TVS selection guide - Is it wrong, or i am not comprehending?

[eTobey]

In the Manual it states:
"For example, if the protected line operates at 5 V nominal with a maximum variance up to 7 V, ensure that the VRWM is 7 V or greater."

So if i choose a Diode with 7V Vrwm, then the diode starts to really conduct just from lets say 7.5V in a ESD event. So that gives me a voltage, that exceeds the maximum values of 7V, wouldnt it?

[tooki]

Sorry, what’s confusing you here?

They’re simply saying “even if the nominal voltage (=Nennspannung) is 5V, if its signal normally can reach 7V even temporarily, then choose a diode with Vrwm above 7V”.

The “variance” they mention doesn’t mean the absolute maximum value from the component datasheet; if your signal exceeded that, it would fry the chip even without ESD. A 7V signal would already be far outside the 5.5V absolute maximum of typical TTL, for example, but well within 4000-series CMOS logic.

So if you’re protecting TTL with an absolute maximum of 5.5V, you’d need to choose a 5V TVS.

[sparkydog]

As noted, "maximum variance" is talking about your maximum normal operation voltage, as in, you don't want the TVS mucking with this. A TVS should never conduct under normal input voltage. A TVS is not intended to be used as a voltage regulator, and doing so can damage it prematurely because they are not meant to conduct continuously.

(That said, note that it's not unusual to use a TVS to "eat" inductor spikes e.g. from relays / solenoids. Although that is "normal operation", it isn't continuous conduction because the spikes are brief.)

So if you’re protecting TTL with an absolute maximum of 5.5V, you’d need to choose a 5V TVS.

Uh... if your nominal voltage is 5V and 5.5V releases the magic smoke, I don't think a TVS is going to work. Now, to be sure, SMBJ is a commodity TVS and maybe more "sensitive" TVSs exist, but a 5V nominal SMBJ has a breakdown voltage (V_BR) rating of  6.40V - 7.00V and a maximum clamping voltage (V_C) of 9.2V. Say goodbye to your TTL.

Personally, if I'm using a TVS to protect something, I try to arrange for its V_C to be around 60-70% the max voltage rating of the device it's protecting. (For example, I have designs that use SMBJ18s with components rated for 48V.)