What this TVS diode protects and where it goes
The STMicroelectronics SM4T33AY is a unidirectional Zener-type transient-voltage-suppression (TVS) diode from the SM4TY series, rated for a peak pulse power of 400W. The DO-214AC (SMA) surface-mount package suits automated assembly flows common in high-volume automotive PCB production.
400W peak pulse — what that 10/1000 µs rating means for your rail
For the shorter 8/20 µs waveform (often used to represent ESD or faster transients), the device can handle a peak pulse current of 39A. The 28V reverse standoff voltage means this diode will not conduct appreciably below that rail voltage, so a 24V or 28V nominal supply bus stays protected without leakage issues during normal operation. The 59V clamping voltage is the maximum voltage the load sees during the transient — the load's own voltage tolerance must stay above that ceiling to avoid secondary damage. Because the part is a Zener-type (not an avalanche type with a different snap-back characteristic), the clamping voltage versus current curve is relatively flat, giving predictable protection across the current range.
DO-214AC SMA — footprint and assembly notes
The package is the standard DO-214AC (SMA) two-lead surface-mount package. The cathode band identifies the unidirectional orientation.
The SM4T33AY is a current-production part from STMicroelectronics, available through independent distribution channels. For a BOM line requiring a 28V rail clamp with automotive qualification, this diode is a direct fit. The part is not subject to a last-time-buy or end-of-life notice, so it can be specified for production runs without a short-term obsolescence review.