DC905A

DEMO CIRCUIT 905 LT3491 QUICK START GUIDE LT3491 LED Driver in SC70 With Built-in Schottky WARNING! DO NOT LOOK AT OPERATING LED. This circuit produces light that can damage eyes. DESCRIPTION Demonstration circuit 905 is a LED Driver in SC70 W ith Built-in Schottky featuring the LT3491. The LT3491 is a fixed frequency step-up current mode DC/DC converter. There are tw o circuits on demonstration board 905. The “Small Size” circuit generates 15mA LED current at the smallest possible solution size. The “High Efficiency” circuit generates 20mA LED current at higher efficiency. The circuit outputs share 6 LEDs. At any given time, depending on the positions of jumpers, only one circuit is in operation. Other features of the LT3491, such as the internal schottky diode,the high sw itching frequency and the internal open circuit protection, allow tiny components for both circuits. The input range of the demo board is from 3V to 5V. Different dimming controls can be implemented on the demo board. Please refer to the quick start procedure. The high side current sensing feature of the LT3491 allow s “one w ire” current source, i.e. the low side of the LED string can return to ground anyw here. The LT3491 datasheet gives a complete description of the part,operation and application information. The datasheet must be read in conjunction w ith this quick start guide for w orking on or modifying demo circuit 905. Design files for this circuit board are available. Call the LTC factory. ,LTC and LT are registered trademarks ofLinear Technology Corporation. ThinSOT and Pow erPath are trademarks ofLinear Technology Corporation. PERFORMANCE SUMMARY Specifications are at TA = 25°C SYM BO L VIN IOUT PAR AM ETER Input Supply Range ** Output Current VOUT Output Voltage η Efficiency VOPEN IQ IQSHDN Fs CAP pin Over-voltage Protection Quiescent Current Quiescent Current W hile in Shutdow n Sw itching Frequency CO NDITION S Small Size circuit High Efficiency circuit Small Size circuit High Efficiency circuit VIN = 4.2V,Small Size circuit VIN = 4.2V,High Efficiency circuit LED Open VIN = 4.2V,CTRL = ON VIN = 4.2V,CTRL = OFF M IN 3 14.25 19 TYP 26 15 20 20.3 20.6 73.6 75.5 27 1.8 2.3 M AX 5 15.75 21 UN ITS V mA V % 28 4 10 2.8 V mA µA M Hz ** The Input Supply R ange ofdemo circuit 905 is limited by the input capacitors. The input voltage absolute maximum rating ofthe LT3491 is 12V. 1 LT3491 QUICK START PROCEDURE Demonstration circuit 905 is easy to set up to evaluate the performance of the LT3491. R efer to Figure 1 for proper measurement equipment setup and follow the procedure below : NOTE. W hen measuring the input or output voltage ripple,care must be taken to avoid a long ground lead on the oscilloscope probe. M easure the input or output voltage ripple by touching the probe tip directly across the Vin or Vout and GND terminals. See Figure 2 for proper scope probe technique. 1. Place jumpers in the follow ing positions to test the “SM ALL SIZE” circuit: ON JP1 O FF JP2 SIZE JP3 SIZE JP4 2. Place jumpers in the follow ing positions to test the “H IG H EFFICIEN CY” circuit: JP1 JP2 JP3 JP4 2 O FF ON EFFICIEN CY EFFICIEN CY 3. Prior to connection, preset the input pow er supply to the desired voltage (3V to 5V). W ith pow er off, connect the input pow er supply to VIN and G N D. 4. Turn on the pow er at the input. NOTE. M ake sure that the input voltage does not exceed 5V. 5. Check for the proper voltages and currents. NOTE. Ifthe LEDs do not light up,check all the jumper settings. 6. O nce proper operation is established, adjust the input w ithin the operating range and observe the parameters ofinterest. 7. To test the filtered PW M dimming or the direct DC dimming, remove the jumper JP1 or JP2, apply the PW M or the DC signal to the CTRL1 or CTR L2 and observe the brightness ofthe LED output. 8. To test the direct PW M dimming, follow the Direct PW M Dimming section in the datasheet. LT3491 Figure 1. Proper M easurem entEquipm entSetup GND VIN Figure 2. M easuring Inputor OutputRipple 3 LT3491 4