LM5145 datasheet(28/60 Pages) TI2

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In high step-down ratio applications, the low-side MOSFET carries the current for a large portion of the switching

period. Therefore, to attain high efficiency, optimizing the low-side MOSFET for low RDS(on) is critical. In cases

where the conduction loss is too high or the target RDS(on) is lower than available in a single MOSFET, connect

two low-side MOSFETs in parallel. The total power dissipation of the low-side MOSFET is the sum of the losses

due to channel conduction, body diode conduction, and typically one-third of the net loss attributed to body diode

reverse recovery. The LM25139 is well-designed to drive TI's portfolio of NexFET™ power MOSFETs.

7.1.1.5 EMI Filter

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.

ZIN= −VINmin2PIN

(19)

The passive 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 1μH and 10μH, but can be lower to reduce losses in a

high-current design.

• Calculate input filter capacitor CF.

Figure 7-2. Passive π-Stage EMI Filter for Buck Regulator

By calculating the first harmonic current from the Fourier series of the input current waveform and multiplying by

the input impedance (the impedance is defined by the existing input capacitor CIN), use Equation 20 to show a

formula derived to obtain the required attenuation.

Attn=20log ILPEAK

π2×FSW×CIN×sinπ×DMAX × 11μV −VMAX

(20)

where

• 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. Use Equation 21 to

determine the passive EMI filter capacitance CF.

LM25139

SLVSJ80 – OCTOBER 2025

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LM5145 Datasheet(PDF) 28 Page - Texas Instruments