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BQ24600: Battery charging and power gate (using LP73100RPW) in a dual battery design schematic review request

hamasaki1818
Intellectual 370 points
Part Number: BQ24600
Other Parts Discussed in Thread: BQ24610,

bq_lp_charger.pdf

I'm making a thing that's powered by 2 2S batteries and user/firmware has ability to choose A, B, or both batteries as power source.

Originally it used some expensive and no longer available LT parts, so I'm redoing it with TI. The schematics have other stuff (mcu etc) so I trimmed just the charger + power gate bits.

DC_IN is the power supply (15V DC nominal), BATT and SYS_VCC is the main system load power rail (goes to 3.3V DC/DC and so on, not relevant to the charger bits). batteries and NTCs are connected via CN1. To auto-select a battery before MCU boots the circuits using Q14/Q16 are made which control OVLO pins on LP73100.

Charger circuit is pretty much copy of reference, the batteries are 2S 16850 in series with protection circuit and NTC inside the pack.

Does the overall schematic/design look sane? Anything to do differently? Would this actually work to charge 2 batts at same time with DC_IN going to BATT?

  • 0
    TI__Mastermind 24075 points

    Hi Hamasaki,

       Since you have a SYS rail have you considered using the BQ24610 instead? It is the same family and has integrated BATDRV to drive the FET between BAT and SYS and your SYS rail will always be on as long as either adapter or battery are present.

  • 0 in reply to Kedar Manishankar
    Intellectual 370 points

    Hello, thanks for the suggestion. However BQ24600 is what we have a lot in stock so that's the part that should be used. The rest of the system I'm designing doesn't care if BATT/SYS_VCC is 8.4V or 15V (the 1st stepdown converter is rated at 28Vin max), so I thought a simple diode would be sufficient. I'm open to the idea of using a 3rd LP73100 (since they're so cheap) to switch DC_IN to BATT/SYS_VCC but only if its really necessary.

  • 0 in reply to hamasaki1818
    TI__Mastermind 24075 points

    Hi Hamasaki,

    It depends on whether you would like SYS to be powered when there is no battery present and adapter only, or in the case of a deeply discharged battery.

  • 0 in reply to Kedar Manishankar
    Intellectual 370 points

    Hiya,

    Powering from AC adapter only should work in current schematic, right? Adapter -> Diode -> BATT rail, powering everything up.

    Very discharged battery would not allow the system to turn on (due to the protection circuit in the battery itself).

    If user placed a discharged batterie(s) into the system, it would not turn on, but plugging in AC adapter will turn on (via the above diode) and BQ24600 will try to slow-charge the battery at low current in attempt to revive it. Right?

    In schematic it may seem like BATT and SYS_VCC are separate but they are actually  same rail, shorted by a slide power switch.

  • 0 in reply to hamasaki1818
    TI__Mastermind 24075 points

    Hi Hamasaki,

       Yes BQ24600 will charge deeply discharged battery, but from your schematic the SYS rail will not be powered. The benefit of BQ24610 is that SYS can always be on when battery is absent or battery is deeply discharged. 

    Regardless, if choosing to use BQ24600 or BQ24610, using 2 different charger IC to charge 2 different batteries from the same input source is fine. BQ24610 is more desirable in such a situation as it also has an added input current regulation feature which allows you to maximize the input current from the DC_IN.

    Apart from that everything else looks good.

  • 0 in reply to Kedar Manishankar
    Intellectual 370 points

    Hi,

    I am really at a loss here why you think that "SYS rail will not be powered".

    Schematic page 2, DC_IN is connected via diode to BATT which is in turn directly shorted to SYS_VCC by slide switch (not shown in schematic, mentioned in previous post).

    Are you familiar with how LP73100 works? Will it enter "shutdown" state (despite being turned on by OVLO low) when BATT side voltage is higher than battery voltage? ex. V_BAT_A is at 6V, V_BAT_B is at 0V (disconnected), BAT_A_OFF_CTL is low (enable battery A) and 15V DC adapter is plugged in to DC_IN rail. Charger handling battery A will start charging, but the system should be powered by DC_IN, at least in my understanding.

  • +1 in reply to hamasaki1818
    TI__Mastermind 24075 points

    Hi Hamasaki,

      I think the confusion is regarding that the control for the FET to disconnect ground return path for battery is called SYS and I was referring to if you had a SYSTEM rail present in your system. From schematic page 2 the test mentioning that SYS is connected should be fine to use as a ship mode to reduce leakage current (I did miss this portion of the text earlier). I am not familiar with the LP73100. For that part I would create a new post.