Well, let’s see. The Femtobuck driver uses a AL8805 which, according to the data sheet, “is a step-down DC/DC converter designed to drive LEDs with a constant current”.
Now, the AL8805 is a switching regulator, basically connecting the power rail to an inductor for a short period of time until the requested current level is reached. After that, the switch is opened, but the inductor keeps supplying a somewhat decaying current to the LED. After a short while, the decaying current is sensed by the chip, and the power rail is again switched on. This switch-on-and-off cycle continues at a fast rate to provide an approximately constant current to the LED - albeit with a small ripple, simply because of the principle of operation.
Note that this has nothing to do with the way the input of the IC is driven (be it analog input or PWM), but with the basic principle of operation of this chip.
The ripple inherent in the operating principle will certainly not matter if this IC is used for home illumination purposes, as our human eyes will have difficulties to resolve any frequencies higher than, say, 100 Hz and the on-off cycle of the IC is much much faster than this. But I am not so sure if we are talking about cameras operating in the msec and faster regime.
With this small cutout from the AL8805 datasheet:
Let me also cite directly from the datasheet:
Reducing Output Ripple
Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor C2 across the LED(s) as shown already in the circuit schematic.A value of 1μF will reduce the supply ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with higher capacitor values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise of LED voltage. By adding this capacitor the current waveform through the LED(s) changes from a triangular ramp to a more sinusoidal version without altering the mean current value.
So, even the datasheet is talking about output ripple, and ways to reduce it. And the last sentence indicates that it will never really go away (just be “more sinusoidal”).
This is in contrast to the LED driver described in this thread (or the driver I designed and described elsewhere in this forum) - here, the current supplied to the LEDs is indeed constant over time, without any intrinsic output ripple.