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BQ27320: PCB GND routing

G Cant
Intellectual 335 points
Part Number: BQ27320

Hello,

I  am using this part to monitor my LIPO battery. The Gauge needs a sense resistor in series with GND.

I have the battery mounted in some clips, that means that positive terminal and negative terminal are not together, but distanced the length of the battery.

Regarding the routing of the PCB, how do you recommend to route GND plane?

Should I place the sense resistor + Gauge as near as possible to the negative terminal and generate my GND plane from there? Or maybe route with a thick trace the negative terminal of the battery closer to the positive terminal and generate my GND plane closer to my power plane?

Thank you. 

  • 0
    TI__Guru 57697 points

    Place the sense resistor close to the gauge negative terminal. Use thick traces for power path. Other than the gauge related components, everything else should be connected to Pack-

  • 0 in reply to Shirish
    Intellectual 335 points

    So if I have a Device with a Battery (Pack+ and Pack- Terminals), the Gauge and other circuits like DCDC and microcontroller, should I connect all my returns GND directly to Pack-? Or is the sense resistor my GND for the whole circuit?

  • 0 in reply to G Cant
    TI__Guru 57697 points

    The gauge circuit uses the gauge GND. For everything else Pack- is ground.

  • 0 in reply to Shirish
    Intellectual 335 points

    When you say "Gauge Circuit" you mean just the BQ27320 and related resistors/Capacitors attached to it? 

    So my microcontroller, DCDC, LEDs, etc should all be connected directly to PACK-?

    I thought Gauge was monitoring the current flowing through the sense resistor and that is why all the circuits of my board should be connected directly to the Sense resistor. Between this virtual GND point for my circuit and PACK- there will be only the Sense Resistor in series.

    Please clarify.

    Thank you very much.

  • 0 in reply to G Cant
    TI__Guru 57697 points

    Yes, "Gauge circuit" means all the resistors capacitors attached to it. In short use the EVM as a reference and anything not on it connects between Pack+ and Pack-.

  • 0 in reply to Shirish
    Intellectual 335 points

    Could you please double check? Datasheet for this part says the opposite (or that is what I understand):

    "The device measures charge and discharge activity by monitoring the voltage across a small-value series sense resistor (5 mΩ to 20 mΩ typical) located between the system’s VSS and the battery’s PACK– pin"

    It seems that all my circuits should be connected to system's VSS which is in series with the sense resistor connecting PACK-.

    Could you please post a schematic or simplified block diagram with some elements + gauge + battery so I can understand better?

    Thank you.

  • 0 in reply to G Cant
    Intellectual 335 points

    Even the schematic of the EVM says that Load - is connected to the sense resistor NOT directly to PACK -.

  • 0 in reply to G Cant
    TI__Guru 60846 points

    You'll have to make sure that the system current flows through the sense resistor. Reading this thread, it appears that there's ambiguity with net names (PACK-, GND, LOAD- etc.).

    The rule is simple: Design your system so that the current to/from the battery flows through the sense resistor.