DEMO MANUAL DC1909A LT3840 60V Current Mode Step-Down DC/DC Converter with Accurate Current Limit loads. Continuous current mode will maintain a constant Description switching frequency regardless of the load current. Demonstration circuit 1909A is a 150kHz, 60V current- The average current control loop regulates the maximum mode DC/DC step-down converter with accurate current output current of the switching regulator. An Optional limit featuring the LT 3840. LDO circuit is provided on the bottom side of the board The circuit operates from a V range of 4.5V to 60V, sur- for programming the LT3840 current limit set point. IN vives an 80V transient, and outputs 3.3V at 20A (66W). This board is suitable for a wide range of automotive, An integrated buck-boost switching regulator generates telecom, industrial, and other applications. a 7.5V bias supply voltage for MOSFET gate drive and IC The LT3840 data sheet gives a complete description of power to increase the overall efficiency by reducing the the part, operation and application information. The data power loss. A soft-start feature controls the output voltage sheet must be read in conjunction with this demo manual slew rate at start-up, reducing current surges and voltage for DC1909A. overshoots. The modes of operation (Burst Mode , dis- continuous current mode and continuous current mode) Design files for this circuit board are available at are jumper selectable. Both Burst Mode operation and DEMO MANUAL DC1909A Quick s p D DC1909A is easy to set up to evaluate the performance 3. Turn on the power at the input. of the LT3840. Refer to Figure 1 for proper measurement NOTE: Make sure that the input voltage never exceeds 80V. equipment setup and follow the procedure below: 4. Check for the proper output voltage of 3.3V. Turn off NOTE: When measuring the input or output voltage ripple, the power at the input. care must be taken to avoid a long ground lead on the 5. Once the proper output voltage is established, connect oscilloscope probe. See Figure 2 for the proper scope a variable load capable of sinking 20A at 3.3V to the technique. output terminals V and GND. Set the current for 0A. OUT 1. Set an input power supply that is capable of 60V/20A. a. If efficiency measurements are desired, an ammeter Then turn off the supply. or a resistor shunt that is capable of handling 20ADC 2. With power off, connect the supply to the input terminals can be put in series with the output load in order to V and GND. IN measure the DC1909As output current. a. Make the ENABLE jumper is in the EN position. b. A voltmeter with a capability of measuring at least b. Input voltages lower than 4.5V can keep the converter 3.3V can be placed across the output terminals in from turning on due to the undervoltage lockout order to get an accurate output voltage measurement. feature of the LTC3840. 6. Turn on the power at the input. c. If efficiency measurements are desired, an ammeter NOTE: If there is no output, temporarily disconnect the capable of measuring 20A DC or a resistor shunt load to make sure that the load is not set too high. can be put in series with the input supply in order 7. Once the proper output voltage is again established, to measure the DC1909As input current. adjust the load and/or input within the operating range d. A voltmeter with a capability of measuring at least and observe the output voltage regulation, ripple volt- 60V can be placed across the input terminals in order age, efficiency and other desired parameters. to get an accurate input voltage measurement. dc1909af 2 ure roce tart