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BQ25703A: Using BQ25703A as pure solar charger

Alex Dukhovny94
Prodigy 70 points
Part Number: BQ25703A

I'd like to use BQ25703A as always connected solar charger for 4S Li-ION battery. I am going to connect the load to the battery output, omitting VSYS branch. I am going to take care of the battery under-voltage issue in the pack's BMS, so I don't need linear regulation provided by P-MOSFET.

Can I remove the BATDRV MOSFET, as follows :

Thanks.

  • 0
    TI__Guru 56650 points

    Even the charger doesn't have the system load, bq25703A charger cannot save the system capacitors to keep output stable.

    Even the battery pack can take care of the battery under voltage condition, bq25703A cannot save the BATFET to avoid output OVP at charge disable condition (battery fully charged or input source plug-in without charge enable command).

  • 0 in reply to Wang5577
    Prodigy 70 points

    Wang5577 said:

    Even the charger doesn't have the system load, bq25703A charger cannot save the system capacitors to keep output stable.

    So, If I am moving the output caps to the BAT branch instead of the VSYS branch I am solving that issue ?

    Wang5577 said:

    Even the battery pack can take care of the battery under voltage condition, bq25703A cannot save the BATFET to avoid output OVP at charge disable condition (battery fully charged or input source plug-in without charge enable command).

    Can you elaborate on that, because I can't see the direct dependence of BATFET presence and OVP at charge disable condition

    Thanks

  • 0 in reply to Alex Dukhovny94
    TI__Guru 56650 points

    If charge is disable, converter regulates the VSYS pin voltage about 160mV higher than VSRN. If without BATFET, the VSYS is almost short to VSRN, VSYS cannot meet VSRN+160mV set point. So, converter will keep pushing VSYS high until trigger OVP or other protection.

    Since the BATFET is needed, bq25703A needs enough VSYS capacitors to keep converter stable.

  • 0 in reply to Wang5577
    Prodigy 70 points
    In case I'd like to connect the load directly to the battery rail, and not to use the VSYS output ( I'd like to draw more than 6.35A from the battery ) I just can use a small P-MOSFET to regulate the VSYS to the correct level . Am I correct ?
  • 0 in reply to Alex Dukhovny94
    TI__Guru 56650 points
    The P-MOSFET should be able to take care the maximum charging current.
    Make sure the system load current is much smaller than the pre-charge current when the battery voltage is lower than Vsys_min.