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bb229x layout considerations

Natallia Holubeva1
Expert 6550 points
Other Parts Discussed in Thread: BQ24295

Hello e2e,

My customer has a question for you about the layout you provided:

There is no wall between analog and digital on the silicon however, so my first intuitive thought would be wrong.  They bring out the ground to separate pins because there’s inductance in the bond wires.  So my thought is to connect them together with a fat trace right at the part.  Maybe that’s not right, though, because the via has some inductance, similar to the bond wires although maybe not as bad.  By this reckoning, I should have separate vias going to the same copper plane.

I’m trying to do this on two layers, so ground might be fat traces on the back.  Is two-layers not recommended for this part?

Thank you,

Natallia

  • 0
    TI__Genius 13700 points

    Hello Natallia,

    I am not sure that you have provided the correct part number making understanding the exact case that you are referring to difficult.

    Onto a general discussion of your question.

    The purpose of only providing a thin trace that connects the Analog ground to the Power ground is that while they two planes interface within the device by not having these planes shorted together with a very low impedance connection that is an element of damping occurs which helps to filter some of the switching noise that can be generated from on the power or digital ground plane which will reduce the noise that influences that comparators and ADCs within the device

    Refer to this document here which provides a lot of great information but the information about Grounding specifically starts at 17.3 on Pg. 8.

    http://www.ti.com/lit/ml/sloa089/sloa089.pdf

    Let me know if there are any follow up questions and also if you are able to clarify which devices you are looking at.

  • 0 in reply to Ryan May
    TI__Expert 3160 points

    Natallia,

    I know we already provided some support via email regarding this question, but let me also answer here so we can close the thread.  

    The bq24295 EVM user's guide has an example layout for a 2-layer solution: http://www.ti.com.cn/cn/lit/ug/sluuao5b/sluuao5b.pdf

    Please note that we do recommend a 4-layer board, but if you are going to go with a 2-layer solution we recommend a 24-mil PCB or thinner with a component/routing layer and a GND plane layer.  

    Best Regards,

    Rachel