Self-Powered Hub Port Protection Topologies

6

SLVA049

Self-Powered Hub Port Protection Topologies

The self-powered hub power distribution system can be designed with two

distinct topologies for port protection: individual port protection or ganged port

protection. The ganged port protection is the most economical and the least

flexible topology for fault isolation.

The ganged topology uses a common current limit element for two or more ports.

In a ganged solution, more current flows through the current limit device. The

resistance of the current limit device must be low, so as not to exceed the

maximum voltage drop limit under full operating conditions. Fault isolation is

another limitation of a ganged solution. To isolate the fault in a ganged solution,

power will be interrupted to the adjacent port. This may not be desirable in some

USB applications. An individual port protection topology provides a more robust

configuration and a better end-user experience.

The individual port protection is the most flexible topology for fault isolation in the

USB power distribution system. The individual port protection topology uses

current limit elements for each port, making isolating faults with minimal

interruption to the adjacent ports possible.

Ganged Port Protection

When designing a ganged topology a low-resistance switch is needed to

minimize voltage drop in the hub, because current for all downstream ports

passes through the current limit device (see Figure 3). The TPS202x power

switches are low 50-m

Ω devices with current limit and overcurrent response flag

as required in USB self-powered hubs. The current limit level of the devices has

been optimized for ganged two (TPS2022), three (TPS2023), and four

(TPS2024) port protection topologies.

The TPS2022 is ideal for a two-port ganged solution; it is rated for a continuous

current of 1 A and has a current limit of 1.5 A. Using ferrite beads with a resistance

of 10 m

Ω or less, a two-port TPS2022 ganged port solution can be designed with

5.05 V

±3% power supply with up to 88 mΩ of trace resistance on the higher

current path. If more trace resistance is required, use the following equation to

calculate the required source voltage for a tighter tolerance power supply.

V

4.75

V

) n

0.5

A

R

R

2

0.5

A

R

BEAD

n

+ number of ports

R

50

m

W

In a manner similar to the two-port ganged protection solution, a three- or four-

port ganged solution can be designed with the TPS2023 or TPS2024 devices,

respectively. By using the above equation, the right power supply can be

calculated for a specific number of ports, power switches, and trace resistances.

range and line regulation. The output voltage of the hub must be greater than

4.75 V and no greater than 5.25 V over all legal load conditions on the upstream

side of the connector.