LTC3786: High Efficiency Li-Ion Battery-to-USB Boost Converter

The LTC3786 is a high performance synchronous boost converter controller that drives all N-channel power MOSFETs. Synchronous rectification increases efficiency, reduces power losses and eases thermal requirements, enabling the LTC3786 to be used in high power boost applications. A 4.5V to 38V input supply range encompasses a wide range of system architectures and battery chemistries. When biased from the output of the boost converter or another auxiliary supply, the LTC3786 can operate from an input supply as low as 2.5V after startup. The 55μA no-load quiescent current extends operating run time in battery-powered systems.

The schematic shown above and associated LTspice simulation are designed to be highly efficient and small converter for powering rechargeable USB devices such as smart phones and tablets from Li-Ion power packs with a cell voltage range of 2.7V to 4.2V. Since these power packs are very compact, heat generation could be problamatic and a small highly efficient converter is required. The synchronous operation of the LTC3786 fits this application perfectly thanks to its very high efficiency.

To improve efficiency, MOSFETs with just the right amount of R_DS(ON) were selected. Also, the switching frequency was set to 350kHz in order to minimize the switching losses and to keep the inductor small. The LTC3786 circuit requires an external 5V, 10mA power source for bias. However, this could be biased internally from it’s own 5V output if startup from fully charged Li-Ion cell (4.2V) is acceptable. To achieve higher efficiency, DC resistance sensing is used with an inductor that has a DCR under 2mΩ. However, this produces a higher current limit than the required 5A output. If current limiting is needed a Hot Swap™ circuit like that provided by the LTC4231 µPower Hot Swap Controller can provide current protection.

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