Tool/software:
Hello,
We based our design off the EVM for this part, which isolates a VSS net local to the BQ40Z80. So this creates three distinct "Ground zones." The first is Cell-, the second is VSS, and the third is the output Pack-. See below for our schematic interpretation of the EVM schematic:
When we lay this out the internal plane layer gets cut up like this:
Where the portion on the bottom left is Cell-, the island in the middle is VSS, and everything else is Pack-. My question is whether we need to isolate the BQ over VSS or if we can simply tie it to Cell-. The problem is that the SMBus is referenced to the "outside world" which only sees Pack-. But internally, it is referenced to VSS. So there is a convoluted return path, first through the sense resistor to get to Cell-, then through the net tie to get to VSS. This is ugly at best, and probably an EMI nightmare. It would be better to only have Cell- and Pack-. It doesn't totally eliminate the problem but then we can at least make things better with some plane stitching caps. What is your advice?
Thanks,
Jamie
