DE M O CIR CU IT 1221 Q U ICK S TA R T GLT3502 U IDE LT3 5 0 2 1.1M H z , 5 0 0 m A , S te p -d o w n R e g u la to r in 2m m ×2m m DF N DESCRIPTION Demonstration circuit 1221 is a monolithic step-dow n DC/DC sw itching regulator featuring the LT3502. The demo circuit is designed for 3.3V output from a 5V to 40V input. The w ide input range of the LT3502 allow s a variety of input sources including automotive batteries and 24V industrial supplies. The high sw itching frequency allow s the use of tiny, low cost external components. The current mode control scheme creates fast transient response and good loop stability. The boost diode is internal to reduce component count. The SHDN pin can also be used to program soft start. In this mode, the SHDN 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. Cycle by cycle current limit and frequency foldback provide the protection against shorted outputs. DA pin can further protect the silicon under fault conditions by sensing the diode current. The LT3502 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 1221. D esign files for this circuit board are available. Call the LTC factory. ,LTC and LT are registered trademarks ofLinear Technology Corporation. Perform ance Sum m ary (TA = 25oC) PAR AM ETER Minimum Input Voltage Maximum Input Voltage O utput Voltage VOUT Sw itching Frequency Maximum O utput Current Efficiency Voltage R ipple CO N D ITIO N VIN = 12V,IOUT = 500mA VIN = 12V,IOUT = 500mA VALU E 5V 40V 3.328V ±2.8% 1.1MH z ±18% 500mA 82% 5mV Q U ICK STA RT PROCEDU RE Demonstration circuit 1221 is easy to set up to evaluate the performance of the LT3502. 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. Measure 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 O 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. NOTE. Make sure that the input voltage does not exceed 40V. 4. Check for the proper output voltages. 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 LT3502 output voltage regulation, ripple voltage, efficiency and other parameters. Figure 1. Proper M easurem entEquipm entSetup GND VIN Figure 2. M easuring Inputor OutputR ipple 2 LT3502 3