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Original question:

BQ25790: IC internal circuit check: circuit between VBUS and PMID

BQ25790: Function of Rsns between VBUS and PMID

Evan Peng
Intellectual 1375 points
Part Number: BQ25790
Other Parts Discussed in Thread: BQ25798

According to the datasheet, I think the Rsns serves as the input current detection. However, this block diagram also shows another element near VBUS PIN to detect input current. So I would like to know the actual function of Rsns.

  • 0
    TI__Guru**** 165786 points

    Hi Evan,

    The resistor is actually a FET that is used for sensing input current.  The block diagram is an oversimplification.  The resistor (FET) and current loop should be combined.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Intellectual 2820 points

    Thanks, Jeff,

    Could you please help solve these further question?

    1. Could this sensing FET be controlled? Will it be constantly on?

    2. Must we add external RBFET? Can Q4 be used as RBFET?

    3. BATFET body diode is from BAT to SYS. So if we need system from SYS power down, must we add external ship FET?

    4. If BQ25790 works on OTG mode, must load be powered from VBUS pin? can it be powered from PMID pin? We can see VOTG is sampled near VBUS pin, but we cannot see IOTG. Where does IOTG sample? Do we have any OTG current protection?

    5. Does BQ25790 have Short-circuit-protection function? Should we add e-fuse to protect SCP situation?

  • 0 in reply to Dillen Chen
    TI__Intellectual 2820 points

    These question is from Customer PMI to evaluate whether our BQ25790 can be used in E-Cig holder.

  • 0 in reply to Jeff F
    TI__Intellectual 1375 points

    Hi Jeff,

    Thanks for your answer. About this FET(Resistor), I have another several questions to go further.

    1. So I think the real circuit is the same as below, the Rsns is actually Q1 in this block diagram, am I right?

    2. How to use this FET to measure current? Does it work in Ohmic region to serve as a 6mΩ resistor?

    3. In which mode this FET will be off, is it can be controlled by us? Can I use PMID as my output PIN in OTG mode?

    Regards,

    Evan

  • +1 in reply to Evan Peng
    TI__Guru**** 165786 points

    Hi Evan,

    Regarding 1, yes.

    Regarding 2, there is a parallel, much smaller FET that is used to measure a scaled down current.

    Regarding 3, unfortunately no. You might consider BQ25798 which has backup mode with OTG at PMID. 

    Regards,
    Jeff