• State Resolved
  • Locked Locked
  • Replies 13 replies
  • Answers 1 answer
  • Subscribers 62 subscribers
  • Views 426 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS25740B: Operating USB PD 2.0 in legacy mode for a pass-through adapter

Perry Watson
Prodigy 30 points
Part Number: TPS25740B
Other Parts Discussed in Thread: TPS65988, TPS55288

My company offers a USB-C charging port for use in airplanes that uses the TPS25740B USB Type-C PD source controller.

We are attempting to design a right-angle adapter that would pass the USB-C connection through, while simultaneously illuminating an LED when not in use.

I have experimented with operating TPS25740B in "legacy mode" by simply attaching a 5.6k ohm resistor from one of the CC lines to ground, effectively forcing the TPS25740B to provide a default 5V on the VBUS pin, which we use to illuminate an LED.

While in this mode, I have also hooked up a PassMark PM125 programmable USB load. I have found that the addition of the 5.6k ohm resistor does not appear to impede the TPS25740B's ability to communicate with the programmable load or receive requests for voltage changes.

However, we want to be sure this will work. Can any of you provide any documentation or assurance that operating the TPS25740B in this manner will be reliable long term and will not interfere with the charging of external devices? If we are going to implement this solution, we must have some clear evidence in the form of documentation.

Any resources related to this topic would be welcome. Thank you.

  • 0
    TI__Mastermind 20985 points

    Hi Perry,

    May I know the reason for choosing TPS25740B? This device is listed not recommended for new design on TI.com.

    Regards

  • 0 in reply to Tommy Lin
    Prodigy 30 points

    The project is to make an external adapter to a legacy product. The TPS25740B was designed into this product, presumably, before it was listed as not for new designs. So this is the hardware we have to work with.

  • 0 in reply to Perry Watson
    TI__Mastermind 20985 points

    Hi Perry,

    Understandable.

    I am not understanding why you place a 5.6k ohm pull down resistor on the CC line when TPS25740B (PD source controller) already have internal pull up for source capability advertisement. In Type-C, source side should only have pull up resistor (Rp) and sink side should only have pull down (Rd). You don't need any addition resistors on CC line to make this a source device, this is already a source device.

    Regards

  • 0 in reply to Tommy Lin
    Prodigy 30 points

    The idea is that by putting a pull down resistor in this right-angle adapter we're designing, we can basically force the TPS25740B to output voltage even when an external device is not connected. We want the TPS25740B to be providing power to an indicator LED in the adapter even when not in use. So the adapter is effectively another sink device itself, and the pull-down is its Rd. Our hope is that this won't interfere with the normal operation of the USB PD protocol once an external device is plugged in to the adapter.

  • 0 in reply to Perry Watson
    TI__Mastermind 20985 points

    Hi Perry,

    What you are doing, force the TPS25740B to output voltage even when an external device is not connected, is a violation of the Type-C/PD spec. In a Type-C port, VBUS cannot be hot when nothing is connected. Please also note that this device will not pass the latest PD compliance.

    You can run the LED off the /ENSRC pin if you plan to use LED to indicate the port is not used (no sink attached)

    Regards.

  • 0 in reply to Tommy Lin
    Prodigy 30 points

    Yes, I thought that might be the case. I was thinking that one way we could get around that would be to have a MOSFET in the adapter in series with the VBUS line, and only switch it on once a device is connected. I was thinking we could monitor the voltage on one of the CC pins to switch the MOSFET on/off when an external device is connected. Do you think that could work? And if we did get it to work, would it be compliant with the USB PD specification?

  • 0 in reply to Tommy Lin
    Prodigy 30 points

    I should also add that we can't run an LED off of the ENSRC pin because the purpose of this project is to create a right-angle USB adapter with a built-in LED, that would enable us to pass-through the connection to the existing charging device without having to modify the charging device in any way.

  • 0 in reply to Perry Watson
    TI__Mastermind 20985 points

    Hi Perry,

    Perry Watson said:
    And if we did get it to work, would it be compliant with the USB PD specification?

    Even if you get this to work this device will still not pass compliance.

    Perry Watson said:
    I should also add that we can't run an LED off of the ENSRC pin because the purpose of this project is to create a right-angle USB adapter with a built-in LED, that would enable us to pass-through the connection to the existing charging device without having to modify the charging device in any way.

    I am not understanding this reason why /ENSRC is no usable here. Is this pin used for something else in the original design therefore not able to run a LED? Can you provide a block diagram of this right-angle USB adapter?

    Regards

  • 0 in reply to Tommy Lin
    Prodigy 30 points

    Concept Block Diagram.pdf

    Here is a block diagram showing the concept of the right-angle adapter project. The charging device which contains the TPS25740B is a legacy product that cannot be changed. The right-angle adapter needs to plug into it, draw some power from it in order to light an indicator LED, and pass power through to an external device to be charged. We can't use the ENSRC pin because we can't modify the existing design, and the LED needs to be on the adapter anyway, not the old device.

  • 0 in reply to Tommy Lin
    Prodigy 30 points
    Tommy Lin said:

    Hi Perry,

    Perry Watson said:
    And if we did get it to work, would it be compliant with the USB PD specification?

    Even if you get this to work this device will still not pass compliance.

    Tommy,

    Thanks for all your help so far. One last thing: could you please point me to the specific section of the USB PD specification that says we can't get this to work? It would be very helpful to convince my boss that this can't be done if I had it in writing.

  • +1 in reply to Perry Watson
    TI__Mastermind 20985 points

    Hi Perry,

    The device (TPS25740B) itself will not pass compliance now as this was designed based on PD2.0. However, USB-IF EOL all PD2.0 certification on Jan. 2019 (https://compliance.usb.org/index.asp?UpdateFile=PowerDelivery&Format=Standard).

    For the right angle adapter, the proposal in the block diagram will not work well

    Perry Watson said:
    I was thinking that one way we could get around that would be to have a MOSFET in the adapter in series with the VBUS line, and only switch it on once a device is connected. I was thinking we could monitor the voltage on one of the CC pins to switch the MOSFET on/off when an external device is connected.

    There are 3 major issues here that will violate Type-C/PD spec.

    First is the time required for the VBUS to come up once the contract is established. The MOSFET turn on time might exceed this timer.

    Second is the IR drop. With this right-angle adapter you consider the voltage drop due to connector to connector impedance (one side), PCB impedance, connector to connector impedance (the other side), and cable IR drop. This may reduce the voltage below a safe threshold and cause the far end device to disconnect.

    Third is the PD communication on CC line. The source side have a set of fixed pull up Rp and sink side have a fixed pull down Rd. The CC line voltage is essentially a voltage divider (Rd Rp divider) output and is expected to be in a specific range. The added resistor in the adapter block diagram will impact PD communication on CC line because you introduced extra resistance that will move the voltage out of the range.

    One possible solution is to use a dual port PD controller + a DC/DC converter in the right-angle adapter. You can use one port to sink high power from the existing charger and the other port to source power to far end device.

    Regards

  • 0 in reply to Tommy Lin
    Prodigy 30 points

    Tommy,

    Thanks for your help. My boss is convinced that this approach isn't going to work, and has asked me to investigate the dual port PD controller idea. Can you recommend an IC that would serve that function, please?

  • 0 in reply to Perry Watson
    TI__Mastermind 20985 points

    Hi Perry,

    The dual port PD controller will be TPS65988 and the DC-DC converter will be TPS55288.

    Regards.