SC5010H View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC5010H
Semtech Corporation
'SC5010H' PDF : 36 Pages View PDF
SC5010H
Components Selection (continued)
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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.
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
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.
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 40mA/channel current is
needed, the total output current for 8 channels will be
320mA, and typically a configuration of 3x 4.7µF output
capacitors is recommended.
During load transient, the output capacitor supplies or
absorbs additional current before the inductor current
reaches its steady state value. Larger capacitance helps
with the overshoot/undershoots during load transient
and loop stability.
Input Capacitor Selection
X5R or X7R ceramic capacitor is recommended for input
bypass capacitor. A 1µF capacitor is sufficient for the VCC
input. Bypass the VIN input with a 4.7µF or larger ceramic
capacitor.
Output Freewheeling Diode Selection
Schottky diodes are the ideal choice for SC5010H due to
their low forward voltage drop and fast switching speed.
Table 4 shows several different Schottky diodes that work
properly with the SC5010H. Verify that the diode has a
voltage rating greater than the maximum possible output
voltage. The diode conducts current only when the power
switch is turned off. The diode must be rated to handle the
average output current. A diode rated for 1A average
current will be sufficient for most designs.
Table 4 — Recommended Rectifier Diodes
Rectifier Diode
DFLS140
Vendor Web site
www.diodes.com
External Power MOSFET Selection
The boost converter in SC5010H uses an external power
MOSFET to regulate the output voltage and output power
to drive LED loads. This boost switching structure has an
advantage in that the SC5010H is not directly exposed to
high voltage, only the external power MOSFET, freewheel-
ing diode and the inductor will be exposed to the output
voltage. The external power MOSFET should be selected
with its voltage rating higher than the output voltage by
minimum 30%. The current rating should be enough to
handle the inductor peak current. Low RDS(ON) MOSFETs are
preferred for achieving better efficiency.
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