LTC3807 - Switching frequency variation


I am using LTC3807 under following configuration

12V Input to 5V Buck Converter, 3A.

Switching Frequency - 350KHz

I have configured 350KHz switching frequency by connecting FREQ pin to GND via 0 ohm resistor. However, we are seeing the switching frequency as 280KHz in oscilloscope.

We are also seeing that the Frequency is varying with the Load. Please find the switching frequencies at different Load. (0.3A - 336KHz, 1.5A - 320KHz, 2.5A - 300KHz, 3A - 280KHz). Should'nt the Frequency be constant irrespective of Load? What am I missing?. We are seeing this problem in all our Boards.

We also have another LTC3807 circuit in our Board to generate 3.3V output (5A). Here we do not see any variation in frequency with Load change. Why is the issue present only in 5V Regulator?

Appreciate your help.