The RT pin is configurable. This pin can be tied to VCC for 400-kHz operation, grounded for 2.2-MHz operation, or a resistor to AGND can be used to set an adjustable operating frequency. See Figure 8-6 for resistor values. Note that if a resistor value falls outside of the recommended range, it can cause the LM6x4xx-Q1 to revert to 400 kHz or 2.2 MHz. Do not apply a pulsed signal to this pin to force synchronization. If synchronization is needed, see the SYNC/MODE pin in Section 8.3.3.
For example, for fSW = 400 kHz, RT = (16.4 / 0.4) - 0.633 = 40.37, so a 40.2-kΩ resistor is selected as the closest choice.