JAJSFH6D May 2018 – October 2022 TPS65987D
PRODUCTION DATA
The TPS65987D transmits and receives USB-PD data over one of the C_CCn pins for a given CC pin pair (one pair per USB Type-C port). The C_CCn pins are also used to determine the cable orientation (see the Section 8.3.4 section) and maintain cable/device attach detection. Thus, a DC bias exists on the C_CCn pins. The transmitter driver overdrives the C_CCn DC bias while transmitting, but returns to a Hi-Z state allowing the DC voltage to return to the C_CCn pin when not transmitting. Figure 8-5 shows the USB-PD BMC TX and RX driver 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.
The transmitter drives a digital signal onto the C_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.