In theory, a linear battery charger with a separate power path for the system is a fairly simple design concept and can be built with an LDO adjusted to 4.2 V; a current-limit resistor; three p-channel FETs to switch the system load between the input power and the battery source; and some bias parts. In reality, there is much more to a good design than the basic topology. This article will discuss dynamic power-path management (DPPM) and explore safety features that turn a basic topology into a complete design.