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SELF-POWERED AND BUS-POWERED HUBS

LOW-POWER BUS-POWERED AND HIGH-POWER BUS-POWERED FUNCTIONS

USB POWER-DISTRIBUTION REQUIREMENTS

TPS2550

TPS2551

SLVS736 – FEBRUARY 2008

devices depending on its maximum current draw. A device classified as low-power can draw up to 100 mA as

defined by the standard. A device classified as high-power can draw up to 500 mA. It is important that the

minimum current-limit threshold of the current-limiting power-switch exceed the maximum current-limit draw of

the intended application. The latest USB standard should always be referenced when considering the

current-limit threshold

The USB specification defines two types of devices as hubs and functions. A USB hub is a device that contains

multiple ports for different USB devices to connect and can be self-powered (SPH) or bus-powered (BPH). A

function is a USB device that is able to transmit or receive data or control information over the bus. A USB

function can be embedded in a USB hub. A USB function can be one of three types included in the list below.

• Low-power, bus-powered function

• High-power, bus-powered function

• Self-powered function

SPHs and BPHs distribute data and power to downstream functions. The TPS2550/51 has higher current

capability than required for a single USB port allowing it to power multiple downstream ports.

A SPH has a local power supply that powers embedded functions and downstream ports. This power supply

must provide between 4.75 V to 5.25 V to downstream facing devices under full-load and no-load conditions.

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.

A BPH obtains all power from an upstream port and often contains an embedded function. It must power up with

less than 100 mA. 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 is 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 100 mA. 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; 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

µF at power up, the device must implement inrush current limiting.

USB can be implemented in several ways regardless of the type of USB device being developed. Several

power-distribution features must be implemented.

• SPHs must:

–

Current-limit downstream ports

–

Report overcurrent conditions

• BPHs must:

–

Enable/disable power to downstream ports

–

Power up at <100 mA

–

Limit inrush current (<44

Ω and 10

µF)

• Functions must:

–

Limit inrush currents

–

Power up at <100 mA

The feature set of the TPS2550/51 meets each of these requirements. The integrated current-limiting and

overcurrent reporting is required by self-powered hubs. The logic-level enable and controlled rise times meet the

need of both input and output ports on bus-powered hubs and the input ports for bus-powered functions.

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