SLVS875C January 2009 – November 2014 TPS54332
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. A high-quality ceramic type X5R or X7R is recommended. The voltage rating should be greater than the maximum input voltage. A smaller value may 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 may 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 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, a single 10-μF capacitor is used for the input decoupling capacitor. It 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 IOUT(MAX) is the maximum load current, fSW is the switching frequency (derated by a factor of 0.8), CBULK is the bulk capacitor value and ESRMAX is the maximum series resistance of the bulk capacitor.
The maximum RMS imput 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 would be 98 mV and the RMS ripple current would be 1.75 A. It is also important to note that the actual input voltage ripple will be 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 would be 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. It is important that the maximum ratings for voltage and current are not exceeded under any circumstance.