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 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC190

Applications Information (Cont.)

Output Filter

The SC190 series of synchronous step-down converters

have internal loop compensation. The internal

compensation is designed to work with a certain output

filter corner frequency defined by the equation:

The internal compensation is optimized to operate with

an output filter, L=4.7µH & COUT = 10µF. When selecting

output filter components the LC product should not vary

over a wide range.

Table 1: Output Filter combinations

L(µH)

C

4.7

4.7

4.7

10

10

10

Inductor Selection

As previously stated the value of the inductor should be in

the range 4.7µH to 10µH. The magnitude of the inductor

current ripple is dependant on the inductor value and can

be determined by the following equation:

This equation demonstrates the relationship between

V

higher inductance & increases with higher V

To maximize efficiency the inductor should have a low

DCR to minimize the conduction losses. As a minimum

requirement the DC current rating of the inductor should

be equal to the maximum load current plus half of the

inductor current ripple and can be determined by the

following equation:

The selection of smaller inductor & capacitor values will

move the corner frequency, having an impact on system

stability. Due to this issue the practical lower limit for the

inductor value is 4.7µH.

Alternatively, set the inductor saturation current to be

greater than the switch current limit as a maximum

limit. I

on various design considerations such as efficiency, EMI,

size and cost. Table 2 lists the manufacturers of practical

inductor options.

Table 2 - Recommended Inductors

Manufacturer

Part Number

Value

(µH)

DCR (Ω)

Rated

Current (A)

Saturation

Current (A)

Tolerance

%

Dimensions

(LxWxH mm)

TDK

LDR655312T-4R7W

4.7

0.206

0.9

-

20

6.5 x 5.3 x 1.2

Sumida

CDRH3D16LD

4.7

0.073

0.68

-

30

4 x 4 x 1.8

Taiyo Yuden

LMNP04SB100M

10

0.066

0.9

-

20

5 x 5 x 2

Coilcraft

LPS3015

10

0.044

-

0.65

20

3 x 3 x 1.5

Coilcraft

LP06610-103M

10

0.41

-

0.8

20

6.9 x 3.8 x 1.0

C

L

2

1

•

π

=

f

L

Vi

Vo

1

Vo

IL

•











 −

=

∆

2

IL

(max)

Iout

ILpk

∆

+

=