Help needed with LM3478 in a sepic config

[gandalf_be]

lm3478 sepic

Hello ,

I have some questions regarding the LM3478 from national in a sepic configuration .

On the website from national there is a schematic available that is able to drive luxeon led strings .

h**p://www.national.com/appinfo/power/LEDrefdesign.html


The description in the table doesn't match with the info in the document .So i would like to do the calculations by myself to see if i understand everything.
In a later stage i would like to change this design with the following specifications:

Vin between 4.5 - 12V
Vout = 28 V
Iout = 0.7A

I already tried to follow the steps in the data sheet over and over again without much succes.

Is there somebody that can help with this process/calculations step by step .

I find out that i struggle with :

1) Vq : Voltage over the mosfet when it is on (needed to calculate duty cycle)
How can you calculate this if you don't know at front what the peak current is that will flow through the mosfet.

2)calculate : Rsense

3)determine Vsl and delta Vsl.

4)inductor current ripples

Thanks on advance for replying and helping,

Regards,

Gandalf