SC441DEVB datasheet(16/21 Pages) SEMTECH

16 / 21 page

SC441D

IN - Input current;

OUT – Output current;

OUT – Boost output voltage;

IN – Input voltage;

η – Efficiency of the boost converter.

Then the duty ratio is,

D – Forward conduction drop of the output rectifying

diode

WhentheboostconverterrunsinDCM(L<L

boundary), it takes

the advantages of small inductance and quick transient

response while avoiding the bandwidth limiting instability

of the RHP zero found in CCM boost converters.

The inductor peak current is,

peak

⋅

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 SC44D. 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 SC44D 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. Althoughthe

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.

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)

SC441DEVB 数据表(PDF) 16 Page - Semtech Corporation

SC441DEVB 数据表(HTML) 16 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

数据表下载