The TPS54332 requires an input decoupling capacitor and depending on the application, a bulk-input capacitor. The typical recommended value for the decoupling capacitor is 10 μF. TI recommends a high-quality ceramic type X5R or X7R. The voltage rating must be greater than the maximum input voltage. A smaller value can be used as long as all other requirements are met; however 10 μF has been shown to work well in a wide variety of circuits. Additionally, some bulk capacitance can be needed, especially if the TPS54332 circuit is not located within about 2 inches from the input voltage source. The value for this capacitor is not critical but must be rated to handle the maximum input voltage including ripple voltage, and must filter the output so that input ripple voltage is acceptable. For this design, a single 10-μF capacitor is used for the input decoupling capacitor that is X5R dielectric rated for 25 V. The equivalent series resistance (ESR) is approximately 3 mΩ, and the current rating is 3 A.

This input ripple voltage can be approximated by Equation 6.

Where I_OUT(MAX) is the maximum load current, f_SW is the switching frequency (derated by a factor of 0.8), C_BULK is the bulk capacitor value and ESR_MAX is the maximum series resistance of the bulk capacitor.

The maximum RMS input ripple current also needs to be checked. For worst case conditions, this can be approximated by Equation 7.

In this case, the input ripple voltage is 98 mV and the RMS ripple current is 1.75 A. Make sure to note that the actual input voltage ripple is greatly affected by parasitic associated with the layout and the output impedance of the voltage source. The actual input voltage ripple for this circuit is shown in Design Parameters and is larger than the calculated value. This measured value is still below the specified input limit of 200 mV. The maximum voltage across the input capacitors is VIN max plus ΔVIN/2. The chosen bypass capacitor is rated for 25 V and the ripple current capacity is greater than 3 A, providing ample margin. Make sure that the maximum ratings for voltage and current are not exceeded under any circumstance.