LED Lighting PWM Dimming (LM3404)
The datasheet (http://www.ti.com/lit/ds/symlink/lm3404hv.pdf) describes the DIM pin as follows:


  Input for PWM dimming. Connect a logic-level PWM signal to this pin to enable and disable the power MOSFET and reduce the average light output of the LED array


Therefore, the LEDs are dimmed by turning on and off along with the PWM input. There is no analog dimming.