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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
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
or less, a two-port TPS2022 ganged port solution can be designed with
5.05 V
±
3% power supply with up to 88 m
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
PS
4.75 V
n
0.5 A
R
TRACE
R
SWITCH
2
0.5 A
R
BEAD
n
number of ports
R
SWITCH
50 m
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.
V
PS
is the minimum voltage of the power supply at full load over the temperature
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.