TVS Diode Selection for RS-485 & CAN Bus Protection: Key Specs Compared

TVS Diode Selection for RS-485 & CAN Bus Protection: Key Specs Compared

Why This Matters in 2026

IEC 61000-4-2 and IEC 61000-4-5 have both moved to Edition 3 in the last two years, bringing tighter calibration tolerances and stricter waveform verification. Designs that marginally passed ESD or surge testing under the old editions may now fail recertification. We've seen boards that sailed through Edition 2 come back flagged on waveform verification under Edition 3 — same layout, same part, nothing changed. For RS-485 and CAN bus interfaces — ubiquitous in industrial automation, automotive ECUs, and building management — selecting the right TVS diode is no longer a "slap on any bidirectional part" decision. Three parameters determine whether your protection works or your communication link drops: clamping voltage, capacitance, and asymmetry.

Understanding the Three Critical Parameters

1. Clamping Voltage (VC) — The Hard Limit

The TVS must clamp below the absolute maximum rating of the transceiver IC. A common rule of thumb: VC(max) ≤ 0.95 × Abs Max of the protected device. Check this one first — if clamping is wrong, no other parameter saves you.

For RS-485, transceivers like the TI THVD1500 have a ±18 V absolute maximum at the bus pins. The SM712 (Semtech / Littelfuse) was designed specifically for this: its asymmetrical design provides a 26 V clamp on the positive side (pins 1→3) and 12 V on the negative side (pins 3→1). When a fault drives the bus negative relative to ground, the 12 V clamp engages before the transceiver sees damaging voltage.

For CAN bus, transceivers have higher headroom — the TI SN65HVD1040A tolerates -27 V to +40 V. The CDSOT23-T24CAN (Bourns) clamps at 36 V (at 5 A, 8/20 µs) and has a minimum breakdown voltage of 26.2 V — meaning it won't conduct during a 24 VDC power cross event. The NUP2105LT1G (onsemi) clamps at 44 V and is AEC-Q101 qualified for automotive use.

2. Capacitance — The Signal Integrity Killer

Every TVS adds shunt capacitance to the bus. Too much, and your signal edges slow down, closing the eye diagram.

The NUP2105LT1G has ~30 pF per line, acceptable for CAN 2.0 but marginal for CAN FD. The CDSOT23-T24CAN comes in at 22 pF (line-to-ground). For high-speed RS-485, these older parts won't cut it — look at ultra-low capacitance arrays like the AQ4024 (Littelfuse) at 1.3 pF, designed specifically to avoid eye closure at high bit rates.

3. Asymmetrical vs. Symmetrical — RS-485 Is the Tricky One

This is where most designs go wrong. RS-485 transceivers operate over a common-mode range of -7 V to +12 V. A standard symmetrical bidirectional TVS (like the NUP2105LT1G) has identical breakdown in both directions — it must be rated above 12 V to avoid clipping the positive signal, but that same rating leaves the negative side under-protected. We've pulled dead transceivers off boards that made exactly this mistake.

The SM712 solves this with an asymmetrical design: 12 V reverse stand-off on the positive channel and 7 V on the negative channel. This matches the RS-485 common-mode profile exactly. For general symmetrical applications — CAN bus, where both lines swing symmetrically around a common-mode midpoint — a standard bidirectional part is fine.

Part Comparison Table

PCB Layout Rules That Actually Matter

Parts get you most of the way; layout is where the last few dB come from.

• Place the TVS right at the connector, within 5 mm of the exposed pin. Every millimeter of trace adds inductance that degrades clamping response.
• Keep the loop tight: the current path from connector pin → TVS → ground plane must be as short and wide as possible.
• Put the TVS before the common-mode choke (closer to the connector), not after. A surge that saturates the choke renders it useless; the TVS must shunt the energy before it reaches the choke.
• Match trace lengths on differential pairs to avoid skew.

Bringing It All Together

For a typical RS-485 design at 10 Mbps with a ±18 V transceiver: use the SM712 (Semtech or Littelfuse). The asymmetrical clamping matches the common-mode range, and 45 pF is safe up to 10 Mbps. For CAN 2.0: the NUP2105LT1G or CDSOT23-T24CAN both work — pick the Bourns if you need lower capacitance for margin. For CAN FD at 5+ Mbps: step up to the AQ4024 at 1.3 pF.

Need to verify availability and pricing? Search TVS diode inventory on PartsCube Global — we stock these parts with real-time market pricing and cross-reference by manufacturer.

References

• IPC Standards Library
• IEEE Xplore Digital Library