Datasheet

bq51013B

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SLUSB62A –MARCH 2013–REVISED OCTOBER 2013

Figure 36. Capacitive Load Modulation

Adaptive Communication Limit

The Qi communication channel is established via 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 in order to modulate the Tx coil voltage/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. In order 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 the table below:

Table 5.

OUT

Communication Current Limit

< 300 mA Fixed 400 mA

> 300 mA I

OUT

+ 50 mA

The first mode is illustrated in Figure 23. 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 V

RECT

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 V

OUT

drops to 4.5 V instead of GND. A

charger IC 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 24. 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 (I

OUT

+ 50 mA) is enabled. In this mode the

bq5101xB measures the active output current and sets the regulators 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 V

OUT

is reduced to 4.5 V and 5 V when not in current limit.

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