LM5145 datasheet(42/69 Pages) TI

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10.2.1.2.2 Custom Design With Excel Quickstart Tool

Select components based on the regulator specifications using the LM25143 Quickstart Calculator available for

download from the LM25143 product folder.

10.2.1.2.3 Inductor Calculation

1. Use Equation 33 to calculate the required buck inductance for each channel based on a 30% inductor ripple

current at nominal input voltages.

IN(nom)

OUT1

IN(nom)

OUT2

IN(nom)

3.3 V

12 V

3.3 V

0.54 +

12 V

2.1A 2.1MHz

5 V

12 V

5 V

0.66 +

12 V

2.1A 2.1MHz

' ˜

(33)

2. Select a standard inductor value of 0.68 µH for both channels. Use Equation 34 to calculate the peak

inductor currents at maximum steady-state input voltage. Subharmonic oscillation occurs with a duty cycle

greater than 50% for peak current-mode control. For design simplification, the LM25143 has an internal

slope compensation ramp proportional to the switching frequency that is added to the current sense signal to

damp any tendency toward subharmonic oscillation.

LO1

OUT1

LO1(PK)

OUT1

IN(max)

LO2

OUT2

LO2(PK)

OUT2

IN(max)

3.3 V

7 A

7.94 A

2 L

2 0.68 +

0+]

5 V

7 A

2 L

2 0.68 +

0+]

5 V

8.27 A

18 V

(34)

3. Based on Equation 10, use Equation 35 to cross-check the inductance to set a slope compensation equal to

the ideal one times the inductor current downslope.

OUT

O1(sc)

OUT

O2(sc)

(V) R (m )

3.3 V 7m

0.46 +

24 F

(MHz)

24 2.1 MHz

(V) R (m )

5 V 7m

0.69 +

24 F

(MHz)

24 2.1 MHz

(35)

10.2.1.2.4 Current-Sense Resistance

1. Calculate the current-sense resistance based on a maximum peak current capability of at least 20% higher

than the peak inductor current at full load to provide sufficient margin during start-up and load-on transients.

Calculate the current sense resistances using Equation 36.

LM25143

SNVSC10 – MARCH 2022

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LM5145 Datasheet(PDF) 42 Page - Texas Instruments

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