In sections 7.2.3 and 7.2.4.1, allowable device actions are specified in terms of voltage droop at the hub. How do I design for this?
In the case of 7.2.4.1, the spec indicates the high power device’s configuration can be treated as another hot plug inrush. Thus, from a hub designer’s perspective, nothing needs to be done except to have the standard 120F of low ESR capacitance on the hub. For a device, up to an additional 10F can be hard started. In practice, however, very few high power devices need less than 20F to operate, so some sort of soft start circuit is almost always required to charge additional capacitance. In order to minimize the chance of a hub having to deal with a hot plug inrush and a high power device configuration inrush at the same time, the USB-IF strongly recommends this soft start circuit be designed such that the device’s current draw doesn’t exceed the MaxPower specified in the device’s configuration descriptor. For most designs, this requires nothing more than tweaking the value of the resistor that sets the gate voltage time constant of the soft start MOSFET. A similar recommendation exten
