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BQ500511: The LED indication problem with wireless charging

Part Number: BQ500511
Other Parts Discussed in Thread: BQ50002

The problem occurs when I use bq50002EVM-607 or BQ500511+BQ50002:

After the battery is fully charged, the green LED on the power transmitter lights up:

And then, I repeat the following steps for several times: removing the power receiver for a short distance -> puting the receiver back on the transmitter -> removing -> putting back......

During above process, the green LED which indicates fully charged keeps lighting.

At last, I remove the power receiver completely, the green LED keeps lighting for 10+ seconds, and then off:

The green LED should be off immediately after the receiver is away, right?

If there's something wrong?

  • Check the time between pings on the TX with a scope.

    After the RX sends charge complete TX will go to sleep for about 5 seconds.  Wakes up and pings the RX if it replies charge complete TX goes back to sleep.

  • Dear sir:

    Which pin or TP on the TX should I test to check the ping?

  • The ping can be seen on TX coil.

    Digital ping will drive the coil at 175kHz for about 100mS, enough power and time for the RX to reply.

  • Dear sir:

    During the period between the RX is removed and the fully charged LED on the TX off, the time between pings on the TX is 5 seconds.

    If the RX is normally removed, the fully charged LED on the TX will be off immediately, and the time between pings on the TX is also 5 seconds.

  • This is normal behavior.

    When charge complete is sent by the RX the TX will stop power transfer and go into a low power mode but monitoring the charging pad / RX.
    TX will send a short series of pulses every 500ms (Analog Ping) and a longer higher power pulse (Digital Ping) every 5 sec.

    Analog ping is low power, does not power up the RX. But the TX will look at the pulse amplitude for changes, if the amplitude changes by 10% to 20% it could indicate the RX has been replaced. Next step would be to send a digital ping to get a response from the RX.

    Digital ping is a higher power signal sent long enough for the RX to reply. RX could send charge complete and TX will turn off and LED turns on.

    When the RX is removed it is first detected by the Analog ping then a Digital ping will confirm that it is removed and LED turns off.
    But if the Analog ping value or running average is not valid it may not detect the change and immediately trigger a Digital ping, no indication RX is removed.

    Now we need the Digital ping to detect the RX is removed. It is looking for no reply from the RX to indicated it is removed and after the 3 or 4 Digital ping with no reply from RX the TX exits charge complete standby mode, LED off. This is not done immediately to prevent unwanted restarts in the dead of night.

    No risk of lock up or unwanted power transfer during this time.