LT1930 Dual Output Converter Topology

Question

Hi, 
 I notice that LT1930 has a special output circuit ( datasheet P8).

How can I calculate the valus as Vin, L1, Iout, Vout, C1 and C2 if I want to change them? 
 For instance, I need 12V input while dual 18V ouputs. 
 It seems that the method and fomula are not mentioned in the part of operations. 
 Is it essentially a SEPIC topology? 
 What do the diodes D2 and D4 play in that place? Are they equivalent to inductors? 
 Finally, can this topology be applied to LT1935 and LT3489? 
 Thanks. 
 Best Regards

Fil · Accepted Answer

Hi Jeronym, 
 Sorry for late reply. 
 The circuit shown is essentially a charge pump. The capacitance values are sized for the 1930 switch current limit and therefore are optimized. The positive rail is regulated with the FB divider network and the negative rail is an inversion of the positive rail. Change the FB divider ratio to change the positive rail voltage and the negative rail will follow. The negative rail is only as accurate as the matching of the diodes forward voltage drop. If more accuracy is needed than I recommend the customer look at parts like the LT8580. Yes, you could use this same topology with the LT1935 and LT3489. 
 The understanding of this topology is best understood using LTSpice. See attached. 1930_ dual plus-minus 15V.asc)

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LT1930 Dual Output Converter Topology