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POWER MANAGEMENT

7

SC191

Table 1: Output Filter Combinations

The selection of smaller inductor and 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.

Inductor Selection

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:

V

OUT

V

OUT

ΔI

L × f

OSC

V

IN

This equation demonstrates the relationship between V

V

efficiency, the inductor should have a low DC resistance.

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 described by the

following equation:

ΔI

L

I

2

L (μH)

C

4.7

4.7

4.7

10

10

10

Alternatively, the inductor saturation current should be

greater than the switch current limit. Final inductor selec-

tion will depend on various design considerations such

as efficiency, EMI, size and cost (see Table 2 for a list of

practical inductor options).

CIN Selection

To prevent large input voltage ripple caused by discon-

tinuous source input current, a low ESR ceramic capacitor

is required. A minimum value of 4.7μF should be used

for sufficient input voltage filtering, and a 10μF capacitor

is strongly recommended for optimum input voltage fil-

tering. Input voltage ripple of approximately 1mV can be

achieved when CIN = 10μF and the front-end LDO regu-

lator is active and pre-regulating the input supply to the

switching regulator.

COUT Selection

A 10μF ceramic capacitor is recommended for the out-

put filter capacitor. Output voltage ripple is mainly deter-

mined by the filter capacitor in the following equation:

ΔV

X7R or X5R ceramic dielectric capacitors should be used

because of their low ESR and superior temperature and

voltage characteristics. Y5V ceramic capacitors should

be avoided due to their widely-varying temperature coef-

ficients.

Manufacturer

Part Number

Value

(μH)

DCR

Ω

Rated Current

(A) (40°C rise)

I

L drop 25%

Tolerance

(± %)

Dimensions

(LxWxH mm)

TDK

LDR655312T-4R7W

4.7

0.206

0.9

-

20

6.5x5.3x1.2

Sumida

CDRH3D16LD

4.7

0.073

0.68

-

30

4x4x1.8

Taiyo Yuden

LMNP04SB100M

10

0.066

0.9

-

20

5x5x2

Coilcraft

LPS3015

10

0.044

-

0.65

20

3x3x1.5

Coilcraft

LP06610-103M

10

0.41

-

0.8

20

6.9x3.8x1.0

Table 2: Recommended Inductors

Applications Information (Cont.)