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BQ78350: Safe-Pin

Paul K52
Intellectual 640 points
Part Number: BQ78350
Other Parts Discussed in Thread: TIDA-00108, BQ76920, , TIDA-00792

Hello,

I read about the SAFE-Pin, that I can use it to connect an in-line fuse to it. Can you recommend what kind of fuse I could use for this?

If a fault is detected the BQ78350 will switch the state of this Pin to High, is that correct?

Refering to the datasheet of the BQ78350 it is said that: " If the Pin is High, an additional safety protection is given."

With safety protection it is meant, that a fuse can be "activated". Is that right? Or does "safety protection" mean, that it can detect, whether which fault is detected (secondary protection)

If yes how?

Can you show me an example, how to connect the fuse to the Pin and to the battery? Im not sure about this.

I think I need some further informations, to understand this. Unfortunately I havent found a schematic with a fuse.

Thanks for helping me

  • 0
    TI__Guru 54715 points
    Hi Paul,
    It would be a thermally controlled fuse. You might see an example at the TIDA-00108 or the bq40z50EVM. A drive or activation circuit is needed for such a fuse.
    If the fault is enabed for the SAFE output and conditions are met it will activate. The voltage must be below 32767, search for e2e posts for limit and explanation, use scaling if above this voltage and you want SAFE.
    "additional safety protection" might be the fuse activation, or you might choose to do some other system design.
    See the configurations for the PF and SAFE output. PF is permanent fail, a fuse activated by SAFE would be a permanent action.
    I don't know of a schematic with fuse for bq78350, these go into systems which are more varied and may not use a fuse. See the bq40z50EVM as an example of a system which has a secondary protector and fuse.
  • 0 in reply to WM5295
    Intellectual 640 points
    For me it is necessary to be able to activate or deactivate the charge/discharge fets ( of the bq67920). The datasheet of the bq76920 (page 28/29) says that it controls the DSG and CHG fets by itself....Do I need the Safe pin for this and what should I do with the pin if it is not needed?

    Can I leave that pin unconnected as it shown in the datasheet of the bq78350-r1 on page 14?
  • 0 in reply to Paul K52
    TI__Guru 54715 points
    Hi Paul,
    You may note in the EVM and the TIDA-00792 that the SAFE pin is not used. Check your system requirements, if your system needs that function implement it, often accompanied by a secondary voltage or second-level protector. The bq40z50EVM mentioned above includes that type circuit since it is a common requirement in systems which use that device. If you don't need it, SAFE is an output and may be left open. The bq76920 will control FETs, but note it can only turn them on under command of the attached host. The bq76920 can turn the FETs off under the programmed fault conditions, as can the host.