LM5145 datasheet(35/66 Pages) TI2

35 / 66 page

• 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 19.

Attn

(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.

res

S ˜

(20)

The purpose of RD is to reduce the peak output impedance of the filter at the 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 filter formed by LIN and

CIN is too high). An electrolytic capacitor CD can be used for damping with a value given by Equation 21.

4 C

(21)

Select the damping resistor RD using Equation 22.

(22)

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LM5146

SNVSBV0A – JUNE 2021 – REVISED JUNE 2021

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LM5145 Datasheet(PDF) 35 Page - Texas Instruments