SNVA406C May   2010  – January 2022 LM27402

 

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9.2 Input Capacitors, Cin1–Cin5

Input capacitors should be selected based on the required input voltage ripple and maximum RMS current rating. The required RMS current rating of the input capacitor for a buck regulator can be estimated by the following equation:

Equation 2. GUID-A87C92C9-5ADD-486E-801E-1C7D595C4549-low.gif

From this equation, it follows that the maximum ICIN(RMS) requirement will occur at a full 20-A load current with the system operating at 50% duty cycle. Under this condition, the maximum ICIN(RMS) is given by:

Equation 3. GUID-99781077-22B1-4C44-A7AC-4789ED621ED7-low.gif

The voltage ripple can be calculated by:

Equation 4. GUID-4B81D914-E9B0-43B2-A453-8443F36F7845-low.gif

Ceramic capacitors feature a very large IRMS rating in a small footprint, making a ceramic capacitor ideal for this application. Five 22-µF X5R 25-V ceramic capacitors were selected to provide the necessary input capacitance for the evaluation board. Neglecting the effects of ESR, at 12-V VIN and 20-A IOUT the selected input capacitors yield an input voltage ripple of:

Equation 5. GUID-2140EBE0-9255-49F2-923D-F76FC445B004-low.gif

If desired, two extra capacitors can be added in the Cin6 and Cin7 footprints.