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SELF-POWER TO BUS-POWER OR BUS-POWER TO SELF-POWER TRANSITION
UNIVERSAL SERIAL BUS (USB) APPLICATIONS
USB POWER-DISTRIBUTION REQUIREMENTS
SLVS287B – SEPTEMBER 2000 – REVISED SEPTEMBER 2007
An autoswitching function between bus-powered mode and self-powered mode is a feature of the TPS2070 and
TPS2071. When this feature is enabled (BP_DIS is inactive) and SP is removed or applied, a transition is
initiated. The transition sequence begins with the internal LDO being turned off and its external capacitance
discharged. Any enabled switches are also turned off and the external capacitors discharged. Once the LDO and
switch outputs are low, the internal logic turns the LDO back on. This entire sequence occurs whenever power to
the SP input is removed or applied, regardless of the source of power, i.e., an external power supply or the use
of the external regulator.
The universal serial bus interface is a 1.5/12-Mb/s (for USB), or 480 Mb/s (for Hi-Speed USB), 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 or 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 TPS2070 and
TPS2071 can provide power-distribution solutions for hybrid hubs that need switching between BPH and SPH
according to power availability and application requirements.
USB can be implemented in several ways, and, regardless of the type of USB device being developed, several
power-distribution features must be implemented.
Hosts/self-powered hubs must:
–
Current-limit downstream ports
–
Report overcurrent conditions on USB VBUS
–
Output 5.25 V to 4.75 V at 500 mA
Bus-powered hubs must:
–
Enable/disable power to downstream ports
–
Power up at <100 mA
–
Limit inrush current (<44
and 10 μF)
–
Output 5.25 V to 4.4 V at 100 mA
–
Not send power back upstream
Functions must:
–
Limit inrush currents
–
Power up at <100 mA
–
Not send power back upstream (SP functions)
The feature set of the TPS2070 and TPS2071 allows them to meet each of these requirements. The integrated
current-limiting and overcurrent reporting is required by hosts and self-powered hubs. The logic-level enable and
controlled rise times meet the needs of both input and output ports on hubs, as well as the input ports for
bus-powered functions.
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