SC440AEVB datasheet(15/21 Pages) SEMTECH

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SC440A

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Applications Information (continued)

Output Capacitor Selection

The next task in SC440A 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;

– Input voltage;

– 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

814-237-1431

www.murata.com

Taiyo Yuden

408-573-4150

www.t-yuden.com

Output Rectifying Diode Selection

Schottky diodes are the ideal choice for SC440A due to

their low forward voltage drop and fast switching speed.

Table 4 shows several different Schottky diodes that work

well with the SC440A. 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 1A will be sufficient for

most designs.

Layout Guidelines

The SC440A contains a boost converter and the

placements of the power components outside the

SC440A should follow the layout guidelines of a general

boost converter. The application circuit on page 17 will

be used as an example. The layout illustration diagram is

shown on page 19. R5 and C7 form a decoupling filter for

the SC440A. C7 should be placed as close as possible to

the VIN and PGND to achieve the best performance. C6 is

the input power filtering capacitor for the boost converter

power train. L1 is the boost converter input inductor. D1 is

the output rectifying diode and it is recommended that a

Schottky diode be used for fast reverse recovery.

To minimize switching noise for the boost converter, the

output capacitor, C2, should be placed at the bottom, as

displayed on page 19, so that the loop formed by C2, D1,

and the internal switch, is the smallest. The output of the

boost converter is used to power up the LEDs. R6, C9 and

C10 (open, not used), form the compensation network for

the boost converter. C9 should return to analog ground.

Table 4. Recommended Rectifying Diodes

Part

Vendor

SS13

SS14

Vishay

www.vishay.com

C8 determines the soft-start time and should be connected

to analog ground. R8 is the output current programming

resistor for IO1 through IO6 and should return to analog

ground. IOGND should also be connected to AGND.

Since there is pad at the bottom of the SC440A for heat

dissipation, a copper area right underneath the pad is

used for better heat spreading. On the bottom layer of the

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