SC5014 datasheet(21/34 Pages) SEMTECH

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SC5014

Application Information (continued)

RIPPLE–Peaktopeakoutputripple.

The ripple voltage should be less than 200mV (pk-pk) to

ensure good LED current sink regulation. For example, a

typical application where 20mA/channel current is

needed, the total output current for 4 channels will be

480mA, and 6x 4.7µF capacitors are 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/undershoot during load transient and

loop stability.

Input Capacitor Selection

X5R or X7R ceramic capacitor is recommended for input

bypass capacitor. A µF capacitor is sufficient for the VCC

input. Bypass the VIN input with a 0µF or larger ceramic

capacitor.

Output Freewheeling Diode Selection

Schottky diodes are the ideal choice for the SC504 due to

their low forward voltage drop and fast switching speed.

Table 5 shows several different Schottky diodes that work

properly with the SC504. 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 A average

current will be sufficient for most designs.

Table 5. Recommended Rectifier Diodes

Rectifier Diode

Vendor Website

DFLS40

www.diodes.com

SS4/5/6, SS24/25/26

www.vishay.com

External Power MOSFET Selection

The boost converter in SC504 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 SC504 is not exposed to high

voltage. Only the external power MOSFET, freewheeling

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 R

DS(on) MOSFETs

are preferred for achieving better efficiency.

The GD (gate driver) on SC504 provides A (peak) current

driving capability which is suitable for most MOSFETs for

high frequency operation. The average current required to

drive the MOSFET is given by the following equation.

GATE=QGxfSW

G—Gatecharge

The R

DS(ON) and its RMS current IS_RMS of the power MOSFET

will generate the conduction loss using the following

equation.

COND=IS_RMS

DS(on)

The MOSFET’s switch loss can be calculated using the fol-

lowing equation.

SW=½xVINxIL_PEAKxfSWx(TON+TOFF)

WhereT

ONandTOFFaretheMOSFET’sonandofftime

and they can be estimated by the following

equations.

(

)

(

)

plateau

−

(

)

plateau

OFF

Where t

r, tf, Qgd and Vplateau can usually be found from data-

sheet of the selected MOSFET. R

g is the resistance of the

optional resistor connected in series on the gate of the

MOSFET.

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