Definitions

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TPS202x/3x and TPS204x/5x USB Power Distribution

Definitions

Bus-powered hub – Bus-powered hubs (BPH) obtain all power from

upstream ports and often contain a nonremovable function. One typical BPH

application is a keyboard. The total current drawn by a BPH is the sum of the

hub controller current, nonremovable function, and downstream port and is

no greater than 500 mA or 5 unit loads. Each port must distribute 1 unit load.

Current limit – USB specification requires SPHs to limit current to

downstream ports and report over-current conditions to the controller.

Downstream port – A port that is electrically further from the host and

generates downstream data traffic from the hub.

Dynamic attach and detach – To attach or remove devices while host is in

operation (hot-plug, unplug event).

High-power, bus-powered function – All power to these devices comes

from the upstream port. Its current draw can be 1 unit load before

configuration and up to 5 unit loads after configuration.

Host – The host computer system where the USB host controller is installed.

Inrush current – The current surge into uncharged downstream input

capacitors.

Low-power, bus-powered function – All power to these devices comes

from the upstream port. Its current draw is less than 1 unit load.

Power Switching – USB specification requires BPHs to power switch

downstream ports to limit aggregate current draw of the controller, function,

and ports to less than 100 mA before enumeration.

Self-powered function – May draw up to 1 unit load from the upstream port.

The remainder of power is provided from a local power supply.

Self-powered hub – Self-powered hubs (SPH) have a local power supply

that power nonremovable functions and the downstream ports. Typical

self-powered hubs are PCs, monitors, printers, and stand alone hubs. The

SPH is required to have current limit and report overcurrent conditions. The

SPH must be able to supply 5 unit loads to all external downstream ports even

when the hub is in a suspended state. A battery-powered SPH may configure

for 1 unit load or 5 unit loads of output current per port.

Unit load – One unit load is 100 mA.

Upstream port – The port on a device that is electrically closer to the host

and generates upstream data traffic from the hub.

USB – Universal Serial Bus

Voltage droop – The momentary drop in voltage under transient load

conditions.

Voltage regulation – The difference in DC voltage levels under continuous

load and no-load conditions.