SM4T33A - 33V 400W TRANSIL TVS Diode | STMicroelectronics
MPN: SM4T33A ✓ Active| Qty | Unit Price | Extended |
|---|---|---|
| 1 | $0.42 | $0.42 |
| 10 | $0.36 | $3.60 |
| 100 | $0.28 | $28.00 |
| 500 | $0.21 | $105.00 |
| 1,000 | $0.16 | $160.00 |
SM4T33A Overview
A TRANSIL is a specific class of avalanche TVS diode whose role is to clamp transient overvoltages (ESD, inductive load switching, lightning-induced surges) to a safe level, protecting downstream semiconductors. TVS diodes sit within the broader protection-component hierarchy of surge protectors, which themselves fall under discrete semiconductors used in power management and interface protection. SM4T-series parts are positioned as general-purpose surface-mount TVS diodes with a low clamping factor and a fast response time, intermediate in power handling between SMA-packaged 400W parts and larger SMB/SMC 1000W-1500W parts.
Key electrical features of the SM4T33A include a stand-off voltage (VWM) of 33V, a minimum breakdown voltage (VBR min) around 36.7V, a maximum clamping voltage (VC) of approximately 53.3V at IPP, and a peak pulse current handling compatible with the 400W/1ms rating. The device is offered in unidirectional (-A suffix) and bidirectional (-CA suffix) variants, providing flexibility for DC rail protection and AC line protection respectively. The low clamping factor minimizes the voltage excursion seen by protected circuitry during a transient event.
Architecturally, the SM4T33A uses a silicon pn-junction avalanche structure optimized for fast turn-on (typically sub-nanosecond response), and the SMA package's C-bend terminations provide a robust solder joint capable of withstanding the thermal stress of repeated surge events. Maximum junction temperature is 150C with operating range typically -55C to +150C (per the SMA TRANSIL datasheet family).
Typical applications include DC power rail protection in 24V industrial systems, protection of microcontroller I/O pins, secondary protection on telecom line interfaces, USB and CAN bus protection, and general ESD/surge protection on PCB signal lines exposed to external cables. The unidirectional polarity suits DC rails, while bidirectional variants suit AC or signal-line applications.
When designing with the SM4T33A, ensure the working voltage is below VWM and that the protected device's maximum rating exceeds VC. Place the TVS as close as possible to the entry point of the transient, with short, wide PCB traces to minimize parasitic inductance that would otherwise let voltage overshoot during fast-edge transients.
This page synthesizes distributor pricing, drop-in SMA-package alternatives, and practical PCB-layout design notes not collected in the manufacturer datasheet alone, giving the engineer a single reference for sourcing and design-in decisions.
Drop-in alternatives for SM4T33A — same package, pin-to-pin compatible. Different-package parts requiring PCB rework are excluded.
Quick Comparison Tool — Select alternative parts for side-by-side comparison:
SMAJ33A
✅ Drop-In📋 Reference alternative (not in catalog)
SMAJ33A-E3/61A
✅ Drop-In📋 Reference alternative (not in catalog)
SMAJ33CA
✅ Drop-In📋 Reference alternative (not in catalog)
SMAJ33CA-E3/61A
✅ Drop-In📋 Reference alternative (not in catalog)
SM4T33CA
✅ Drop-In📋 Reference alternative (not in catalog)
SMAJ30A
✅ Drop-In📋 Reference alternative (not in catalog)
SM4T33A Maximum Ratings & Electrical Characteristics
| Part Number | SM4T33A |
| Manufacturer | STMicroelectronics |
| Product Family | SM4T (TRANSIL) series |
| Device Type | Transient Voltage Suppressor (TVS) diode |
| Polarity | Unidirectional |
| Package | SMA / DO-214AC |
| Mounting Type | Surface Mount |
| Stand-off Voltage (VWM) | 33 V |
| Operating Temperature Range | -55C to +150C (per SM4T family) |
| Maximum Junction Temperature | 150 C |
| JEDEC Registration | Yes (SMA package outline) |
| RoHS Status | Compliant (typical for SM4T family) |
| Terminal Form | C-Bend |
| Terminal Position | Dual |
SM4T33A Pin Configuration
| Pin 1 | Cathode (K) — Banded terminal - connects to protected rail/signal |
| Pin 2 | Anode (A) — Unbanded terminal - connects to ground for DC-rail clamping |
Peak Pulse Power vs. Pulse Duration (SM4T series)
Typical Applications
SM4T33A is suitable for 8 applications: 24V Industrial DC Rail Surge Protection, USB-C VBUS and CC Line Protection, Automotive 24V Truck/Bus System Protection, RS-485 / CAN Bus Transient Protection, Industrial Sensor I/O Protection, Secondary Protection on Telecom Line Interfaces, Power Supply Output Protection, LED Driver Output Protection.
