The thing with PWM dimming LEDs in say a sepic, where an in-line dimming fet is used, is that even with PWM dimming frequency being at the crossover frequency ,it shouldn't matter if the controller is advanced enough....
...If every time the dimming fet turns off, the "comp" pin voltage is preserved at the voltage it stood at when the dimming fet first turned off, then when the dimming fet turns back on again, then even though its say 10ms later, its as if time had frozen, and no conditions in the control loop changed, and so it just carrys on driving the leds almost as if the leds had never actually been turned off at all.
I am wondering if the LM3421 has this facility? (preservation of 'comp' pin voltage when dimming fet is off)....some of the linear.com controllers have this, but I am wondering if the LM3421 does this?