bandwidth of digital control design in conventional dc-dc converter
For a reasonable guarantee of stability your process/measurement/control bandwidth should be significantly greater than the bandwidth of the analogue LC filter that smoothes the output.

It's not a 100% guarantee but a reasonable guarantee of stability.



Reason: -

If you are switching at 1 MHz then your LC filter (yes you must have an output capacitor although not shown in your circuit) MUST be significantly more "sluggish" than the switching frequency to obtain adequately small ripple voltage on the output. This of course is easy to achieve with pretty small components so there's no excuse for bad selection. 

Sampling/controlling at a higher speed can achieve not much else other than a slight improvement in accuracy due to the ability to numerically average the ripple voltage but, as I said earlier, it's sensible to make the natural ripple voltage low by use of an appropriately scaled LC filter.

From the L and C values you quote, the analogue cut-off frequency is about 11 kHz. If you are alternately switching at 500 kHz then the size of the ripple voltage will be determined by a 40 dB / decade roll-off: -



Resonance is about 11 kHz and, at 1 MHz, the attenuation of the switching waveform will be about 80 dB and, at 500 kHz will be about 78 dB or 8000:1. So, if Vin is 2V, the switching waveform will be 2V p-p and, with a bit of hand-waving the ripple will be about 8000 times lower i.e. sub milli-volts.

In other words you have chosen an L and a C combination that are more than adequate for this design. Your control loop could probably work at a speed of 100 kHz without much problem.