buck boost converter

[pencilneck]

i am trying to design a buck boost converter to output a controled 12 volts dc with a variable voltage input of 8-16V dc, i am having a hard time controling the voltage output, if anyone has a circuit like this or has any helpful suggestions please let me know.

 

[Lewish]

Please describe more fully what you mean by "having a hard time controlling the output voltage". Is it anywhere close to 12V? Does it start close and wander off? What do you mean?

 

[tiki]

All significant dcdc-ic manufacturers publish circuits for this "problem": SEPIC-circuits are one of the easiest solutions. Look at

http://www.maxim-ic.com,

they are specialists for those ICs.
If the output voltage must be very stable (better than 1%), you should combine a reference and a low offset OPA to feed back the (divided) output voltage signal to the error amp pin of the dcdc-ic. This second control loop shoud be at least 5 times slower than the initial circuit (often a current controlled PWM) to maintain stability.
If you have more detailed questions, let me know.

tiki