Features and Benefits
- Expands Up to 4 Phases per Chip
- Up to 80V Input or Output Voltage
- Cascade with Multiple Chips for High Current Applications
- Supports Up to 18 Distinct Phases from 20° to 180°
- Phases Can Share Phase Angle
- Excellent DC and Transient Current Sharing
- Phase-Lockable Fixed Frequency 125kHz to 1MHz
- Supports Bidirectional Current Flow
- RSENSE or DCR Current Sensing
- Eliminates the Need to Route Sensitive Feedback and Control Signals.
- 52-Lead (7mm × 8mm) QFN Package
The LT8551 is a multiphase expander for synchronous boost DC/DC converters. It operates in tandem with any synchronous boost DC/DC converter to increase the load current capability by adding additional phases, which are clocked out-of-phase to reduce ripple current and filtering capacitance. It easily adds phases without the need to route sensitive feedback and control signals.
The LT8551 integrates 5V gate drivers and can support up to four boost phases per device. Multiple LT8551 devices can be used for up to 18 phases. It accurately monitors and adjusts the current of each channel to achieve excellent DC and transient current sharing.
The LT8551 operates over a fixed frequency from 100kHz to 1MHz or can be synchronized to an external clock.
- High Current Distributed Power Systems
- Telecom, Datacom, and Storage Systems
- Industrial and Automotive
Product Lifecycle Recommended for New Designs
This product has been released to the market. The data sheet contains all final specifications and operating conditions. For new designs, ADI recommends utilization of these products.
Evaluation Kits (2)
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  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.
Demonstration circuit 2332A-A is a multiphase boost converter expander that features the LT8551. The DC2332A-A uses the LTC3769 as primary controller and LT8551 as a phase extender. The increased number of power phases, allowed by the LT8551, results in higher output power without a corresponding increase in design difficulty or a sacrifice of the primary controller LTC3769 features.
The input voltage range of DC2332A-A is from 6V to 46V and the output is 48V. The maximum input current is 10A per power section, so the maximum input current of DC2332A-A is 50A. It can produce a 25A output current and output power of 1,200W with an input voltage of 24V. Correspondingly, output current will drop to 12A and output power to 576W at 12VIN. Higher output power can be achieved at a 12VIN if heat sinks are installed, see Parts List.
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