DEMO CIRCUIT 1167 LT3685 QUICK START GUIDE LT3685EMSE 2A, 38V Step-down Switching Regulator with SYNC Function DESCRIPTION Demonstration circuit 1167 is a monolithic step-dow n DC/DC sw itching regulator featuring LT3685. The LT3685 can be synchronized over a 250 KH z to 2 M H z range. The demo board is designed for 5V output from a 6.3V to 38V input w ith transient up to 60V. The w ide input range of the LT3685 allow s a variety of input sources. The typical sources are automotive batteries, w all adaptors and industrial supplies. start. In this mode, the R U N /SS pin is driven through an external R C filter to create a voltage ramp on this pin. The soft start function reduces the input current surge during start-up. The current mode control scheme creates fast transient response and good loop stability. The gate drive of the internal sw itch is boosted to a voltage that is higher than the Vin to ensure saturation of the sw itch. The LT3685’s integrated boost diode reduces the parts count. The R U N /SS pin can be used to set the part in micropow er shutdow n mode,reducing the supply current to less than 1uA. The R U N /SS pin can also be used to program soft N ote:It is best to ground the SYN C pin if the SYN C function is not being used. The LT3685 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 demo circuit 1167. 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. Perform ance Sum m ary for Step-dow n Sw itching Regulator (TA = 25oC ) PAR AM ETER FO R BU CK R EG U LATO R M inimum Input Voltage M aximum Input Voltage O utput Voltage VOUT M aximum O utput Current Typical Sw itching Frequency CO N DITIO N VALU E 6.3V 38V 5V +/- 4% 2A 550kH z QUICK START PROCEDURE Demonstration circuit 1167 is easy to set up to evaluate the performance of the LT3685. 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 JP1 on the R U N position. 2. W ith pow er off,connect the input pow er supply to Vin and G N D. 3. Turn on the pow er at the input. 4. Check for the proper output voltage. NOTE. Ifthere is no output,temporarily disconnect the load to make sure that the load is not set too high. 5. O nce the proper output voltage is established, adjust the load w ithin the operating range and observe the 1 LT3685 output voltage regulation,ripple voltage,efficiency and other parameters. 6. An external clock can be added to the SYN C pin w hen SYN C function is used. See synchronization section in the datasheet for details. A A + + + V + _ _ _ V _ Figure 1. Proper M easurem ent Equipm ent Setup GND VIN Figure 2. M easuring Input or Output Ripple 2 + Load _ LT3685 3