Selecting an ESD Suppressor

suppressor selection

Littelfuse offers three device technologies

for the suppression of ESD events and

other EMC-related transients.These are

the MultiLayer Metal Oxide Varistor,Silicon

(SCR/Diode Array and TVS Avalanche

Diode Arrays) and Polymeric Voltage

Variable Material technologies.Though

these products have different characteris-

tics,they have the common goal of

protecting sensitive components from tran-

sients that threaten their survivability and

functionality.Additionally,EMC legislation,

or other immunity requirements,may mean

that existing products require suppression

devices in order to be compliant with the

applicable immunity test.

How to use this selec-

tion guide

As with all decision-making processes,infor-

mation is the key when considering the use

of ESD suppressors.It is critical that the

circuit characteristics are defined.For

example,the capacitance value of the ESD

suppressor will limit its usage on communi-

cation lines.As the speed of the data

increases,the capacitance (if it is too large)

can distort or attenuate the data.Ideally,

the Design Engineer will gather basic circuit

information such as data rate,allowable

leakage current,line voltage,size constraints,

and number of lines that require protection.

This information can then be used to

compare the circuit conditions to the ESD

suppressor characteristics to find the best

“match”.T o help with the “matching”

process,a suppressor characteristic table is

included below,and a selection flowchart

can be found on the next page.

ESD suppressor func-

tional similarities

The table at the bottom of this page shows

the details of the ESD suppressor products

offered by Littelfuse.The unique features of

each product family are used to define the

applications (circuit conditions) for which

they are best suited.While these tech-

nologies have different electrical and

physical characteristics,they share many

basic traits.In general terms,they:

• Reduce the ESD voltage that is seen by

the circuit in order to protect the

data/signal/control lines

• Are effective protection against fast

rise-time ESD transients,such as those

specified in IEC 61000-4-2.They are not

applicable to ESD specifications such as

Charged Device Model (CDM),Machine

Model (MM),or Human Body Model

(HBM).These three specifications are used

to typify chip survival in the chip foundry

and board manufacturing environments and

reflect the built-in ESD protection of IC’s

and ASIC’s.They do not guarantee the

survival of the chip during its lifetime in the

end product (computer,cell phone,PDA,

etc.).User-generated ESD events (such as

those specified in IEC 61000-4-2) that will

be seen by the chip in the end-product are

much more severe and typically require the

use of supplemental ESD suppression treat-

ments (such as Littelfuse’s ESD

suppressors).

• Are bi-directional,working in (+) or (-)

polarity circuits and suppressing positive

and negative transients

• Are rated for the most common board-

level operating voltages

• Can often replace TVS Zeners (e.g.

“Transorbs”or “Mosorbs”) while saving

board space - SP723/SP724 or ML series

• Can often replace diode or Zener arrays Selecting an ESD Suppressor

TECH

BRIEF

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.

EC623A Copyright ©2001 Littelfuse, Inc., All Rights Reserved. Printed in U.S.A. JANUARY2002