SC120 datasheet(18/21 Pages) SEMTECH

18 / 21 page

PRELIMINARY

SC120

The programmed value of V

OUT

is constant. I

PSAVE-entry

is a

polynomial function of V

. Equating dI

PSAVE-entry

/dV

= 0

and solving for V

reveals that there is one non-zero extre-

mum of this function, a maximum, at V

= 2/

OUT

. Applying

this value of V

OUT

max

entry

PSAVE

The value of the inductor determines the PSAVE entry

output load current. Evaluate I

PSAVE-entry

at the smallest and

largest expected values of V

. If the input range includes

2/3 V

OUT

, also determine I

PSAVE-entry-max

. If the largest V

exceeds approximately 90% of V

OUT

, then instead evaluate

PSAVE entry at V

= 0.9V

OUT

. Since PSAVE exit will require

an unusually high output load current at high V

. This was

explained in a previous section, therefore PSAVE entry

under this condition may not be relevant.

The inductor selection should also 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 allowable

ΔI

, based on the largest expected I

L-avg

the minimum n-channel FET current limit, and the

inductor tolerance. Ensure that in the worst case,

L-avg

ΔI

/2 < I

LIM(N)

These calculations include the parameter

η, efficiency.

Efficiency varies with V

, I

OUT

, and temperature. Estimate

using the plots provided in this datasheet, or from experi-

mental data, at the operating condition of interest when

computing the effect of a new inductor value on PSAVE

entry and I-limit margin.

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.

Table 1 lists the manufacturers of recommended inductor

options.

Table 1 — Recommended Inductors

Manufacturer/

Part #

Value

(μH)

DCR

(Ω)

Rated

Current

(mA)

Tolerance

(%)

Dimensions

LxWxH

(mm)

Murata

LQH43MN4R7K03L

4.7

0.4

500

4.5 x 3.2 x 2.6

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.

Additional output capacitance may be required for V

close to V

OUT

to reduce ripple in PSAVE mode and to ensure

stability in PWM mode, especially at higher output load

currents.

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 rec-

ommended due to possible reduction in capacitance seen

at the switching frequency of the SC120. Ceramic capaci-

tors of type Y5V are not recommended as their tempera-

ture coefficients make them unsuitable for this application.

Table 2 lists the manufacturer of the recommended

capacitor.

Table 2 — Recommended Capacitor

Manufacturer/

Part Number

Value

(μF)

Rated Voltage

(VDC)

Type

Case

Size

Murata

GRM21BR60J226ME39B

6.3

X5R

0805

Applications Information (continued)

SC120 Datasheet(PDF) 18 Page - Semtech Corporation

SC120 Datasheet(HTML) 18 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Datasheet Download

Link URL