24V Industrial DC Rail Surge Protection
The SM4T33A clamps inductive switching transients on 24V industrial DC rails (PLCs, motor-drive auxiliaries, sensor power buses) before they reach sensitive downstream microcontrollers and signal-conditioning ICs. Its 33V VWM sits comfortably above the 24V nominal rail plus tolerance (typically 28V at full load), so the device stays in high-impedance standby during normal operation and only conducts during an overvoltage event. The 400W peak pulse power on 10/1000 us is sufficient to absorb typical IEC 61000-4-5 24V-system surge waveforms; for harsher environments (outdoor, long cables), step up to the SM6T68A in SMB. Place the TVS close to the connector entry with a low-impedance ground return.
Recommended
USB-C VBUS and CC Line Protection
The SM4T33A placed on the VBUS rail of a USB-C source or sink protects the downstream PD controller and power path from inductive kickback and cable-discharge transients. Its 33V VWM is well above the 20V USB-PD maximum, so it does not clamp during normal negotiation, and its sub-nanosecond response time catches ESD events faster than the PD controller's internal protection. For USB-C CC/SBU lines where signal integrity matters, the bidirectional SM4T33CA is the right choice. SMB or SMC-packaged parts are not needed because USB-C cable lengths are short and the surge environment is bounded by the source-side cable inductance.
Recommended
Automotive 24V Truck/Bus System Protection
In heavy-vehicle (24V truck, bus, agricultural machinery) electrical systems, the SM4T33A clamps load-dump and inductive transients that exceed the 32V jumpstart/load-dump envelope defined in ISO 7637-2. Its 33V stand-off sits below the ISO 7637-2 24V-system suppressed load-dump peak, so it clamps the bulk of the surge energy before reaching ECUs and sensor modules. For production automotive designs, prefer the AEC-Q101-qualified SMAJ33A-Q or equivalent instead, since the standard SM4T33A is not explicitly AEC-Q qualified per the family datasheet. Place on each connector pin and on internal DC rails feeding MCUs.
Recommended
RS-485 / CAN Bus Transient Protection
The SM4T33A protects RS-485 and CAN bus transceiver I/O pins from outdoor/cable-induced surge transients that exceed the transceiver's built-in ESD rating. Although transceiver ICs include +-8 kV HBM and +-15 kV air-gap ESD, system-level surges (lightning-induced, motor-drive switching) require external TVS clamping. Placing the SM4T33A between each data line and ground at the cable-entry connector limits the surge voltage to the SM4T33CA clamping voltage, preserving the transceiver. Choose the bidirectional SM4T33CA since RS-485 and CAN are differential signals with both positive and negative excursions; the unidirectional SM4T33A suits single-ended DC rail only.
Recommended
Industrial Sensor I/O Protection
The SM4T33A clamps ESD and surge events on 24V-loop-powered sensor inputs (pressure, level, flow transmitters in process automation) at the cable-entry terminal block, before the signal reaches the analog front-end ADC or the 4-20 mA loop driver. Its low clamping factor keeps the overvoltage excursion tight, which is important for protecting precision analog stages with limited input range. Pair with a TVS protection diode on the loop supply rail and a series PTC/rheostat for sustained overcurrent events. SMA footprint fits standard industrial terminal-block PCB layouts without sacrificing density.
Recommended
Secondary Protection on Telecom Line Interfaces
The SM4T33A provides secondary surge protection on telecom line-interface cards (FXS, FXO, ISDN, xDSL) downstream of the primary gas-discharge tube or thyristor-based protector. After the primary shunts the bulk of the surge current to ground, the SM4T33A clamps the residual voltage to below the line-driver IC's absolute-maximum rating. Its 33V VWM is suitable for the -48V telecom rail when used with appropriate series resistance or coordination with the primary protector. The bidirectional SM4T33CA is preferred since telecom lines carry both polarities of ringing voltage and transient events.
Recommended
Power Supply Output Protection
The SM4T33A is placed at the output of an isolated AC-DC or DC-DC converter to protect downstream electronics from overvoltage events caused by load-side faults, external miswiring, or upstream supply transients passed through the isolation barrier. Its 33V VWM is suited for 24V-output converter rails; for 12V outputs the SM4T12A or SM4T15A in the same SMA footprint is appropriate. Adding the SM4T33A at the output rail reduces field-failure rates by absorbing fault energy that would otherwise reach the load. Combine with an upstream fuse for sustained-event protection.
Recommended
LED Driver Output Protection
The SM4T33A clamps inductive kickback from LED string wiring on industrial LED drivers (24V or higher architectures), protecting the driver output stage from flyback transients when an LED string opens or shorts. Its 33V stand-off suits 24V LED strings with margin; for higher-voltage LED strings (48V, 60V), step up to the SM4T47A or SM4T68A in the same SMA package. Place close to the LED connector with a wide, short ground-return trace to maximize clamping effectiveness during the fast-edge flyback event.
Recommended
Recommended Products Summary
Engineering reference data for SM4T33A — comparison, design guidance, and compliance information.
