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 during in an output short-circuit condition.

Choosing a 120-μH inductor in this design results in 250-mA peak-to-peak ripple current at a nominal input

voltage of 48 V, equivalent to 50% of the 500-mA rated load current. For designs that must operate up to the

maximum input voltage at the full-rated load current of 2.5 A, the inductance must 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 LM5012. TI recommends that the saturation current to be greater than 5 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

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

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.

For example, the LM5012EVM uses the V8P12-M3/86-A Schottky diode. The 120-V breakdown voltage rating

and 8-A current rating ensure that the design can support a 100-V input and a short-circuit condition without

any reliability concern. Furthermore, being that it is a Schottky diode with a low forward voltage and has small

switching losses due to its low junction capacitance, the efficiency figure of the design can be optimized. With

what loss does occur in the device, the package of the diode must be selected so it can have good heat

conduction out of it into the copper ground plane.

9.2.2.5 Output Capacitor (COUT)

Select a ceramic output capacitor to limit the capacitive voltage ripple at the converter output. This 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 250 mA gives COUT greater than 3.1 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 LM5012. The input capacitors conduct a

square-wave current of peak-to-peak amplitude equal to the output current. It 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.

www.ti.com

LM5012

SNVSCA9 – OCTOBER 2022

Copyright © 2022 Texas Instruments Incorporated

Submit Document Feedback

19

Product Folder Links: LM5012