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Original question:

BQ78350-R1: Protection FET control

BQ78350-R1: SAFE pin use

Jeffrey Wright
Expert 2875 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ78350, BQ76920, BQ77180, TIDA-010030,

Hello; My client's design will use the bq76920 + bq78350. I note in your reference design (tidrqy7) that there is no PTC fuse controlled by the 78350's SAFE output. What is TI's recommended approach for this? Is the 78350's SAFE output suited for this purpose, or do you recommend the bq77180 used on the tida-010030 reference design, or ??

Thanks in advance

  • 0
    TI__Guru 54715 points

    Hi Jeffrey,

    The use of an activated fuse is apparently more system dependent for the higher cell count designs where the BQ78350-R1 is used.  SAFE can be used to activate a fuse as noted in the data sheet section 8.3.2. That is interesting that we don't seem to have an example showing SAFE used to blow a fuse.  SAFE could be used either with or without a secondary protector.  The closest example which comes to mind would be http://www.ti.com/lit/pdf/sluuav7 section 5.2 schematic, although note the voltage limits of the BQ78350-R1 SAFE pin which is an output only.  The FET must handle the system voltage and the current to activate the fuse, but also have a low enough VGSth to be turned on by the SAFE output high level and any components in series.  Sometimes separate FETs are used for the gauge and protector.

    Also note that the FUSE output does not activate above 32767V use voltage scaling when needed.