V
vasile
- 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.
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.
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.
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.
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.
jasen said:The regulator is DC coupled, has a single-ended output, more gain, and much higher
bandwidth.
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.
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."
vasile said:
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.
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.
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.
SioL said:Really (LM317)?
What bandwidth?
John said:I like LM2941, and any regulator has a legal right to scream if you
drive it from 100 ohms.
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."
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.
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.
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.
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.
If I need one I either roll my
own or design a switcher around a Schmitt inverter and some discretes.
Works every single time
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.
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...
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.