74HC(T)245 quiescent current

Hello

This is from the datasheet, and I don't really understand it:

"Icc quiescent supply current 80uA. deltaIcc additional quiescent supply current per input pin Vi=Vcc - 2.1V, other inputs at Vcc or GND. Vcc=4.5 to 5.5V An or Bn inputs 180uA OE input 675uA DIR input 405uA"

I get the bit about Icc at 80uA, and I get the bit (not quoted) about dynamic power. But what's the additional Icc per input pin about? And why is it at Vcc - 2.1V?

Sorry if it's a dumbo question. Ta for any hints.

Reply to
OldGit
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I thought that might be the case (honest ;) but only just before I read your post. Thanks. So I need to calculate the amount of time the inputs spend in this halfway point. Should be easy enough.

Reply to
OldGit

On a sunny day (Tue, 14 Jul 2009 13:10:57 +0100) it happened "OldGit" wrote in :

Have not read that datasheet, but likely when halfway 'on' both of some MOSFETS (pull up + pull down) are partly on, so then it draws some power.

Reply to
Jan Panteltje

I know, my PC clock is wrong. Corrected now (I hope).

Reply to
OldGit

some power.

I would think that it's the minimum high level input voltage that you might expect when interfacing to TTL (and the resultant current due to the voltage being 'halfway on', as noted above).

--
Jim
Reply to
Jim

some power.

Jan is right about "half way on". The input inverter (even that of a Schmitt trigger such as the 74HC14) will conduct substantial quiescent current when the input pin resides in the middle. The middle is defined by the IC designers, for HC logic it is truly in the middle, meaning at VCC/2.

--
Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

is

some power.

The spec for "middle" is actually 30%-70% :-(

In my custom designs I often make such input devices either quite long or use current sources and sinks to limit "overlap" current.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

is

some power.

Yeah, you's guys are always +/-8dB while us guys limit ourselves to

+/-3dB :-)

When it comes to very speedy devices the chip designer is probably between a rock and a hard spot.

--
Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

is

some power.

Wouldn't "speedy" imply very little time spent in the overlap region?

Only slow stuff, or stuff that can float around, requires such limiting.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

2.1V,

why is

draws some power.

Sure, but when the input signal isn't always under your control the overlap region remains a concern. I get that question over and over again: "Why is this draining the battery so fast" ... measure, measure, measure ... "Well, those two inputs are not anywhere close to logic levels" ... "But it's a Schmitt" ... "Doesn't matter, the input inverter still seeps" ... "Oh".

--
Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

2.1V,

why is

draws some power.

You need to learn to speak harshly to your customers when they're doing dumb-ass things ;-)

Of course, in my case, I can contrive my own frontend to prevent/solve such issues.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

2.1V,

why is

draws some power.

Nope, old rule: Always be mild-mannered and never point fingers, consultants aren't supposed to run around with smirky faces :-)

Lucky you. I sometimes have to interface with stuff where I get sick when I see it the first time.

--
Regards, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

:
2.1V,

why is

draws some power.

I didn't know that

Of course. But there _can_be_ off-the-shelf fixes. Ask me the next time you run into such a situation (free home demonstration ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Is the tool du jour to fix it a Beretta or a Glock?

:-)

--
SCNR, Joerg

http://www.analogconsultants.com/
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Reply to
Joerg

No! No! Try me. I can likely suggest off-the-shelf components for such situations ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

2.1V,

why is

draws some power.

No, you speak softly and carry a big bill.

But, "speedy" and long devices (and other protection means) don't tend to go together. C == bad

Reply to
krw

:
2.1V,

why is

draws some power.

All my "floaty" inputs are slow.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

:
2.1V,

about

And why is

draws some power.

Because you do that simple analog crap, you can afford to be slow [*]. ;-)

[*] Now that I'm retired, I can too.
Reply to
krw

:
2.1V,

about

And why is

draws some power.

defined

at

Speak for yourself. As recently as two years ago I was doing stuff at

5.5GHz ;-)

A present project includes a 500MHz VCO.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

"OldGit"

:

- 2.1V,

about

And why is

it draws some power.

to

quiescent

defined

at

I did. You didn't do that with long channel FETs or current limiting input devices.

Same.

Reply to
krw

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