Thursday 18 May 2017

integrated circuit - Is it safe to run Logic ICs at their maximum output current?


I am building a circuit that will ultimately output to 8 7-segment displays. I am going to be multiplexing these displays.


The displays I chose are https://uk.farnell.com/kingbright/cc56-12surkwa/display-seven-segment-14-22mm/dp/2478714 (common cathode) which lists the DC forward current as 30mA on the datasheet.


I am planning on running the data lines for this display from the 4094 (SIPO register) and the cathodes (to do the multiplexing) from a 4017 (Decade counter).


In the case of the 4094, the datasheet (https://assets.nexperia.com/documents/data-sheet/HEF4094B.pdf) lists the maximum input/output current at 10mA.


My question is two part. Firstly, am I right in assuming these values given are per led / per pin and secondly, if that's the case, am I safe to run the 4094 at the maximum current output without using a resistor to go to the display?


I know I should probably use a driver chip or something, but I just wanted to clarify my query about pushing the maximum ratings on an IC.




Answer




In the case of the 4094, the datasheet (https://assets.nexperia.com/documents/data-sheet/HEF4094B.pdf) lists the maximum input/output current at 10mA.



Let's be clear about the current you can supply from the 4094: -


enter image description here


Yes, the IO current is 10 mA but you can only drive 5 lines at 10 mA before you hit power supply current problems and, in any case you should not be driving the IO lines at 10 mA because that is their absolute maximum rating AND, without current limiting resistors this could easily be exceeded by your LED load. You should not assume that the IO lines are current limited at 10 mA in other words.


Regarding the LEDs, 30 mA is the absolute max rating and you should not approach it. I would say 20 mA is a fairly reasonable and safe limit as per this graph in their data sheet: -


enter image description here


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