ESD Zener diode shunt to ground or power rail?

[kean]

Hi,
I normally connect Zener diodes for ESD protection to ground so that
the ESD event will be shunted to ground. However i have noticed a
design that actually connects the ESD zener diode to the power rail
(3.3V).
Does anyone know why it is done this way instead of shunting it to
ground? What would be the advantages and disadvantages of doing it
this way?

Thanks in advance,
Kean

 

[Robert Baer]

Hi,
I normally connect Zener diodes for ESD protection to ground so that
the ESD event will be shunted to ground. However i have noticed a
design that actually connects the ESD zener diode to the power rail
(3.3V).
Does anyone know why it is done this way instead of shunting it to
ground? What would be the advantages and disadvantages of doing it
this way?

Thanks in advance,
Kean

1) you said "zener". Reasonably abrupt knee zeners are above 5.6V
which is not too good for such low voltage devices.
2) tied to the supply line, the forward region would prevent the
voltage from going about 0.6V more positive, and the negative zener
region would prevent the input from going (6.0V-3.3V =) 2.7V below
ground (ASSuMEing the power is on).
3) i am assuming the devices can withstand short negative input energy
bursts without damage. CMOS cannot withstand energy bursts above the
supply rail.

 

[James Meyer]

Hi,
I normally connect Zener diodes for ESD protection to ground so that
the ESD event will be shunted to ground. However i have noticed a
design that actually connects the ESD zener diode to the power rail
(3.3V).
Does anyone know why it is done this way instead of shunting it to
ground? What would be the advantages and disadvantages of doing it
this way?

Thanks in advance,
Kean

Voltages are always two terminal sources. In other words, a voltage at
one point must be specified with respect to another point somewhere.

If you expect ESD voltages to be referenced to ground, then it makes
sense to return the diode to ground. If, on the other hand, the ESD is expected
to be referenced to the power rail, it makes sense to return the diode to the
rail.

ESD protection design is no more or less important and worthy of careful
consideration than any other part of a design.

Jim

 

[nospam]

[email protected] (kean) wrote:

And another

Hi,

Solve my problem

Thanks in advance,

post from a google information leech.

 

[Rich Grise]

Voltages are always two terminal sources. In other words, a voltage at
one point must be specified with respect to another point somewhere.

If you expect ESD voltages to be referenced to ground, then it makes
sense to return the diode to ground. If, on the other hand, the ESD is expected
to be referenced to the power rail, it makes sense to return the diode to the
rail.

ESD protection design is no more or less important and worthy of careful
consideration than any other part of a design.

Whenever I've ever done inputs, I just use a couple of ordinary 1N4148s or
1N914s, one to each rail. Sometimes a 10R resistor in series, or so,
depending.

Cheers!
Rich

 

[Pooh Bear]

Hi,
I normally connect Zener diodes for ESD protection to ground so that
the ESD event will be shunted to ground. However i have noticed a
design that actually connects the ESD zener diode to the power rail
(3.3V).
Does anyone know why it is done this way instead of shunting it to
ground? What would be the advantages and disadvantages of doing it
this way?

Well..... it would avoid 'ground noise' that might cause malfunctioning.

As others have noted, zeners aren't really good at this. Small signal
diodes to ground and supply make more sense. You'll need a smallish
value limiting resistor before them too. Most chips now actually have
ESD specs for their I/O but the above gives extra 'security'.

Graham

 

[[email protected]]

Thanks for the answers. They were really helpful.

I was wondering also another possible problem with connecting a zener
diode to ground would be that if the ground is too noisy, the diode may
actually turn on causing noise signals to travel from ground into the
signal line.

 

[R.Lewis]

Thanks for the answers. They were really helpful.

I was wondering also another possible problem with connecting a zener
diode to ground would be that if the ground is too noisy, the diode may
actually turn on causing noise signals to travel from ground into the
signal line.

Noisy with respect to what?

If the signal ground is noisy the noise is already injected.