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Linear 3.3V power supply anywhere?

T

Tam/WB2TT

Jan 1, 1970
0
Joerg said:
Well, this is medical and everything newly designed needs to go through
the usual hoops. Also, the LT1764 is a LDO and I am pretty much cured of
using those. A quote from the LT1764 data sheet might illustrate the
point: "Ceramic capacitors have extremely low ESR, and while they are a
good choice in many cases, placing a small series resistance element will
sometimes achieve optimum stability and minimize ringing."

That gives me the goose pimples. Sounds like marketing speak for "it could
go ballistic if you don't watch the ESR". At least they offer a table on
page 12 but still, I'd rather not.

Actually, with 12 V available, you don't need an LDO. Run the regulator off
+12, with only the 3.3 V current coming from the +5. You have enough
voltage available to use an N channel source follower.

Tam
 
J

John Larkin

Jan 1, 1970
0
Well, this is medical and everything newly designed needs to go through
the usual hoops. Also, the LT1764 is a LDO and I am pretty much cured of
using those. A quote from the LT1764 data sheet might illustrate the
point: "Ceramic capacitors have extremely low ESR, and while they are a
good choice in many cases, placing a small series resistance element
will sometimes achieve optimum stability and minimize ringing."
That gives me the goose pimples. Sounds like marketing speak for "it
could go ballistic if you don't watch the ESR". At least they offer a
table on page 12 but still, I'd rather not.


All my alarms go off when a datasheet uses the word "optimum." It's
code for "something is really wrong with this part and you can work
around it with luck and difficulty, if at all."

John
 
E

ehsjr

Jan 1, 1970
0
Joerg said:
That's almost what I am going to use. We need a crowbar module with the
right certs and all. Per suggestion from Condor Applications
Engineering we'll take their OV12 module, yank the zener and replace it
with something like a LMV or TLV431.

Nice, low current.

Someday you're going to be asked to design something
where you have a lot of footprint available, no low
current/low voltage constraints, and a fat budget.
You'll faint from the shock. :)

Ed
 
T

Tam/WB2TT

Jan 1, 1970
0
Joerg said:
Well, this is medical and everything newly designed needs to go through
the usual hoops. Also, the LT1764 is a LDO and I am pretty much cured of
using those. A quote from the LT1764 data sheet might illustrate the
point: "Ceramic capacitors have extremely low ESR, and while they are a
good choice in many cases, placing a small series resistance element will
sometimes achieve optimum stability and minimize ringing."

That gives me the goose pimples. Sounds like marketing speak for "it could
go ballistic if you don't watch the ESR". At least they offer a table on
page 12 but still, I'd rather not.

This may be a duplicate, but after 5 hours, I don't see my original posting.

Tam

Actually, with 12 V available, you don't need an LDO. Run the regulator off
+12, with only the 3.3 V current coming from the +5. You have enough
voltage available to use an N channel source follower.
 
J

Joerg

Jan 1, 1970
0
ehsjr said:
Nice, low current.

Someday you're going to be asked to design something
where you have a lot of footprint available, no low
current/low voltage constraints, and a fat budget.
You'll faint from the shock. :)

:)

Actually I did just that. Finished the netlist checks about 10 minutes
ago, whew, ready to take a long walk with the dogs and then start the
barbeque.

Big med system, cost is a relatively minor concern and it was no problem
to use really nice DACs in the $5 class. To me that feels like sipping a
fine Chateau Neuf du Pape. One of the other consultants couldn't get his
board into 160mm 6U so the client said "Ok, just make it longer then".
Meaning mine could be longer, too. So here I am, looking at a vast
space, giving me the luxury to go back to 0805 parts. Nowadays those
look like boulders to me.
 
J

Joerg

Jan 1, 1970
0
jasen said:
The regulator is DC coupled, has a single-ended output, more gain, and much higher
bandwidth.

Which doesn't mean it can't be used as an audio amp. I am guilty of
having used a TL431 in that function and a LM317 as a modulator.
 
J

Joerg

Jan 1, 1970
0
Tam/WB2TT said:
Actually, with 12 V available, you don't need an LDO. Run the regulator off
+12, with only the 3.3 V current coming from the +5. You have enough
voltage available to use an N channel source follower.

