LM5145 datasheet(34/67 Pages) TI1

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Attn

PEAK

MAX

Attn

20log

1 9

VLQ

VIN

GND

LIN

CIN

VOUT

COUT

GND

IN(min)

LM5145

SNVSAI4 – JUNE 2017

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The power supply designer now has all the necessary expressions to optimally position the loop crossover

frequency while maintaining adequate phase margin over the required line, load and temperature operating

ranges. The LM5145 Quickstart Calculator is available to expedite these calculations and to adjust the bode plot

as needed.

9.1.4 EMI Filter Design

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.

(17)

The EMI filter design steps are as follows:

•

Calculate the required attenuation of the EMI filter at the switching frequency, where CIN represents the

existing capacitance at the input of the switching converter;

•

Input filter inductor LIN is usually selected between 1 μH and 10 μH, but it can be lower to reduce losses in a

high current design;

•

Calculate input filter capacitor CF.

Figure 45. Buck Regulator With π-Stage EMI Filter

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 18.

(18)

VMAX is the allowed dBμV noise level for the applicable EMI standard, for example EN55022 Class B. CIN is the

existing input capacitance of the buck regulator, DMAX is the maximum duty cycle, and IPEAK is the peak inductor

current. For filter design purposes, the current at the input can be modeled as a square-wave. Determine the EMI

filter capacitance CF from Equation 19.

(19)

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. The resonant frequency of the

filter is given by Equation 20.

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