LTC3444EDD#PBF Datasheet LTC3444EDD#TRPBF, LTC3444EDD#PBF | P7-P9

LTC3444

3444fb

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 amplifier’s voltage rail from standby to transmit

and transmit to standby levels in < 25μs while minimizing

output overshoot or undershoot.

Efficiency under low output voltage conditions

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

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.

LTC3444

3444fb

Error Amp

The LTC3444 error amplifier is a voltage mode amplifier.

The internal loop compensation is designed to optimize

transient response to control input change when the

proper output L-C and R1 values are used. Refer to

Figure 1.

Internal loop compensation is selected by grounding the

pin. The loop is designed to exhibit a single pole roll-off

(–20dB/dec) with a crossover frequency of ~100KHz.

External compensation can be used by connecting the

compensation components from FB to V

. The V

must be allowed to float when using external compensa-

tion. If external compensation is used the internal com-

pensation is automatically disabled. A Type III compensa-

tion network is typically required to meet the output

transient requirements of WCDMA.

During start-up, the ramp rate of the error amp output is

controlled to provide a soft-start function. Refer to

Figure 2.

Internal Current Limit

There are two different current limit circuits in the LTC3444.

The two circuits have internally fixed thresholds.

The first circuit sources current out of the FB pin to drop

the output voltage once the peak input current exceeds

2.5A minimum. During conditions where V

OUT

is near

ground, such as during a short circuit or during startup,

this threshold is cut in half, providing current foldback

protection.

The second circuit is a high-speed peak current limit

amplifier that shuts off P-channel MOSFET switch A if the

input current exceeds 3.5A typical. The delay to output for

this amplifier is typically 50ns.

OPERATIO

0.5μA

20μA

OUT

CONTROL

3444 F01

INTERNAL

COMPENSATION

NETWORK

GND = INTERNAL

OPEN = EXTERNAL

TO PWM

COMPARATORS

–

ERROR AMP

INT

1.22V

Figure 1. Error Amplifier with Compensation Select Function

LTC3444

3444fb

Reverse Current Limit

The LTC3444 always operates in forced continuous con-

duction mode. The reverse current limit amplifier moni-

tors the inductor current from the output through switch

D. Once the negative inductor current exceeds 3A mini-

mum, the LTC3444 will shut off switch D. The high reverse

current is required to meet the transient slew require-

ments for WCDMA power amplifiers.

Output Overvoltage Protection

The LTC3444 provides output overvoltage protection. If

the output voltage exceeds 5.3V typical, P-channel MOSFET

switches A and D are turned off and N-channel MOSFET

switches B and C are turned on. Normal switching will

resume once the output voltage drops below ~5.1V. If the

condition which caused the output overvoltage is still

present the output will charge up to 5.3V again and the

overvoltage cycle will be repeat. Normal output regulation

will resume once the condition responsible for the output

overvoltage is removed.

Soft-Start

The soft-start function is initiated when the SHDN pin is

brought above 1.4V and the LTC3444 is out of UVLO

(above minimum input operating specs). The LTC3444 is

enabled but the PWM duty cycle is clamped via the error

amp output. The soft-start time is internally set to 250μs

to minimize output overshoot. A detailed diagram of this

function is shown in Figure 2.

OPERATIO

20μA

SHDN

3444 F02

TO PWM

COMPARATORS

SOFT-START

CLAMP

–

ERROR AMP

1.22V

Figure 2. Soft-Start Circuitry

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P20

LTC3444EDD#PBF

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Description:

Switching Voltage Regulators Micropower Sych Buck-Boost DC/DC Converter

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