Re: Switching power supply question

Rough ascii art mechanical drawing: [should be][is]

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

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

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.

Nonsense, see

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

Better read the one below ;-)

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

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 ...

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?

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.

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

Thanks for the information!

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

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 @

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 :-)

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

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.

I was a kid BT (before transistors :-)

...Jim Thompson

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

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.

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