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bq51050B FOD Values and Filtering

Other Parts Discussed in Thread: TIDA-00318

The data sheet shows FOD being connected with a resistor, yet the EVM has a parallel 0.1uF cap. Is this cap recommended? What would be the effect of not having it?

Also, the EVM has RFOD @ 200 ohms, the data sheet says use 188 with no explanation. 

I am not the first to ask. 

http://e2e.ti.com/support/power_management/wireless_power/f/693/t/268619.aspx

Again, we need a better explanation of how the FOD pin works and how it interacts with ILIM and also TERM. My application is not standard compliant so I don't care about that as much as how to make my device work reliably and safely in its unique environment. 

  • Mike,

    FOD is a quite complex issue and solution.  I'm not sure I can cover it all in this post, but I hope to shed some light on your questions and concerns.

    The resistor is required on FOD, but the capacitor is not required.  The FOD pin is monitored internally to aid in determination of whether or not a foreign object is present.  As such, adding the capacitor may help alleviate any noise interference.  We have released a couple of reference designs (for instance:  http://www.ti.com/tool/TIDA-00318) that do not include the capacitor on FOD.  You can search the TI Designs area for other devices to see more reference designs.  http://www.ti.com/tool/TIDA-00329 shows a non-Qi solution in a very small form factor - this is intended for lower power applications.

    The relationship between the FOD resistor and the ROS resistor (from RECT to FOD) aids in setting the proper balance between the friendly metals in your application and the foreign objects that will interfere with the power transfer process.

    During the calibration for FOD, the values ROS and RFOD will change.  As a result, 188 and 200 are nominal values that work on the EVM and are not meant to imply that they will work with any form factor.

    A little on FOD.  The RX measures the power it receives and sends it back to the TX.  The TX measures the power it is transmitting and compares it to the power that the RX reports.  The algorithms required to process the data and determine if there is a foreign object are proprietary and cannot be shared in this forum.  

    If necessary, please contact your local TI sales team to contact the factory for the next steps of resolution.

    Regards,

    Dick