[sdiy] Re: Protecting Microcontrollers

[ASSI]

On Thursday 21 May 2009, Scott Nordlund wrote:
> As far as I understand (and I'm certainly open to being corrected),
> you don't want to forward-bias these diodes when the device is
> powered.  I guess it would depend on the specific design, but in
> general these diodes are only parasitic structures that exist as
> by-products of the CMOS process.

That used to be the case, but almost any sub-micron CMOS process will 
have specific ESD structures.  These may be "dumb" diodes with a 
latchup protection guardring, sized to survive the ESD pulse or highly 
elaborate structures with grounded-gate-nMOS and thyristor.  The 
parasitic diodes are still there as well as the more problematic 
parasitic thyristor that will cause the latchup if triggered, though.

> Sending any current through them
> while power is applied risks switching on parasitic thyristors that
> could short the power supply rails until power is removed
> (latchup...).

Sending a current in from a pin is indeed called external latchup test 
in IC qualification.  The current levels used depend on a number of 
standards that your product should comply to and are quite high, 
however these external current injections are generally assumed to be 
transient events, as this is the more critical condition for latchup.  
Any DC current into a pin must be limited to a much lower current.  If 
it is permissible to use the internal protection diodes for this 
purpose, the datasheet normally spells out which pin(s) can be used 
(not all of them might have the same protection structures) and which 
level of DC current is tolerable.

To get back on the original topic: analog inputs to an ADC almost 
certainly have diode clamps to the internal supply rails.  As long as 
the internal and external supply is the same, you can avoid using 
these diodes by adding Schottky clamps externally.  For any clamping 
scheme, the clamping current must be limited to a safe value (the 
internal diodes having a much lower tolerance than external ones), 
most often by a series resistor that unfortunately can adversely 
affect the sampling bandwidth.  To circumvent this problem, you can 
now get special buffer opamps (e.g. LMH6553) that limit the output 
amplitude safely without increasing the output impedance.

For audio you can do the same with opamps, for some discussion check
http://www.audiodesignline.com/204802026?printableArticle=true
(I already posted this link to an earlier thread this year, so you've 
maybe already read it).


Achim.
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