On Sun, 21 Feb 2010 11:06:45 -0800, John Larkin
wrote:
On Sun, 21 Feb 2010 13:59:58 -0500, Phil Hobbs
wrote:
On 2/21/2010 12:29 PM, Jim Thompson wrote:
On Sun, 21 Feb 2010 19:11:51 +0200,
wrote:
"Jim kirjoitti
om...
The internal schematics of classics like LM317 and LM1117 and such are
on the data sheets. Some people (?) could deduce their behavior from
that.
John
Those aren't LDO's, they're NPN "followers"; not PNP or PMOS, whose
behavior would be radically different, and quite process dependent.
LP2950 has schematics on datasheet (National version).
-ek
Thanks! I'll check that out. Although my past experiences in the
"jelly bean" business is those schematics are usually "simplified" to
hide IP.
This is getting to be a really amusing mental endeavor! For instance,
how might an LM7805 behave with a zener inserted in its ground lead to
boost the voltage ?:-)
...Jim Thompson
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
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
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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