Hi everyone,
I have the LM2577 step-up step-down voltage regulator. R1 resistor is replaced with a potentiometer. Conversion ranges are 1.2V - 34V. Thing is I need to limit the output voltage up to 12V (in step-up mode) even if potentiometer is turned over 12V level. Putting a 7812 regulator on the output stage - doesn't sound right. I guess something has to be done with the feedback circuit, but I can't think of anything, that would work. Any ideas ?
P.S. Sorry for my English, it's not my native language.
Make life easier - choose the values for the resistors and potentiometer so that in the worst case scenario, the maximum output voltage is 12v.
If I were doing this...
Select a potentiometer value to replace R1 - 10k
Work out the value for R2 to give 12v out at full 10k resistance.
Vout=1.23* (1+R1/R2)
Rearrange:
R2=R1*(1.23/(Vout-1.23))
For R1=10k, and Vout=12v
R2 this gives me 1.14k for R2.
1.14k isn't standard, so we choose 1.2k and see how this affects the voltage:
Vout=1.23*(1+10/1.2) = 11.48v
A little on the low side, so we use the Vout equation to work out a new value for R1 at 12v with 1.2k
R1=((Vout-1.23)/1.23)*R2
This gives me R1=10.50k
So for R1 lets put a 510R resistor in series with the potentiometer.
This gives us R1=0.51k to 10.51k, and R2=1.2k and we see if these values work.
Putting these values into
Vout=1.23* (1+R1/R2)
gives us a range of 1.75v to 12.00v
Note that this assumes that a 10k potentiometer will give exactly zero ohms at one end and exactly 10k at the other, and that the fixed resistors are exactly 1.2k and 510R.
In the real world, this won't happen so (for example) your 510R resistor might have to be 560R or 470R, and/or you'll be happy with something other than exactly 12v max.
/John