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LM7321: How do they do that?

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tomizett

Active Member
Purely out of curiosity...
Recently I was looking for an op-amp to generate a mid-rail/split a supply (rather like Large Ghostman is doing in a current thread), and stumbled across the LM7321 (http://www.alldatasheet.com/view.jsp?Searchword=LM7321). This part's party-trick is that it can drive unlimited capacitive loads - the datasheet alludes to it being something inherent in the architecture of the amplifier, but doesn't give much away.

Does anyone know what is different about this amplifier that makes this possible? I know that most (internally compensated) op-amps use a dominant-pole compensation scheme - does this use something different? I see lots of references to multi-pole compensation schemes, but rarely any detail on how one might design one. Or perhaps is this the standard dominant-pole scheme with the feedback path somehow decoupled from the capacitance on the output?

If anyone can point me in the right direction I'd be interested - even if it's only handing me a few key phrases to google.
 

MrAl

Well-Known Member
Most Helpful Member
Hello,

The output current is limited so the device does not burn up. Since the output current is limited, that means the slew rate gets slower and slower with increased output capacitance. The bandwidth also changes. So it doesnt look like anything really that surprising. The main point is that the device does not burn up.
 

dknguyen

Well-Known Member
Most Helpful Member
I thought the main point was that it doesn't become unstable with a large capacitance on the output?
 

tomizett

Active Member
Well, I was thinking about this the other night, and wondered if the current limit meant that the open-loop gain decreased as the capacitive load increased, and therefore kept it stable. But then I considered that the current limit is not active all the time (only when trying to slew fast into large C loads) and so should not affect the small-signal response. So I'm not sure that Mr Al's response quite covers it.... or am I missing something?

Thanks for the interest; any other insights welcome!
 
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