STMicroelectronics SMDB3

Overview

SMDB3 from STMicroelectronics is a surface-mount DIAC (Diode for Alternating Current) trigger device in SOT-23-3L package, designed for bidirectional breakover voltage control with V_BO = 28–36 V (typ. 32 V), breakover voltage symmetry ≤3 V, and I_BO ≤10 µA. It functions as a precise voltage threshold switch in phase-control circuits for motor dimmers and CFL/LED lamp ballast startup.

For engineers reviewing the SMDB3 datasheet, SMDB3 pinout, SMDB3 application, or SMDB3 equivalent, key selection criteria include its low breakover current, tight V_BO tolerance, symmetric forward/reverse triggering, and compatibility with triac gate drive timing in compact SMD lighting and power control designs.

Technical Context

The SMDB3 operates as a two-terminal, bidirectional semiconductor switch that conducts only after voltage across its terminals exceeds the symmetrical breakover threshold in either polarity. Its dynamic breakover voltage ΔV ≥10 V and rise time t_r ≤0.5 µs ensure fast, repeatable triggering under transient conditions.

Designed for use with external capacitor-resistor networks (e.g., C = 10 nF, R = 20 Ω), it delivers consistent peak current I_P ≥1 A and maintains leakage current I_R ≤1 µA at 0.5×V_BO(max), enabling reliable oscillator and phase-shift timing in AC line-synchronized circuits.

Key Specifications

Pinout & Package

SOT-23-3L surface-mount package with 1.7 mm × 1.3 mm footprint, 0.95 mm height, and gull-wing leads; JEDEC-compliant for automated pick-and-place and reflow soldering.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar DIAC trigger applications.

Compared with DB3 and SMDB4, SMDB3 offers the lowest breakover current (≤10 µA) and tightest V_BO symmetry (≤3 V) in an SMT package, making it optimal for precision timing in compact, high-reliability LED/CFL ballasts and low-power motor controls.

Availability

SMDB3 is available at Aetrix Electronics and suitable for LED lamp ballast startup, universal motor speed control, CFL starter replacement, and relaxation oscillator timing requiring stable component supply and SMT manufacturability.

Supply support for SMDB3 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, specializing in power management, analog, and discrete devices for industrial, automotive, and consumer applications.

The SMDB3 belongs to ST's DIAC series optimized for AC line-triggered power control, with design focus on reliability, symmetry, and SMT compatibility in lighting and motor control subsystems.

FAQ

What is the required external capacitor value for reliable SMDB3 triggering in a 230 V AC dimmer?

A 10 nF capacitor is specified in the datasheet for standard V_BO testing and provides stable triggering across the full 28–36 V range. For 230 V AC operation, a 4.7–10 nF Class X2 capacitor ensures safe, repeatable breakover while limiting I_BO-induced timing drift over temperature.

Why must Pins 1 and 3 be shorted on the SMDB3?

Pins 1 and 3 are electrically identical terminals of the symmetrical DIAC structure. Shorting them externally ensures balanced current distribution and prevents unintended voltage division or parasitic coupling in high-frequency gate drive paths, as confirmed in ST's application schematics and layout guidelines.

Can SMDB3 replace DB3 in existing through-hole designs without circuit modification?

No - SMDB3 uses SOT-23-3L packaging and requires PCB layout changes including pad geometry, thermal relief, and shorting trace between Pins 1 and 3. Electrical substitution is valid, but mechanical integration demands redesign due to footprint and soldering process differences.

How does SMDB3's V_BO vary with junction temperature?

V_BO decreases linearly with rising temperature: typical drift is −0.05%/°C, resulting in ~3% reduction from −40 °C to +125 °C. This behavior is documented in Figure 4 of DS2125 and must be accounted for in timing-critical ballast designs operating at elevated ambient temperatures.