LM3207TL datasheet(14/19 Pages) NSC

14 / 19 page

Operation Description

The LM3207 is a simple, step-down DC-DC converter with a

VREF LDO optimized for powering RF power amplifiers

(PAs) in mobile phones, portable communicators, and similar

battery powered RF devices. The DC-DC converter is de-

signed to allow the RF PA to operate at maximum efficiency

over a wide range of power levels from a single Lithium-Ion

battery cell. The DC-DC is based on current-mode buck

architecture, with synchronous rectification for high effi-

ciency. It is designed for a maximum load capability of

650mA in PWM mode.

Maximum load range may vary from this depending on input

voltage, output voltage and the inductor chosen.

The device has two pin-selectable operating modes required

for powering RF PAs in mobile phones and other sophisti-

cated portable devices. Fixed-frequency PWM operation of-

fers regulated output at high efficiency while minimizing in-

terference with sensitive IF and data acquisition circuits.

Shutdown mode turns the device off and reduces battery

consumption to 0.01uA (typ).

Efficiency is typically around 95% for a 400mA load with

3.9V

IN, 3.4VOUT. The output voltage is dynamically program-

mable from 0.8V (typ) to 3.6V (typ) by adjusting the voltage

on the control pin without the need for external feedback

resistors. This ensures longer battery life by being able to

change the PA supply voltage dynamically depending on its

transmitting power.

Additional features include current overload protection, and

thermal overload shutdown.

The LM3207 is constructed using a chip-scale 9-pin micro

SMD package. This package offers the smallest possible

size, for space-critical applications such as cell phones,

where board area is an important design consideration. Use

of a high switching frequency (2MHz) reduces the size of

external components. As shown in Figure 1, only four exter-

nal power components are required for implementation. Use

of a micro SMD package requires special design consider-

ations for implementation. (See Micro SMD Package Assem-

bly and use in the Applications Information section.) The fine

bump-pitch requires careful board design and precision as-

sembly equipment. Use of this package is best suited for

opaque-case applications, where its edges are not subject to

high-intensity ambient red or infrared light. Also, the system

controller should set EN low during power-up and other low

supply voltage conditions. (See Shutdown Mode in the De-

vice Information section.)

Circuit Operation (DC-DC

Converter)

Referring to Figure 1 and Figure 2, the LM3207 operates as

follows. During the first part of each switching cycle, the

control block in the LM3207 turns on the internal PFET

(P-channel MOSFET) switch. This allows current to flow

from the input through the inductor to the output filter capaci-

tor and load. The inductor limits the current to a ramp with a

slope of around (PV

IN -VOUT) / L, by storing energy in a

magnetic field. During the second part of each cycle, the

controller turns the PFET switch off, blocking current flow

from the input, and then turns the NFET (N-channel MOS-

FET) synchronous rectifier on. In response, the inductor’s

magnetic field collapses, generating a voltage that forces

current from ground through the synchronous rectifier to the

output filter capacitor and load. As the stored energy is

transferred back into the circuit and depleted, the inductor

current ramps down with a slope around V

OUT / L. The

output filter capacitor stores charge when the inductor cur-

rent is high, and releases it when low, smoothing the voltage

across the load.

The output voltage is regulated by modulating the PFET

switch on time to control the average current sent to the load.

The effect is identical to sending a duty-cycle modulated

rectangular wave formed by the switch and synchronous

rectifier at SW to a low-pass filter formed by the inductor and

output filter capacitor. The output voltage is equal to the

average voltage at the SW pin.

PWM Operation

While in PWM (Pulse Width Modulation) mode, the output

voltage is regulated by switching at a constant frequency

and then modulating the energy per cycle to control power to

the load. Energy per cycle is set by modulating the PFET

switch on-time pulse width to control the peak inductor cur-

rent. This is done by comparing the signal from the current-

sense amplifier with a slope compensated error signal from

the voltage-feedback error amplifier. At the beginning of

20165336

FIGURE 3. Typical Application Circuit

LM3207TL Datasheet(PDF) 14 Page - National Semiconductor (TI)

LM3207TL Datasheet(HTML) 14 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Datasheet Download

Link URL