Datasheet

SLVA037A
Designing a USB Power Distribution System Using the TPS2014 and TPS2015 Power Distribution Switches
11
r
mV V
An
switch
PCB
ports
=
−
×
100
01.
If the voltage drop across the PCB is limited to 10 mV, the maximum resistance for the switch is
225 m
Ω for four ports ganged together. The TPS2014/15, with its maximum 120 mΩ on-
resistance over temperature, easily meets this requirement.
3.2.3 Transient Regulation
A bus-powered hub has the same hot-insertion transients and minimum 120-µF output
capacitance requirement as a self-powered hub. Its maximum normal load transient of 100 mA
is less than the 500-mA load transient for a self-powered hub. The discussions in section 3.1.3
for ganged and non-ganged self-powered hubs thus also apply to bus-powered hubs.
3.2.4 Bus-Powered Hub Reference Designs
Although a bus-powered hub can be designed with one switch per output port, the most cost-
effective solution is to gang all of the output ports together. The reference designs for ganged
and non-ganged bus-powered hubs are essentially the same as those for self-powered hubs
except for the bulk input capacitance (see section 3.1.4 for self-powered hubs reference
designs). Because a bus-powered hub is a hot-insertable device, the USB requirement for the
maximum input capacitance is 10
µF. In this case, a 4.7-µF input capacitor is recommended in
place of the 1000-
µF bulk capacitor.
The TPS2014 has a 120-m
Ω maximum on-resistance over temperature. With a four-port load of
400 mA, it has a maximum voltage drop of 48 mV instead of the 90 mV allotted in section 3.1.2.
The TPS2014/15 allows this difference in voltage drop to be allotted to the cable, thus making it
easier to meet the USB voltage regulation requirements.
4 ESD
Because USB is a hot insertion and removal system, USB components (especially the connector
pins) are subject to electrostatic discharge (ESD) and should be qualified to IEC801.2. The
TPS2014 and TPS2015 are designed to withstand a 6-kV human body model, as defined in MIL-
STD-883C. The requirements in IEC801.2 are much more stringent and require additional
capacitors for the TPS2014 and TPS2015 to withstand the higher ESD energy.
Low ESR 1-
µF ceramic bypass capacitors and output tantalum capacitors should be placed as
closely as possible to the IN and OUT pins to increase the ESD immunity. The TPS2014 and
TPS2015 pass the level-4 15-kV air discharge and level-4 8-kV contact discharge tests when
these capacitors are added.
5 Summary
Three basic types of devices provide overcurrent protection. A fuse provides overcurrent
protection but is not maintenance free. A fault blows its element to isolate the fault. The fuse
must then be replaced before the port is operational. In addition, a fuse does not provide fault
indication. Additional circuitry must be implemented to provide the host with fault information.