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SC250

Step-Down DC-DC Converter with Bias LDO for RF Power Amplifiers

Semtech Corporation 

SC250

POWER MANAGEMENT

Applications Information (Cont.)

Under-Voltage Lockout

Under-voltage lockout protection is used to prevent

erroneous operation. As the input decreases, the device

shuts down when the voltage drops below 2.35V and will

not restart until the input voltage exceeds approximately

2.5V.

Inductor Selection

The SC250 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:

PRELIMINARY

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.

COUT Selection

The internal compensation is designed to work with a

certain output filter corner frequency defined by the

equation:

fC

1

2S L u COUT

' IL

V OUT ¨©§ 1



V OUT

V IN

¸¹·

L u f OSC

This equation demonstrates the relationship between

input voltage, output voltage, and inductor ripple current.

The inductor should have a low DC resistance 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:

'IL

I I  LPK

OUT (MAX )

2

Final inductor selection depends on various design

considerations such as efficiency, EMI, size, and cost.

Table 1 lists the manufacturers of practical inductor

options.

Table 1 — Recommended Inductors

Manufacturer/Part #

Value

(μH)

DCR Saturation

(Ω) Current (A)

Tolerance

(%)

Dimensions

LxWxH (mm)

BI Technologies

HM66304R7

4.7 0.072

1.32

Coilcraft

D01608C-472ML

4.7

0.09

1.5

TDK

VLCF4018T- 4R7N1R0-2

4.7

0.101

1.07

20

4.7 × 4.7 ×3.0

20

6.6 × 4.5 ×3.0

30

4.3 × 4.0 ×1.8

This single pole filter is designed to operate with an output

capacitor value of 4.7μF.

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 that is dependent

upon this ESR, as can be seen in the following equation:

'V 'I u ESR OUT(ESR)

L (ripple)

( COUT )

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 2 — Recommended Capacitors

Manufacturer/Part Number

Murata

GRM21BR60J226ME39L

Value

(μF)

22

Rated

Voltage

(VDC)

6.3

Type

X5R

Case Size

0805

Murata

GRM188R60J106MKE19

10

6.3

X5R

0603

TDK

C2012X5R0J106K

10

6.3

X5R

0603

Murata

GRM188R60J475KE19D

4.7

6.3

X5R

0603

CIN Selection

The source input current to a buck converter is non-

continuous. To prevent large input voltage ripple, a low

© 2006 Semtech Corp.

9

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