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LTC2876/LTC2877

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For more information www.linear.com/LTC2876

The IEC standard for ESD, IEC 61000-4-2, specifies a very

fast (sub-nanosecond) edge transient stimulus intended

for system level ESD testing and not specified at the device

level. However, if it is applied directly to the bus interface

pins,withoutanyexternalprotectiondevices,theLTC2876/

LTC2877 is protected to 4kV IEC when used in a typical ap-

plication, powered or unpowered, and terminated with the

standard PROFIBUS load. This is not tested in production.

EFT

Electrical fast transients can result from arcing contacts

in switches and relays, common when switching induc-

tive loads. The IEC standard for EFT is IEC61000-4-4 and

specifies a repetitive burst pattern lasting 60 seconds. The

LTC2876/LTC2877 is robust to EFT events and passes the

highest level recognized in the IEC standard: level 4, ±2kV

on the PA and PB pins, without any external protection.

Auxiliary Protection for Surge

Surge events represent the most severe transient condi-

tions caused by such things as load switching in power

distribution systems, high current short circuit faults,

and lighting strikes. These are addressed in standard IEC

61000-4-5, which specifies repetitive voltage and current

waveformsusedtodeliverhighpowerstimuluslastingtens

ofmicrosecondseach.TheLTC2876/LTC2877isdesigned

for high robustness against ESD and EFT, but the on-chip

protection is not able to absorb the energy associated with

the IEC 61000-4-5 surge transients. External protection is

necessary to achieve a high level of surge protection, and

can also extend the ESD and EFT protection to extremely

high levels.

Inadditiontoprovidingtransientprotection,externallycon-

nected devices must preserve the ability of the LTC2876/

LTC2877 to operate over a wide common mode voltage

and yet safely clamp the pin voltage low enough to avoid

damage during the overstress event. The added protection

must be low in capacitance to avoid excessively loading

applicaTions inForMaTion

the transceiver bus, allowing operation at full data rate.

The LTC2876/LTC2877’s ±60V pin rating makes it easy

to find protection devices meeting these requirements.

Figure 21 shows a solution providing ±4kV protection of

the bus Interface pins (PA and PB) for all three IEC 61000

standards as follows:

IEC 61000-4-5 2nd Ed. 2005-11 Surge Level 4: ±4kV

(line to GND, 8/20µs waveform, each line coupled to

generator through 80Ω resistor per Figure 14 of the

standard)

IEC 61000-4-4 2nd Ed. 2004-07 EFT Level 4: ±4kV

(line to GND, 5kHz repetition rate, 15ms burst duration

every 300ms, 60s test duration, discharge coupled to

bus pins through 100pF capacitor per paragraph 7.3.2

of the standard)

IEC 61000-4-2 2nd Ed. 2008-12 ESD Level 3: ±4kV

contact (line to GND, direct discharge to bus pins with

transceiver and standard PROFIBUS resistor load and

protection circuit mounted on a ground referenced test

card per Figure 4 of the standard)

The TVS devices in Figure 21 have a typical clamp voltage

ofabout36V,comfortablybeyondtheLTC2876/LTC2877’s

common mode operating range of ±25V and well below

the ±60V rating. Since the LTC2876/LTC2877 bus pins are

rated for ±60V, the clamping device must maintain volt-

ages less than this when conducting peak current during

the overstress event. This relatively wide voltage window

permits the use of smaller, more resistive clamps, which

generally also have less capacitance.

Two of these TVS devices are used in an antiparallel con-

figuration because each can only protect in one polarity.

Thebenefitoftheseuni-directional TVSdevicesistheirlow

capacitance, offering a total load of only about 50pF to the

signal lines in this configuration, permitting the LTC2876/

LTC2877 to communicate at maximum data rates with no

significant performance degradation.

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