn power switch, floating current sense amplifier, LED current dimming control, inrush protection, programmable switching frequency, soft-start, and overvoltage protection make the LT3478-1/LT3478 an extremely powerful and versatile LED driving IC. It can be used for boost, buck mode, and buck-boost mode LED drivers. The LT3478-1/LT3478 has programmable overvoltage protection in case the LEDs are open or not properly attached to the terminals on the PCB. The shutdown function is activated by pulling the shutdown terminal to ground. In shutdown the boost configuration allows the input voltage to be seen on the output and if the attached LED array leaks current at the input voltage, the LEDs may not completely turn off although IC switching will cease. Tying the PWM terminal low will disable the LEDs in shutdown or during normal running operation. PWM dimming can be applied to DC949A as built. CTRL dimming can be used for analog control of LED current if the resistor networks attached to the CTRL1 and/or CTRL2 pins are removed. Inrush protection limits the VS input current to 6A during a hotplug or startup condition. Soft-start prevents high inrush current in most conditions. See details in the ‘Soft-Start’ section in the LT34781/LT3478 datasheet. The switching frequency is set at 1MHz and can be adjusted by changing a single resistor. Please read the datasheet for details on setting the switching frequency and selecting components such as the inductor and capacitors. The maximum rating on VS is 36V. However, in a boost converter, the maximum normal operating voltage of VS is the maximum LED voltage connected to the output. Using a higher voltage on VS results in loss of control of LED current. Modifications can be made to DC949A in order to convert the board from a boost LED driver to either a buck mode or buck-boost mode LED driver. The LT3003 three string ballaster is also available as an option for any configuration of the DC949A. Please consult the factory for customization details. Some schematics are supplied in this quick-start guide. The LT3478-1, LT3478 datasheet gives a complete description of the part, operation and applications information. The datasheet must be read in conjunction with this Quick Start Guide for demonstration circuit 949A-A/949A-B. The LT3478-1, LT3478 is assembled in a small 16-lead plastic TSSOP FE package with a thermally enhanced ground pad. Proper board layout is essential for maximum thermal performance. See the datasheet section ‘Layout Considerations’. Design files for this circuit board are available. Call the LTC factory. LT is a trademark of Linear Technology Corporation 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 VS Range VLED > 25V, ILED = 350mA 2.8 – 25V Input Voltage VIN Range Non-transient 2.8 – 16V ILED 350mA Maximum Output Voltage 36V Open LED Output Voltage 36V to 40V Switching Frequency R6 = 31.6k 1MHz Efficiency VS=12V VIN=5V VLED=31.5V ILED=.35A 88% Maximum PWM Dimming Ratio VS=12V VIN=5V VLED=31.5V ILED=.35A 3000:1 3. With power off, connect the VS power supply to the VS terminal and connect the VIN power supply to the VIN terminal within the ranges specified on the PCB. 4. Connect the PWM and VIN terminals. 5. Turn the VS and VIN power supplies on. 6. Release the shutdown/GND connection. 7. Observe the LED string running at full 350mA LED current at 100% duty cycle. 8. For PWM dimming, connect a PWM 100Hz or higher signal to the PWM terminal. 9. Observe the reduction of brightness in the LED string. QUICK START PROCEDURE Demonstration circuit 949A-A/949A-B is easy to set up to evaluate the performance of the LT3478-1, LT3478. Follow the procedure below: NOTE: Make sure that the VS DC input voltage does not exceed 25V and the VIN DC input voltage does not exceed 16V. 1. 2. Connect a string of LEDs with forward voltage less than 36V, but greater than the VS input voltage, to the LED+ and LED- terminals on the PCB as shown. Connect the shutdown terminal to GND. 2 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B 4.5A BOOST LED DRIVER WITH PWM DIMMING Figure 1. Test procedure setup drawing for DC949A-A DC949A-B 3 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B 4.5A BOOST LED DRIVER WITH PWM DIMMING for details regarding the use of the sense resistor and setting the LED current. Both DC949A-A and DC949A-B are built to provide 350mA LED current in the boost LED driver topology. 