DEMO C IR C U IT 127 3 Q U IC K S TA R T GLT1938 U IDE L T 19 3 8 EDD 2.2A , 25 V S te p -d o w n S w itc h in g R e g u la to r DESCRIPTION Demonstration circuit 1273 is a monolithic step-dow n DC/DC sw itching regulator featuring LT1938. The demo board is designed for 3.3V output from a 4.5V to 25V input. The w ide input range of the LT1938 allow s a variety of input sources. The typical sources are automotive batteries, w all adaptors and industrial supplies. The programmed 800 kHz sw itching frequency allow s the use of small, low cost inductor and ceramic capacitors, resulting in low , predictable output ripple. 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 LT1938’s integrated boost diode reduces the parts count. The RUN/SS pin can be used to set the part in micropow er shutdow n mode, re- ducing 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 LT1938 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 1273. 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 4.5V 25V 3.3V +/- 4% 2.2A 800kH z Q U ICK STA RT PROCEDU RE Demonstration circuit 1273 is easy to set up to evaluate the performance of the LT1938. 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. NOTE. M ake sure that the input voltage does not exceed the maximum input voltage. 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 voltages is established, adjust the load w ithin the operating range and observe the output voltage regulation, ripple voltage, efficiency and other parameters. 1 LT1938 _ A + V _ + + V + _ _ A + _ Figure 1. Proper M easurem ent Equipm ent Setup GND VIN Figure 2. M easuring Input or Output R ipple 2 _ Load + LT1938 3