72-V hybrid step-down DC/DC controller reduces solution size

Jan. 19, 2018

Norwood, MA. Analog Devices has announced the Power by Linear LTC7821, which the company describes as an industry-first hybrid step-down synchronous controller that merges a switched-capacitor circuit with a synchronous step-down controller, enabling up to a 50% reduction in DC/DC converter solution size compared with traditional step-down solutions. This improvement is enabled by a 3X higher switching frequency without compromising efficiency. Alternatively, when operating at the same frequency, an LTC7821-based solution can provide up to a 3% higher efficiency. Other benefits include low EMI and reduced MOSFET stress due to a soft-switched front end, suitable for next-generation nonisolated intermediate bus applications in power-distribution, datacom, and telecom as well as emerging 48-V automotive systems.

The LTC7821 operates over a 10-V to 72-V input voltage range and can produce an output voltage from 0.9 V to 33.5 V with currents in multiple tens of amps, depending on the choice of external components. In a typical 48-V to 12-V/20-A application, an efficiency of 97% is attainable with the LTC7821 switching at 500 kHz. The same efficiency can only be achieved in a traditional synchronous step-down converter by switching at one-third the operating frequency, resulting in the use of much larger magnetics and output filter components. External MOSFETs switch at a fixed frequency programmable from 200 kHz to 1.5 MHz. The LTC7821’s 1-Ω N-channel MOSFET gate drivers maximize efficiency and can drive multiple MOSFETs in parallel for higher power applications. Due to its current mode control architecture, multiple LTC7821s can be operated in a parallel, multiphase configuration with current sharing to enable higher power applications.

The LTC7821 implements many proprietary protection features for robust performance in a range of applications. An LTC7821-based design eliminates the inrush current typically associated with switched-capacitor circuits by prebalancing the capacitors during start-up. The LTC7821 also monitors system voltage, current, and temperature for faults, and it uses a sense resistor for overcurrent protection. It stops switching and pulls the /FAULT pin low when a fault condition occurs. An onboard timer can be set for appropriate restart/retry times. The EXTVCC pin enables the LTC7821 to be powered from the lower voltage output of the converter or other available source up to 40 V, reducing power dissipation and improving efficiency. Additional features include ±1% output voltage accuracy over temperature, a clock output for multiphase operation, a power good output signal, short-circuit protection, monotonic output voltage start-up, optional external reference, undervoltage lockout, and internal charge balance circuitry.

The LTC7821 is available in a 5-mm x 5-mm QFN-32 package. Extended and industrial temperature versions are available from –40 to 125°C.

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