TPD2E001: Data Line impedance "Rr" in application

Brian Wang0928

Part Number: TPD2E001

On the d/s of the TPD2E001 typ application diagram, there is a Rr line impedance on the data line. I'm wondering to know what is it?

Is this internal impedance of the USB controller? What's the purpose?

over 2 years ago

0 Matt Smith3 over 2 years ago

TI__Expert 8471 points

Hello Brian,

It actually says "R_T", which is most likely to symbolize the trace resistance on the board. Unfortunately, the original author of this datasheet is no longer at TI. While I do not know the original purpose of putting that in the datasheet, I would infer that it is to show the reader that there will be some type of isolating impedance between the TPD2E001 and the internal ESD cell on the USB controller. This would cause the TPD2E001 to conduct before the internal ESD cell during an ESD event.

Regards,

Matt Smith

0 Brian Wang0928 over 2 years ago in reply to Matt Smith3

TI__Genius 11640 points

Thanks. I'm wondering to know how to select the resistor value of the isolating impedance. In the real system, the TPD2E001 is for the F28384 I/O. So would you suggest isolating impedance in between? and how to choice the best resistor?

0 Matt Smith3 over 2 years ago in reply to Brian Wang0928

TI__Expert 8471 points

Hi Brian,

You can read about how to calculate your isolating impedance value starting on page 18 of this app note: https://www.ti.com/lit/an/slaa530b/slaa530b.pdf?ts=1631806992921&ref_url=https%253A%252F%252Fwww.ti.com%252Finterface%252Fcircuit-protection%252Fesd-surge-protection-ics%252Ftechnical-documents.html . To make these calculations, you will need to know the IC's internal ESD cell minimum turn on voltage on-resistance, and acceptable current as well as the external diode specs. Unfortunately, I'm having a hard time finding that info for the device you mentioned.

From what I've seen, most IC's don't really need an extra isolating resistor on their boards. That is mainly for modeling and simulation. Unless the IC's datasheet or EVM specifically calls out for an extra isolating impedance, you probably won't need it.

Regards,

Matt Smith

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TPD2E001: Data Line impedance "Rr" in application