I did that on the boards at lower power. But on the backplane I'd rather
use something commercial. Well, for now, later it'll be all custom anyhow.
 
J

Joerg

Jan 1, 1970
0
John said:
All my alarms go off when a datasheet uses the word "optimum." It's
code for "something is really wrong with this part and you can work
around it with luck and difficulty, if at all."

Can sometimes also be code for "This is how it works and we don't really
know why" or "There are some issues and Marketing doesn't want us to
make too much of a fuss about it".

But LTC is pretty honest and reliable. Still, I won't use LDOs from any
source anymore. The worst calls for a consultant are post-design issue
fixes. Clients may not and mostly do not fully understand how a circuit
works and that it's really the fault of xyz corporation. All they know
is "it don't work".
 
J

John Larkin

Jan 1, 1970
0
Can sometimes also be code for "This is how it works and we don't really
know why" or "There are some issues and Marketing doesn't want us to
make too much of a fuss about it".

But LTC is pretty honest and reliable. Still, I won't use LDOs from any
source anymore.

I like the LM1117, which has an npn output stage, so runs about a volt
of dropout, but is cheap, very rugged, and stable.

But the LP2985 is a "true" ldo that is stable with very low esr. This
works great with just ceramics on the output, which avoids the dangers
of tantalums or the temperature and lifetime problems of aluminum
'lytics.

Things like the TPS769 series are oscillators waiting to happen.

John
 
J

Joerg

Jan 1, 1970
0
John said:
I like the LM1117, which has an npn output stage, so runs about a volt
of dropout, but is cheap, very rugged, and stable.

But the LP2985 is a "true" ldo that is stable with very low esr. This
works great with just ceramics on the output, which avoids the dangers
of tantalums or the temperature and lifetime problems of aluminum
'lytics.

Don't know about this one but I had a LM29xx series LDO go berserk when
the source impedance became a bit higher, around 100 ohms or so. I
received a "Ahem, yes, you are right, they seem to do that". So they
went onto my "no-no" list.
 
J

John Larkin

Jan 1, 1970
0
Don't know about this one but I had a LM29xx series LDO go berserk when
the source impedance became a bit higher, around 100 ohms or so. I
received a "Ahem, yes, you are right, they seem to do that". So they
went onto my "no-no" list.

I like LM2941, and any regulator has a legal right to scream if you
drive it from 100 ohms.

John
 
S

SioL

Jan 1, 1970
0
Joerg said:
Which doesn't mean it can't be used as an audio amp. I am guilty of having used a TL431 in that function and a LM317 as a
modulator.

Really (LM317)?
What bandwidth?

SioL
 
J

Joerg

Jan 1, 1970
0
John said:
I like LM2941, and any regulator has a legal right to scream if you
drive it from 100 ohms.

IMHO it has to say so in the datasheet. There are lots of applications
where you must feed power over a huge cable run.
 
H

Haude Daniel

Jan 1, 1970
0
Joerg said:
A quote from the LT1764 data sheet might illustrate the
point: "Ceramic capacitors have extremely low ESR, and while they are a
good choice in many cases, placing a small series resistance element
will sometimes achieve optimum stability and minimize ringing."

I was about to rant about the fluffy choice of words,

"small resistive element"
"sometimes"
"optimum stability"

but decided to download the LT1764 datasheet and see for myself. Text
searches for "resistive", "ringing", "sometimes" or "optimum" turned up
nothing ("Optimum" appears once in a different context). Instead, the
datasheet dedicates half of page 11 to output capacitor choice, which itself
can be seen a warning against the part, but at least they provide a fairly
thorough treatment of the problem including a discussion of different
dielectrica.

In short: in order to do LT justice you need to update your datasheet. But
LDOs are still nasty.

Somewhere I have kicking around a pdf about a "zero-drop" linear
regulator using a P-MOSFET as pass element. I can email it to interested
parties.

--Daniel
 
J

Joerg

Jan 1, 1970
0
Haude said:
Joerg wrote:




I was about to rant about the fluffy choice of words,

"small resistive element"
"sometimes"
"optimum stability"

but decided to download the LT1764 datasheet and see for myself. Text
searches for "resistive", "ringing", "sometimes" or "optimum" turned up
nothing ("Optimum" appears once in a different context). Instead, the
datasheet dedicates half of page 11 to output capacitor choice, which itself
can be seen a warning against the part, but at least they provide a fairly
thorough treatment of the problem including a discussion of different
dielectrica.

