Well, everything gets converted to analog when it hits the real world!
For any sort of
luminaire that isn't
LED tape, there's typically going to be a switch-mode constant
current driver for each string of LEDs. The best way to dim those is to put a transistor in parallel with the
LED string--turn the transistor on and it shorts the LEDs out, turning them off. That seems counterintuitive, but since the
driver is going to very efficiently hold the output
current constant, and the transistor has very low resistance, the output
voltage drops to near zero and very little
power is dissipated this way. This is a common enough method that a lot of
LED driver ICs include provisions for
shunt dimming--sometimes they include a
driver for the
shunt transistor, or at least a
PWM sense input so that the internal control loop can
switch operating modes between the on and off times for better efficiency/response time. You could also
PWM the whole
driver on and off, but it takes time for the
driver to go from zero to steady state operation at the desired setpoint, so in practice the
PWM frequency has to be pretty low. With
shunt dimming the drive
current continues to circulate through the transistor when the LEDs are off, so as soon as the transistor turns off the LEDs go right back to running at their setpoint, and the ultimate rise/fall times are much shorter, allowing for much faster
PWM. Neither of those requires a D-A conversion, so they can provide very good precision with minimal cost.
On the other
hand, you could implement dimming by adjusting the
current setpoint of the
driver, which would require some sort of analog input to the
driver. That analog signal could be provided by filtering a digital
PWM signal to derive an analog DC
voltage, but there's a big tradeoff between effective resolution and response time with that method. Even with a proper
DAC, though, analog dimming generally doesn't provide nearly as much dimming range or precision. Adjusting the
current setpoint is better reserved for trimming the
LED strings for color/brightness matching independent of the
PWM dimming.