Single-cell Li-Ion charge controller, 4.1V regulation
The Texas Instruments BQ2057SNG4 is a linear charge management IC designed for single-cell Lithium Ion and Lithium Polymer battery packs, with a regulation voltage of 4.1V. It delivers a constant, programmable charge current — set by an external resistor — making it straightforward to tailor the charge rate to the cell capacity without firmware. The 8-SOIC package (0.154", 3.90mm Width) is a common, rework-friendly footprint that handles well under a hot-air station if the board needs a swap — the body is small enough that the part lifts cleanly with preheat, and the pin 1 indicator is clear on the top mark.
The 4.1V battery pack voltage is the float voltage for a single Li-Ion cell, which is lower than the more common 4.2V termination used in many consumer chargers. This part is intended for cells or applications where a 4.1V ceiling is specified — typically for extended cycle life or certain Li-Polymer chemistries that do not tolerate 4.2V. The charge current is programmable via a single resistor from the ISET pin to ground, giving the designer direct control over the charge rate without software overhead. Maximum supply voltage is 15V, so the input rail can be a 5V USB or a 12V wall adapter without an additional pre-regulator, as long as the input stays below that ceiling.
Temperature range and operating environment
The 8-SOIC package with surface-mount termination means the board layout needs a decent copper area under the part for thermal dissipation during constant-current charging — the package itself is not a power package, so the charge current should be derated if the ambient is near the 70°C upper limit.