In short: in order to do LT justice you need to update your datasheet. But
LDOs are still nasty.

That was a LTC datasheet. I didn't make that up:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1040,C1055,P1820,D1316
See page 11, right column, 2nd paragraph.

Ok, how about this text in the version you may have looked at, can
undoubtedly be seen on page 11 (upper right): "The LT1764 regulators are
designed to be stable with a wide range of output capacitors. The ESR of
the output capacitor affects stability, most notably with small
capacitors. A minimum output capacitor of 10µF with an ESR in the range
of 50m? to 3? is recommended to prevent oscillations."

Sounds nice, but every bit just as scary to me. What exactly is a wide
range here? What is "in the range of"? I'd expect hardcore min max
datasheet values, not some nice looking graphs and carefully minced words.

BTW, in the other datasheet above they are talking about down to 20mohms
ESR for optimum transient response. Hmm...

Somewhere I have kicking around a pdf about a "zero-drop" linear
regulator using a P-MOSFET as pass element. I can email it to interested
parties.

Thanks but I'll pass. I am cured of LDOs. If I need one I either roll my
own or design a switcher around a Schmitt inverter and some discretes.
Works every single time :)
 
H

Haude Daniel

Jan 1, 1970
0
Joerg said:
That was a LTC datasheet. I didn't make that up:
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1040,C1055,P1820,D1316
See page 11, right column, 2nd paragraph.

Ah, that's the LT1764A. I looked at the LT1764.

Anyway, I wasn't trying to convince you to use LDOs -- in fact it was
reading *your* posts on this NG that, lacking experience of my own,
is going make me steer clear of LDOs should I ever feel the urge to use
one.
A minimum output capacitor of 10uF with an ESR in the range
of 50m? to 3? is recommended to prevent oscillations."

Sounds nice, but every bit just as scary to me. What exactly is a wide
range here? What is "in the range of"? I'd expect hardcore min max
datasheet values, not some nice looking graphs and carefully minced words.

It does sound scary, but those damn buggers seem to be hard to get stable
by their very nature. And since the mfg doesn't know the impedance you're
going to hang in parallel to the output cap they necessarily have to be vague
about the impedance of the cap as well. What it boils down to is that you
have to use DS values as starting points in prototyping, and then work your
way from there.

And then your gadget goes into production with LDOs from a different batch
or a second source and all hell breaks loose...
If I need one I either roll my
own or design a switcher around a Schmitt inverter and some discretes.
Works every single time :)

Just how does your Schmitt inverter switcher work, and over which range of
duty cycles? I've built simple PWMs myself, but they always relied on a
sawtooth generator/comparator combo.

--Daniel
 
J

Joerg

Jan 1, 1970
0
Haude said:
Joerg wrote:




Ah, that's the LT1764A. I looked at the LT1764.

Anyway, I wasn't trying to convince you to use LDOs -- in fact it was
reading *your* posts on this NG that, lacking experience of my own,
is going make me steer clear of LDOs should I ever feel the urge to use
one.

I wouldn't mind them if there were guaranteed stability limits in the
datasheets. I just will not design per "typical" graphs or text that
sounds too vague to me.
It does sound scary, but those damn buggers seem to be hard to get stable
by their very nature. And since the mfg doesn't know the impedance you're
going to hang in parallel to the output cap they necessarily have to be vague
about the impedance of the cap as well. What it boils down to is that you
have to use DS values as starting points in prototyping, and then work your
way from there.

And then your gadget goes into production with LDOs from a different batch
or a second source and all hell breaks loose...

Been there. It hurts. A lot.
Just how does your Schmitt inverter switcher work, and over which range of
duty cycles? I've built simple PWMs myself, but they always relied on a
sawtooth generator/comparator combo.

Sawtooth is fine, too. Basically just an oscillator where a BJT or FET
tugs on the input, pulling it down as the desired output voltage is
being reached. IOW slowly "starving" the oscillator by reducing its duty
cycle. You'll have to make sure there is enough oomph to drive FETs and
the like so the whole thing reaches the desired enough efficiency. And
always mind any RHP zero issue, of course ;-)
 
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