Analog Devices LT8705 | Demoboard DC2285A

Description

Demonstration circuit 2285A features two high power highperformance buck-boost LT®8705A converters operatingin master-slave configuration. The circuit can operate withinput voltages above, below or equal to the output voltage.The demo board input range is 36V to 75V. The output isoptimized for 48V at 18A, with the output current limit setat 20A. The circuit will operate with lower input voltagethan 36V if load current is reduced.The LT8705A controller has integrated input current, inputvoltage, output current and output voltage regulators. Theone regulator that wants to decrease current gets controlover the compensation pin VC. The inductor current iscontrolled by the VC signal that is fed into a current comparatortogether with a ramp compensation signal.While the current mode control limits the inductorboth 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 high impedance power sources,and will reduce the current if input voltage drops belowthe set point.The operating mode of the controller is determinedthrough the MODE pin (jumper JP1) and can be setto discontinuous mode, forced continuous mode andBurst Mode® operation.The LT8705A is capable of bidirectional current flow whenoperating in forced continuous operation. It is importantto know that additional circuitry is required for controlledreverse current operation, and this additional circuitry canbe elaborate in high power applications where tight controlof current amplitude is required. The SYNC inputs are used to synchronize the two LT8705Apower stages with 180-degree phase shift. The DC2285Ademo board contains a multiphase clock.The LT8705A has internal series regulators that provideINTVCC voltage (6.3V Typ) from VIN or EXTVCC. Especiallyin high power applications the internal dissipation of theseseries regulators can be high. The DC2285A demo boardincludes an LT8631 buck regulator that is configured for7.8V. By connecting this low voltage to VIN and EXTVCCthe power dissipation is reduced. The auxiliary 7.8V canalso be used to power external circuitry.The LT8705A 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 2285A. The input voltage range ofthe LT8705A itself is 2.8V (with external auxiliary voltage> 6.4V) to 80V and the output range is 1.3V to 80V. TheDC2285A demo board has an optional signal interfacethat makes it easy to add potentiometers and switches(or a microcontroller) for accurate external control of thepower stages.The LT8705AEUHF 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. The DC2285A circuitboard has mounting holes for an optional heat sink andNTC resistor for monitoring heat sink temperature

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