ULN2805A

The ULN2805A is an array of eight open collectors, controlled by high voltage input signals and intended for use with TTL or CMOS devices. It has a parts code designation of "HC/HL/ HCT/LSTTL/SHT/SOS Logic". In order to understand its application field and working principle, it is important to understand its connection types, input/output staging, and features.

Connection Types

ULN2805A is available in three connection types: 8 pin PDIP (plastic dual in-line package), 8 pin SOIC (small outline integrated circuit) and 8 pin TSSOP (thin shrink small outline package). The first two connection types are preferred as they offer better reliability and less impedance. The TSSOP is best used when board space is limited.

Input/Output Staging

ULN2805A has two types of staging: common emitter and common base. In common emitter staging, the input signals (typically TTL or CMOS levels) appear at the base terminal. This causes the voltage at the emitter to rise and the transistor turns on. The collector is then connected to the load and the current flows through the load. In common base staging, the input signals are connected to the emitter and the voltage at the base rises, turning the transistor on. The collector is then connected to the load and the current is allowed to flow through the load.

Features

ULN2805A has a typical isolation voltage of 3.5V and is resistant to reverse voltage up to 10V for each pin. It is capable of handling up to 800mA of current and up to 100mA of constant current, making it ideal for driving medium power loads. It is also manufactured with a number of protection circuits that protect the transistors from overvoltage and overcurrent. These protection circuits are implemented using diodes, zeners, and resistors.

Working Principle

ULN2805A is basically an array of eight transistors or transistors integrated in a single package. It is designed to provide a low impedance path for a current to flow through an H-bridge circuit. When the gate is high, the transistor will be turned on and the current will flow through the collector. When the gate is low, the transistor will be turned off and no current will flow through the collector. This allows the current to be controlled using a low voltage signal.

Applications

ULN2805A is suitable for use as a motor driver, relay driver and/or lamp driver in low frequency switching applications. It is also suitable for driving high power loads such as lamps and relays. Additionally, it can be used to drive multiple loads in parallel, thus increasing the number of available circuits.

In conclusion, the ULN2805A is an array of eight transistors and is designed for use with TTL or CMOS devices. It has three connection types, two types of staging and features a number of protection circuits. It can handle up to 800mA of current and is suitable for driving high power loads. Its application fields include motor, relay, and lamp driving for low frequency switching applications.