STMicroelectronics SM4T39AY

Overview

SM4T39AY from STMicroelectronics is an AEC-Q101-qualified unidirectional automotive TVS diode in SMA package, designed to clamp transient surges up to 53.3 V (V_CL) at 7.5 A (I_PP, 10/1000 µs), with 400 W peak pulse power and 0.2 µA leakage at 25 °C. It protects CAN, LIN, and sensor interfaces in 12 V automotive systems against ISO 7637-2 pulses (Pulse 1: −150 V; Pulse 2a: +112 V) and ISO 10605 ESD (±30 kV).

For engineers reviewing the SM4T39AY datasheet, SM4T39AY pinout, SM4T39AY application, or SM4T39AY equivalent, this device is selected for robust front-end surge protection where low leakage, high T_j (150 °C), and JEDEC-standard SMA footprint are mandatory - especially in engine control units, body electronics, and ADAS sensor nodes requiring long-term reliability under thermal stress.

Technical Context

The SM4T39AY uses planar silicon junction technology to achieve stable clamping performance across −55 °C to +150 °C junction temperature, with dynamic resistance (R_D) of 0.884 Ω enabling precise voltage limiting during fast transients. Its unidirectional configuration provides asymmetric breakdown behavior optimized for DC-biased automotive signal lines.

It complies with ISO 7637-2 (Pulse 1, 2a, 3a, 3b) and ISO 10605 (150 pF/330 Ω and 330 pF/330 Ω), delivering verified clamping at 53.3 V (10/1000 µs) and 69.7 V (8/20 µs) while maintaining <1 µA leakage at 85 °C - critical for low-power always-on vehicle networks.

Key Specifications

Pinout & Package

SM4T39AY is housed in a standard SMA (DO-214AC) surface-mount package per IPC 7531, with JEDEC-registered outline and UL94 V0-compliant resin. The two-terminal device has no polarity marking on cathode band per Figure 19; orientation follows standard SMA tape-and-reel configuration (Figure 20–21).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar unidirectional automotive TVS applications.

Compared with SMAJ39A and 1.5SMC39A, SM4T39AY delivers tighter clamping (53.3 V), lower leakage (0.2 µA), and AEC-Q101 validation in a space-constrained SMA package - making it optimal for modern automotive ECUs where voltage margin, thermal density, and qualification compliance are non-negotiable.

Availability

SM4T39AY is available at Aetrix Electronics and suitable for engine control units, ADAS camera modules, and body control modules requiring stable component supply with full automotive qualification and traceable sourcing.

Supply support for SM4T39AY includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

STMicroelectronics is a global semiconductor leader headquartered in Geneva, Switzerland, designing and manufacturing microcontrollers, power devices, sensors, and protection components for automotive, industrial, and consumer markets.

The SM4TxxAY series belongs to ST's automotive-grade transient voltage suppression portfolio, engineered specifically to meet stringent ISO 7637-2 and ISO 10605 requirements for 12 V and 24 V vehicle electrical systems.

FAQ

Is SM4T39AY bidirectional or unidirectional?

SM4T39AY is a unidirectional TVS diode, meaning it clamps only in reverse bias (cathode positive relative to anode). Its marking "CMY" (per Table 6) and electrical characteristics table confirm unidirectional configuration - suitable for DC-biased lines like 12 V supplies or CAN_H referenced to ground.

What is the maximum clamping voltage at 8/20 µs surge?

Per Table 2, SM4T39AY clamps to 69.7 V at 7.5 A under 8/20 µs waveform. This value is measured at T_amb = 25 °C and defines the absolute upper voltage imposed on protected circuitry during fast-rising transients such as EFT or lightning-induced coupling.

Does SM4T39AY require derating above 25 °C ambient?

Yes - its peak pulse power decreases with rising junction temperature, as shown in Figure 3. At T_j = 125 °C, PPP drops to ~220 W (10/1000 µs); design must ensure thermal resistance (R_th(j-a)) and copper area (Figure 12) keep T_j ≤150 °C during worst-case surge repetition.

Can SM4T39AY replace SM4T33AY in the same footprint?

Yes - both share identical SMA package, pinout, and thermal profile. However, SM4T39AY has higher V_RM (33 V vs 28 V) and V_CL (53.3 V vs 45.4 V), so substitution requires verifying that protected circuit can tolerate the 7.9 V higher clamping voltage during transients.