The TPS25772-Q1 transmits and receives USB-PD data over one of the Px_CCy pins for a given CC pin pair (one pair per USB Type-C port). The Px_CCy pins are also used to determine the cable orientation (see the Cable Plug and Orientation Detection section) and maintain cable/device attach detection. Thus, a DC bias exists on the Px_CCy pins. The transmitter driver overdrives the Px_CCy DC bias while transmitting, but returns to a Hi-Z state allowing the DC voltage to return to the Px_CCy pin when not transmitting. While either Px_CC1 or Px_CC2 may be used for transmitting and receiving, during a given connection only the one that mates with the CC pin of the plug is used; so there is no dynamic switching between Px_CC1 and Px_CC2. Figure 9-22 shows the USB-PD BMC TX and RX driver block diagram.

Figure 9-22 USB-PD BMC TX/Rx Block Diagram

Figure 9-23 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 and the maximum threshold for detecting a Sink attach to a Source. This means that the DC bias can be above or below the V_OH of the transmitter driver.

Figure 9-23 TX Driver Transmission with DC Bias

The transmitter drives a digital signal onto the Px_CCy lines. The signal peak, V_TXHI, is set to meet the TX masks defined in the USB-PD Specifications. Note that the TX mask is measured at the far-end of the cable.

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

Figure 9-24 ZDRIVER Circuit