UDN2981 vs ULN2803

Hi Folks
I have started using a very nice chip called the UDN2891. It has been mentioned in passing a few times on the forum but I think that it is very under rated.

The UDN2981 is a 8 buffer LED/Relay driver with a Common Ground.

http://www.datasheetcatalog.org/datasheet/allegromicrosystems/2981.pdf

Put simply A ULN2803 required you to have this set up which does not work for Tri LED's or LED displays with a common ground etc

PicAxe Out <- ULN2803 <- LED <- 5v+

But the UDN works the opposite way

PicAxe Out -> UDN2981 -> LED -> Common Ground.

This is also very handy if you want to apply a MOSFET at the common ground and PWM all of the LED's at the same time.

Hope this help someone. It certainly helped me when I used the chip

Dave

The use of a high side Darlington array has, as you say, been mentioned here previously and does have advantages as a high side switch.
Certainly some on rue forum run into grief trying to implement high side FET switching so this Darlington array will be useful for those seeking to use in conjunction with control on the "other" side of the load for purposes such as PWM for speed or intensity control.

Well done on bringing this to the general attention of all.

Marcwolf said:

The UDN2981 is a 8 buffer LED/Relay driver with a Common Ground.

Click to expand...

The UDN2981 is a 8 buffer LED/Relay driver which allows you to drive loads with a Common Ground.

Does anybody know if you can bridge inputs and outputs like on the 2803? It doesn't seem to mention it in the data sheet.

I have some 38mm Bi colour displays that are common anode. I want to drive the segments with a 2803 and the colours with a 2981. They seem to take about 50mA per segment.

Thanks for bringing this to my attention. I have often needed a current supplying (high side) buffer but just have not found the time to research it.

Dave

StigOfTheDump said:

Does anybody know if you can bridge inputs and outputs like on the 2803? It doesn't seem to mention it in the data sheet.

I have some 38mm Bi colour displays that are common anode. I want to drive the segments with a 2803 and the colours with a 2981. They seem to take about 50mA per segment.

Click to expand...

I can see no problem with bridging inputs and outputs to gain extra power. I can give it a try tonight as I have a 2981 on my breadboard for testing.
Take Care
Dave

While paralleling will work (to varying degrees), transistors in directly connected parallel do not always share the load well.

If limiting resistors are required then put one in series with each transistor. Otherwise provision of a small resistance in series with each transistor can still be done. Then connect the resistors together only at the load side.

It is worth mentioning that the 2981 is not a pin for pin replacement for the 2803. The 0V and V+ at the bottom pin either side is reversed. Therefore it is not a 'drop-in' replacement for project boards, but can be used with a bit of tweaking.

One common way to adapt a ULN2803 project board socket is to bend these two bottom legs out so they do not enter the socket. Then solder wires direct to these two legs to make the required connection.