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bq40z60 Power Ground (PGND) and Analog Ground (GND)

electronicjohn
Intellectual 330 points
Other Parts Discussed in Thread: BQ34Z100-G1

The document SLUSAW3C section 12.1.1 #8 tells us we should connect power ground to device ground (analog ground) with a zero-ohm resistor or thermal pad at one point. However, for the bq40z60EVM board it seems like the PGND and GND are only connected through the sense resistor R43. Is this what I should do on my own design? If I connect PGND to GND then wouldn't R43 stop having current flow through it?

  • 0
    TI__Guru 73830 points
    PGND is equivalent to Pack- on gauge only packs. We originally had GND, PGND and ChassisGnd (charger), but consolidated PGND and ChassisGnd into one. Originally PGND was the analog GND for the gauge/charger sections and it was single point connected to GND at the cells. The error slipped by in the datasheet and will need to be corrected. GND and PGND are not connected at the cells.
  • 0 in reply to ThomasCosby
    Intellectual 330 points
    Thanks Thomas, that is an excellent explanation.
  • 0 in reply to ThomasCosby
    Intellectual 330 points
    Hi Thomas,

    Another question regarding PGND/GND. The 3.3V regulator on the bq40z60-EVM is referenced to Analog ground (GND), and not PGND. Can you explain the reason for this? I was hoping to use this regulator to power a microcontroller in addition to the 4 LEDs, but all of the SMBus signals are connected to PGND. I worry there will be potential problems if the microcontroller is referenced to a different ground than the regulator, or if the SMBus signals are referenced to a different ground than the microcontroller.
  • 0 in reply to electronicjohn
    TI__Guru 73830 points

    John,

    The sole purpose of the 3.3V regulator on the EVM is to support the LEDs. The problem with referencing it to PGND is that the regulator current will return to the battery through the sense resistor and it will be logged as a discharge current. You can connect it from VSYS to PGND, if you only want it to be enabled when the DSG FET is on.

    Tom

  • 0 in reply to ThomasCosby
    Intellectual 360 points
    Hi Tom,
    Why are the VFB resistors connected to GND? ? Shouldn't they be connected to PGND?
  • 0 in reply to Pietro Di Buono 88
    TI__Guru 73830 points
    Pietro,
    The current and voltage measurement circuits on the device are referenced to VSS, so the current sense filter and feedback resistors are also connected to this node.
    Tom
  • 0 in reply to ThomasCosby
    Prodigy 60 points
    Hi Thomas,

    I have a similar question. I am using the BQ32Z100-G1 to monitor a 2s/8.4V LiIon battery pack. My application is powerd from this battery. As indicated in the datasheet I connected the GNDs of the BQ34Z100-G1 directly to batteries ground. But the Microprocessor uses the ground level which has the sense resistor in series. In my application load currents of up to 5A are possible. Can the ground shift between the MCU and the BQ34Z100-G1 lead to failing data transmission using the I2C bus?

    BR

    Michael
  • 0 in reply to Michael Moitzi
    TI__Guru 73830 points
    Ground bounce can cause communications problems, but there should be enough Vil margin to allow reliable operation. Assuming that you are using a 10 mohm sense resistor, the voltage drop for a 5A current will only be 50mV.