LM2577 step-up voltage regulator design problem

[BishopB7]

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.

[Rigolon]

A power regulator, as you thought, will not work since they usually have some drops in them, usually around 2V. So if you have 5V on the input of the 7812, your output will get down to 3V and your step-up won't work as intended.

What you want is a voltage limiter a.k.a Zener Diode. I believe that it will do the job.

[BishopB7]

Yes I know the part about linear regulator that it requires 2V more at the input than its regulating voltage. Zener diode? Where to add it ? I forgot to mention that minimum input voltage of step up regulator is 3V.

[ramonest]

Maybe playing with the value of the potentiometer? So that when you have 12 V  at the outut the potentiometer is already maxed? I do believe 12(R2/(R1+R2))=1.23 is the expression you need to look at.

Regarding the zenner thing I guess he ment to put a zenner pararell to the potentiomenter.

[johnkenyon]

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

[BishopB7]

Thanks John, that really seems like simplest solution (I always start from difficult ones )