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TIDA-010030: Sense resistor selection question

jediengineer
Prodigy 140 points
Part Number: TIDA-010030
Other Parts Discussed in Thread: BQ76940, , BQ34Z100-G1

I am designing a BMS for a 12s stack, very similar to TIDA-010030 reference design, but when it came to the sense resistor selection, I ran into some trouble. So naturally, I have a few questions about the sense resistor selection as outlined in the BQ76940 datasheet. In the datasheet on page  46, it outlines the exact current I'm planning to sense, but I think there's a mistake. I chose cells that had a max continuous current of 10A. So the example in the datasheet work for me, BUT - something is wrong with the math presented. RSNS has a maximum range of +/-200mV with an absolute max of 300mV. If we're considering an Iscd of 25A over the absolute maximum range, that calls for a 12.5mR sense resistor, not 7.5mR as the datasheet says. I've been trying to figure out where that number comes from and can't seem to get it close. Can you elaborate? Also, in your TIDA-010030 reference design, they're using 1mR as a sense resistance, and that tells me that you're expecting an 18650 cell to output a maximum of 200A - something doesn't' seem right - can you elaborate on this?

  • 0
    TI__Expert 3145 points

    Hi, 

    Regarding the pack current sensing resister in TIDA-010030, SCD is just one of the filter, I also considered the thermal performance and current accuracy during normal charging/discharging current. I chose 1mR current sense resister in TIDA-010030, that gave me 200A maximum SCD threshold, but I didn't have to configure 200A, actually I used a much smaller value as you can see in the user guide. Also, there is another device BQ34Z100-G1 which will also sense the voltage across the current sense resister, and the recommended voltage range is just +/-125mV. So I had to consider it too. 

    For your case, please note that the SCD threshold is just up to 200mV, you need to identify your SCD threshold (<200mV) and peak SCD current (<300mV) to choose the maximum current sense resister. 

    Please click "This resolved my issue" if no more questions. Thanks.

    Best Regards,

    Ryan Tan

  • 0 in reply to Ryan Tan
    Prodigy 140 points

    Let me make sure I understand - I can use a 1mR sense resistor, so long as I configure the overcurrent properly...? I don't need to select one, per se, as in the datasheet? I'm fine with that if that's the case, it will reduce IIR losses across the resistor. My pack cannot exceed 13 amps regardless, so as long as this works I can go with it. This would just mean that the ADC is reading a maximum of 13.3mV out of the 200mV range. As long as this is an acceptable scenario as far as design criteria is concerned... 

  • 0 in reply to jediengineer
    TI__Expert 3145 points

    Hi, 

    You can configure Protect2 (0x07) and achieve your preferred OCD threshold. I am not saying that 1mR is good for all projects, 1mR is just what TIDA-010030 uses. If I understand you correctly, your constant current is 10A, maximum working current is 13A, so OCD should be set to 13A or a little bit smaller, let's say 12A, and SCD threshold is 25A. If with above conditions, that means that your current sense resister should be less than min (100mV/12A, 200mV/25A)=8mR. If consider some tolerance, you may choose 5mR or 6mR. Then you need to check if 5mR or 6mR can achieve your required OCD/SCD threshold, if you peak current will hit the voltage limitation (300mV), if the thermal a problem. Normally, larger current sense resister gives your better current accuracy and worse thermal performance. 

    Please click "This resolved my issue" if no more questions. Thanks.

    Best Regards,

    Ryan Tan

  • 0 in reply to Ryan Tan
    Prodigy 140 points

    Thanks Ryan, I see where your numbers come from now - I wasn't exactly sure at first. I have a few other questions, but I put them in another post.  Thanks!

  • 0 in reply to jediengineer
    TI__Expert 3145 points

    Hi,

    Sure. glad to help. Let's discuss in your another post. 

    Best Regards,

    Ryan Tan