Part overview — what the BQ771807DPJR does on a Li-ion pack
The Texas Instruments BQ771807DPJR is a dedicated battery protection IC for lithium-ion packs built from 2 to 5 series cells. It monitors each cell voltage and triggers a fault output when any cell exceeds the over-voltage threshold — a single-purpose protector, not a full fuel gauge or secondary protector with over-current or over-temperature detection.
The 2-5 cell range is the headline selection parameter: this IC fits a 2S to 5S Li-ion pack. The over-voltage-only fault protection means the designer must add separate over-current (fuse, PTC, or current-sense supervisor) and over-temperature (NTC + comparator) protection externally. The -40°C to 110°C operating range is wide enough for most portable and light industrial packs, but a hot engine-bay or downhole application would exceed the 110°C ceiling.
The 8-WSON (3x4) package with exposed pad is a small, leadless DFN. Stencil aperture design for the pad is critical — too little solder and the part lifts during reflow; too much and the pad floats.