Electronic – Relationship between switching frequency and controller’s crossover frequency in buck SMPS

From the details below:

At 400mA and 20V in, you're running in continuous mode with a duty cycle of around 18%, with a peak inductor current of around 0.7A. At 4.8V, it's also CCM with a duty cycle near 70% and a peak inductor current of around 0.5A.

You may want to consider having a footprint on the PCB for the feed-forward capacitor Cff, just in case the internal compensation needs a speed-up. (You could also add a resistor in series with Cff, making what's referred to as type-3 compensation when combined with the ICs internal feedback). Bucks can be tricky to stabilize, especially with ceramic output capacitors (and with most of the compensation inside the chip!)

If you can afford some copper, add some around the SW and/or Vin pins. Copper here can pull some of the heat out of the internal MOSFET and improve reliability.

The 2% reference may cause a setpoint error of up to +/- 66mV, not including the tolerance of the resistors used in the feedback divider. You may want to add another footprint in parallel with the bottom resistor in the divider or add a trim pot if the setpoint is critical for your application.

Don't expect great efficiency at 20V in - the duty cycle is very small. It'll be much better than a linear regulator of course, but not great.

The first point is correct. As a general rule of thumb, the loop crossover frequency should be a fraction of the converter switching frequency. I've heard both one-fifth and one-sixth cited as the maximum.

Crossing over at -1 is not a firm requirement for stability. If the closed-loop response at the crossover frequency has sufficient phase margin (typically 45 degrees or more) and gain margin (-10 dB or more when the phase is at 0 degrees), the converter is stable.

The reason -1 slope is cited as the target for gain crossover is that, in general, the phase is small and isn't changing rapidly during that part of the curve. When the gain slope is -2, the phase shift is greater and changes rapidly, making it difficult to ensure phase margin if the crossover frequency shifts (due to component tolerances, etc.) That doesn't make it impossible to choose a compensation network, but it does make it more sensitive to changes.