8.2.2.1 Selecting the Switching Frequency

The first step is to decide on a switching frequency for the regulator. Typically, the user wants to choose the highest switching frequency possible, because this produces 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 the designer can select has limits imposed by the minimum on-time of the internal power switch, the input voltage, the output voltage, and the frequency-shift limitation.

Use Equation 8 and Equation 9 to find the maximum switching frequency for the regulator; choose the lower value of the two equations. Switching frequencies higher than this value result in pulse skipping or a lack of overcurrent protection during a short circuit.

The typical minimum on-time, t_onmin, is 130 ns for the TPS57140-EP. For this example, the output voltage is 3.3 V and the maximum input voltage is 18 V, which allows for a maximum switch frequency up to 1600 kHz when including the inductor resistance, on-resistance, and diode voltage in Equation 8. To ensure overcurrent runaway is not a concern during short circuits in the design, use Equation 9 or the solid curve in Figure 34 to determine the maximum switching frequency. With a maximum input voltage of 20 V, assuming a diode voltage of 0.5 V, inductor resistance of 100 mΩ, switch resistance of 200 mΩ, and an output current of 2.8 A, the maximum switching frequency is approximately 1600 kHz.

Choosing the lower of the two values and adding some margin, this example uses a switching frequency of 1200 kHz. To determine the timing resistance for a given switching frequency, use Equation 7 or the curve in Figure 32.

Resistor R_t sets the switching frequency as shown in Figure 52.