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© 2006 Semtech Corp.

www.semtech.com

SC194A

PRELIMINARY

POWER MANAGEMENT

Output voltage ripple is a combination of the voltage

ripple from the inductor current charging and discharging

the output capacitor and the voltage created from the

inductor current ripple through the output capacitor ESR.

Selecting an output capacitor with a low ESR will reduce

the output voltage ripple component, as can be seen in

the following equation:

ΔV

Capacitors with X7R or X5R ceramic dielectric are

strongly recommended for their low ESR and superior

temperature and voltage characteristics. Y5V capacitors

should not be used as their temperature coefficients

make them unsuitable for this application. Table 2 lists

the manufacturers of recommended capacitor options.

Table 1: Recommended Inductors

Manufacturer/Part #

Value

μH

DCR

Ω

Saturation

Current

A

Tolerance

±%

Dimensions

(LxWxH)

mm

BI Technologies

HM66304R7

4.7

0.072

1.32

20

4.7 × 4.7 × 3.0

Coilcraft

D01608C-472ML

4.7

0.09

1.5

20

6.6 × 4.5 × 3.0

TDK

VLCF4018T- 4R7N1R0-2

4.7

0.101

1.07

30

4.3 × 4.0 × 1.8

Table 2: Recommended Capacitors

Manufacturer/Part #

Value

μF

Rated

Voltage

VDC

Temperature

Characteristic

Case Size

Murata

GRM21BR60J226ME39L

22

6.3

X5R

0805

Murata

GRM188R60J106

MKE19

10

6.3

X5R

0603

TDK

C2012X5R0J106K

10

6.3

X5R

0603

Note: Where PSAVE operation is required 22μF must be used for C

Applications Information (Cont.)

The SC194A is designed for use with a 4.7μH inductor.

The magnitude of the inductor current ripple is dependent

on the inductor value and can be determined by the

following equation:

V

OUT

V

OUT

ΔI

L × f

OSC

V

IN

This equation demonstrates the relationship between

input voltage, output voltage, and inductor ripple current.

The inductor should have a low DCR to minimize the

conduction losses and maximize efficiency. 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 as shown by the following

equation:

ΔI

L

I

2

Final inductor selection will depend on various design

considerations such as efficiency, EMI, size and cost.

Table 1 lists the manufacturers of practical inductor

options.

C

The source input current to a buck converter is non-

continuous. To prevent large input voltage ripple a low

ESR ceramic capacitor is required. A minimum value of

10μF should be used for sufficient input voltage filtering

and a 22μF should be used for improved input voltage

filtering.

C

The internal compensation is designed to work with a

certain output filter corner frequency defined by the

equation:

This single pole filter is designed to operate with a

minimum output capacitor value of 10μF. Larger output

capacitor values will improve transient performance. If

PSAVE operation is required the minimum capacitor value

is 22μF.

1

2

π√L × C

OUT