Hello ,
I'm having problem with understanding this circuit
from what I have read in the datasheet , Vo=1.26(1+R1/R2)
http://www.ti.com/lit/ds/symlink/lm3488.pdf

I have a working circuit with
R1= 28K , R2=18K and I'm getting Vo=12V
how could it be?
what am I missing?

this is the circuit -

 

C7 and C9 are higher values than shown in the datasheet. Could they be slugging the feedback too much and preventing the FET from turning off as quickly as it should?

 

You think this is the problem?
but in such a big difference? 7V~

 

I don't quite understand you circuit. You appear to have a flyback transformer connection but with a large capacitor from input to output. How is that supposed to work? The circuits in the data sheet use a simple inductor.

 

Is the actual circuit switching? Maybe the feedback resistors are not having the values you want or maybe the inductor pins are crossed and you may get at the output the supply voltage not the generated one.
Can you post any voltages around the U6 IC? FB, DR ?

 

let me explain - someone gave my this circuit,and wanted to know why it's doesn't work. from what he have told\show me , in all the cards C6 has explode.

this is what you have ask:
Vin =18VDC
V3(FB) - 1.26V
V6(DR) - 2.3V

and the Vo= 12.1V
I just can't understand it

Thanks,

 

I don't see why C6 should fail so dramatically if it's been correctly specified. C5 is a more likely candidate since its ripple current is higher.
If the dual-inductor is wrongly specified perhaps the inductor(s) is/are saturating and its inductance hence drops? A reduced inductance value would also increase the cap ripple current.

 

OP probably meant U6 failing not C6.

 

what would be a good value of a new inductor?
also how can I calculate it by my own?

Thanks ,

 

I suggest you research DC-DC converters, specifically SEPIC converters. Semiconductor vendors (Linear Technology, T.I etc) probably have online calculators to aid inductor selection.