Datasheet

SLVA037A
Designing a USB Power Distribution System Using the TPS2014 and TPS2015 Power Distribution Switches
7
3.1.4 Self-Powered Hub Reference Designs
Figure 2 shows the schematic of the non-ganged reference design, which uses the TPS2014 as
the overcurrent protection device. The following assumptions apply:
• Power supply regulation: 5 V (± 5 percent)
• Total PCB voltage drop: 10 mV
Using these assumptions, the required resistance of the overcurrent protection device must be
less than 180 m
Ω to meet USB voltage regulation requirements (see section 3.1.2). The
TPS2014 has a typical on-resistance of 85 m
Ω with an over-temperature maximum of 120 mΩ.
Figure 2. Self-Powered Hub Non-Ganged Reference Design
A 150-
µF tantalum capacitor is chosen for the output port capacitor because it is the closest
standard value that meets the minimum 120-
µF specification requirement. The 0.1-µF input
capacitor is a bypass capacitor for the TPS2014. It is recommended that some bulk capacitance
be on the 5-V input to the USB overcurrent protection devices. Bulk capacitance provides the
following two functions:
• Improves the voltage droop for hot-insertion transients
• Helps keep transients on the output ports from affecting other hub functions using the 5-V
power
Figure 3 shows the schematic of the ganged reference design, which uses the TPS2015 for
overcurrent protection. Applying the same assumptions applied in the non-ganged reference
design, the required on-resistance of the TPS2015 is 90 m
Ω. Although the TPS2015 has a
nominal on-resistance of 85 m
Ω, the on-resistance at high temperature is 120 mΩ. Therefore,
for this design to be valid, the power supply must have a 4-% regulation to allow for the
additional voltage drop.