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>> 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

That's at 250 uA !

However,

        http://www.irf.com/product-info/datasheets/data/irl3103.pdf

Fig 1 in the above datasheet says you can get 6A at 0.1V 
drop TYPICAL with 3.3V gate drive and about 4.5A at 0.1V 
drop with 3V gate drive. so the PIC has enough voltage.

What the PIC doesn't have is the vast oodles of drive that 
would be ideal to drive the gate hard and fast at very high 
PWM frequencies - 100mA plus gate currents are "nice".

HOWEVER, the PWM doesn't need to be fast - faster than a 
speeding eye is fast enough - 200 Hz may do, 1 KHz almost 
certainly would. At that speed the waveform edge can be a 
bit rounded with minimal losses overall.

A trickier questionis, how are you going to ensure the LED 
is properly driven?
This happens to be an area that I've been giving a lot of 
thought to lately.
As it's a headlamp I assume it's 1 Watt plus and maybe 3 
Watt or 5 Watt?
FWIW the Edison White LEDs seem to offer superb lumens per 
watt per dollar at present. YMMV.

I'd be interested in which White LEDs you choose to use and 
why and how much. I can share what I've found of late.
You can even get a GENUINE 100 lumens/Watt 9at under 1 Watt 
level) LED now and it's getting better all the time. As an 
eg Luxeon Star 1 Watt is rated at around 40 l/W typical, use 
of a higher l/W LED can save battery at the same light 
levels.

I have an Edison Star 3W that at 500 mW gives several times 
the power of a 2 year old 1 Watt Luxeon and over 50% more 
light than a 3W new Luxeon Star (also at 500 mW). Retail the 
Edison is half the cost of the Luxeon. I have 100,000 
pricing info... :-).

As LED power rises Vf typical tends to rise as well. While 
you can get binned LEDs with VFs' in the low 3+V region the 
norm is mid 3V range and the spec sheets say you can get 
them at > 4V at rated current. Even with a low to medium Vf 
that doesn't give a lot of voltage margin for the Isense 
resistor for the constant current driver. You were of course 
definitely going to drive it in constant current mode, 
weren't you? :-)

Rather than a voltage doubler for your FET gate you may wish 
to end up with a constant current output boost regulator for 
your LED ! :-).

This is actually not too hard to do [tm] and the experience 
can be "lots of fun" [tm again]. The magazines have quite a 
few of them and the web has various examples. See Olins 
website for some examples using a PIC 10Fxxx. [The 10F gives 
access to all 3 pins of the comparator - there's a reason 
that Olin always uses pin 3 as the sense pin :-) ] As the 
FET is low side drive is easyish. With a boost SMPS you do 
need higher frequency to get smaller inductor so you will 
need a gate driver - but 3V is still ample. The classic FET 
gate driver would be fine (attached) either for a SMPS of if 
you wanted one for PWM drive. For even greater low drive D1 
can be replaced by a PNP - E to E of Q8, B to B of Q8 and C 
grounded, remove D1.

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

Yes - but the cap wants to be VERY small to avoid loading 
the OSC fatally.
But, as above, you don't need it ;-)




            Russell

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