The LT3757 datasheet includes the following schematic:
The resistor in the bottom left corner is annotated with a frequency (100kHz). What is the significance of that annotation?
That resistor sets the frequency of the DC to DC converter, the frequency will be set at 100KHz. Shown below is a table from the datasheet of other values for the Rt resistor that can change the switching frequency.
If you'll notice in the features section of the linked datasheet:
It states that one resistor can be used to set its operating frequency. Evidently, a 140k resistor sets it to its minimum of 100kHz.
On page 11 of the datasheet, Table 1 (shown below) provides resistances to be used for several example frequencies.