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BQ24078: BQ24078

Shawn Zhou
Prodigy 90 points
Part Number: BQ24078

Tool/software:

Dear Sir,

I would need the help of your technical team for the following question concerning the BQ24078.

In the datasheet of the BQ204078 (page 4, table describing the pin functions) is reported that pins EN1 and EN2 are internally pulled down with ~285kΩ resistors. It is recommended to not leave EN1 or EN2 unconnected to ensure proper operations.

In my design those pins are connected to the MCU that drives them. However, I want to ensure that if the MCU is powered OFF and the USB is connected, the charger input current is limited to 100mA (EN2=EN1=0) until the MCU is capable of driving those pins. Can I rely on those internal pull-down to ensure EN2=EN1=0 until the MCU starts driving them or should I place external pull-down for that ?

Best Regards

Shawn

  • +1
    TI__Expert 8515 points

    Hi Shawn,

    Whether you can rely on the 285kΩ internal pulldown resistors on the EN1 and EN2 pins depends mainly on the leakage current flowing into these pins from the MCU GPIOs when the MCU is off.

    To guarantee the ENx pins stay at logic LOW, they need to be held below VIL(max) = 0.4V. With an internal pulldown of about 285kΩ, this means the maximum allowable leakage current into an ENx pin is about 1.4uA.

    If the leakage current from your MCU's GPIOs is in the nanoamp range, then it should be fine to rely on the internal pulldowns of the BQ24078. Otherwise, if the leakage current is on the order of microamps when the MCU is unpowered, then I'd recommend adding external pulldown resistors to ensure the pins don't float high enough to be interpreted as logic HIGH.

    I'd suggest weighing the BOM savings against the leakage characteristics of your MCU.

    Best regards,

    Alec