SLLS372E
–
MARCH 2000
–
REVISED NOVEMBER 2011
APPLICATION INFORMATION
A major advantage of USB is the ability to connect 127 functions configured in up to 6 logical layers (tiers) to a
single personal computer (see
PC
With Root Hub
Monitor
With 4-Port Hub (Self-Powered)
Printer
With 4-Port Hub
(Self-Powered)
Scanner
Digital
Scanner
Figure 5. USB-Tiered Configuration Example
Another advantage of USB is that all peripherals are connected using a standardized four-wire cable that
provides both communication and power distribution. The power configurations are bus-powered and
self-powered modes. The maximum current that may be drawn from the USB 5-V line during power up is 100
mA. For the bus-powered mode, a hub can draw a maximum of 500 mA from the 5-V line of the USB cable. A
bus-powered hub must always be connected downstream to a self-powered hub unless it is the only hub
connected to the PC and there are no high-powered functions connected downstream. In the self-powered mode,
the hub is connected to an external power supply and can supply up to 500 mA to each downstream port.
High-powered functions may draw a maximum of 500 mA from each downstream port and may only be
connected downstream to self-powered hubs. Per the USB specification, in the bus-powered mode, each
downstream port can provide a maximum of 100 mA of current, and in the self-powered mode, each downstream
port can provide a maximum of 500 mA of current.
Both bus-powered and self-powered hubs require overcurrent protection for all downstream ports. The two types
of protection are individual-port management (individual-port basis) or ganged-port management (multiple-port
basis). Individual-port management requires power-management devices for each individual downstream port,
but adds robustness to the USB system because, in the event of an overcurrent condition, the USB host only
powers down the port that has the condition. The ganged configuration uses fewer power management devices
and thus has lower system costs, but in the event of an overcurrent condition on any of the downstream ports, all
the ganged ports are disabled by the USB host.
Using a combination of the BUSPWR and EEDATA/GANGED inputs, the TUSB2036 supports four modes of
power management: bus-powered hub with either individual-port power management or ganged-port power
management, and the self-powered hub with either individual-port power management or ganged-port power
management. Texas Instruments supplies the complete hub solution because we offer this TUSB2036 along with
the power-management devices needed to implement a fully USB compliant system.
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