I want to calculate buck input current ripple. How can choose from formula(5) and formula(6)? I don't know how to judge high and low capacitance,can you kindly teach me please?
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Appropriate input capacitor can usually sustain the voltage in case of any line transients. Increasing the input capacitance helps reduce input voltage ripple as well. There is usually a minimum requirement of input capacitors stated in the datasheet for stability. This will usually follow equation 5. In Equation 6 you can see poor input capacitance can lead to the input current not sustaining and hitting the floor every switching cycle.