LM3150: Where does RonD come from?

Hi experts!

Just curious, why is there an "RonD" included in the section 9.2.2.3 of the datasheet?:

I notice if I follow equation (4) of the datasheet using passive values from the 250KHz eval board, I calculate Fsw = 287KHz:

• Fs= Vout/K * Ron, where Vout = 3.3V, K = 100pC, Ron = 115Kohm

BUT when I calculate Ron from section 9.2.2.3, I get:

• RonD = -[(Vin – 1) * (Vin * 16.5 +100)] – 1000. For Vin, I use Vin(nom) = 12
• RonD = -[(12-1) * (Vin * 16.5 + 100)] – 1000 = -4278ohm
• Ron = [(3.3V * 12V) – 3.3V] / (12V * 100pC * 250KHz) – 4278ohm = ~116Kohm... which is what the evaluation board uses! 

Thank you!