SNVSB29C October 2018 – June 2021 LM5143-Q1
Switching regulators exhibit negative input impedance, which is lowest at the minimum input voltage. An underdamped LC filter exhibits a high output impedance at the resonant frequency of the filter. For stability, the filter output impedance must be less than the absolute value of the converter input impedance.
The EMI filter design steps are as follows:
By calculating the first harmonic current from the Fourier series of the input current waveform and multiplying it by the input impedance (the impedance is defined by the existing input capacitor CIN), a formula is derived to obtain the required attenuation as shown by Equation 23.
For filter design purposes, the current at the input can be modeled as a square-wave. Determine the EMI filter capacitance CF from Equation 24.
Adding an input filter to a switching regulator modifies the control-to-output transfer function. The output impedance of the filter must be sufficiently small such that the input filter does not significantly affect the loop gain of the buck converter. The impedance peaks at the filter resonant frequency. Use Equation 25 to calculate the resonant frequency of the filter.
The purpose of RD is to reduce the peak output impedance of the filter at its resonant frequency. Capacitor CD blocks the DC component of the input voltage to avoid excessive power dissipation in RD. Capacitor CD must have lower impedance than RD at the resonant frequency with a capacitance value greater than that of the input capacitor CIN. This prevents CIN from interfering with the cutoff frequency of the main filter. Added damping is needed when the output impedance of the filter is high at the resonant frequency (Q of the filter formed by LIN and CIN is too high). An electrolytic capacitor CD can be used for damping with a value given by Equation 26.
Use Equation 27 to select the damping resistor RD.