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As the needed inductance is reduced, the amount of generated ripple current is also reduced. A smaller ripple current means smaller ripple voltage, allowing higher ESR capacitors for a given voltage output. This slide shows an example of a buck converter switching regulator for an automotive application. In a buck regulator, the output voltage is expected to be lower than the input voltage. A switch is driven on and off, allowing current to flow into the output. The current is constrained by the inductor during the ON phase. The current starts to ramp up, and it is during this ramping up that the capacitor gets charged. Then the switch is turned off. Naturally, the current wants to stop flowing, but the inductor will use its stored energy to keep the current flowing. During the OFF phase, the power will be sourced by the output capacitor. Before the energy is depleted from the inductor, the switch is turned back on to repeat the cycle. Looking at the current on the inductor, it has a triangular waveform. This change is called di. The average of that waveform is the design output current. The ratio di to the output current is an important parameter to keep in mind. Looking at the equations that govern this current ripple ratio (rcr), it should be kept within 0.25 to 0.4.
