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BQ25570 UV Threshold

Ardeno Wibisatria
Prodigy 40 points
Other Parts Discussed in Thread: BQ25570, BQ25504, TPS62736

Hi,

BQ25570 really suits my design, but the problem is VBAT_UV threshold which is set internally to 1.95V.

Since I want to use Lipo battery with cut-off voltage at around 3V, is there anything that I can do to protect my Lipo battery?

Thanks in advance.

Best Regards,

Ardeno

  • 0
    TI__Guru**** 166176 points

    You could add an external PFET in series with the VBAT pin that is controlled by an inverted (requiring a NFET) VBAT_OK signal where VBAT_OK =3.0V.

  • 0 in reply to Jeff F
    Prodigy 40 points

    Okay, this is the difference which I discover from the another version BQ25504. 

    I am now thinking more or less the same idea. I will try that.

    Thanks a lot.

  • 0 in reply to Ardeno Wibisatria
    TI__Guru**** 166176 points

    If you have the board space, the bq25504 + TPS62736 is essentially the same as the bq25570.

  • 0 in reply to Jeff F
    Prodigy 40 points

    Yes, now I have to choose:

    BQ25504 + TPS62736(buck)  or  BQ25570 + FET(regulate Vcutt-off)

    Thanks for the solution. I will look into it.

  • 0 in reply to Jeff F
    Prodigy 40 points

    Jeff F said:

    You could add an external PFET in series with the VBAT pin that is controlled by an inverted (requiring a NFET) VBAT_OK signal where VBAT_OK =3.0V.

    Hi Jeff, there is one thing that I would like to ask you.

    When using BQ25570+FET, I can set my Lipo cutoff voltage at 3V to the system load. But the harvester undervoltage is still at 1.95V. Then if the harvester enters the cold-start again and my Lipo is empty (at cut-off 3V). It will somehow drains my Lipo to V_storage (in order to exit cold-start and enable main booster). In this case, my Lipo can decrease below 3V and will certainly be damaged.

    1. So, what do you think about that? Anything that I miss?

    2. Can you give me hint about using BQ25570+FET compared with BQ25504+buck? << Sorry I haven't read the datasheet too deep

  • 0 in reply to Ardeno Wibisatria
    TI__Guru**** 166176 points

    Regarding 1, the PFET will not turn on and the body diode will prevent current flow from the battery to VSTOR.  The PFET source would be connected to the battery and the drain connected to VSTOR.  The gate is connected to the drain of NFET with source at GND and gate at VBAT_OK.  There will be a pull up resistor from battery to gate.  Note that VBAT_OK monitors VSTOR not VBAT pin.  

    Regarding 2, the bq25504 has user selectable VBAT_UV using external resistors.  The TPS62736 is a 50mA version of the 100mA buck converter that is in the bq25570.  The charger in the bq25570 is the next gen of the bq25504.  The bq25504 does not have /EN (i.e. ship mode) and you will higher quiescent current  with the two chip solution versus the bq25570+external FET.