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From: larwe on 19 Apr 2010 05:41
On Apr 18, 7:49 pm, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:

> >Optoisolation is better still if you can manage it.
>
> Why? ESD will kill an LED or a phototransistor, so you've got to

ESD shouldn't reach the phototransistor, in theory; the whole purpose
of an optoisolator is that there is only an optical path between the
tx and rx side, and the breakdown voltage between them is a device
parameter. Yes an LED will fry, as all things do, but it's much more
robust than a CMOS input.

Also when properly employed, the LED will be completely isolated from
the circuit (i.e. running a single wire out of the box, going to say
the anode of the LED, with the cathode going to a resistor straight to
the same ground as your IC is NOT the right way to do it). So the ESD
pulse will reach both sides of the LED at the same time.
From: keithw86 on 19 Apr 2010 08:26
On Apr 19, 4:41 am, larwe <zwsdot...(a)gmail.com> wrote:
> On Apr 18, 7:49 pm, John Larkin
>
> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> > >Optoisolation is better still if you can manage it.
>
> > Why? ESD will kill an LED or a phototransistor, so you've got to
>
> ESD shouldn't reach the phototransistor, in theory; the whole purpose
> of an optoisolator is that there is only an optical path between the
> tx and rx side, and the breakdown voltage between them is a device
> parameter. Yes an LED will fry, as all things do, but it's much more
> robust than a CMOS input.

John was referring to outputs. The phototransistor is hanging out in
space.

> Also when properly employed, the LED will be completely isolated from
> the circuit (i.e. running a single wire out of the box, going to say
> the anode of the LED, with the cathode going to a resistor straight to
> the same ground as your IC is NOT the right way to do it). So the ESD
> pulse will reach both sides of the LED at the same time.

Nonsense. ESD protection has to be designed for a single-pin hit.
You cannot rely on common mode.
From: Paul E. Bennett on 19 Apr 2010 14:21
linnix wrote:

[%X]

> I am thinking about clamping diodes. Caps might slow it down too much
> and not fast enough for xxxxKV ESD.

You should really do some SPICE analysis on your circuitry to understand
what is going on in your input and output circuits. Both are vulnerable (and
this includes the Power Supply as an input). You will need some added
capacitance to slow down the rise for the clamping diodes anyway (to allow
them to operate properly).

>>
>> Optoisolation is better still if you can manage it.
>
> Too expensive.

In comparison to losing the whole unit?

Think of your precious circuitry as needing the protective barrier of
isolation from the nasty real-world. Opto-isolators, isolation transformers,
series resistance, capacitance filtering and clamping diodes all working in
harmonious unison will keep your sensitive micro-controllers safe from harm.
Changing a burnt out opto-isolator is cheaper than dumping the whole board-
full of components.

I would normally point you at my articles "Reading the World" and "Writing
the World" but I am between web-sites at present and I don't know what has
happened to the UK Forth Interest Group web-site.

[%X]

--
********************************************************************
Paul E. Bennett...............<email://Paul_E.Bennett(a)topmail.co.uk>
Forth based HIDECS Consultancy
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-510979
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************

From: linnix on 22 Apr 2010 12:55
Thanks for the many suggestions.
In particular, the following ideas:

Larwe: spark gaps
Jamie: TVS diodes
Johns (Larkin/Devereux): RC clamps

Additionally, I found that HCT claims better ESD immunity than ALS.
Both are much better than CMOS micros.

The QFN package has 15 mils gap between signals and ground pad. I
will reduce it to 7 mils in PCB.

The unnamed manufacturer (many of you would have guessed) could have
done a better job in ESD protection, but the high failure rate
indicates otherwise.

I am going to install two diodes back to back between two pins (around
8 mils apart). 0603 is slightly too big. 0402 would be perfect. It
would help if I can pre-joint two diodes before mounting on the PCB.
What can I use to pre-join them? High temperature soldering? Micro-
welding?

http://linnix.com/esd.jpg

From: Robert Latest on 21 Apr 2010 13:48
["Followup-To:" header set to sci.electronics.design.]
linnix wrote:

> common power and ground. Isn't this a very common need?

It is, for instance NXP has a large portfolio of ESD protection devices.
Many are CSP though (just a bare die with solder balls). Maybe your assy
house can process them.

robert
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