reg + reference
in [Design]
|
From: John Larkin on 28 Mar 2010 00:35 On Sun, 28 Mar 2010 00:29:31 -0500, Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> wrote: >On Sat, 27 Mar 2010 09:58:06 -0700, the renowned John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Fri, 26 Mar 2010 13:43:32 -0800, "Artemus" <bogus(a)invalid.org> >>wrote: >> >>> >>>"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >>>news:13rpq5lqd1lj1k9flb9orfr2vcl2tuucpi(a)4ax.com... >>>> >>>> ftp://jjlarkin.lmi.net/V220_reg.JPG >>>> >>>> >>>> John >>>> >>> >>>How much current can that 1117 source from it's ADJ pin? >>>Art >>> >> >>Oh, I see what you're getting at, fig 4 >> >>ftp://jjlarkin.lmi.net/ARM_power.JPG >> >>if I catch your drift. Nice. >> >>John > >How about something like this? > >http://speff.com/ARM_ref.jpg > > > > >Best regards, >Spehro Pefhany Hi, Spehro, Actually, the dynamic impedance of an LM4040-3.0 is low enough - about 1 ohm - that I can run it from the 1117 output through just a resistor. I'll probably run the 1117 output at about 3.4 volts to make sure the drop in the resistor is reasonable. 400 ohms would put about 1 mA into the 4040. John
From: markp on 28 Mar 2010 13:02 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:uddsq5ldu791ingq021n2fik76mic9taj4(a)4ax.com... > On Sat, 27 Mar 2010 07:08:36 -0700 (PDT), "langwadt(a)fonz.dk" > <langwadt(a)fonz.dk> wrote: > >>On 27 Mar., 02:40, John Larkin >><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>> On Fri, 26 Mar 2010 17:01:38 -0700 (PDT), "langw...(a)fonz.dk" >>> >>> >>> >>> <langw...(a)fonz.dk> wrote: >>> >On 26 Mar., 22:24, John Larkin >>> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >> On Fri, 26 Mar 2010 19:21:00 GMT, Jan Panteltje >>snip >>> >> Or maybe there exists a 3 volt 3-wire reference that would work from >>> >> 3.3 volts. >>> >>> >use the ~3.3V as reference, measure a 1.2V reference voltage and >>> >correct for it? >>> >>> I could use a 1.2 volt shunt reference, with a reasonable resistor >>> from 3.3, and use an opamp to scale the 1.2 up to 3. More parts! The >>> National low-dropout reference is ideal, if it doesn't oscillate. >>> >>snip >> >>I meant connect the 1.2V reference to a spare ADC channel, no scaling >>needed. >> >> >>-Lasse > > Oh. Then we'd use 3.3 as the ADC reference, measure 1.2, and normalize > measured values against the 1.2. That would work, with a bit more > math... unfortunately a divide, but not too often. I could do that at > 2.5 volts, too, and still have a reasonable voltage drop across a > resistor to 3.3. Nice idea. > > John > Except that these 'voltage refrences' are normally bandgap references, very stable over temperature and voltage but have low accuracy in absolute terms. You'll need to calibrate it to get any decent absolute accuracy. Mark.
