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Protecting the Universal Serial Bus from Over Voltage and Overcurrent Threats

Posted: 26 May 2003     Print Version  Bookmark and Share

Keywords:power 

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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.

PTC's 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

single-port 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,T\V 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

LogResistance(ohms)

Temperature (0C)

Trip Point

OutsideWorld

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-

GND

D+

D-

ML1

ML2

Table 1. Symbol Definition

Shield (Chassis Ground)

GND (Signal Ground)

PTC 1 1206L/1812L Series Resettable PTC

ML1, 2 V5.5MLA0603

PG1, 2 PGB0010603

SOT1 PGB002ST23

Physical Tie

SOT1

Figure 2. PTC's resistance as a function of temperature. Figure 3. USB 2.0 port protection reference design.

and chassis ground should not be tied

together at the board level.This may allow

transients to propagate via signal ground

with respect to chassis ground, especially

while the board is being handled or when a

USB cable is inserted.

The V5.5MLA0603 device was chosen for

transient suppression of the Vbus and GND

for several reasons. First, since board real

estate is always a consideration, a small

0603 size device was chosen.This device

has a continuous DC voltage rating of

5.5Vdc, consistent with the 5V USB power

supply.The MLVs are fully cable of handling

ESD transients or higher energy threats

(EFT, lightning remnants, etc). Since we are

protecting a dc bus, added capacitance is

useful.The V5.5MLA0603 device has a typi-

cal capacitance of 660pF.

ESD Protection of

USB Data Lines

ESD protection should be an integral part

of the USB design process.While USB

Transceiver ICs will have some level of

protection on the chip, additional protec-

tion is required to augment immunity.While

many ICs are specified to ESD levels of 1-

4kV per Mil-STD-883, Method 3015, they

may have little or no capability to the

Human Body Model (HBM) standard IEC-

61000-4-2.The two standards have very

different test methods. Additionally, ESD

discharge transients are very fast events,

which may reach peak voltages in sub

nanosecond time frames, and peak voltages

and currents can reach 25kV and 100A,

respectively.Transients of this magnitude or

lower can cause gate oxide ruptures or

other silicon damage, transmission signal

degradation, loss of stored data and equip-

ment latch-up.

An ESD suppressor is a circuit protection

component that reduces the ESD transient

to a level which prevents damage to elec-

tronic components.The suppressor is

installed from line to ground and shunts

most of the ESD energy to ground.The

remaining energy is either dissipated within

the suppressor or is reflected back towards

the source of the ESD event. Figure 3 illus-

trates how an ESD suppressor is

incorporated into USB data line circuitry.

The Littelfuse PulseGuard.

ESD suppressor

utilizes a polymer composite voltage-vari-

able material exhibiting a highly nonlinear

resistance response to applied voltage.

Under normal circuit operating conditions,

the ESD suppressor exhibits a high resist-

ance and remains transparent to the circuit.

When an ESD transient develops, the

suppressor resistance drops sharply.This

low resistance path redirects most of the

ESD energy away from the circuit. After the

energy is dissipated, the suppressor auto-

matically returns to its normal

high-resistance state.

USB 2.0, 480Mbps waveforms can be

distorted by any added capacitance.

Littelfuse PulseGuard.

surface mount ESD

suppressors present almost no capacitive

loading to the circuit with a typical capaci-

tance of 0.05pF. Higher capacitance surge

suppression devices distort the data wave-

form rounding the leading and trailing

edges. Figure 10 illustrates how a digital

wave shape is altered during its transition

between high and low logic states if the

capacitance of the surge suppression

element is too large.

Data recorded for Figures 8-10 illustrate

the effect capacitance has on a digital wave-

form similar to a USB signal. Figures 8 and 9

show the effect on a digital pulse train at

6MHz and 240MHz, respectively. Since USB

utilizes a differential pair, a bit change can

occur as often as every half cycle, therefore,

a square wave at frequencies of 6MHz and

240MHz were used to generate the data.

Figure 10 shows the effect capacitance has

on a typical USB rise time.

Each figure shows five curves representing

different loads - No load, PulseGuard

suppressor (0.05pF), 1pF Capacitor, 10pF

Capcitor, and a Multilayer Varistor (660pF).

As can be expected, the device with the

lowest capacitance, the Pulse Guard

suppressor, provides the lowest level of

signal distortion.The benefit of Pulse Guard

suppressors are more apparent at the

0.0E+00

2.0E-08

4.0E-08

6.0E-08

1.0E-07

1.2E-07

1.4E-07

1.6E-07

1.8E-07

2.0E-07

3.0E.01

2.0E-01

1.0E-01

0.0E+01

-1.0E-01

-2.0E-01

-3.0E-01

Peak to Peak Voltage Amplitude at 480mV

Signal Frequency at 6MHz Equal to 12Mbit/sec

Time (sec)

Voltage(V)

Without Device

10.0 pF 0603 Capacitance

PGB0010603 1.0 pF 0603

CapacitanceV5.5MLA0603

+

1

2

3

4

+

1

2

3

4

+

1

2

3

4

V5.5MLA0603

V5.5MLA0603

V5.5MLA0603

LF PTC

LF PTC

LF PTC

Power Vdc

Source +5V

+

+

+

V5.5MLA0603

V5.5MLA0603

V5.5MLA0603

LF PTC

Power Vdc

Source +5V

1

2

3

4

1

2

3

4

1

2

3

4

+

V5.5MLA0603 1

2

3

4

+

V5.5MLA0603 1

2

3

4

+

V5.5MLA0603 1

2

3

4

1

2

3

4

Logic

Controller

LF PTC

LF PTC

LF PTC

+

V5.5MLA0603 1

2

3

4

+

V5.5MLA0603 1

2

3

4

+

V5.5MLA0603 1

2

3

4

1

2

3

4

Logic

Controller

LF PTC

0.0E+00

5.0E-10

1.0E-09

1.5E-09

2.0E-09

2.5E-09

3.0E-09

3.5E-09

4.0E-09

4.5E-09

5.0E-09

3.0E.01

2.0E-01

1.0E-01

0.0E+01

-1.0E-01

-2.0E-01

-3.0E-01

Peak to Peak Voltage Amplitude at 480mV

Signal Frequency at 240MHz Equal to 480Mbit/sec

Time (sec)

