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BQ34Z100-G1: Tying two different grounds on the Bq34z100 compared to Bq28z610 and Bq78z100

Graham Cameron
Prodigy 30 points
Part Number: BQ34Z100-G1
Other Parts Discussed in Thread: BQ78Z100, BQ34Z100, BQ28Z610,

Short Description: Tying two different grounds on the Bq34z100 compared to Bq28z610 and Bq78z100  (See TI case number CS0173441)

I have a question about TI impedance-tracker charger ICs. This discussion centers around bq34z100-G1 Section 10.2.1 and the following quote from the datasheet: "The gas gauge requires a low-current ground system separate from the high-current PACK(-) path.  ESD ground is defined along the high-current path from the Pack(-) terminal to low-side protector FETs (if present) or the sense resistor.  It is important that the low-current ground systems only connect to the PACK(-) path at the sense resistor Kelvin pick-off point." 

A colleague of mine reads the data sheet for bq34z100-G1 Sec. 10.2.1 and is troubled. He contends that connecting ESD ground, GND, to Pack- and analog ground, AGND, to Bat- makes sense to him. He states this is the way GND and AGND are connected with two other fuel -gauge impedance-tracking TI parts: Bq28z610 Section 9.2 Fig 21,and Bq78z100 Section 9.2 Fig. 22.

I have connected GND and AGND as shown in bq34z100-G1 Section 8.2 Fig. 15 and as discussed in bq34z100-G1 Section 10.2.1. Both GND and AGND are connected only at Bat-. I understand the concept of the high current flowing through the sense resistor and current sensing (Coulomb counting) being done with a low current ground (AGND). Is the reason, for tying GND and AGND together only at Bat-, because of the coulomb counter (delta-sigma ADC)? How do I answer my colleague about this grounding situation?

  • 0
    TI__Mastermind 34775 points

    Hey Graham,

    The strongest recommendation is that ESD ground connect to PACK(-) and AGND connected to BAT(-). The grounds for your external circuitry (GND) is kind of your call. If it's just FET control then it can be connected to PACK(-), which is on the other side of Rsense resistor (SRN). If it's circuit use to measure battery voltage, etc it would be better to connect it to BAT(-) for more accuracy and not be subjected to IR changes of the Rsense due to high currents. The trade-off of connecting GND to BAT(-) is that the current used in these external circuits won't be coulomb counted.

  • 0 in reply to Damian Lewis
    Prodigy 30 points

    I want to clarify something. I took ESD ground and GND to be the same thing. What is the difference between them? I see what appears to be an ESD ground in section 10.2.4, Fig. 26 of the bq34Z100-G1 data sheet. I have no FET control in my circuit. I am unaware of internal battery pack circuitry on my battery packs (Ultralife UBBL23, UBBL22, UBBL20).

  • 0 in reply to Graham Cameron
    TI__Mastermind 34775 points

    Graham,

    As I mentioned above, if that's the case and you connected them to PACK(-) / SRN, then it's fine.