Hi Marco,

> I'm starting to make an LED headlamp with a small PIC12F629
> as the "brain". The PIC use PWM to drive a IRL3103 Mosfet
> connected to a single power LED. The power source is a 3.7V
> Li-Ion battery so I need a voltage doubler to drive the gate

If the FET is on the low side (and it's an N-ch device), do you ?
The IRL3103 spec says gate threshold is 1.0V

> Final question:  as the PIC is INTOSC running (4Mhz), can I use
> the CLKOUT signal on OSC2/GP4 (Fosc/4) for the voltage doubler? 

Some of the voltage conversion ICs use switching at fairly HF. Up
around 1MHz IIRC for chips like the ICL7660, and external caps
are a few uF. That's definitely on a par with CLKOUT = 1MHz of
the PIC so it should be doable. Many/most switcher topologies have
a switched inductor to get even higher voltages. You might want to
look at some app notes, eg

http://www.maxim-ic.com/getds.cfm?qv_pk=1017

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