LTC3636 datasheet(13/28 Pages) LINER

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LTC3636/LTC3636-1

3636fb

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APPLICATIONS INFORMATION

core loss, greater ESR loss in the output capacitor, and

larger output voltage ripple. Generally, highest efficiency

operation is obtained at low operating frequency with

small ripple current.

A reasonable starting point is to choose a ripple current

of 2.4A which is about 40% of IOUT(MAX). Exceeding 60%

of IOUT(MAX) is not recommended. Note that the largest

ripple current occurs at the highest VIN. To guarantee that

ripple current does not exceed a specified maximum, the

inductance should be chosen according to:

L =

VOUT

f • ΔIL(MAX)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟ 1–

VOUT

VIN(MAX)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

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 is very dependent on the

inductance selected. As the inductance increases, core

losses decrease. Unfortunately, increased inductance

requires more turns of wire, leading to increased DCR

and copper loss.

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.

Differentcorematerialsandshapeswillchangethesize/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 FDVE0603 Series

0.33

0.47

0.68

0.75

2.7

3.7

6.2

8.5

15.9

15.6

10.4

10.9

9.5

7.4 × 6.7

3.0

Coilcraft XAL6030 Series

0.20

0.33

0.56

0.82

1.0

3.04

5.18

11.8

13.25

17.2

15.4

13.8

11.5

9.6

6.4 × 6.6

TDK SMP5030 Series

0.2

0.35

0.75

2.31

4.29

9.35

11.44

14.9

9.7

8.5

5.2 × 5

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

ESRinputcapacitorsizedforthemaximumRMScurrentis

recommended. The maximum RMS current is given by:

IRMS = IOUT(MAX)

VOUT VIN − VOUT

(

)

VIN

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.

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