LM5145 datasheet(36/74 Pages) TI2

36 / 74 page

IN(min)

(18)

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 regulator.

• The input filter inductor LIN is usually selected between 1μH and 6.8μH, but can be lower to reduce losses in

a high-current design.

• Calculate input filter capacitor CF.

VIN

GND

LIN

CIN

VOUT

COUT

GND

Figure 8-2. Buck Regulator With π-Stage EMI Filter

By calculating the first harmonic current from the Fourier series of the input current waveform and multiplying

by the input impedance (the impedance is defined by the existing input capacitor CIN), Equation 19 presents an

expression to obtain the required attenuation.

L(PEAK)

MAX

Attn

20log

sin

1 9

(19)

where

• VMAX is the allowed dBμV noise level for the applicable conducted EMI specification (for example, CISPR 25

Class 5)

• CIN is the existing input capacitance of the buck regulator

• DMAX is the maximum operating duty cycle (at minimum input voltage)

• IL(PEAK) 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 20.

Attn

(20)

Adding an input filter to a switching regulator as shown in Figure 8-2 modifies the control-to-output transfer

function. The output impedance of the filter must be sufficiently low such that the input filter does not significantly

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LM5145 Datasheet(PDF) 36 Page - Texas Instruments

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