SLVSD02D March 2015 – June 2019 TPS65982
The TPS65982 transmits and receives USB-PD data over one of the C_CCn pins. The C_CCn pin is 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 will exist on the C_CCn. The transmitter driver will overdrive the C_CCn DC bias while transmitting, but will return to a Hi-Z state allowing the DC voltage to return to the C_CCn pin when not transmitting. Figure 14 shows the USB-PD BMC TX/Rx driver block diagram.
Figure 15 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) defined in the Cable Plug and Orientation Detection section. 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 adjustable by application code and sets the VOH/VOL for the BMC data that is transmitted, and is defined in USB-PD TX Driver Voltage Adjustment Parameter. Keep in mind that the settings in a final system must 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 16 shows the simplified circuit determining ZDRIVER. It is specified such that noise at the receiver is bounded.
ZDRVER is defined by Equation 1.