The TPS65988DK transmits and receives USB-PD data over one of the Cx_CCn pins for a given CC pin pair (one pair per USB Type-C port). The Cx_CCn pins are also used to determine the cable orientation (see Port Power Switches) and maintain the cable/device attach detection. Thus, a DC bias exists on the Cx_CCn pins. The transmitter driver overdrives the Cx_CCn DC bias while transmitting, but returns to a Hi-Z state allowing the DC voltage to return to the Cx_CCn pin when not transmitting. Figure 8-5 shows the USB-PD BMC TX and RX driver block diagram.

Figure 8-5 USB-PD BMC TX/Rx Block Diagram

Figure 8-6 shows the transmission of the BMC data on top of the DC bias. Note, The DC bias can be anywhere between the minimum threshold for detecting a UFP attach (VD_CCH_USB) and the maximum threshold for detecting a UFP attach to a DFP (VD_CCH_3P0). This means that the DC bias can be below VOH of the transmitter driver or above VOH.

Figure 8-6 TX Driver Transmission with DC Bias

The transmitter drives a digital signal onto the Cx_CCn lines. The signal peak, VTXP, is set to meet the TX masks defined in the USB-PD Specifications.

When driving the line, the transmitter driver has an output impedance of ZDRIVER. ZDRIVER is determined by the driver resistance and the shunt capacitance of the source and is frequency dependent. ZDRIVER impacts the noise ingression in the cable.

Figure 8-7 shows the simplified circuit determining ZDRIVER. It is specified such that noise at the receiver is bounded.

Figure 8-7 ZDRIVER Circuit