TUSB8020B behavior when not powered

SimonF

Other Parts Discussed in Thread: TUSB8020B, TIDA-00287

Hi Team,

As the TUSB8020B can be used in a self-powered hub configuration I expect that, if it is not powered but is plugged into a powered hosts USB cable, the USB signals (SSTX, SSRX, DP and DM) will not draw current from the cable. Can you confirm that?

Best Regards,

over 8 years ago

0 Elias Villegas M. over 8 years ago

TI__Guru* 76130 points

Hello,
The TUSB8020B only supports self-power configuration, when there is no self-power the hub will not draw current from vbus.
Regards

0 SimonF over 8 years ago in reply to Elias Villegas M.

TI__Expert 3370 points

Hi Elias,

I got the following feedback:
"I want to know that it will also not draw current from the signal traces. Basically, what my concern is, is that typically CMOS inputs have protection diodes (or perhaps just parasitic diodes) that clamp the inputs to the power rail. When the power rail isn’t being supplied current from the inputs flows through these diodes and attempts to power the supply rail. I believe that USB hubs are required to ensure isolation so that this doesn’t occur but I would like to verify that this true for the TUSB8020B."

Best Regards,
SF

0 Elias Villegas M. over 8 years ago in reply to SimonF

TI__Guru* 76130 points

Hello,
Are you talking about the USB2.0 or the USB3.0 lines?
Regards

0 SimonF over 8 years ago in reply to Elias Villegas M.

TI__Expert 3370 points

Hi Elias,

the question is about the USB 2.0 and the USB3.0 data lines.

Best Regards,

0 SimonF over 8 years ago in reply to SimonF

TI__Expert 3370 points

Hi Elias,
any update?
Best Regards,
SF

0 Diego Cortes over 8 years ago in reply to SimonF

TI__Mastermind 36085 points

Hello SF,

The USB interfaces are not CMOS implementation but differential, each pair needs to be polarized in order to provide/draw any current; therefore when there is no power supply these differential pairs are deactivated and can be seen as High impedance nodes.

Regards,
Diego.

0 Alex Arango over 8 years ago in reply to Elias Villegas M.

Prodigy 240 points

Hi Elias. If it only supports self-power configuration, how is the TIDA-00287 reference design expected to work?

0 Elias Villegas M. over 8 years ago in reply to Alex Arango

TI__Guru* 76130 points

It can work in bus-power mode on a closed system where the current budget is met.
Regards

0 Alex Arango over 8 years ago in reply to Elias Villegas M.

Prodigy 240 points

Copy that. Thanks.

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TUSB8020B behavior when not powered