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good starting point. (ΔiL = (1/5 to 2/5) × IOUT). The peak-to-peak inductor current ripple can be found by Equation

15 and the range of inductance can be found by Equation 14 with the typical input voltage used as VIN.

OUT

) D

L f

(14)

OUT

L-MAX

) D

0.4 f

0.2 f

LOAD-MAX

OUT

L-MAX

) D

0.4 f

0.2 f

u LOAD-MAX

(15)

D is the duty cycle of the converter which in a buck converter it can be approximated as D = VOUT / VIN,

assuming no loss power conversion. By calculating in terms of amperes, volts, and megahertz, the inductance

value comes out in micro henries. The inductor ripple current ratio is defined by:

OUT

(16)

The second criterion is the inductor saturation-current rating. The inductor must be rated to handle the maximum

load current plus the ripple current:

IL-PEAK = ILOAD-MAX + Δ iL / 2

(17)

The LM76005 has both valley current limit and peak current limit. During an instantaneous short, the peak

inductor current can be high due to a momentary increase in duty cycle. The inductor current rating must be

higher than the HS current limit. TI recommends selection of an inductor with a larger core saturation margin and

preferably a softer roll off of the inductance value over load current.

In general, it is preferable to choose lower inductance in switching power supplies, because it usually

corresponds to faster transient response, smaller DCR, and reduced size for more compact designs. However,

too low of an inductance can generate too large of an inductor current ripple such that overcurrent protection at

the full load can be falsely triggered. It also generates more conduction loss because the RMS current is slightly

higher relative that with lower current ripple at the same DC current. Larger inductor current ripple also implies

larger output voltage ripple with the same output capacitors. With peak-current-mode control, it is not

recommended to have an inductor current ripple that is too small. Enough inductor current ripple improves

signal-to-noise ratio on the current comparator and makes the control loop more immune to noise.

Once the inductance is determined, the type of inductor must be selected. Ferrite designs have very low core

losses and are preferred at high switching frequencies, so design goals can concentrate on copper loss and

preventing saturation. Ferrite core material saturates hard, which means that inductance collapses abruptly

when the peak design current is exceeded. The hard saturation results in an abrupt increase in inductor ripple

current and consequent output voltage ripple. Do not allow the core to saturate.

For the design example, a standard 6.8-μH inductor from Wurth, Coiltronics, or Vishay can be used for the 5-V

output with a saturation current rating over 8 A.

8.2.2.6 Output Capacitor Selection

The device is designed to be used with a wide variety of LC filters. TI generally recommends using as little

output capacitance as possible to keep cost and size down. Choose the output capacitor or capacitors, COUT,

with care as it directly affects the steady-state output-voltage ripple, loop stability, and the voltage over/

undershoot during load current transients.

The output voltage ripple is essentially composed of two parts. One is caused by the inductor current ripple

going through the equivalent series resistance (ESR) of the output capacitors:

ΔVOUT-ESR = ΔiL × ESR

(18)

The other is caused by the inductor current ripple charging and discharging the output capacitors:

www.ti.com

LM76005

SNVSBK5A – FEBRUARY 2020 – REVISED JULY 2020

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