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LTC3413

3413fc

APPLICATIONS INFORMATION

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current

ΔIL increases with higher VIN or VOUT and

decreases with higher inductance.

Δ=

⎛

⎝⎜

⎞

⎠⎟

OUT

()( )

–

Having a lower ripple current reduces the core losses in

the inductor, the ESR losses in the output capacitors and

the output voltage ripple. Highest efﬁciency operation is

achieved at low frequency with small ripple current. This,

however, requires a large inductor.

A reasonable starting point for selecting the ripple current

ΔIL = 0.4(IMAX). The largest ripple current occurs at the

highest VIN. To guarantee that the ripple current stays

below a speciﬁed maximum, the inductor value should

be chosen according to the following equation:

OUT

LMAX

OUT

IN MAX

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

()

–

Inductor Core Selection

Once the value for L is known, the type of inductor must

be selected. Actual core loss is independent of core size

for a ﬁxed inductor value, but it is very dependent on the

inductance selected. As the inductance increases, core

losses decrease. Unfortunately, increased inductance

requires more turns of wire and therefore copper losses

will increase.

Ferrite designs have very low core losses and are used

often 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. This results in an abrupt increase in inductor

ripple current and consequent output voltage ripple. Do

not allow the core to saturate!

Different core materials and shapes will change the size/

current and price/current relationship of an inductor. Toroid

or shielded pot cores in ferrite or permalloy materials are

small and don’t radiate much energy, but generally cost

more than powdered iron core inductors with similar

characteristics. The choice of which style inductor to use

mainly depends on the price versus size requirements and

any radiated ﬁeld/EMI requirements.

Table 1 shows some recommended surface mount induc-

tors for LTC3413 applications.

Table 1. Recommended Surface Mount Inductors

MANUFACTURER

PART NUMBER

VALUE

(μH)

DCR

(mΩ)

Murata

LQH55DNR47M01

0.47

13.0

Vishay/Dale

IHLP252CZPJR47M01

0.47

4.2

Pulse

P1166.681T

0.44

6.0

Cooper

SD20-R47

0.47

20.0

CIN and COUT Selection

The input capacitance, CIN,isneededtoﬁlterthetrapezoidal

wave current at the source of the top MOSFET. To prevent

large voltage transients from occurring, a low ESR input

capacitor sized for the maximum RMS current should be

used. The maximum RMS current is given by:

RMS

OUT MAX

OUT

()

–1

This formula has a maximum at VIN = 2VOUT, where IRMS =

IOUT/2.Thissimpleworst-caseconditioniscommonlyused

for design because even signiﬁcant deviations do not offer

much relief. Note that ripple current ratings from capacitor

manufacturers are often based on only 2000 hours of life

which makes it advisable to further derate the capacitor,

or choose a capacitor rated at a higher temperature than

required. Several capacitors may also be paralleled to meet

size or height requirements in the design.

The selection of COUT is determined by the effective series

resistance (ESR) that is required to minimize voltage ripple

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