LM5145 datasheet(24/60 Pages) TI2

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7 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

7.1 Application Information

7.1.1 Power Train Components

A comprehensive understanding of the buck regulator power train components is critical to successfully

completing a synchronous buck regulator design. The following sections discuss the output inductor, input and

output capacitors, power MOSFETs, and EMI input filter.

7.1.1.1 Buck Inductor

For most applications, choose a buck inductance such that the inductor ripple current, ΔIL, is between 30% to

50% of the maximum DC output current at nominal input voltage. Use Equation 12 to choose the inductance

based on a peak inductor current given by Equation 13.

L= VOUT

∆IL×FSW× 1− VOUT

VINnom

(12)

ILpeak =IOUT+∆IL2

(13)

Check the inductor data sheet to make sure that the saturation current of the inductor is well above the peak

inductor current of a particular design. Ferrite designs have very low core loss and are preferred at high

switching frequencies, so design goals can then concentrate on copper loss and preventing saturation. Low

inductor core loss is evidenced by reduced no-load input current and higher light-load efficiency. However, ferrite

core materials exhibit a hard saturation characteristic and the inductance collapses abruptly when the saturation

current is exceeded. This action results in an abrupt increase in inductor ripple current and higher output voltage

ripple, not to mention reduced efficiency and compromised reliability. Note that the saturation current of an

inductor generally decreases as the core temperature increases. Of course, accurate overcurrent protection is

key to avoiding inductor saturation.

7.1.1.2 Output Capacitors

Ordinarily, the output capacitor energy storage of the regulator combined with the control loop response are

prescribed to maintain the integrity of the output voltage within the dynamic (transient) tolerance specifications.

The usual boundaries restricting the output capacitor in power management applications are driven by finite

available PCB area, component footprint and profile, and cost. The capacitor parasitics—equivalent series

resistance (ESR) and equivalent series inductance (ESL)—take greater precedence in shaping the load transient

response of the regulator as the load step amplitude and slew rate increase.

The output capacitor, COUT, filters the inductor ripple current and provides a reservoir of charge for step-load

transient events. Typically, ceramic capacitors provide extremely low ESR to reduce the output voltage ripple

and noise spikes, while tantalum and electrolytic capacitors provide a large bulk capacitance in a relatively

compact footprint for transient loading events.

Based on the static specification of peak-to-peak output voltage ripple denoted by ΔVOUT, choose an output

capacitance that is larger than that shown with Equation 14.

COUT≥ LO×∆IOUT2

VOUT+∆VOVERSHOOT2−VOUT2

(14)

LM25139

SLVSJ80 – OCTOBER 2025

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