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SC5014 View Datasheet(PDF) - Semtech Corporation

Part Name
Description
MFG CO.
'SC5014' PDF : 34 Pages View PDF
SC5014
Components Selection
Inductor Selection
The choice of the inductor affects the converter’s steady
state operation, transient response, and its loop stability.
Special attention needs to be paid to three specifications
of the inductor, its value, its DC resistance and saturation
current. The inductor’s inductance value also determines
the inductor ripple current. The boost converter will
operate in either CCM (Continuous Conduction Mode) or
DCM (Discontinuous Conduction Mode) depending on its
operating conditions. The inductor DC current or input
current can be calculated using the following equation.
IIN
=
VOUT × IOUT
VIN × η
IIN - Input current;IOUT – Output current;
VOUT – Boost output voltage;
VIN – Input voltage;
η – Efficiency of the boost converter
Then the duty ratio under CCM is shown by the following
equation.
D = VOUT VIN + VD
VOUT + VD
VD – Forward conduction drop of output rectifying diode
When the boost converter runs under DCM ( L < Lboundary),
it has the advantages of small inductance and quick tran-
sient response; where as if the boost converter works
under CCM (L > Lboundary), normally the converter has higher
efficiency.
When selecting an inductor, another factor to consider is
the peak-to-peak inductor current ripple, which is given
by the following equation:
ΔI
L
=
V
IN
×
D
f
SW
×L
Usually this peak-to-peak inductor current ripple can be
chosen between 30% to 50% of the maximum input DC
current. This gives the best compromise between the
inductor size and converter efficiency. The peak inductor
current can be calculated using the following equation:
IL-peak
= IIN
+
VIN ×D
2 × fSW ×L
For most applications, an inductor with a value of 2.2µH to
22µH should be acceptable, (refer to the detailed applica-
tion circuit on page 23, Figure 2). The inductor peak
current must be less than its saturation rating. When the
inductor current is close to the saturation level, its induc-
tance can decrease 20% to 35% from the 0A value depend-
ing on the vendor specifications. Using a small value
inductor forces the converter in DCM, in which case the
inductor current ramps down to zero before the end of
each switching cycle. It reduces the boost converter’s
maximum output current and produces a larger input
voltage ripple. The DCR of the inductor plays a significant
role for the total system efficiency and usually there is a
trade-off between the DCR and size of the inductor. Table
4 lists some recommended inductors and their vendors.
Table 4. Recommended Inductors
Inductor
HCM0703, 2.2uH~10uH
Vendor Website
www.cooperindustries.com
IHLP-2525CZ-01, 4.7uH~10uH www.vishay.com
MLPC0730L, 2.2uH~4.7uH
www.nec-tokin.com/english
Output Capacitor Selection
The next design task is targeting the proper amount of
output ripple voltage due to the constant-current LED
loads. Usually X5R or X7R ceramic capacitor is recom-
mended. The ceramic capacitor minimum capacitance
needed for a given ripple can be estimated using the fol-
lowing equation:
( ) C
OUT
=
V
OUT
V
IN
×
I
OUT
V
OUT
×f
×V SW RIPPLE
20
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