From: markp on 28 Mar 2010 13:18 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:63ksq59j6m3u6hqnboijnngb5qqi52fbf5(a)4ax.com... > On Sat, 27 Mar 2010 10:58:04 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >>John Larkin wrote: >>> On Sat, 27 Mar 2010 08:47:41 -0700, Joerg <invalid(a)invalid.invalid> >>> wrote: >>> >>>> John Larkin wrote: >>>>> On Fri, 26 Mar 2010 17:22:36 -0700, Joerg <invalid(a)invalid.invalid> >>>>> wrote: >>>>> >>>>>> John Larkin wrote: >>>>>>> On Fri, 26 Mar 2010 15:36:09 -0700, John Larkin >>>>>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>>>> >>>>>>>> On Fri, 26 Mar 2010 14:34:19 -0700, Joerg <invalid(a)invalid.invalid> >>>>>>>> wrote: >>>>>>>> >>>>>>>>> John Larkin wrote: >>>>>>>>>> ftp://jjlarkin.lmi.net/V220_reg.JPG >>>>>>>>>> >>>>>>>>> Just curious, why not replace all this with one of these: >>>>>>>>> >>>>>>>>> http://www.analog.com/static/imported-files/Data_Sheets/ADR01_02_03_06.pdf >>>>>>>>> >>>>>>>>> Much less space, cheaper. Unless you need the 3.3V somewhere. >>>>>>>> Yes, I need 3.3 to run an ARM processor, 40 mA or so maybe. >>>>>>>> >>>>>>>> The ADR could run off the +15, so maybe that's a better way to go. >>>>>>>> It >>>>>>>> would cost a little more, but the TC would be a lot better. >>>>>>>> >>>>>>>> Maybe I'll do that, the LM1117 for power and the ADR for the >>>>>>>> reference. >>>>>>>> >>>>>>>> Wait! LM4120 will work. +3 ref with 120 mV dropout! >>>>>>>> >>>>>>>> >>>>>>>> John >>>>>>> And of course, the LM4120 is a stability land-mine of its own. >>>>>>> >>>>>>> ThestandardapplicationcircuitfortheLM4120isshownin >>>>>>> Figure1.Itisdesignedtobestablewithceramicoutput >>>>>>> capacitorsintherangeof0.022�Fto0.047�F.Notethat >>>>>>> 0.022�Fistheminimumrequiredoutputcapacitor.These >>>>>>> capacitorstypicallyhaveanESRofabout0.1to0.5. >>>>>>> SmallerESRcanbetolerated,howeverlargerESRcannot. >>>>>>> Theoutputcapacitorcanbeincreasedtoimproveloadtran- >>>>>>> sientresponse,uptoabout1�F.However,valuesabove >>>>>>> 0.047�Fmustbetantalum.Withtantalumcapacitors,inthe >>>>>>> 1�Frange,asmallcapacitorbetweentheoutputandthe >>>>>>> referencepinisrequired.Thiscapacitorwilltypicallybein >>>>>>> the50pFrange.Caremustbetakenwhenusingoutput >>>>>>> capacitorsof1�Forlarger.Theseapplicationmustbethor- >>>>>>> oughlytestedovertemperature,lineandload. >>>>>>> >>>>>>> >>>>>>> Thanks, National. >>>>>>> >>>>>> I was just gonna tell you. If there was one I'd have a bumper sticker >>>>>> on >>>>>> my car "LDO, no, no, no!" >>>>>> >>>>>> My advice to clients there is always the same, don't ever use an LDO >>>>>> unless there is absolutely no other way (and in my 25 years on the >>>>>> beat >>>>>> there always was). No matter what the marketeers sing from the trees. >>>>> Unless I work off the +15, which has its own complications, I need >>>>> this LDO voltage reference. After all, they say it's stable over - >>>>> wait for it - a 2:1 load capacitor range! Assuming you get the ESR >>>>> right. >>>>> >>>> And nothing seems to be guaranteed. When I read page 9 in the datasheet >>>> I am beginning to feel sick. >>> >>> Cute, how they say the compensation capacitor is in the "50 pF range" >>> and how they expect *me* to compensate and extensively >>> temperature/line/load test *their* part to ensure stability. I'm >>> expected to do a couple thousand dollars worth of testing on their >>> behalf. >>> >>> National is very sloppy as regards stability of lots of their parts. >>> >> >>Which is why I haven't designed in any of their power parts lately, >>except legacy stuff such as the LM317. LTC is pretty good, very >>responsive, great support engineers who really know electronics. Plus >>LTSpice models for just about anything and that alone ist worth