ULN IC are darlington pair IC’s. These Darlington arrays are furnished in 16-pin plastic 3 dual in-line packages (PDIP) (suffix “A”) and 16-lead surface-mountable Small Outline Integrated Circuit (SOICs) (suffix“L”).
The Series ULN2003A/L is high-voltage, high-current Darlington transistor pair arrays. IC comprises of seven NPN Darlington pairs that feature high-current, high-voltage outputs with common-cathode clamp diodes for switching inductive loads.
The collector-current rating of a single Darlington pair is 500 mA for each of the seven drivers. The output of collector pins can be combine/paralleled for high current (in case of more than 500mA) load requirements. That’s why, ULN20xxA/L IC’s are ideally suited for interfacing between low-level logic circuitry and multiple peripheral power loads.
ULN is actually comprises of transistor that’s why it has three pins base, emitter and collector. Enable (emitter) pin 8 is connected to ground for proper operation of IC (all units feature a common emitter), input are base pins from 1-7 and output are collector pins from 10-16 and pin 9 is Common cathode for freewheel diode. This free wheel diode protects from back EMF because we know diode not works in reversed direction.
|PIN No.||NAME||I/O DESCRIPTION|
|1||1B||I – Channel 1 through 7 Darlington base INPUT|
|8||E||Common emitter (enable) shared by all channels (typically tied to ground)|
|9||COM||Common cathode node for flyback diodes (required for inductive loads)|
|10||7C||O – Channel 1 through 7 Darlington collector OUTPUT|
INTERNAL CIRCUITRY OF ULN IC
The ULN20xxA/L have series base input resistors to each Darlington pair, allowing operation directly with 5 V TTL or CMOS.
The ULN2003A devices have a 2.7-kΩ series base resistor for each Darlington pair for operation directly with TTL or 5-V CMOS devices.
The ULx2004A devices have a 10.5-kΩ series base resistor to allow operation directly from CMOS devices that uses supply voltage of 6V to 15V.
When we apply a positive signal to input base of ULN2003 IC it will provide negative output to collector because of inverter of the IC you can see it in figure. A small current is able to provide signal to the ULN IC.
The purpose of the pin is to provide a flyback diode aka (also known as) catch diode aka (also known as) freewheeling diode on each output for inductive loads, so that the inductive energy stored in the coil of the relay does not break down the output transistor of the ULN2003 and can possibly damage ULN IC transistors.
Common (not best) practice is to connect that pin (the internal diodes) to the relay supply voltage.
ABSOLUTE MAXIMUM RATINGS
Output Voltage, VCE
Output Voltage to (ULN200xA and ULN200xL) ……..50 V
Output Voltage to (U LN202xA and ULN202xL) ……..95 V
Max Input Voltage, VIN …………………………..30 V
Continuous Output Current
IC Collector Current (Single Output Current)…………………………………………. 500 mA (max)
ULN ICs can operate over a wide temperature range (–40°C to 105°C).