The datasheet clamping voltage is quoted at a current your surge probably exceeds. This works out the real clamp from dynamic resistance, adds the L·di/dt your layout contributes, and compares it to what the pin can survive.
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A TVS quotes its clamping voltage at a specific peak current, and that current is often far below what your actual surge delivers. Between breakdown and that rated point the device has a dynamic resistance, and the clamp rises linearly along it.
A part specified at 9.2 V clamping for 10 A has about 280 mΩ of dynamic resistance. Hit it with an IEC 61000-4-2 8 kV contact discharge — 30 A peak — and it clamps at roughly 14.8 V, not 9.2. If the pin behind it is rated 6 V absolute maximum, the datasheet number was reassuring and wrong.
Rdynamic = (VC − VBR) / IPP
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Vclamp(I) = VBR + Rdynamic · IVpin = Vclamp + L · di/dt — the track is part of the circuit
An ESD strike rises in about a nanosecond. Thirty amps in one nanosecond through five nanohenries of track is 150 V — an order of magnitude more than the clamp itself. That voltage appears at the protected pin regardless of how good the TVS is.
Which means TVS placement matters more than TVS selection. The part belongs immediately at the connector, with the shortest possible path to a solid ground, and the protected trace routed past it rather than teeing off it. A perfect TVS 20 mm from the connector protects considerably less than a mediocre one placed properly.
A TVS is a large semiconductor junction and it hangs across the line. On a slow signal that is irrelevant; on USB, HDMI or Ethernet it rounds edges and closes the eye. Above roughly 3–5 pF you will see it on a high-speed line.
The standard answer is steering diodes: small, fast, low-capacitance diodes route the surge up to a rail or down to ground, where a single larger TVS absorbs it. Only the small diodes' capacitance loads the signal.
VRWM must exceed your signal's maximum, including tolerance and any ringing, or the TVS conducts during normal operation — which shows up as a mysteriously loaded line or a part that runs warm. But every volt of stand-off above what you need is a volt added to the eventual clamp. The gap between "does not conduct in service" and "clamps low enough to protect" is where the selection actually happens, and on 3.3 V logic with a 4 V absolute maximum it can be uncomfortably narrow.
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