What do you think, the SEPIC application from page 17 can be used
(with minor modifications) to generate negative output voltage ?

Vasile

On 11/25/08, apptech <apptech@paradise.net.nz> wrote:
> > 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
>
>
> Full datasheet here
>
>   http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1094,C1766,P49780,D25708
>
> The circuit is somewhat unusual for a buck due to the ICs capabilities.
>
> FET MUST be a P channel FET.
>
> Simplified:
>
> Sw pin is a switch to ground.
> This is the main switch for Buck operation and NOT the external FET.
>
> With Sw turned on the LED current increases via inductor (Call it L)
>
> When current reaches target level (sensed by 100 mohm resistor) Sw turns off
> BUT external FET stays on.
>
> ....
>
>
>   Russell
>
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