SC446TETRT Datasheet, PDF (15/19 Pages) Samtec, Inc – High Efficiency Integrated Driver for 3-Strings of 100mA LEDs

SC446
Applications Information (continued)
The inductor peak current is,
I _{L − peak}
=
V_IN ⋅ D
F_S ⋅ L
The converter will work in CCM if L > L . _boundary Generally
the converter has higher efficiency under CCM and the
inductor peak current is,
,_/SHDN
,_,1

9_,1 ˜ '
˜ )₆ ˜ /
For many applications, an inductor with value of 4.7µH to
22µH should be fine, such as for the typical case shown
on page 1. 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 under
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
large input voltage ripple. An inductor with larger
inductance will reduce the bandwidth of the feedback
loop, possibly higher DC resistance (DCR). Inductor’s DCR
plays a significant role for the total efficiency since the
power transistor is integrated inside the SC446. Of course,
there is a trade-off between the DCR and inductor size.
Table 2 lists recommended inductors and their vendors.
Table 2. Recommended Inductors
Inductor
DR74, 4.7μH ~ 15μH
IHLP-2525CZ-01, 4.7μ ~ 10μH
DS85LC, 6.8μH ~ 10μH
Website
www.cooperet.com
www.vishay.com
www.tokoam.com
Output Capacitor Selection
The next task in SC446 design is targeting the proper
amount of ripple voltage due to the constant-current
LED loads. The two error amplifiers that control the PWM
converter sense the delta between requested current
and actual current in each output current regulator. On
a cycle-by-cycle basis, a small amount of output ripple
ensures good sensing and tight regulation, while the
output current regulators keep each LED current at a fixed
value. Overall, this allows usage of small output caps while
ensuring precision LED current regulation. Although
the mechanics of regulation and frequency dependence
may be complex, actual selection of output capacitor can
be simplified because this capacitor is mainly selected
for the output ripple of the converter. Assume a ceramic
capacitor is used. The minimum capacitance needed for a
given ripple can be estimated by,
C_OUT
(V_OUT  VI_N ) x I_OUT
V_OUT ˜ F_S ˜ V_RIPPLE
V_RIPPLE – Peak to peak output ripple;
I_OUT – Output current;
V_OUT – Boost output voltage;
V_IN – Input voltage;
F_S – Switching frequency.
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 and undershoots during load transient,
and loop stability. Recommended ceramic capacitor
manufacturers are listed in Table 3.
Table 3. Recommended Ceramic Capacitor
Manufacturers
Vendor
Kemet
Murata
Taiyo Yuden
Phone
408-986-0424
814-237-1431
408-573-4150
Website
www.kemet.com
www.murata.com
www.t-yuden.com
Output Rectifying Diode Selection
Schottky diodes are the ideal choice for SC446 due to their
low forward voltage drop and fast switching speed. Table
4 shows several different Schottky diodes that work well
with the SC446. Make sure that the diode has a voltage
rating greater that the possible maximum ouput voltage.
The diode conducts current only when the power switch
is turned off. A diode of 2A will be sufficient for most
designs.
Table 4. Recommended Rectifying Diodes
Part
SS23
SS24
Vendor
Vishay
www.vishay.com
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