Current calculation for TL4051A

I think you have that wired up wrong.  See the datasheet page 13 figure 3.  In that case the worst case currents should be easy to calculate from your corner conditions using the equations provided.

HI

Thank's

I need the shunt reference for variable voltage reduction (between about 5 to 10V Reduction of source voltage)

See

->DIfference is here is the TL431 and i would like to use the more precise TL4051A

Do you think my design is wrong?

Andy

Andreas,

Is the goal to scale a 6V to 16.8V signal into a 0 to 3.3V output?
There is a better way.

The TL4051 can be used to reduce a voltage by fixed amount. But there is a minimum current that must flow to work. That is I[R1] + 70uA.  

Yes, 6V to 16.8V into 0 to 3.3V output.

A better way? which way? and why better?

I'm very interested to learn more. I need also a simple but also quite precise way. (otherwise i can just use a simple votlage divider for 16.8V to 3.3V.....

Thank's

Andy

Andy,

My first idea was a passive 3 resistor divider plus voltage reference. 

However, to get all positive value resistors, the power supply "P" must be < -3V.

Also a zero input would output a negative voltage which is probably not what you want.
 

Here is one of many possible solutions for P, A, B, and C

Andy,

My second idea was an unbalanced differential amplifier. 
The math took me far longer than I dare to admit to get it right.

I reduced maximum VIN to match upper common mode range of TLV341 running at 3.3V  (2.2V is good)
Then gain and outset was calculated. R1 and R3 are scaled in terms of R2. R4 is scaled in terms of R5. It is the ratios that set the transfer function.

I recommend resistors 10X to 50X bigger than the values shown below. OPamp can be TLV341.

Thank's Ronald. Very helpful!

I  have found these forum entry:

http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/t/360833.aspx

Is this about the same solution that you propose? LPV521 vs TLV341?

I have only 5V...

I have to read more about op amp....

Andy

i have just found this:

http://www.ti.com/lit/an/slyt173/slyt173.pdf

Maybe a good read for me...

Andy

Andy,

It is curious that you need 0V to 3.3V, but only have 5V supply.
The nice thing about a rail to rail output opamp running on a 3.3V supply won't exceed to output range.

If it is safe to have an output greater than 3.3V then 5V can be used for the op amp supply.
Otherwise a LP2950-33 can be used to drop 5V to 3.3V.

The control board with   the ADC is on a different board that my board. I feed the control board with 5V (and voltage level) but i have no access to the 3.3 Voltage rail in the control board. Thanks i will study the op AMP solution in deep