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ESD protection

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alex

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Hi everybody,

I realize this may be an overly-general question, but I'm wondering how to add ESD protection to my circuits. I know there are about a million ways to do this, and some are better for high-speed, some for low speed. Right now I only need it for very low speed logic levels, but who knows what I will try and build in the future? :)

So my question, is there a simple way to protect inputs that will work for most cases (including serial port and I2C speeds)? Like a single SMT component I can just solder between the input and ground?

Thanks,
Alex
 
TVS Diode- unidirectional for unipolar inputs, bidirectional ones are also available. THey tend to be expensive though...well I find them expensive for the amount I would like to use.
 
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TVS Diode- unidirectional for unipolar inputs, bidirectional ones are also available. THey tend to be expensive though...well I find them expensive for the amount I would like to use.

Thanks! I just looked on digikey, and they have a whole lot of parameters to choose. Does unipolar mean a signal that is just biased one way (+ or -) and does not swing past the 0-line? For example 0-5v is unipolar, and if I had a differential signal like -3.3 to +3.3 I'd use a bidirectional?

next, there's power. How do you know what kind of power ESD would dissipate?

Breakdown voltage I understand - it will prevent the signal from exceeding this voltage. And if you set it too low (say 4.5v in a 5v circuit) the TVS will need to dissipate the rest of the voltage. Do you need a resistor in series so it doesn't burn up? Or is that what you use the power rating for?

Finally, they give you Voltage - Reverse Standoff (Typ). What is that?

One last question, can these be used for analog as well?

Thanks so much,
Alex
 
Unipolar means there is only one TVS diode, meaning it will conduct normally one direction and bidirecitonal means two back to back TVS diodes. So bipolar will clamp a line symmetrically above and below the the ground, and unipolar will clamp assymetricaly (it will clamp within breakdown voltage above the ground rail, but below the ground rail, it will conduct like a normal diode and clamp to the forward voltage of the diode so it better protects unipolar systems).

Power for ESD...get the smallest one you can? THe energy is so small in an ESD pulse because it's so transient so it does not have a continuous rating. It's kind of a black art to be honest with different ESD waveform shapes and shocking scenarios.

You do not need a resistor for ESD...the energy is so short that the diode can dissipate the heat all on it's own. A resistor would add additional protection though because unlike the TVS diode that has to switch on to protect the circuit, the resistor is always "on". Another result is that it allows for smaller TVS diodes to handle larger transients. But for ESD, don't worry about burning up. It's not for continuous or heavy energy clamping like other overvoltage circuits.

As for standoff voltage or working voltage...there's a difference between that and breakdown voltage. Breakdown voltage is what it the TVS clamps to but there are manufacturing tolerances of the breakdown voltage and breaking down also isn't a clean-cut threshold (you don't want the diode to be semi-breaking down near the extremes of operation). So there is a gap between the breakdown voltage and the regular operating voltage rating- this is the standoff/working voltage.

They can be used for analog, though depending on the signal they may affect integrity (like adding stray capacitors to RF circuits). It tends to be fine for most hobbiest level applications though (you're not running RF or MHz serial signals right?)
 
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Unipolar means there is only one TVS diode, meaning it will conduct normally one direction and bidirecitonal means two back to back TVS diodes. So bipolar will clamp a line symmetrically above and below the the ground, and unipolar will clamp assymetricaly (it will clamp within breakdown voltage above the ground rail, but below the ground rail, it will conduct like a normal diode and clamp to the forward voltage of the diode so it better protects unipolar systems).

Power for ESD...get the smallest one you can? THe energy is so small in an ESD pulse because it's so transient so it does not have a continuous rating. It's kind of a black art to be honest with different ESD waveform shapes and shocking scenarios.

You do not need a resistor for ESD...the energy is so short that the diode can dissipate the heat all on it's own. A resistor would add additional protection though because unlike the TVS diode that has to switch on to protect the circuit, the resistor is always "on". Another result is that it allows for smaller TVS diodes to handle larger transients. But for ESD, don't worry about burning up. It's not for continuous or heavy energy clamping like other overvoltage circuits.

As for standoff voltage or working voltage...there's a difference between that and breakdown voltage. Breakdown voltage is what it the TVS clamps to but there are manufacturing tolerances of the breakdown voltage and breaking down also isn't a clean-cut threshold (you don't want the diode to be semi-breaking down near the extremes of operation). So there is a gap between the breakdown voltage and the regular operating voltage rating- this is the standoff/working voltage.

They can be used for analog, though depending on the signal they may affect integrity (like adding stray capacitors to RF circuits). It tends to be fine for most hobbiest level applications though (you're not running RF or MHz serial signals right?)

Hi,

Ok that helps a lot. I understand most of that, but I'm still a bit confused as to the voltages. For the "Voltage - Reverse Standoff (Typ)" parameter, I'm not sure what they mean by "reverse". Is the operating voltage the breakdown voltage minus the "Voltage - Reverse Standoff (Typ)"? So if you had a chip that operates at 5v and is damaged above 9v, you'd pick breakdown=9v and reverse standoff=4v?

Alex
 
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