DEMO CIRCUIT 976 LT3476EUHF QUICK START GUIDE LT3476EUHF Quadruple High Power LED Driver in Buck Mode DESCRIPTION frequency results in higher maximum duty cycle. WARNING The input voltage is limited by the CAP pin over- voltage protection threshold. As a result, each Do not look directly at operating LED. channel of the demo circuit can drive up to 27V This circuit produces light that can damage eyes. total LED voltage. Adjustable switching frequency allows optimization of the efficiency and the exter- Demonstration circuit 976 is a Quadruple High nal component size. At 1MHz, 96% efficiency can Power LED Driver in Buck Mode featuring the be achieved at 1A without sacrificing the solution LT3476EUHF. The LT3476 is a quad output size. DC/DC converter designed to operate as a current Each of the four regulators on the demo circuit is source for driving high current LEDs. On the demo independently operated. The PWM dimming fea- circuit, the default current for each output channel ture of the LT3476 allows True Color PWM dim- is 350mA but that can be adjusted to up to 1.05A ming with the dimming ratio up to 1000:1. Addi- via resistor dividers at the VADJ pins. The output tional analog dimming is possible. current should not exceed the rated current of the LEDs used. If 1W LEDs, such as the Lumileds The LT3476 datasheet gives a complete descrip- Luxeon LEDs, are used, 350mA current is the tion of the part, operation and application informa- maximum. 1A output current is reserved for 3W tion. The datasheet must be read in conjunction LEDs, such as Luxeon III. The LED current thermal with this quick start guide for working on or modify- de-rating should be considered to protect the LEDs. ing the demo circuit 976. The maximum LED string voltage the demo circuit Design files for this circuit are available. Call can drive is limited by the duty cycle and the input the LTC factory. voltage of the LT3476. The demo circuit default , LTC and LT are registered trademarks of Linear Technology Corporation. switching frequency is 1MHz. At 1MHz, the maxi- mum duty cycle is 84% (MIN). Lower switching PERFORMANCE SUMMARY Specifications are at TA = 25C SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS PV Power Input Supply 33 V IN V Chip Bias Input Supply 2.8 16 V IN V Output LED Voltage, per Output PV = 33V, I = 350mA 27 V LED IN LED I Output LED Current, per Output R2, R25, R27, R29 = 66.5K 350 mA LED I Output LED Current, per Output R2, R25, R27, R29 = 0 1013 1050 1087 mA LED F Switching Frequency R11 = 21K 0.85 1 1.15 MHz SW Conversion Efficiency I = 350mA 94 % h LED Conversion Efficiency I = 1050mA 96 % h LED P Maximum Total Output Power I = 1050mA 100 W OUT LED PWM PWM Dimming Ratio PWM Frequency = 100Hz 1000:1 D 1 LT3476EUHF QUICK START PROCEDURE Demonstration circuit 976 is easy to set up to the last LED to the LEDn-. All four channels can evaluate the performance of the LT3476EUHF. Re- operate simultaneously or individually. fer to Figure 1 for proper measurement equipment 5. Turn on PVIN power supply. setup and follow the procedure below: 6. Turn on VIN power supply. 7. Check for the proper LED current and LED 1. With power off, connect an input power supply string voltage. to VIN and GND. The maximum rating is 16V. To NOTE. If there is no output, turn off the power supplies. Check all achieve highest efficiency, a low voltage supply, connections. such as 3.3V or 5V, is recommended. 8. Once the proper output current/voltage are 2. With power off, connect a second input power established, adjust the parameters within the supply to PVIN and GND. The maximum PVIN operating range and observe the output current is 33V. regulation, efficiency and other parameters. 3. With power off, connect PWM1, PWM2, PWM3, 9. To observe PWM dimming, turn off all power PWM4 to VIN. If an output n is not used, the supplies. Disconnect PWMn from VIN. Connect PWMn can be left unconnected. PWMn to a pulse generator. Then turn on PVIN, 4. With power off, connect a LED (in series) string the PWM pulse generator and VIN. to an output channel. Connect the anode of the 10. For even faster LED current turn-off, remove first LED to the LEDn+. Connect the cathode of R20, R21, R22, R23 and repeat step 9. 2