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The resistor value at the FOD pin --in BQ51013B

Soldier 76
Intellectual 555 points
Other Parts Discussed in Thread: BQ51013B, BQ51013BEVM-764, BQ51013

Hi

i have two questions about BQ51013B,

1.how to select the resistor value for FOD pin ,the value in the datasheet is 188 ohm,can i use any other value ,such as 140 150 or any else?

2.in the picture,how to select the value of resistor Ros1 & Ros2 between out pin and FOD pin,

thanks in advance

  • 0
    Prodigy 35 points

    I have the same question. Can anybody answer? Also, why is Ilim connected to FOD pin with R1 instead of ground?

  • 0 in reply to Louis-Philippe Perron
    Prodigy 10 points

    Same question here. If I have a shielding which absorbs a little energy, have I to make Rfod smaller or bigger?

  • 0 in reply to Idontlike Registrations
    TI__Genius 14890 points

    Calculating the correct values for the FOD and ROS1/2 resistors is not a simple smaller or bigger question.  There is interaction between the FOD and ROS resistors.  Note, only ROS1 or ROS2 is used, not both.

    The detailed process for calculating RFOD and the ROS resistors is quite involved.  The best way to continue this discussion is to contact your local TI support (or distributor).  They will be able to contact me directly and we can work towards a final solution for your application.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 80 points

    Hi, 

    You have stated either of the ROS1/2 resistors are to be used. 

    Can you state the purpose of these resistors and mounting either one of them.

  • 0 in reply to Vikram Chandrasekaran1
    TI__Genius 14890 points

    Vikram,

    The ROS1/ROS2 resistors are used as part of the Foreign Object Detection (FOD) scheme, along with RFOD.  These resistors are used as part of a calculation to estimate the power level to be sent to the transmitter.

    The ROS1 resistor would be tied between OUT and FOD.  Or, the ROS2 would be tied between RECT and FOD.

    Using ROS1 on OUT gives a more stable voltage source than RECT, but then there would always be a resistive load on the OUT pin (on the battery).  The RECT pin changes voltage levels based on the load.  The key to this is that the FOD concern is during higher power levels when RECT is much more stable (close to OUT voltage).  Most systems use the resistor between RECT and FOD.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 70 points

    Hi Dick,

    Can you please provide some default values for ROS2 connected between RECT and FOD. I have found default value for RFOD = 188 Ohm from BQ51013B datasheet. For ROS resistor there is no reference. I have looked the bq51013BEVM-764 datasheet. There is a resistor R17 on the following link : BQ51013B EVM.

    That seems to be the ROS2 I am talking about. Its value is 20k. Can I use that one as default?

    Sincerely,

    Stjepan Gornik

  • 0 in reply to Stjepan Gornik
    TI__Genius 14890 points

    Stjepan,

    You are correct, 20k is a good starting point for the Ros resistor from RECT to FOD.  I recommend that the board layout for both Ros and RFOD include 2 resistors in series.  This will allow more accurate resistance values after the FOD tuning is complete.

    Please see the "Foreign Object Detection (FOD) Calibration/Tuning Tool" in the "Tools & software" tab of the bq51013 product folder.  When the calibration is complete, the RFOD will usually be close to 188, and the Ros depends on the system solution.

    There is a "User Manual" in the tool discussed above.  It gives a good overview of the process and some insight as to why we recommend the 2 resistors.

    Regards,

    Dick