quick 4049 data sheet question

Nov 27, 2006 2 Replies
T

Hey all;



I'm driving a 2222 transistor with the 5v output high from a 4049 inverter. I'm not sure which figure to refer to on the data sheet regarding how much current I can safely draw from it though. I was hoping to use a 1-3k resistor on the base of the transistor (it's being used as a simple switch).



One of the figures says "Output Drive Current (for 5v supply) -1.6mA (typ.) Another says: "Output Sink Current (for 5v) 6.4mA (typ.)



Which one do I use to decide on my base resistor? Here is a link to the datasheet:


formatting link



Thanks


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D

It is common for ICs to have two different figures for "source current" and "sink current". It is common for "sink current" to be the higher of these two when they differ.

"Sink Current" is what the output can drain/suck/sink when it is "low". "Source Current" is what the output can push out/source when it is "high".

I suspect that the "Output Drive Current" of 1.6 mA is what you need to consider for use with driving an NPN transistor. Given the B-E junction of the transistor having a voltage drop likely to exceed .5 volt at 1.6 mA and the IC likely being allowed to have output up to at least .5V below +5V at full sink current and +5V likely to be allowed to be as high as +5.25 volts, I would say "fairly worst case" the voltage across the resistor between IC output and transistor base is is

4.25 volts. Divide that by 1.6 mA, and the current is 2.656 mA. A 3K resistor - which is typically +/- 5% so should not be less than 2.7K - should be just fine!

- Don Klipstein ( snipped-for-privacy@misty.com)

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T

Music to my ears - the circuit I need to switch just shuts off with 3K. Thanks for the explanation Don.

inverter.

much

switch).

(typ.)

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