TUSB522P: long PCB trace about 24inch application

Brian

The PCH design guide gives the trace length recommendation, max Length 0 is 13in. Total Length 1 and Length 2 are 3in.

Please see section 8.2.3 of TUSB522 datasheet for the measurement result.

Thanks
David

David,

The PCH design guide is just a reference while customer's design is much worse (~24inch PCB traces). From the section 8.2.3 of datasheet, it shows 25inch PCB trace with good performance. (Do we have more test results with different PCB trace length?)

So whether our TUSB522P can support customer's case? If yes, how to set Length 0/1/2 for customer's case?

Many thanks.

Brian Zhao

Brian

On the TX side, assume trace length of 24 inches with a 4 mil trace width. This would give us about 0.63 dB per inch of insertion loss at 2.5 GHz. This equates to approximately 15 dB of loss for the entire 24 inches of trace. An additional 1.5 dB of loss is added due to package of the Host, TUSB522P, and the A/C coupling capacitor. This brings the entire channel loss at 2.5 GHz to 15 dB + 1.5 dB = 16.5 dB. A typical USB 3.1 host/device will have around 3 dB of transmitter de-emphasis. Transmitter de- emphasis pre-compensates for the loss of the output channel. With 3 dB of de-emphasis, the total equalization required by the TUSB522P is in the 12.5 dB range. The maximum equalization TUSB522P supports is 9dB, which leaves you 3.5dB uncompensated, adds this to the total loss of channel after TUSB522P which needs to be handled by the device RX.

On the RX side, I have to assume 24in TX trace would have 24in RX trace plus Host/TUSB522P package and AC coupling capacitor, which is 16.5dB loss. The device plugged into receptacle will have either a short or long channel. USB3.2 standard defines 8.5dB for device, and 6.0dB for cable. So the total is 16.5+8.5+6 = 31dB. You can't set TUSB522P equalizer to max of 9dB, set to maximum would over-equalize the channel for the short channel case. Assume we set to 3dB equalizer, plus 6.2dB de-emphasis on the TX side, you still have 31-3-6.2 = 21.8dB which needs to be handled by the host RX.

So you have to see how much loss host/device TX (de-emphasis) and RX (equalizer) can handle. If customer wants to use 24in, you can also reduce the insertion loss by use wide trace width, better PCB material (above calculation assumes FR4 board material).

Thanks
David

David,

Thank you so much for your detail guide. I got customer's insertion loss data and did some calculation according to your guide. I sent it to you via email and can you help to check and reply?

Best Regard,

Brian Zhao