reg + reference

Man! High class engineering :-) ...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

| 1962 | The only thing bipartisan in this country is hypocrisy

On a sunny day (Fri, 26 Mar 2010 10:19:55 -0700) it happened John Larkin wrote in :

No money for a real current source or current mirror? Does that 1M depending on beta circuit change a lot over temperature? Its Vbe will change, so will the Ib.

The current into the reference will be more constant than if I just used a resistor with a nominal 0.3 volt drop across it. The BCX71K has a very tight beta spec. It would probably be better than using the current mirror discussed in another thread recently. And we have BCX71Ks in stock for under 4 cents each.

The +15 is not very well regulated, and I'm tight for space.

I guess I could use a depletion mode fet from +15. That's just one part. The current regulation would be fair, but I wouldn't have any flexibility on setting current. An LND150 is 33 cents, no problem there, but Idss is 1 to 3 mA and it may change a bunch with temperature.

Or maybe there exists a 3 volt 3-wire reference that would work from

3.3 volts.

This will be the ADC voltage reference for an NXP ARM processor. I'm doing a 12-channel 4-20 mA thing, electrically isolated per channel, with an ARM per channel. The ARM and other stuff might need 45 mA or so, less if we can sleep the code a lot.

Or I could use a very good regulator to make 3.3 and run everything off that, Vref included. More parts.

Or add another regulator, 15-to-5 and derive the Vref from the +5.

Or find a good 3-volt reference that works directly from +15.

I figured using the beta thing would predictably get the geezers all worked up. There's nothing wrong with it.

John

Just curious, why not replace all this with one of these:

Much less space, cheaper. Unless you need the 3.3V somewhere. There's also a ref that can generate 3.0V from as low as 3.2V but I don't remember the P/N.

--
Regards, Joerg

http://www.analogconsultants.com/

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How much current can that 1117 source from it's ADJ pin? Art

Ha. And this after pre-coffee pouncing on my designs?

Pot, Kettle, Larkin ;-)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Ha. And this after pre-coffee pouncing on my designs?

Pot, Kettle, Larkin ;-)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Ha. And this after pre-coffee pouncing on my designs?

Pot, Kettle, Larkin ;-)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Funny, you didn't seem to be a geezer, up to now.

BCX71K is beta graded from 380 to 630 at 2 mA.

John

Repeating yourself is a sign of impending geezerdom.

John

Since the PNP is in the saturation (resistive) region, the improvement over just a series resistor is... ?? ...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

| 1962 | The only thing bipartisan in this country is hypocrisy

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

Eww, Eternal September wasn't sending. Or it seemed like it wasn't.

Tim

-- Deep Friar: a very philosophical monk. Website:

And of course, the LM4120 is a stability land-mine of its own.

ThestandardapplicationcircuitfortheLM4120isshownin Figure1.Itisdesignedtobestablewithceramicoutput capacitorsintherangeof0.022µFto0.047µF.Notethat

0.022µFistheminimumrequiredoutputcapacitor.These capacitorstypicallyhaveanESRofabout0.1to0.5. SmallerESRcanbetolerated,howeverlargerESRcannot. Theoutputcapacitorcanbeincreasedtoimproveloadtran- sientresponse,uptoabout1µF.However,valuesabove 0.047µFmustbetantalum.Withtantalumcapacitors,inthe 1µFrange,asmallcapacitorbetweentheoutputandthe referencepinisrequired.Thiscapacitorwilltypicallybein the50pFrange.Caremustbetakenwhenusingoutput capacitorsof1µForlarger.Theseapplicationmustbethor- oughlytestedovertemperature,lineandload.

Thanks, National.

John

Or you're drinking way too much coffee!

-- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85.

Michael A. Terrell Central Florida

On a sunny day (Fri, 26 Mar 2010 14:24:39 -0700) it happened John Larkin wrote in :

It is senile, the base will pick up all the noise it can get from the universe.

use the ~3.3V as reference, measure a 1.2V reference voltage and correct for it?

why not pick an ARM with a build in reference? something like stm32... ?

12 bits too, think the nxp is only 10bits

I'm sure analog has some too

snip

-Lasse

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.

It's like press-fit where I sat in a meting where a company pitched it to a client. Tappa-di-tap went my calculator. "I calculated that 3 out of all the contacts will be open. Can you tell us which ones those will be?". The meeting and the sales pitch ended that instant ...

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

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.

I sold a lot of CAMAC crates that used pressfit backplanes. They always worked. Luck'o the Irish I guess.

John