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LTC4081

4081f

Accepting larger values of ΔIL allows the use of low

inductances, but results in higher output voltage ripple,

greater core losses, and lower output current capability. A

reasonable starting point for setting ripple current is ΔIL

=0.3 • ILIM, where ILIM is the peak switch current limit.

The largest ripple current occurs at the maximum input

voltage. To guarantee that the ripple current stays below a

speciﬁed maximum, the inductor value should be chosen

according to the following equation:

OUT

IN MAX

≥−

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

()

•

For applications with VOUT = 1.8V, the above equation

suggests that an inductor of at least 6.8μH should be used

for proper operation.

Many different sizes and shapes of inductors are

available from numerous manufacturers. To maximize

efﬁciency, choose an inductor with a low DC resistance.

Keep in mind that most inductors that are very thin or

have a very small volume typically have much higher core

and DCR losses and will not give the best efﬁciency. Also

choose an inductor with a DC current rating at least 1.5

times larger than the peak inductor current limit to ensure

that the inductor does not saturate during normal opera-

tion. To minimize radiated noise use a toroid or shielded

pot core inductor in ferrite or permalloy materials. Table

1 shows a list of several inductor manufacturers.

APPLICATIO S I FOR ATIO

Table 1. Recommended Surface Mount Inductor Manufacturers

Coilcraft

www.coilcraft.com

Sumida

www.sumida.com

Murata

www.murata.com

Toko

www.tokoam.com

Input and Output Capacitor Selection

Since the input current waveform to a buck converter is a

square wave, it contains very high frequency components.

It is strongly recommended that a low equivalent series

resistance (ESR) multilayer ceramic capacitor be used to

bypass the BAT pin which is the input for the converter.

Tantalum and aluminum capacitors are not recommended

because of their high ESR. The value of the capacitor on

BAT directly controls the amount of input voltage ripple for

a given load current. Increasing the size of this capacitor

will reduce the input ripple.

To prevent large VOUT voltage steps during transient

load conditions, it is also recommended that a ceramic

capacitor be used to bypass VOUT. A typical value for this

capacitor is 4.7μF.

Multilayer Ceramic Chip Capacitors (MLCC) typically have

exceptional ESR performance. MLCCs combined with a

carefully laid out board with an unbroken ground plane will

yield very good performance and low EMI emissions.

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