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ESD Protection USB 2.0 Data Lines

Posted: 29 May 2003     Print Version  Bookmark and Share

Keywords:power 

/ARTICLES/2003MAY/A/2003MAY29_ID_PD_POW_AN2.PDF

Background

Universal Serial Bus (USB) products

(PCI adapter cards, digital cameras,

peripherals, etc.) can be suscepti-

ble 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 suffi-

cient 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.

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

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 capaci-

tance. 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:

7 Computers (desktops and

laptops)

7 Peripherals (printers, scanners,

etc.)

7 Handheld devices (PDA's, cell

phones,

MPEG players, etc.)

7 Repeaters and hubs

7 Digital still cameras

ESD Protection

USB 2.0 Data Lines

TECH

BRIEF

USB 2.0

Controller

PWR

GND

Circuit Board

USB 2.0 Product

USB2.0

Connector

D+

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).

Capacitance and Signal

Integrity

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.

The signals replicate the USB, rev.

2.0 (Template 5 - section 7.1.2.2)

protocol (1 0.480V, 12 and 480

Mbps); created on Agilent 81250

ParBERT equipment and measured

with an Agilent Digital

Communications Analyzer. Littelfuse, Inc.

800 E. Northwest Highway

Des Plaines, IL 60016

(847) 824-1188

www.littelfuse.com/esd

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 Copyright ) 2001 Littelfuse, Inc., All Rights Reserved. Printed in U.S.A. JANUARY 2002

Figure 3. PulseGuard. suppressor traces for 12Mbps USB

2.0 data line

Figure 2.Test board traces for 12Mbps USB 1.1 data line

Figure 4. Surface mount capacitor (3 pF) traces for 12Mbps

USB 2.0 data line

Figure 6. PulseGuard. suppressor traces for 480Mbps USB

2.0 data line

Figure 5.Test board traces for 480Mbps USB 2.0 data line

Figure 7. Surface mount capacitor (3 pF) traces for

480Mbps USB 2.0 data line

12 Mbps 480 Mbps





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