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PMP41115: TPS26750 + BQ25756 setup

Rain Haase
Prodigy 101 points
Part Number: PMP41115
Other Parts Discussed in Thread: BQ25756, TPS26750

Tool/software:

Hi,

I was just wondering, will a design similar to PMP41115, which can charge and discharge at 240W through USB-C, work with a 9S battery pack instead of a 10S - 12S one? Pack voltage range is 26.1V to 37.8V. (2.9V/cell low-voltage cutoff)

I can afford to cutoff at up to 3.25V/cell (so 29.25V) without significant decreases in runtime, but no higher, and this would be difficult because I can't find any no-programming BMS chips by TI that offer a low-voltage cut-off this high.

Total pack capacity is 100Wh. Board area can be 10725mm^2, and ideally I would use 4 layers, but I'm ok with 6 or 8 too.

Thank you very much.

  • 0
    Prodigy 101 points

    Hi, I'd just like to follow up on this please, as I need to start designing the circuit board soon. I am assuming it would be acceptable since the voltage is similar to that of a 10S Li-ion within the bottom half of the discharge curve, but I would just like to make sure please.

  • 0 in reply to Rain Haase
    TI__Intellectual 2030 points

    Hi Rain.

    it is ok to change battery voltage and current, BQ25756 can support up to 14S Li-ion battery, you can change the FB pin voltage divider to fit for your target cell voltage

     

  • 0 in reply to Max Wang1
    Prodigy 101 points

    Hi Max,

    Thank you very much for your response.

    I was aware that the charging voltage could be changed, however, I am also interested in the OTG mode when outputting 240W from the battery as a source.

    When you guys say '10 to 12 cell batteries are supported in OTG mode' within the test report, it is unclear what the low-voltage cutoff is in this situation to still be able to achieve acceptable efficiency.

    If the low-voltage cutoff in mind was 2.9V/cell or less when this recommendation was made (the standard for cylindrical cells), I don't have any issues, because, at that point, using the 9S battery pack has an identical minimum operating voltage.

    But if it's closer to 3.5V/cell, it makes my whole project not viable. This is exacerbated by the fact that tests at 240W seem to have only been done at a 50V battery voltage, meaning the converter would be significantly more efficient than if they had been done at the minimum intended operating voltage of the device... which I am still unsure of.

    I'm just trying to find out - is there anything in software stopping me from outputting 240W in OTG mode at a voltage as low as 26V? I'd imagine I'd just have to make the inductor a big bigger and same with the FETs. I'm not sure how long you did the tests for but the longest the device would be running at these power levels is 20 minutes.

    I understand this sort of inquiry is best solved by purchasing the development board and running tests to ensure thermals will be fine. But I unfortunately cannot do that as this stage, so what I'd like to know is that, if there is a limiting factor, it'll be thermals, and nothing else.

    Thank you so much.

  • +1 in reply to Rain Haase
    TI__Intellectual 2030 points

    Hi Rain

    if you request a 240W at Lower battery voltage, it would be less efficient, as the voltage gap is larger, this is depend on your power stage thermal design, for software, you can change those information in TPS26750 GUI, you can reach out or raise another E2E request for specific GUI and software support, some of the expert on this device will help you on the GUI generation based on your input.

    BR

    Max

  • 0 in reply to Max Wang1
    Prodigy 101 points

    Hi Max,

    Ok, great. Thank you so much. I really appreciate your fast reply. I will ensure my power stage thermal design is good, and I will make another E2E request for GUI and software support after I receive the assembled PCBs.

    Kind regards,

    Rain