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BQ25505: External Power Mux Topology and System Isolation

Dale Maxwell
Prodigy 20 points
Part Number: BQ25505
Other Parts Discussed in Thread: TLV840

Hi, I am using the BQ25505 as a solar harvester along with a single secondary battery (VB_SEC) as my main power supply, and I have no primary battery attached (VB_PRI). I am currently having issues exiting the cold start mode upon connecting VB_SEC and the solar harvester. I have a 2 to 1 power mux with V_STOR and VB_SEC as the inputs and my system load as the output. I need this configuration to disable BQ25505 from charging VB_SEC at very low temperatures. My current configuration does not isolate the system load from either power source and I believe this is why it will not exit cold start. Are there any TI applications that execute a similar topology? 

  • 0
    TI__Guru**** 166306 points

    Hi Dale,

    The charger should exit cold start whenever VSTOR>VSTOR_CHGEN~-1.8V. Are you trying connect a battery at VBAT_SEC at this time?  The charger does not support battery hot swap/attach.  A battery >2.0V must be attached when VSTOR and VBAT_SEC are < 100mV in order for the PFET between VSTOR and VBAT_SEC to turn on. 

    Once the PFET turns on at battery attach or after VBAT_SEC rises above VBAT_UV=2.0V max, it stays on, tying VSTOR to VBAT, until VBAT_SEC=VSTOR < VBAT_UV =1.9Vmin.  Therefore, the 2 to 1 power mux may not be needed or you might be able to use the integrated mux that is controlled by the VBAT_OK signal. 

    Regards,
    Jeff 

  • 0 in reply to Jeff F
    Prodigy 20 points

    Hi Jeff, 

    I am first connecting VBAT_SEC = 3.0V. Then I am attaching the the solar cell to VIN. On the BQ25505EVM board VSTOR >VSTOR_CHGEN but on my PCB implementation VSTOR<VSTOR_CHGEN and I am assuming because a load is attached. Am I attaching these in the correct order? Do I also need VBAT_PRI to properly initialize? Assuming I am connecting these correctly, are there any examples or applications for removing the system load other then what is shown in the BQ25505 datasheet (using VB_SEC_ON_not to drive an external PFET)?  

  • 0 in reply to Dale Maxwell
    TI__Guru**** 166306 points

    Hi Dale,

    Yes, connecting the battery to VBAT_SEC before powering up is correct. 

    If the load is attached while the charger is in cold start, the charger may not exit cold start.  You can use an external PFET from VSTOR to system load to isolate the load.  To drive the PFET gate, you would either need to use /VB_SEC_ON or some other comparator with reference to drive the gate low sometime after VSTOR is higher than VSTOR_CHGEN.  TLV840 is a nano-power SVS that could work.  You would want the active high version in order to drive the PFET directly.  An active low SVS would require an NFET for inversion.

    Regards,
    Jeff