That was an interesting experiment. I thought about how cool the LP2950
was when it was first introduced, but remembered it was a bipolar LDO
and uses a PNP pass element, so I looked up the ground current. Yes,
it's dependent on the pass current and quite significant. Only a few
microamps at idle, but 8 mA at 100 mA out.

So I tried it just for giggles, with a resistor divider made of two
1Kohm resistors. That'll gobble 5 mA, so much for "micropower", but I
don't need that.

I figured at best I'd get a regulator with negative output impedance.
What I got was an oscillator, no matter how I bypassed it. I started
with tantalum caps from input and output to system ground, then added a
third cap from LP2950 ground pin to system ground, then tried it to
output. No good.

I think next I'll try a CMOS LDO, I have some HT7136-1. Not as great
ratings, 30 mA and 24V max in.

Back to real work for a while, though :)

Best regards,

Bob

On Sat, Aug 31, 2013, at 11:06 AM, davidcou@aol.com wrote:
> Maybe LP2950 and use as an adjustable (using GND as ADJ). I believe the
> Vdrop is 0.6 Volts. Just a thought. LF33 and LFxx are coming to mind
> also. =20

>=20
> While we're on the subject of analog, I need 10V in car for a personal
> project, driving sensors. It doesn't need to be operational during
> starting, but I'd like it to work down to 11 V input. Required current
> is only 25 mA. I can provide enough filtering and protection so it
> doesn't need to be overly tough.
>=20
> Adjustable or fixed 10V if fine.
>=20
> Anybody have a suggestion for a favorite part?
>=20
> Thanks!
>=20
> Bob

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