Demonstration circuit 2596A is a high performance bidirectional buck-boost converter featuring the LT®8708 that can operate from input voltages above, below or equal to the output voltage. The demo board input range is 36V to 75V. The output voltage is set at 48V and the output current limit at 6A.
The controller has integrated input voltage and output voltage regulators and two sets of input and output current regulators that control current flow in forward or reverse direction. Features are included that simplify bidirectional power conversion in battery/capacitor backup systems and other applications that may need regulation of VIN, VOUT, IIN and/or IOUT.
While the current mode control limits the inductor current both in normal and in reverse direction these current limits have some variation as input/output voltage changes. The forward and reverse input and output current regulators offer four accurate current limits that can be set individually.
The input voltage regulator is often used in applications with high impedance power sources and will reduce the forward current if the input voltage drops below the set point. When operating with reverse current flow the input voltage regulator regulates the voltage at the input side.
The operating mode of the controller is determined through the MODE pin (jumper JP7 Pins 5 to 12) and can be set to discontinuous mode, hybrid discontinuous mode, forced continuous mode and Burst Mode® Operation.
The LT8708 is capable of bidirectional operation when operating in the continuous conduction mode (CCM). DCM, HCM and Burst Mode operation only allow power to flow in one direction. Additional circuitry may be needed depending on the application.
By feeding the LT8708 from a separate low voltage supply the power dissipation can be reduced. The LT8708 will start when >5.5V voltage is applied at the input (VIN pin), and when it is running it will draw current from the EXTVCC pin if the voltage at the pin is > 6.4V.
Typical efficiency with 5A load is above 96.3% across a 36V to 75V input range.
The LT8708 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 DC2596A. The input voltage range of the LT8708 itself is 2.8V (need EXTVCC > 6.4V) to 80V and the output range is 1.3V to 80V.
The LT8708EUHG is assembled in a 40-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 Circuit Board Layout Checklist section.