LM5145 datasheet(32/70 Pages) TI

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• VMAX is the allowed dBμV noise level for the applicable conducted EMI specification, for example CISPR 32

Class B.

• 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 passive

EMI filter capacitance CF from Equation 22.

Attn

(22)

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. The resonant frequency of the

passive filter is given by Equation 23.

res

S ˜

(23)

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 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 given by Equation 24.

4 C

(24)

Select the input damping resistor RD using Equation 25.

(25)

9.1.2 Error Amplifier and Compensation

Figure 9-3 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 26.

O-EA

EA(openloop)

O-EA

(s)

1 s R

(26)

The EA high-frequency pole is neglected in the above expression. Equation 27 calculates the compensator

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

feedback resistor network.

LM5148

SNVSC01 – FEBRUARY 2023

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