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BQ25708: SMBUS 1.1 PEC compliant?

k1mgy
Expert 1680 points
Part Number: BQ25708
Other Parts Discussed in Thread: BQ24725A, , BQ24765

The BQ25708 data sheet is a bit weak in describing specific compliance with SMBUS 1.1  In our case, we've already invested time into a design using the BQ24725A to charge a smart battery pack, only to learn that it (1) loses all register settings when powered down and (2) crashes when the smart battery pack includes a packet error correction byte in its alert broadcasts.

Will the BQ25708 handle SMBUS PEC?  If not, can TI recommend a comparable charge controller that will give us the nice (seamless) buck/boost, operation without battery connected and the ability to respond to the smart battery pack?

Our pack is the excellent RRC2054.  Unfortunately we (and the manufacturer) cannot turn off the PEC as it's too deep within the protection device.

  • 0
    TI__Guru 56650 points

    Both bq25708 and bq24725A don't support SMBus PEC. Seamless buck/boost transition isn't related to SMBus communication.

    So far, TI doesn't have charger controller to support SMBus PEC. Please let me know if you find any SMbus charger IC on the market can support PEC.

  • 0 in reply to Wang5577
    Expert 1680 points
    I am under the impression that the LTC4100 does.

    It's fine if the controller doesn't support PEC. But that byte shouldn't send the chip into the tank. The only way around this is to shut off the PEC, but we can't with a pre-packaged battery pack. Quite a conundrum.
  • 0 in reply to k1mgy
    TI__Guru 56650 points
    I don't find PEC in LTC4100 (cds.linear.com/.../4100fc.pdf )
    I get this PEC feedback. May investigate it later for future new charger development.
  • 0 in reply to Wang5577
    Expert 1680 points
    Another Engineer tells me that the BQ24765 seems to work with a smart battery and PEC. However, the recommended implementation includes a fet to switch on the battery charge voltage, and this is controlled by the host processor (not the BQ24765 in response to the smart battery), so I am less than convinced.

    I've proposed putting a small microcontroller between the battery and the smbus so the PEC is stripped out. It's a hack, but might solve our particular issue. Thoughts on that?
  • 0 in reply to k1mgy
    TI__Guru 56650 points

    Yes. I agree "putting a small microcontroller between the battery and the smbus so the PEC is stripped out. It's a hack, but might solve this particular issue".
    When write "How to Operate Fuel Gauge and Charger Without Host Controller" apps (www.ti.com/.../slua608.pdf), gauge team asks to turn off the PEC. Even "BQ24765 seems to work with a smart battery and PEC", I cannot guarantee.

  • 0 in reply to Wang5577
    Expert 1680 points
    Thank you.
    Please put PEC compatibility in planning. I think it will be very helpful.