I'd recommend configuration c) - using a buck driver which has an external
current control, and drive that by low pass filtering the PWM output of the
PIC to give you a voltage to feed in.

On 6/7/07, Ariel Rocholl <foros@arocholl.com> wrote:
>
> I think you meant diode opposite right? FET Drain-->ADiodeK--> +supply
>
> Nevertheless, your proposal is a good simple switching regulator, but I do
> have already a buck DC-DC converter that theoretically can get about 85%
> efficiency, so I just want to use it but connecting the FET driver
> properly.
>
> Would you recommend configuration a) or b)?
>
>
> 2007/6/6, Harold Hallikainen <harold@hallikainen.org>:
> >
> > Since you're using an LM317 as the current source, I'll assume your
> supply
> > voltage is higher than the LED voltage. So, how about this for a
> circuit?
> >
> > +supply-->inductor-->LED-->FET Drain
> >
> > FET Source-->small current sense R-->ground
> >
> > FET Drain-->kDiodeA--> +supply
> >
> > Turn on the FET when the FET source current is below the LED current you
> > want. Turn off the FET when the FET source current is above the current
> > you want.
> >
> > Harold
> >
> > > Hi there,
> > >
> > > I am controlling a number of Luxeon LED with PWM using PIC12F family
> > > capabilities.
> > >
> > > I've been using a simple LM317 constant current configuration and now
> > > wanted
> > > to move to a DC-DC circuit to reduce power loss.
> > >
> > > Now the 1st question: In my current LM317 setup, I am using a FET to
> > > switch
> > > the whole constant current circuit on/off as per PWM duty cycle, and I
> > am
> > > not sure that is the right way. I mean I have two options, right? I am
> > > currently using option a) below
> > >
> > > a) PWM switches on/off the constant current circuit, or
> > > b) PWM switches on/off the LOAD, having the constant current circuit
> > > connected all the time.
> > >
> > > It was somehow easier to guarantee the LM317 is not having any option
> to
> > > work on the off side, but I wonder if a LM317 is designed to be
> switched
> > > at
> > > 10KHZ in its input - probably not. So maybe is a more adequate
> solution
> > to
> > > move to option b).
> > >
> > > Now the same question but for a DC-DC circuit (step down): I would
> think
> > > using option b) is again more natural, but I may need to protect the
> FET
> > > with a diode.
> > >
> > > Thanks in advance
> > > --
> > > Ariel Rocholl
> > > Madrid, Spain
> > > --
> > > http://www.piclist.com PIC/SX FAQ & list archive
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> >
> >
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>
>
> --
> Ariel Rocholl
> Madrid, Spain
> --
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