CMOS Output drive current

[treez]

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 …

http://www.st.com/stonline/books/pdf/docs/2069.pdf

-

Figure 1: Circuit to be used…..

…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….

http://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.

 

[sreid]

You have interpreted the IOH corectly. The 4000 series CMOS do not have much drive current. To increase the drive, often 4049 or 4050 devices are used. If you need more current drive fron the transistor you could consider using two 2N3904's in a Darlington connection.

 

[IRstuff]

Only partly correct. The 360 uA value applies to a Voh of 4.6V. Ioh at Voh=2.5V is >1 mA.

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

FAQ731-376

 

[itsmoked]

Or just go with an N-FET and just use a pull down resistor that's 680k. Dump the series resistor.

http://search.digikey.com/scripts/DkSearch/dksus.dll?pname?site=us;lang=en;name=ZVN4424A-ND

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jimkirk]

2N7000 MOSFET should do what you want. Connect the gate directly to the 4094 output. Source corresponds to the emitter, drain corresponds to the collector. High, LED goes on, low, LED goes off. The pull down resistor Keith suggests isn't strictly necessary, but it's nice to define a known state in case something goes wrong.

And it's much cheaper than Keith's 240 volt rated part.

 

[itsmoked]

Yeah thanks jimkirk; I couldn't recall a cheap MOSFET with "logic" levels so I went to Digikey for a quick hunt. Then was so horrified by their "new" terrible catalog display that I lost focus. Wow have they messed things up bad.

Anyway, yeah the 2N7000 is a cheaper solution. Fore shore.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[felixc]

A part that I've used is the FDV303N. They call it a digital FET, as it is intended to be driven directly by a logic gate. Very similar to the 2N7000. A cent cheaper.

 

[OperaHouse]

As said before, you can use a 4049/50 if you have some spare sections. What wasn't mentioned is the outputs can be paralleled for higher output.