Datasheet

SLVA037A
6 Designing a USB Power Distribution System Using the TPS2014 and TPS2015 Power Distribution Switches
The USB specification defines the maximum voltage drop across the overcurrent protection
device and the PCB traces to be 100 mV. This leaves 250 mV (or 5 %) for the power supply
regulation (V
reg
). The voltage drop across the overcurrent protection device is a function of its
output current and the on-resistance of the device. The output current is 500 mA times the
number of ports ganged together on its output. The maximum on-resistance of the overcurrent
protection device is defined by:
r
mV V
An
OCP
PCB
ports
=
−
×
100
05.
This equation shows that for a 10-mV voltage drop for the PCB trace, the switch must have a
resistance of 180 m
Ω to support one port and 90 mΩ for two ports. With the resistance of the
TPS2015, two ports is the maximum that can feasibly be ganged with one overcurrent protection
device.
3.1.3 Transient Regulation
During normal operation, only moderate transients occur. The maximum and peak current that a
function may draw is 500 mA. Appendix B contains the transient testing results for both normal
transients and hot-insertion transients. The results show that 500-mA load transients cause
almost no transient voltage droop. The required 120-
µF output port capacitance is more than
sufficient to meet the 330-mV droop requirement.
The real concern is the effect on other output ports of the hub during hot insertion of a function.
The USB specification requires that hot insertion operations not interrupt the operation of
existing connections. The USB specification limits the maximum load that can be hot inserted to
44
Ω paralleled by 10 µF. The specification also limits the maximum voltage droop this insertion
causes on other ports to 330 mV.
In the non-ganged architecture of the single port per overcurrent protection device, the minimum
required output capacitance of 120
µF is sufficient to meet the maximum voltage droop (see
Figure 2).
The testing described in Appendix B shows that the hot insertion transient produced a voltage
droop of 280 mV. The impedance of the two TPS2014 switches between the output port
capacitors provided sufficient filtering to limit the voltage droop. The voltage droop can be
improved with the addition of bulk capacitance on the 5-V input to the ports. The bulk
capacitance also helps prevent the transients from being reflected back to the system.
In the ganged architecture, where multiple output ports are connected in parallel to one
overcurrent protection device, there is no impedance between the output port capacitors (see
Figure 3). Testing shows that the hot insertion produced a 1-V transient on the adjacent port,
even with the required 120
µF output capacitance per port (see Appendix B). Even with the
addition of 1000
µF of bulk capacitance to the 5-V input, the 330-mV droop could not be met.
Impedance must be added between the ports to filter transients between ports.
Ferrite beads inserted in series with 5-V and GND lines before the output port capacitors provide
lossless transient filtering. Testing shows that with the addition of the ferrite beads, the hot
insertion transient produced a 300-mV voltage droop on the other port (see Appendix B). The
voltage droop can be reduced even more with the addition of bulk capacitance to the 5-V input.