OUT

OUT

O

SW

L

IN(nom)

V

V

L

1

F

I

V

§

·

˜ ¨

¸

¨

¸

˜ '

©

¹

(12)

For applications in which the device must support input transients exceeding 72 V, select the inductor to be at

least 22 μH, which ensures that excessive current rise does not occur in the power stage due to the potential

large inductor current slew that can occur in an output short-circuit condition.

Choosing a 22-μH inductor in this design results in 1.36-A peak-to-peak ripple current at a nominal input voltage

of 48 V, equivalent to 39% of the 3.5-A rated load current. For designs which must operate up to the maximum

input voltage at the full-rated load current of 3.5 A, the inductance needs to increase to ensure current limit

(IPEAK current limit) is not hit.

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

limit setting of the LM5013. TI recommends a saturation current greater than 7 A. Ferrite-core inductors have

relatively lower core losses and are preferred at high switching frequencies, but exhibit a hard saturation

characteristic – the inductance collapses abruptly when the saturation current is exceeded. This collapse results

in an abrupt increase in inductor ripple current, higher output voltage ripple, and reduced efficiency, and in turn

compromising reliability. Note that inductor saturation current levels generally decrease as the core temperature

increases.

9.2.2.4 Schottky Diode (DSW)

The breakdown voltage rating of the diode is preferred to be 25% higher than the maximum input voltage. In the

target application, the power rating for the diode must exceed the maximum DC output current and support the

peak current limit (IPEAK current limit) for best reliability in most applications.

9.2.2.5 Output Capacitor (COUT)

Select a ceramic output capacitor to limit the capacitive voltage ripple at the converter output, which is the

sinusoidal ripple voltage that is generated from the triangular inductor current ripple flowing into and out of the

capacitor. Select an output capacitance using Equation 13 to limit the voltage ripple component to 0.5% of the

output voltage.

L

OUT

SW

OUT(ripple)

I

C

8 F

V

'

t

˜

˜

(13)

Substituting ΔIL(nom) of 1.36 A gives COUT greater than 10 μF. With voltage coefficients of ceramic capacitors

taken in consideration, a 22-µF, 25-V rated capacitor with X7R dielectric is selected.

9.2.2.6 Input Capacitor (CIN)

An input capacitor is necessary to limit the input ripple voltage while providing AC current to the buck power

stage at every switching cycle. To minimize the parasitic inductance in the switching loop, position the input

capacitors as close as possible to the VIN and GND pins of the LM5013. The input capacitors conduct a

square-wave current of peak-to-peak amplitude equal to the output current. The device follows that the resultant

capacitive component of AC ripple voltage is a triangular waveform.

Along with the ESR-related ripple component, use Equation 14 to calculate the peak-to-peak ripple voltage

amplitude.

OUT

IN(ripple)

OUT

ESR

SW

IN

I

D 1 D

V

I

R

F

C

˜

˜

˜

˜

(14)

Use Equation 15 to calculate the input capacitance required for a load current, based on an input voltage ripple

specification (ΔVIN).

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