KEMI Docs · circuit practice
ESD Protection Design — Choosing and Placing TVS Diodes
Last updated: 2026-09-16
A human discharge is thousands of volts lasting nanoseconds. The MCU's internal protection diodes survive a few of them, then the pin dies, and the symptom is 'it resets sometimes' or 'USB stopped enumerating one day'. This article sets out which pins, which TVS ratings and where to put them, with practical values.
1
Which pins need protection
One criterion: every conductor that leaves the enclosure or can be touched. USB, UART, SD card, audio jack, buttons, touch pads, external sensor connectors, power input. Signals that stay on the board (an internal I2C sensor, SPI flash) do not qualify. Because protection must act at the entry point, once the pins are chosen the part location follows automatically: right beside the connector.
2
Three numbers on a TVS datasheet
| Item | Meaning | Selection rule |
|---|---|---|
| VRWM (working voltage) | stays quiet with no leakage up to here | above the signal maximum — 5V class for 3.3V signals |
| VC (clamping voltage) | what it actually holds the line to during a surge | below the protected IC's absolute maximum |
| CJ (capacitance) | parallel capacitance added to the line | USB 2.0 ≤ 3 pF, USB 3/HDMI ≤ 0.5 pF, UART/buttons don't care |
The usual target is IEC 61000-4-2 contact ±8 kV, air ±15 kV. Most small-signal TVS parts meet it, so the real choice comes down to those three numbers. Read the clamping voltage at the '1 A, 8/20 µs' condition, not the 8 kV headline, to match reality.
3
Placement — distance and path are everything
- Connector pin to TVS within 5 mm, and the signal continues to the IC only after the TVS (no stubs — if the trace runs to the IC first with the TVS on a branch, there is no protection).
- TVS ground short and wide, ideally two vias straight to the plane. Inductance in this path adds tens of volts to the clamp.
- Connector shells go to chassis ground, meeting signal ground at one point or through 1 MΩ‖1 nF.
- A series resistor after the TVS (22-100 Ω) knocks down the remaining energy. Effective on UART, buttons and external I2C, at essentially zero cost.
4
Practical values per interface
| Interface | TVS example | Note |
|---|---|---|
| USB 2.0 D+/D− | VRWM 5V, CJ ≤ 1 pF, 2-channel array | separate 5V TVS on VBUS |
| External UART/GPIO | VRWM 5V, capacitance irrelevant | + series 100 Ω |
| Buttons/touch | VRWM 5V or a cheap varistor | + series 1 kΩ + 100 nF doubles as debounce |
| 12-24V power input | VRWM 24-30V, 400-600W peak pulse (SMA/SMB) | with reverse diode and fuse |
| RS-485/CAN | dedicated bidirectional TVS (±12 to ±36V) | alongside a common-mode choke |
The most common failure
The TVS is present but sits next to the IC, or its ground runs through a thin trace. It is on the BOM and protects nothing. With ESD parts, 'where' is 90% and 'which' is 10%.
FAQ
Aren't the MCU's internal diodes enough?
They are rated for a few mA, survive a single ~2 kV event, and degrade with repetition. External TVS is the rule for any touchable pin.
Varistor or TVS?
TVS responds faster (ps) with an accurate clamp — use on signal lines. Varistors are cheap and absorb more energy — good for AC inputs and slow lines like buttons.
What if I use a high-capacitance TVS on a fast line?
The waveform collapses and communication fails. USB 3.0, HDMI and LVDS require ultra-low-capacitance parts under 0.5 pF.
See also
This article summarises the working rules KEMI uses in real design and fabrication work. For production, part datasheets and fab specifications take precedence.