Texas Instruments ULN2004AD

Overview

ULN2004AD from Texas Instruments is a 7-channel high-voltage, high-current Darlington transistor array designed for interfacing low-voltage logic (6–15 V CMOS) with inductive loads. It delivers 500 mA per channel, supports 50 V collector-emitter voltage, integrates clamp diodes for flyback suppression, and uses 10.5 kΩ input base resistors for direct CMOS compatibility - commonly used in relay and stepper motor control circuits.

For engineers reviewing the ULN2004AD datasheet, ULN2004AD pinout, ULN2004AD application, or ULN2004AD equivalent, key selection considerations include per-channel current capability, integrated clamp diode functionality, input resistor value (10.5 kΩ), thermal derating at elevated ambient temperatures, and SOIC-16 package footprint compatibility with industrial PCB layouts.

Technical Context

The ULN2004AD implements seven independent NPN Darlington pairs, each with a dedicated 10.5 kΩ series input resistor enabling direct drive from 6–15 V CMOS logic without external biasing. Its open-collector outputs sink current to a common emitter (Pin 8), while the COM pin (Pin 9) provides a shared cathode node for internal clamp diodes - essential for safe switching of relays, solenoids, and other inductive loads.

Each Darlington stage achieves high current gain (β² ≈ 10⁴), allowing ≤1.45 mA input current at 12 V to fully saturate the output at 350 mA collector current. The device operates across –40°C to 105°C and features ESD protection (±2000 V HBM), with absolute maximum ratings including 50 V V_CE, 500 mA I_C, and 2.5 A total emitter current.

Key Specifications

Pinout & Package

ULN2004AD is housed in a 16-pin SOIC package (9.90 mm × 3.91 mm), optimized for surface-mount assembly and thermal performance (R_θJA = 88.6°C/W).

Key Features

Applications

Equivalent & Alternatives

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

Compared with ULN2004AD, ULN2003AD demands lower input voltage but lacks 6–15 V compatibility, while ULQ2004A shares electrical specs but sacrifices 20°C of operational headroom - making ULN2004AD the optimal choice for wide-voltage CMOS interfacing in thermally demanding environments.

Availability

ULN2004AD is available at Aetrix Electronics and suitable for relay driver modules, stepper motor interfaces, and industrial lamp control systems requiring stable component supply and long-term production continuity.

Supply support for ULN2004AD 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

Texas Instruments is a global semiconductor leader specializing in analog, embedded processing, and power management technologies with over 90 years of innovation in industrial and automotive electronics.

The ULN200x family was engineered specifically for robust logic-to-load interfacing in industrial automation, offering integrated protection, wide supply compatibility, and proven field reliability across decades of deployment.

FAQ

What is the maximum input voltage the ULN2004AD can accept?

The ULN2004AD supports an absolute maximum input voltage (V_I) of 30 V, but its 10.5 kΩ internal base resistor is optimized for reliable operation with 6–15 V CMOS logic levels. At 12 V input, it draws ≤1.45 mA - well within safe operating limits and ensuring full saturation of the Darlington pair at 350 mA output current. Exceeding 15 V offers no functional benefit and increases power dissipation unnecessarily.

Does the ULN2004AD require external flyback diodes when driving relays?

No - the ULN2004AD includes seven integrated common-cathode clamp diodes connected between each collector (Pins 10–16) and the COM pin (Pin 9). When COM is tied to the positive side of the relay coil supply, these diodes provide a safe path for inductive kickback energy during turn-off. External diodes are unnecessary unless the load voltage exceeds 50 V or requires bidirectional clamping.

Can multiple ULN2004AD outputs be paralleled to increase current capacity?

Yes - the ULN2004AD's Darlington outputs can be paralleled to handle higher load currents. For example, two channels driven together support up to ~700 mA (derated for thermal limits), and all seven channels can collectively sink up to 2.5 A total emitter current. However, per-channel V_CE(sat) rises slightly under parallel operation, and PCB copper area must be increased to manage cumulative power dissipation.

What is the thermal resistance (R_θJA) of the ULN2004AD in SOIC package?

The ULN2004AD in SOIC-16 (Package D) has a junction-to-ambient thermal resistance (R_θJA) of 88.6°C/W under standard JEDEC test conditions. This value assumes a 1-inch² 2-oz copper pad on the board. With enhanced thermal layout (e.g., 2-inch² copper pour and thermal vias), R_θJA can be reduced to ~55°C/W, enabling sustained 500 mA operation across all channels at 70°C ambient.

How does the ULN2004AD differ from the ULN2003AD in terms of input compatibility?

The ULN2004AD uses a 10.5 kΩ input base resistor, enabling direct interface with 6–15 V CMOS logic families (e.g., 74HC, CD4000), whereas the ULN2003AD uses a 2.7 kΩ resistor optimized for 3.3 V/5 V TTL and CMOS. As a result, ULN2004AD draws less input current at 12 V (≤1.45 mA vs. ~4.4 mA for ULN2003AD), reducing MCU loading and improving noise immunity in mixed-voltage systems.