TPS25751D datasheet(28/89 Pages) TI1

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8.3.1.4 USB-PD BMC Transmitter

The TPS25751 transmits and receives USB-PD data over one of the CCy pins for a given CC pin pair (one

pair per USB Type-C port). The CCy pins are also used to determine the cable orientation and maintain the

cable/device attach detection. Thus, a DC bias exists on the CCy pins. The transmitter driver overdrives the CCy

DC bias while transmitting, but returns to a Hi-Z state, allowing the DC voltage to return to the CCy pin when it is

not transmitting. While either CC1 or CC2 can 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 CC1 and

CC2. Figure 8-7 shows the USB-PD BMC TX and RX driver block diagram.

Digitally

Adjustable

VREF (VRXHI, VRXLO)

Level Shifter

Driver

PD_TX

PD_RX

LDO_3V3

CC2

CC1

USB-PD Modem

Figure 8-7. USB-PD BMC TX/Rx Block Diagram

Figure 8-8 shows the transmission of the BMC data on top of the DC bias. Note that the DC bias can be

anywhere between the minimum and maximum threshold for detecting a Sink attach. This note means that the

DC bias can be above or below the VOH of the transmitter driver.

DC Bias

VOH

VOL

DC Bias

VOH

VOL

Figure 8-8. TX Driver Transmission with DC Bias

The transmitter drives a digital signal onto the CCy lines. The signal peak, VTXHI, 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 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.

TPS25751

SLVSH93 – OCTOBER 2023

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