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BQ25792: VSYS cylcing between 12.5V, 9.5V, and 9.1V when no host or battery present.

Lane Brooks
Prodigy 70 points
Part Number: BQ25792
Other Parts Discussed in Thread: BQ25798

I have the BQ25792 in a system where the host processor is on a daughter board. I have removed the daughter board so that the host is not present. I have removed the battery. When I plug in USB power (from an Unknown Adapter), the SYS output then enters a cycle between 12.5V, 9.5V, and 9.1V every few seconds. Current draw from the USB supply cycles between 120mA, 80mA, and 0mA. The device does work as expected when the battery is present and when the host and battery are present. If I plug in the battery and the USB and boot the host and then unplug the battery, it behaves correctly and puts a stable 12.5V on Vsys. It's only when plugging in USB power when no battery is present that it gets stuck in this cycle. When the host is present, it does the same thing but cannot finish booting because the 9.1V triggers it's POR. This is setup for 3s mode.

Here is a photo from the scope:

The VREGN is stable at 5V. The TS voltage is stable and connected to VREGN.

Here is a picture of the schematic:

  • 0
    TI__Guru**** 152956 points

    Hi Lane,

    With no battery connected and charge enabled (/CE pin or CE bit) and no TS fault due to missing thermistor, the charger turns on charge current so fast that it overvoltages BAT and then SYS.  The converter then briefly turns off.  If you can disable charge when the battery is removed, this will not happen.  If not, you will need to add at least 200uF to BAT and lower the charge current <0.5A (possibly by lowering the input current limit clamp with the ILIM_HIZ resistors).  The exact amount of capacitance is a function of input voltage, battery regulation voltage and input current limit.

    The BQ25798 later version of this device has this issue fixed.  Unfortunately, the BQ25798 is still not available to order.

    Regards,

    Jeff