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NOTES
This application note addresses the various
requirements for protecting the Universal
Serial Bus (USB) from overcurrent and over
voltage environmental threats.The solutions
presented cover both USB 1.1 and the
higher speed USB 2.0 circuitry. Specific
emphasis is placed on USB 2.0 with infor-
mation directed at hot connection over
current conditions and electrostatic
discharge (ESD) induced in the USB system.
The USB Standard
The USB specification provides a uniform
protocol for the addition and configuration
of computer peripherals. USB is designed
around one uniform port size and a match-
ing connector. It uses the concept of a
single host and multiple hubs designed to
provide uniform and simple methods for
adding and connecting various peripherals.
The goal of USB is to reduce the number
of cable connections and configurations. A
single USB port has the capability of driving
up to 127 USB peripherals such as mice,
modems and keyboards (see Figure 1).
Additionally, a single hub permits the
connection of several USB devices by
providing power through the communica-
tion cable itself, eliminating the need for
individually powered peripherals. It also
allows mixed high-speed communications
between USB and other protocols
such as Ethernet, DSL, ISDN or
satellite communications
USB is an external bus standard that
supports data transfer rates of up to 12Mbps
for USB 1.1 and 480Mbps for the new USB
2.0 standard. USB also supports Plug-and-
Play installation and hot plug operation.
USB 2.0 addresses the evolution to higher
data transmission rates between computers
and peripherals or networked LAN or
WAN systems. Recent data protocols now
reach millions to billions of characters
per second.
The integrated circuits required to support
this high-speed technology become increas-
ingly complex, shrinking feature size and
making them more susceptible to over
current and over voltage occurrences.
Over current Protection
for USB Power Rails
The USB port consists of four lines - two
data lines (D+ and D-),Vbus and GND.
which connect the USB Hub to the USB
peripheral. Overcurrent protection is not
normally required on the two data lines
or GND.
USB ports can be configured two different
ways: As Self-powered ports or Bus-
powered ports. A Self-powered USB Hub
must have the capability to source up to
500mA on Vbus on all of its ports. A Bus-
powered Hub does not draw power from
the USB stream, but may utilize up to
100ma from upstream devices or hubs to
allow for functionality of the hub when it is
powered down.
Bus-powered Hubs can draw up to 500mA
from an upstream self-powered connection.
Typically 100mA is available for functions
and processors internal to the hub. External
ports in a Bus-powered Hub can supply up
to 100mA per port, with a maximum of 4
ports per hub.
USB Bus Transceiver ICs or Power
Management ICs may include current limit-
ing functions that satisfy USB requirements,
however, when the ICs do not include
current limiting features, are cost prohibi-
tive, or supplemental protection is required,
the circuit designer may choose external
passive current limiting elements for Vbus.
There is a choice of two low cost technolo-
gies for developing over current protection
circuitry.The traditional fuse and the
Polymer-based PTC (positive temperature
coefficient) device are the most common.
Understanding differences between these
two components facilitates choosing the best
protection device for a specific application.
Fuses are one time devices, since they
provide protection from the overload by
opening only once, after which they must
be replaced.The heart of a traditional fuse
is a metal element, which is heated to its
melting point by the excessive current.The
circuit current flow drops to zero as the
element melts open.
Protecting the Universal Serial Bus from
Over Voltage and Overcurrent Threats
APPLICATION
NOTES
Direct Connection to Computer USB Port(s)
Connection to USB-Powered Hub
Keyboard
Mouse
Monitor
Power
Adapter
Printer
Power
Adapter
Personal Digital Assistant
Power
Adapter
Scanner
Power
Adapter
USB Port
USB Port
Figure 1. Typical USB communication structures. The PTC also reacts to excessive current,
but it is self re-settable.The conductive
polymer inside the device increases in
resistance when heated by the overload,
thereby limiting the circuit current.
