HD3SS3220: Power management forum

Charles Lai

Part Number: HD3SS3220

Dear experts:

I have read SLLU237A.pdf - HD3SS3220 DFP Dongle Evaluation Module. In Figure 3. Capacitor Placement, 0.47uF (470nF) AC coupling capacitors are selected in the schematics. According to USB 3.2 V1.2 specification (the latest version), the AC decoupling capacitors on RX side should be between 297nF and 363nF (please see Table 6-22 Receiver Normative Electrical Parameters).

This issue also can be found in the EVB boards of other vendors sometimes. ITE USB switch IT5205 even uses 0.68uF in the reference schematics.

I am trying to get the answer of this issue.

By the way, I found an ECN (20131608-ECN) on TI E2E forum by accident (the link information shows, it's from TI E2E forum). But it seems not be a formal ECN from USB-IF.

google.com.tw/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjn-Njpp9aCAxVIQ94KHZGSCawQFnoECAkQAQ&url=https%3A%2F%2Fe2e.ti.com%2Fcfs-file%2F__key%2Fcommunityserver-discussions-components-files%2F138%2FUSB-3.1-ECN-Rx-AC-Coupling-Capacitor-Option.pdf&usg=AOvVaw3CGzxn4HkEKy5tDLaeb49m&opi=89978449

Best regards.

Charles Lai

9 months ago

0 David (ASIC) Liu 9 months ago

TI__Guru**** 153401 points

Charles

Two implementation options on the RX pins:

1. DC couple RX pins to USB connector, no 330nF capacitors

2. Follow the USB-IF ECN and have 330nF capacitors on the RX pins to USB connector

Thanks

David

0 Charles Lai 9 months ago in reply to David (ASIC) Liu

Prodigy 50 points

Dear David:

Sorry! I do not quite understand your answers. As I know, USB 2.0 should be DC coupling. But HD3SS3220 is an USB 3.2 SuperSpeed mux. AC coupling capacitors is necessary, though AC coupling capacitors are optional on RX side.

If we take all capacitors (TX side and RX side) into consideration, the total AC coupling capacitance still can meet 75nF to 265nF (mentioned in the ECN and would not affect RX detection).

100nF (TX) + 330nF (RX) in series = 76.74nF

100nF (TX) + 470nF (RX) in series = 82.46nF

470nF RX side AC coupling capacitors are just out of range (specified by USB-IF standard 297nF to 363nF).

That’s what I wonder about.

Your answers are appreciatted!

Charles Lai

0 Shane Hauser 9 months ago in reply to Charles Lai

TI__Genius 16180 points

Hi Charles,

As you said, the USB spec allows for the total capacitance on a data line to be within 75 - 265nF, a good explanation for this can be found in this E2E post.

When taking into account that the 3220 EVM is between a USB host and device, you would need to add all capacitors on the data line. Here is an example of how that could look.

The total capacitance of the data line should be within 75 - 265nF, so having a higher capacitance near the MUX gives more margin for the TX/RX capacitors near the USB host and device.

In this example, the total capacitance on both TX and RX lines would be 103nF, as you are adding 220nF, 470nF, and 330nF.

Best,

Shane

0 Charles Lai 9 months ago in reply to Shane Hauser

Prodigy 50 points

Dear Shane:

I just wonder. Why the recommended values do not meet USB 3.2 standard?

I also got some feedback from other vendors. These values are selected by the experience of each vendor. (A balance between RX detection and Vcm DC Blocking)

As system designers, it's difficult to us. Sometimes you must follow the standard. Sometimes the standard is just for reference only.

The last question:

Has the ECN (20131608-ECN) been approved by USB-IF, or it is just a proposal from TI?

I can not find it on USB-IF.

google.com.tw/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjp8-ORoduCAxUZ_GEKHUcECzQQFnoECAgQAQ&url=https%3A%2F%2Fe2e.ti.com%2Fcfs-file%2F__key%2Fcommunityserver-discussions-components-files%2F138%2FUSB-3.1-ECN-Rx-AC-Coupling-Capacitor-Option.pdf&usg=AOvVaw3CGzxn4HkEKy5tDLaeb49m&opi=89978449

I will close this issue after the ECN question.

Thanks!

Charles Lai

0 David (ASIC) Liu 9 months ago in reply to Charles Lai

TI__Guru**** 153401 points

Charles

The ECN came from the USB-IF. It was part of the USB3.1 Specification and then included into the USB3.2 Specification.

Thanks

David

0 Charles Lai 9 months ago in reply to David (ASIC) Liu

Prodigy 50 points

Dear David and Shane:

Thank you for your replying. I do not have questions anymore.

Charles Lai