8.2.2.2 Selecting the Switching Frequency

The first step is to choose a switching frequency for the regulator. Typically, the designer uses the highest switching frequency possible since this produces the smallest solution size. High switching frequency allows for lower value inductors and smaller output capacitors compared to a power supply that switches at a lower frequency. The switching frequency that can be selected is limited by the minimum on-time of the internal power switch, the input voltage, the output voltage and the frequency foldback protection.

Equation 9 and Equation 10 should be used to calculate the upper limit of the switching frequency for the regulator. Choose the lower value result from the two equations. Switching frequencies higher than these values results in pulse skipping or the lack of overcurrent protection during a short circuit.

The typical minimum on time, t_onmin, is 135 ns for the TPS54360. For this example, the output voltage is 5 V and the maximum input voltage is 60 V, which allows for a maximum switch frequency up to 710 kHz to avoid pulse skipping from Equation 9. To ensure overcurrent runaway is not a concern during short circuits use Equation 10 to determine the maximum switching frequency for frequency foldback protection. With a maximum input voltage of 60 V, assuming a diode voltage of 0.7 V, inductor resistance of 25 mΩ, switch resistance of 92 mΩ, a current limit value of 4.7 A and short circuit output voltage of 0.1 V, the maximum switching frequency is 902 kHz.

For this design, a lower switching frequency of 600 kHz is chosen to operate comfortably below the calculated maximums. To determine the timing resistance for a given switching frequency, use Equation 7 or the curve in Figure 6. The switching frequency is set by resistor R₃ shown in Figure 34. For 600 kHz operation, the closest standard value resistor is 162 kΩ.

Equation 25.

Equation 26.

Equation 27.