SC441AEVB datasheet(16/20 Pages) SEMTECH

16 / 20 page

SC441A

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The inductor peak current is,

The converter will work in CCM if L > L

boundary. Generally

the converter has higher efficiency under CCM and the

inductor peak current is,

For many applications, an inductor with value of 4.7µH to

22µH should be fine, such as for the typical case shown

on page . 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 SC44A. 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

Output Capacitor Selection

The next task in SC44A 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,

RIPPLE

OUT

)

RIPPLE – Peak to peak output ripple;

OUT – Output current;

OUT – Boost output voltage;

IN – Input voltage;

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

Phone

Website

Kemet

408-986-0424

www.kemet.com

Murata

84-237-43

www.murata.com

Taiyo Yuden

408-573-450

www.t-yuden.com

Output Rectifying Diode Selection

Schottky diodes are the ideal choice for SC44A due to

their low forward voltage drop and fast switching speed.

Table 4 shows several different Schottky diodes that work

well with the SC44A. 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

Vendor

SS23

SS24

Vishay

www.vishay.com

peak

⋅

−

Inductor

Website

DR74, 4.7μH ~ 5μH

www.cooperet.com

IHLP-2525CZ-0, 4.7μ ~ 0μH

www.vishay.com

DS85LC, 6.8μH ~ 0μH

www.tokoam.com

Applications Information (continued)

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