8.2.2.4 Output Capacitor

In a switch mode power supply, the minimum output capacitance is typically selected based on the capacitor ripple current rating and the load transient requirements. The output capacitor must be large enough to absorb the inductor energy and limit over voltage when transitioning from full-load to no-load, and to limit the output voltage undershoot during no-load to full load transients. The worst-case load transient from zero to full load occurs when the input voltage is at the maximum value and a current switching cycle has just finished. The total output voltage drop ΔV_OUT is the sum of the voltage drop while the inductor is ramping up to support the full load and the voltage drop before the next pulse can occur.

The output capacitance required to maintain the minimum output voltage drop (ΔV_OUT) can be calculated as follows:

Equation 27.

Equation 28.

where

• I_STEP (the transient step load current for this example) = 4 A
• ΔV_OUT = 1% of 3.3 V, or 33 mV

For this example a total of 211 μF of capacitance is used, two 82-μF aluminum capacitors for energy storage and one 47-μF low ESR ceramic capacitor to reduce high frequency noise.

Generally, when sufficient capacitance is used to satisfy the undershoot requirement, the overshoot during a full-load to no-load transient will also be satisfactory. After the output capacitance has been selected, calculate the output ripple current and verify that the ripple current is within the capacitor ripple current ratings.

Equation 29.

Equation 30.