E.S.D. Protection Basics
... part three ...
3.0 Actual Circuits and Hot Fixes
(last update 25th March 2004 - updated links to external sites)
If this looks useful to you, but I havn't finished it yet...send me an email on
me at tonywilk.co.uk
3.1 Some Devices
3.1.1 Ordinary Chippery...
Although many integrated circuits now claim they have 'static protection' built-in, this
often only refers to very basic protection.
Years ago, early PMOS and CMOS devices were extremely sensitive - it was easy to kill a device
just by handling it, some would stop working if you just stared at 'em hard
(Ok, so I exaggerate a little!). Later devices were designed with extra circuitry to protect
the device during handling, but gave circuit designers the problem of stopping the device from
cooking itself when its protection circuits latched on and tried to draw loads of current.
Newer IC's have better built-in protection, but this is still only meant to save the device
from low-level static during assembly - NOT to withstand 'real world' ESD.
For example, the data sheet on a TI TLV2352 comparator boasts:
However, the datasheet later states:
The TLV2352 has internal electrostatic-discharge (ESD)-protection circuits and has been
classified with a 2000-V ESD rating tested under MIL-PRF-38535. However, care should be
exercised in handling this device as exposure to ESD may result in degradation of the device
In other words: it won't blow up by just looking at it, but you need to add some real
protection if the device is handling external signals.
If you are looking at datasheets for 'protected' devices, you need to look for specifications
which say it can handle 8Kv to 15Kv and can withstand transients of 100+ Watts, not just
a 2000volt handling rating.
3.1.2 TransZorbs - Transient Voltage Suppressors
As I mentioned earlier, there are some devices which are specifically designed
for ESD protection, an example is the General Semiconductor "TransZorb" which
is in the general category of Transient Voltage Suppressors.
The following is an extract from a .pdf format datasheet from the General Semiconductor
The complete data is available on: the
Vishay (formerly General Semiconductor)
web site in the "SAC5.0 thru SAC50 Series" category.
Note that these devices can withstand 100's of watts for a few microseconds - there are other devices
available from other companies, some are specific protection components and others have other
funtions with protection built in.
For example, the following is an extract from a TI datasheet:
3.1.3 Protected comms drivers
SN75LBC184 DIFFERENTIAL TRANSCEIVER
WITH TRANSIENT VOLTAGE SUPPRESSION
Document: SLLS236A - OCTOBER 1996 - REVISED MAY 1998
- Integrated Transient Voltage Suppression
- ESD Protection for Bus Terminals:
- ±15 kV Human Body Model
- ±8 kV IEC1000-4-2, Contact Discharge
- ±15 kV IEC1000-4-2, Air-Gap Discharge
- Circuit Damage Protection of 400 W Peak (Typical)
- Controlled Driver Output-Voltage Slew Rates Allow Longer Cable Stub Lengths
- 250-kbit/s in Electrically Noisy Environments
- Open-Circuit Fail-Safe Receiver Design
- 1/2 Unit Load Allows for 64 Devices Connected on Bus
- Thermal Shutdown Protection
- Power-Up/-Down Glitch Protection
- Each Transceiver Meets or Exceeds the Requirements of EIA RS-485 and ISO/IEC 8482:1993(E)
- Low Disabled Supply Current 300 uA Max
- Pin Compatible with SN75176
The SN75LBC184 and SN65LBC184 are differential data line transceivers in the trade-standard
footprint of the SN75176 with built-in protection against high-energy noise transients.
This feature provides a substantial increase in reliability for better immunity to noise
transients coupled to the data cable over most existing devices. Use of these circuits
provides a reliable low-cost direct-coupled (with no isolation transformer) data line interface
without requiring any external components.
The SN75LBC184 and SN65LBC184 can withstand overvoltage transients of 400 W peak...etc. etc.
The Texas Instruments web site also contains a .PDF-format document
INTERFACE CIRCUITS FOR TIA/EIA-485 which covers circuits for serial communications
with reference to ESD protection, fail-safe operation and galvanic isolation.
...more to go in here....
All-Spec Static Control Inc.
have some interesting info on their
About ESD page plus
some good ESD on the Web links.
The Electrostatic Discharge Association
has a couple of good pages in the About ESD
section (see parts One, Three and Five).
Most of the rest of this site is about ESD protection in the workplace.
More info on the Web?
Give your favorite search engine "Electrostatic Discharge" to chew on
(I find that Google or
is often a good starting point), although most of the hits tend to be
for static control in the workplace rather than actual circuit design.
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