SNVSBI9 October 2019 LMR33610
The value of the output capacitor and its ESR determine the output voltage ripple and load transient performance. The output capacitor bank is usually limited by the load transient requirements rather than the output voltage ripple. Use Equation 6 to estimate a lower bound on the total output capacitance and an upper bound on the ESR, which is required to meet a specified load transient.
Once the output capacitor and ESR have been calculated, use Equation 7 to check the peak-to-peak output voltage ripple, Vr.
The output capacitor and ESR can then be adjusted to meet both the load transient and output ripple requirements.
This example requires a ΔVOUT ≤ 250 mV for an output current step of ΔIOUT = 1 A. Equation 6 gives a minimum value of 25 µF and a maximum ESR of 0.21 Ω. Assuming a 20% tolerance and a 10% bias de-rating, there is a minimum capacitance of 35 µF. This can be achieved with a bank of 2 × 22-µF, 16-V, ceramic capacitors in the 1210 case size. More output capacitance can be used to improve the load transient response. Ceramic capacitors can easily meet the minimum ESR requirements. In some cases, an aluminum electrolytic capacitor can be placed in parallel with the ceramics to help build up the required value of capacitance. In general, use a capacitor of at least 10 V for output voltages of 3.3 V or less, while a capacitor of 16 V or more must be used for output voltages of 5 V and above.
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 must always be completed before the application goes into production. In addition to the required output capacitance, a small ceramic placed on the output can help reduce high frequency noise. Small case size ceramic capacitors in the range of 1 nF to 100 nF can be very helpful in reducing voltage spikes on the output caused by inductor and board parasitics.
The maximum value of total output capacitance must be limited to about 10 times the design value, or 1000 µF, whichever is smaller. Large values of output capacitance can adversely affect the start-up behavior of the regulator as well as the loop stability. If values larger than noted here must be used, then a careful study of start-up at full load and loop stability must be performed.