Current limit a bus-powered usb 3.1 hub

Question

I'm currently designing a bus-powered usb 3.1 hub. It will use a USB-C for UFP and 3 USB-A downstream. The hub will only be acting as a sink.

As of my understanding, since it may be connected to USB 1 (low speed), I need to limit the possible current drain to 100mA before negotiations, and only after negotiations, set the maximum current drain that can be provided upstream?

I have been looking at two reference designs from TI that specifically states "USB 3.0 compliant hub". Is that really correct?

• TIDA-00287: http://www.ti.com/tool/TIDA-00287
• TIDA-00288: http://www.ti.com/tool/TIDA-00288

Neither of these reference designs have implemented current limiting upstream, only downstream.

I have been looking for IC's that specifically manage current limiting upstream, but so far I have only found down-stream limiting.

Do I need to limit the hub's power drain?

Do anyone know of a chips that manage this or reference design which have implemented power limiting upstream based on the current negotiation?

Thanks!

Answer 1

There are several things to consider if you want to design a USB-IF compliant hub.

1. A bus-powered USB hub must have means to disable power to downstream ports, see Section 7.2.1.1 of USB 2.0 Specifications, which says: "Power to external downstream facing ports of a bus-powered hub must be switched."

2. The 100 mA unconfigured limit is usually maintained by controller IC itself, otherwise the silicon won't get certified. Unconfigured hub IC won't engage downstream power, so no-one else will draw it from UFP but IC itself, and this is managed by IC.

So no, you don't need to "limit the hub's power drain" by any special means, but you must use high-side power switches on all DFPs to be compliant.