Peter L. Peres wrote:
> .I have mixed experiences with 78S40. Unless you
> guarantee a minimum load it will be 'chaotic' <g> in operation.

 Why not use some Microchip stuff:
TC115 or TC120 PFM/PWM Step-Up DC/DC Controllers?

From 21361b.pdf, 21365b.pdf:
" Unlike conventional PWM step-up converters, the TC115
(TC120) automatically shifts to pulse frequency modulation
(PFM) at low loads, resulting in reduced supply current and
improved efficiency."

               >The chaos is elsewhere.<
 Mike.
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