TPS2045A, TPS2046A, TPS2047A, TPS2048A

TPS2055A, TPS2056A, TPS2057A, TPS2058A

CURRENT-LIMITED POWER-DISTRIBUTION SWITCHES

SLVS251A – SEPTEMBER 2000 – REVISED FEBRUARY 2001

21

POST OFFICE BOX 655303

APPLICATION INFORMATION

undervoltage lockout (UVLO)

An undervoltage lockout ensures that the power switch is in the off state at power up. Whenever the input voltage

falls below approximately 2 V, the power switch will be quickly turned off. This facilitates the design of

hot-insertion systems where it is not possible to turn off the power switch before input power is removed. The

UVLO will also keep the switch from being turned on until the power supply has reached at least 2 V, even if

the switch is enabled. Upon reinsertion, the power switch will be turned on, with a controlled rise time to reduce

EMI and voltage overshoots.

universal serial bus (USB) applications

The universal serial bus (USB) interface is a 12-Mb/s, or 1.5-Mb/s, multiplexed serial bus designed for

low-to-medium bandwidth PC peripherals (e.g., keyboards, printers, scanners, and mice). The four-wire USB

interface is conceived for dynamic attach-detach (hot plug-unplug) of peripherals. Two lines are provided for

differential data, and two lines are provided for 5-V power distribution.

USB data is a 3.3-V level signal, but power is distributed at 5 V to allow for voltage drops in cases where power

is distributed through more than one hub across long cables. Each function must provide its own regulated 3.3 V

from the 5-V input or its own internal power supply.

The USB specification defines the following five classes of devices, each differentiated by power-consumption

requirements:

Self-powered and bus-powered hubs distribute data and power to downstream functions. The TPS204xA and

TPS205xA can provide power-distribution solutions for many of these classes of devices.

host/self-powered and bus-powered hubs

Hosts and self-powered hubs have a local power supply that powers the embedded functions and the

downstream ports. This power supply must provide from 5.25 V to 4.75 V to the board side of the downstream

connection under full-load and no-load conditions. Hosts and SPHs are required to have current-limit protection

and must report overcurrent conditions to the USB controller. Typical SPHs are desktop PCs, monitors, printers,

and stand-alone hubs.

Bus-powered hubs obtain all power from upstream ports and often contain an embedded function. The hubs

are required to power up with less than one unit load. The BPH usually has one embedded function, and power

is always available to the controller of the hub. If the embedded function and hub require more than 100 mA

on power up, the power to the embedded function may need to be kept off until enumeration is completed. This

can be accomplished by removing power or by shutting off the clock to the embedded function. Power switching

the embedded function is not necessary if the aggregate power draw for the function and controller is less than

one unit load. The total current drawn by the bus-powered device is the sum of the current to the controller, the

embedded function, and the downstream ports, and it is limited to 500 mA from an upstream port.