BQ51013B qi receiver

[mrmp17]

Hi!

I'm putting together qi wireless charging receiver based on TI's BQ51013B. Everythink seems straight forward except connecting ILIM and FOD pins. In pin functions section in the datasheet it's written that you should connect ILIM to VSS (voltage out right?) via calculated resistor and connect FOD pin to ground via 188ohm resistor. But in typical aplication shematics those pins are connected together with resistors and they also connect to RECT and OUT via two resistors (see shematics in datasheet). How do I know how to connect them and what are the resistor values?


Datasheet: http://www.ti.com/lit/ds/symlink/bq51013b.pdf

[PA0PBZ]

Yes, it looks very confusing.

First: VSS is normally used for ground or minus, but in the schematics it is connected to Vout.
The ILIM pin is, as the name suggests, the current limiter and the datasheet tells you how to calculate the resistor:

Programming pin for the over current limit. Connect external resistor to VSS. Size RILIM with the
following equation: RILIM = 250 / IMAX where IMAX is the expected maximum output current of the
wireless power supply.

Not sure what to believe here, because they say to connect it to VSS?
The FOD (Foreign Object Detect) pin seems happy with a 188 Ohm resistor to ground,
But then you have to use either Ros1 or Ros2, and the value is a mystery to me. Here is what a TI Employee has to say about it:

(http://e2e.ti.com/support/power_management/wireless_power/f/693/t/275054.aspx)

The ROS1/ROS2 resistors are used as part of the Foreign Object Detection (FOD) scheme, along with RFOD.  These resistors are used as part of a calculation to estimate the power level to be sent to the transmitter.

The ROS1 resistor would be tied between OUT and FOD.  Or, the ROS2 would be tied between RECT and FOD.

Using ROS1 on OUT gives a more stable voltage source than RECT, but then there would always be a resistive load on the OUT pin (on the battery).  The RECT pin changes voltage levels based on the load.  The key to this is that the FOD concern is during higher power levels when RECT is much more stable (close to OUT voltage).  Most systems use the resistor between RECT and FOD.


That doesn't really help. I also wonder what that does to the value of Ri (the ILIM resistor) because it is in series with either Ros1 or Ros2.

Here is another discussion, this time even with a schematic with values:

http://e2e.ti.com/support/power_management/wireless_power/f/693/t/282054.aspx

Sorry if this didn't help, it is all I could find at the moment.

[mrmp17]

Thanks a lot!
I guess I'll just try it out and see what works best. Looking at evaluation module user guide, resistor from ILIM to OUT is open and ILIM to RECT is fitted with 20K resistor. But then I have no idea what to place between ILIM and FOD. On evaluation module it's selectable - 5k pot or 110ohm resistor (but in BOM it's marked as 75ohm).