74HCT with 3.3V supply

Oct 08, 2013 10 Replies
A

Hi,



Due to a shortfall in supply, I may have to substitute a 74HCTxxx part that I can get for a 74HCxxx device that I can't get.



The supply voltage is 3.3V. 74HCT is only specified to run from 4.5V to



5.5V, so this would be operating outside the datasheet Recommended Operating Conditions.

I understand that 'HCT is just like 'HC but with different W/L ratios on the input P and N ch transistors, and as such, it ought to work at 3.3V. My application doesn't need to be fast or need lots of output drive. The input thresholds do concern me though.



Question 1: What sort of input thresholds should I expect? (I understand it won't be guaranteed, but still...)



Question 2: Are there any gotchas? I'm thinking of things like an (undocumented) brownout detector resetting flip flops when the supply gets low.



Thanks, Allan


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J

Is there any reason that you can't use 74LVCXXX?

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J

Those tend to be a bit vicious (but may be OK). There is also LV, LVX, AUP, ... depending on the actual device function of course.

John Devereux
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A

It's an older type of shift register, not readily available in newer logic families.

I searched again and I found it in "LV" in the right package from Digikey. LV can be made to work in this application.

I'd still like to know about the 'HCT thresholds when run from 3.3V though. If I had some I test them in the lab.

Regards, Allan

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J

Roughly, HCT threshold is 28% of supply, because it's set by the size ratios of the input devices.

So threshold at VDD=3.3V, threshold will be ~0.924V

It will be slightly slower.

...Jim Thompson

| James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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B

Did you see this post from JT !

Roughly, HCT threshold is 28% of supply, because it's set by the size ratios of the input devices.

So threshold at VDD=3.3V, threshold will be ~0.924V

It will be slightly slower.

                                                         ...Jim Thompson
Best Regards: Baron.
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A

Thanks very much Jim.

Allan

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J

You are quite welcome, Allan! ...Jim Thompson

| James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
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C

See fig. 24 of this document:

formatting link

They put a diode in series with the source of the input stage PMOS device to reduce the quiescent supply current when the input voltage is TTL high but not as high as CMOS high. Not sure what other manufacturers do.

I guess they could have altered the PMOS devices' threshold voltage instead, but that would worsen the performance of the PMOS output devices too, unless they had another mask and two threshold voltage options.

Chris

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A

Thanks for the link. Extrapolating the graph in figure 25 of that same document shows that the input threshold might be below 1.0V with a 3.3V supply. It should still be ok for this (non critical) application though.

I guess I'll find out in a week or so when the boards come back from the contract manufacturer.

Thanks, Allan

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