DEMO MANUAL DC1736A LT3975 42V, 2.5A, 2.7A Micropower Buck Regulator DESCRIPTION Demonstration circuit 1736A is a monolithic step-down the EN pin is low and active when the pin is high. The DC/DC switching regulator featuring the LT 3975. The threshold of the EN pin is accurate at 1.02V with 60mV demo circuit is designed for 3.3V, 2.5A output from a 4.3V of hysteresis. Users can populate R7 and R8 to provide a to 42V input. The wide input range of the LT3975 allows programmable undervoltage lockout. During a short-circuit a variety of input sources, such as automotive batteries fault, the LT3975 has current limit foldback to limit the and industrial supplies. The switching frequency can be power dissipation. programmed either via oscillator resistor or external clock The LT3975 data sheet gives a complete description of up to 2MHz. When the circuit is synchronized to an external the part, operation and application information. The data clock connected to the SYNC terminal, the R resistor (R5) T sheet must be read in conjunction with this demo manual should be chosen to set the LT3975 internal switching for demo circuit 1736A. The LT3975 is assembled in a frequency at least 20% below the lowest synchroniza- 16-lead plastic MSOP package with an exposed pad for tion input frequency. Low ripple Burst Mode operation low thermal resistance. Proper board layout is essential increases the efficiency at the light load while keeping the for both proper operation and maximum thermal perfor- output ripple below 15mV. mance. See the data sheet section PCB Layout and High The circuit consumes only 2.7A of quiescent current. Temperature Considerations. The LT3975 internal boost diode and loop compensation Design files for this circuit board are available at reduce the components count and solution size. The cur- DEMO MANUAL DC1736A QUICK START PROCEDURE Demonstration circuit 1736A is easy to set up to evalu- 5. Turn on the power at the input. ate the performance of the LT3975. Refer to Figure 2 for NOTE. Make sure that the input voltages do not exceed 42V. proper measurement equipment setup and follow the 6. Check for the proper output voltage (3.3V). procedure below: NOTE. If there is no output, temporarily disconnect the load NOTE. When measuring the input or output voltage ripple, to make sure that the load is not set too high or is shorted. care must be taken to avoid a long ground lead on the oscilloscope probe. Measure the input or output voltage 7. Once the proper output voltages are established, adjust ripple by touching the probe tip directly across the V IN the load within the operating ranges and observe the or V and GND terminals. See Figure 3 for the proper OUT output voltage regulation, ripple voltage, efficiency and scope technique. other parameters. 1. Place JP1 on the ON position. 8. An external clock can be added to the SYNC terminal when SYNC function is used (JP2 on the SYNC posi- 2. Place JP2 on the RT FREQ position. tion). Please make sure that the SYNC frequency is at 3. With power off, connect the input power supply to V IN least 20% higher than the set switching frequency. See and GND. the data sheet section Synchronization. 4. With power off, connect load from V to GND. OUT 90 85 80 75 70 65 60 V = 3.3V OUT 55 V = 12V IN f = 600kHz SW 50 0 0.5 1 2 2.5 1.5 LOAD CURRENT (A) DC1735A F01 Figure 1. Efficiency vs Load Current dc1736af 2 EFFICIENCY (%)