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:

Built-In ESD Protection

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 parametric performance.

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 web site.:

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

features

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) Standards
Low Disabled Supply Current 300 uA Max
Pin Compatible with SN75176

description

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 called 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....

Web Info

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 MetaCrawler 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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