a ton, >>has won them many design-ins on my part. Although I recently unearthed >>an ugly power-up bug in a part but that was a comparator, LT6700. But >>they went to the ground of it, confirmed it, fessed up, and apologized. >>Way to go. > > > Yes, LTC is great. And LT Spice is a gift to the world. > > >> >>> >>>> Why does working off of the +15V have complications? >>> >>> I'd need a series reference that can tolerate unregulated +15, so I'd >>> probably have to preregulate into it... more parts. Shunt regs have >>> mediocre TCs and would get not-very-constant current if I just use a >>> resistor from +15. I expect that my 4-20 mA (really 0 to 25 maybe) >>> load and uP power patterns will whack the DC-DC output voltage all >>> over the place. >>> >>> There will be 12 channels per VME board, so I want low parts count and >>> cheap parts! >>> >> >>An RC filter or capacitor multiplier should suffice. Input ripple >>rejection is usually huge as long as it doesn't contain high frequency >>stuff. Or use one preregulator per board. Or even better, one reference >>per board. > > > Sadly, all 12 channels are individually floating. > > It's looking like the safest thing to do is use an LM1117 to make > about 3.4 volts - close enough to 3.3 - and use a ca 500 ohm resistor > into an LM4040-3.0 or equivalent bandgap with a 10 uF ceramic bypass. > Typical TC will be about 15 PPM (guaranteed is not as good, but not > lethally bad), sensitivity to 1117 output is pretty good (500 ohms > into Zd~~1 ohm), and it's simple and cheap and most important, it's > safe. > > That out of the way, my next task is to explode some SSRs. Certain > user abuse modes will put, say, 50 volts across an ON-state SSR. The > ARM will sense the hazard and turn off the SSR, so it will have to > tolerate the abuse for a millisecond maybe. I'm considering some Clare > 8-ohm, 100 volt, 150 mA parts. So what happens if you turn them on and > put a bunch of voltage across them? > > I love to test parts to destruction. > > John > 1% tolerance is pretty bad for a precision reference. Here's a better one: http://www.farnell.com/datasheets/71410.pdf Not avaialble at 3V unfortunately, but +/-1mV at 2.5V is 0.04%! 20ppm per deg C temperature coefficient max. Mark.
From: Artemus on 28 Mar 2010 18:14 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:8sdsq5h1eaastj51m4ter75ido4npmp1i4(a)4ax.com... > On Fri, 26 Mar 2010 13:43:32 -0800, "Artemus" <bogus(a)invalid.org> > wrote: > > > > >"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message > >news:13rpq5lqd1lj1k9flb9orfr2vcl2tuucpi(a)4ax.com... > >> > >> ftp://jjlarkin.lmi.net/V220_reg.JPG > >> > >> > >> John > >> > > > >How much current can that 1117 source from it's ADJ pin? > >Art > > > > Oh, I see what you're getting at, fig 4 > > ftp://jjlarkin.lmi.net/ARM_power.JPG > > if I catch your drift. Nice. > > John Just a silly comment re the labeling of the pins of the 1117 in your dwg. Art
From: Fred Bartoli on 28 Mar 2010 17:20
Artemus a �crit : > "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message > news:8sdsq5h1eaastj51m4ter75ido4npmp1i4(a)4ax.com... >> On Fri, 26 Mar 2010 13:43:32 -0800, "Artemus" <bogus(a)invalid.org> >> wrote: >> >>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >>> news:13rpq5lqd1lj1k9flb9orfr2vcl2tuucpi(a)4ax.com... >>>> ftp://jjlarkin.lmi.net/V220_reg.JPG >>>> >>>> >>>> John >>>> >>> How much current can that 1117 source from it's ADJ pin? >>> Art >>> >> Oh, I see what you're getting at, fig 4 >> >> ftp://jjlarkin.lmi.net/ARM_power.JPG >> >> if I catch your drift. Nice. >> >> John > > Just a silly comment re the labeling of the pins of the 1117 in your > dwg. > Art > > That's just to make copycats life harder :-))) -- Thanks, Fred. |