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LTC4080

4080fb

APPLICATIO S I FOR ATIO

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 nor-

mal operation. To minimize radiated noise, use a toroid,

or shielded pot core inductors in ferrite or permalloy

materials. Table 1 shows a list of several inductor manu-

facturers.

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. The 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.

There are several types of ceramic capacitors with con-

siderably different characteristics. Y5V and X5R ceramic

capacitors have apparently higher packing density but

poor performance over their rated voltage or temperature

ranges. Under given voltage and temperature conditions,

Y5V, X5R and X7R ceramic capacitors should be com-

pared directly by case size rather than speciﬁed value for

a desired minimum capacitance. Some manufacturers

provide excellent data on their websites about achiev-

able capacitance. Table 2 shows a list of several ceramic

capacitor manufacturers.

Table 2. Recommended Ceramic Capacitor Manufacturers

Taiyo Yuden

www.t-yuden.com

AVX

www.avxcorp.com

Murata

www.murata.com

TDK

www.tdk.com

Board Layout Considerations

To be able to deliver maximum charge current under all

conditions, it is critical that the exposed metal pad on the

backside of the LTC4080’s package has a good thermal

contact to the PC board ground. Correctly soldered to a

2500mm2 double-sided 1 oz. copper board, the LTC4080

has a thermal resistance of approximately 43°C/W. Failure

to make thermal contact between the exposed pad on the

backside of the package and the copper board will result

in thermal resistances far greater than 43°C/W.

Furthermore due to its high frequency switching circuitry,

it is imperative that the input capacitor, BAT pin capaci-

tor, inductor, and the output capacitor be as close to the

LTC4080 as possible and that there is an unbroken ground

plane under the LTC4080 and all of its high frequency

components.

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