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UNIVERSAL SERIAL BUS (USB) APPLICATIONS
HOST/SELF-POWERED AND BUS-POWERED HUBS
LOW-POWER BUS-POWERED FUNCTIONS AND HIGH-POWER BUS-POWERED FUNCTIONS
IN
OC
EN
GND
0.1 µF
2,3
5
4
6, 7, 8
0.1 µF 10 µF
GND
1
OUT
TPS2045A
Power Supply
D+
D–
V
BUS
USB
Control
3.3 V
10 µF
Internal
Function
TPS2045A , , TPS2046A
TPS2047A , TPS2048A , TPS2055A
TPS2056A , TPS2057A , TPS2058A
SLVS251C – SEPTEMBER 2000 – REVISED JANUARY 2008
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:
• Hosts/self-powered hubs (SPH)
• Bus-powered hubs (BPH)
• Low-power, bus-powered functions
• High-power, bus-powered functions
• Self-powered functions
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.
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.
Both low-power and high-power bus-powered functions obtain all power from upstream ports; low-power
functions always draw less than 100 mA (see Figure 26); high-power functions must draw less than 100 mA at
power up and can draw up to 500 mA after enumeration. If the load of the function is more than the parallel
combination of 44 Ω and 10 µ F at power up, the device must implement inrush current limiting.
Figure 26. Low-Power Bus-Powered Function (Example, TPS2045A)
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Product Folder Link(s): TPS2045A TPS2046A TPS2047A TPS2048A TPS2055A TPS2056A TPS2057A TPS2058A