SLVSAA4G June 2010 – February 2018 TPS65251
There are two primary considerations for selecting the value of the output capacitor. The output capacitors are selected to meet load transient and output ripple’s requirements.
Equation 20 gives the minimum output capacitance to meet the transient specification. For this example,
LO = 4.7 µH, ΔIOUT = 1.5 A – 0.75 A = 0.75 A and ΔVOUT = 120 mV. Using these numbers gives a minimum capacitance of 18 µF. A standard 22-µF ceramic capacitor is chose in the design.
Equation 21 calculates the minimum output capacitance needed to meet the output voltage ripple specification. Where fsw is the switching frequency, VRIPPLE is the maximum allowable output voltage ripple, and IRIPPLE is the inductor ripple current. In this case, the maximum output voltage ripple is 30 mV. From Equation 17, the output current ripple is 0.46 A. From Equation 21, the minimum output capacitance meeting the output voltage ripple requirement is 1.74 µF.
Additional capacitance de-rating for aging, temperature and DC bias should influence this minimum value. For this example, one 22-µF, 6.3-V X7R ceramic capacitor with 3 mΩ of ESR will be used.