uln2803 interfacing

[trdon]

I have interfaced the uln2803a with an addressable latch. the load that the 2803 is driving is 1.2A so i have paralleled three of the arrays. my problem is that the device driving the 2803 can't support the current requirements and the voltage is dropping, causing the latch to shut down. is this due to the internal resistors' parallel network? if so would it be a good idea to have a 2n4400 to help out?

 

[udi_hakim]

Did you put the ULN like my scheme?
The inputs goe's to the latch and the outputs of the latch should go to the inputs of the ULN.

You might consider a Pulldown method if you need a lot of current!

 

[trdon]

i paralleled 3 of the 8 pairs but the current requirements of the 2803 were pulling down the voltage output of the latch below ttl levels. none of the applications that i have seen show a setup where both the inputs and the outputs are paralleled but it has been mentioned. the motorola data sheet, for the darlington pairs, mentioned that each input required 25mA. one pin would push the limit of the latches ability to source the current. i was thinking of putting a npn in the path as a switch to source the current. would this be a good idea or am i wrong from the start?

 

[trdon]

i do have the latch as shown above. the latch has a tendency to shut down after a couple of seconds of running because it is also sourcing to another pair on the same 2803. the shutdown only occurs when the two devices are turned on, a dc pump and a dc motor driven by the 2803.