DC1924A: 48V @ 5A, 36 ~ 80V in, Buck-Boost

Demonstration circuit 1924A is a high performance buck/boost converter featuring the LT®8705 that can operate from input voltages above, below or equal to the output voltage. The demo board input range is 36V to 80V. The output is optimized for 48V at 5A, with the output current limit set at 7A. (The circuit will operate with lower input voltage than 36V if load current is reduced).

The controller has integrated input current, input voltage, output current and output voltage regulators. The one regulator that wants to decrease current gets control over the compensation pin VC. The inductor current is controlled by the VC signal that is fed into a current comparator together with a ramp compensation signal.

While the current mode control limits the inductor current both in normal and in reverse direction, these current limits have some variation as input voltage changes. The input and output current regulators offer more accurate current limits.

The input voltage regulator is typically used in applications with solar panels or other high impedance power sources, and will reduce the current if the input voltage drops below the set point.

The operating mode of the controller is determined through the MODE pin (jumper JP1) and can be set to discontinuous mode, forced continuous mode and Burst Mode® operation.

The LT8705 is capable of bidirectional operation when operating in forced continuous mode. Additional circuitry may be needed depending on the application.

The CLKOUT output and the SYNC input can be used to synchronize two DC1924A circuits with 180 degree phase shift.

By feeding the LT8705 from a separate low voltage supply, the power dissipation can be reduced. To supply the LT8705 chip from an external voltage supply (> 6.4V), cut the trace as marked on the board to disconnect the EXTVCC pin from VOUT. The LT8705 will start when voltage is applied at the input (VIN pin), and when it is running it will draw current from the EXTVCC pin if the voltage is > 6.4V.

Typical efficiency with 5A load is above 97% across a 36V to 72V input range using the supplied inductor. Lower core loss can be achieved by using a ferrite core inductor.

The LT8705 data sheet gives a complete description of the part, operation and application information. The data sheet should be read in conjunction with this quick start guide for demo circuit 1924A. The input voltage range of the LT8705 itself is 2.8V (need EXTVCC > 6.4V) to 80V and 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 underneath the chip. Proper board layout is essential for maximum thermal and electrical performance. See the data sheet section Circuit Board Layout Checklist.