LM5145 datasheet(33/62 Pages) TI

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Referring to the bode plot in Figure 9-2, the phase margin, indicated as φM, is the difference between the loop

phase and –180° at crossover. A target of 50° to 70° for this parameter is considered ideal. Additional phase

boost is dialed in by locating the compensator zeros at a frequency lower than the LC double pole. This helps

mitigate the phase dip associated with the LC filter, particularly at light loads when the Q-factor is higher and

the phase dip becomes especially prominent. The ramification of low phase in the frequency domain is an

under-damped transient response in the time domain.

The power supply designer now has all the necessary expressions to optimally position the loop crossover

frequency while maintaining adequate phase margin over the required line, load and temperature operating

ranges. The LM5145-Q1 Quickstart Calculator is available to expedite these calculations and to adjust the bode

plot as needed.

9.1.4 EMI Filter Design

Switching regulators exhibit negative input impedance, which is lowest at the minimum input voltage. An

underdamped LC filter exhibits a high output impedance at the resonant frequency of the filter. For stability,

the filter output impedance must be less than the absolute value of the converter input impedance.

IN(min)

(17)

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 converter.

• Input filter inductor 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 capacitor CF.

VIN

GND

LIN

CIN

VOUT

COUT

GND

Figure 9-4. Buck Regulator With π-Stage EMI Filter

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 18.

L(PEAK)

MAX

Attn

20log

sin

1 9

(18)

where

• VMAX is the noise specification in dBμV from the applicable EMI standard, for example CISPR 25 Class 5

• CIN is the existing input capacitance of the buck regulator

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LM5145-Q1

SNVSBU9 – JUNE 2021

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