LM5145 datasheet(38/69 Pages) TI

38 / 69 page

• The input filter inductance, 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 capacitance, CF.

• Calculate damping capacitance, CD, and damping resistance, RD.

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.

L(PEAK)

MAX

Attn

20log

sin

1 9

(23)

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 duty cycle.

• 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 24.

Attn

(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.

res

S ˜

(25)

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.

4 C

(26)

Use Equation 27 to select the damping resistor RD.

(27)

LM25143

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LM5145 Datasheet(PDF) 38 Page - Texas Instruments

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