LTC3550 datasheet(21/24 Pages) LINER

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LTC3550

3550fa

APPLICATIO S I FOR ATIO

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

Figure 7. DC-DC Converter Suggested Layout

VCC

VFB

VOUT

RUN

GND

VIA TO

VOUT

VIA TO

GND

CIN

3550 F07

VIA TO VCC

COUT

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