Hi, I used the forumula Vout = Vref(1+R5/R3) with a vref of 2.495V i get a v out of 3.0938 V( I want ~ 3.1V) is this correct?

Thanks.

Using a TL431 shunt reference
Shunt reference data sheet


Also, what is the difference between a tl431 and lm431?


Thanks!

 

hi chog,
LTSpice confirms your circuit.
What are the resistor tolerances?
E

 

hi chog,
LTSpice confirms your circuit.
What are the resistor tolerances?
E
View attachment 340730

I’m not sure, what do you recommend?

 

don't you know what your application demands are?
if your resistors are 1% (very common these days) then you can expect 1% different voltage due to resistors alone. if you are building bunch of units (an actual product) this can be acceptable or not.
1% of 3.1V is 0.031V or 31mV.

Are you ok that your 3.1V is anything between 3.069...3.131V? and actually the variation would be even bigger because ICs also have tolerances. so maybe replace 2.4k with 2.2k and add 500 ohm Cermet trimmer in series with it to allow for calibration. do not use carbon resistors/trimmer here as they drift too much when temperature changes. use only metal film / cermet.

 

Hi ch,
As an exercise, consider they could be +/-5%
Start with the 2k4 and see what is the effect on the vout.
Post your results.
E

 

don't you know what your application demands are?
if your resistors are 1% (very common these days) then you can expect 1% different voltage due to resistors alone. if you are building bunch of units (an actual product) this can be acceptable or not.
1% of 3.1V is 0.031V or 31mV.

Are you ok that your 3.1V is anything between 3.069...3.131V? and actually the variation would be even bigger because ICs also have tolerances. so maybe replace 2.4k with 2.2k and add 500 ohm Cermet trimmer in series with it to allow for calibration. do not use carbon resistors/trimmer here as they drift too much when temperature changes. use only metal film / cermet.

I’m assuming the deviation is a continuous and not variable offset, if so this is fine. Having not exactly 3.1 is fine as long as the value is constant. I’m worried about adding a trimmer as there is very limited room on the final PCB. Is the footprint for metal film resistors 0204?

 

footprint like 0402 is a form factor or physical size of the SMT resistor. this has nothing to do with technology or material used to make it (thin/thick film, wirewound or whatever). to know the specs you need to read the datasheet.
same goes for ICs. TL431 is available in different grades.

Standard grade is 2%. B grade is 0.5%. if you think the A grade is even better, you would be making mistake, as A grade is actually 1% in this case.

 

footprint like 0402 is a form factor or physical size of the SMT resistor. this has nothing to do with technology or material used to make it (thin/thick film, wirewound or whatever). to know the specs you need to read the datasheet.
same goes for ICs. TL431 is available in different grades.

Standard grade is 2%. B grade is 0.5%. if you think the A grade is even better, you would be making mistake, as A grade is actually 1% in this case.

Yes but I’m asking if that kind of resistor is available in that form factor.

What is the % representing for tl431? Deviation from reference?

 

What is the % representing for tl431? Deviation from reference?

Yes.
It's the tolerance of the reference voltage from unit to unit.
The change of that voltage for any particular unit is given by the temperature coefficient for that in the data sheet.