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LTC3550 Datasheet, PDF (21/24 Pages) Linear Technology – Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter
APPLICATIO S I FOR ATIO
LTC3550
CF
VIA TO
VOUT
R2
VIA TO
GND
R1
VFB
VIA TO VCC
RUN
SW
CIN
L1
VCC
GND
COUT
VOUT
3550 F07
Figure 7. DC-DC Converter Suggested Layout
1. The power traces, consisting of the GND trace, the SW
trace and the VCC trace should be kept short, direct
and wide.
2. Does the VFB pin connect directly to the feedback resis-
tors? The resistive divider R1/R2 must be connected
between the (+) plate of COUT and ground.
3. Does the (+) plate of CIN connect to VCC as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
4. Keep the switching node, SW, away from the sensitive
VFB node.
5. Keep the (–) plates of CIN and COUT as close as
possible.
6. Solder the exposed pad on the backside of the package
to PC board ground for optimum thermal performance.
The thermal resistance of the package can be further
enhanced by increasing the area of the copper used for
PC board ground.
Design Example
As a design example, assume the LTC3550 is used in
a single lithium-ion battery-powered cellular phone
application. The battery is charged by either plugging
a wall adapter cable into the phone or putting the phone in
a USB cradle. The optimum charge current for this parti-
cular lithium-ion battery is determined to be 800mA. The
buck regulator output voltage needs to be 1.8V.
Starting with the charger, choosing RIDC to be 1.24k
programs the charger for 806mA. Choosing RIUSB to
be 2.1k programs the charger for 475mA when charging
from the USB cradle, ensuring that the charger never
exceeds the 500mA maximum current supplied by the
USB port. A good rule of thumb for ITERMINATE is one-
tenth the full charge current, so RITERM is picked to be
1.24k (ITERMINATE = 80mA).
Moving on to the step-down converter, VCC will be pow-
ered from the battery which can range from a maximum
of 4.2V down to about 2.7V. The load current requirement
3550fa
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