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BQ20Z65-R1 Circuit Layout

Nick56152
Intellectual 545 points
Other Parts Discussed in Thread: BQ20Z65-R1, BQ20Z65

I am in the final stages of finalizing a design using the BQ20Z65-R1 gas gauge and am basing my circuit off that used in the Evaluation Module for this gas gauge. Before I send the product off for fabrication, there a few minor points which I'd really appreciate if anyone could clear up:

1. The single-point connection to the low-current ground system from the sense resistor:

Should the connection be made at the PACK- side or the 1N side of the sense resistor?

The EVM datasheet shows the connection at the PACK- side while in SLUA534 it is made at the 1N side: which is correct?

2. Can the chemical fuse be put in between PACK+ and the MOSFETS instead of between the top (+ terminal) cell of the pack and the MOSFETS? (as it might be too late to alter my design...)

3. Which capacitor (designator) in the EVM schematic is used to decouple the power supply to the BQ20Z65? C20? C21?

Thanks :) Any help or tips would be appreciated!

Nick

  • 0
    Intellectual 545 points

    UPDATE:

    I managed to move the chemical fuse so that it is now between the top (+ terminal) cell of the pack and the MOSFETS (as in the System Partitioning Diagram in the BQ20Z65-R1 datasheet).

    Is anyone able to shed any light on the other two points?

    Thanks again

  • 0
    Prodigy 140 points

    Nick,

    I'm also working on a bq20z65 design, so thought I'd eventually benefit from knowing the answers to your questions anyway.  So I did a little digging myself...

    Item 1:  Are you referring to Figure 6 in SLUA534?  I believe that figure 6 is just a diagram error by someone.  Note that both fig 7 and 8 show the ground on the Pack- side which agrees with the datasheet as well.

    Item 3:  According to SLUA534, C21 is for decoupling of the power supply of the bq20z65.  Note that C21 is attached to VCC, and CLUA534 makes note in item 3 to keep this connection between VCC, this cap, and VSS as short as possible.  C20 connects to the VCELL+ pin which per the bq20z65 datasheet is "Internal cell voltage multiplexer and amplifier output."  So this is not really supplying power to the IC, so will be less picky about layout length.

    Hope I was of some help.

    Brian

  • 0 in reply to Brian Fritz
    Intellectual 545 points

    Thanks a lot for your reply Brian! :)

    Item 1: Yes - that's right. Figure 6 was the diagram I assumed to be incorrect... Maybe TI should change the ground location to avoid confusion.

    Item 3: Thanks for that! I'm sure you're right (I was looking at SLUA529 (Section 4.2) where it seems that the C14 they are referring to is actually C21 in our reference design...) But due to the fact that the bq20z65 is basically the result of the combining of the two ICs mentioned throughout that document, VCC becomes shared and then the C21 must be the capacitor for supply decoupling.

    By the way, how is your design going?

    Any issues so far?

    Thanks again,

    Nick