DEMO CIRCUIT 1526 LT3991 QUICK START GUIDE LT3991 55V, 1.2A, MicroPower Buck Regulator DESCRIPTION Demonstration circuit 1526 is a monolithic stepdown DC/DC switching regulator featuring the LT®3991. The switching frequency is adjustable up to 2MHz. The demo circuit is designed for 3.3V, 1.2A output from a 4.5V to 55V input. The wide input range of the LT3991 allows a variety of input sources including automotive batteries and 24V industrial supplies. Low ripple Burst Mode increases the efficiency at the light load while keeping the output ripple below 15mV. The part is in shutdown when the EN pin is low and active when the pin is high. The threshold of the EN pin is accurate at 1.01V with 30mV of hysteresis. When Vin is above 6V, adding a resistor divider from Vin to EN can program the LT3991 to regulate the output only when Vin is above a desired voltage. The circuit consumes only 2.8µA of quiescent current. The boost diode, control and logic circuit are integrated to reduce the components count and solution size. The current mode control scheme creates fast transient response and good loop stability. The LT3991 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 1526. Design files for this circuit board are available. Call the LTC factory. , LTC and LT are registered trademarks of Linear Technology Corporation. Performance Summary (TA = 25oC) PARAMETER Minimum Input Voltage Maximum Input Voltage Output Voltage VOUT Typical switching Frequency Maximum Output Current Typical efficiency Typical output voltage Ripple CONDITION VIN=24V, Iout=1.2A VIN=24V, Iout=1.2A VALUE 4.5V 55V 3.307±2.5% 400kHz 1.2A 79.4% 20mV QUICK START PROCEDURE Demonstration circuit 1526 is easy to set up to evaluate the performance of the LT3991. 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 LT3991 1. 2. 3. Place JP1 on the ON position. 5. With power off, connect the input power supply to Vin and GND. Turn on the power at the input. 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. NOTE. Make sure that the input voltage does not exceed 55V. 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. Figure 1. Proper Measurement Equipment Setup GND VIN Figure 2. Measuring Input or Output Ripple 2 LT3991 3