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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