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BQ51003: FOD problems

Wout Geeurickx
Prodigy 235 points
Part Number: BQ51003
Other Parts Discussed in Thread: BQ25100,

Hi,

We have a setup where we have mounted our RX coil directly on top of a small coin cell battery. The ferrite of the coil has the same diameter as the battery by coincidence (12mm). When I try the BQ51003EVM (adapted to our application) with only the coil, there is no problem. The battery charges through a bq25100 nicely as expected. However, when I put the battery under the Qi coil, the Qi connection is started, but again aborted after about 2.5 seconds. Tuning is good, since I see pulses every 30ms on VRECT. My guess is that the battery is triggering the FOD which causes the RX to send an EPT to the TX. I have included a screenshot from the scope measuring VFOD on TP17 from the bq51003 EVM. The top line is from the 'refernce case' where only the coil is connected and the charging works, the bottom line is when there is also a battery included and there is no charging. The VFOD is lower in the case with the battery included and the connection is broken after about 2.5s. After a while, the connection is again established to disappear again.

Is this already enough information for you to verify that this indeed indicates a problem with the FOD?

If so, is there a way to disable the FOD? We are only using the Qi charging with a custom-made Qi TX so we do not need Qi certification (or do we?) as we don't require our product to be compatible with other Qi TX. I have looked through the FOD Calibration manual, but we do not have the AVID calibrated TX. If this is indeed a FOD problem and there is no option to turn it off, we might try to get one.

Best regards,

Wout

  • 0
    TI__Guru* 75856 points
    Hello Wout
    This could be FOD, try increasing R-FOD resistor (Pin F2 to gnd). The total resistance in ILIM (G1) and FOD (F2) string can be kept the same by reducing ILIM to FOD and increasing FOD to GND. This will result in the RX reporting more received power and shift the TX calculation away from a fault condition.

    The RX does not set an FOD fault it only reports how much power it has received to the TX. The TX know how much power has been sent to RX and how much the RX is reporting it has received, difference is loss. When loss exceeds threshold and FOD fault is set.

    The other possible problem is RX coil tuning I would recommend measuring the coil L with battery in position and unit on the TX. TX should not be powered up for this test. RX coil tuning will use this L value Ls'. See section 9.2.1.2.2
  • 0 in reply to Bill Johns
    Prodigy 235 points
    Increasing the R-FOD resistor did the trick for me, thanks!

    Best regards,
    Wout