SC5010
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 u IOUT
VIN u Ș
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
V – Forward conduction drop of output rectifying diode
D
L-peak
IN
IN
SW
For most applications, an inductor with value of 2.2μH to
22μH should be acceptable, (refer to the Typical
Application Circuit on page 21). The inductor peak current
must be less than its saturation rating. When the inductor
current is close to the saturation level, its inductance can
decrease 20% to 35% from the 0A value depending 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 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 3 lists some recommended
inductors and their vendors.
Table 3 — Recommended Inductors
Inductor
XFL4020, 2.2μH ~ 4.7μH
Web site
www.coilcraft.com
DR73, 4.7μH ~ 22μH; DR74, 4.7μH ~ 22μH
www.cooperet.com
IHLP-2525CZ-01, 4.7μH ~ 10μH
www.vishay.com
DS84LC, 4.7μH ~ 10μH; D62LCB, 4.7μH ~ 22μH
www.toko.co.jp
When the boost converter runs under DCM ( L < L ),
boundary
it takes the advantages of small inductance and quick
transient response; where as if the boost converter works
under CCM (L > L ), normally the converter has higher
boundary
efficiency.
When selecting an inductor, another factor to consider is
the peak-to-peak inductor current ripple, which is given
by the following equation.
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.
ǻIL
VIN u D
fSW u 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.
VRIPPLE – Peak to peak output ripple.
The ripple voltage should be less than 200mV (pk-pk) to
ensure good LED current sink regulation. For example, a
typical application where 20mA/channel current is
needed, the total output current for 8 channels will be
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