variable linear DC-DC switching regulator

[maminej]

Gentlemen,

I have been trying to build a DC-Dc switching voltage regulator using the LT3991. I was successful in designing the board for a fixed voltage of around 36V with a 48V power supply. the purpose of this is to drive a CO2 laser fans.
the next step was to make the output voltage variable and tune the fan speed. I am not a big fan of using PWM with FET or a Darlington to change the fan speed as I believe this introduces additional vibration and the optical setup is sensitive to that.
I re-watched Dave's tutorial on the variable power supply as a reference and tried to change the Feedback pin voltage of the LT3991.
So I am using a dual stage dual stage sallen key active filter to get a dc from a 3khz pwm signal, the output has low ripple.
I had a little success in changing the voltage but the result was highly non-linear. below is the schematic I used for this:

here is some data for the Vout vs Vfb I got :


any idea of what could have caused the huge discontinuity in the curve ?

Thanks in advance

[maminej]

forgot to mention that I also tried to add a voltage divider at the output of my filter to get a higher resolution in the range of 0-1.2V :
for some reason I don't understand, I was not able to adjust the output voltage of the LT when changing the FB voltage all the way from 0 to 1.2/
here is the second schematic I tried.

[rob77]

you must have a voltage divider on your feedback pin set to the max voltage - so resistor from Vout to FB and resistor from FB to ground... then you can add a resistor between FB and your filter's output - this way you can inject current and fool the feedback to lower the Vout.

[maminej]

Thanks for the reply.
I just made the changes you suggested and it's working just like you said.   

Thank yo very much