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Re: Switching power supply question

[...]

> National Semiconductor has an article on LDO ESR:

>> "Capacitors are key to voltage regulator design" >>
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>> The LP2985 specs the minimum ESR at 5 milliohms. This is >> compatible with ceramic output caps: >>
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> Then one sunny day the cap mfg "improves" a cap series, resulting > in even lower ESR, phssst. BANG!

That should be caught in incoming inspection.

Besides, the ESR "stable range" is usually a figure and not > guaranteed data. That alone is a turn-off point for me. In medical > electronics where I work a lot you can't rely on typical data.

It is never a good idea to use typical values in a design. From the datasheet:

"The LP2985 was designed to work with ceramic capacitors on the output to take advantage of the benefits they offer: for capacitance values in the 2.2 uF to 4.7 uF range, ceramics are the least expensive and also have the lowest ESR values (which makes them best for eliminating high- frequency noise). The ESR of a typical 2.2 uF ceramic capacitor is in the range of 10 mohm to 20 mohm, which easily meets the ESR limits required for stability by the LP2985."

It sounds like they intend the part to be used with low ESR caps.

I'm sure you could require a guaranteed performance spec in a purchase contract if you were willing to pay for it.

Anyway, this was just an example to show it is possible to improve the ESR performance without requiring an extra pin with external filter components.

I also had some non-datasheet-mentioned pathologies. For example > on a LM29xx series chip from National many moons ago. Someone > absolutely wanted to have it in there despite my advice against > LDOs. Turned out it did not "like" too high a source impedance. > Made it oscillate, heat up and then fry. Lengthy calls, a team at > National on the other side. You could hear large sheets being > handled. Suddenly a faint "oh drat" could be heard over the phone > and then I knew we were in deep doodoo.

This is an implementation problem in the design. Obviously it is something you check before committing a part to production.

[...]

Take a look at the far right where the price is listed. The

> ADP3331 has a realistic quantity street price around a Dollar, way > above budget for a lot of designs. Especially if you can do it > without an LDO.

I checked the parts in Digikey before posting. Certainly the price is high - that's what you pay for in a monopoly. However, the part is not affected by ESR, and it does not require an extra pin with external filter components. So a clever design might get around the patent and give the same results.

> For reference, here's a bunch of articles from various sources on >> LDO ESR. The url's are not shown since they degrade with time. >> But google finds them quickly. >> AN1148 PDF 205,195 Compensating Low-Dropout Regulators >> AN682 PDF 879,026 New Generation of Low Dropout Regulators >> AND8028 PDF 38,766 Precision Sub-One Volt 1.7 Ampere Output LDO >> SLVA068 PDF 68,595 LDO Fundamental Theory >> SLVA072 PDF 291,538 LDO Technical Review >> SLVA079 PDF 202,342 LDO Terms and Definitions >> SLVA115 PDF 85,754 Regulator ESR Stability >> SLYT151 PDF 468,245 Compensation Transient Response >> SLYT187 PDF 254,973 Understanding the Stable ESR Range >> SLYT194 PDF 246,355 LDO Linear Stability Analysis >> SR003AN PDF 73,945 Compensation for Linear Regulators >> SR004AN PDF 41,598 Linear Regulator Output Structures > Thanks for the links. I don't generally use LDOs but there may > come a situation where it has to be one.

Your complaints about LDO's are well known. You spoke of rolling your own when needed. Any chance you could post a schematic so we could see how these problems are solved?

Reply to
NoName
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Rough ascii art mechanical drawing: [should be][is]

I have seen many "improved" parts get past incoming inspection.

Reply to
MooseFET

Are you suggesting that every lot of ceramic caps be tested, at receiving, for ESR? And by extension, every received part (like 16-bit octal DACs, FPGA's, laser diodes, kep nuts) be tested for all their specs at receiving inspection?

John

Reply to
John Larkin

On ESR? Manufacturers routinely spec a guaranteed maximum, no minimums. They will refuse to take back and give credit if you return a shipment because ESR is too low. And rightfully so. I am not sure 1206 caps would be suitable as tiles for the lobby or pavers for the patio but you'd have to do something with all the "rejects" ;-)

Excellent example. Quote from that very datasheet: "The output capacitor must maintain its ESR within the stable region over the full operating temperature range of the application to assure stability." Or how about "...utilizing circuitry which allows the regulator to be stable across the entire range of output current with an output capacitor whose ESR is as low as 5 mohm." a page earlier? So what exactly does "as low as" mean? Is it guaranteed? I doubt that. And how do you maintain that value? Put a 5mohm resistor in series?

