That's good info. What input-ouput differential did you use? 5V-3.3V?
I've noticed that some R-R op-amps get a bit wrangy as you get very close to the rail.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
...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
That doesn't buy you LEAD (pronounced (LEEEED)) ;-)
See my example from a few days ago...MY stuff IS stable. To see how I test my designs, see the LoopGain discussion, macros, and symbols, on my website.
I am presently working with Agustin Ochoa (Zarlink) on methods to analyze multi-path loops.
...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
Is there a danger that the next batch of parts will, in fact, oscillate?
-- John Devereux
I don't understand why you don't understand. Unless the LDO has a emitter-follower output like the LM1117, it suffers from the output stage having a gain proportional to the load impedance.
Actually, the LM1117 also suffers from that. Looking carefully you see the output-stage gain is proportional to g_m * h_fe * X_L, where g_m is for the PNP transistor and h_fe for the NPN. The load capacitor makes a pole, which is added to the control-loop pole(s), and the regulator benefits greatly from a zero provided by the load capacitor's esr. If I understand Jim's assertion earlier, he designs a LEAD into his control loop, to compensate for the LAG of the load capacitor. Or was he using another one of the definitions of LEAD, as in the dense-metal element? :-) Anyway, yes of course the IC designers should have, must have, woulda-liked-to have added a high-frequency zero to cancel their control-loop pole, but then again maybe somehow they didn't.
Maybe you could ask him some day whether they'll make another round of the SL6440 mixer ;-)
AFAIK inherited much of the IP of the former Plessey.
-- Regards, Joerg http://www.analogconsultants.com
Of course. That's why I measure transient-whacked Q, not just oscillation/no-oscillation, to see if there's margin. I wouldn't go into production on anything that rings with Q=2 or better, ideally on nothing that rings at all. Looks like I'm back to tantalums, grossly derated on voltage.
But ICs from one mfr are usually pretty consistant. Our purchasing notes on the LM1117 say *no substitutes*, apparently based on experience.
John
Then you might want to ship your units together with one of these:
SCNR ....
That would be a huge red flag for me.
-- Regards, Joerg http://www.analogconsultants.com
Tantalums aren't bad if they are used at half of rated voltage or less, and if they're not exposed to nasty current surges. In fact, they are very reliable if not abused.
Aluminums dry out, and esr goes bonkers below 0 degrees C
Polymer aluminums have a wearout mechanism, worse at high temp and humidity.
Ceramics are low density, worse if you have to add esr padding resistors.
Don't know enough about polymer tantalums or niobium.
What's a boy to do?
Hmmm, the purchasing note says
DO NOT ORDER FAIRCHILD EQUIVALENT -- PER JL
I wonder what that means?
John
Sounds familiar. I issued one at GenRad, "NO MOTOROLA LM324's!"
...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 | |
[snip]
That's my standard design procedure, my LoopGain analysis method PLUS "twang it" ;-)
...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
Just follow the 11th commandment: Thou shalt not use LDOs ;-)
Got to confess that I did something similar. ECO'd a whole range of mfgs for a logic chip that was used in, ahem, analog fashion. Pretty much all of them but Fairchild because their datasheet was a bit too silent about substrate diodes and they weren't willing to give me some more info in writing. All I could get on the phone was folks from sales.
-- Regards, Joerg http://www.analogconsultants.com
Wasn't it Sheriff Taylor in Mayberry who played the "twang box"? At least that's what Charlene's father used to say.
-- Regards, Joerg http://www.analogconsultants.com
Consider this anonymous message from early this year:
"I am currently in LDO Hell. We have almost 100 different voltage regulators in stock, and it looks like I'm going to add a few more..."
-- JL
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
The box I have in layout now, the tiny 4-channel arb, gets +12 in from a wall wart. I need +12, -12, +5, -5, +3.3, +2.5, and +1.2. I'm making the -12 from +12 (the inverter circuit I posted to abse), linears for
+-5, buck switcher from +12 to +3.3, then linears from 3.3 to 2.5 and 1.2. So many opportunities to screw up! LM1117 is an MDO, just able to do the 2.5 from 3.3, but not quite as quirky as most pnp-type LDOs.Really, the power supplies are more trouble than the waveform generators.
One other project we're doing, we need to generate regulated 3.3 from as low as, well, 3.3, so we plan to make our own LDO using an opamp and a milliohm nfet pass transistor. We can run the opamp off +12, so we can really slam the fet gate. AND we have absolute control over loop dymamics.
John
Since I just buy the regulators, and the datasheets do their best to obscure what's going on inside, I can't do loop analysis. So we just guess, test, and hope.
John
Looks like you can get 200mOhms easily (and cheaply).
Then you could combine one with a ceramic capacitor.
-- John Devereux
I'd most likely do them all with home-cooked switchers. +12V is perfect for a CD40106. It's got six inverters that can be used as PWM generators and you need six voltages besides the +12V coming in :-)
Why not a buck? At 100% duty cycle all there is between Vin and load is the FET and the inductor which should cover the 3.3V Vin situation.
-- Regards, Joerg http://www.analogconsultants.com
Maybe for digital, but noise considerations for the analog supply often dictate the use of a linear regulator or post regulator.
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Sure, but it can be done. I had to do it many times. For example on patient interfaces where weak signals had to be received but power had to be brought out to that isolated head unit. Couldn't use DC ;-)
Actually, on one system they did use DC before I redesigned that area. Two huge camera batteries, lead acid, 2lbs each, from the good old days. Could really smash your foot if one fell out but then again you were already in a hospital when that happened ....
-- Regards, Joerg http://www.analogconsultants.com
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