It is rated for 400W peak pulse power (10/1000µs waveform) and clamps at 39.3V maximum with a 59A peak pulse current. The DO-214AC (SMA) surface-mount package suits automated assembly and dense PCB layouts.
For a BOM destined for a production vehicle, this qualification is the gatekeeper: a non-qualified part introduces risk of field failures under thermal or surge conditions.
Breakdown voltage and clamping: the numbers that matter
The minimum breakdown voltage is 20V, with a reverse standoff of 18V. That means the diode starts conducting (and clamping) when the rail exceeds 20V, protecting downstream circuitry from transients up to 39.3V at the clamping point. For a 12V or 18V automotive rail, this provides a comfortable margin above normal operating voltage while keeping the clamp below the typical 40V absolute maximum of many ICs. The 400W peak pulse power rating is the energy-handling capability—enough for ISO 7637-2 pulse 1 and 2a in most modules, though pulse 5a (load dump) may require a higher-power device.
Package and mounting: DO-214AC SMA
The SM4T21CAY comes in the DO-214AC (SMA) package, a standard surface-mount outline for TVS diodes. The SMA footprint is shared across many TVS families, so a layout designed for one SMA TVS can accept this part without board respin.