25˚C, unless otherwise noted.) (Continued)

Forced Bypass Operation

Vcontrol Response

200409A3

200409A4

Output Voltage Ripple

(V

Output Voltage Ripple

(V

200409A5

200409A6

Device Information

The LM2706 is a simple, step-down DC-DC converter and

bypass switch optimized for powering RF power amplifiers

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

battery powered RF devices. It is designed to allow the RF

PA to operate at maximum efficiency over a wide range of

power levels from a single LiION battery cell. It is based on

a current-mode buck architecture, with synchronous rectifi-

cation for high efficiency. It is designed for a maximum load

capability of 300 mA. Maximum load range may vary from

this depending on input voltage, output voltage and the

inductor chosen.

The device has all four of the pin-selectable operating

modes required for powering RF PAs in mobile phones and

other sophisticated portable devices with complex power

management needs. Fixed-frequency PWM operation offers

regulated output at high efficiency while minimizing interfer-

ence with sensitive IF and data acquisition circuits. Forced

Bypass mode turns on an internal FET bypass switch to

power the PA directly from the battery. Automatic Bypass

mode turns on the bypass switch when the input voltage gets

close to the set output voltage. This helps the RF power

amplifier maintain its operating power during low battery

conditions by reducing the dropout voltage across the

LM2706. Shutdown mode turns the device off and reduces

battery consumption to 0.1 µA (typ).

is typically around 95% for a 200 mA load with 3.25V output,

3.9V input. The output voltage is dynamically programmable

from 1.5V to 3.25V by adjusting the voltage on the control pin

without the need for external feedback resistors. This en-

sures longer battery life by being able to change the PA

supply voltage dynamically depending on its transmitting

power.

Additional features include soft-start, current overload pro-

tection, over voltage protection and thermal overload protec-

tion.

The LM2706 is constructed using a chip-scale 10-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 (600 kHz) reduces the size of

external components. As shown in Figure 1, only three ex-

ternal power components are required for implementation.

Use of a micro-SMD package requires special design con-

siderations for implementation. (See Micro SMD Package

Assembly and Use in the Applications Information section.)

Its fine bump-pitch requires careful board design and preci-

sion assembly equipment. Use of this package is best suited

for opaque-case applications, where its edges are not sub-

ject 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 Device Information section.)

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