Is it OK to have output clamp diod conducting in 74HC07?

Dec 02, 2005 5 Replies
A

Hi,



Well, a simple problem. I have a design with all logic at Vdd = 3.3V. I need then to control a number of high side PNP transistors powered from



12V rail. I use currently NPN transistors for that purpose (bases of PNP transistors being connected through resistors to collectors of NPN's).

I would like to simplify this and replace all NPN's with one chip of open collector drivers like 74xx07.



The problem is I cant use TTL part as I have VDD only 3.3V, and for CMOS variant the problem is that they are specified for Vout no more than Vdd (they have clamping diodes to Vdd at they output pins).



My question is whether it is OK to use these parts somewhat beyond the recommended conditions, connecting output collectors through resistors to say 12V (to bases of PNP transistors in my case). It seems I am still safe since the diodes will not allow Vout to rise above Vdd +



0.5V which is still within "Absolute maximum ratings" of the part.

Is there any better solution?



TIA


-Arkady-



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K

ark1 skrev:

With the 4000 logic series it is possible to drive current in the protection diodes. Just a couple of days ago we did some tests on the CD4093 and the current through the diode in the input (causing an simelar die current) was over 20mA before the supply collapsed and the function of the gate failed due to SCR latchup.

But, we did opt to do a work-around instead of allowing current through the diodes. Mainly because the manufactors does not specify the allowed current and the latest datasheets specify the input voltage to the chip to be between the GND/VDD rails and not as in the older days when they specified 0.5V in excess of the supply rails. (the old 4000 series datasheets has lots more info than the newer ones)

Regards

Klaus

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M

or use the gate to turn on NPNs and the collector of the NPN pulls down on the base of the PNP....ie you make your own open collector outputs... you might need that to get enough current gain anyway,,,how much current are the high side PNPs carrying?

Mark

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S

Then the current will flow through the protection diodes and turn your PNPs partially on.

Why not a jellybean ULN2003/2803? Or if you just need a few, there are dual NPNs with base resistors built in in tiny packages. Or use source drivers and get rid of all your discrete transistors.

Best regards, Spehro Pefhany

"it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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J

On 02/12/2005 the venerable ark1 etched in runes:

UDN2981 or TD62783

John B
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A

This is exactly the configuration I have now. What I want is to avoid using discrete components. I think I'll try one of the source drivers ICs as was suggested by others.

-Arkady-

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