ec631.

ec631.
TECH
BRIEF
ESD Protection
USB 2.0 Data Lines
For high-speed protocols like that
of USB 2.0 (480 Mbps), suppressors
with extremely low capacitance
levels should be used. Suppressors
with high capacitance can affect the
data stream by distorting the data
waveforms
USB 2.0 Product
USB 2.0
Connector
Circuit Board
PWR
GND
D+
USB 2.0
Controller
D-
Figure 1. Simple USB 2.0 circuit diagram (ESD suppressors protect the data lines which have exposure to the outside world.
The USB 2.0 protocol uses 2 high-speed data lines).
Background
Universal Serial Bus (USB) products
(PCI adapter cards, digital cameras,
peripherals, etc.) can be susceptible to ESD events when the cable
is disconnected from the data port.
The ESD pulse can be introduced
directly into the open port, or into
the disconnected end of the cable.
Either way, the integrated circuitry
that controls the USB functionality
can be compromised.
The Problem
After the ESD pulse is introduced
into the data port, it will travel
through the connector to the PC
board. Once on the PC board, it
will propagate down the data lines
toward the integrated circuitry.
Specifically, the IC of concern is the
USB 2.0 Controller. Without sufficient protection, the Controller chip
can be rendered inoperable.
The Solution
In order to provide the IC with
protection against ESD transients,
the use of suppression products is
recommended. The suppressors
are installed between the data line
and the chassis ground (parallel
connection) and shunt the ESD
transient from the data line to the
ground.
Littelfuse offers PulseGuard® ESD
suppressors as a solution for
USB 2.0 data line ESD protection.
These products are surface mount
devices with 0.050 pF of capacitance. So they will provide ESD
protection and maintain the
integrity of the data signals.
Examples of USB 2.0 products
which can benefit from ESD
protection include:
• Computers (desktops and
laptops)
• Peripherals (printers, scanners,
etc.)
• Handheld devices (PDA’s, cell
phones,
MPEG players, etc.)
• Repeaters and hubs
• Digital still cameras
Capacitance and Signal
Integrity
12 Mbps
In previous generations of I/O
protocols (RS232, USB 1.1, 10BaseT
Ethernet, etc.), the signal speed
(rise/fall time of logic states) was
sufficiently slow that the parasitic
components of circuit protection
devices was of minimal concern.
However, as data rates have
increased, the capacitive loading
that is presented to the data
circuitry by the protection device
becomes more of a concern. For
example, the diagrams to the right
show the effect capacitive loading
has on USB full and high speed
waveforms.
In the first set of diagrams, data is
shown for the 12 Mbps data rate.
The first diagram shows the eye
diagram for a control board which
only contain traces for the data
lines. The second diagram shows
the response of a data line which
has a PulseGuard® ESD (0.050 pF)
suppressor installed. The third
diagram is included for reference,
and shows the response when a 3
pF capacitor is installed on a data
line. The PulseGuard device and
capacitor were referenced to
ground. The second set of
diagrams replicates the same tested
devices, but raises the data rate to
480 Mbps. Notice how the slower
12 Mbps data speed can tolerate a
higher level of capacitance.
Figure 2. Test board traces for 12Mbps USB 1.1 data line
Figure 5. Test board traces for 480Mbps USB 2.0 data line
Figure 3. PulseGuard® suppressor traces for 12Mbps USB
2.0 data line
Figure 6. PulseGuard® suppressor traces for 480Mbps USB
2.0 data line
Figure 4. Surface mount capacitor (3 pF) traces for 12Mbps
USB 2.0 data line
Figure 7. Surface mount capacitor (3 pF) traces for
480Mbps USB 2.0 data line
The signals replicate the USB, rev.
2.0 (Template 5 - section 7.1.2.2)
protocol (± 0.480V, 12 and 480
Mbps); created on Agilent 81250
ParBERT equipment and measured
with an Agilent Digital
Communications Analyzer.
Specifications, descriptions and illustrative material in this literature are as accurate as known at time of publication, but are
subject to change without notice. Littelfuse is a registered trademark of Littelfuse Incorporated.
EC631
480 Mbps
Copyright © 2001 Littelfuse, Inc., All Rights Reserved. Printed in U.S.A. JANUARY 2002
Littelfuse, Inc.
800 E. Northwest Highway
Des Plaines, IL 60016
(847) 824-1188
www.littelfuse.com/esd
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