lm2706 National Semiconductor Corporation, lm2706 Datasheet - Page 8

• Manufacturer Part Number: lm2706
• Description: Miniature, Variable, Step-down Dc-dc Converter With Bypass For Rf Power Amplifiers
• Manufacturer: National Semiconductor Corporation

www.national.com
Typical Performance Characteristics
25˚C, unless otherwise noted.) (Continued)
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).
DC PWM mode output voltage precision is
is typically around 95% for a 200 mA load with 3.25V output,
Forced Bypass Operation
Output Voltage Ripple
(V
OUT
= 1.5V)
±
2%. Efficiency
200409A5
200409A3
(Circuit of Figure 1 , V
8
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.)
IN
Output Voltage Ripple
Vcontrol Response
= EN = 3.6V, ABD = BYP = 0V, T
(V
OUT
= 3.25V)
200409A4
200409A6
A
=