ZHCSM89 july 2021 BQ51013B-Q1
PRODUCTION DATA
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 coils. Essentially, the switching in-and-out of the communication capacitor or resistor adds a transient load to the RX coil in order 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 device 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 9-5.
IOUT | COMMUNICATION CURRENT LIMIT |
---|---|
< 300 mA | Fixed 400 mA |
> 300 mA | IOUT + 50 mA |
The first mode is illustrated in Figure 8-18. In this plot, an output load pulse of 300 mA is periodically introduced on a DC current level of 200 mA. Therefore, the 400 mA current limit is enabled. The pulses on VRECT indicate that a communication packet event is occurring. When the output load pulse occurs, the regulator limits the pulse to a constant 400 mA and, therefore, preserves communication. Note that VOUT drops to 4.5 V instead of GND. A charger device with an input voltage regulation set to 4.5 V allows this to occur by offloading the load transient support to the mobile device’s battery.
The second mode is illustrated in Figure 8-19. In this plot, an output pulse of 200 mA is periodically introduced on a DC current level of 400 mA. Therefore, the tracking current mode (IOUT + 50 mA) is enabled. In this mode, the BQ51013B-Q1 measures the active output current and sets the regulator's current limit 50 mA above this measurement. When the load pulse occurs during a communication packet event, the output current is regulated to 450 mA. As the communication packet event has finished the output load is allowed to increase. Note that during the time the regulator is in current limit VOUT is reduced to 4.5 V and 5 V when not in current limit.