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BQ29700EVM-610: Understanding the layout of Cell- and VSS pin

Nikos Antoniou
Intellectual 820 points
Part Number: BQ29700EVM-610
Other Parts Discussed in Thread: BQ76920

Hi everyone,

I believe this is a trivial designing question but I do not have the experience to understand it.. I design a PCB using the BQ29700 battery protector and I am advising the sluuaz3 reference design. I have a question regarding the VSS pin of the protector and the battery negative terminal.

Looking the reference design I cannot understand why there is a net tie from Cell- to the ground... What's the purpose? It is like there are two grounding systems connected together to a signle point (like AGND and DGND), but I do not understand the reason of that here.. Why you don't just connect Cell- to VSS?

Thanks

Nick

  • 0
    TI__Guru 54715 points

    Hi Nick,

    In figure 1 of the user guide the connection is the "Net-Tie" shown near the J1 connector on the schematic.  It is a feature which allows connecting 2 nets with different names, on the board is done with a piece of copper having a footprint shape which you have shown above.

    It does connect Cell- to VSS, with this simple circuit there was likely not needed.  In more complex designs it can be useful to keep multiple capacitors connected to VSS referencing the same potential of a high current trace such as Cell-.  In this design the net tie could have been omitted. 

  • 0 in reply to WM5295
    Intellectual 820 points

    Thank you for you answer  

    WM5295 said:
    In more complex designs it can be useful to keep multiple capacitors connected to VSS referencing the same potential of a high current trace such as Cell-

    Could you please explain more on this? I cannot understand why it could be usefull to keep multiple capacitors to VSS referencing the same potential of a high current trace such as Cell-

    Thanks

    Nick

  • 0 in reply to Nikos Antoniou
    TI__Guru 54715 points

    Hi Nick,

    You might see figures 19 and 20 in the BQ76920 data sheet to help in understanding.