On Fri, 01 Jun 2007 14:09:35 -0300, you wrote:

>
>	Hi,
>
>	I need to power a uP circuit with 3.3V @ 6mA from a 4700uF capacitor 
>charged to 12V. The circuit has to run until the capacitor discharges.
>	Have tried the Roman Black's SMPS circuits, but could'n go over 50% 
>efficiency at this 6mA current output. Even the Zener diode current is 
>excessive...
>
>	Any ideas? Low cost and part number is a constrain.
>
>	Thanks.
>	Mark Jordan

For maximum efficiency at low voltages you should look at SMPS chips with synchronous rectification.
These can be up to 95% efficient.
Unfortunately some of the niceer ones won't go up to 10v input ( e.g. TI TPS62050 series), however
at this low output current you may have a reasonable selection of parts to choose from.



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