Voltage(V)

Without Device

10.0 pF 0603 Capacitance

PGB0010603 1.0 pF 0603

CapacitanceV5.5MLA0603

7.0E-10 9.0E-10 1.1E-09 1.3E-09 1.5E-09 1.7E-09 1.9E-09

3.0E.01

2.0E-01

1.0E-01

0.0E+01

-1.0E-01

-2.0E-01

-3.0E-01

Rise Time (Without Device):

Rise Time (PGB0010603):

Rise Time (1.0 pF):

Rise Time (10.0 pF):

Rise Time (V5.5MLA0603):

226 pSec

225 pSec

275 pSec

526 pSec

NA

Peak to Peak Voltage Amplitude at 480mV

Signal Frequency at 240MHz Equal to 480Mbit/sec

Time (sec)

Voltage(V)

Figure 4. Self powered hub individual port protection.

Figure 6. Bus powered hub individual port protection.

Figure 5. Self powered hub multiple port protection.

Figure 7. Bus powered hub multiple port protection.

Figure 10. 10-90% USB signal rise time response of

several loads.

Figure 8. Capacitive loading effects on USB 1.1 signal.

Figure 9. Capacitive loading effects on USB 2.0 signal.

higher USB 2.0 rates (480Mbps) as there is

almost no effect to the waveform or rise

time. Low capacitance TVS (Transient

Voltage Suppression) diodes in the 3-60pF

range would show appreciable capacitive

loading effects.

Off-state leakage currents are another

consideration when choosing an ESD

suppressor.This is especially important in

portable applications where batteries are

used to supply circuit power.The ESD

suppressor leakage currents must be mini-

mized for maximum battery life.The

leakage current for a PulseGuard.

ESD

suppressor is <1 nA, which is much less

than the amount for a TVS (Transient

Voltage Suppression) diode (0.1-

20microamp).

The chassis or shield ground of the system

is the recommended ESD surge suppressor

ground return (see Figure 3.), not the signal

ground (GND).The design objective is to

force the ESD currents flowing on the data

or GND directly to the chassis and or

shield. By returning the ESD currents to the

chassis ground we effectively accomplish

the following:

* Any ESD event entering the shield of the

USB cable, the USB device housing, or

connector will be shunted to the external

shielding of the USB host or possibly to

the shield of the computer containing the

USB port preventing current from

coupling into the data lines.

* Lowers the likelihood of an ESD gener-

ated upset.

* Electric fields resulting from the ESD

event do not reach internal circuitry.

* Energy in the ESD transient gets reflected

back towards the ESD generator (i.e., the

human or other charged structure).

Figure 11 shows two possible printed circuit

board structures.The first uses the 0603

PulseGuard.

suppressor case style and the

second uses the SOT-23 case style. Both

options show the pad layout for the MLVs.

Figure 12 shows the physical layouts for the

top of the USB port.

Summary

The USB standards present a means to

improve serial data communication. As with

any bus architecture, the threat from fault

currents and induced voltage transients exist.

Littelfuse offers a variety of technologies to

address these situations. Highlighted in this

note is Littelfuse PTCs for over current and

Pulseguard.

suppressors for ESD.

The Polymer PTCs and Zinc Oxide MLVs

have been shown to effectively reduce over

current and transient over voltage events,

respectively, on the USB Power Rails.

In order to maintain the signal integrity on

the high-speed USB 2.0 data lines, the

designer must take into consideration the

addition of any inserted capacitance.

Littelfuse PulseGuard.

suppressors provide

the necessary ESD suppression while at the

same time have negligible capacitance of

their own.

Bibliography and References

1. Jim Colby,The ESD Problem

2. Hyatt, et al, ESD Symposium

paper 1, 2000

3. Hyatt et al, ESD Symposium

paper 2, 2000

4. Littelfuse, Protecting Universal Serial

Bus (Reference design)

5. Hyatt, ESD Symposium (resistive

phase paper)

6. Littelfuse Suppression Products

Databook, DB450.5

7. Littelfuse, Electronic Designer's Guide,

EC101-F

FORM NO. EC606 Printed in U.S.A. MARCH 2001 Copyright ) 2001 Littelfuse, Inc., All Rights Reserved

Littelfuse, Inc.

800 E. Northwest Highway

Des Plaines, IL 60016 USA

(847) 824-1188

www.littelfuse.com

Figure 11a: Option 1 layout for 0603 case style PulseGuard

Supressor and MLVs.

Figure 11b. Option 2 layout for SOT-23 case style

PulseGuard Suppressor and MLVs.

Figure 12.Top view of Layout showing Polymer PTC pads.

Vcc

GND

D- D+

Shield Ground

V5.5MLA0603

PGB0010603

Bottom Side of Board

(Option 1)

Vcc

GND

D- D+

Shield Ground

V5.5MLA0603

PGB001ST23

Bottom Side of Board

(Option 2)

Vcc GND

D- D+

1812L150

Shield Ground

Top Side of Board





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