DEMO CIRCUIT 1122 QUICK START LT3681 GUIDE LT3681 Step-Down Regulator DESCRIPTION Demonstration circuit 1122 features LT3681 in a 500kHz step-down converter circuit, designed for 3.3V output from a 4.5V to 34V input. The wide input range of the LT3681 allows a variety of input sources. The typical sources are automotive batteries, wall adaptors and industrial supplies. The current mode control scheme creates fast transient response and good loop stability. In addition, the LT3681’s integrated boost diode and catch diode reduce the parts count. The low ripple Burst Mode Operation of LT3681 not only allows the circuit to have high efficiency over a broad current range but also provides the low output ripple at light load conditions. The RUN/SS pin can be used to set the part in micropower shutdown mode. 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 LT3681 datasheet gives a complete description of the part, operation and application information. The datasheet must be read in conjunction with this quick start guide for demo circuit 1122. Design files for this circuit board are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT and PowerPath are trademarks of Linear Technology Corporation. Performance Summary for Step-Down Regulator ( TA = 25oC ) PARAMETER FOR BUCK REGULATOR Minimum Input Voltage Maximum Input Voltage Output Voltage VOUT Maximum Output Current Typical Switching Frequency CONDITION VALUE 4.5V 34V 3.3V +/- 4% 2A 500kHz QUICK START PROCEDURE Demonstration circuit 1122 is easy to set up to evaluate the performance of the LT3681. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: 1 LT3681 NOTE. When 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. NOTE. Make sure that the input voltage does not exceed the maximum input voltage. 4. Check for voltages. the proper output 1. Place JP1 on the RUN position: NOTE. If there is no output, temporarily disconnect the load to make sure that the load is not set too high. 2. With power off, connect the in- 5. Once the proper output voltage put power supply to Vin and GND. 3. Turn on the power at the input. is established, adjust the load within the operating range and observe the output voltage regulation, ripple voltage, efficiency and other parameters. Figure 1. Proper Measurement Equipment Setup GND VIN Figure 2. Measuring Input or Output Ripple 2 LT3681 3