Google Groups no longer supports new Usenet posts or subscriptions. Historical content remains viewable.
Dismiss
74HCT with 3.3V supply
391 views
Skip to first unread message
Allan Herriman
Oct 8, 2013, 6:04:59 AM10/8/13
to
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
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
JW
Oct 8, 2013, 6:39:19 AM10/8/13
to
On 08 Oct 2013 10:04:59 GMT Allan Herriman <allanh...@hotmail.com>
wrote in Message id:
<5253d8cb$0$29984$c3e8da3$5496...@news.astraweb.com>:
Is there any reason that you can't use 74LVCXXX?
wrote in Message id:
<5253d8cb$0$29984$c3e8da3$5496...@news.astraweb.com>:
John Devereux
Oct 8, 2013, 7:05:29 AM10/8/13
to
AUP, ... depending on the actual device function of course.
--
John Devereux
Allan Herriman
Oct 8, 2013, 7:45:08 AM10/8/13
to
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
Jim Thompson
Oct 8, 2013, 10:45:14 AM10/8/13
to
On 08 Oct 2013 10:04:59 GMT, Allan Herriman
ratios of the input devices.
So threshold at VDD=3.3V, threshold will be ~0.924V
>
>Question 2: Are there any gotchas? I'm thinking of things like an
>(undocumented) brownout detector resetting flip flops when the supply
>gets low.
It will be slightly slower.
>
>Thanks,
>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.
<allanh...@hotmail.com> wrote:
>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...)
Roughly, HCT threshold is 28% of supply, because it's set by the size
>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...)
ratios of the input devices.
So threshold at VDD=3.3V, threshold will be ~0.924V
>
>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
...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.
Baron
Oct 8, 2013, 12:45:34 PM10/8/13
to
Did you see this post from JT !
>Question 1: What sort of input thresholds should I expect? (I
>understand it won't be guaranteed, but still...)
Baron.
>Question 1: What sort of input thresholds should I expect? (I
>understand it won't be guaranteed, but still...)
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
>
ratios of the input devices.
So threshold at VDD=3.3V, threshold will be ~0.924V
>
>Question 2: Are there any gotchas? I'm thinking of things like an
>(undocumented) brownout detector resetting flip flops when the supply
>gets low.
>(undocumented) brownout detector resetting flip flops when the supply
>gets low.
It will be slightly slower.
>
>Thanks,
>Allan
...Jim Thompson
--
Best Regards:
>
>Thanks,
>Allan
...Jim Thompson
--
Baron.
Allan Herriman
Oct 8, 2013, 5:31:49 PM10/8/13
to
On Tue, 08 Oct 2013 07:45:14 -0700, Jim Thompson wrote:
> On 08 Oct 2013 10:04:59 GMT, Allan Herriman <allanh...@hotmail.com>
> wrote:
>
>>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...)
>
> 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
>
>
>>Question 2: Are there any gotchas? I'm thinking of things like an
>>(undocumented) brownout detector resetting flip flops when the supply
>>gets low.
>
> It will be slightly slower.
Thanks very much Jim.
> On 08 Oct 2013 10:04:59 GMT, Allan Herriman <allanh...@hotmail.com>
> wrote:
>
>>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...)
>
> 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
>
>
>>Question 2: Are there any gotchas? I'm thinking of things like an
>>(undocumented) brownout detector resetting flip flops when the supply
>>gets low.
>
> It will be slightly slower.
Allan
Jim Thompson
Oct 8, 2013, 7:26:34 PM10/8/13
to
On 08 Oct 2013 21:31:49 GMT, Allan Herriman
You are quite welcome, Allan!
JW
Oct 9, 2013, 7:09:29 AM10/9/13
to
On 08 Oct 2013 11:45:08 GMT Allan Herriman <allanh...@hotmail.com>
wrote in Message id:
<5253f044$0$29993$c3e8da3$5496...@news.astraweb.com>:
>
>It's an older type of shift register, not readily available in newer
>logic families.
What is the exact part number?
wrote in Message id:
<5253f044$0$29993$c3e8da3$5496...@news.astraweb.com>:
>
>It's an older type of shift register, not readily available in newer
>logic families.
Chris Jones
Oct 9, 2013, 8:31:28 AM10/9/13
to
http://www.nxp.com/documents/user_manual/HCT_USER_GUIDE.pdf
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
Allan Herriman
Oct 10, 2013, 8:06:18 AM10/10/13
to
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
0 new messages