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BQ76PL455EVM: Maximum number of BQ76PL455EVM evaluation modules that can be connected in series via the isolated daisy-chain

Joachim Sturm
Prodigy 30 points
Part Number: BQ76PL455EVM
Other Parts Discussed in Thread: BQ76PL455A-Q1, , BQ76PL455A

In the bq76PL455A-Q1 datasheet https://www.ti.com/lit/ds/symlink/bq76pl455a-q1.pdf?ts=1596059302879&ref_url=https%253A%252F%252Fwww.google.com%252F it is stated on page 1 that the maximum number of bq76PL455A-Q1 connected in series via the daisy-chain is 16. So that would give you a maximum total battery voltage of 4,2*16*16 = 1075V which would be achievable with this IC.

But in the user guide for the bq76PL455EVM evaluation module https://www.ti.com/lit/ug/sluuba7a/sluuba7a.pdf?ts=1596059608499&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FBQ76PL455EVM%253Futm_source%253Dgoogle%2526utm_medium%253Dcpc%2526utm_campaign%253Dapp-null-null-GPN_EN_EVM-cpc-evm-google-wwe%2526utm_content%253DBQ76PL455EVM%2526ds_k%253DBQ76PL455EVM%2526DCM%253Dyes%2526gclid%253DEAIaIQobChMIosKvqLnz6gIVVeDtCh0LrgisEAAYASAAEgIQ0_D_BwE%2526gclsrc%253Daw.ds it is stated on page 3:

“For battery stacks with more than 16 cells, stack up to 6 bq76PL455EVMs in series to support up to 96 cells. Communication with stacked bq76PL455EVMs is via an isolated daisy-chain differential bus capable of withstanding 500 V.”

So do I assume correctly that this limitation is board specific because of the isolation limit of the daisy-chain differential bus on the evaluation module? Would it therefore be possible to connect more than 6 bq76PL455EVM in series if I would for example use LiFePO4 cells with a maximum cell voltage of 3,6V? In this case for example 8 bq76PL455EVM with a maximum total voltage of 8*16*3,6 = 461V < 500V?

Thank you in advance for your answer!

Best regards,

Joachim

  • 0
    TI__Genius 17440 points

    Hi Joachim,

    Thanks for reaching out. Let me look into the reason behind the EVM stacking limitation and get back to you on Monday with some options.

    Cheers,

  • 0 in reply to Kristin Jones93
    TI__Genius 17440 points

    Hi Joachim,

    Thanks for your patience. The 500V rating for the daisy-chain communication actually comes from the ESD ratings for those pins. You can find this in section 6.2 of the datasheet. You won't be able to communicate with additional BQ76PL455A EVMs if you use lower voltage cells. I recommend using the GUI for initial evaluation of a smaller number of devices, and once you are comfortable with the features you can switch to the example code: 

    The example code will let you stack up to 16 devices.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 30 points

    Hi Kristin,

    thanks for your response!

    So do I assume correctly that only the GUI limits the maximum number of bq76PL455EVMs that can be connected/controlled via the daisy chain to 6? So if I do not use the GUI (use the provided sample code or control the bq76PL455EVM stack via a microcontroller) I can connect up to 16 bq76PL455EVMs via the daisy chain?

    That would be ok for us because we will use the GUI only for initial evaluation (as you also mentioned), I just wanted to make sure that this limitation is not related to the bq76PL455EVM hardware, because later we want to connect up to 8 bq76PL455EVMs via the daisy chain and control the stack via a microcontroller.

    Can you also maybe elaborate more on the correlation between the ESD ratings of the bq76PL455A-Q1 pins and the stack limitation of the bq76PL455EVM board, because I couldn't make a connection there?

    best regards,

    Joachim

  • 0 in reply to Joachim Sturm
    TI__Prodigy 110 points

    Hello,

    Yes with a micro-controller you will be able to communicate with all 16 devices and not be limited by the GUI to 6.

    There is not a connection between the two as they are separate ratings. The stack rating is for the maximum voltage that can be applied to a single PL455 given the maximum rating of the cell inputs. These are DC values.

    The ESD ratings have things like the charged device model and human body model, the pins can tolerate much higher ESD voltage as these are transient values and the device has internal ESD structures to mitigate ESD events that occur on the pins.

    In short, there is not a connection between the two as they are separate entities.

    Thank you, David

  • 0 in reply to David Dolt
    Prodigy 30 points

    Hi David,

    thanks for the clarification, that makes sense!

    best regards,

    Joachim