DEMO C IR C U IT 12 12 Q U IC K LT3480/LT3685 S TA R T G U IDE L T 3 4 8 0 EDD/ L T 3 6 8 5 EDD 2 A , 3 8 V S te p -d o w n S w itc h in g R e g u la to r w ith S Y N C F u n c tio n a n d EMC filte rs DESCRIPTION Demonstration circuit 1212 is a monolithic step-dow n DC/DC sw itching regulator featuring LT3480/LT3685. The LT3480 is on DC1212A-A circuit and the LT3685 is on DC1212A-B circuit. Both the LT3685 and LT3480 can be synchronized over a 250 KHz to 2 M Hz 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 allow s a variety of input sources. The typical sources are automotive batteries, w all adaptors and industrial supplies. The EM C filter at the input makes the circuit compatible w ith EM C stardard, EN55022 Class B. The current mode control scheme creates fast transient response and good loop stability. The RUN/SS pin can be used to set the part in micropow er shutdow n mode,reducing the supply current to less than 1uA. The RUN/SS pin can also be used to program soft start. In this mode, the RUN/SS pin is driven through an external RC filter to create a voltage ramp on this pin. The soft start function reduces the input current surge during start-up. The LT3480/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 1212. Note:It is best to ground the SYNC pin ifthe SYNC function is not being used. 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 R egulator ( TA = 25oC ) PAR AM ETER FO R BU CK R EG ULATO 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 800kH z Q U ICK STA RT PROCEDU RE Demonstration circuit 1212 is easy to set up to evaluate the performance ofthe LT3480/LT3685. Refer 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 GND. 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. Once the proper output voltage is established, adjust the load w ithin the operating range and observe the 1 LT3480/LT3685 output voltage regulation, ripple voltage, efficiency and other parameters. 6. An external clock can be added to the SYNC pin w hen SYNC function is used. See synchronization section in the datasheet for details. Figure 1. Proper M easurem ent Equipm ent Setup GND VIN Figure 2. M easuring Input or Output R ipple 2 LT3480/LT3685 90dBµV 10dBuV/div EN55022 Class B Limit 50dBµV Figure 3. 12V to 5V @ 2A conductive em ission result EN55022 Class B Limit Figure 4. 12V to 5V @ 2A radiated em ission result 3 LT3480/LT3685 4