Hi Michael,

SW pin will short only inductor to ground to charge I do not see how it
Becomes LED return path? Cathode of LED needs ground to turn on?
The board is at work I am going to check details tomorrow 

Thanks for your replay

Andre

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Michael Algernon
Sent: Tuesday, November 25, 2008 4:41 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE]: buck boost mode question

>
> On Nov 25, 2008, at 4:47 PM, Andre Abelian wrote:
>
> Hi to all,
>
> I built buck boost led driver LT3517 and one thing I do not understand
> is
> If you can open LT3517 datasheet page 15 M1 FET Drain is connected to
> main power +Vin. I do not understand how the LED gets ground  
> "negative"
> to operate. I asked this question to linear tech support they said do
> not worry that's how buck boost mode works but now I built pcb and it
> doesn't work. I am sorry I wouldn't be able to explain without you
> looking at the datasheet page 15.
> My question is how led gets negative voltage to turn on?
> http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1094,C1766
> ,P49780
>
> thank you
>
> Andre
>
>

The SW pin on the LT3517 provides a FET switch to ground.  This  
provides the return path for the current that flows through the LEDs.   
If the pcb is not working, then we would need more information from  
you to find the problem.  Do you have an oscilloscope ?  If so, what  
is the voltage waveform at  ISP, ISR, Vin, Vref, PWM .
MA


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