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TPS65950: How to disable automatic USB charging

Ryan Milne
Prodigy 70 points
Other Parts Discussed in Thread: TPS65950, OMAP3530

I have a design using an OMAP3530 and TPS65950 development board.  The system is turned on and off by supplying VBAT to the TPS from an external regulator (the backup battery is not connected).  I have a problem where even though I am not supplying VBAT from my system, the VBAT power rail is partially powered up to 1.2V when VBUS power is present on the OTG port.  It seems like the TPS chip is trying to charge up the VBAT power rail from VBUS.  Currently, the BCI is configured to support charging.  If I change the circuit to match Figure 8-3 (Typical Application Schematic BCI Not Used) in the TPS65950 Data Manual, will it stop powering up VBAT from VBUS?

Thanks,

Ryan

  • 0
    TI__Genius 15100 points

    Hi,

    Based on your explanation it looks like you have the BCI circuit for charging from USB. That may be the case and hence you see VBAT at 1.2V, I cannot certainly be sure unless I see your schematic.

    If you do not use BCI as you have proposed then VBAT will not have any source from VBUS.

    As a test you can disconnect the charging path (remove the FETs) and check if you get 0V on VBAT.

     

    Regards,

    Gandhar.

     

  • 0 in reply to Gandhar Dighe
    Prodigy 70 points

    I did try removing the FETs but I still see 1.2V on VBAT.  My circuit is identical to circuit (A) in Figure 8-1 of the TPS65950 data manual.  In addition I've also removed Rs which seems like should have disconnected the charging circuit, but I still see the voltage.  Is there a connection between VBUS and VBAT internal to the TPS65950?

  • 0 in reply to Ryan Milne
    TI__Genius 15100 points

    Hi,

    Can you help with the following questiuons, we would need to know more about the board/test condition on your platform.

    1.Is the battery pack connected or is the vbat node open?
    2.If the battery pack is connected what is the voltage of the battery pack.Is it totally discharged?
    3.If the battery pack is totally discharged then yes small precharge will charge the battery to 1.2V but then the voltage shouldn't stop at 1.2V it should keep on increasing upto 2.0v due to slow precharge. Is VBAT rising slowly above 1.2V?

    Regards,
    Gandhar.

  • 0 in reply to Gandhar Dighe
    Prodigy 70 points

    Our system does not use a battery pack.  The VBAT rail is supplied by a fixed 3.7V regulator that is disconnected through a FET switch in the off state.

    1.  The VBAT node is open.

    2.  N/A

    3.  VBAT does not increase above 1.2V.  I believe the PMIC is just charging up the capacitors on the VBAT rail.

    Thanks,

    Ryan

  • 0 in reply to Ryan Milne
    Prodigy 150 points

    Hi,

    This a feature in the system in which if VBAT node is open or shorted or completely discharged then the node is charged upto 1.2V.After that if VBAT presence is checked and if battery pack is not present then the charging is completely disabled.So the behaviour your are seeing is normal.

    Thanks,

    Rohit

  • 0 in reply to Rohit Bhan
    Prodigy 70 points

    If charging is completely disabled after it doesn't detect a battery pack, why does the 1.2V remain on the VBAT node?  If the PMIC stops charging I would expect the voltage to return to 0V, unless it is continually trying to detect a battery.

  • 0 in reply to Ryan Milne
    Prodigy 150 points

    As the VBAT node reaches 1.2V PMIC will look for battery presence and if the battery pack is not present then any further charging of VBAT node is disabled however the VBAT node is maintained to 1.2V by a providing the node with very small current from voltage domain dependent on VBUS presence.The system makes sure that VBAT node doesn't increase beyond this voltage without battery pack being present.

    Thanks,

    Rohit