I think there is a misuse of protection diodes being propagated here. Input
protection diodes are generally designed to prevent the destruction of the
input of the device. The input of the IC is usually a transistor base, of a
small geometry, or a gate of a MOSFET. These devices have minimum or no
ability to handle even static discharges from handling. You may remember
FET's that had all leads shorted together with an extra wire that was cut
away after the device was soldered into a board. Gate punch through was, and
is, a problem. The protection diodes are there to protect the gate and not
as a line termination device.
Protection diodes are not intended for cycle by cycle usage. These devices
may conduct to the substrate and have a long path back to ground. This is
injecting charge into the die and can produce unintentional current flow in
the device. This effect is well known in memory systems where there is
usually specific warning about not conducting significant and repeated
current into the silicon substrate. Many memory system failures can be
traced to this problem, which is very difficult to isolate, can be very
pattern sensitive and usually fails at a customer site. Memory vendors, in
general, guarantee the products with input voltages that are just short of
conducting current in the protection diodes. There are some vendors that
seem to have a bullet proof scheme and don't care on the data sheet (a
source of worry).
The performance of the protection diode is not guaranteed and may only be
spot tested due to the cost of tester time. Also remember that different
vendors may implement the protection diodes differently based on there own
design parameters and process rules as, again, this is not a guaranteed
function of the device. It is much wiser not to depend on input protection
devices except to protect the device during required handling.
Back to thinking about the energy and where it goes. Diodes do not absorb
the excess energy except for some consumed accidentally in unavoidable
internal resistance. The current goes to ground and must be distributed back
via the holey ground plane causing additional ground plane shift and input
reference shifting in the target device due to ground lead inductance and
resistance. This conduction still causes a high energy reflection back along
the transmission line and this energy still needs to be absorbed somewhere
along the line.
Resistors still provide the best general solution to line termination. As
with all design issues there is the exception that needs to be handled with
a different approach such as diodes. Diodes, MOV's and similar devices are
still needed to handle unknown and extreme excursions of voltage from long
lines subject to acts of nature.
When running Quad I often see the characteristic snubbing of the negative
excursion caused by the shunting characteristics of the input protection
diode in the models. This is always a concern for the long term viability of
the circuit and council repeatedly for the addition of some resistance to
keep that energy from being conducted through the input protection diodes in
any critical or high reliability applications.
Resistors are cheap compared to just one field repair trip, not including
the ones that need a rocket ride to fix.
That's my $0.02.
Summit Computer Systems, Inc
Signal Integrity Specialists - High Speed, Critical PCB Design
firstname.lastname@example.org , www.scsi.com
[mailto:owner-si-list@silab.Eng.Sun.COM] On Behalf Of Mike Monett
Sent: Thursday, October 29, 1998 3:17 AM
Subject: Re: [SI-LIST] : Schottky diode termination
Robert Muir wrote:
> Some years ago a colleague of used the input protection diodes of
> logic devices 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 input voltages from 0 to 60V. If I recall
> correctly the input was taken through a 200K current limiting
> resistor to the input of a 74HC244 buffer, with a 2M resistor also
> on the 74HC244 buffer to ground. Even with a 60V input the maximum
> current through the protection diode is only 0.3mA, well below the
> limit of 20mA. The circuit worked nicely and was put into
> Robert Muir
Sounds like the 0-60V signals were logic levels.
Why not change the 2Meg resistor to 18k? This would give 0-5V at the
input to the 74HC244, which should allow it to detect if the signal level
was high or low.
This is a simple change to the bill of materials. Then, any 74HC244
should work. No need for special part selection and procurement.
Michael R. Monett,
Automated Production Test
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