Datasheet

SLVA037A
Designing a USB Power Distribution System Using the TPS2014 and TPS2015 Power Distribution Switches
5
• Fault reporting
• Low on-resistance
The TPS2014 and TPS2015 have an extremely fast trip time of approximately 10 µs (see
Appendix A). By comparison, fuses have trip times in the 100s of microseconds and PTC
resistors have trip times in the 100s of milliseconds. The faster trip time of the TPS2014 and
TPS2015 greatly increases a system’s ability to operate continuously through a fault.
The USB specification requires that the host be informed of any overcurrent event. The
TPS2014 and TPS2015 provide fault reporting through the overcurrent logic output. A system
using a fuse or PTC resistor requires additional external circuitry to provide the fault reporting
information.
The TPS2014 and TPS2015 have a low on-resistance, typically 85 mΩ. Fast-acting fuses have
a typical resistance of 170 mΩ for a 1-A fuse and 40 mΩ for a 2.5-A fuse. PTC resistors have a
resistance of 240 mΩ for a 1-A rating and 65 mΩ for 2.5-A rating. As discussed in section 3.1.2,
the higher resistance of fuses and PTC resistors make it difficult, if not impossible, to meet the
stringent USB voltage regulation requirements.
Unlike fuses and PTC resistors, the TPS2014 and TPS2015 are controlled switches, which allow
the system to turn off ports that have failed or to support low-power states for portable products.
3.1.2 DC Voltage Regulation
All resistive voltage drops must be accounted for to ensure that the required voltage regulation is
met (see Figure 1). The minimum output voltage (V
Omin
) at the output connector is equal to the
power supply voltage (V
PS
) minus its regulation (V
reg
) and the resistive voltage drops in the PCB
traces (V
PCB
) and overcurrent protection device (V
OCP
). The following equation shows the
relationship:
V
O
V
PS
V
reg
V
OCP
V
PCBmin
=−− −
4.75 V
min
GND
IN
IN
EN
OUT
OUT
OUT
OC
TPS2014
R
PCB
4.75 V
min
Self-Powered Hub or Host
GND
IN
IN
EN
OUT
OUT
OUT
OC
TPS2014
USB
Conn
R
PCB
4.75 V
min
USB
Conn
Figure 1. USB Self-Powered Hub Voltage Drops