In my experiment with LT3476 for LED dimming, I saw a phenomenon that seems like a limit cycle. I need some help to explain this. For my circuit employing the LT3476, I used 21K for Rt resistor, that means the switching frequency is 1[MHz], and used 20[KHz] PWM dimming signal for PWM input pin, and set reference current for 1.5[A] with Vadj and shunt resistor. I did not use shutdown feature. Power supply is 24[Vdc]. And this is important point as I think, 20K resistor and 220[pF] capacitor connected in series to Vc pin. I did suspect that this controller gain is too large, so some non-linearity in the LT3476 might cause the limit cycle. The limit cycle itself was seen at the zoomed waveform of switching node voltage, and it was observed during LTspice simulation and real experiment. When I used 10K resistor and 220[pF] capacitor, the limit cycle disappeared,
One more question : I used an inductor that is about half an inch distant from the LT3476, value 10[uH] for switching LED current. And I know this inductor is not directional, that is it does not matter whether I connected the inductor in one direction or connected in reverse direction. The data sheet of inductor also ensures this point. But the limit cycle mentioned above seems that it does receive some effect from the orientation of inductor on the PCB. That means, with one orientation of inductor, limit cycle was seen, but with other orientation limit cycle is not shown. Is there any explanation on this observation?
When designing a controller for LT3476, we may need a small signal AC model of the LT3476. Would you provide such a model?
[edited by: dbsuh at 2:40 AM (GMT 0) on 20 Dec 2018]