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BQ25792: Potential state machine instability issues?

Kiran Kerai
Intellectual 380 points
Part Number: BQ25792
Other Parts Discussed in Thread: BQ25713, TPS25750

In our application, we have a 3 cell battery (12.6V) and a heavy load (7 amps) connected to the system. This load is switched on for about 8-10 seconds and then switched off and the resulting voltage across the battery steps down (droops) by about 1V. If we are going to charge the battery whilst powering the switching system load, I understand that VBUS will supply the system and battery will supplement where necessary. Will the drop in battery voltage affect stability of the internal state machine and force it to inadvertently switch from CV charge mode to CC under any charge scenario (i.e. when the battery is only just in taper charge phase). Will this cause any issues in our application?

  • 0
    Intellectual 380 points

    Just spotted that IBAT discharge is recommended < 6A continuous or <10A for 1 sec. Confirm that this effectively prevents from connecting our 7A load to SYS.

    If this is true, it appears we would only have option of connecting our load directly across the battery, this would then prevent the user from powering the load when the battery is depleted, even if VBUS is connected. The battery would need to recharge sufficiently. Would we be able to use the battery (once sufficiently charged) whilst charging given that we will be drawing 7 Amps for 8-10 seconds and reducing voltage across the battery by 1V? Or will any protection mechanism activate to disable charging while in use?

  • 0 in reply to Kiran Kerai
    TI__Guru*** 143910 points

    Hi Hiran,

    In battery only mode, IBAT discharge is not rated high enough for your load.  With VBUS attached, the load would be supplied by ISYS+IBAT.

    If battery is depleted and VBUS is applied, the charger's NVDC provides a minimum system voltage at SYS even with a depleted battery.

    We have not tested your application with a large pulsed load on BAT.  I see the potential for spikes that would trigger BATSHORT and BATOVP faults.  The charger should recover from those faults.

    I think BQ25713 with external, higher voltage rated FETs might be a better fit for your application.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 380 points

    If we connect our load directly to the battery, not to the SYS load, the battery charger can effectively be turned off whilst our device is in use.

    So the charger would only be active whilst charging the battery, during which time there will be no load.

    Would this work?

    We have very tight space constraints in our design. I will look into BQ25713 anyway, is there an EVM for integrated BQ25713 and TPS25750?

  • 0 in reply to Kiran Kerai
    TI__Guru*** 143910 points

    Hi Kiran,

    Your proposal should work.

    I am not aware of an EVM with BQ25713 and TPS25750.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Intellectual 380 points

    Great so I have removed all capacitors on SYS pin.

    What is the maximum current that can be drawn from Battery (BAT pin) whilst the battery is being charged?

  • 0 in reply to Kiran Kerai
    TI__Guru*** 143910 points

    Kiran,

    You cannot remove the SYS caps.  There are the output of the converter.  Without those caps, the charger will damage itself.

    Regards,

    Jeff