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BQ25858-Q1: Automotive BMS

frank zhang
Prodigy 20 points
Part Number: BQ25858-Q1

Tool/software:

Technical Inquiries Regarding BQ25858-Q1 During Schematic and BOM Cost Evaluation​​

We are currently in the schematic and BOM cost evaluation phase for a project using the BQ25858. We have several technical questions and would appreciate your advice:

  1. 1.If input current detection is not implemented, do the ​​ACN and ACP pins​​ still need to be connected? Or can they be left floating?
  2. 2.Do the ​​ACN, ACP, and VAC​​ pins require ​​reverse polarity protection​​?
  3. 3.If only ​​buck-boost operation​​ is needed, but ​​bypass mode​​ is also required, can the ​​Q3 and Q4 switches​​ still be omitted?
  4. 4.In ​​Constant Current (CC) mode​​, are the ​​charging current value​​ and the ​​maximum charging voltage​​ set by hardware design, or are they both configured via the ​​I2C​​ interface?
  5. 5.When the BQ25828 is not actively operating, does it have a ​​low-power mode​​ or ​​sleep mode​​? What is the ​​quiescent current​​ (in mA)? Which pin controls this mode?

     

  • 0
    TI__Expert 6545 points

    Hi Frank,

    Thanks for working with this device.

    1. I would short these pins together and tie them to VAC.
    2. Yes, the BQ25858-Q1 does not have the ability to handle reverse polarity. The minimum voltage value is -0.3V.
    3. Q3 and Q4 are required for buck-boost. Did you mean buck only?
    4. The charging current and maximum charging voltage, or the CC and CV values of this device are configured via I2C. You can set a hardware limit of the input current and output current as a failsafe, however. The device will select the lower value of the two current settings.
    5. There is no sleep mode, but you can set EN_HIZ via I2C to turn off the REGN and stop switching. The datasheet lists the typical quiescent current as 0.75 mA and the maximum value as 1 mA.

    Let me know if there are any further questions.

    Best regards,
    Michael Bradbourne

  • 0 in reply to Michael Bradbourne
    Prodigy 20 points

    schematic15b.pdf

    Hi thanks for information.could you please help check the SCH applicaiotion for 12V/20A, only bulk mode.

    another question is the DRV_sup can use internal Voltage, just link directly connect to REGN pin?

    thanks.

    best regards.

    frank 

  • 0 in reply to frank zhang
    TI__Expert 6545 points

    Hi Frank,

    Thanks for working with this. I reviewed the schematic. Here are the notes:

    • The Coss of the MOSFET is too high. The BQ25856 datasheet has a section on power MOSFET selection that recommends the total switch node capacitance is less than 160/Vin (in nF). This is left out of the BQ25858 datasheet for some reason, but the information still applies. The MOSFET Coss here is 5.7 nF.
    • It would be helpful to place some ceramic (or other low ESR) capacitors close to the switching network on the T30 node
    frank zhang said:
    another question is the DRV_sup can use internal Voltage, just link directly connect to REGN pin?

    This is correct. DRV_SUP can be from an external power supply. We also offer the REGN supply for DRV_SUP. When using REGN for the gate drive power, connect REGN directly to the DRV_SUP pin.

    Best regards,
    Michael

  • 0 in reply to Michael Bradbourne
    Prodigy 20 points

    Hi, Michael

    thanks for feedback, for the typical quiescent current, if we add a mosfet on VAC pin to control VAC on/off,  could this reduce the quiescent current?
    when VAC off, how about the chip typical quiescent current?

    Best regards

    Frank zhang

  • 0 in reply to frank zhang
    TI__Expert 6545 points

    Hi Frank,

    frank zhang said:
    if we add a mosfet on VAC pin to control VAC on/off,  could this reduce the quiescent current?

    I think that the quiescent current would then depend on the MOSFET selection.

    frank zhang said:
    when VAC off, how about the chip typical quiescent current?

    Do you mean that VOUT is still powered but VAC is not?

    Best regards,
    Michael Bradbourne

  • 0 in reply to Michael Bradbourne
    Prodigy 20 points

    hi Michael 

    Do you mean that VOUT is still powered but VAC is not?

    yes,  Vout is still power,but VAC is not.

  • 0 in reply to frank zhang
    TI__Expert 6545 points

    Hi Frank,

    The quiescent currents are the same:

    IQ_REV will the quiescent current when Vout is powered.

    Best regards,
    Michael