LTC3444_15 Datasheet, PDF(7/20 Page) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter for WCDMA Applications

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LTC3444_15 Datasheet, PDF (7/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter for WCDMA Applications

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OPERATIO

The LTC3444 is a highly efficient, fixed frequency, buck-

boost DC/DC converter, which operates from input volt-

ages above, below, and equal to the output voltage. The

topology incorporated in the IC provides a continuous

transfer function through all operating modes, making the

product ideal for single Lithium-Ion or multi-cell applica-

tions where the output voltage can vary over a wide range.

The LTC3444 is designed to provide dynamic voltage

control in space constrained 3G WCDMA applications.

Due to the high operating frequency and integrated loop

compensation a complete WCDMA application requires

only six additional components; input and output capaci-

tors (ceramic), an inductor, and three resistors. The high

speed error amplifier and integrated loop compensation

provide the fast transient response required to slew the RF

power amplifierâs voltage rail from standby to transmit

and transmit to standby levels in < 25Î¼s while minimizing

output overshoot or undershoot.

Efficiency under low output voltage conditions

(standby mode) is improved by using an N-channel

LTC3444

MOSFET in parallel to P-channel MOSFET switch D.

This parallel MOSFET eliminates the need for an external

Schottky. Output overvoltage protection protects the RF

power amplifier from voltages greater than 5.5V.

When used with the proper inductance and output capaci-

tance, the LTC3444 internal compensation is designed to

be consistent with the transient requirements of a typical

WCDMA application. External compensation can be used

with other combinations of inductance and output capaci-

tance, however, the transient response may not be

consistent with typical WCDMA requirements.

Output voltage programming is accomplished via a sum-

ming resistor input to the feedback resistive divider string.

The output voltage varies inversely with the command

voltage. When using the internal loop compensation,

resistor R1 in the feedback resistive divider string must be

340k. There are no constraints on R1 when using external

compensation. However, lower value resistors will de-

crease the resistance value required for programming the

output voltage. Care must be taken not to load down the

control voltage source.

3444fb

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