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BQ500511A: FOD tripping but disabled

Part Number: BQ500511A
Other Parts Discussed in Thread: BQ51003, BQ50002A


We are using the BQ500511A in combination with the BQ50002A for a wireless charger in combination with the BQ51003 on the receiver side. We noticed a strange error state which can be triggered by slowly removing the RX from the TX. We have LED mode 4 (Rled = 66.5kOhm) and in the mentioned state, the LED B is constant on for 5 minutes. According to the datasheet (if I understand it correctly), the 5 minute fault indication could be due to overtemperature of the receiver, overtemperature detected through the NTC on the charger, or FOD trip. However, I am almost certain that the error could not be temperature related since I left the devices to cool before triggering this state, and it should also not be FOD tripping since we disabled FOD by tying FOD_THR to 3V.

I was wondering if there are other cases where there is a fault that triggers a 5 minute indication on the LED, or if it is possible that an EPT-3, NTC or FOD trip is falsely communicated/received?

Some extra observations: sometime when I try to trigger this error state, I also see LED_B blinking fast. Following the dataseet with LED mode 4, this should indicate FOD warning, but as I mentioned we should have disabled FOD detection. Am I missing something that could still trigger FOD with FOD_THR tied to 3V?

Best regards,


  • Hi Wout
    I have set up the a BQ500511A & BQ51003 EVM in LED Mode 4 and tested in the above condition but did not see the behavior you described. Unit would stop power transfer and all LEDs would be off.
    One thing you could check is the TX input voltage it may be dropping as the RX is moved away. During this condition the TX will generate a larger field to overcome the increased distance, higher input current.
  • Hi Bill,

    Thank you for your insight. It was indeed the TX input voltage dropping. This also explained why the problem only occurred sometimes: we noticed that some of our USB cables we use for powering the TX had significantly larger resistance compared to other cabels. When we used these cables the problem was frequently obeserved. Switching the cable with a lower resistance one mostly solved the problem.

    Best regards,


  • Hi Wout
    Thanks for the update, good to hear that was the problem.