SLVS839F July 2008 – October 2014 TPS54331
The TPS54331 device 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. A high-quality ceramic type X5R or X7R is recommended. The voltage rating should be greater than the maximum input voltage. A smaller value can be used as long as all other requirements are met; however a value of 10 μF has been shown to work well in a wide variety of circuits. Additionally, some bulk capacitance may be required, especially if the TPS54331 circuit is not located within about 2 inches from the input voltage source. The value for this capacitor is not critical but should be rated to handle the maximum input voltage including ripple voltage, and should filter the output so that input ripple voltage is acceptable. For this design two 4.7-μF capacitors are used for the input decoupling capacitor. The capacitors are X7R dielectric rated for 50 V. The equivalent series resistance (ESR) is approximately 2 mΩ and the current rating is 3 A. Additionally, a small 0.01-μF capacitor is included for high frequency filtering.
Use Equation 6 to calculate the input ripple voltage.
The maximum RMS ripple current must also be checked. For worst case conditions, use Equation 7 to calculate the maximum-RMS input ripple current, ICIN(RMS).
In this case, the input ripple voltage is 143 mV and the RMS ripple current is 1.5 A.
The actual input voltage ripple is greatly affected by parasitics associated with the layout and the output impedance of the voltage source.
The actual input voltage ripple for this circuit is listed in Table 3 and is larger than the calculated value. This measured value is still below the specified input limit of 300 mV. The maximum voltage across the input capacitors would be VIN(MAX) + ΔVIN / 2. The selected bulk and bypass capacitors are each rated for 50 V and the ripple current capacity is greater than 3 A, both providing ample margin. The maximum ratings for voltage and current must not be exceeded under any circumstance.