TVS clamping voltage checker

The comparison a TVS datasheet is asking you to make: the standoff voltage against the line's normal swing, the IEC strike translated into TLP current, and the clamping voltage at that current — read off the dynamic-resistance line — against what the protected pin tolerates.

Fig 1 — the TLP line: 10.3 V at 16 A against a 12 V pin — clamped below the limit.

Line voltage (V)

TVS standoff voltage (V)

Strike

Clamping voltage (V)

…specified at (A)

Dynamic resistance (Ω)

Downstream pin limit (V)

Strike current: 16 A TLP for ±8 kV contact (SLLA305 Table 4-10)
Clamp at that current: 10.29 V along the TLP line
Standoff: OK — 2.20 V above the 3.3 V line
Downstream pin: protected — 1.71 V under its 12 V limit

The comparison passes on paper. The layout still decides: the diode belongs at the connector, its ground return must not detour, and a strike that can bypass it through a nearby trace will. TI SLVA680 covers the placement rules.

How this is calculated

Standard: TI SLLA305 — Reading and Understanding an ESD Protection Data Sheet

$V_{RWM} \geq V_{line}$: The reverse standoff (working) voltage is the recommended operating voltage of the protected line: below it the diode leaks nanoamps, above it the diode is part of the circuit.
$I_{TLP} = 2 \,\text{A/kV}: \quad 2\,\text{kV} \to 4\,\text{A}, \;\; 8\,\text{kV} \to 16\,\text{A}$: SLLA305 Table 4-10, the equivalence between IEC 61000-4-2 contact levels and TLP current — clamping voltages are most often specified at the 16 A / 8 kV point.
$V_{clamp}(I) = V_{clamp}(I_{ref}) + R_{dyn} \cdot (I - I_{ref})$: The TLP line: SLLA305 derives clamping voltages from the TLP plot’s breakdown point and dynamic resistance. The tests reproduce its example device (8.2 V at 5 A, 0.19 Ω → 10.4 V at 16 A) to within 0.15 V.
$V_{clamp}(I_{peak}) \leq V_{pin,max}$: The verdict: what the downstream pin sees during the strike, against what it tolerates.

Assumptions

The TLP line is a straight line. Snap-back devices (SCRs, some deep-snapback clamps) fold back after triggering and this extrapolation over-estimates their clamp — read their TLP plot directly.
The strike reaches the diode first. Layout that lets the transient couple past the diode, or a ground return that detours, adds L·di/dt the datasheet never saw — TI SLVA680 covers placement.
TLP current, not the raw IEC waveform: the table’s 2 A/kV equivalence is how TI specifies parts, and it is the comparison the datasheet numbers support.
The downstream limit is yours to choose honestly: interface pins survive far above their DC absolute maximum for nanoseconds, and the DC figure is the conservative stand-in when nothing better is published.
Surge (IEC 61000-4-5, 8/20 µs) carries far more energy at a given current — use the custom current with the surge I_PP rating, and check the part’s surge power rating separately.

What a TVS actually clamps to

A TVS diode datasheet is a comparison waiting to happen, and most of the bad picks come from comparing the wrong pair of numbers. The breakdown voltage is not what the pin sees. The standoff voltage is not the protection level. The number that reaches the protected pin during a strike is the clamping voltage at the strike current, and TI SLLA305 — a guide to reading these datasheets — is where this page's method comes from.

Three checks, in order. The standoff voltage must sit at or above the line's normal swing, or the diode conducts every day instead of on the bad day. The strike level translates to current — SLLA305's table puts IEC 61000-4-2 contact levels at 2 A of TLP current per kV, which is why clamping voltages are usually specified at 16 A for the 8 kV level. And the clamp at that current, extended along the dynamic-resistance line from wherever the datasheet specified it, must land under what the protected pin tolerates.

Worked example: a 3.3 V line against an 8 kV strike

The defaults are SLLA305's own example device on a 3.3 V line: 5.5 V standoff, clamping specified as 8.2 V at 5 A TLP, dynamic resistance 0.19 Ω, designing for ±8 kV contact.

standoff   5.5 V ≥ 3.3 V                        OK, 2.2 V of margin
strike     ±8 kV contact  →  16 A TLP           (2 A per kV)
clamp      8.2 + 0.19 × (16 − 5)  =  10.29 V
           (datasheet's own 16 A row says 10.4 V — the line is honest)
pin limit  12 V  →  protected, 1.7 V of margin

The instructive failure is picking a much higher-standoff part for the same 3.3 V line "for safety": breakdown scales up with standoff, the clamp scales up with breakdown, and the 12 V pin takes the strike anyway. Standoff close to the line, not far above it, is what protects.

Where the clamping model stops being valid

The straight TLP line is the model, and two device families bend it. Snap-back clamps trigger high and fold back low — extrapolating their clamp linearly overstates it, which at least errs safe. Deep-breakdown varistors do the opposite at high currents. When the datasheet gives a TLP plot, the plot wins over this arithmetic.

The comparison also assumes the strike actually flows through the diode. That is a layout property, not a datasheet one: the diode goes at the connector, before anything else the trace touches; its ground return goes straight down, not on a stub; and no protected trace runs close enough to the unprotected side to take the hit inductively. A perfect part in the wrong place clamps nothing.

Surge is a different animal wearing the same units. An IEC 61000-4-5 8/20 µs pulse at 6 A carries orders of magnitude more energy than an ESD strike at 16 A; the clamp arithmetic here still applies via the custom current, but the part must also carry a surge power rating (P_PP) that covers it, which this page does not check.

Common TVS selection mistakes

Comparing V_RWM to the pin limit. Standoff is where the diode is off; the pin sees the clamp, which is always higher — sometimes by a factor of two.
Grabbing the highest-voltage TVS on the shelf. Every volt of standoff above the line raises the breakdown and the clamp with it. The right part hugs the line voltage.
Reading the clamp at 1 A when designing for 8 kV. The datasheet's most flattering row is the lowest-current one; the strike happens at 16 A.
Treating an IEC-rated diode as system-level compliance. The rating says the diode survives; whether the system does depends on the clamp, the pin behind it, and the layout between them.
Adding series resistance nowhere. Where the interface tolerates it, a few tens of ohms between diode and pin divides the residual spike for free — often the difference when the clamp margin is thin.