Selecting an ESD Suppressor By Littelfuse

Selecting an ESD Suppressor By Littelfuse
TECH
BRIEF
Selecting an ESD Suppressor
Introduction to ESD
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 characteristics, they have the common goal of
protecting sensitive components from transients 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 selection guide
As with all decision-making processes, information 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 communication 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”. To 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 functional 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 technologies 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
MultiLayer Varistors
SP72x Arrays
Product Families
MLA, MLE,
MHS, MLN
Device Technology
MultiLayer
Metal Oxide Varistor
Silicon
SCR/Diode Array
Package Outlines
EIA 0402, 0603,
1206, 1210
8 or 16 pin
JEDEC DIP or SOIC
Lines of Protection
Capacitance
Working Voltage
Operating Temperature
Protection Method
General Applications
1, 4
3 - 4,500 pF
3.5 - 120 VDC
SP720, SP721,
SP723, SP724
SP05xx Diodes
SP05xxAA, SP05xxBA,
SP05xxBAC, SP05xxBBC,
SP05xxCAA, SP05xxCAB
Silicon
TVS Avalanche
PGB
Polymeric
Voltage Variable Material
EIA 0402,
JEDEC SOT23, SOT143
TSSOP, MSOP
EIA 0402, 0603, 0805
JEDEC SOT23
4, 6, 14
1, 2, 3, 4, 5, 6, 8,
16, 18
1, 2, 4
3 - 5 pF
3 - 39 pF
0.050 pF
Up to 35 VDC
(except SP724)
Up to 20 VDC (SP724)
-40˚C to +125˚C
-40˚C to +105˚C
Clamps transient to
ground or common
Clamps transient to
power supply rails
Data, signal, control lines,
Data, signal, control lines
Power supply lines
Table1. Comparison of Littelfuse ESD suppressor characteristics
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 treatments (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 boardlevel 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
Up to 5.5 VDC
Up to 24 VDC
-40˚C to +85˚C
-65˚C to +125˚C
Clamps transient to
common or supply rails
Data, signal, control lines
Clamps transient to
ground or common
Data, signal, control lines
Example
Number
Application Desciption
Data Rate
Frequency
1
Keypad of wireless
device
< 10 Mbps
Array
MLN: SurgeArray
suppressors
SP05x TVS Avalanche
2
Audio Input
(Microphone/Speaker)
20 - 20,000 Hz
Single Line
MLA/MLE
SP0501x TVS Avalanche
3
USB 2.0 Data Port
480 Mbps
Not Determined
4
USB 1.1 Data Port
12Mbps
Not Determined
5
RS-485
25 Mbps
Form Factor
Preferences
Recommended
Solution
TM
Single Line
MHS
SP724
SP05x - Rail Clamp
MHS
Table 2. Application examples and recommended Littelfuse ESD suppressor solutions
Selecting an ESD
Suppressor
The data protocols and transmission
rates that are used in various applications (cell phone, PDA, computer, etc.)
will help determine the applicability of
ESD suppressors. As data rates
increase, the amount of stray capacitance that the circuit can withstand,
and still function properly, decreases.
In turn the capacitance of ESD
suppressors becomes an important
characteristic, and can be used as one
criterion for their selection. In short, at
very low speeds (audio and keypad
circuits), capacitance of the ESD
suppressor is not an issue. In fact, the
capacitance can help minimize EMI
noise like a low band pass filter.
However, as speeds increase, capacitance plays an important role. Too
much capacitance can cause degradation to the data signals and interfere
with signal integrity. So lower capacitance devices are necessary. Lastly,
keep in mind that other suppressor
characteristics (leakage current, clamping level, number of channels, etc.) will
also need to be kept in mind when
selecting the most appropriate
suppressor for your circuit.
Is
Data Rate > 10 Mbps
Frequency > 20 MHz?
Begin
Yes
Is
Data Rate > 125 Mbps Yes
Frequency > 250 MHz?
Example 3
No
No
Array
Is
Preferred Package
Single-line Protector
or
Multi-line Array?
Is
Preferred Package
Single-line Protector
or
Multi-line Array?
Array
Consider:
SP72x
SP05xxA-Rail Clamp
Multi-line Arrays
Example 4
Single
Consider:
MLN-SurgeArray TM
SP05x TVS Avalanche Diode
Multi-line Array
Consider:
MLA or MLE
SP0501x
Single-line Protectors
Example 1
Example 2
Single
Consider:
MHS
Single-line Protector
Example 5
Figure 1. Process flowchart for selecting a Littelfuse ESD suppressor
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.
EC623A
Copyright © 2001 Littelfuse, Inc., All Rights Reserved. Printed in U.S.A. JANUARY 2002
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