I'm debugging a fast, low level preamp board, and I found a +5 V analogue rail sitting at +6V.
It turns out that the reference voltage of this particular LM317L is about 1.5V. Not oscillating, just being weird.
Has anybody else seen that symptom?
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Seem to remember seeing something like with not enough minimum load
-Lasse
Yep, You must draw much more than the ADJ pin current spec with your divider, otherwise "weirdness" ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
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I love to cook with wine. Sometimes I even put it in the food.
Never, and it's odd. The data sheet
does say 1.20V to 1.30V. Lasse suggests that you might not have been drawing the 10mA minimum output current
10mA ? IO ? IMAXbut everybody designs their voltage-setting resistive divider to soak up that 10mA.
-- Bill Sloman, Nijmegen
Are you sure you don't have something backing up to the rail from elsewhere? THe 317 is a sourcing device the last time I looked, it won't sink..
Jamie
Well, if they're thinking. I'll admit that I've failed at that rather basic test on occasion, and I've debugged circuits for folks that did the same thing.
-- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
The ADJ pin current and min load are not really related. ADJ is only
100uA. For the TI parts the minimum load current is 25 times that, or 2.5mA.-- Regards, Joerg http://www.analogconsultants.com/
Which just means that even if you have a guaranteed minimum load you still need to pay attention to the ADJ current.
-- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
You missed my point. If your divider, by itself, pulls only 200uA, the output voltage will be...
6.875V !!!Assuming the reference potential stays put. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Well, sure, at least you have to calculate that into the resistor ratio.
-- Regards, Joerg http://www.analogconsultants.com/
So, guys, be honest, who else has forgotten the min load in the heat of the game at least once?
It's not like a gear-up landing but it sure is something where you'd think "I hope nobody saw that".
-- Regards, Joerg http://www.analogconsultants.com/
up
ther
he
My wife studied speech errors at one stage in her career, and I got exposed to some of the literature on "errors of action".
Basically it's typo's - you intend to do something and actually do something else. Sometimes you omit a planned action (omission), sometimes you do it twice (perseveration) and sometimes you do something familiar but inappropriate (deviation).
Programmers typically make one mistake per thirty lines of code, and people drawing circuit diagrams and making up parts lists get stuff wrong pretty regularly, though - sadly - not at predictable intervals.
Lots of engineering is simply checking for errors and correcting them. Programmers have walk-throughs. We have design reviews.
-- Bill Sloman, Nijmegen
I always use a 240 ohm resistor from Vout to Adj, which never fails to use up the required current, regardless of the output voltage, but hey, s**t happens, and I've similar things from time to time.
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
On a sunny day (Thu, 04 Oct 2012 16:31:28 -0400) it happened Phil Hobbs wrote in :
No
The last time I saw this effect (on a LD1117S12TR) my FPGA was getting
1.4V on its 1.2V rail before I stopped winding the higher supplies up. I'd changed the regulator before I found the short elsewhere that was pulling the rail up !Michael Kellett
Yup, pilot error and not monsters under the bed --I was just under the
2.5 mA minimum.Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
This board is full of MSOPs and SC70s and 0402s, which is _not_ my natural habitat, so debugging it is irritatingly slow. It's that 1 nA/100 MHz biochip front end gizmo with the Really Triangular Triangle calibrator. The layout guy did a nice job, especially an array of 39 pF
0201 caps to get rid of most of the ground inductance on the calibrator's integrating cap.So far I found a voltage reference oscillating because its bypass was too small. Fussy modern thing, the LTC6655--a 1N823 or LM329 wouldn't do that. ;) It seems to have an LDO-type architecture, with a high-Z output that's sensitive to the bypass value. (Of course in a precision voltage reference, I'd far rather have that than an internal charge pump.)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
AFAIR they need somthing big (>3uF?). Comes in 0402 even with 6.3V rating. But ... no X7R, yet. So you have to take capacitance derating into consideration, to make sure things aren't "riding on the edge". Also, mind the microphonics and piezo effects if this is a super-low-noise app. Then ceramic caps may not be so cool.
-- Regards, Joerg http://www.analogconsultants.com/
I stuck in a 10 uF 0603 X5R, which seems to be okay. Since the shot noise limit is equal to half the number of electrons you collect in one inverse bandwidth
SNR_max = n/(2*B)
the shot noise limited SNR of 1 nA in 1 MHz is
SNR_max = (6.2E9 electrons/s)/(2.0E8 Hz) = 31.
That's not 31 dB, it's 31, i.e. 15 dB.
I don't know how close I got, yet. We shall see, assuming the rate of beginner mistakes drops to something reasonable. ;(
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
LDOs often has a requirement for both the minimum and maximum ESR of the ceramic cap to be unconditionally stable
Cheers
Klaus
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