Two reasons - it does avoid switching the driver on and off at high frequency which won't actually be that good for a switch mode driver - note that with a switch mode current driver you do really want to this this on the supply side, so as not to leave it trying to drive current into an open circuit. Also it is more efficient to drive LEDs at lower current than at full current with PWM switching - typically when using current dimming you get 60% of the light for only 50% of the power, whereas using PWM dimming you only get 50% of the light for 50% of the power, so a 20% efficiency improvement. BTW the low pass filter required to convert a PWM output to a voltage is only a single resistor and capacitor (or two resistors and one capacitor if you also want to divide the voltage down to a lower level), so hardly vast amounts of components. On 6/7/07, Ariel Rocholl wrote: > > Chris, can you elaborate a bit on the advantage of c) configuration? Is it > to avoid any switching of the driver (by using the low pass filter) worth > the component count increment? > > > 2007/6/7, Chris McSweeny : > > > > 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 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 : > > > > > > > > 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 > > > > > View/change your membership options at > > > > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > > > > > > > > > > > > > -- > > > > FCC Rules Updated Daily at http://www.hallikainen.com - Advertising > > > > opportunities available! > > > > -- > > > > http://www.piclist.com PIC/SX FAQ & list archive > > > > View/change your membership options at > > > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > > > > > > > > > > > > -- > > > Ariel Rocholl > > > Madrid, Spain > > > -- > > > http://www.piclist.com PIC/SX FAQ & list archive > > > View/change your membership options at > > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > -- > > http://www.piclist.com PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > > > > -- > Ariel Rocholl > Madrid, Spain > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist