SLUSBS8B December 2013 – November 2019 BQ40Z50
PRODUCTION DATA.
The FUSE pin of the BQ40Z50 is designed to ignite the chemical fuse if one of the various safety criteria is violated. The FUSE pin also monitors the state of the secondary-voltage protection IC. Q5 ignites the chemical fuse when its gate is high. The 7-V output of the bq294700 is divided by R16 and R6, which provides adequate gate drive for Q5 while guarding against excessive back current into the bq294700 if the FUSE signal is high.
Using C3 is generally a good practice, especially for RFI immunity. C3 may be removed, if desired, because the chemical fuse is a comparatively slow device and is not affected by any sub-microsecond glitches that come from the FUSE output during the cell connection process.
If the AFEFUSE output is not used, it should be connected to VSS.
When the BQ40Z50 is commanded to ignite the chemical fuse, the FUSE pin activates to give a typical 8-V output. The new design makes it possible to use a higher Vgs FET for Q5. This improves the robustness of the system, as well as widens the choices for Q5.