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SC4505MLTRT

High Efficiency Boost Converter for Backlight/Flash LED Driver

Semtech Corporation 

POWER MANAGEMENT

Applications Information

To minimize switching noise for boost converter, the

output capacitor, C3, should be placed right at the bottom

as displayed in Figure 6b so that loop formed by C3, D5

and the SC4505 internal switch is the smallest. The

output of the boost converter is used to power up the

LEDs. The backlight LED string includes D6, D7, D8 and

D9. The flashligh/torch-light string is composed of D1,

D2, D3 and D4. C5 and C6 are the filtering capacitors

for the IO2 and IO1 pins and they are optional to

customers. If they are adopted, C5 should be placed as

close as possible to IO2 and PGND and C6 should be

placed as close as possible to IO1 and PGND. R2, C7

and C8 form the compensation network for the boost

converter. C7 should return to analog ground. C4, on

the bottom layer, determines the flash timeout duration.

It should be connected to analog ground. R3 and R4 are

the output current programming resistors for IO1 and IO2

respectively. R3 and R4 should return to analog ground.

Since there is pad at the bottom of the SC4505 for heat

dissipation, as shown in Figure 6a, a copper area right

underneath the pad is used for better heat spreading.

On the bottom layer of the board, another square copper

area, connected through vias to the top layer, is used

for better thermal performance. The pad at the bottom

of the SC4505 should be tied to the analog ground of

the SC4505. The analog ground should be kelvin

connected to the power ground near the input filtering

capacitors for better noise immunity as shown in Figure

6a.

SC4505

 2007 Semtech Corp.

13

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