Datasheet

SLVA037A

Designing a USB Power Distribution System Using the TPS2014 and TPS2015 Power Distribution Switches

3.2 Bus-Powered Hub

The bus-powered hub draws power for internal functions and output ports from its input port. It

distributes data and/or power to:

• Low power, bus-powered functions

• Self-powered functions

• Self-powered hubs

The bus-powered hub is allowed to consume a maximum of 500 mA for its internal functions and

output ports. Each output port must be able to supply a continuous 100 mA to downstream

loads. Therefore, the number of output ports a bus-powered hub can support depends on the

maximum amount of current consumed for its internal functions. A bus-powered hub can support

a maximum of four output ports.

Since a bus-powered hub does not know if it is being plugged into a port that can support a high-

power function, its total current consumption on power up must be less than 100 mA. A power

distribution switch is required to keep the output ports off during power up. Once configured, the

hub can confirm that it is plugged into a high-power port, and then turn on power for its

downstream ports.

Because of the high output capacitance (120

µF minimum) on each output port, inrush current

limiting is required to prevent drooping of the 5-V power supply from inrush currents.

Overcurrent protection is not required for a bus-powered hub because it is protected by the

upstream port (either a self-powered hub or host). As with the self-powered hub, the output ports

can be ganged together and powered from a single switch, or each port can have an individual

switch.

3.2.1 Power Switch

The TPS2014/15 offers several advantages over a normal MOSFET for the bus-powered hub

power switch. The 3.3-V- or 5-V-compatible logic input allows easy interface with the hub

controller. The on-resistance of the TPS2014/15 is low enough to meet USB voltage regulation

requirements even when the maximum of four outputs are ganged together. The controlled rise

time provides the required inrush current limiting without the need for external circuitry. The

TPS2014/15 undervoltage lockout guarantees that the switch is off during power up.