SC120 SEMTECH [Semtech Corporation], SC120 Datasheet - Page 18

• Manufacturer Part Number: SC120
• Description: Low Voltage Synchronous Boost Converter
• Manufacturer: SEMTECH [Semtech Corporation]

Applications Information (continued)
The programmed value of V
polynomial function of V
and solving for V
mum of this function, a maximum, at V
this value of V
The value of the inductor determines the PSAVE entry
output load current. Evaluate I
largest expected values of V
V
exceeds approximately 90% of V
PSAVE entry at V
an unusually high output load current at high V
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
expected smallest V
largest allowable ΔI
the minimum n-channel FET current limit, and the
inductor tolerance. Ensure that in the worst case,
I
These calculations include the parameter η, efficiency.
Effi ciency varies with V
using the plots provided in this datasheet, or from experi-
mental data, at the operating condition of interest when
computing the eff ect of a new inductor value on PSAVE
entry and I-limit margin.
Any chosen inductor should have low DCR, compared to
the R
though for DCR << R
provide diminishing benefi t. The inductor I
exceed the expected I
quency should exceed 5×f
properties should provide satisfactory performance.
Table 1 lists the manufacturers of recommended inductor
options.
L-avg
IN
=
I
+ ΔI
PSAVE
2
DS-ON
/
3
L
V
entry
/2 < I
OUT
of the FET switches, to maintain efficiency,
, also determine I
max
IN
LIM(N)
,
IN
IN
.
reveals that there is one non-zero extre-
= 0.9V
L
L
, based on the largest expected I
T
IN
DS-ON
L-max
IN
and largest I
, I
27
2
OUT
OUT
. The inductor self-resonant fre-
IN
, further reduction in DCR will
. Equating dI
. Since PSAVE exit will require
, and temperature. Estimate η
osc
IN
V
OUT
. If the input range includes
PSAVE-entry-max
. Any inductor with these
OUT
PSAVE-entry
is constant. I
OUT
, then instead evaluate
OUT
IN
L-avg
at the smallest and
=
. Determine the
. If the largest V
2
PSAVE-entry
will occur at the
/
SAT
3
V
OUT
value should
PSAVE-entry
IN
. Applying
. This was
/dV
PRELIMINARY
IN
L-avg
is a
= 0
IN
,
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 suffi cient 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
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 fi ltering. 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 coeffi cients make them unsuitable for this application.
Table 2 lists the manufacturer of the recommended
capacitor.
Manufacturer/
Part #
Murata
LQH43MN4R7K03L
Manufacturer/
Part Number
Murata
GRM21BR60J226ME39B
OUT
Table 1 — Recommended Inductors
Table 2 — Recommended Capacitor
to reduce ripple in PSAVE mode and to ensure
Value
(μH)
4.7
DCR
(Ω)
0.4
Value
(μF)
22
Current
Rated
(mA)
500
Rated Voltage
(VDC)
6.3
Tolerance
(%)
10
Type
X5R
SC120
4.5 x 3.2 x 2.6
Dimensions
LxWxH
(mm)
Case
0805
Size
18
IN