9.2.1.2.1.2.2 Output Capacitor Selection

The output capacitor of a switching converter absorbs the AC ripple current from the inductor, reduces the output voltage ripple, and provides the initial response to a load transient. The ripple voltage at the output of the converter is the product of the ripple current flowing through the output capacitor and the impedance of the capacitor. The impedance of the capacitor can be dominated by capacitive, resistive, or inductive elements within the capacitor, depending on the frequency of the ripple current. Ceramic capacitors have very low ESR and remain capacitive up to high frequencies. Their inductive component can be usually neglected at the operating frequency range of the converter.

The LMS3655 is designed to work with low-ESR ceramic capacitors. TI recommends X5R and X7R type capacitors. The effective value of these capacitors is defined as the actual capacitance under voltage bias and temperature. All ceramic capacitors have a large voltage coefficient, in addition to normal tolerances and temperature coefficients. Under DC bias, the capacitance value drops considerably. Larger case sizes or higher voltage capacitors are better in this regard. To help mitigate these effects, multiple small capacitors can be used in parallel to bring the minimum effective capacitance up to the desired value. This can also ease the RMS current requirements on a single capacitor. Table 6 shows the nominal and minimum values of total output ceramic capacitance recommended for the LMS3655.The values shown also provide a starting point for other output voltages. More output capacitance can be used to improve transient performance and reduce output voltage ripple.

In order to minimize ceramic capacitance, a low-ESR electrolytic capacitor can be used in parallel with minimal ceramic capacitance. As a starting point for designing with an output electrolytic capacitor, Table 7 shows the minimum ceramic capacitance recommended when paired with a 120-µF Aluminum-polymer (ESR = 25 mΩ) in order to maintain stable operation. Depending on load transient design requirements, the designer may choose to add additional capacitance.

In practice, the output capacitor has the most influence on the transient response and loop phase margin. Load transient testing and bode plots are the best way to validate any given design and should always be completed before the application goes into production. Make a careful study of temperature and bias voltage variation of any candidate ceramic capacitor in order to ensure that the minimum value of effective capacitance is provided. The best way to obtain an optimum design is to use the Texas Instruments WEBENCH Design Tool.

In adjustable applications the feed-forward capacitor, C_FF, provides another degree of freedom when stabilizing and optimizing the design. Refer to Optimizing Transient Response of Internally Compensated DC-DC Converters With Feedforward Capacitor (SLVA289) for helpful information when adjusting the feed-forward capacitor.

In addition to the capacitance shown in Table 6, a small ceramic capacitor placed on the output can help to reduce high frequency noise. Small case-size ceramic capacitors in the range of 1 nF to 100 nF can be very helpful in reducing spikes on the output caused by inductor parasitics.

Limit the maximum value of total output capacitance to between 800 μF and 1200 μF. Large values of output capacitance can prevent the regulator from starting up correctly and adversely effect the loop stability. If values greater than the given range are to be used, then a careful study of start-up at full load and loop stability must be performed.

Consult Output Ripple Voltage for Buck Switching Regulator (SLVA630) for more details on the estimation of the output voltage ripple for this converter.