SLVS839F July 2008 – October 2014 TPS54331
The important design factors for the output capacitor are DC voltage rating, ripple current rating, and equivalent series resistance (ESR). The DC voltage and ripple current ratings cannot be exceeded. The ESR is important because along with the inductor current it determines the amount of output ripple voltage. The actual value of the output capacitor is not critical, but some practical limits do exist. Consider the relationship between the desired closed-loop crossover frequency of the design and LC corner frequency of the output filter. In general, keeping the closed-loop crossover frequency at less than 1/5 of the switching frequency is desired. With high switching frequencies such as the 570-kHz frequency of this design, internal circuit limitations of the TPS54331 device limit the practical maximum crossover frequency to about 25 kHz. In general, the closed-loop crossover frequency should be higher than the corner frequency determined by the load impedance and the output capacitor. Use Equation 12 to calculate the limits of the minimum capacitor value.
For a desired maximum crossover of 25 kHz the minimum value for the output capacitor is approximately 5.8 μF. This value may not satisfy the output ripple voltage requirement. The output ripple voltage consists of two components; the voltage change because of the charge and discharge of the output filter capacitance and the voltage change because the ripple current times the ESR of the output filter capacitor. Use Equation 13 to estimate the output ripple voltage.
The maximum ESR of the output capacitor can be determined from the amount of allowable output ripple as specified in the initial design parameters. The contribution to the output ripple voltage because the ESR is the inductor ripple current times the ESR of the output filter. Therefore, use Equation 14 to calculate the maximum specified ESR as listed in the capacitor data sheet.
Use Equation 15 to calculate the maximum RMS ripple current.
For this design example, two 47-μF ceramic output capacitors are selected for C8 and C9. These capacitors are TDK C3216X5R0J476MT, rated at 6.3 V with a maximum ESR of 2 mΩ and a ripple current rating in excess of
3 A. The calculated total RMS ripple current is 161 mA (80.6 mA each) and the maximum total ESR required is 43 mΩ. These output capacitors exceed the requirements by a wide margin and result in a reliable, high-performance design.
The actual capacitance in circuit may be less than the catalog value when the output is operating at the desired output of 3.3 V.
The selected output capacitor must be rated for a voltage greater than the desired output voltage plus half of the ripple voltage. Any derating amount must also be included. Other capacitor types work well with the TPS54331 device, depending on the needs of the application.