Then there is the typical graph on top of top of page 15. Where are those limits guaranteed? Why are they not in the min-max tables? I'll never use a part like that.

IME companies are often not even willing to simulate the thing when it did go wrong. Maybe because then a dirty little secret could pop out.

Sorry, I do not see that. Same old, same old, there is a minimum ESR mentioned and IMHO that's not a good thing.

Absolutely not. This was a stability problem of a chip that was not mentioned in the datasheet. IOW not supposed to happen but did happen. Needless to say the guys where rather apologetic on the phone because they knew it wasn't supposed to happen.

Then I'd get shot ;-)

But seriously, all you have to do is run thorough simulations and pay special attention to loop stability. In a discrete design it is not a problem to add a 4700pF cap somewhere, on a chip that is quite impossible. But I rarely design discrete LDOs these days because when you are this close to the input voltage you might as well go straight to a SEPIC or forward converter, which is what I usually do.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Nonsense, see

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, explains simply what is going on in sections 8 and 9 and why you end up with a range of ESR. You can use a compensating series resistor...

Reply to
Fred Bloggs

But it's so easy to add esr without adding series resistance...

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Thanks,
Fred.
Reply to
Fred Bartoli

Better read the one below ;-)

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--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Then someone hangs a heavily bypassed cicuit onto it ... WHADDABANG!

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg
[...]

Ok, one hint, don't want to leave you hanging here: The statement in most papers that an emitter- or source-follower cannot be pulled into saturation and thus cannot become an LDO is not necessarily true ;-)

There are ways to generate voltages above Vin level ...

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

You mean even without any switching circuitry (which obviously makes it easy to generate higher voltage) and only DC input (AC is the same as switching...), right?

Reply to
Joel Koltner

You do need a wee oscillator :-)

Sometimes you can hitch a ride on an oscillator that is in the system anyway, as long as that comes on line well before the regulator needs to go into really low dropout. Or use a free Schmitt somewhere. For example a CMOS variety that already starts at 2-3V.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

And here I thought you were going really exotic on us. :-)

Thanks for the information!

Reply to
Joel Koltner

I remember a really exotic version from the tube days. Someone needed a negative grid voltage at next to nothing in current. He put a couple of

9V batteries in there and they pretty much lasted their usual shelf life (possibly they even survived him). You just had to peek into the amplifier every other year, making sure they didn't leak. 
--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

A C battery? Pffbt, that's *ancient*...

Now how about this:

- Run the phono amp heaters from the output tubes' cathode current (class A)

- Run the phono/tape head amp heaters from an RF oscillator (or a number of other possible uses)

Tim

-- Deep Fryer: A very philosophical monk. Website @

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Reply to
Tim Williams

Goes back to Jim's old question: How does it start? There ain't no cathode current before the filament has heated up ...

And then the Federales come swooping in because the FCC got too many complaints :-)

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Yep. I had a separate switch to turn on filament power first, back in my audiophool tube days ;-)

Later I used one of those huge mercury-filled time delay relays.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

Mercury? Yuck! I used regular relays and two-transistor time delay circuits. And only because Schmitt Inverters were too expensive when I was a kid. One-shots cost as much as 1/4 crate of Pilsener beer, no way.

On transmitter tubes you had to make sure the direct-heat cathode was up to spec temperature before allowing quiescent current. Those tubes cost a fortune.

[...] 
--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

I was a kid BT (before transistors :-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

C'mon, you couldn't have been older than 12 or so when these showed up on the market:

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Whenever I couldn't get hold of or afford the required transistors I used tubes. For example when I needed a 170V linear regulator. Hopped on my bicycle, spotted a discarded B/W TV, plucked the PL500 from that, done. I would have carted the whole set home except that I already had a dozen in various states of organ donation and the baggage rack had collapsed during the previous schlepping of a large set.

[...] 
--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

I saw my first CK760 in 1956, when I was sixteen. I used the mercury delay relay because it could do 30 seconds.

BTW, I celebrate my 17th birthday on Friday ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

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