Understanding the reference design layout of the BQ29700 Battery protector

[Nikos A.]

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.

BQ29700 Battery protector datasheet
https://www.ti.com/lit/ds/symlink/bq2970.pdf?ts=1619080532853&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ2970%253FkeyMatch%253DBQ2970%2526tisearch%253Dsearch-everything%2526usecase%253DGPN

sluuaz3 reference design
https://www.ti.com/lit/ug/sluuaz3/sluuaz3.pdf?ts=1618864542494&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ2970

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

The following screenshot is the top layer layout provided from the reference design and I have denoted for your convenience the ground, the VSS pin, the negative and positive battery terminals



Thanks in advance
Nick

[Nikos A.]

After talking with Texas Instruments they gave me the following answer:

" The net tie 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."

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-

Could anyone explain this to me?