SC121 datasheet(19/23 Pages) SEMTECH

19 / 23 page

SC121

Note that this is a negative quantity, since V

OUT

> V

and

0 < D < 1. For a constant load in steady-state, the inductor

current must satisfy

ΔI

L-on

ΔI

L-off

= 0. Substituting the two

expressions and solving for D, obtain D = 1 – V

OUT

Using this expression, and the positive valued expression

ΔI

L-on

for current ripple amplitude, obtain expanded

expression for I

L-max

and I

L-min

OUT

max,min

From this result, obtain an alternative expression for

ΔI

OUT

min

max

The inductor selection should consider the n-channel FET

current limit for the expected range of input voltage and

output load current. The largest I

L-avg

will occur at the

expected smallest V

and largest I

OUT

. Determine the

largest expected

ΔI

. Then for the largest expected I

L-avg

ensure that the n-channel FET current limit is not exceed.

That is, for the minimum n-channel FET current limit,

worst case inductor tolerance, highest expected output

current, and lowest expected V

, ensure that

L-max

= I

L-avg

ΔI

/2 < I

LIM(N)

Many of these equations include the parameter

η, effi-

ciency. Efficiency varies with V

, I

OUT

, and temperature.

Estimate

η using the plots provided in this datasheet, or

from experimental data, at the operating condition of

interest.

Any chosen inductor should have low DCR compared to

the R

DS-ON

of the FET switches to maintain efficiency,

though for DCR << R

DS-ON

, further reduction in DCR will

provide diminishing benefit. The inductor I

SAT

value should

exceed the expected I

L-max

. The inductor self-resonant fre-

quency should exceed 5×f

osc

. Any inductor with these

properties should provide satisfactory performance.

L = 4.7μH should perform well for most applications.

The following table lists the manufacturers of recom-

mended inductor options. The specification values shown

are simplified approximations or averages of many device

parameters under various test conditions. See manufac-

turers’ documentation for full performance data.

Manufacturer/

Part #

Value

(μH)

DCR

(Ω)

Rated

Current

(mA)

Tolerance

(%)

Dimensions

LxWxH

(mm)

Murata

LQM31PN4R7M00

4.7

0.3

700

3.2 x 1.6 x 0.95

Coilcraft

XFL2006-472

4.7

0.7

500

2 x 2 x 0.6

Capacitor Selection

Input and output capacitors must be chosen carefully to

ensure that they are of the correct value and rating. The

output capacitor requires a minimum capacitance value

of 10μF at the programmed output voltage to ensure sta-

bility over the full operating range. This must be consid-

ered when choosing small package size capacitors as the

DC bias must be included in their derating to ensure this

required value. For example, a 10μF 0805 capacitor may

provide sufficient capacitance at low output voltages but

may be too low at higher output voltages. Therefore, a

higher capacitance value may be required to provide the

minimum of 10μF at these higher output voltages.

Low ESR capacitors such as X5R or X7R type ceramic

capacitors are recommended for input bypassing and

output filtering. Low-ESR tantalum capacitors are not

recommended due to possible reduction in capacitance

seen at the switching frequency of the SC121. Ceramic

capacitors of type Y5V are not recommended as their tem-

perature coefficients make them unsuitable for this appli-

cation. The following table lists recommended capacitors.

For smaller values and smaller packages, it may be neces-

sary to use multiples devices in parallel.

Manufacturer/

Part Number

Value

(μF)

Rated Volt-

age (VDC)

Type

Case

Size

Case

Height

(mm)

Murata

GRM21BR60J226ME39B

6.3

X5R

0805

1.25

Murata

GRM31CR71A226KE15L

X7R

1206

1.6

Murata

GRM185R60G475ME15

4.7

X5R

0603

0.5

TDK

C2012X5R1A226M

X5R

0805

0.85

Taiyo Yuden

JMK212BJ226MG-T

X5R

0805

1.25

Applications Information (continued)

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