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BQ51051B: Question about disabling FOD and charge termination

Garry Lund
Prodigy 20 points
Part Number: BQ51051B
Other Parts Discussed in Thread: BQ51050B

Hi,

I have a design that I am getting ready for production, and I would like to maximize the amount of current that I am able to safely transmit with reliable results.  My wireless coils are both around 20 mm, and the coil distance is maybe 6 to 7 mm.  Given my small coil size and extra distance, my practical desired charging current is around 400 mA.  I have R1 = 578 Ohms and RFOD = 187 Ohms.  When I put these values in my charger, it will try to charge but fails after a couple of seconds with a red flashing light on the charger.  I decoded the response to the digital ping during this error condition, and it appeared to be sending a RX signal strength packet to the charger, which appeared to be correctly sent according to the checksum of the packet.  This is why I suspect that FOD is the problem.

If I go up to 1K for R1, which is around 250 mA charge current, I can get the system to charge reliably.

I would like to disable FOD, and my understanding is that by increasing RFOD it will make the system less likely to fail.  Can I take RFOD all the way up to RILIM, with R1 being a 0 ohm jumper, to completely disable FOD?

The other problem that I am having is that at the 250 mA charging current, with Rterm at 2.2K, the charge will terminate when the battery is full, but will almost immediately restart the charging.  This will occur repeatedly.  Is this due to the very low termination current of around 25 mA?

Thanks,

Garry

  • 0
    TI__Guru* 75866 points
    Hi Garry
    "Can I take RFOD all the way up to RILIM, with R1 being a 0 ohm jumper, to completely disable FOD?"---Bill J - I recommend keeping R1 at 100 ohms and increasing RFOD to adjust current limit.

    "The other problem that I am having is that at the 250 mA charging current, with Rterm at 2.2K, the charge will terminate when the battery is full, but will almost immediately restart the charging. This will occur repeatedly. Is this due to the very low termination current of around 25 mA?" -- Bill J -- Would not be due to low termination current. After charge complete the RX will send End Power Transfer Charge Complete (EPT01) to TX to turn off. TX will turn on after some time and the RX should respond with a first packet EPT01. Sounds like something is not working quite right in this sequence. Can you take a scope capture of the RECT voltage when you see this problem?
  • 0 in reply to Bill Johns
    Prodigy 20 points

    Hi Bill,

    I ended up with 330 Ohms for both R1 and RFOD, and this looks good for the charging profile, just under 500 mA charging.

    As for the problem with termination, I accidentally put the BQ51051B instead of the BQ51050B, so the charger will try to charge the battery to 4.35 volts instead of 4.2 volts.  I suspect the battery protection circuity kicks in before we reach 4.35 V, and the current drops and we stop charging with a charge complete.  After a few minutes the protection circuity releases and since we are around 4.2 volts, the charger circuit kicks in and tries to recharge again.  I suspect that if I had the proper part with the 4.2 V limit, the charge would terminate before the protection circuit kicks in and would not recharge until the battery dropped to 4.065 Volts. 

    Thanks for your help.

    Garry