DC1073A Analog Devices

DEMO CIRCUIT 1073 LT3845 QUICK START GUIDE LT3845 High Voltage Synchronous Buck Controller with Programmable Frequency and Sync mm3mm DFN DESCRIPTION Demonstration Circuit 1073 is a output voltages. In such applica- 100kHz-500kHz programmable frequency, tions, the optional circuit can in- high voltage, current-mode DC/DC crease the overall efficiency by re- step-down converter featuring the ducing the power loss in the LT3845. LT3845. The operating frequency can The demonstration board has also been be synchronized up to 600kHz. The laid out with the option of doubling demo board is designed for 12V the switching MOSFETs to facilitate 6.25A (75W) output from a 20V to 55V higher output current. This board is input. A soft-start feature controls suitable for a wide range of indus- the output voltage slew rate at trial control systems and particu- start-up, reducing current surges and larly suitable for 12V/42V automotive voltage overshoots. The modes of op- applications and 48V Telecom power eration (Burst Mode, Discontinuous supplies. Current Mode and Continuous Current The LT3845 datasheet gives a complete Mode) are jumper selectable. Both description of the part, operation Burst Mode and Discontinuous Current and application information. The Mode increase the efficiency at light datasheet must be read in conjunction loads. Continuous Current Mode will with this quick start guide for demo maintain a constant switching fre- circuit 1073. quency regardless of the load cur- Note: It is best to ground the SYNC rent. pin if the SYNC function is not being An Optional Boost Bias circuit is used. provided on the bottom side of the board for back-driving the LT3845 in- ternal regulator from the output Design files for this circuit board voltage. Customers may want to use are available. Call the LTC factory. this optional circuit with modified , LTC and LT are registered trademarks of Linear Technol- ogy Corporation. ThinSOT and PowerPath are trademarks of Lin- applications that have relatively ear Technology Corporation. high input voltages and low (~ 3.3V) o Performance Summary ( T = 25 C ) A PARAMETER FOR STEP-DOWN CONVERTER CONDITION VALUE Minimum input voltage 20V Maximum input voltage 55V Output voltage V 12V +/- 4% OUT Maximum output current 6.25A Typical switching frequency 300kHz 1 LT3845 QUICK START PROCEDURE Demonstration circuit 1073 is easy to 3. Turn on the power at the input. set up to evaluate the performance of NOTE. Make sure that the input voltage does not the LT3845. Refer to Figure 1 for exceed 60V. proper measurement equipment setup 4. Check for the proper output volt- and follow the procedure below: ages NOTE. When measuring the input or output voltage 5. Once the proper output voltages are ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. Measure the input established, adjust the loads or output voltage ripple by touching the probe tip within the operating range and ob- directly across the Vin or Vout and GND terminals. serve the output voltage regula- See Figure 2 for proper scope probe technique. tion, ripple voltage, efficiency 1. Place JP1 on the RUN position: and other parameters. 2. With power off, connect the input power supply to Vin and GND. 2