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LTC3633

3633f

APPLICATIONS INFORMATION

Ferrite designs exhibit very low core loss and are pre-

ferred at high switching frequencies, so design goals

can concentrate on copper loss and preventing satura-

tion. 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, so it is important to ensure that

the core will not saturate.

Different core materials and shapes will change the size/cur-

rent 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 gives a

sampling of available surface mount inductors.

Table 1. Inductor Selection Table

INDUCTANCE

(μH)

DCR

(mΩ)

MAX

CURRENT

(A)

DIMENSIONS

(mm)

HEIGHT

(mm)

Würth Electronik WE-HC 744312 Series

0.25

0.47

0.72

1.0

1.5

2.5

3.4

7.5

9.5

10.5

7 × 7.7

3.8

Vishay IHLP-2020BZ-01 Series

0.22

0.33

0.47

0.68

5.2

8.2

8.8

12.4

11.5

5.2 × 5.5

Toko FDV0620 Series

0.20

0.47

1.0

4.5

8.3

18.3

12.4

9.0

5.7

7 × 7.7

2.0

Coilcraft D01813H Series

0.33

0.56

1.2

7.7

5.3

6 × 8.9

5.0

TDK RLF7030 Series

1.0

1.5

8.8

9.6

6.4

6.1

6.9 × 7.3

3.2

CIN and COUT Selection

The input capacitance, CIN, is needed to ﬁlter the trapezoi-

dal wave current at the drain of the top power MOSFET.

To prevent large voltage transients from occurring, a low

ESR input capacitor sized for the maximum RMS current is

recommended. The maximum RMS current is given by:

RMS

OUT MAX

OUT

−

()

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

IRMS ≅ IOUT/2. This simple worst case condition is com-

monly used 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 de-

rate 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. For low input voltage

applications, sufﬁcient bulk input capacitance is needed

to minimize transient effects during output load changes.

Even though the LTC3633 design includes an overvoltage

protection circuit, care must always be taken to ensure

input voltage transients do not pose an overvoltage hazard

to the part.

The selection of COUT is determined by the effective series

resistance (ESR) that is required to minimize voltage ripple

and load step transients as well as the amount of bulk

capacitance that is necessary to ensure that the control

loop is stable. Loop stability can be checked by viewing

the load transient response. The output ripple, ΔVOUT, is

approximated by:

VOUT < IL ESR+

8• f • COUT

When using low-ESR ceramic capacitors, it is more useful

to choose the output capacitor value to fulﬁll a charge stor-

age requirement. During a load step, the output capacitor

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