CMOS Output drive current
CMOS Output drive current
(OP)
Dear Readers,
Sorry for this novice-like email, -I am really from a computing background.
I wish to turn on a 2N3904 NPN BJT with a “Q” output from a HCF4094B shift register ...
(Datasheet …
www.st.com/stonline/books/pdf/docs/2069.pdf -
Figure 1: Circuit to be used…..
[IMG]http://i16.tinypic.com/82j57nl.jpg[/IMG]
…On page 5 of the HCF4094B datasheet it states that “IOH” , the “Output drive current” for a 0/5V supply is 360 micro-Amps.
Therefore if I am to drive on the 2N3904.......
(2N3904 NPN DATASHEET….
www.fairchildsemi.com/ds/2N/2N3904.pdf -
.....then I should limit the current from the 4094’s “Q” output to 360 micro-Amps….
..I have chosen to do this with an 18K resistor (please see Figure 1 above) –this will limit the output current of the 4094 to ~5/18K = 278 Micro-Amps.
I am wondering and would be grateful if any reader could inform myself whether or not I have interpreted the “IOH” parameter correctly, since this really does seem to be a tiny amount of current.
Sorry for this novice-like email, -I am really from a computing background.
I wish to turn on a 2N3904 NPN BJT with a “Q” output from a HCF4094B shift register ...
(Datasheet …
www.st.com/stonline/books/pdf/docs/2069.pdf -
Figure 1: Circuit to be used…..
[IMG]http://i16.tinypic.com/82j57nl.jpg[/IMG]
…On page 5 of the HCF4094B datasheet it states that “IOH” , the “Output drive current” for a 0/5V supply is 360 micro-Amps.
Therefore if I am to drive on the 2N3904.......
(2N3904 NPN DATASHEET….
www.fairchildsemi.com/ds/2N/2N3904.pdf -
.....then I should limit the current from the 4094’s “Q” output to 360 micro-Amps….
..I have chosen to do this with an 18K resistor (please see Figure 1 above) –this will limit the output current of the 4094 to ~5/18K = 278 Micro-Amps.
I am wondering and would be grateful if any reader could inform myself whether or not I have interpreted the “IOH” parameter correctly, since this really does seem to be a tiny amount of current.





RE: CMOS Output drive current
RE: CMOS Output drive current
The next question is really what are you trying to do with the series resistor? 18K with 278 uA is a 5V drop, which doesn't allow for your Vbe drop. And why are you making the Ioh worse?
Even with the larger drive, you'll never saturate the transistor at its worst-case Hfe. Something like this http://www.fairchildsemi.com/ds/TI/TIP122.pdf is probably more apropos. Either that, or you'll need to buffer output with a line driver or equivalent.
TTFN
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RE: CMOS Output drive current
h
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: CMOS Output drive current
And it's much cheaper than Keith's 240 volt rated part.
RE: CMOS Output drive current
Anyway, yeah the 2N7000 is a cheaper solution. Fore shore.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: CMOS Output drive current
RE: CMOS Output drive current