AD7887: Overvoltage protection

Q 

Our project uses the AD7887 in a mixed voltage system. During a fault /
failure it is possible that the component is not supplied with 5V, but that
one of it's inputs (CS and/or CLK) is set high by a 3.3V component on the
PCB. I presume there are protection diodes between the chip's inputs and the
supply voltage rails within the chip. What is the maximum current that can
flow  through these diodes?

 

A 

In general you should not rely on internal ESD protection diodes to protect
against fault conditions. ESD hits are not the same as a sustained overvoltage
condition. If you are likely to see the fault condition you describe, I suggest
you add external Schottky diodes and current limiting resistor to CS, CLK and
any other input which is likely to exceed the abs max ratings. Note the diodes
need to be mounted physically close to the AD7887. Ideally, you should design
your system to ensure that the AD7887 is power up fully before any analog or
digital signals are applied. I've also included a few general notes on
overvoltage.

Any other semiconductor IC has basic ESD protection diodes which protect the
device from possible ESD hits due to handling and production. It is the
designers responsibility to provide external protection circuitry if the input
is likely to exceed the supplies at any time.

These ESD diodes can protect the IC from ESD hits up to about 1.5kV. These ESD
protection diodes will act to clamp the voltage at any pin to within 0.5V of
the supplies. (So that's the problem solved right? No not quite.) ESD
protection diodes can carry quite high currents but only for a short period of
time so they can protect the IC from large pulses of short duration (the total
energy is still quite low). The maximum DC current which these protection
diodes can carry is 10mA. Therefore unless you can guarantee that the current
into in pin will me less then 10mA you need some kind of external protection.
External protection could be as simple as a series resistor to limit the
current into a pin. For example if the maximum overvoltage voltage applied to a
pin will be 5V you need to add a 500Ohm series resistor in each digital line to
limit the current to <10mA.  The higher you can make this series resistance the
better.

A high series resistance in a digital IO line can cause other problems such as
slowing the rise and fall time of high speed digital signals. If connections 
will be accessible by the user, you'll also need to consider ESD protection to
much higher levels than +-1.5kV. You might also want to protect against higher
overvoltages but you don't want to add any more series resistance, so what can
you do? Well, you can do this by adding external Schottky diodes between each
digital IO line and the supply lines. A schottky diode will clamp applied
voltages to within ~0.3V of the supply so the majority of the current will be
diverted via the external diodes (which can carry higher current) and not
through the internal ESD protection diodes. There are other protection
techniques which include use of spark gaps, large capacitors to earth ground,
small choke inductors and more. One the best structures I have seen for
protecting against both overvoltage and ESD is a small series resistor followed
by Schottky diodes to the supplies followed by another small series resistor.

You can see that designing suitable protection circuitry is not a trivial
matter. You need to decide how much protection you need, how much abuse you
expect the card to be subjected to, how much board space and component cost you
can allow, and what test levels you need to meet. Check app notes AN202 and
AN397 for more info.