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LM66100: Battery pack insertion/removal in unpowered device

Nicholas Zambetti
Prodigy 190 points
Part Number: LM66100
Other Parts Discussed in Thread: TPS22908

Tool/software:

I am starting a project that requires a single "3.2V" (2V-3.65V) LiFePO4 18650 cell to be in a housing with exposed contacts, I will refer to this a the "battery pack", which is intended to be easy to swap in and out of the unpowered device. The device will only be battery-powered, the batteries will be charged by a dedicated charger outside the device, so this is not a "hot-swap" scenario. The plan is to use spring contacts to connect the battery to the device. The battery cell will be charged at 2.5A max and discharge with 1A max.

My concerns/thoughts I'm hoping for clarification/feedback on are...

  1. On PCB of the device, to protect from reverse-polarity during battery insertion, I am considering using TI LM66100. In the datasheet for the LM66100, 8.3.1 shows reverse polarity protection RPP. Also on that same page, 8.3.2 shows reverse current blocking RCB. Looking at both of these, I it seems the reverse current blocking approach shown in 8.3.2 will also provide reverse polarity protection, is this correct?
  2. On the PCB inside the "battery pack" am hoping to add short-circuit protection. It needs to allow for bi-directional current so I can charge the battery in the charger. Does TI offer an IC that can provide short-circuit protection in the battery pack? Perhaps an eFuse or similar?
  3. When connecting the battery pack to the device with spring contacts, I'd like to protect the device against voltage spikes or other power anomalies. Does TI offer an IC that is intended to protect an unpowered device during battery insertion? I am considering an inrush-current limiter of some kind. Many of TI's hot-swap controllers do not support the voltage range of LFP cells 2V-3.65V. A possible candidate that I've found is the TPS22908, but maybe there's a better option?
  • 0
    Prodigy 190 points

    For the battery pack, I found the BQ297xx might be a good option since there are variants of it that have under voltage protection UVP of 2.3V, which is ok for LFP batteries since it will offer some headroom before over-discharging them, the OVP triggers at 4.280V, which is too high to be useful, but at least it won't conflict with the battery cell's in-built PCM during charging. The BQ29701 in particular seems like perhaps a good solution for the battery pack.

    The more modern BQ298xyz family would be better since it offers a transport/shutdown function, but unfortunately the lowest UVP is 2.5V in the section "5 Device Comparison Table" (even if in the "7.5 Electrical Characteristics" section, it says 2.2V is the lowest, strange). Which might be ok since the LFP discharge curve drops off rapidly around 2.75V, but maybe it will limit runtime due to triggering prematurely due to voltage drop due to system load. Another thing that confuses me about the BQ298xyz, is that the datasheet does not discuss short circuit protection well enough. It is not clear if it is for internal pack short circuit protection, or short circuit protection across pack+ and pack-, any guidance you may have here would be very helpful.

    These two chips might solve point #2 of my question above, perhaps there are better options?

    •Update* I'm not the first one who has investigated this https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/951174/lifepo4-battery-protection-ic

  • 0
    TI__Mastermind 28235 points

    Nicholas, 

    I can speak to the LM66100 and the TPS22908, however once those questions are answered I can assign the thread to the battery management team to answer the question on the BQ device.

    For the LM66100, this will provide reverse current blocking, however do keep in mind that the reverse activation current has a maximum specification of 1A. This means that as much as 1A needs to be seen in the reverse direction before the device shuts off. 

    For the TPS22908 device, this would help with inrush currents as it will have a controlled slew rate, however I am a but concerned that the hotplug event would cause voltage spikes on the input of the switch that would not be supported.

    Maybe a better option here would be an actual eFuse device? (https://www.ti.com/power-management/power-switches/efuse-hotswap-controllers/overview.html). This would integrated the LM66100 and the load switch on the input power side and would also give you overvoltage protection. I can assign this to the eFuse team if you want to get you a more clear recommendation.

    Best Regards,
    Tim

  • 0 in reply to Timothy Logan
    Prodigy 190 points

    Thank you Tim, the guidance you provided is very helpful.

    Timothy Logan said:

    For the LM66100, this will provide reverse current blocking, however do keep in mind that the reverse activation current has a maximum specification of 1A. This means that as much as 1A needs to be seen in the reverse direction before the device shuts off. 

    1A is a lot, I will instead use the LM66100 for reverse polarity protection as shown in the datasheet 8.3.1

    Timothy Logan said:
    For the TPS22908 device, this would help with inrush currents as it will have a controlled slew rate, however I am a but concerned that the hotplug event would cause voltage spikes on the input of the switch that would not be supported.

    So it will be a combination of voltage spikes and unstable current? Maybe a TVS with a low working voltage might offer some protection from voltage spikes? I will look at eFuses and see if there is a good fit for my application. Considering the unstable current as well, I am also considering a delayed/debounced activation of an ideal diode or load switch.

    Timothy Logan said:
    once those questions are answered I can assign the thread to the battery management team to answer the question on the BQ device.

    Is there another forum specifically for battery management? Advice/recommendations about the BQ297xx (or other) from the battery management team for my application scenario above would be very helpful.

    Many thanks.