There are a few ways to select the proper value of output capacitor (C_OUT). The output capacitor value can be selected based on output voltage ripple, output overshoot, or output undershoot due to load transient. In this example, C_OUT is selected based on output undershoot because the wake-up performance is similar with no-load to full-load transient performance.

The output undershoot becomes smaller by increasing F_CROSS or by decreasing F_LP. A smaller C_OUT is allowed by increasing F_CROSS or by decreasing F_LP.

To increase F_CROSS, F_SW and F_RHP must be increased because the maximum F_CROSS is, in general, limited at 1/10 of F_RHP at V_SUPPLY(MIN) or 1/10 of F_SW, whichever is lower.

F_RHP is calculated using Equation 28.

Equation 28.

F_CROSS is selected at 1/10 of F_RHP or 1/10 of F_SW, whichever is lower.

Equation 29.

Equation 30.

In this example, 1.59 kHz is selected as a target F_CROSS and F_LP is selected to be 286 Hz (K1 = 0.18).

In general, there is about 5% or less undershoot with F_LP = 0.1 × F_CROSS (K1 = 0.1) and 10% or less undershoot with F_LP = 0.2 × F_CROSS (K1 = 0.2) during 0% to 100% load transient. The recommended K1 factor range is from 0.02 to 0.2.

F_LP is calculated using Equation 31.

Equation 31.

The minimum required output capacitance value is calculated using Equation 32.

Equation 32.

The maximum output ripple current is calculated at the minimum input supply voltage using Equation 33:

Equation 33.

The ripple current rating of the output capacitors must be enough to handle the output ripple current. By using multiple output capacitors, the ripple current can be split. In practice, ceramic capacitors are placed closer to the diode and the MOSFET than the bulk aluminum capacitors to absorb the majority of the ripple current.

In this example, three 100-µF capacitors are placed in parallel to ensure ripple current capability. If high-ESR capacitors are used for the output capacitor, additional 10-µF ceramic capacitors can be placed close to the switching components to minimize switching noise.