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bq51050b component value justifications

John Baek
Prodigy 100 points
Other Parts Discussed in Thread: BQ51050B

Hello,

I am implementing bq51050b for a proof of concept project, and I had a few questions about the suggested component values for various pins, and other generl questions.

  • the datasheet says that capacitive communication is the "most common" - why is that? why does the EVM use resistive communication despite this? what are the tradeoffs? Also, how is the value is 47nF (as per datasheet) or 22nF (as per EVM) chosen for the COMM pin? if I were to switch to resistive communication, how would I choose the value?
  • for clamping capacitors a value of 0.47uF is recommended - why is that?
  • caps connected at the RECT pin are recommended to 10.1uF - why is that?
  • the minimum charging current per datasheet is 500mA. what happens when the resistors are set for a charging current lower than this? will it not function or default to 500mA?
  • the procedure to calibrate Rfod and Ros seems to require at least a few iterations of design, which i do not have the time for in my project schedule. how can i make a best estimate for these values (if possible)? for now i have values recommended by the datasheet as a starting point - will i run into serious errors with this setup? the inductance of my coil is roughly the same as the inductance assumed by the examples in the datasheet.

Thanks,

John

  • 0
    TI__Expert 7730 points
    - The bq51050B is using capacitive communication because it is more efficient than the resistive communications. The COMM pin capacitor is chosen based on your own design to provide enough modulation depth to send message to TX. You can start with either one of the values and tune it with your own system.
    - The CLAMP capacitor value is determined based on our design simulation. You can start with it too.
    - We have two 10uF caps to filter out the low frequency noise and one 0.1uF cap to filter out the high frequency noise at RECT pin.
    - It will still work with less than 500mA ICHG, but the accuracy of termination is not as good and it's possible the part will not terminate at all when ICHG reduces to certain level.
    - The Rfod and Ros are part of the FOD circuit which is used when you are worried about the foreign objects. If it's not an issue for you, then you don't need to tune it, just pick the EVM value. If you do have the concern for foreign objects, you need to have a AVID calibrated TX and the below tool to tune your FOD circuit.
    www.ti.com/.../sluc577

    Thanks,
    Wenjia
  • 0 in reply to Wenjia Liu
    Prodigy 100 points

    Thanks Wenjia! I had a couple more questions. Why are the bootstrap caps 10nF? And how do I go about tuning COMM caps? How will I know if they need to be tuned - will the device just not function? How will I know what capacitance I need?

    Thanks,

    John

  • 0 in reply to John Baek
    TI__Expert 7730 points
    Bootstrap caps are used to boost the gate voltage to allow the RECT FETs t turn on. 10nF is what's needed for that boosting.
    COMM cap is used to make sure the communication between RX and TX is successful. So the easiest way is to have the initial COMM cap value the same as the EVM, and then turn on the system, if the RX cannot start up, then your communication may have failed. Then you can increase the cap value until the system starts up. Please note high in capacitance will impact the efficiency.

    Thanks,
    Wenjia