SC121 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC121' PDF : 23 Pages View PDF
SC121
Applications Information (continued)
Note that this is a negative quantity, since V > V and
OUT
IN
0 < D < 1. For a constant load in steady-state, the inductor
current must satisfy ΔI + ΔI = 0. Substituting the two
L-on
L-off
expressions and solving for D, obtain D = 1 – V /V .
IN OUT
Using this expression, and the positive valued expression
ΔI = ΔI for current ripple amplitude, obtain expanded
L
L-on
expression for I and I .
L-max
L-min
ILmax,min
VOUT uIOUT
VIN u K
r T u VIN
2 u L VOUT
u
VOUT VIN
From this result, obtain an alternative expression for ΔIL.
'IL I I Lmax Lmin
T u VIN
L VOUT
u
VOUT VIN
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 will occur at the
L-avg
expected smallest V and largest I . Determine the
IN
OUT
largest expected ΔIL.
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
IN
I = I + ΔI /2 < I .
L-max
L-avg
L
LIM(N)
Many of these equations include the parameter η, effi-
ciency. Efficiency varies with V , I , and temperature.
IN OUT
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 of the FET switches to maintain efficiency,
DS-ON
though for DCR << R , further reduction in DCR will
DS-ON
provide diminishing benefit. The inductor I value should
SAT
exceed the expected I . The inductor self-resonant fre-
L-max
quency should exceed 5×fosc. 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/ Value DCR
Part #
(μH) (Ω)
Murata
LQM31PN4R7M00
Coilcraft
XFL2006-472
4.7 0.3
4.7 0.7
Rated
Current
(mA)
Tolerance
(%)
Dimensions
LxWxH
(mm)
700
20
3.2 x 1.6 x 0.95
500
20
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
Murata
GRM21BR60J226ME39B
Murata
GRM31CR71A226KE15L
Murata
GRM185R60G475ME15
TDK
C2012X5R1A226M
Taiyo Yuden
JMK212BJ226MG-T
Value
(μF)
22
22
4.7
22
22
Rated Volt-
age (VDC)
6.3
10
4
10
20
Type
X5R
Case
Size
Case
Height
(mm)
0805 1.25
X7R 1206 1.6
X5R 0603 0.5
X5R 0805 0.85
X5R 0805 1.25
19
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