Updating source driver IC with new resistor?

Thread Starter

trherzfeld

Joined Jul 2, 2015
9
Hey all, I currently have LED numeric displays that are being driven by an old UDN2585 8 channel source driver and a 100 Ohm resistor network. I'm trying to replace it with some new drivers, but the displays now are just on all the time without actually displaying a number. Does this mean there is too much current going into the LED displays? If so, would changing the 100 Ohm resistor network to a different value possible fix this?


Thank you so much for any possible.
 

eetech00

Joined Jun 8, 2013
1,660
Replacement is TI ULN2803a but is 5v max input and 18SOIC package
 
Last edited:

AnalogKid

Joined Aug 1, 2013
8,103
Nope. The 2585 is a source driver; its outputs are emitter followers, and are non-inverted WRT the input signals. An input high drives the output high. The 2803 is a sink driver; its outputs are open collector, and are logically inverted from the input. An input high drives the output low.

ak
 

AlbertHall

Joined Jun 4, 2014
8,364
The UDN2585 is inverting but the MIC2981 is not, so this will not work (unless you invert the data being fed to it).
The ULN2803A is inverting and would be a good replacement.
Nope. The 2585 is a source driver; its outputs are emitter followers, and are non-inverted WRT the input signals. An input high drives the output high. The 2803 is a sink driver; its outputs are open collector, and are logically inverted from the input. An input high drives the output low.

ak
Nope, The UDN2585A is inverting.
upload_2016-11-7_16-50-51.png
 

AnalogKid

Joined Aug 1, 2013
8,103
The UDN2585 is inverting
Yup, you're right. The very low res datasheet I have blurs the first transistor being a PNP. oops.
The ULN2803A is inverting and would be a good replacement.
Still don't think so. Replacing a source driver with a sink driver probably will not work with an LED display. Besides the problem with the direction of current, there are those pesky suppression diodes. They point the opposite direction in a 2803. If the COMMON pin is tied to GND as it should be for a source driver, then a sink driver's diodes will hold all of the outputs low all of the time. Among the three parts being discussed (2585, 2981, 2803), the power, ground, and suppression pins are in different places and do different things. Don't know if we're talking 18 pin DIPs or 20 pin SOWs, but there are pinout issues either way.

ak
 

AlbertHall

Joined Jun 4, 2014
8,364
Still don't think so. Replacing a source driver with a sink driver probably will not work with an LED display.
Yes, my bad, it is indeed a sink driver so will not work and the diodes are wrong. But it does match the pinout.
Back to inverting the data and using the MIC device.
 

JWHassler

Joined Sep 25, 2013
273
Yes, my bad, it is indeed a sink driver so will not work and the diodes are wrong. But it does match the pinout.
Back to inverting the data and using the MIC device.
Won't the LEDs be connected to the wrong supply?
A source driver would want the LEDs to be commoned to ground, and a sink-driver to Vcc.
 
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