ULN2004AINSR Discrete Semiconductor Products

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Bipolar junction transistors, or BJTs, are commonly used in many types of electronic devices such as amplifiers and digital logic devices. An array of BJTs can be used in applications such as relays and motor switching circuits. The ULN2004A, a high-voltage, high-current Darlington transistor array, is one such array. This article will discuss the application field and working principle of the ULN2004A.

Application Field

The ULN2004A is a high-voltage and high-current Darlington array containing seven NPN Darlington pairs. It is designed for use in applications such as relay-driving circuits, lamp coupling, DC and stepping motor control, as well as other general purpose switching. It is capable of sourcing and sinking large currents, typically up to 500 mA per Darlington pair, making it suitable for power control applications. The ULN2004A can also be used as an inverter for logic-level signals, as it has an open-collector design.

Working Principle

The ULN2004A operates using a Darlington pair arrangement. A Darlington pair is formed by two transistors connected in a special configuration such that the current through the collector of one transistor is amplified and fed into the base of the other. This arrangement greatly increases the current gain available, making it possible to source and sink higher currents than a single transistor can. The ULN2004A consists of seven Darlington pairs, each one connected in parallel with an output terminal that can be pulled up to the voltage at the VCC terminal. The Darlington pairs are driven by the input from the five logic input pins, each connected to the base of one transistor in the pair. When the input logic is high, the transistor in the pair is turned on, thus allowing current to flow from either the VCC or GND terminals to the output.

The ULN2004A is designed for use with TTL, CMOS, and PMOS logic. It has two supply voltages—VCC and VEE—and can be operated from a wide range of supply voltages from +6 V to ±20 V. It has a supply current of 1 mA and a forward current gain (Hfe) of 500–700.

In summary, the ULN2004A is an array of seven high-voltage, high-current Darlington transistors designed for applications such as relay-driving circuits, lamp coupling, DC and stepping motor control, as well as other general purpose switching. It has two supply voltages—VCC and VEE—and can be operated from a wide range of supply voltages from +6 V to ±20 V. It is capable of sourcing and sinking large currents, typically up to 500 mA per Darlington pair, making it suitable for power control applications.

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