Datasheet

SLVA037A
10 Designing a USB Power Distribution System Using the TPS2014 and TPS2015 Power Distribution Switches
Bus-Powered HubSelf-Powered
Hub
GND
IN
IN
EN
OUT
OUT
OUT
OC
TPS2014
R
PCB
4.40 V
min
USB
Conn
USB
Conn
USB
Conn
4.75 Vmin
Cable
4.40 V
min
USB
Conn
Figure 4. USB Bus-Powered Hub Voltage Drops
3.2.2 DC Voltage Regulation
The minimum input voltage at the upstream port connector is 4.75 V. The USB specification
requires a minimum voltage of 4.4 V at the output port (V
Omin
) of the bus-powered hub. As with
the self-powered hub, all resistive voltage drops for the bus-powered hub must be accounted for
to guarantee voltage regulation (see Figure 4).
The following calculation determines V
omin
for multiple ports (n
ports
) ganged together through one
switch (if using one switch per port, n
ports
is equal to 1):
VIrrAnrV
O I conn cable ports switch PCBmin
.( ).=−×⋅ +⋅
−
×
×
−
465 4 2 01
where
r
conn
= resistance of connector contacts (two contacts per connector)
r
cable
= resistance of upstream cable wires (one 5 V and one GND)
r
switch
= resistance of power switch
V
PCB
= PCB voltage drop
The USB specification defines the maximum resistance per contact (r
conn
) of the USB connector
to be 30 m
Ω and the drop across the PCB and switch to be 100 mV. This basically leaves two
variables in the equation: the resistance of the switch and the resistance of the cable. By
defining the voltage drop across the PCB (V
pcb
) to be 10 mV, the equation can be rearranged to
define the resistance of the cable as:
r
mV
I
m
cable
I
=⋅ −






1
2
0150
0120
.
. Ω
If the hub consumes the maximum current (I
I
) of 500 mA, the maximum resistance of the cable
is 90 m
Ω. This defines the cable length allowed for various wire gauges.
The resistance of the switch is defined as follows: