4 Reducing Output Voltage Ripple

The output ripple is essentially composed of two parts. One part is caused by the inductor current ripple going though the Equivalent Series Resistance (ESR) of the output capacitors. And the other part is caused by the inductor current ripple charging and discharging the output capacitors. The two components in the voltage ripple are not in phase, so the actual peak-to-peak ripple is smaller than the sum of the two peaks. If the lower output ripple is desired, there are mainly three ways. One way is to reduce the amount of the ripple current through the inductor. The second way is to reduce the magnitude of the impedance of the capacitor. For a ceramic capacitor with very low ESR, the most effective way is to use a larger capacitance. Due to the effect of ESR, the effect of increasing the capacitance can have a negligible effect when the capacitance becomes larger. The third way is to choose higher switching frequency. For TPS56325x, the switching frequency is 1.2 MHz which is high frequency part compared with last generation devices. The high-switching frequency allows smaller inductors and output capacitors with the same output ripple requirement.

Figure 5-6 shows the output voltage ripple of TPS563257 when using 2.2 uH inductor with a 10 uF output capacitor. Figure 4-2 shows the output voltage ripple of TPS563257 when using 2.2 uH inductor with 22 uF *3 output capacitor. Using a larger ceramic capacitor is an effective way to reduce the ripple.