8.3.10 Communication Modulator

The bq51003 provides two identical, integrated communication FETs which are connected to the pins COMM1 and COMM2. These FETs are used for modulating the secondary load current which allows bq51003 to communicate error control and configuration information to the transmitter. Figure 26 shows how the COMM pins can be used for resistive load modulation. Each COMM pin can handle at most a 24-Ω communication resistor. Therefore, if a COMM resistor between 12 Ω and 24 Ω is required COMM1 and COMM2 pins must be connected in parallel. The bq51003 does not support a COMM resistor less than 12 Ω.

Figure 26. Resistive Load Modulation

In addition to resistive load modulation, the bq51003 is also capable of capacitive load modulation as shown in Figure 27. In this case, a capacitor is connected from COMM1 to AC1 and from COMM2 to AC2. When the COMM switches are closed there is effectively a 22-nF capacitor connected between AC1 and AC2. Connecting a capacitor in between AC1 and AC2 modulates the impedance seen by the coil, which will be reflected in the primary as a change in current.

Figure 27. Capacitive Load Modulation