Lithium-Ion and Lithium Polymer batteries are common in portable consumer products because of their relatively high energy density—they provide more capacity than other available chemistries within given size and weight
constraints. USB battery charging is also becoming commonplace, as many portable devices require frequent interfacing with a PC for data transfer.
As portable products become more complex, the need for higher capacity batteries increases, with a corresponding need for more advanced battery chargers. Larger batteries require either higher charging current or additional time to charge to their full capacity. Most consumers look for shorter charge times, so increasing the charge current seems obviously preferable, but increasing charge current presents two major problems. First, with a linear charger, increased current creates additional power dissipation (i.e., heat). Second, the charger must limit the current drawn from the 5V USB bus to either 100mA (500mW) or 500mA (2.5W) depending on the mode that the host controller has negotiated.
Download the full Design Note as a PDF.