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TIDA-00792: Ground shift

KIM
Intellectual 550 points
Part Number: TIDA-00792
Other Parts Discussed in Thread: BQ78350-R1, BQ78350-R1A,

Hi,

I am looking at the TIDA 0792 reference design.

I notice the SMBus ground is reference to Pack-.

one query for my application: Rsense = 3mOhm and Discharge current I = 35A. The potential difference across Rsense is 0.105V. 

Pack- will be 0.105V with respect to B-. Please refer to attached figure.

Will this cause the ground shift problem?

The ground reference of IC on BMS is B- 0V. However the ground reference of load and SMBus is Pack- (which is 0.105V higher than the B-).

Or is it better to connect the ground of low voltage circuit on the load to the B- instead of PACK-? So that the BMS IC and low voltage circuit on load have same ground B-?

Please help to clarify if this will cause ground shift issue.

  • 0
    TI__Guru 54715 points

    Hi Kim,

    The current will modulate the communication lines reducing the noise margin and if large enough it can cross the threshold and corrupt the signal, so it is good to consider this.

    The load current is  typically higher in a system and will put the Load at a higher voltage than the BQ78350-R1.  The high level should be set by the pull up in the Load and should be above the VIH of the host.  The low level from the pull down by the BQ78350-R1 may appear lower than normal, be sure the host can tolerate the level.

    With the BQ78350-R1A used in the TIDA-00792, VIH is 6V maximum, check that the combination of the host pull up value and the ground shift does not exceed this level.  VIL is 0.8V maximum, be sure that the VOL of the host and the ground shift still meets this level. Using the PEC on the SMBus interface can reject communication errors from current surges.

    If the ground shift offset is too large you might use a smaller sense resistor for less offset which also reduces current resolution, or use an isolated communication interface which requires more parts and power.  For low current systems grounding might be done on the pack side of the sense resistor but is not recommended.  Note the small recommended range of the SRP and VC0 of the BQ769x0.  Short circuit currents in many batteries can be very high and the BQ769x0 SRP and SRN inputs have a higher abs max for positive transients than negative, so grounding is recommended on the battery side of the sense resistor.