8.2.1.2.2 Selecting the Switching Frequency

The first step is to decide on a switching frequency for the regulator. Typically, the user will want to choose the highest switching frequency possible since this will produce the smallest solution size. The high-switching frequency allows for lower valued 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 and the output voltage and the frequency shift limitation.

Equation 12 and Equation 13 must be used to find the maximum switching frequency for the regulator, choose the lower value of the two equations. Switching frequencies higher than these values will result 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 TPS54260. For this example, the output voltage is 3.3 V and the maximum input voltage is 13.2 V, which allows for a maximum switch frequency up to 2247 kHz when including the inductor resistance, on resistance output current and diode voltage in Equation 12. To ensure overcurrent runaway is not a concern during short circuits in your design use Equation 13 or the solid curve in Figure 41 to determine the maximum switching frequency. With a maximum input voltage of 13.2 V, assuming a diode voltage of 0.7 V, inductor resistance of 26 mΩ, switch resistance of 200 mΩ, a current limit value of 3.5 A and a short circuit output voltage of 0.2 V. The maximum switching frequency is approximately 4449 kHz.

For this design, a much lower switching frequency of 300 kHz is used. To determine the timing resistance for a given switching frequency, use Equation 11 or the curve in Figure 40.

The switching frequency is set by resistor R₃ shown in Figure 50 For 300 kHz operation a 412 kΩ resistor is required.