PTC Protection
Function
The PTC functions by limiting potentially
damaging over currents if they exceed the
specified device rating. Heat caused by the
over current condition produces thermal
expansion in the polymer material. As the
polymer expands, it becomes more resistive
thereby reducing and limiting the current
flowing through it to a safe level.The
increase in resistance is non-linear and
occurs when the operating current exceeds
the trip point. Once the PTC has reached
its trip point, its resistance will remain high
until the power source is removed. Figure 2
illustrates a typical resistance vs. tempera-
ture curve of PTC devices.The PTC
element usually cuts the current to the
circuit as a result of a very small change in
temperature.The current limiting function
occurs at the point when the resistance of
the PTC element matches the impedance
of the circuit.This is also the peak of the
power dissipated in the PTC element.
PTCs are offered in many different sizes,
operating voltages and amperage ratings.The
Littelfuse 1812L Series, measuring only
0.179 x 0.127, is the size of choice for
most computer designers. Littlefuse has
recently released its 1206L series of PTCs.
This product allows designers to provide
singleport protection for ratings up to 1.5A
while utilizing 1/3 of the onboard space of an
1812L PTC
1
.The 3425L Series of PTCs are
also used by computer manufacturers due to
their surface mount packaging and cost
competitiveness relative to other solutions
for USB power management.
1
The 1206L series is available in ratings ranging from
.5A to 1.5A. Samples are available by contacting
electronics@littlefuse.com.
(UL,CSA,TÜV approvals pending.)
Protection of USB
Power Rails with a PTC
Various manufacturers implement designs
for USB power management using PTCs.
Some manufactures use a lower amperage
device for individual protection of each
port.This individual-port solution provides
excellent protection since each single port
can be isolated. Other manufacturers
protect multiple ports on a bus with a
single higher amperage device, thereby
creating a lower-cost solution.The higher
amperage solution will be somewhat less
sensitive to marginal over current condi-
tions than the individually protected
port solutions.
Figure 3 illustrates the placement of the
PTC element (1206L OR 1812L Series).
(The over voltage suppression devices are
shown on the same figure and will be
described in a later section.)
When designing these USB ports, the engi-
neer must insure that the voltage drop
does not fall below 4.75V for a Self-
powered Hub port or 4.40V for a
Bus-powered Hub port.The upstream volt-
age supplied to a Bus-powered Hub is 4.75.
To demonstrate that Littelfuse
®
PTC
devices meet these requirements, voltage
drop calculations are shown on the Sample
Calculations page of this document for
several USB port protection applications.
The calculations for bus-powered hubs
include a resistance budget for the connect-
ing cable.The USB specification specifies
that the connection cables for host to hub
and peripherals have a maximum length of
5 meters and a maximum resistance of
190mohms.The circuit trace was assumed
to be 4 inches with a trace resistance of
5mohms/inch. Bus-powered circuits include
control logic circuitry, which enables soft-
ware control of bus power and port reset
capabilities.The voltage drop for over
current protection with PTC devices
easily meets the requirements in the
USB specifications.
Overcurrent circuit protection scenarios for
the Self-powered hubs and Bus-powered
hubs depict individual port and multiple
(ganged) port protection (Figures 4-7)
Individual port protection offers advantages
over ganged port protection; if one port
fails, the other ports are unaffected.
Additionally, knowledge of the time-to-trip
parameter allows the design engineer to
eliminate false circuit trips due to power-
on-currents.
Over voltage
Suppression of the
USB Power Rails
Transient over-voltage suppression of the
USB power supply rails (Vbus and Signal
GND) is achieved in these circuits with the
addition of two multilayer varistors (MLVs)
and is illustrated as ML1,2 in Figure 3.
Figures 4-7 also depict a varistor in several
port configurations for the protection of
Vbus. A Littelfuse V5.5MLA0603 MLV is
shown in all examples.
Data signal ground (GND) and Vbus tran-
sients must be suppressed. Good layout
practices prescribe that data signal ground
Log Resistance (ohms)
Temperature (
°
C)
Trip Point
Outside W
o
r
l
d
USB
Port
Shield
Shield
PG1
PG2
SOT1
GND
Option:
SOT23 PulseGuard Surface Mount
ESD Suppressors provide High
Speed Data Line Protection on both
D+ and D- lines in one package
PTC1
USB
Controller/
Transceiver
VBUS
D+
D