When I analyze this I don’t see a problem, but 5000 pairs of eyes might catch something I missed.

Basically I need to know if connecting the outputs of the UDN2981A in this way could be a problem.

I included the internals of the chip in question.

The basic concept is to light the RGB LEDs but to create yellow when the green signal is present. (but has no bearing on the question)

 

Using the bare outputs of the UDN2981, I wonder how much current is being used to drive the bases of the darlington ULQ2003. I guess what I am saying is...do you need resistors between the UDN2981 outputs and the ULQ2003 inputs?

 

Both chips have built in input resistors.

The 2003 has 2.7k

And I believe the 2981 has a 10k (calculated from the data sheet)

Sorry, I should have been clearer…the question refers to the diode.

 

While I am familiar with the driver chips you used and I can't see any reason what you have won't work I have a question. It seems you are using a driver to drive a driver? You are coming off an EPROM or similar with what I guess to be 5 volt logic. Should that be true why not just use a single MOSFET to drive each channel of RGB? Here is what I am getting at:


The above example just uses a single logic level MOSFET for each channel. The LEDs are just an example of a generic 5050 RGB LED in a string. I just don't understand why you are driving a driver? Yes, what you have should work fine.

Ron

 

Well, originally I was only going to use the ULQ2003 connected directly to the EPROM, but then the typical project creep started to creep in and the current requirements for the LEDs became much higher than a single output on that chip could handle so I started to parallel the drivers, and decided I better buffer the EPROM outputs, also customer decided to change green to yellow, so I needed a stage where I could combine the outputs to create the yellow.

So I started working on a non-inverting stage to put in between the EPROM and the ULQ2003, where it was safe to combine outputs.

Decided not to work with discrete components right from the start because there are way too many EPROM outputs to deal with.

Good question tho....

 

I was just curious and your diode combining Red and Green to get Yellow should work. The MOSFETs I used above will handle 30 Amps. I was using them in a PWM scheme off a uC. Anyway, just me being curious.

Ron