DEMO CIRCUIT 1578A QUICK START LT3695 GUIDE LT3695-3.3 and LT3695-5 1A Fault Tolerant Micropower Step-down Switching Regulator DESCRIPTION Demonstration circuit 1578A-A and 1578A-B are micropower monolithic step-down DC/DC switching regulators featuring LT3695-3.3 and LT3695-5 respectively. The demo boards have fixed output voltages of 3.3V and 5V. The wide input range of the LT3695 allows a variety of input sources. The typical sources are automotive batteries, wall adaptors and industrial supplies. The switching frequency can be programmed either via oscillator resistor or external clock. The modes of operation (Burst Mode or synchronization) are jumper selectable. The Burst Mode operation increases the efficiency at light loads. The LT3695 family tolerates adjacent pin shorts or an open pin without raising the output voltage above its programmed value. NC pins on the LT3695 family must be left floating. 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 LT3695 family'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 LT3695 family can run in low ripple burst mode or pulse skipping mode. JP2 selects the mode. When in burst mode, the no load supply current is reduced to 80uA for the demo circuit 1578A-A and 100uA for the demo circuit 1578A-B. The LT3695 family 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 1578A-A and 1578A-B. Design files for this circuit board are available. Call the LTC factory. LTM, LTC, LT and Module are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 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 Efficiency Typical Switching Frequency CONDITION Demo Circuit 1578A-A Demo Circuit 1578A-B Operating Transient Demo Circuit 1578A-A Demo Circuit 1578A-B VALUE 4.5V 6.9V 36V 60V 3.3V +/- 1.5% 5V +/- 1.5% 1A Demo Circuit 1578A-A; VIN = 12V; IOUT = 1A 85% Demo Circuit 1578A-B; VIN = 12V; IOUT = 1A 88% 800kHz QUICK START PROCEDURE Demonstration circuit 1578A-A and 1578A-B are easy to set up to evaluate the performance of the LT3695-3.3 and LT3695-5. 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 LT3695-3.3 AND LT3695-5 1. Place JP1 = ON; JP2 = Burst Mode. 2. With power off, connect the input power supply to VIN and GND. Connect load to VOUT 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. 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. 6. An external clock can be added to the SYNC pin when SYNC function is used. See synchronization section in the datasheet for details. Figure 1. Proper Measurement Equipment Setup Figure 2. Measuring Input or Output Ripple 2 LT3695-3.3 AND LT3695-5 3