DEMO CIRCUIT 1149 QUICK START LT3680 GUIDE LT3680EMSE 3.5A,36V Step-down Switching Regulator with SYNC Function DESCRIPTION Demonstration circuit 1149 is a monolithic stepdown DC/DC switching regulator featuring LT3680. The LT3680 can be synchronized over a 250 KHz to 2 MHz range. The demo board is designed for 5V output from a 6.3V to 36V input. The wide input range of the LT3680 allows a variety of input sources. The typical sources are automotive batteries, wall adaptors and industrial supplies. The modes of operation (Burst Mode or synchronization) are jumper selectable. The Burst Mode operation increases the efficiency at light loads. The current mode control scheme creates fast transient response and good loop stability. The gate drive of the internal switch is boosted to a voltage that is higher than the Vin to ensure saturation of the switch. The LT3680’s integrated boost diode reduces the parts count. The RUN/SS pin can be used to set the part in micropower shutdown 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 LT3680 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 1149. 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 Switching Regulator ( TA = 25oC ) PARAMETER FOR BUCK REGULATOR Minimum Input Voltage Maximum Input Voltage Output Voltage VOUT Maximum Output Current Typical Switching Frequency CONDITION VALUE 6.3V 36V 5V +/- 5% 3.5A 600kHz QUICK START PROCEDURE Demonstration circuit 1149 is easy to set up to evaluate the performance of the LT3680. Refer to Figure 1 for proper measurement equipment setup and follow the procedure below: 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. 1. Place JP1 on the RUN position. 2. With power off, connect the input power supply to Vin and GND. 3. Turn on the power at the input. 4. Check for the proper output voltage. NOTE. If there is no output, temporarily disconnect the load to make sure that the load is not set too high. 1 LT3680 6. An external clock can be added to the SYNC pin when SYNC function is used. See synchronization section in the datasheet for details. + _ A + + + V + _ _ _ A + V _ Figure 1. Proper Measurement Equipment Setup GND VIN Figure 2. Measuring Input or Output Ripple 2 _ Load 5. Once the proper output voltage is established, adjust the load within the operating range and observe the output voltage regulation, ripple voltage, efficiency and other parameters. LT3680 3