Selection Guide
Comparison with Alternatives
| Parameter | This Product | SMAJ33A | SMAJ33A-E3/61A | SMAJ33CA | SMAJ33CA-E3/61A | SM4T33CA | SMAJ30A |
|---|---|---|---|---|---|---|---|
| Package | SMA (DO-214AC) | SMA (DO-214AC) - same | SMA (DO-214AC) - same | SMA (DO-214AC) - same | SMA (DO-214AC) - same | SMA (DO-214AC) - same | SMA (DO-214AC) - same |
| Brand | STMicroelectronics | Littelfuse | Vishay Intertechnology | Littelfuse | Vishay Intertechnology | STMicroelectronics | Littelfuse |
| Stand-off Voltage (VWM) | 33 V | 33 V | 33 V | 33 V | 33 V | 33 V | 30 V (-9%) |
| Polarity | Unidirectional | Unidirectional | Unidirectional | Bidirectional | Bidirectional | Bidirectional | Unidirectional |
| Peak Pulse Power (10/1000 us) | 400 W | 400 W | 400 W | 400 W | 400 W | 400 W | 400 W |
| Series / Family | ST SM4T TRANSIL | Littelfuse SMAJ TRANSZORB | Vishay SMAJ | Littelfuse SMAJ TRANSZORB | Vishay SMAJ | ST SM4T TRANSIL | Littelfuse SMAJ TRANSZORB |
| Automotive Grade (AEC-Q101) | Not qualified (standard) | Standard (AEC-Q101 in -Q suffix) | Standard (Vishay -AQ/E3 suffix for automotive) | Standard (AEC-Q101 in -Q suffix) | Standard (Vishay automotive variants exist) | Not qualified (standard) | Standard (AEC-Q101 in -Q suffix) |
| Approx. Unit Price (qty 1000, USD) | 0.16 | 0.14 | 0.13 | 0.15 | 0.14 | 0.17 | 0.14 |
Key Differentiators
- Mature ST TRANSIL family with global distributor stock (vs SMAJ33A (Littelfuse))
- Unidirectional polarity optimizes DC rail clamping with lower IR (vs SM4T33CA (bidirectional ST))
- Right-sized 400W rating avoids over-spec cost (vs SMCJ33A (1500W, SMC package))
Design Notes
Place the SM4T33A as close as physically possible to the connector entry or the point where the transient enters the PCB. Keep the ground-return trace short and wide - parasitic inductance in series with the TVS ground path allows voltage overshoot during fast-edge transients that can exceed the protected IC's absolute-maximum rating. A 4-8 mil (0.1-0.2 mm) trace longer than 5 mm can add several volts of overshoot at a 1 kV/us transient edge rate. Use a dedicated ground pour stitched with vias near the TVS for lowest-impedance return.
Do not select a TVS by peak pulse power alone. The protected IC must survive the clamping voltage (VC) seen at peak pulse current (IPP). For the SM4T33A on a 24V rail, confirm the downstream IC's Vmax exceeds the VC max of approximately 53-59V. If your protected device has Vmax < 50V, derate to a lower-VWM part (e.g., SM4T30A) - this reduces VC proportionally. Always cross-check both VC max and IR max on the substitute's datasheet when qualifying an alternative.
Estimated: peak pulse power 400W for 1ms on the SM4T33A dissipates 0.4 J of energy per event. With theta_JA approximately 150 C/W on a 1 oz copper SMA pad (JEDEC EIA-481 land pattern), a single 400W/1ms pulse raises junction temperature roughly 60-80C above ambient - well within the 150C TJmax. For repetitive surges, accumulate via thermal time constant; for sustained overvoltage events, the TVS will enter thermal runaway and fail short - verify the system's worst-case surge duty cycle against the SM4T PPPM-vs-tp derating curve.
The SM4T33A's sub-nanosecond turn-on time and low clamping factor minimize high-frequency ringing on the protected rail during a transient event. To preserve this EMI performance, minimize the loop area formed by the TVS, the protected IC, and the bypass capacitors - place all three within 5 mm of one another. For multi-layer boards with a dedicated ground plane, stitching vias near the TVS ground terminal keep the return path inductance low.
When protecting high-inductance loads (solenoids, relays, motors), the SM4T33A absorbs the inductive kickback energy (E = 0.5 * L * I^2) directly. Verify the per-event energy does not exceed 0.4 J for the SM4T33A's 1ms rating; for larger energies, choose a higher-power TVS (SMBJ33A in SMB package for 600W, SMCJ33A in SMC for 1500W) or add a freewheel diode in parallel with the inductive load to recycle the kickback energy.
Compliance Information
RoHS and lead-free per ST SM4T family datasheet. Not AEC-Q100/101 qualified - choose the automotive-graded variant (e.g., SMAJ33A-Q from Littelfuse) for automotive designs. Halogen-free status varies by date code; consult the lot-specific compliance report for definitive status.
