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TPS65994QFNEVM: USB-C PD / DP Alt Mode Reference Design Question

Jacob Voss
Prodigy 110 points

Part Number: TPS65994QFNEVM
Other Parts Discussed in Thread: TPS65994

Hello,

I am looking for guidance on a possible TI reference design or evaluation-board approach that could help me prove out a USB-C PD / DisplayPort Alt Mode design concept.

I have a motherboard with a USB-C port that supports DisplayPort 2.1 output and USB 3.2 data. However, that port only sources 5 V at 1.5 A. When I connect a 15 W portable USB-C monitor directly to the motherboard using a single USB-C cable, the monitor only receives about 7.5 W, which limits the backlight to roughly half brightness.

What I would like to build is an intermediate board with:

  1. One upstream USB-C connection to the motherboard/host
  2. One downstream USB-C connection to the portable monitor/display
  3. An auxiliary DC power input, ideally accepting something in the 12 V to 20 V range

The goal is for the downstream USB-C port to act as a PD source to the monitor and negotiate a higher-voltage contract, preferably 20 V, using power supplied from the auxiliary input. At the same time, the board would need to pass or route the host’s DisplayPort Alt Mode video signals, and ideally USB 3.2 data as well, from the upstream USB-C connection to the downstream display connection.

In other words, the intended topology is:

Motherboard USB-C port → intermediate PD/Alt Mode board → portable USB-C monitor

The motherboard would provide the video/data source, while the auxiliary input would provide the power needed for the downstream USB-C port to supply the monitor at its full 15 W operating level. The desired end result is still a single USB-C cable from the intermediate board to the monitor, carrying both DisplayPort Alt Mode video and sufficient negotiated PD power.

Could you recommend a TI reference design, EVM combination, or device architecture that would support this? I am especially interested in whether a TPS65994-based design could be configured for this type of upstream USB-C host connection, downstream USB-C display connection, auxiliary power input, and downstream 20 V PD source negotiation while maintaining DP Alt Mode signal routing.

Thank you in advance for the help.

Best regards,
Jacob

  • 0
    TI__Mastermind 39030 points

    I have assigned this to a team member and he will respond to you shortly

  • 0 in reply to Tommy Lin
    Prodigy 110 points

    Hey  , do you have any update here?

  • 0 in reply to Jacob Voss
    TI__Intellectual 800 points

    Hi Jacob,

    Sorry on the delay here, this e2e accidentally slipped my radar. I'll give you an update in a few hours today.

    Thanks and Regards,
    Annchit

  • 0 in reply to Annchit Bhatnagar
    TI__Intellectual 800 points

    Hi Jacob, 

    "I have a motherboard with a USB-C port that supports DisplayPort 2.1 output and USB 3.2 data. However, that port only sources 5 V at 1.5 A."

    Does the PD controller have its PP5V rail at 5V? or is the device in sleep mode by any chance? These could lead to a 5V1.5A contract instead of the 5V3A contract. Another thing that can be checked here would be what the received sink Capabiilties message to PD Controller looks like.

    For the overall system requirement that you have explained here, there is a way to achieve this. The TPS65994 device treats the USB lines as a passthrough without altering any of its characteristics. If the need is to directly relay the same USB communication that is being received from the motherboard on port 1 to the monitor connected on port 2, you can look to externally route the USB lines coming in to the PD controller port1, on to the USB lines going out to the monitor, which will take care of the communication part.

    For Power, Does your system have an embedded controller? if so we can leverage the EC to control the power on the two ports. if that is not possible, then the only way would be to passthrough the VBUS lines which will only allow the power contract to be forwarded as is (which means that if the input contract is 20V3A, the same would get relayed to next port).
    If we have an EC, then it would help in allowing different contracts on each port (which means that if input contract is 20V3A, i can still decide to allow a 12V contract (or any PDO) on the other port.

    Thanks and Regards,
    Annchit

  • 0 in reply to Annchit Bhatnagar
    Prodigy 110 points

    Hi  ,

    Thank you for the detailed response. I want to clarify the system requirement and make sure I am thinking about the architecture correctly.

    The portable monitor supports USB PD 3.0 contracts. I have tested it at 5 V, 15 V, and 20 V. The issue is that when the monitor negotiates a 5 V contract, even at either 1.5 A or 3 A, the monitor firmware limits the display to roughly half brightness as a protection/power-management behavior. To allow full brightness control, the monitor needs to negotiate a voltage level other than 5 V, such as 15 V or 20 V.

    I understand that this likely means the intermediate board would need an auxiliary power input and would need to act as a PD Source to the monitor, rather than simply passing through the laptop’s VBUS.

    As additional background, I tested the laptop USB-C ports with a PD trigger, and those ports will only negotiate 5 V as a source. They do not appear to offer 9 V, 15 V, or 20 V source PDOs to external devices.

    My understanding is that the difficult part is the CC/PD architecture. The CC pins are used both for USB-C attach/orientation and PD negotiation, but they are also involved in discovering and entering DisplayPort Alt Mode. In my application, the laptop still needs to see a valid USB-C/DP Alt Mode sink so that it will enable DisplayPort Alt Mode output. At the same time, the monitor needs to see a separate PD Source that can advertise and provide 15 V or 20 V.

    My initial concept, which is not fully vetted, is as follows:

    The laptop-facing port on the intermediate board would connect to the laptop USB-C port. A PD controller, likely managed by an EC, would connect to the laptop-facing CC lines and present itself as a valid USB-C sink / DP Alt Mode sink so that the laptop enables DisplayPort Alt Mode output. The high-speed USB/DisplayPort lanes would then be routed through the intermediate board toward the monitor.

    On the monitor-facing port, the monitor’s CC lines would connect to a PD Source controller implemented on the intermediate board. That PD Source would advertise the desired PDOs, such as 15 V and/or 20 V, powered from the board’s auxiliary power input. Once the monitor negotiates that higher-voltage contract, the board would provide that voltage on the monitor-facing VBUS while also passing the DisplayPort Alt Mode signals from the laptop to the monitor.

    In simplified form, I am envisioning:

    Laptop USB-C port
    → intermediate board presents as USB-C / DP Alt Mode sink to laptop
    → DP Alt Mode high-speed lanes pass through board
    → intermediate board presents as PD Source to monitor
    → monitor receives DP Alt Mode video plus 15 V or 20 V VBUS from auxiliary power

    Does this architecture make sense for the TPS65994 family? More specifically, can the TPS65994 support this type of dual-port policy where one port negotiates with the laptop to enable DP Alt Mode while the other port independently negotiates a higher-voltage source contract with the monitor, assuming the actual high-speed DP/USB lines are routed externally and the auxiliary power rail is provided separately?

    Also, would this require an EC to manage the two-port policy, or is there a standalone configuration approach that could support this use case?