LMV793, odd clause in datasheet

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Page 12,

-=-=- An innovative positive feedback scheme is used to boost the current drive capability of the output stage. This allows the LMV793/LMV794 to source more than 40 mA of current at 1.8V supply. This also limits the performance of these parts as comparators, and hence the usage of the LMV793 and the LMV794 in an open-loop configuration is not recommended.

-=-=-

What the heck does that mean? Why would it be okay in linear (closed loop) operation, and in saturated operation, but not in open loop operation?

Maybe it does weird bouncy stuff when recovering from saturation, so the comparator output edge isn't monotonic?

The elephant in the room is the input differential +/- 0.3V range, which means they put diodes across it. Why the hell would they do that on a CMOS amp?!

(I'm considering this part for a current shunt amplifier, so these wrinkles shouldn't be a problem, at least, but it worries me that there should ever be even a single quirk, in a stupid freaking op-amp, in 2017.)

Tim

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Seven Transistor Labs, LLC 
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Tim Williams
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Would the positive feedback make for greater hysteresis, or greater recovery time?

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John
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quiasmox

[snip]
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Tim, Where is that on the datasheet? I can't find that. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

Too bad there's no chip schematic. People used to do that.

I wonder if there's some capacitive boost into the output mosfet gates, to get more transient swing or something.

Some famous person said "Never use an opamp as a comparator", which is not absolutely true. Most modern rro amps make decent comparators. But many opamps do weird things if their outputs hit a rail, like stick for a long time or sort of bobble around randomly or draw lots of supply current. One Intersil opamp would take seconds to come off the positive rail.

It's thoughtful of that data sheet to point out the comparator limitation.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

Too big? Or it confuses the folks who aren't used to transistors? It's "proprietary"? All three?

I learned a lot of practical electronics from looking at the schematics.

I can understand not showing the whole shebang, but particularly for analog parts, I appreciate it when they show equivalent input and output schematics.

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Tim Wescott 
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Tim Wescott

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Abs max ratings.

Tim

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Seven Transistor Labs, LLC 
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Tim Williams

Thought so. You misread what it is saying. That's the inputs relative to rails. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

???

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If one were to connect the supply pins to +5V and GND, and also connect one input pin to +5V and the other input pin to GND, do you recommend this connection?

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Tim Williams

Missed that. That's really odd. Perhaps it's current-mode feedback? Any actual users to report back?

(May well be that the input is actually CMOS plus a bipolar slew-rate over-ride stage ??) ...Jim Thompson

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

If the amp has some internal +ver feedback and you don't provide external negative (as per normal), I think they're hinting that it's likely to oscillate.

John

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jaybsykes

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