BQ51050B Application

I think so. I have designed but not tested my board yet with the intent of charging at 75 mA and terminating at 9.6 mA. I didn't mess with RFOD because that seems like a safety issue, but I suspect it may require adjustment. 

The values I came up with are:

• R12 RLIM 3.83K
• R13 RFOD 200
• R11 RTERM 3.09K

ILIM=300/R12-RFOD

R11=%ILIM * 240

If anyone wants to check my work , I'm all ears. 

James,

Yes, you can use the bq51050B to charge at 100mA or 200mA.  Termination current tolerance must be understood for proper operation.  Termination current is set as a percentage of the fast charge current.  As termination currents get lower, the accuracy becomes relatively loose.  For example, at a 10mA termination current,  you may expect to have 50% variation.  Tolerances of every component used to create this level must be accounted for as well.

Alternatively, using the bq51003 (2.5W optimized wireless power supply) and possibly the bq24040 or bq24232 stand-alone chargers.

Regards,

Dick

Mike,

I've reviewed your numbers and have a few comments.

I(ILIM) is calculated based on the resistance from the ILIM pin to ground.  This includes resistors R1 and RFOD (from figure 2 in the bq51050B datasheet).  If I understand your values correctly, that would be R12 + R13 (3.83k + 0.2k = 4.03k).  Calculating for I(ILIM) gives:

300/(4.03k) = 74mA.

Termination current would be:

%ILIM = R11 / 240 = 3.09k / 240 = 12.875% (Termination current = 74mA * 0.12875 = 9.66mA)

Please note the other response I have on this string related to low termination currents.

The (Foreign Object Detection) FOD circuit (includes RFOD and ROS) is essential for Qi certification.  Basically this circuit is used to identify what is "normal" power.  The bq51050B reports back to the transmitter how much power it is using.  The transmitter calculates how much it is sending and compares it to what is reported.  If the delta is too large, then something on the receiver pad is consuming the excess power (possible coins or a key).  The FOD circuit allows the bq51050B to be calibrated to take into account its own "excess power" to make sure that it does not report a value that is too low and terminate power transfer unnecessarily.

Regards,

Dick

Dick, 

Thanks for checking my values. It looks like you agree I got R1 and RFOD right. 

Your other reply about the Iterm "50% variation" is concerning though. I do not want to quit charging too early, nor charge indefinitely. What tolerances make this vary so widely? How can I minimize this variation?

Note that in my case, I expect to have very well controlled coupling, we will have positive mechanical orientation of the coils with good shielding. So my coupling efficiency should be high and consistent. 

Mike,

The internal comparators are optimized for higher currents.  The charge termination currents you suggest are quite a bit lower than where we have optimized the device.  Once the currents are as low as 10mA, then the inherent offset becomes more of a factor.  If you note the tolerance of the different constants and then factor in the tolerances of the resistors you use, you can understand and minimize the variation.  Once you have minimized the impact the circuitry has, you will still have the inherent offset of the device.

As noted in a previous post related to this string:  Alternatively, using the bq51003 (2.5W optimized wireless power supply) and possibly the bq24040 or bq24232 stand-alone chargers.

Regards,

Dick