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the losses due to conduction, switching (voltage-current overlap), output charge, and typically two-thirds of the

net loss attributed to body diode reverse recovery.

The low-side (synchronous) MOSFET carries the inductor current when the high-side MOSFET is off (or 1–D

interval). The low-side MOSFET switching loss is negligible as it is switched at zero voltage – current just

commutates from the channel to the body diode or vice versa during the transition dead-times. LM5148, with its

adaptive gate drive timing, minimizes body diode conduction losses when both MOSFETs are off. Such losses

scale directly with switching frequency.

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, it is critical to optimize the low-side MOSFET for low RDS(on). 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 LM5148 is well suited to drive TI's portfolio of NexFET™ power MOSFETs.

9.1.1.5 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.

IN(min)

(20)

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

high-current design.

• Calculate input filter capacitor CF.

Figure 9-2. 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 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 21.

L(PEAK)

MAX

Attn

20log

sin

1 9

(21)

where

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LM5148

SNVSC01 – FEBRUARY 2023

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