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BQ79606A-Q1: Placement of daisy chain communication components and keep out

Carlos Zelayeta
Prodigy 40 points
Part Number: BQ79606A-Q1

Hello.

I have a question regarding the recommended placement of the components involved in the daisy chain communication between BQ79606A ICs. We have boards with multiple BQ (for example, 2 on each board). In addition to this, several PCBs are connected with cables. As recommended in the datasheet SLUSDQ4  and also in the design recommendations SLUA811C we use capacitor isolation between BQs on the same board and capacitor and choke for communication between boards.

Section 13 (Layout considerations) of SLUA811C recommends placing isolation capacitors near the edge of the board and chokes near the cable connector. Then it says "Place the series resistors and TVS diodes next to the BQ79606A-Q1." My question is how to interpret this sentence. So, my first question is: Does it means that series resistors and TVS should be as close to the BQ as possible?

Another issue is the keep out on internal copper layers and bottom layer (assuming components are placed in top layer, of course).

  • Which components should be placed above keep out of internal layers? Should TVS and series resistors have keep out on all layers below them or this requirement is only for choke and isolation capacitors?
  • Assuming TVS and resistors are close to the BQ and choke and capacitor are close to board edge: Should there be a keep out on all layers below the tracks that connects them? (In our pcb, this distance is 10 cm aprox.)

Thanks in advance for your reply.

Regards

  • 0
    TI__Genius 14890 points

    Hello Carlos,

    Short answer - yes the closer to the BQ79606A the better, but we placed them 1-2cm away on our EVM and it worked fine. Take a look at the EVM user's guide: https://www.ti.com/lit/ug/slou471e/slou471e.pdf?ts=1643139273483&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ79606A-Q1%253FkeyMatch%253DBQ79606A-Q1%2526tisearch%253Dsearch-everything%2526usecase%253DGPN.

    Inserting a snapshot of the relevant image below:

    Which components should be placed above keep out of internal layers? Should TVS and series resistors have keep out on all layers below them or this requirement is only for choke and isolation capacitors?

    [Alex] Isolation capacitor, choke, and the first resistor (as seen in the diagram below) are recommended to have the keep-out layer. The TVS and other series resistor don't need the keep out layer (the TVS will need to be referenced to the board ground anyways).

    Assuming TVS and resistors are close to the BQ and choke and capacitor are close to board edge: Should there be a keep out on all layers below the tracks that connects them? (In our pcb, this distance is 10 cm aprox.)

    [Alex] Yes. Again I'll refer to the diagram below.

  • 0 in reply to Alexander Scheuermann
    Prodigy 40 points

    Thank you for your answer. Unfortunately, due to geometric constrains, I am forced to violate the keep out in at least one layer. I was expecting to be able to place the first series resistors close to the capacitors to avoid this rule violation, but since you mention this is not recommended, I will proceed as you suggest.

    One more comment: I had already seen the diagram of figure 28. It is confusing since the capacitors are placed between connectors and chokes, but all schematics in the datasheet are the other way around: choke is placed between connector and capacitor. Why is the figure inverted? Is it a mistake?

    Thanks again for your answer.

  • +1 in reply to Carlos Zelayeta
    TI__Genius 14890 points

    Hello Carlos,

    Yes, it looks like the cap and the choke is swapped in the diagram above. I think this was just a mistake, sorry for the confusion.