Analog Devices LT8705 | Demoboard DC1924A

Description

Demonstration circuit 1924A is a high performance buckboostconverter featuring the LT®8705 that can operatefrom input voltages above, below or equal to the outputvoltage. The demo board input range is 36V to 80V. Theoutput is optimized for 48V at 5A, with the output currentlimit set at 7A. (The circuit will operate with lower inputvoltage than 36V if load current is reduced).The controller has integrated input current, input voltage,output current and output voltage regulators. The oneregulator that wants to decrease current gets control overthe compensation pin VC. The inductor current is controlledby the VC signal that is fed into a current comparatortogether with a ramp compensation signal.While the current mode control limits the inductor currentboth in normal and in reverse direction, these currentlimits have some variation as input voltage changes. Theinput and output current regulators offer more accuratecurrent limits.The input voltage regulator is typically used in applicationswith solar panels or other high impedance powersources, and will reduce the current if the input voltagedrops below the set point.The operating mode of the controller is determinedthrough the MODE pin (jumper JP1) and can be set todiscontinuous mode, forced continuous mode and BurstMode® operation.The LT8705 is capable of bidirectional operation whenoperating in forced continuous mode. Additional circuitrymay be needed depending on the application.The CLKOUT output and the SYNC input can be usedto synchronize two DC1924A circuits with 180 degreephase shift.By feeding the LT8705 from a separate low voltage supply,the power dissipation can be reduced. To supply theLT8705 chip from an external voltage supply (> 6.4V), cutthe trace as marked on the board to disconnect the EXTVCCpin from VOUT. The LT8705 will start when voltage is appliedat the input (VIN pin), and when it is running it willdraw current from the EXTVCC pin if the voltage is > 6.4V.Typical efficiency with 5A load is above 97% across a 36Vto 72V input range using the supplied inductor. Lowercore loss can be achieved by using a ferrite core inductor.The LT8705 data sheet gives a complete description ofthe part, operation and application information. The datasheet should be read in conjunction with this quick startguide for demo circuit 1924A. The input voltage range ofthe LT8705 itself is 2.8V (need EXTVCC > 6.4V) to 80Vand the output range is 1.3V to 80V.The LT8705EUHF is assembled in a 38-Lead (5mm ×7mm) plastic QFN package with a thermal pad underneaththe chip. Proper board layout is essential for maximumthermal and electrical performance. See the data sheetsection Circuit Board Layout Checklist.

More information

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