The switching frequency and operation mode are configured by the resistor on the MODE pin. Select one of three switching frequencies: 600 kHz, 800 kHz, or 1 MHz. Refer to Table 7-1 for the relationship between the switching frequency, operation mode, and R_MODE.

Switching frequency selection is a tradeoff between higher efficiency and smaller system solution size. Lower switching frequency yields higher overall efficiency, but relatively bigger external components. Higher switching frequencies cause additional switching losses, which impact efficiency and thermal performance. For this design, short the MODE pin to AGND to set the switching frequency to 0.6 MHz and set the operation mode as FCCM.

When selecting the switching frequency of a buck converter, the minimum on time and minimum off time must be considered. Equation 7 calculates the maximum f_SW before being limited by the minimum on time. When hitting the minimum on-time limits of a converter with D-CAP3 control, the effective switching frequency will change to keep the output voltage regulated. This calculation ignores resistive drops in the converter to give a worst case estimation.

Equation 7 calculates the maximum f_SW before being limited by the minimum off time. When hitting the minimum off-time limits of a converter with D-CAP3 control, the operating duty cycle will max out and the output voltage will begin to drop with the input voltage. This equation requires the DC resistance of the inductor, R_DCR, selected in the following step, so this preliminary calculation assumes a resistance of 1.1mΩ. If operating near the maximum f_SW limited by the minimum off time, the variation in resistance across temperature must be considered when using Equation 8. The selected f_SW of 600 kHz is below the two calculated maximum values.