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Behavior of Regulators Near and Below Drop-out

Some people could also try running the LT Spice models of their LDOs.

Or even fire up some chips and see how they actually work... if they don't mind using a soldering iron.

John

Reply to
John Larkin
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Works great as long as the zener isn' too big. 7800s push about 5 mA through their ground pins at zero load, iirc. LM317s put most of the quiescent current out their outputs, and only 50 uA or so out the adjust pin.

Cheers

Phil Hobbs

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Reply to
Phil Hobbs

One can stack regulators too, putting the adj pin of one reg on the output of another. I've done that three deep on occasion. Works great with LM1117s for 1.25 and 2.5 for FPGA core and Vaux voltages... no resistors!

You can also drive the adj pin from an opamp.

John

Reply to
John Larkin

Yep. But what do they do when pulled into current limit when stacked, say the simple zener case?

I know that bipolar types will forward bias the substrate, at least momentarily.

LTspice models are already behavioral, not device level, so they model behavior under "normal" circumstances.

Analog Devices' models are like that, as many have discovered ;-)

I guess that the lab is the only way to get data for a reasonable model. ...Jim Thompson

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Reply to
Jim Thompson

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