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BQ25898: Connecting SYS Output of two BQ25898 devices together

Rejeesh Raj
Intellectual 330 points
Part Number: BQ25898
Other Parts Discussed in Thread: TPS22976

Hi
In our design I am using two independent and identical, charger (Bq25898) and battery (18650 - Li-Ion) configuration.
The system output (SYS) from each charger is tied together and connected to load.
MSP430 configures the charger devices independently. I use this configuration, to isolate the battery in case of failur.
I am planning to do isolation this by disabling the charger using CE# pin / EN_TERM (REG07.7) and by disabling BATFET.
In evm when I pulled up CE input the charger functionality is not halted, but the software configuration worked well. Please clarify?
I have following queries on the design too.
1. Is it recommended to tie the system outputs of two identical chargers together?
2. In case of failure in battery will it draw power from SYS through BATFET if is isolated by software by BATFET_DIS (REG09.5).
Thanks
Rejeesh S Raj

  • 0
    TI__Guru**** 165786 points

    Hi Rejeesh,

    Regarding 1, it is not recommended to tie the SYS outputs of multiple chargers in parallel because chargers control charge current by modulating the duty cycle of the buck converter.  If both are powered and both have /CE low, then the charger with lower battery voltage has no way to control charge current into its battery.  If only one is powered and the other is not, then the unpowered charger is in HiZ mode and has the battery turned on fully, allowing current to flow out of but also into the battery.   The 898 has the feature to completely turn off the BATFET between BAT and SYS.  So, in theory you could turn one charger on and the other charger's bat FET off but the timing is difficult so it is not recommended.

    Regarding 2, as mentioned above, you can turn off the BATFET of each charger, thereby isolating the SYS pin from the battery.  The buck converter still functions and provides power to SYS, assuming that charger is powered.

  • 0 in reply to Jeff F
    Intellectual 330 points

    Hi Jeff,

    Thanks for the reply.

    From your mail I conclude that if I tied the SYS outputs together, battery having lower terminal voltage then it will be difficult to charge right?

    From my testing I observed that the SYS output also depend on Battery terminal voltage. If I tied SYS outputs together then the battery-charger pair which is having lower terminal voltage may sink SYS output current from the battery-charger pair having higher SYS output right?

    So I expect that in the above situation the total current will be reduced. So the battery charging will be delayed. Please comment me if I am wrong    

    Also please comment me on using the Dual channel load switches like TPS22976 for paralleling SYS outputs of two chargers 

    Thanks

    Rejeesh S Raj

  • 0 in reply to Rejeesh Raj
    TI__Guru**** 165786 points
    Rajeesh,

    The higher battery voltage charger will have part of its charge current go to the lower voltage battery. The lower voltage battery will then have its charger's current plus this extra, unregulated current.

    The TPS22976 used with VINx=SYSx and VOUT1=VOUT2 still has a body diode that would allow current from higher battery voltage flowing to lower battery voltage through the body diode even if the FET is off. If that IC can be used in reverse, i.e. SYSx=VOUTx and VIN1=VIN2=system, then it might work. We will need to confirm with the load switch team whether this is possible through the forum.
  • 0 in reply to Jeff F
    TI__Guru**** 165786 points
    Rajeesh,

    On second thought, I don't think using the TPS22976 by itself in reverse will help because it does not have current monitoring and so will not know when to turn off the FET if current is flowing from high SYS to low SYS. We would have to add two comparators to monitor the voltage drop from VOUTx to VINx and then turn on the FET when the voltage reverses, indicating reverse current flow. Further, we would probably want to add external Schottky diodes, across the internal body diodes, to reduce the min SYS turn on voltage from 2.5V+0.6V to 2.5V+0.3V.