Datasheet

2.7 kΩ

7.2 kΩ 3 kΩ

COM

Output C

Input B

ULN2803A

SLRS049G –FEBRUARY 1997–REVISED JANUARY 2015

www.ti.com

9 Detailed Description

9.1 Overview

This standard device has proven ubiquity and versatility across a wide range of applications. This is due to its

integration of 8 Darlington transistors that are capable of sinking up to 500 mA and wide GPIO range capability.

The ULN2803A comprises seven high voltage, high current NPN Darlington transistor pairs. All units feature a

common emitter and open collector outputs. To maximize their effectiveness, these units contain suppression

diodes for inductive loads. The ULN2803A has a series base resistor to each Darlington pair, thus allowing

operation directly with TTL or CMOS operating at supply voltages of 5.0 V or 3.3 V. The ULN2803A offers

solutions to a great many interface needs, including solenoids, relays, lamps, small motors, and LEDs.

Applications requiring sink currents beyond the capability of a single output may be accommodated by paralleling

the outputs.

9.2 Functional Block Diagram

9.3 Feature Description

Each channel of ULN2803A consists of Darlington connected NPN transistors. This connection creates the effect

of a single transistor with a very high current gain (β2). This can be as high as 10,000 A/A at certain currents.

The very high β allows for high output current drive with a very low input current, essentially equating to

operation with low GPIO voltages.

The GPIO voltage is converted to base current via the 2.7 kΩ resistor connected between the input and base of

the pre-driver Darlington NPN. The 7.2 kΩ & 3.0 kΩ resistors connected between the base and emitter of each

respective NPN act as pull-downs and suppress the amount of leakage that may occur from the input.

The diodes connected between the output and COM pin is used to suppress the kick-back voltage from an

inductive load that is excited when the NPN drivers are turned off (stop sinking) and the stored energy in the

coils causes a reverse current to flow into the coil supply via the kick-back diode.

In normal operation the diodes on base and collector pins to emitter will be reversed biased. If these diode are

forward biased, internal parasitic NPN transistors will draw (a nearly equal) current from other (nearby) device

pins.

9.4 Device Functional Modes

9.4.1 Inductive Load Drive

When the COM pin is tied to the coil supply voltage, ULN2803A is able to drive inductive loads and supress the

kick-back voltage via the internal free wheeling diodes.

9.4.2 Resistive Load Drive

When driving a resistive load, a pull-up resistor is needed in order for ULN2803A to sink current and for there to

be a logic high level. The COM pin can be left floating for these applications.

Product Folder Links: ULN2803A