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BQ25895: Load Sharing

Donald Bosley
Prodigy 180 points
Part Number: BQ25895

I have incorporated BQ25895 into a design where a high current load is driven from the battery - to drive the load V+ from the battery is connected to that load, while the main system load is powered through VSYS into an LDO with only ground common between the two power systems - is there any chance the high current return directly connected to the battery will effect the chargers internal current limits? I realize thermal control on the PCB is an issue/concern/consideration, and we've accounted for dissipation of heat, but I'm wondering if there is anything else I should be cautious of and if this "load sharing" for lack of a better term, is going to cause any issues (like if you use two transformers and don't couple both of them to the same ground...)

  • 0
    TI__Guru**** 166246 points
    Donald,

    If I understand correctly, you have a load, separate from the system load, connected directly to the battery. While fast charging, the BAT pin acts as a constant current source, outputting the charge current per the I2C register. But once the charger reaches constant voltage regulation (i.e. V(BAT) = VBATREG) and tries to terminate, the load on BAT may prevent accurate charge termination. For example, if the termination current is set lower than the load on BAT and the battery is fully charged, the charger will think that load current is battery current, preventing the charge current from tapering down to the termination current. If the charger never terminates, the safety timer will eventually expire.
  • 0 in reply to Jeff F
    Prodigy 180 points
    The high current load will be disconnected from the battery via mosfet during any charge cycle - I'm more concerned with current limiting during runtime.
  • 0 in reply to Donald Bosley
    TI__Guru**** 166246 points
    Donald,

    If charging is disabled or the BATFET is turned off, no current flows from SYS to BAT from the BATFET.
  • 0 in reply to Jeff F
    Prodigy 180 points
    "If I understand correctly, you have a load, separate from the system load, connected directly to the battery." - Correct.

    "While fast charging, the BAT pin acts as a constant current source, outputting the charge current per the I2C register. But once the charger reaches constant voltage regulation (i.e. V(BAT) = VBATREG) and tries to terminate, the load on BAT may prevent accurate charge termination." - The external MOSFET which switches the higher amperage second load will never have its gate driven during a charge cycle, so there are megs and megs of resistance between the second load and the battery - I can't see the charger not being able to terminate a charge cycle properly.

    "If charging is disabled or the BATFET is turned off, no current flows from SYS to BAT from the BATFET." - OK...not sure what this is telling me or how this is relevant to the original question.

    "...if this "load sharing" for lack of a better term, is going to cause any issues (like if you use two transformers and don't couple both of them to the same ground...)" <--- Is this potentially a problem during runtime, not charging, with BATFET and external MOSFET "on" and the battery running two loads.
  • 0 in reply to Donald Bosley
    TI__Guru**** 166246 points
    Donald,

    When charging is disabled, no current flows from SYS to BAT so the buck converter's input current limit will not be affected by the load on BAT. The entire input current will be available to the SYS load.