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TUSB321: DFP integration with VBUS switch

Stephen Leahey
Prodigy 40 points
Part Number: TUSB321
Other Parts Discussed in Thread: HD3SS3212

Hi, 

We are implementing a DFP solution using the TUSB321 and HD3SS3212.

We require a 900mA output, and as per the USB-C Specification, Table4-11, P.136, VBUS should not be powered until after a device is attached.

We need the power on and off of the VBUS output to be autonomous, using the ID output of the TUSB321 to enable an VBUS switch component.

The datasheet shows on page 13 for the typical application that the power switch is driven from a micro, which might be suitable for some applications, but not in our case.

It isn't clear from reading the datasheet that the TUSB321 will only assert ID as low when a valid USB Type-C connection is detected (i.e., unless a SINK is connected, ID should stay high-Z).

The details on how ID works from the datasheet is,

Section 5: Open drain output; asserted low when the CC pins detect device attachment when port is a source (DFP), or dual-role (DRP) acting as source (DFP).

Section 7.4.2: When in active mode, the TUSB321 device communicates to the AP that the USB port is attached. This happens through the ID pin if TUSB321 is configured as a DFP or DRP connect as source.

This reads like ID is asserted low when ANY device is connected..

It is very unclear. Could someone please confirm the behavior of ID.

  • 0
    TI__Genius 16150 points
    Hi Stephen,

    In the application of datasheet, a VBUS switch was placed. Basically the switch is not required, just make sure that VBUS_DET pin is connected to VBUS signal through a 900k resistor. Thus, ID pin is an open drain output, a pull-up resistor (that is 200k) must be installed for proper operation of the device.

    Best Regards,
    Luis Omar Moran
    High Speed Interface]
    SWAT Team
  • 0 in reply to Luis Omar Moran
    Prodigy 40 points

    Hi, 

    Sorry, I am not sure if I understand your response. please could you clarify for me what you mean?

    Here is an example of why a switch is needed,

    Lets say I leave out the switch, (and put a 900K pull up between VBUS_DET and VBUS..), then if I connect two of my DFP devices together to get a failure mode ( USB Type-C <--> USB Type-C is possible), they will back-power each others bus and cause damage.

    Please explain in more detail how the ID pin can be used with an external switch.

    If it cannot be used this way, we will investigate using another part.

    Perhaps a microchip UTC2000, which seems to have the correct functionality.

    Thanks,

    Stephen Leahey

  • 0 in reply to Luis Omar Moran
    Prodigy 40 points
    it is still unclear
  • 0 in reply to Stephen Leahey
    TI__Genius 16150 points

    Stephen,

    The case that you mentioned DFP to DFP is not possible if both devices are compliant. According to the USB Type-C specification, the interoperability between Source-only <==> Source only is Non-functional (please refer to Table 4-9 of spec, page 133). As well, the ID pin is enabled until an attachment is performed (Rd detected), then a sensing for CC detach must be performed (refer to Table 4-11 for source behavior, page 136).

    Otherwise, a P-ch FET is an easy solution to replace the VBUS switch. Just keep on mind that the ID pin is an open drain output and it needs a pull-up resistor.

    Regards,

    Luis

  • 0 in reply to Luis Omar Moran
    Prodigy 40 points

    Hi Luis, 

    P-FET or some IC to switch.. same thing.

    In your previous comment you say a switch is not required, and in this comment you say to use a switch?

    If I connect  (MY DEVICE) Source-only <==> Source only (LEGACY USB2.0 Type A) with TUSB321 connected with CC pins connected to type C connector, VBUS_DET pin connected to VBUS via 900K, ID output pullup to VDD with 200K, does ID stay high?

  • 0 in reply to Stephen Leahey
    TI__Genius 16150 points

    Your device MUST HAVE VBUS OFF before CC detection per Type-C spec. Our device enables ID if and only if the detection is achieved. Despite ID pin is forced, the device manages this output according to the specification. If your device enables VBUS before CC detection, is wrong. Please take a look into these diagrams:

    Regards,

    Luis