13
2007 Semtech Corp.
www.semtech.com
SC4505
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 I
O1
and I
O2
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.
