For both loop stability and filtering of ripple voltage, an output capacitor is required, again in close proximity to the regulator. For best performance, low-ESR aluminum electrolytics are recommended, although standard aluminum electrolytics may be adequate for some applications as shown in Equation 2.
Equation 2. Output ripple voltage = (ESR of COUT) × (inductor ripple current)
Output ripple of 50 mV to 150 mV typically can be achieved with capacitor values of 220 μF to 680 μF. Larger COUT can reduce the ripple 20 mV to 50 mV peak to peak. To improve further on output ripple, paralleling of standard electrolytic capacitors may be used. Alternatively, higher-grade capacitors such as high frequency, low inductance, or low ESR can be used.
The following should be taken into account when selecting COUT:
- At cold temperatures, the ESR of the electrolytic capacitors can rise dramatically (typically 3× nominal value at –25°C). Because solid-tantalum capacitors have significantly better ESR specifications at cold temperatures, they should be used at operating temperature lower than –25°C. As an alternative, tantalums can also be paralleled to aluminum electrolytics and should contribute 10% to 20% to the total capacitance.
- Low ESR for COUT is desirable for low output ripple. However, the ESR should be greater than 0.05 Ω to avoid the possibility of regulator instability. Hence, a sole tantalum capacitor used for COUT is most susceptible to this occurrence.
- The ripple current rating of the capacitor, 52 kHz, should be at least 50% higher than the peak-to-peak inductor ripple current.