Automotive-grade bidirectional TVS in a compact SMA package
The STMicroelectronics SM4T68CAY is a bidirectional TRANSIL™ TVS diode from the SM4TY series, designed for automotive applications and qualified to AEC-Q101. It provides 400W peak pulse power protection with a 58V reverse standoff voltage, making it suited for 48V automotive bus protection, sensor lines, and general-purpose surge clamping on power rails in engine control units, body controllers, and infotainment modules. The DO-214AC SMA surface-mount package fits standard pick-and-place assembly and reflow profiles used in high-volume automotive PCB production.
Voltage thresholds and clamping performance
With a minimum breakdown voltage of 64.4V and a maximum clamping voltage of 121V at the peak pulse current, this part protects downstream circuitry from transients up to 19A (8/20µs waveform). The 58V reverse standoff voltage means it does not conduct at normal operating levels up to that DC voltage, avoiding unnecessary leakage in steady-state conditions. For a 48V nominal automotive rail that sees load-dump or alternator-field decay events, the 64.4V breakdown threshold provides margin above the normal operating range before the device starts to clamp.
Temperature range and thermal reliability
The 400W peak pulse power rating is derated with temperature per the TRANSIL datasheet curves — at 150°C junction, derate to approximately 50% of the room-temperature rating. For high-energy pulses or elevated ambient temperatures, consider the pulse-width derating factors for the specific waveform.
Package and mounting for automotive production
The DO-214AC SMA package (also designated SMA) is a standard JEDEC outline with a small footprint — roughly 4.6 mm by 2.8 mm body — suitable for dense PCB layouts. The surface-mount design supports reflow soldering with typical lead-free profiles. The bidirectional configuration uses a single die, so no polarity marking is needed, simplifying pick-and-place orientation.
For production builds requiring AEC-Q101 qualified TVS protection, this part is a stable BOM line item. No immediate LTB risk, and the TRANSIL series has broad automotive market adoption with multiple package variants for design flexibility.