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Analysis and Design of Input Filter for DC-DC Circuit

 

When designing DC-DC circuits, it is common to add the input filter circuit before the power stage. An LC filter is one of most frequently used input filter circuit. However, if a design is not optimal, input filter circuit may cause large output noise instead of suppressing the noise, and may even cause loop stability problems. This application report analyzes the influence of the input filter on the DC-DC control loop transfer function, and the influence of a closed loop on the input filter, explains why input filter causes unexpected problem, and suggests how to eliminate the side effect of the input filter.