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BQ25710: BQ25710 I2C / SMBus interface when VBUS = 0v.

Tim Warren13
Prodigy 110 points
Part Number: BQ25710

Hi,

I am designing BQ25710 into a charging circuit which uses a 1S lithium smart battery (Level 2), and a USB C PD charging source. The system MCU must be able to communicate with the smart battery at all times when the system is powered, including when VBUS = 0v due to USB source disconnection (when system will be battery powered). The MCU, Battery and BQ25710 share the same SMBus. It is not clear to me what happens tothe SDA / SCL lines of the SMBus interface of BQ25710 when VBUS = 0v, and VSYS is approaching low battery state (<3v).

  1. Shall the BQ25710 ensure that the SMBus lines are not affected by the low supply voltage and remain high impedance?
  2. Is the maximum voltage allowed on the SMBus lines affected by VBUS or VSYS voltage?
  3. Does the BQ25710 remain functioning in low voltage situation? Ie communicable with over SMBus when VBUS = 0v and VSYS = 3v?
  4. The nominal 6v rail from REGN LDO shall be operating from VSYS rail, but it cannot regulate at 6v. Is REGN and/or VDDA required for operation other than during charging of battery when VBUS shall be present? What is minimum voltage requirement on VSYS when VBUS = 0v?

Thank you,

Tim Warren

  • 0
    TI__Mastermind 25800 points
    Hey Tim,

    For 1. The SMBus lines on the charger are open-drain. So unless the charger is being communicated with, the SDA/SCL lines will not be affected by the charger. The SDA/SCL lines are tri-stated and subsequently pull up to your system's main SMBus rail.

    The bigger question is where you derive the pull-up rail from?

    For 2. Please do not exceed the absolute maximum voltage specification on the datasheet for the SDA and SCL lines on the charger.

    For 3. The minimum voltage before the I2C is characterized as VBAT_UVLO in the datasheet Electrical Characteristics table.

    For 4. REGN is primarily used for the internal gate drivers to drive the external power stage for the converter.


    Regards,
    Joel H