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TUSB521-Q1: Placement/Sequence of AC-coupling capacitor, R ESD and ESD Protection Diode

Markus Bölzle
Prodigy 85 points
Part Number: TUSB521-Q1
Other Parts Discussed in Thread: TUSB1004, TUSB1104, STRIKE, TUSB522P, TUSB521Q1-EVM, TUSB1002A

Tool/software:

Hello,

for implementing the TUSB521 I study the schematic on page 22 of the datasheet. See Figure 8-1.
I noticed that after the chip the first Part is the R ESD then the ESD diode and direct on the connector comes at final the AC-coupling capacitor. Is this really right?
In chapter 8.2.2.3 ESD Protection the datasheet says “Place the ESD component near the USB connector.”
In my opinion the AC-Coupling capacitor should be the first part after the TUSB521. Only than it is possible that the ESD diode is near es possible to the connector.
In other datasheets from TUSB522, TUSB1004, TUSB1104 and so on, is it in this way.
1. What is the right sequence of the parts and why?
2. Is the ESD series resistant for an USB-C design really necessary?

Thaks!
Markus

  • 0
    TI__Mastermind 36690 points

    Hi Markus,

    Markus Bölzle said:
    In chapter 8.2.2.3 ESD Protection the datasheet says “Place the ESD component near the USB connector.”
    Markus Bölzle said:
    1. What is the right sequence of the parts and why?

    For automotive designs, I believe we typically recommend placing the capacitor closer to the connector to help prevent VBUS/VBATT shorts to the data lanes. The concern of having VBUS short to the data lanes is more of an issue for automotive designs, so by implementing the capacitor like this, it should help reduce the chance of this being an issue.

    Alternatively, the capacitor can be placed AFTER the resistor to help lower the rate at which the signal rises/falls, giving the ESD diode more time to clamp the voltage in the case of an ESD strike, and prevent damage to the redriver. It should, however, always be ESD diode first, and then the ESD resistor. Whether the capacitor is before the diode or after the resistor depends on design decision.

    The TUSB522P highlights the typical application, and also mentions when the capacitor should be between the diode and the receptacle/connector:

    Markus Bölzle said:
    2. Is the ESD series resistant for an USB-C design really necessary?

    The ESD series resistor helps to create a sort-of RC circuit in the system, all of which helps to slow down how fast the signal rises and falls, giving the ESD diode more time to clamp the voltage in the case of a high-voltage event.

    Please let me know if there are any other questions.

    Thanks,
    Ryan

  • 0
    Prodigy 85 points

    Hello Ryan,

    thanks for your fast detailed answer.

    Ok then this design with the capacitor direct on the connector is special for automotive, right? Is it not difficult to find a 0402 220nF capacitor with a voltage rating for an ESD-pulse?

    For a normal USB-C design (not automotive) the design like the TUSB522p is better?

    Thanks Markus

  • 0 in reply to Markus Bölzle
    TI__Mastermind 36690 points

    Hi Markus,

    Markus Bölzle said:
    Ok then this design with the capacitor direct on the connector is special for automotive, right? Is it not difficult to find a 0402 220nF capacitor with a voltage rating for an ESD-pulse?

    In the cases with these devices, its typically not too difficult to find a capacitor that meets these requirements. However, we typically recommend an 0201 size over an 0402 size. The TUSB521Q1-EVM BOM has an example of a 220nF capacitor that should work.

    Markus Bölzle said:
    For a normal USB-C design (not automotive) the design like the TUSB522p is better?

    For non-automotive, If you are looking for only the redriver application and not the mux application, I believe the TUSB522P is a good choice. We also have redrivers like the TUSB1002A that are dual-channel linear redrivers.

    Please let me know if you have any other questions.

    Thanks,

    Ryan