Demonstration circuit 2896A-B demonstrates a high power boost converter and features the LT®8551 the phase expander with internal gate drivers. The DC2896A-B employs LTC3769, a single-phase boost controller as primary controller and LT8551 as the phase expander which added four more power phases. The increased number of power sections connected to the LT8551, results in greater higher level output power without a corresponding increase in the difficulty of the design or a sacrifice of primary controller LTC3769 features.
The input voltage range of DC2896A-B is from 10V to 46V and the output is 48V. The maximum input current is 12A per power section, so the maximum input current of DC2896A-B is 60A. It’s reflected to 30A of output current and output power of 1,440W at an input voltage of 24V. Correspondingly output current will drop to 14.5A and output power will to 720W at a 12V input voltage, to keep the input current at the same level of 60A. Both values will increase at higher voltages. The input current value can be increased above 60A, if heat sinks are installed, see parts list below.
The switching frequency is 250kHz and at a 24V input to 48VOUT at full load, the efficiency of DC2896A-B is 98%. As the input voltage decreases, the output current should be decreased as well, to prevent saturation of the DC2896A-B inductors, the derating curve presented on Figure 3. Efficiency curves presented on Figure 4.
DC2896A-B resistors R114 to R119 program the phase relationship between the power sections, by setting the Total Distinct Phase Number (TDPN). For increasing efficiency and reducing switching losses at light loads DC2896A-B supports disconnecting some power sections. This mode of operation designated as the stage shedding and it is programed by the jumper J3 STAGE SHED. Jumper JP1 programs the LT8551 SYNC pin and it allows DC2896A-B to be synchronized to an external clock. The print circuit board of DC2896A also supports two phase expander controllers, the LT8561 and LT8551, with versions DC2896A-A and DC2896A-B correspondingly. The table [1] in the electrical schematic describes differences between both versions, including controller biasing and gate drive voltages. DC2896A-B has jumper SHDN (JP2) for manually enable or disable controller. The SYNC (E19) input can be used to sync LT8551 with an external signal. There are also terminals that make it easy to monitor the control logic of the LT8551 functional. The current sense resistors RSNS1 to RSNS5 used for implementation of current mode control, monitoring and sharing current between the phases. To further increase efficiency DC2896A-B supports optional DCR sensing of inductors L1 to L5.
DC2896A-B includes sense line filters and filter options for inputs to the LT8551 in accordance with data sheet recommendations. Undervoltage shutdown, switching frequency and LT8551 ILIM current limit levels can all be adjusted by resistors or resistor jumpers. DC2896A-B comes with a housekeeping circuit based on U3 controller. It reduces the power dissipation that would otherwise occur at high input voltages in regulator transistor Q21. The housekeeping circuit also stabilizes converter biasing at sudden input voltage drops.
A high level of available output power without a corresponding high level of design complexity makes the LT8551 attractive for high power DC bus and battery systems in commercial, industrial and automotive settings. DC2896A-B features the LT8551EUKG in a thermally enhanced 52-pin 7mm × 8mm QFN package. The LT8551 and LTC3769 data sheets must be read in conjunction with this demo manual to properly use or modify DC2896A-B.
