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bq27x00 datasheet: GND and PACK- connection

Anonymous
Anonymous

Hi,

I would like to ask a question on bq27x00 connection.

Question 1

I could not understand the "Typical Application" schematics in bq27x00 datasheet. In the right upper corner of the schematics, the current should come out of the positive end of the battery, and after flowing through components of the system, eventually come back to the negative end of the battery through the GND network.

If this is the path of the current, then why would current through sense resistor RS? It seems that no current would flow between GND and PACK- at all.

I guess the GND network in the right upper corner might be wrong. Could anyone confirm it?

Question 2

What does the Li-ion Protector stand for? Does it refer to circuit/temperature circuit in the battery pack?

 

 

Zheng

  • 0
    TI__Intellectual 2340 points

    The application schematic is correct. The ground symbol should not be confused with the Pack- connection. The Vss of the gauge is connected typically to the negative node of the cell. Pack- is the the most negative reference that the connecting system has access to. The Cell- and the Pack- are offset by the protector ICs FETs and the sense resistor,

    The Li-Ion Protector is the battery protection IC that is commonly included in battery packs. These ICs snese the voltage and curret of the cell and open the appropriate FETs to cut current path in cases of over voltage, under voltage and overcurrent. The protector IC can also detect recovery states from these fault conditions.

  • Anonymous
    0 Anonymous in reply to MikeV

    Mike,

    MikeV said:

    The Vss of the gauge is connected typically to the negative node of the cell. Pack- is the the most negative reference that the connecting system has access to.

    If PACK- is the most negative reference a connecting system can access, then how can bq27x00 connect to GND? Does it mean it has to "penetrate" or "***" into the external shell(cover) to go even beyond PACK-?

    I compared bq27000/27200 datasheet connection with its EVM, SpectrumDigital EVMAM1808, datasheet and EVM of {bq26200, 26220, 26231} and datasheet of bq26221 alone in the latest post of bq27000: does GND & GND_PACK order (topology) matter? and proposed some explanations based on the rules I have observed. Could you comment on those points?

     

    Zheng

  • 0 in reply to Anonymous
    Prodigy 45 points

    Zheng,

    I know this is an old post but it deserves a clear answer. Because we did it wrong once.

    The application note is drawn as if the circuitry is on the battery. I assume you are not making a battery pack but rather are using an existing pack and in your design and want to add a fuel gauge that is not on the pack, so the application note is confusing, I know it was for us.

    The bq's ground connects to the negative terminal of the battery but not the ground of your circuit. The ground of your circuit is Pack- in that application note and the negative terminal of the battery is only connected to your ground through the sense resistor. 

    As per question 2 in your first post, the Li-ion Protector refers to the protection built in to the battery pack.

    Figure 8 and 9 in the below document explain better then I can.

    http://www.ti.com/lit/ug/sluu157a/sluu157a.pdf

    --Morgan