100% 12.5V L ED 95% 19V L ED 90% 31.5V L ED 85% 80% 75% 70% 65% 60% 55% 5V IN 50% 0 5 10 15 20 25 VS INP UT (V) Figure 2. DC949A-A efficiency TWO ICS, THREE TOPOLOGIES DC949A has two build versions. DC949A-A uses the LT3478-1 1A fixed LED current IC with internal sense resistor. It can be set at full 1A current or less than 1A LED current with CTRL1,2 resistor divider network. DC949A-B uses the LT3478 adjustable LED current IC with a required external sense resistor to set the LED current. It can be set at LED currents above and below 1A. Please see the datasheet Although DC949A is a boost LED driver, it can be reconfigured for buck-boost mode LED driving and buck mode LED driving. The datasheet details some of these changes and the figures below show sample schematics with the demonstration circuit reference designators that can be used in the different modes. There may be a cut or two required on the PCB to make these changes. Please ask for further documentation detailing the changes in DC949A for conversion to buck mode or buck-boost mode LED driving. OPTIONAL LT3003 BALLASTER The LT3003 is available for three-channel LED ballasting. With 1A or less output using the DC949A, three parallel channels of LED strings can be driven and PWM dimmed in all three topologies using the LT3003 with the LT3478-1 or LT3478. For details regarding the connection of the LT3003 and the LT3478 please see the LT3003 and LT3478 datasheets. When not being used, the LT3003 circuit should be left unstuffed. Please note that there is a small pad near the LT3478 Vc pin that connects internally to the LT3003 /OT2 output for overtemp protection. It is shown in the schematic of the LT3478 and it is easy to connect these functions using the DC949A. 4 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B 4.5A BOOST LED DRIVER WITH PWM DIMMING VS 2.8V-25V 36VDC MAX C1 4.7uF 25V VIN C6 3.3uF 25V 11 SHDN 13 CTRL2 L1 10uH 3 4 VIN VS 5 SW SW SHDN CTRL2 U1 OUT * LED LT3478 R3 93.1k 10 12 CTRL1 R4 36.5k 350mA C21 4.7uF 50V 1 2 LED String 6 7 VREF 15V max PWM OVPSET R10 100k C3 1uF LED- PWM CTRL1 R9 24k 8 LED+ PDS560 C2 4.7uF 50V L * 3 R1 100k CIN 1uF 25V R11 OUT SS GND RT 16 17 15 VC Q1 Si2318DS 14 1 9 R7 10k C4 0.1uF R6 31.6k 1% 1 MHz ASSEMBLY VERSION 2 2.8V-16V 36V transient D1 * VERSION TABLE U1 DC949A-A LT3478EFE-1 DC949A-B LT3478EFE R11 DO NOT STUFF 0.1 ohm 1% Figure 3. Customer friendly schematic for boost (as-built) DC949A. 40V is the maximum that the output will see, but OVP might be as low as 36V without any adjustments. 5 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B 4.5A BOOST LED DRIVER WITH PWM DIMMING Q1 Si2302ADS R21 PWM 510 Q2 Si2315BDS VS R7 10k R20 510 3.8V-6.5V LED+ 1A LEDC1 10uF 10V VIN D1 L1 6.8uH R1 100k SHDN 11 13 10 12 3 4 VIN VS 5 2 SW SW SHDN OUT * C2 4.7uF 16V B320 1 L R11 6 CTRL2 * U1 VREF LT3478 LED PWM CTRL1 OVPSET SS R9 8 100k R10 34k 16 GND VC 17 9 C3 1uF RT 7 14 15 R6 69.8k 1% C4 0.1uF 500kHz * VERSION TABLE ASSEMBLY VERSION U1 DC949A-A LT3478EFE-1 DC949A-B LT3478EFE R11 DO NOT STUFF 0.1 ohm 1% Figure 4. Customer friendly schematic for buck-boost mode LED driver DC949A. This circuit requires changes to the standard demo circuit such as addition of new components and a trace cut on the top-side of the board between the drain of Q1 and the LED- terminal. 6 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B 4.5A BOOST LED DRIVER WITH PWM DIMMING PVIN This is the OUT terminal on DC949A. C2 3.3uF 50V 32V R11 0.068 LED+ 1.5A LEDR20 365 Q2 Si2319DS COUT_buck 10uF VIN 3.3V C6 4.7uF 10V R1 100k L1 R21 510 10uH VIN L VS OUT LED D1 SW SHDN SHDN PDS560 LT3478EFE VREF Q1 2N7002 PWM CTRL2 R7 CTRL1 10k R9 PWM 24k OVPSET VC SS RT GND R10 100k C3 1uF R6 69.8k 1% 500kHz C4 0.1uF Figure 5. Customer friendly schematic for buck mode LED driver DC949A-B. This circuit requires changes to the standard demo circuit such as addition of new components and a trace cut on the top-side of the board between the drain of Q1 and the LED- terminal. The input PVIN must be connected to the OUT terminal. The connection between pin L and the inductor must be cut on the topside of the board. Pin L can be connected to VIN terminal via R22 with a zero ohm resistor. Terminal Vs can be left floating. The COUT_buck capacitor must be added to the board. 7 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 949A-A/949A-B 4.5A BOOST LED DRIVER WITH PWM DIMMING 8