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BQ25798: IBUS Limit

Michael Balourdas
Expert 5335 points
Part Number: BQ25798

Hello,

In the Recommended Operating Conditions, IBUS MAX is 3.3A. The IBUS overcurrent rating is listed at 6.5A. What issues might arise if IBUS is used at say 5A?

Thanks,

Michael

  • 0
    TI__Guru**** 155606 points

    Hi Michael,

    If the host does not disable the IBUS OCP protection using EN_IBUS_OCP in REG0x13[0] and the average IBUS exceeds the threshold for more the deglitch time, the charger auto enters HiZ mode (i.e. stops switching). More details in the datasheet section:  8.3.13.1.8 Input Over-current Protection (IBUS_OCP). 

    While it is possible to disable the charger's input current limit by setting EN_IBUS_OCP=0, doing so is only recommended for brief periods to handle intermittent SYS load transients.  There is a cycle-by-cycle current limit feature (9-10A) that turns off the FETS to prevent damage.  But practically, the package and FETs are not rated to run at that much power so the IC die quickly reaches its thermal regulation temp and clamps the duty cycle and therefore output power.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Expert 5335 points

    Hi Jeff,

    Thanks for the info. My question is more centered around the 3.3A recommended maximum for IBUS. Can the device be operate at 5A without any issues? Since that's still a ways below the OCP limit.

    Thanks,

    Michael

  • 0 in reply to Michael Balourdas
    TI__Guru**** 155606 points

    Michael,

    The maximum register setting for the input current limit and therefore maximum regulated input current is 3.3A.  The 3.3A value is related to the max output current from USB ports and some wall warts.  If you disable the input current limit, the device will function until average input current hits IBUS_OCP = 6.5A and then it will enter HiZ (stop switching).  If only operating at average input current 5A with input current limit disabled, the package will likely get too hot and the device will enter thermal regulation which clamps the converter duty cycle and reduces the operating current below 5A.  If you are going to attempt it, I recommend setting fsw=750kHz and using a 2.2uH inductor.  

    Regards,

    Jeff