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10 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.

10.1 Application Information

The LM25143 is a synchronous buck controller used to convert a higher input voltage to two lower output

voltages. The following sections discuss the design procedure for a dual-output implementation using a specific

circuit design example. To expedite and streamline the process of designing of a LM25143-based regulator, a

comprehensive LM25143 Quickstart Calculator is available for download to assist the designer with component

selection for a given application.

10.1.1 Power Train Components

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

completing a synchronous buck regulator design. The subsequent subsections discuss the following:

• Output inductor

• Input and output capacitors

• Power MOSFETs

• EMI input filter

10.1.1.1 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. Choose the inductance using Equation 15

based on a peak inductor current given by Equation 16.

OUT

˜ ¨

' ˜

(15)

L(peak)

OUT

(16)

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 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 its core temperature increases. Of course, accurate overcurrent protection is key to

avoiding inductor saturation.

10.1.1.2 Output Capacitors

Ordinarily, the output capacitor energy store 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.

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LM25143

SNVSC10 – MARCH 2022

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