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bq25505 rechargeable battery and system load connection

K. Sasaki14
Guru 10200 points

Our customer would like to flow large current from rechargeable battery. So they would like to design as attached circuit because the voltage drop at internal PFET between VSTOR and VBAT_SE becomes big.

Is there any concern about attached circuit?

Best Regards,

Kohei Sasaki

  • 0
    TI__Guru*** 143950 points
    They will be bypassing the internal PFET which protects the battery from overdischarged by the IC itself (which is powered from VSTOR) down to 1.8V.
  • 0 in reply to Jeff F
    Guru 10200 points

    Jeff-san,

    Thank you for your answer.
    I know the internal PFET bypasses between VSTOR and VBAT_SEC. However the ON resistance of this FET is about 1 ohm according to datasheet. This is large value for our customer. So they would like to connect battery to system load directly instead of VSTOR.
    So we would like to know whether is there any concern about this using method.


    Best Regards,

    Kohei Sasaki

  • 0 in reply to K. Sasaki14
    TI__Guru*** 143950 points

    There is no concern for the IC.  However, the battery will need protection from over-discharge.

  • 0 in reply to Jeff F
    Intellectual 480 points

    Jeff,

     I have a very similar issue.  

     Couple of questions:

    a) from what I can tell the proposed solution above cuts the line from the battery to Vstor.  The IC will still get powered?

    b) my solution is to use replace the proposed red trace jumper above with another dual FET (exactly the same as the one used for switching between the secondary and primary) that is in parallel to the internal switch.  I am not planning on cutting the trace back to VSTOR.

    Thanks,

    Kurt

  • 0 in reply to Kuenrg
    Intellectual 480 points
    Also, any particular reason why the bidirectional configuration of FETs are recommended? Maybe it's obvious...
  • 0 in reply to Kuenrg
    TI__Guru*** 143950 points

    The internal FET between VSTOR and VBAT_SEC has a body diode that always allows current from VSTOR to VBAT_SEC.  The back to back external FETs prevent the body diode of 1 single FET from allow current flow.