RE: [SI-LIST] : Schottky diode termination

Robert Muir (Muir@ali.com.au)
Thu, 29 Oct 1998 09:46:06 +1100

Some years ago a colleague of used the input protection diodes of logic
devices=20
as part of the normal operation of a circuit and experienced an
interesting problem.

The circuit was an 8-channel digital input module of an industrial
controller taking=20
input voltages from 0 to 60V. If I recall correctly the input was taken
through=20
a 200K current limiting resistor to the input of a 74HC244 buffer, with
a 2M=20
resistor also on the 74HC244 buffer to ground. Even with a 60V input =
the
maximum=20
current through the protection diode is only 0.3mA, well below the =
limit
of 20mA.=20
The circuit worked nicely and was put into production.

Of course once in production problems occurred. All 74HC244 being the
same, naturally=20
purchasing bought parts from a different manufacturer. When a high
voltage was=20
input the circuit read not only the actual input as being high, but
others as well.=20
The current into the diodes somehow caused other inputs on the same =
chip
to read=20
high, even though their actual input was low. The problem was solved by
testing which=20
manufacturers worked and specifying those to purchasing.=20

Even when the circuit failed none of the chips were damaged, which is =
as
the data=20
sheet specified. I'd be concerned using the protection diodes as
terminators if
this is not specified by the manufacturers. The chips may not be
damaged, but=20
reliability of the product could be suspect.
=20
Regards,
Robert Muir

-----Original Message-----
From: Dennis Tomlinson [mailto:det@tellabs.com]
Sent: Thursday, 29 October 1998 4:07
To: 'si-list'
Subject: Re: [SI-LIST] : Schottky diode termination

Andrew Ingraham wrote:
>=20
> John,
>=20
> I first saw Schottky diode termination described in a rather old
> Fairchild ECL book. Diode termination was not often used for ECL,
> perhaps because the normal ECL parallel termination usually worked
quite
> well if you could afford the power dissipation.
>=20
> The TTL logic families had clamp diodes in their input structures, =
and
> one of their intended functions actually was to provide some amount =
of
> diode termination. This is one of the reasons why TTL was easy to
work
> with, sometimes even when the wires got long compared to the
falltimes.
> Many people who have used TTL do not realize that they have been
making
> use of diode termination all along.
>=20
> Most CMOS families also have input clamp diodes, and again they
> generally work as both input protection and partial signal
termination.

Andrew, John, list,

It has long been my belief that the input clamp diodes on CMOS are for
the purpose of providing ESD protection, and for clamping the
"occasional"
over/undershoot. They are not provided for the purpose of providing =
line
termination. My belief is based on verbal communiqu=E9 with cohorts and
apps. engineers (which is to say, it could be based on legend and=20
mis-information).

As a crude example, suppose a full 5V transition is launched onto an
electrically long 50 Ohm line, causing the propagation of a 100 mA=20
current wave. With no other termination or loads, and assuming a clamp=20
forward bias of about 1V, this would require the clamp diode to conduct =

a peak current of about 80 mA. If this were a highly repetitive signal, =

the duty cycle for each diode could approach 50%. I would expect =
serious

MTBF issues with this scenario. I would also expect the semiconductor
types on this list to shudder at the thought of such an abusive
application=20
(aside from the strong possibility that the circuit may not function
correctly).

I would like to pose some related questions for anyone on the list -
most
particularly the semiconductor types:

1. Do the semiconductor companies ascribe and/or acquiesce to the use =
of
CMOS
input clamp diodes for line termination?

2. What are typical limits for "one time" peak currents?

3. What are typical limits for repetitive peak currents?

4. What if I were to change the example above such that the driver
launched a=20
3.5 volt wave (70 mA)? The clamp current would then peak at roughly
20 mA.

5. And finally, what if the line length were shorter, so that the clamp
duty cycle were only a few percent?

>=20
> Those CMOS devices without input diodes tend to be SI nightmares.
>=20
> In a multi-drop bus configuration, diode terminators may be needed at
> several places, perhaps as many as one per input. For a daisy-chain
> route, they may only be needed at the two ends. The best way to tell
> what works is to try, preferably in simulation with GOOD models.
>=20
> Regards,
> Andy Ingraham
>=20

TIA,

Dennis

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