onsemi BD681STU-ON

Overview

BD681STU-ON from ON Semiconductor (formerly Fairchild) is an NPN epitaxial silicon Darlington transistor in TO-126 package, rated for 100 V collector-emitter sustaining voltage (V_CEO(sus)), 4 A DC collector current, and 40 W power dissipation at T_C = 25°C. It serves as a medium-power switching and linear amplifier device in industrial motor controls and relay drivers.

For engineers reviewing the BD681STU-ON datasheet, pinout, applications, or equivalent options, key selection criteria include its 100 V V_CEO(sus), 750 minimum h_FE at I_C = 1.5 A, 2.5 V V_CE(sat) under specified drive, TO-126 thermal performance, and complementarity to BD682 in push-pull configurations.

Technical Context

This Darlington pair integrates two NPN transistors in cascade to achieve high DC current gain (h_FE ≥ 750 at V_CE = 3 V, I_C = 1.5 A) with low base drive requirements. Its V_CE(sat) of 2.5 V at I_C = 1.5 A and I_B = 30 mA enables efficient medium-current switching without excessive base current sourcing.

The device operates across –65°C to +150°C junction temperature range and features 100 V V_CBO/V_CEO ratings, making it suitable for inductive load switching where voltage spikes exceed standard single-transistor limits. Its safe operating area (SOA) supports pulsed operation up to 6 A with defined pulse width and duty cycle constraints.

Key Specifications

Pinout & Package

BD681STU-ON uses the TO-126 molded plastic package with integral heat sink tab. The case is electrically connected to the collector terminal. Package dimensions conform to JEDEC TO-126 standard: 16.10 mm × 13.06 mm × 4.5 mm (max height), with 3.20 mm mounting hole diameter.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar NPN Darlington transistor applications.

Compared with TIP122 and BD681S, BD681STU-ON offers superior thermal efficiency in TO-126, tighter V_CE(sat) guarantee, and optimized mechanical fit for space-constrained industrial PCBs-making it preferred for relay drivers and linear regulators where thermal density matters.

Availability

BD681STU-ON is available at Aetrix Electronics and suitable for industrial relay drivers, DC motor speed controllers, and linear power regulators requiring stable component supply, long-term lifecycle support, and consistent parametric performance across production batches.

Supply support for BD681STU-ON 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

ON Semiconductor is a global semiconductor supplier delivering energy-efficient, high-performance silicon solutions for automotive, industrial, cloud computing, medical, and IoT applications.

BD681STU-ON belongs to ON Semiconductor's legacy discrete power transistor portfolio, originally developed by Fairchild for robust medium-power linear and switching applications where high gain, voltage ruggedness, and thermal reliability are essential.

FAQ

What is the maximum continuous collector current rating for BD681STU-ON?

The BD681STU-ON has a maximum DC collector current (I_C) rating of 4 A at case temperature T_C = 25°C. This rating decreases with rising case temperature per the derating curve in the datasheet; at T_C = 100°C, the usable I_C drops to approximately 2.2 A. Always verify thermal design margins using the published R_θJC and R_θJA values for your specific board layout and heatsink.

Is BD681STU-ON pin-compatible with BD681S?

Yes, BD681STU-ON and BD681S share identical TO-126 package dimensions, pinout (Emitter–Collector–Base), and full electrical specifications. The only difference is packaging: BD681STU-ON is supplied in tape-and-reel (rail) format for automated placement, while BD681S is in bulk. No PCB or schematic changes are required when substituting BD681STU-ON for BD681S.

Does BD681STU-ON require a base resistor, and how is its value calculated?

Yes, BD681STU-ON requires an external base resistor to limit drive current. For saturation at I_C = 1.5 A, the datasheet specifies I_B = 30 mA. With a 5 V logic drive and V_BE(on) ≈ 2.5 V, R_B = (5 V − 2.5 V) / 0.03 A = 83 Ω. A standard 82 Ω ±5% resistor ensures reliable turn-on while avoiding excessive base current that could degrade h_FE stability in BD681STU-ON.

Can BD681STU-ON be used in place of BD679A or BD677A?

No-BD681STU-ON is not a drop-in replacement for BD679A (80 V) or BD677A (60 V). While all belong to the same family and share TO-126 packaging, their absolute maximum ratings differ: BD681STU-ON is rated for 100 V V_CEO(sus), whereas BD679A and BD677A are limited to 80 V and 60 V respectively. Substituting BD681STU-ON in circuits designed for lower-voltage variants may over-specify cost and thermal design without functional benefit.

What is the safe operating area (SOA) limitation for BD681STU-ON in pulse applications?

The BD681STU-ON SOA curve permits pulsed collector currents up to 6 A (I_CP) at V_CE ≤ 10 V for pulse widths ≤ 300 µs and duty cycle ≤ 1.5%. At higher voltages or longer pulses, the maximum allowable current decreases rapidly to prevent second breakdown. Always consult the SOA graph in the BD681STU-ON datasheet and apply appropriate derating for ambient temperature and heatsink performance.