LMR23625: Rfbt selection

I do not directly support LMR23625, but I believe I can answer this question.  Please refer to figure 24 of the datasheet.  During pulse skipping PFM operation, the LMR23625 will output a single switching pulse followed by a period of off time.  The off time (and observed operating frequency) is a function of the load current.  Lower load current will result in longer time and lower observed operating frequency.  The individual pulse will supply more energy than required for the light load condition and this extra energy will charge the output capacitance to a higher voltage as you can see in the figure 24.  During the off time this voltage decreases back towards the regulation point as the output current is supplied by the charge on the output capacitors.  Once the voltage falls so that the fed back portion of Vout is equal to the internal reference, a new pulse is initiated.  So essentially, you are regulating to the valley point of the sawtooth waveform of Vout.  The Dc voltage is the average voltage, hence there is some "offset" to the output in PFM.  If you look at figure 24, you can see that the amplitude scale is only 5 mV / div, so the offset is only about 2.5 mV.  Decreasing the value of the feedback resistors essentially adds some pre-load to the output.  Lower resistors increase this load and lower the required load current to enter PFM.