SLUSBC8C December 2013 – July 2018
The Qi communication channel is established through backscatter modulation as described in the previous sections. This type of modulation takes advantage of the loosely coupled inductor relationship between the Rx and Tx coil. Essentially the switching in-and-out of the communication capacitor or resistor adds a transient load to the Rx coil to modulate the Tx coil voltage and current waveform (amplitude modulation). The consequence of this technique is that a load transient (load current noise) from the mobile device has the same signature. To provide noise immunity to the communication channel, the output load transients must be isolated from the Rx coil. The proprietary feature Adaptive Communication Limit achieves this by dynamically adjusting the current limit of the regulator. When the regulator is put in current limit, any load transients will be offloaded to the battery in the system.
Note that this requires the battery charger IC to have input voltage regulation (weak adapter mode). The output of the Rx appears as a weak supply if a transient occurs above the current limit of the regulator.
The Adaptive Communication Limit feature has two current limit modes and is detailed in Table 6:
|IOUT||COMMUNICATION CURRENT LIMIT|
|< 300 mA||Fixed 400 mA|
|> 300 mA||IOUT + 50 mA|