The STMicroelectronics SM4T28AY is a single-channel unidirectional Zener-type TVS diode from the automotive-grade SM4TY TRANSIL™ series. The 400W peak pulse power rating (10/1000µs waveform) and 46A peak pulse current (8/20µs) give it enough headroom for load-dump and inductive-load spikes common in automotive ECUs, body controllers, and sensor modules.
AEC-Q101 and the temperature range
The active lifecycle status means STMicroelectronics is still manufacturing it — no last-time-buy clock ticking, no need to hunt for obsolete-stock brokers yet.
24V system fit — the clamping numbers
For a 24V automotive system (nominal 28V alternator float, transients up to 36V or more), the 24V reverse standoff means the diode stays out of conduction during normal operation. The 26.7V minimum breakdown gives a guard band before the part starts clamping. The maximum clamping voltage of 50V at the peak pulse current is what the downstream circuitry sees during a surge — make sure your 24V-rated ICs have at least that much margin, or add a second stage. The single unidirectional channel is polarity-sensitive; the cathode goes to the protected rail, anode to ground.
Sourcing and field reality
In the field, the SMA package is straightforward to replace — no hot-air station needed, just a standard iron and tweezers. The part marking is clear enough to read without magnification, which helps when you are swapping it on a customer site without a lab bench.