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4l stack up

Geombu , 11-09-2019, 04:12 PM
Hell all,
I would like to ask 2 questions about a specific design in attachment. The Altium project is from TI, reagarding a 4L BMS board.

1) On the left side of the board where the Hercules MCU is located that logic area has separated GND Polygon Planes (PGND, CANGND, etc). Is it better than using one single plane for all signals and power areas? What is the benefit?

2) In this design all layers are Sig + Vcc and free area is GND for all layers. Also the signal tracks from BQ79606 to the connector path are not reneferenced to any GND layer. According to Rick Hartley's presentation, one of the best (lower impedance) stack up is GND for Bottom and Top and in the middle layers (2+3) Sig+Vcc. In such a design case (BMS) which is the best option considering battery measurements accuracy and EMC performance?

I would appreciate your feedback.


robertferanec , 11-11-2019, 08:59 AM
@Geombu - what a reference design ..... I really do not like reference designs where schematic is without block diagram and notes - so people have to go through whole schematic and all the chips to try to understand what is there. Thumbs down for Texas Instrument for this.

And because of this unprofessional schematic .. I am not really sure how to trust the layout. The layout is .. interesting .. honestly I do not really know what to think about it. Planes on every layer and every single plane on every layer is broken by tracks .... I am attaching screenshots so everyone can see.

PS: read layout section in datasheet, it looks like for the "BQ79606A-Q1 SafeTI™ Precision Monitor With Integrated Hardware Protector for Automotive Battery Pack Applications" the layout is important: http://www.ti.com/lit/ds/symlink/bq79606a-q1.pdf

So, here are the screenshots .... I am curious what other people think about this. It must be a very very clever layout or a very very poor layout .. I am not sure which one as without block diagram it would take some time to actually understand what is on the board. What confuses me, that it suppose to be probably a Texas Reference design, so I would expect high quality .... hmm

Geombu , 11-11-2019, 09:31 AM
Hello Robert,
I am relieved from your response because I felt dumm for a moment. I posted this because it is very strange from TI. I don't see any high quality to many of their reference designs lately.

I never designed any close to this approach before and I have the same understanding as you, especially for the broken ground planes crossed by various tracks. The schematics and the project structure are awfull since you can't find any continuity. Somehow this project was compiled !

I think that nobody from design team, read the datasheet layout guidelines which are also quite fuzzy at the first paragraph. On the second is getting better. At the same time the ground and the Vcc references are referenced to...somewhere and the broken planes will radiate strongly enough to measure the signals even without anechoic chamber. Another 'nice' practice here is the routing of all the tracks from the BQ devices to the connectors. These tracks are referenced to blank.

There are more designs like this from TI but it is better to post examples to follow, rather than to avoid. So, I will not post more.

It is interesting to see what also other people think for this.

Kind regards,
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