ADP Device requirement

USB 2.0 Specification, Section 7.2.4.2 

The USB 2.0 specification have an ECN that require device to have a capacitance of at least 1�F in order for host with ADP feature to detect the device.

ADP detection is described in the USB 2.0 OTG & Embedded Host specification.

This ECN ensures that all USB peripherals have a detectable change in capacitance on VBus when they are attached. This ensures that OTG devices and Embedded Hosts can detect attachment of peripherals in the absence of VBus. A USB device is required to expose a capacitance on the VBUS pin of its connector of at least 1�F. Make also not sure the capacitance exceed 10�F otherwise the product will fail the inrush test.

Beware that many self-powered device not meet this requirement. So make sure that you have at least 1�F capacitance over Vbus and GND when going for USB-IF compliance.