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BQ25570: Confirmations with BQ75570 VIN_DC connecting to power supply instead of solar panel

Jonathan Geronga
Expert 7930 points
Part Number: BQ25570

Hi Team,

Our customer first inquiry is if the BQ75570 VIN_DC input be connected to a power supply (5V) instead of solar panel. They like to bench test our devices indoors to ensure they're working yet keep their battery topped off, so replacing the solar panel with a 5V power source would enable us to do this.
We answer the customer using this thread below:
https://e2e.ti.com/support/power-management/f/196/t/787040

After sharing the thread above, the customer would like to confirm if connecting any typical 4.2V LiPo charger ~250mA and that will do the trick?

Another question is what are the consequences of connecting a 6V panel to the BQ25570?
They don't have anything connected to VOUT so not using the buck converter and have 4.2V LiPo connected to VBAT with its own over-voltage protection circuitry.
Here's the customer schematic for your reference:
EclipseSolarEV3.pdf

Please let me know if you have any questions with the customer.

Thanks,

Jonathan

what are the consequences of connecting a 6V panel to the BQ25570?

  • 0
    TI__Guru**** 166176 points

    Hi Jonathan,

    The BQ25570 is a boost only charger.  A boost converter always has a path through its output FET's body diode from the input to the output.  If VIN_DC is ever greater than the VBAT_OV setpoint, the charger stops charging and turns on a pull down resistor on VIN_DC in an attempt to prevent the input voltage from appearing at the battery.  In the case you state above, 5V-0.6V diode diode = 4.4V or 6V-0.6V diode drop = 5.4V will be applied to the battery.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Expert 7930 points

    Hi Jeff,

    Thank you for your support in this thread.
    Here's the feedback from the customer upon sharing this information:

    On page 15 on the datasheet in section 7.3.3 the gray Caution box it states that VIN_DC is pulled to ground with this pull down resistor. If that is the case then the solar panel will be brought to ground and present zero voltage at VIN_DC, so no voltage would be applied to the battery. Can you verify this is the case as this battery protection seems to be the proper behavior described in section 7.3.3.

    Please let me know your comment.

    Regards,

    Jonathan

  • 0 in reply to Jonathan Geronga
    TI__Guru**** 166176 points

    HI Jonathan,

    Yes.  If the MPP voltage at VIN_DC (i.e. voltage sensed during switching) is ever greater than VBAT_OV setting per the resistors, the charger turns on a pull down FET at VIN_DC in an attempt to prevent the charger from over-voltaging the storage element.  The FET has a resistance in 1-10 ohm range so will not pull down a  source with low output impedance like a USB port.

    Regards,

    Jeff