USB Voltage Drop and Droop Measurement

The USB specification stipulates that hot plugging a peripheral does not adversely affect the functionality of other peripherals that are connected to USB. In order to meet this condition it is necessary to characterize the behavior of the power delivered to other USB peripherals connected to the host or hub that is undergoing hot plug.

This paper analyzes the factors that affect voltage drop and droop, including IR drops in the host, cabling and peripherals; hot plugging is analyzed with regard to its effect on voltage droop. Included is a review of the parameters defined in the USB specification, description of the measurement technique, test results, and an interpretation of those results. A set of guidelines is given to enable the peripheral designer to meet voltage drop and droop figures and explains possible tradeoffs that the designer has available.

This paper analyzes voltage drop and droop for USB hosts, self powered hubs and for bus powered peripherals. Subsequent references in this document use the term host to refer to either the host or to self powered hubs. The following table summarizes the voltage limits for USB hosts, peripherals, and hubs.

File Name/Size:: vdroop.pdf
187870 bytes
Download From:: Developers' Insight CD-ROM