DC1578A-A Analog Devices

DEMO CIRCUIT 1578A LT3695 QUICK START GUIDE LT3695-3.3 and LT3695-5 1A Fault Tolerant Micropower Step-down Switching Regulator DESCRIPTION Demonstration circuit 1578A-A and 1578A-B are micro- parts count. The RUN/SS pin can be used to set the part power monolithic step-down DC/DC switching regulators in micropower shutdown mode, reducing the supply cur- featuring LT3695-3.3 and LT3695-5 respectively. The rent to less than 1uA. The RUN/SS pin can also be used demo boards have fixed output voltages of 3.3V and 5V. to program soft start. In this mode, the RUN/SS pin is The wide input range of the LT3695 allows a variety of driven through an external RC filter to create a voltage input sources. The typical sources are automotive batter- ramp on this pin. The soft start function reduces the in- ies, wall adaptors and industrial supplies. The switching put current surge during start-up. The LT3695 family can frequency can be programmed either via oscillator resis- run in low ripple burst mode or pulse skipping mode. tor or external clock. The modes of operation (Burst JP2 selects the mode. When in burst mode, the no load Mode or synchronization) are jumper selectable. The supply current is reduced to 80uA for the demo circuit Burst Mode operation increases the efficiency at light 1578A-A and 100uA for the demo circuit 1578A-B. loads. The LT3695 family datasheet gives a complete descrip- The LT3695 family tolerates adjacent pin shorts or an tion of the part, operation and application information. open pin without raising the output voltage above its pro- The datasheet must be read in conjunction with this grammed value. NC pins on the LT3695 family must be quick start guide for demo circuit 1578A-A and 1578A-B. left floating. The current mode control scheme creates Design files for this circuit board are available. Call fast transient response and good loop stability. The gate the LTC factory. drive of the internal switch is boosted to a voltage that is LTM, LTC, LT and Module are registered trademarks of Linear Technology Corporation. higher than the Vin to ensure saturation of the switch. All other trademarks are the property of their respective owners. The LT3695 familys integrated boost diode reduces the o Performance Summary for Step-down Switching Regulator ( T = 25 C ) A PARAMETER FOR BUCK REGULATOR CONDITION VALUE Minimum Input Voltage Demo Circuit 1578A-A 4.5V Demo Circuit 1578A-B 6.9V Maximum Input Voltage Operating 36V Transient 60V Output Voltage V Demo Circuit 1578A-A 3.3V +/- 1.5% OUT Demo Circuit 1578A-B 5V +/- 1.5% Maximum Output Current 1A Typical Efficiency Demo Circuit 1578A-A V = 12V I = 1A 85% IN OUT Demo Circuit 1578A-B V = 12V I = 1A 88% IN OUT Typical Switching Frequency 800kHz QUICK START PROCEDURE NOTE. When measuring the input or output voltage ripple, care must be Demonstration circuit 1578A-A and 1578A-B are easy to taken to avoid a long ground lead on the oscilloscope probe. Measure the set up to evaluate the performance of the LT3695-3.3 input or output voltage ripple by touching the probe tip directly across the and LT3695-5. Refer to Figure 1 for proper measurement V or V and GND terminals. See Figure 2 for proper scope probe IN OUT technique. equipment setup and follow the procedure below: 1 LT3695-3.3 AND LT3695-5 1. Place JP1 = ON JP2 = Burst Mode. 5. Once the proper output voltage is established, adjust the load within the operating range and observe the 2. With power off, connect the input power supply to output voltage regulation, ripple voltage, efficiency V and GND. Connect load to V and GND IN OUT and other parameters. 3. Turn on the power at the input. 6. An external clock can be added to the SYNC pin 4. Check for the proper output voltage. when SYNC function is used. See synchronization section in the datasheet for details. NOTE. If there is no output, temporarily disconnect the load to make sure that the load is not set too high. Figure 1. Proper Measurement Equipment Setup Figure 2. Measuring Input or Output Ripple 2