LM5145 datasheet(29/60 Pages) TI2

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CF= 1LIN× 10Attn40

2π×FSW 2

(21)

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 so 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 22 to calculate

the resonant frequency of the passive filter.

fres= 1

2π× LIN×CF

(22)

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 requirement prevents CIN from interfering with the cutoff frequency of the main filter. Added

input 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 input damping with a value

shown using Equation 23.

CD≥4×CIN

(23)

Use Equation 24 to select the input damping resistor RD.

RD= LINCIN

(24)

7.1.2 Error Amplifier and Compensation

Figure 7-3 shows a type-ll compensator using a transconductance error amplifier (EA). The dominant pole of the

EA open-loop gain is set by the EA output resistance, RO-EA, and effective bandwidth-limiting capacitance, CBW,

as shown by Equation 25.

GEAopenloop s =− gm×RO−EA

1+s×RO−EA×CBW

(25)

The EA high-frequency pole is neglected in the above expression. Equation 26 calculates the compensator

transfer function from output voltage to COMP node, including the gain contribution from the (internal or external)

feedback resistor network.

Gc s = vcsvouts =−VREFVOUT×gm×RO−EA× 1+ sωz1

1+ sωp1 × 1+ sωp2

(26)

where

• VREF is the feedback voltage reference of 0.8V.

• gm is the EA gain transconductance of 1.1mS.

• RO-EA is the error amplifier output impedance of 10MΩ.

ωz1= 1

RCOMP×CCOMP

(27)

ωp1=

RO−EA× CCOMP+CHF+CBW ≅ 1

RO−EA×CCOMP

(28)

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SLVSJ80 – OCTOBER 2025

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